PROVISION OF SUCH PROPERTIES BY ULTRASONIC IMPACT TREATMENT

FIELD OF INVENTION

[0001] The invention is directed to welded joints having new strength and process induced properties and the process of providing such properties to the welded joints by ultrasonic impact treatment (UIT) . The welded joint, of the invention has specific properties providing improved quality and reliability to the welded joint. In a welded joint, the properties to be obtained or enhanced are defined based on the task the welded joint is to serve, such as in the areas of quality, reliability and fabricability.

BACKGROUND OF INVENTION

[0002] U.S. Patent Nos. 6,171,415 Bl and 6,338,765 Bl describe ultrasonic impact methods for treatment of welded structures using pulse impact energy, in particular ultrasonic impact energy. These patents teach fabrication and repair treatments for welded structures based on stochastic ultrasonic impact treatment. The frequency and amplitude of an ultrasonic transducer are basic aspects of the impact. The striction feedback signal allows selection of parameters sufficient and necessary to obtain a specified treatment effect.

[0003] It has now been found to be desirous to customize properties of a welded joint structure. This is in particular beneficial with respect to welded joints in view of the particular task and corresponding

structure of the joint to further enhance quality and reliability of the joint.

OBJECTS AND SUMMARY OF THE INVENTION

[0004] Accordingly, the present invention is directed to non-detachable welded joints with improved properties and the provision of such properties to the welded joints when subjecting the welded joint to ultrasonic impact treatment. New structural properties are obtained in the welded joint in view of the particular task to which the welded joint is intended to perform. The description herein is set forth in relation to welded joints. However, an equivalent non-detachable welded structure may also be treated in accordance with the invention as described herein and the engineering solutions described herein may be applied to any other equivalent non- detachable welded joints and structures formed thereby.

[0005] The invention also involves the selection of parameters for ultrasonic impact application upon welded joints and structures with new and predetermined properties.

[0006] As with the engineering solutions described in U.S. Patent Nos. 6,171,415 Bl and 6,338,765 Bl, the present invention also utilizes stochastic ultrasonic impact to treat welded joints. The present invention, however, demonstrates that certain ultrasonic impact treatment parameters in combination improve technical properties of a welded structure, in particular a welded joint. These parameters include (1) the repetition rate and length (or duration) of the ultrasonic impact, (2) the pressure or pressing force exerted on the ultrasonic impact tool against the surface being treated and (3) the impact amplitude. The new conditions of ultrasonic

impact treatment of the invention also involve an extension of ranges of standard parameters for exciting the ultrasonic transducer that generates the carrier ultrasonic oscillating frequency in the indenter of the ultrasonic impact tool. A certain combination of these parameters make it possible to obtain new properties or modify existing properties in welded joints in view of the task the joint is to serve. The selected parameters for the ultrasonic impact treatment control the ultrasonic impact and create the necessary conditions in order to define new quality and reliability criteria for welded structures and obtaining welded structural properties suitable for serving predetermined tasks of the welded structures.

[0007] The invention can be utilized for any type of non-detachable welded structure, but primarily provides welded joints with properties which result in significant performance enhancement. Examples of welded joint structures of the invention include welded joints in high-strength steels,- welded joints with stress concentration; welded joints subject to unbalanced loading, welded joints having defects or damaged areas, such as cracks; welded joints requiring predetermined manufacturing accuracy; repaired welded joints; welded joints needing repair; lap welded joints; tack welds for joints; corner welded joints,- welded joints prone to liquation, coarse grain and pore formation; welded joints made with preliminary heating; welded joints having predetermined stress corrosion resistance; welded joints with holes; welded joints in brackets or stiffeners,- and welded joints prone to martensite formation.

DESCRIPTION OF DRAWINGS

[0008] FIGURE 1 illustrates, in terms of amplitude and time, vibrations of an ultrasonic transducer which cause ultrasonic impact .

[0009] FIGURE 2 illustrates, in terms of amplitude and time, the force impulse randomly transferred by- ultrasonic impact.

[0010] FIGURE 3 illustrates, in terms of amplitude and time, the lengthened ultrasonic impact obtained using the process of the invention.

[0011] FIGURES 4a and 4b illustrate fatigue limits of high strength steel untreated and treated according to the invention, respectively.

[0012] FIGURE 5 illustrates stress and deformation distribution in a stress concentration area of material of a welded structure.

[0013] FIGURES 6a and 6b illustrate, as an example, girders and loading conditions possible therewith, and the change in the loading conditions as illustrated through change in the stress concentration area following ultrasonic impact treatment which compensates for dangerous effects of external factors.

[0014] FIGURES 7a, 7b and 7c illustrate a socket welded joint before and after treatment according to the invention and the effect on stress of the joint.

[0015] FIGURES 8a, 8b and 8c illustrate a defect retardation mechanism for compressive stresses induced by ultrasonic impact. FIGURE 8a shows the joint before treatment, FIGURE 8b during treatment and FIGURE 8c after treatment.

[0016] FIGURES 9a, 9b and 9c illustrate a technique of weld deformation compensation using, as an example, a symmetric corner welded joint taking into account directional weld shrinkage. FIGURE 9a illustrates the

welded joint and tolerances thereof before ultrasonic impact treatment and FIGURE 9b following treatment. FIGURE 9c shows a schematic of deformation compensation direction matching.

[0017] FIGURES 10a, 10b, 10c and 1Od illustrate a mechanism of action of a repair of a welded joint with crack and stress redistribution due to ultrasonic impact treatment.

[0018] FIGURES 11a and lib illustrate the formation of a weld joint protected from root crack formation by positive flank angles of the weld metal.

[0019] FIGURES 12a and 12b illustrate another weld joint formed to be protected from root crack formation.

[0020] FIGURES 13a to 13e illustrate a spot welded joint before, during and after ultrasonic impact treatment thereof.

[0021] FIGURE 14a illustrates an untreated lap joint; FIGURE 14b illustrates a lap joint during treatment; and FIGURE 14c illustrates the lap joint subsequent to treatment .

[0022] FIGURES 15a and 15b illustrate a corner welded joint before and after treatment in accordance with the invention, respectively.

[0023] FIGURES 16a and 16b illustrate another corner welded joint before and after ultrasonic impact treatment .

[0024] FIGURES 17a and 17b illustrate a weld joint's structural phase homogeneity (enlarged portion) before and after ultrasonic impact treatment, respectively.

[0025] FIGURES 18a and 18b illustrate a weld joint

(including an enlarged portion) untreated and after ultrasonic impact treatment to provide activated crystallization (FIGURE 18b) in the weld joint. FIGURE

18c graphically represents the treated and untreated weld joints.

[0026] FIGURES 19a and 19b illustrate a weld joint without and with ultrasonic impact treatment activated degassing, respectively.

[0027] FIGURES 20a and 20b illustrate a welded joint with and without hydrogen content. FIGURE 20c graphically compares a joint with a permissible hydrogen content and a joint with minimization of residual diffusion of hydrogen content following ultrasonic impact treatment.

[0028] FIGURE 21 graphically illustrates the corrosion rate of welded joints of steel with high carbon content untreated and treated by ultrasonic impact in accordance with the invention.

[0029] FIGURES 22a and 22b illustrate a welded joint with holes at the tips of a crack before and during ultrasonic impact treatment, respectively.

[0030] FIGURES 23a and 23b illustrate a welded bracket joint before and after ultrasonic impact treatment, respectively.

[0031] FIGURE 24 illustrates a diagram of supercooled austenite decomposition in steel.

[0032] FIGURES 25a, 25b and 25c illustrate a welded joint before coating and ultrasonic impact treatment

(UIT) , after application of a protective coating and before UIT, and after UIT over the coating, respectively.

[0033] FIGURE 26 illustrates examples of welded joint structures obtainable.

DETAILED DESCRIPTION OF THE INVENTION

[0034] Ultrasonic impact treatment utilizes vibrations resulting from excitation of an ultrasonic transducer. As shown in FIGURE 1, the vibrations occur at a certain

amplitude over a defined time. The vibrations can be forced when the transducer is activated or free during a pause. The amplitude will lessen during free vibration over time. As shown in FIGURE 2, vibrations as illustrated in FIGURE 1 randomly transfer the force impulse to a freely axially moving impacting element or indenter. The forced vibrations of the ultrasonic transducer, as shown in FIGURE 1, are interrupted to get information about free vibrations of the ultrasonic transducer under load and to correct the oscillator operating mode. The source of this information is the feedback signal delivered from the winding or electrodes of the active element during pause. It is noted that this principle remains general for all types of active materials used in ultrasonic transducers, specifically magnetostrictive or piezoceramic. To analyze and correct the operation of a generator, and hence a transducer, the striction feedback signal is generally used (as described in Russian Patent No. 817931 of March 30, 1981) . Thus, in order to select ultrasonic impact treatment conditions in accordance with a task for a particular welded joint, the striction feedback signal is used and the technical system tuned for frequency and amplitude of transducer vibrations under off-load and on-load conditions. [0035] Besides ultrasonic transducer vibrational parameters, being of importance in ultrasonic impact treatment, it has now been determined that related parameters of the ultrasonic impact are important in obtaining or modifying properties and, thus, characteristics of non-detachable welded joints by ultrasonically impacting material of the joint. Through selection of particular parameters and optimization of these parameters, welded joints having predetermined improved properties can be obtained. The selection of

ultrasonic transducer vibrational parameters and ultrasonic impact parameters are based on the related characteristics of the transducer-indenter-treated object oscillating system wherein the characteristics are interdependent with the pressure applied in treatment against the joint, physical and mechanical properties of the joint material, and acoustic properties of the joint itself. FIGURE 3 illustrates how the invention results in lengthening of the ultrasonic impacts, and thus improving efficiency of the ultrasonic energy transfer to a treated object in order to obtain new predetermined properties in welded joints and structures. Accordingly, the ultrasonic impact efficiency criteria are direct effects upon the joint material and the associated length, frequency and amplitude parameters of the ultrasonic impact.

[0036] Parameters of such an acoustic and mechanical system provide the link for obtaining new or modified properties in welded joint structures. The process of determining the correct combination of select parameters involves:

(a) Defining the actual physical properties of the weld and the material forming a welded joint,

(b) Defining conformity of the properties of (a) to properties desired to meet quality and reliability requirements for a specific joint,

(c) Defining the physical factors resulting from ultrasonic impact treatment on the welded joint in context of providing the desired properties to the joint,

(d) Defining criteria of the effect of ultrasonic impact treatment on providing the desired joint properties,

(e) Defining conditions of the ultrasonic impact treatment to provide the desired properties of the joint,

(f) Defining the ultrasonic impact treatment conditions in combination with parameters of the transducer, ultrasonic impact, indenter, pressure, mechanical properties and acoustic characteristics of the treated joint material, and

(g) Carrying out ultrasonic impact treatment on the joint in accordance with the definitions established above.

[0037] More particularly with respect to the above, to provide non-detachable welded joints with predetermined new or modified properties by ultrasonic impact treatment, the actual physical properties of the welded joint to be treated are initially determined by conventional testing techniques.

[0038] The properties desired in a welded joint following treatment must then be defined and evaluated as to the difference thereof from the properties of a welded joint before treatment. This may be achieved by the present invention referred hereinafter as an algorithm or series of procedural steps to achieve the desired end. The algorithm generally includes (1) defining conformity of the actual properties of the joint material to specified requirements; (2) defining the physical factors and the mechanism of ultrasonic impact treatment on a welded joint; (3) defining criteria in determining desired weld joint quality and reliability; (4) defining the basic criteria of the ultrasonic impact treatment on a welded joint; (5) defining parameters of the ultrasonic impact treatment for providing non-detachable welded joints with desired properties, and (6) determining the results of the ultrasonic impact treatment on a welded

joint to provide predetermined properties. The algorithm of the invention is described in further detail hereafter. More particularly, the algorithm involves initially determining conformity of the actual properties of the non-detachable welded joint to be treated to the properties desired in the joint in view of the task the joint is to serve, and conforming to a set of ultrasonic impact treatment parameters required to obtain the desired properties of the welded joint.

[0039] Physical factors and the mechanism of ultrasonic impact treatment on a welded joint include plastic deformation caused by the low-frequency impact; ultrasonic plastic deformation during the impact; amplitude and attenuation (decrement of damping) of the ultrasonic stress wave in the material of a given joint, while ultrasonic vibrations of a layer saturated with plastic deformations produced by low-frequency impact and ultrasonic plastic deformation occur during the impact; and temperature and heat rejection rate at the contact point during impacting.

[0040] Criteria in determining desired welded joint quality and reliability include geometry accuracy; residual deformations and their nominal dimension tolerance; residual stresses equilibrated within the volume of the joint and structural segments of the joint material; acceptable stress concentration level and configuration of stress raisers responsible for the load- carrying capacity of the joint; fatigue limit and fatigue resistance under low-cycle and high-cycle reversed and fluctuating loading; and fatigue limit and resistance to corrosion and corrosion-fatigue failures in aggressive environment under low-cycle and high-cycle reversed and fluctuating loading, and properties of the welded joint material.

[0041] Basic criteria of the ultrasonic impact treatment effect on a welded joint include the level of induced residual stresses and deformations; relief, roughness and geometry modification of the surface and transitional areas thereof and modification of material properties in the treatment area; relaxation and redistribution of residual stresses produced by the manufacturing technique of a given joint prior to ultrasonic impact treatment; and modification of the joint type and conditions of its resistance to service loads.

[0042] Parameters of the ultrasonic impact treatment

(UIT) for providing non-detachable welded joints with properties desired include (1) pressure on the ultrasonic impact tool in the range of about 0.1 to 50 kg, (2) carrier ultrasonic frequency of the transducer between about 10 and 800 kHz, (3) amplitude of ultrasonic vibrations at carrier frequency between about 0.5 and 120 μm, (4) ultrasonic impact frequency and self-oscillation frequency of the tool-indenter system between about 5 and 2500 Hz with duration of a random ultrasonic impact in the range of about 2 to 50 vibration periods at carrier ultrasonic frequency, (5) self-oscillation amplitude of the tool between 0.05 and 5 mm, (6) the level of connection between a freely axialIy moving indenter and a transducer of the tool, which depends on the range of UIT parameters described above, and (7) free ultrasonic impacts with parameters set within above-mentioned ranges in accordance with the task, properties and sizes of the material and welded joint.

[0043] The results of the ultrasonic impact treatment on a welded joint to provide predetermined properties include at least one of the following positive changes: surface roughness and relief of about 0.1 μm and above; a

radius between surfaces of about 0.5 mm and above; the depth of the groove along the weld toe line or line between any surfaces in the stress concentration area of up to about 2 mm with the width of the groove being up to about 10 mm; improvement of material mechanical properties in the stress concentration area, as to strength by no less than about 1.5 times and impact strength by no less than about 1.2 times,- plastic deformation, favorable compressive stresses and a favorable relative change in microhardness to a depth of up to about 7 mm,- distribution of elastic compressive stresses due to plastic deformation of material in section normal to the surface to the depth of up to 10 mm; relaxation of process induced residual stresses due to ultrasonic fluctuating stress wave with the amplitude of no less than about 0.05 of the material yield strength, to a depth of up to about 12 mm; favorable residual stresses of the first and second kind on and under the surface to a specified depth of no less than material yield strength and ultimate strength depending on the task definition; compensation for residual process induced deformations by not less than about 40% of those which occurred without UIT application with improvement in stress corrosion resistance by up to about 10 times; improvement in corrosion-fatigue strength by up to about 2.5 times and a life span in- a corrosion environment of up to about 20 times under variable loading; improvement in fatigue limit in air under repeated or fluctuating stress by no less than about 1.5 times and a life span by no less than about 10 times, increasing the strength of a joint by no less than 1 category; formation of a white layer and an amorphous structure to a depth of no less than about 50 μm.

[0044] The non-detachable welded joints can be made of any joined material with the use of ultrasonic impact treatment with or without fusion of the interface of the materials being joined, with or without filler materials, and can contain in the aggregate or in any combination the weld material, transition zone of a solid solution of one material in another and zones altered relative to joined and unjoined material structures and modes of deformation. The non-detachable joints may be made by butt, fillet, lap, narrow-gap or spot welding as well as welding along the aperture of structural elements of any given shape with or without complete, partial or incomplete penetration, with or without edge preparation, and produced by varying means e.g. arc, resistance, laser, electron beam, diffusion, friction, pressure, submerged arc, shielded metal, gas shielded, open and submerged arc welding, welding using filler material, open flame of ultrasonic welding, soldering, and the like.

[0045] Particular welded joints of the invention will now be described.

(A) Welded joints in high-strength steels

[0046] In practice, the use of high-strength steels in the fabrication of welded joints is limited by a low fatigue resistance of the welded joints made from such steels as compared to low and average-strength steels, namely, low-carbon and low-alloy steels with yield strengths of a minimum two times as low and fatigue limits up to two times as high as those of high-strength steels. It is understood in the industry that the conditional boundary between these steels is a yield strength or ultimate strength value of up to 500 MPa.

[0047] The welded joints of high strength steel of the invention obtained have a fatigue resistance which is at

minimum twice as high as that of low and average-strength steels. This is graphically illustrated in FIGURES 4a and 4b. FIGURE 4a shows the fatigue limits of a high strength steel 1, a welded joint of low carbon or low alloy steel 2 and a welded joint of high strength steel without ultrasonic impact treatment 3. FIGURE 4b shows the fatigue limits of a welded joint of high strength steel after ultrasonic impact treatment 4 and of a welded joint of low carbon or low alloy steel after ultrasonic impact treatment 5. As shown, the materials subjected to ultrasonic impact treatment in accordance with the invention are significantly improved. The welded joints made of high strength steels and alloy have a yield strength of σ > 500 MPa following ultrasonic impact treatment determined according to the invention and falling within the parameters as set forth above to provide in the material of the welded joint a fatigue limit which is a minimum of 30% greater than that of steels and alloys with σ ≤ 500 MPa. [0048] More specifically, to obtain the above, ultrasonic impact treatment is applied to an area of hazardous stress concentration at the toe of the weld. Thus, in accordance with the invention, the characteristics of the as-welded joint and the base metal are first determined. Taking into account the need to provide the fatigue limit of the welded joint comparable to the strength of the base metal of no less than 500 MPa, ultrasonic impact treatment conditions are determined by calculating the impact energy that suffices to create plastic deformations and compressive stresses. Ultrasonic impact treatment conditions are then experimentally verified and corrected to serve the task. At the oscillating system frequency of about 27 kHz and a tool pressing force of up to about 10 kg, the ultrasonic

impact treatment conditions to provide a non-detachable welded joint with the desired properties are as follows: ultrasonic transducer vibrational amplitude during impact of not less than about 30 μm, impact frequency in the range of about 80 to 250 Hz, tool self-oscillation amplitude of up to about 2 mm, indenter diameter of about 3 to 6.35 mm, and the average length or duration of the indenter being in a range of about 10-35 mm depending on the welded joint type. The above ultrasonic impact treatment conditions are responsible for strengthening hazardous tensile stress concentration area and creation therein of favorable compressive stresses to a depth of no less than about 2 mm, whose magnitude at the surface is greater than the yield strength and fatigue limit of the base material by a factor of up to about 1.5. In such a case, the stress concentration area after ultrasonic impact treatment attains the configuration of a regular groove with a depth up to about 1 mm, which is formed due to plastic deformation caused by the ultrasonic impact and provides a smooth transition between the weld and the base metal.

[0049] Thus, the inclusion of high-strength steels in the fabrication of welded structures and in the resulting welded joint is available.

(B) Welded joints with stress concentration [0050] Physical and mechanical properties of the material at a weld toe of a joint, the nature of operating stresses and their distribution at a stress concentration area are the basic strength and fatigue resistance criteria for welded joints with stress concentration together with the concentration factor that depends on the geometry of the transition between the weld and base metal at the weld toe.

[0051] Weld joints are obtained according to the invention by ultrasonic impact treatment of the stress concentration area to improve the strength, ductility and impact strength of the treated welded joint material above nominal values relative to untreated material forming the welded joint. In addition, the welded joint is modified and adapted to external loads, since the ultrasonic impact treatment of the stress concentration area performed induces favorable residual compressive stresses in the treated area.

[0052] The condition, characteristics and properties of the treated area are determined by the features of ultrasonic and impulse plastic deformations, which are dependent on the amplitude and length of ultrasonic impacts and their repetition rate during ultrasonic impact treatment. As a result, the ultimate strength and fatigue limit of the weld joint material in the stress concentration area are greater than those of materials forming the weld joint.

[0053] The mode of deformation of the weld joint under such conditions is defined by the residual stresses and equivalent plastic and elastic deformations. The favorable residual compressive stresses in the area of ultrasonic plastic deformations due to ultrasonic impact treatment are not less than the greater nominal yield point of the materials. Elastic deformations and respective elastic stresses decrease exponentially in the depth of the treated material from the maximum of the residual compressive stresses equilibrating the elastic stresses while the level and distribution of the residual and elastic stresses on and under the surface are established to compensate for environmental effect and operational stresses.

[0054] Stress and deformation distribution in the stress concentration area are shown in FIGURE 5 together with the change in material properties in this area as a result of ultrasonic impact treatment performed in accordance with the algorithm described herein.

[0055] It is well-known that hazardous stress concentration is generally localized at a weld toe. This is due to the unfavorable sharp transition between the weld and the base metal, the presence in this zone of pronounced welding defects (such as overlaps, irregularities, undercuts) as well as due to tensile residual stresses caused by weld shrinkage on cooling.

[0056] In accordance with the invention, ultrasonic impact treatment produces a smooth transition between a weld and a base metal by forming a groove with radiuses at its boundaries of about 0.5 mm and greater, with widths of greater than zero and up to about 10 mm and depths of greater than zero up to about 2 mm depending on the metal thickness and the weld toe angle. Ultrasonic impact treatment conditions define the relief, groove roughness (not less than Ra = 75 μin) , the magnitude and the nature of induced compressive stresses (not less than the material's ultimate strength), the effect thereof to a depth of not less than about 2 mm in the plastic deformation area and not less than about 5 mm in the elastic deformation area, and residual welding stress relaxation to a point not greater than about 20% of the original state.

[0057] The parameters to provide the welded joint include an ultrasonic vibration amplitude during impact of greater than zero and up to about 50 μm at a frequency of greater than zero and up to about 80 kHz, impact frequency of greater than zero and up to about 500 Hz, tool self-oscillation amplitude of about 0.2 mm and

greater, the off-duty factor of impact impulses of greater than zero and up to about 0.5, a pressing force of at least about 3 kg and as a consequence of the above, impact energy which is equivalent and sufficient to create compressive stresses and modify material ultimate strength properties in the stress concentration area to be greater than the original stress and strength properties and sufficient to compensate for external operational forces.

[0058] Ultrasonic impact treatment of carbon steels performed in accordance with the method under the above- mentioned conditions increases the fatigue limit of a welded joint as a result of a combined action of the physical factors set forth above, as well as the removal of welding defects by plastically deforming the welded joint material.

(C) Welded joints subjected to balanced and unbalanced loading

[0059] A primary requirement that defines the ability of welded joints to resist failure under balanced and unbalanced loading in the original condition is the unbalanced nature of the load on these joints after ultrasonic impact treatment to obtain properties in accordance with the invention. However, the final stressed state of the welded joint will always depend on the condition of external loading on the weld joint. On this basis, ultrasonic impact treatment of the weld joint is performed in accordance with the algorithm of the invention concurrently with balanced or unbalanced loading on the joint, which is close to actual loading.

[0060] The level and nature of external loading on a given weld joint and related parameters of ultrasonic impact treatment performed are determined and matched by the condition of adequacy to compensate for the effect of

factors causing crack formation during operation of a given weld joint.

[0061] The procedure of rating the ultrasonic impact treatment adequacy as a part of the invention can be as set forth below.

[0062] Initially, the varying loading, which is adequate to the actual loading, is applied to a sample or the actual welded joint in the as-welded condition and stresses or equivalent deformations due to the loading are measured by any conventional means. By calculating the required impact energy, the parameters of ultrasonic impact treatment are then determined to compensate for the stresses or deformations. Thereafter, ultrasonic impact treatment is applied together with the varying loading and the level of compensation for hazardous operational stresses or deformations is established by the measuring procedure used before. If required, design parameters of ultrasonic impact treatment are corrected to compensate for stresses or deformations as defined by the task the weld joint is to perform.

[0063] The ultrasonic impact treatment of a welded joint applied in parallel with the load can be performed in the free state on an unfixed structure, in a rigid contour on a fixed structure, or under constant, variable and balanced loading.

[0064] To solve problems as above described, the parameters of ultrasonic impact treatment to provide welded joints made from carbon structural and stainless steels, and aluminum and titanium alloys with the desired properties includes ultrasonic vibration amplitude during impact of greater than zero and up to about 50 μm at a frequency of greater than zero and up to 80 kHz, the impact frequency of greater than zero and up to 500 Hz with the prevailing impact duration on average of no less

than about 1 ms, the tool self-oscillation amplitude of about 0.2 mm and greater, the pressing force of no less than about 3 kg and as a consequence of the above, the impact energy equivalent and sufficient to create compressive stresses and modify material ultimate strength properties in the stress concentration area to be greater than the original compressive stresses and strength properties and are sufficient to compensate for external operational forces.

[0065] The change in the loading condition as a result of concurrent ultrasonic impact treatment which results in compensation for the dangerous effects of external factors is shown in FIGURES βa and 6b through exemplary girder structures. FIGURE 6a shows girders under different stress loadings. Girder 10 illustrates a girder under static loading Fc. Girder 11 is under cyclic, fluctuating or dynamic loading Fv. Girder 12 is under complex loading, i.e., Fc + Fv. FIGURE 6b shows the initial stressed state in the stress concentration area for each of girders 10, 11 and 12 as compared to the stressed state in the same girder after ultrasonic impact treatment .

[0066] Another exemplary structure is a so-called "socket weld joint" as shown in FIGURE 7a. In FIGURE 7a, 20 indicates a socket welded joint and 21 denotes the ultrasonic impact tool in treatment of the weld for the joint. The feature of this "socket weld joint" which is unique is that the joint is generally used in structures having both fluctuating and alternating loading with a relatively small thickness in the material forming the welded joint. In this case, ultrasonic impact treatment of the stress concentration area in accordance with the invention forms a groove of dimensions and depth not greater than about 0.15 mm of thickness of the treated

material. FIGURE 7b illustrates the joint before and after ultrasonic impact treatment. Following treatment, the welded joint has a radius 22 of a minimum of about 0.5 mm, width of greater than zero and up to about 10 mm, depth of greater than zero and up to about 2 mm and about 0.15 mm of web thickness when the overall thickness is about 4 mm.

[0067] Thus, the modification of the material properties in the stress concentration area results in a specific level of compressive stresses induced in the stress concentration area of the joint. Conditions for creating such stresses and groove dimensions related with weld joint dimensions and the thickness of materials forming the socket weld joint give the socket weld joint in the aggregate an excellent breaking strength under fluctuating and cyclic loads that induce stresses above the yield strength of the joint material in the stress concentration area. FIGURE 7c comparatively shows the cycle stress of the joint before and after ultrasonic impact treatment. Accordingly, the loading condition and ultrasonic impact treatment of the weld toe and the load- carrying component on the side of constant loading and/or localization of varying loading, initiate the ultrasonic plastic deformation, creation and distribution of compressive stresses and formation of a transition between the weld and the base metal so as to compensate for the influence of static or cyclic or varying stresses that cause the formation of in-service cracks due to the stress concentration above the yield point of the base metal along the weld toe and/or in the root.

(D) Welded joints with defects and damaged areas (including cracks)

[0068] The practice of fabrication and operation of welded structures presents an independent group of problems associated with the improvement of the life and reliability of welded joints which have welding defects, material structural defects, meso-structure damages and cracks.

[0069] The benefits of ultrasonic impact treatment performed in accordance with the invention makes it possible to provide properties in welded joints in which the above defects are detected so as to result in a reliable joint. Of importance for weld joint modification in such instance are the ultrasonic plastic deformation, deformations due to external force impulse

(impact) and residual compressive stresses that are introduced into the material of the welded joint wherein such are within the above-described parameters for these factors of ultrasonic impact effect on the material condition.

[0070] Of critical importance in modifying defective welded joints is ultrasonic plastic deformation, i.e., deformations caused by the impact and residual compressive stresses introduced into the material of the welded joint that cover the above-described defects and retard their development under external forces due to operational loads.

[0071] The crack is the most common example of a hazardous defect in a welded joint material. Using differing crack sizes, in fact, allows for defining the internal condition and simulating the initial conditions or stages of failure produced by other types of defects under external forces.

[0072] The hazardous area of all types of welding defects, including cracks, is the stress concentration area, as shown in FIGURES 8a-8c. Also shown in FIGURES

8a-8c is the defect retardation mechanism in the field of compressive stresses caused by the ultrasonic impact treatment. In FIGURE 8a, 30 denotes a defective welded joint containing a crack before ultrasonic impact treatment and the stresses present in relation thereto. FIGURE 8b illustrates treatment of the defective area with an ultrasonic impact tool 31 to create a compressive field. FIGURE 8c illustrates the welded joint 32 following ultrasonic impact treatment and the change in the stresses present therein (compare FIGURES 8a and 8c) .

[0073] A defect presents the severest hazard when the tension vector is perpendicular to the plane on which the greatest defect area is projected. In the case illustrated in FIGURES 8a-8c, the crack periphery- outlines the stress concentration area. When the defect is subjected to the compressive stress field by means of ultrasonic impact treatment in accordance with the invention, this makes it possible to compensate for unfavorable tensile stresses in the stress concentration area and displace them to a region of the material where the stress concentration hazard is unlikely.

[0074] In this instance, ultrasonic impact treatment is localized on the surface, whose dimensions suffice to displace possible tensile stresses away from the possible stress concentration at a distance sufficient to maintain resulting compressive stresses under unfavorable conditions of external force action. The dimensions of this surface are determined during simulating defect development and retardation conditions as described herein. Ultrasonic impact treatment parameters in this case to provide the desired welded joint include the following: tool pressing force of greater than zero and no greater than about 10 kg; ultrasonic impact frequency of greater than zero and no greater than about 500 Hz;

prevailing duration of ultrasonic impact of no less than an average about 1 ms; ultrasonic carrier frequency of greater than zero and up to about 100 kHz depending on the properties of the material being treated and the surface condition requirements; ultrasonic oscillation amplitude of the indenter during impact of no less than about 30 μm; and impact amplitude of no less than about 0.2 mm. The impact energy defined in accordance with the process and expressed by the above parameters and corresponding indenter mass is set so as to produce compressive stresses in the plastic deformation area to a depth of no less than about 2 mm and in the elastic deformation area, to a depth that suffices to compensate for the residual effect of tensile stresses.

[0075] New properties and welded joint material conditions so obtained allow compensation for the effect of the dangerous stresses resulting from operational loading on a given welded joint and thus also the retardation of the defect development when the joint is in service.

(E) Welded joints with specified requirements to manufacturing accuracy

[0076] Geometric accuracy of welded joints is a primary quality and reliability characteristic. Ultrasonic impact treatment in accordance with the invention is characterized by a system of features that guarantees meeting this fundamental technical requirement. These features essentially include ultrasonic relaxation (of stresses and deformations) , ultrasonic and impulse plastic deformation (material redistribution) , and creation of compressive stresses

(redistribution of tensile and compressive stresses and deformations) .

[0077] Thus, four ways to obtain a specified accuracy in a welded joint are as follows: (1) ultrasonic impact treatment performed in accordance with the invention using a rigid attachment (fixed position) and ultrasonic relaxation of residual welding stresses caused by- fixation, (2) welding without fixation, ultrasonic and impulse plastic deformation of the weld and base metal in the joint area in accordance with the invention, material redistribution in the joint, compensation for shrinkage and thus welding deformations, (3) combining (1) and (2) above in the ultrasonic impact treatment, and (4) dividing (differentiation of) weld shrinkage by directions and ultrasonic impact treatment taking into account compensation for joint deformations in these directions.

[0078] The above examples of obtaining welded joints with specified configuration accuracy requirements are applied over hot (above ambient temperature) metal during welding or when the weld is cooled down or over cold (at about ambient temperature) metal after welding depending on the task and specific conditions of its solution.

[0079] The technique of weld deformation compensation is shown in FIGURES 9a, 9b and 9c using, as an example, a symmetric corner welded joint taking into account a directional weld shrinkage. FIGURE 9a illustrates the welded joint 40 and the tolerances therein. FIGURE 9b illustrates the welded joint after ultrasonic impact treatment with ultrasonic impact tool 41. Deformations and tolerances are denoted in FIGURE 9b as follows: a and f each indicate residual deformation after ultrasonic impact treatment, b and e each indicate tolerance, and c and d each indicate residual welding deformation. FIGURE 9c illustrates schematically deformation compensation direction matching. While residual welding deformations

in the joints are compensated for by either creating a rigid attachment with subsequent ultrasonic relaxation of residual welding stresses or ultrasonic and impulse plastic deformation and redistribution of the weld metal or by a combination of these effects, and, thus, in so doing match the direction and magnitude of plastic deformation of the weld metal with the ratio between its longitudinal and transverse shrinkage depending on the welded joint type and welding process. [0080] During compensation for deformations in directions specified by the task, the principle is used of selecting the ultrasonic impact treatment tool marks overlap coefficient (K ₀ ) . The greatest value of K ₀ corresponds to the direction of greater residual deformations that should be compensated so as to provide the specified accuracy, while the smallest value of K ₀ corresponds to the direction of smaller residual deformations. The residual deformations in various directions correspond to the shrinkage of weld metal and near-weld zone in these directions, and deformation compensation corresponds to the sum of cumulative displacements of local volumes of weld metal and near- weld zone caused by plastic deformation due to ultrasonic impact treatment. Taking K ₀ to be positive and equal to the relationship between the indentation diameter difference and the indentation center-to-center distance when the surface is fully covered with tool marks, and the ratio of interindentation distance to indentation center-to-center distance corresponds to negative overlap coefficients during intermittent treatment, then ultrasonic impact treatment provides control of deformation compensation in specified directions within the range of values, for which the following is true: 1 > Ko > -1.

[0081] Thus, at a tool or workpiece travel speed of about 90 m/min, K ₀ becomes positive even at an ultrasonic impact frequency of 500 Hz and the indentation diameter of 3 mm. The actual ultrasonic impact treatment speed, however, is within the range of greater than zero and up to about 5 m/min. This emphasizes the reliability of ultrasonic impact treatment in accordance with the method of the invention and possible control of K ₀ within the wide range of treatment conditions, i.e., pressing force on the tool of about 4 kg and above, impact frequency of about 100 Hz and above, impact amplitude of about 0.2 mm and above, impact duration of about 1 ms and above, carrier ultrasonic frequency of no less than about 15 kHz, ultrasonic vibration amplitude during impact of no less than about 30 μm when steels and high-strength alloys are treated and greater than zero and no greater than about 30 μm when aluminum alloys and metals with a yield strength of up to 350 MPa are treated.

(F) Repaired welded joints

[0082] Repaired welded joints covers a wide area of fabrication and operation of welded structures, e.g., repair of weld defects, failures and cracks, strengthening structures and elements thereof, as well as providing additional improvement in structural stability and load-carrying capacity and correcting structural configuration in the process of fabrication and operation. At the same time, repairs of welded joints are a source of residual welding stress, deformation, and stress concentration area and, thus, unregulated metal fatigue.

[0083] Ultrasonic impact treatment conducted in accordance with the invention solves these problems and results in welded joints repaired to have improved properties, i.e., a level of residual stresses not

greater than about 0.5 of the yield strength of the welded joint material, residual welding deformations not greater than 100% of the dimensional tolerance specified for a given joint, and fatigue resistance of not less than that of the base metal of the given welded joint.

[0084] The mechanism of action on a repaired welded joint, and cracks and stress redistribution due to ultrasonic impact treatment are illustrated in FIGURES 10a to 1Od.

[0085] As shown in FIGURE 10a, the crack in a plane perpendicular to tensile forces or in a spatial surface close to the plane creates a concentration of stresses that is many times greater than normal design stresses due to such forces.

[0086] A repaired welded joint somewhat improves the situation. However, it produces a new residual tensile stress concentration at the ends of the repair welding caused by the longitudinal shrinkage of the weld deposition (FIGURE 10b) .

[0087] Ultrasonic impact treatment in accordance with the invention (FIGURE 10c) redistributes unfavorable residual tensile stresses that are replaced by compressive stresses in the hazardous weld deposition area (FIGURE 1Od) . As this takes place, tensile stresses move into the region of normal stresses that is safe for the welded joint load-carrying capacity and can be calculated using standard procedures.

[0088] Ultrasonic impact treatment of a repaired welded joint, as defined by the task served by the joint, is applied in the course of welding to the metal being cooled and to the cold metal.

[0089] Thus, to improve the quality of the weld metal and its resistance to structural defect formation, ultrasonic impact treatment in accordance with the

invention is done during welding. In order to compensate for residual welding deformations and stresses localized in the repair welded area, ultrasonic impact treatment in accordance with the invention is done upon the metal being cooled. Ultrasonic impact treatment is done on the cold (ambient temperature) metal to harden welded joint metal, create favorable compressive stresses in hazardous areas, and replace and relax hazardous tensile stresses.

[0090] In doing so to provide the welded joint, the pressure upon the ultrasonic tool during manual treatment of steels is about 3 kg and above, which may increase up to 20 kg in the case of mechanized treatment, the impact frequency is not less than about 80 Hz, the impact frequency is not less than about 0.2 mm, the impact length is not less than on average about 1 ms, the carrier frequency of indenter ultrasonic vibrations is about 15 kHz and above, the ultrasonic vibration amplitude during impact is not less than about 20 μm when hot (above ambient temperature) metal is treated and not less than about 30 μm when treating metal being cooled and cold metal. When weld deposits of aluminum alloys are treated, the ultrasonic vibration frequency is reduced by up to 40% subject to the strength of material.

(G) Corner joints with incomplete penetration protected from root cracking

[0091] A welded joint protected against root cracking and having a load-carrying capacity is obtained by selecting type and dimensions of a weld joint with complete, partial or incomplete penetration. Achieving such is particularly difficult when the joint has partial or incomplete penetration.

[0092] The cause of root crack formation is primarily associated with the flank angle of the weld metal with

the web end and flange plane in a gap between them, as may be exemplified by a corner joint. In the case of a negative (acute) flank angle, the crack formation directly results from the stress concentration in this area of the welded joint.

[0093] Ultrasonic treatment of a weld joint, performed during welding, solves this problem by changing heat exchange conditions at the boundary between the molten metal and the solid metal in the root of the weld. This phenomenon may be explained as follows. Ultrasonic impact during welding causes an impulse and ultrasonic stress wave to propagate in the weld metal and thus the molten metal. As a result, strong acoustic flows are formed at the molten-solid metal boundary in the weld root that contribute to heat exchange activation and hence greater penetration of the surface of metal forming the gap between web and flange in this area. Thus, based on the procedure invention, an instrument to control the penetration configuration of the web and flange metal in the weld root may be provided, thereby resulting in a substantially new appearance of a welded joint having positive (obtuse) flank angles of the weld metal with the flange surface and web end, which, in turn, insure that a given welded joint is resistant to stress concentration and fatigue crack formation in the weld root.

[0094] The formation of a weld joint protected from root crack formation by positive (obtuse) flank angles of the weld metal with the web and flange metal in the gap between them is shown in FIGURES 11a and lib. FIGURE 11a illustrates a weld 50 made without ultrasonic impact treatment. FIGURE lib illustrates a weld 51 subjected to ultrasonic impact treatment using an ultrasonic impact tool in an initial operating position 52 during welding and a continuing operating position 53.

[0095] The selection of the tool angle and ultrasonic impact treatment areas, as shown in FIGURES 11a and lib, allow formation in the molten pool of acoustic flows specifically directed relative to the pool boundaries. This, in turn, offers possibilities for control of the flange and web metal fusion penetration intensity in directions where the weld metal favorably meets the base metal.

[0096] Thus, when the flange side face is subjected to ultrasonic impact treatment (operating position 53 FIGURE lib) , the prerequisites are created for better fusion of the flange metal as compared to the web. A close effect can be obtained by increasing the tool angle relative to the flange plane by more than 45° (the position 52 in FIGURE lib) . A choice of treatment conditions, tool angles and positions during treatment depends on the welding process, material and dimensions of a welded joint. The above-mentioned preferred ultrasonic impact treatment conditions to provide welded joints of this type made of carbon steels include: tool pressing force of about 3 kg and above during manual treatment, greater than zero and up to about 25 kg during mechanized treatment; impact frequency of greater than zero and up to about 800 Hz; impact amplitude of about 0.2 mm and above; ultrasonic vibration carrier frequency of about 18 kHz and above; ultrasonic vibration amplitude during impact of greater than zero and up to 20 μm in a temperature range of above about 400 ⁰ C and not less than about 30 μm in a temperature range below about 400 ⁰ C; and ultrasonic impact duration of on average of not less than about 1 ms.

[0097] With favorable redistribution of the weld metal between the flange and the web, ultrasonic impact

treatment in accordance with the invention reduces residual welding stresses by a minimum of 40% of standard mode of deformation of the as-welded joint. [0098] Concurrent with the heat exchange activation effect described above, the ultrasonic impact in accordance with the invention initiates a surface tension reduction effect for the molten metal and, as a consequence of this phenomenon, increases the fluidity of the molten metal. That is, ultrasonic and impulse stress waves are transferred to materials being welded through the weld metal as a result of the ultrasonic impact treatment and increase the yielding and hence the flowability of the molten metal on the web and flange ends in the gap between them. The temperature of the molten pool, activated by the acoustic flow, additionally fuses the edges, forming a concave meniscus similar to that in capillary as shown in FIGURES 12a and 12b. It was established that the molten metal fluidity increases within a wide range of ultrasonic vibration carrier frequencies of up to 300 kHz and ultrasonic impact repetition rates of up to 2500 Hz. Ultrasonic impact treatment parameters are defined in accordance with the process of the invention depending on the properties of welded materials and consumables, the type and sizes of welded joints, the welding process and conditions. In the schematic representation of a welded joint as shown in FIGURES 12a and 12b, FIGURE 12a shows a weld 60 not subjected to ultrasonic impact treatment and the crack formed therein. FIGURE 12b shows a weld 61 subjected to ultrasonic impact treatment. The meniscus in the weld root is denoted by 62. The ultrasonic impact tool is shown in an initial operating position 63 on the weld and in a continuing operating position 64 during treatment of the weld. The corner welded joint, with incomplete

and/or partial penetration, made with ultrasonic impact treatment conducted within the parameters of the invention during making a root run over the weld metal, flange or web, results in the molten metal filling the gap (under ultrasonic impacting) between the stiffener or web end and the flange or web plant with or without diffusion or adhesion bonds between the weld and the base metal in the gap producing a meniscus 62 and fusing of the sharp edges upon solidification from smooth transitions between base and weld metals, thereby increasing the resistance of a given welded joint to stress concentration effect and fatigue crack formation in the root of the weld.

[0099] Thus, one further mechanism makes possible positive (obtuse) flank angles of the weld metal with the web end and flange surface as a result of ultrasonic impact treatment in accordance with the invention. This explains how a new welded joint is formed that is protected from root crack formation due to stress concentration and fatigue.

(H) Spot welded joints

[00100] A specific task associated with the need to increase quality and reliability of a welded joint based on fatigue resistance criterion relates to spot welding. A primary problem is that the danger zone in the weld joint area is inaccessible for conventional stress concentration treatment techniques. This necessitates modifying a mode of deformation of a welded joint across the whole thickness of the materials being welded. Thus, the dangerous heat affected zone must be considered to include stress raisers and represent a circle or ring with an average diameter that is equal to the diameter of a circle along the boundary of a welded joint.

[00101] A spot welded joint made using ultrasonic impact treatment in accordance with the invention features a high level of ultrasonic plastic and impulse deformation across the whole metal thickness in the weld area, the fatigue limit being a minimum of about 1.3 times greater than that of an untreated joint and having an ultimate strength of not less than that of the base metal .

[00102] A schematic representation of a spot welded joint is shown in FIGURES 13a-13e. FIGURE 13a illustrates at 70 an untreated spot welded joint and stresses in relation thereto. FIGURE 13b shows an ultrasonic impact tool 71 in treatment of a spot weld in conjunction with a stop plate 73. In FIGURE 13c, two ultrasonic impact tools 71 and 72 are utilized in relation to a spot weld. FIGURE 13d is a close-up of the point of contact of impact from a stop plate or tool 74 and tool 75 as to the spot weld. FIGURE 13e shows at 76 a treated joint and stresses in relation thereto.

[00103] Ultrasonic impact treatment of a spot welded joint can be done during welding (when the welding electrode at the same time presents the vibration velocity concentrator or indenter) and after welding. The indenter can have a round, flat and circumferential working surface depending on the welded joint size and its post-welding condition.

[00104] In fact, ultrasonic impact treatment can be applied using passive or active resonance acoustic decoupling, passive non-resonance acoustic decoupling and a rigid stop block serving as an "anvil" . It means that plastic deformations in the welded joint area may be formed sequentially from each side and simultaneously from both sides.

[00105] As shown in FIGURE 13a, the risk area of the spot welded joint, where the maximum tensile stresses operate, is localized at the "spot weld" boundary and is positioned in the operational stress critical concentration zone.

[00106] Ultrasonic impact treatment in accordance with the invention completely subjects the welded joint to the favorable compressive stress area and displaces the tensile stress area to the zone without any structural prerequisites for stress concentration.

[00107] Thus, based on the experimental data, ultrasonic impact treatment in accordance with the invention, increases the fatigue limit of a spot weld by at least about 1.3 times and improves the fatigue resistance, yield points, ultimate strengths and impact strength to the level not below that of the base material.

[00108] To obtain spot welded joints made of carbon steels and aluminum alloys, ultrasonic impact treatment conditions include the following and vary within the described amounts based on the joint type and material: ultrasonic impact frequency of not less than about 80 Hz, impact duration of not less than on average about 1 ms at an amplitude of not less than about 0.2 mm, indenter ultrasonic vibration carrier frequency during impact of greater than zero and up to about 100 kHz, ultrasonic vibration amplitude during impact in a range of from about 5 to 40 μm, and tool pressure from about 3 to 30 kg. The stabilization of the resonance frequency of the system "tool-welded joint within a structure" during welding with ultrasonic impact treatment or during ultrasonic impact treatment is the method treatment termination criterion for such types of welded joints.

(I) Lap welded joints and tack welds

[00109] Lap or tack welded joints are extremely prone to cracking at weld ends with cracks quickly propagating on short weld portions. Crack formation in these joints is mainly due to welding defects, unfavorable weld toe angles, stress concentration, the loss of the local stability and strength of a joint, and fatigue. These problems can be solved by creating a welded joint, which is subjected to ultrasonic impact treatment in accordance with the invention to result in the formation of a smooth transition between the weld and base metal. At the same time, such transitions at the tack weld end and along the weld toe line are subjected to ultrasonic plastic deformation, while the fatigue limit of the tack weld is a minimum about 1.3 times greater as compared to the untreated condition, and the fatigue resistance, ultimate strength and impact strength are not less than that of the base metal. A schematic representation of a welded joint and the mode of deformation thereof due to ultrasonic impact treatment is shown in FIGURES 14a to 14c. FIGURE 14a shows an untreated lap joint and stresses 80 in relation thereto. FIGURE 14b illustrates a lap joint during treatment with an ultrasonic impact tool 82 to create compressive stress areas as denoted thereon. FIGURE 14c illustrates the treated lap joint 84 and the stresses associated therewith.

[00110] More specifically, FIGURE 14a shows that maximum tensile stresses are localized at tack weld ends due to longitudinal and, to a lesser extent, transverse weld shrinkage. This situation is aggravated by the fact that the tack weld end area coincides with the operational stress concentration area.

[00111] Ultrasonic impact treatment in accordance with the invention changes the nature of the welded joint mode of deformation, redistributes tensile stresses, replaces

these by compressive stresses and displaces tensile stresses due to operational loads to the welded joint region where stress concentration is unlikely to occur. Ultrasonic impact treatment in accordance with the invention improves the resistance of a given welded joint to formation of cracks caused by the stress concentration due to design features of a given joint and metal fatigue under the unfavorable nature of variable and reversed loading cycles.

[00112] Thus, in parallel with residual stress redistribution, the improvement of a given welded joint resistance to crack formation is also achieved by modifying material properties of the welded joint during ultrasonic plastic deformation thereof, as shown in FIGURES 14a-14c.

[00113] Parameters of ultrasonic impact treatment in accordance with the invention which provide the desired welded joint include the following: ultrasonic impact frequency of greater than zero and up to about 2000 Hz, ultrasonic impact length of not less than on average about 1 ms, impact amplitude of not less than about 0.2 mm, indenter ultrasonic vibration carrier frequency of about 18 kHz and above, indenter ultrasonic vibration amplitude during impact of not less than about 25 μm for carbon steels and not greater than about 30 μm for aluminum alloys, tool pressure against a treated surface of about 3 kg and above.

(J) Corner welded joints

[00114] It is a difficult technical problem to obtain manufacturing accuracy and high fatigue resistance of corner welded joints with a groove varying along the joint perimeter, as well as with a varying flank angle of less than 90° and complete weld penetration. This problem

is aggravated by specific welding stress and deformation distribution present, as well as the joint fatigue limit dependence on the geometric conditions of the formation of a complex oriented in the space joint along the weld perimeter.

[00115] Ultrasonic impact treatment performed in accordance with the invention during welding and over cold metal makes possible a specified dimensional accuracy along the perimeter of such a complex joint and increases fatigue limit at a minimum by a factor of 1.3. A schematic representation of a corner welded joint with a groove varying along the perimeter and an angle of less than 90° treated by ultrasonic impact treatment is shown in FIGURES 15a and 15b. The welded joint is denoted as 90 and the weld as 91. The ultrasonic impact tool 93 is shown in different weld treatment positions.

[00116] Corner welded joints with an angle between the web and flange of < 90° and with a through or incomplete penetration are widely used, which brings to the forefront the problem of technical cost minimization, providing therewith a dimensional accuracy and appropriate fatigue limit and life span. Ultrasonic impact treatment in accordance with the invention solves this problem by ultrasonic and impulse compensation for longitudinal and transverse weld shrinkage, symmetric angle deformation of the flange relative to the web, material properties and condition modification in the stress concentration area. This provides for a weld joint wherein the angles between the web and flange are < 90°, and obtaining a specified joint dimensional accuracy as well as increased fatigue limit and life span not less than a factor of 1.3 and 10 respectively.

[00117] A schematic representation of a welded corner joint in accordance with the invention is shown in FIGURES 16a and 16b. FIGURE 16a shows the work pieces 100 for forming a corner prior to welding. FIGURE 16b illustrates the work pieces including corner welds 101 being treated by ultrasonic impact tools 102. Following ultrasonic impact treatment, modifications are present in the properties of the treated material. Deviation from specified dimensions after ultrasonic impact treatment is within the tolerances for longitudinal and cross deformations. The fatigue limit of the welded corner joint after treatment is a minimum of 1.3 times greater over that of a welded corner joint in an untreated condition. The life span of the welded corner joint after treatment is a minimum of 10 times greater than that of the welded corner joint in an untreated condition.

[00118] Thus, the fabrication and maintenance of corner welded joints with varying and "constant" groove beveling angles, as shown in FIGURES 15a-15b and 16a-16b, is associated with the need to search for engineering solutions that through minimum production costs provide, on one hand, the requisite accuracy of such joints and on the other hand, a specified life thereof.

[00119] The accuracy of corner welded joints should ensure their service reliability, design load-carrying capacity and external loading resistance. The endurance of the welded joints should ensure a life time expressed through the resistance of the welded joints to varying and reversed loads.

[00120] The welded joint accuracy is generally achieved by heat treatment and using a costly conductor tool set. The endurance of the welded joint is achieved through special approaches to selection of the base metal and

welding consumables, greater weld dimensions and the heat treatment for residual stress reduction.

[00121] Ultrasonic impact treatment in accordance with the invention minimizes production costs, eliminates the need for heat treatment and the use of large amounts of weld metal in the weld. This is achieved through ultrasonic relaxation and redistribution of residual welding stresses and deformations, as well as by modifying welded joint material properties to be at the level of the base material in the area affected by- ultrasonic plastic deformations of the welded joint material.

[00122] Ultrasonic impact treatment in accordance with the invention may be applied to the hot metal during welding, to the metal during cool down or to cold metal after welding, depending on the production conditions and welding process.

[00123] The results of the ultrasonic impact application in accordance with the invention are obtained by layer treatment of the weld metal, formation of the deconcentration groove in the stress concentration area, and in-process or on-line control of the ultrasonic impact treatment results in the course of treatment.

[00124] Ultrasonic impact treatment conditions for corner welded joints in accordance with the invention include: ultrasonic impact frequency of up to about 1200 Hz, ultrasonic impact length of not less than about 1 ms, impact amplitude of not less than about 0.2 mm, indenter ultrasonic vibration carrier frequency of about 18 kHz and above, indenter ultrasonic vibration amplitude during impact of not less than about 25 μxa for carbon steels and not greater than 30 μm for aluminum alloys, tool pressure against the treated surface of about 3 kg and above subject to manual or mechanized treatment.

(K) Liquation, grain size, degassing and pores [00125] Welded joints made with a high volume of a molten pool under conditions of long duration and long cooling of the weld metal are prone to liquation. This phenomenon is mainly explained by the growth of large grains and the direction of molten pool crystallization from its boundaries with the base metal to the center. [00126] Ultrasonic impact treatment concluded within the parameters of the invention during welding and cooling down of the weld metal solve this problem on the basis of the volume ultrasonic crystallization of the molten metal and the ultrasonic and impulse recrystallization of large grains. Volume crystallization in the molten pool occurs due to acoustic flows and enhanced cavitation caused by ultrasonic vibrations originating from the ultrasonic wave propagating along the weld as a result of the effect thereupon of ultrasonic impacts . Weld metal and near- weld area are recrystallized under direct action of the ultrasonic impact upon the weld and the near-weld metal being cooled down. This provides specified weld metal phase homogeneity across the weld section in all directions. A weld joint with structural phase homogeneity can be formed in accordance with the schematic representation as shown in FIGURES 17a and 17b wherein representative portions are enlarged. FIGURE 17a illustrates a weld having liquation 110 in the center of the weld. FIGURE 17b illustrates an ultrasonic impact tool 112 treating the weld within the parameters of the invention to provide a weld with ultrasonic impact activated crystallization 111. Impact is provided across the weld shown in FIGURE 17b as indicated by the arrows and the tool 112 shown in solid and broken lines.

[00127] The most important characteristics responsible for weld joint reliability, such as impact strength, yield and ultimate strengths, stringiness and crack resistance at sub-zero, and high and ambient temperatures, depend on the grain size. Ultrasonic impact treatment performed within the parameters of the method at a distance from the arc corresponding to the maximum sensitivity of a molten metal to the crystallization center formation and solidifying metal to grain recrystallization in the process of a grain growth successfully solves this problem. A new type of weld joint is thus created which meets the stringent mechanical strength requirements and possesses specified physical and mechanical properties because of the fine grain structure of the weld metal and heat affected zone. A schematic representation of how such a joint is obtained is shown in FIGURES 18a and 18b. FIGURE 18c graphically illustrates the mechanical strength and impact strength, which results from ultrasonic impact treatment, for the joints. FIGURE 18a shows a weld 120

(with enlarged portion for illustration) which was not subjected to ultrasonic impact treatment. FIGURE 18b shows a weld 121 with ultrasonic impact activated crystallization (shown in the illustrative enlarged portion) by treatment with an ultrasonic impact tool 122 which moves across the weld in accordance with the arrows and tool shown in solid and broken lines. FIGURE 18c sets forth data as to weld 120 and weld 121.

[00128] One of the basic quality criteria for a welded joint is the presence or absence of pores in the weld metal. This property is chiefly determined by the molten pool degassing efficiency in the process of welding. Ultrasonic impact treatment in accordance with the invention makes an effective solution for this problem

possible based on the initiation of molten pool ultrasonic degassing in the process of welding.

[00129] This effect is achieved by ultrasonic impact treatment performed over the weld metal or associated metal using the parameters set forth above at a distance from the arc that corresponds to a molten pool liquid phase, which is equivalent to the minimum solubility of gas inclusions in the weld metal. The welded joint and a schematic representation of its degassing are shown in FIGURES 19a and 19b. FIGURE 19a illustrates a weld 130 not subjected to ultrasonic impact treatment and having visible pores in the root area of the weld. In FIGURE 19b, the weld 131 was treated with ultrasonic impact to activate degassing so no pores are visible. Treatment with an ultrasonic impact tool 132 is across the weld as indicated by the arrows and the tool 132 shown in solid and broken lines.

[00130] Thus, described are three possible applications of ultrasonic impact treatment in accordance with the invention during welding that are directed toward producing welded joints with new properties such as liquation resistance at great volumes of molten metal, reliable recrystallization and fine-grain structure formation, and weld metal resistance to pore formation.

[00131] Effects of ultrasonic impact treatment in accordance with the invention upon the molten metal's behavior, structure and properties of the weld metal and the joint as a whole are based on the corresponding method choice of the distance of the ultrasonic impact area from the molten pool and the ultrasonic impact parameters. In each specific case the selection criteria of the ultrasonic impact treatment area location performed in accordance with the invention relative to the welding area are the temperature ranges of effective

crystallization and recrystallization of the molten metal and weld metal respectively, as well as the temperature range of the minimum gas solubility in the molten pool. In this case, the parameters of ultrasonic impact treatment in accordance with the invention, subject to properties of the treated material and the temperature at the ultrasonic impact treatment area, are set within the following ranges: tool pressure from about 0.1 to 50 kg, ultrasonic vibration carrier frequency at the transducer of from about 10 to 800 kHz, ultrasonic vibration amplitude under no-load conditions and during impact at a carrier frequency of from about 0.5 to 120 μm, tool self- oscillation amplitude of from about 0.05 to 5 mm, and the average ultrasonic impact duration of not less than about 1 ms.

(L) Diffusion hydrogen

[00132] Welded joints with stringent brittle fracture resistance requirements made of steels, specifically ferritic steels, are preliminarily or concurrently heated before and during welding to expel diffusion hydrogen from the joint metal. This results in a high temperature at the operator's work place, pollution of the environment and an increase in residual welding deformations caused by the added heating of the structure.

[00133] Ultrasonic impact treatment performed in accordance with the invention during welding at a distance from a molten pool and/or over cold metal of edges or after welding with intensity and spectrum of ultrasonic impact that jointly correspond to the maximum mobility of diffusion hydrogen produces a welded joint with high resistance to brittle fracture. Thus, preliminary and concurrent heating requirements are minimized.

[00134] A schematic representation of a welded joint is shown in FIGURES 20a and 20b. FIGURE 20c is a graph showing the minimization of residual diffusion hydrogen content in the metal of the joint after ultrasonic impact treatment. FIGURE 20a shows a weld 140 (with an illustrative enlarged section) not subjected to ultrasonic impact treatment and thus has visible pores. FIGURE 20b shows weld 141 (with illustrative enlarged section) with activated crystallization (no pores) due to the cooling down or cold edge preparation being accompanied by ultrasonic impact treatment using tool 142 which is moved across the weld during treatment in accordance with the arrows and the ultrasonic impact tool 142 shown in solid and broken lines. Treatment occurs within the parameters described below. FIGURE 20c shows permissible hydrogen content limits for steel. It is conventional that prior to welding, the permissible level of residual hydrogen in the welded joint metal should not exceed 5 cm ³ /l00 g for steel. FIGURE 20c shows the hydrogen content for the welds shown in FIGURES 20a and 20b as indicated by the corresponding reference numbers.

[00135] Ultrasonic impact treatment of welded joints in accordance with the invention is performed, with consideration for the fact that the metal is prone to hydrogen saturation, in any production conditions: over cold edges before welding or over edges some distance ahead of the molten pool during welding, or over the weld metal some distance following the welding pool during welding, or over the weld metal after welding within a certain temperature range in fabrication of new structures, reengineering thereof, preventive maintenance or repair.

[00136] For all conditions referenced above, prior to treatment in accordance with the process of the

invention, the temperature range or temporary conditions are determined that provide for effective diffusion hydrogen removal and maintaining metal in this state. [00137] From the saturation diagram shown in FIGURE 21, it can be seen that ultrasonic impact treatment in accordance with the invention reduces the content of diffusion hydrogen within a wide temperature range by at least 2 times.

[00138] Parameters of ultrasonic impact treatment in accordance with the invention that ensure the results presented above include: ultrasonic impact frequency of up to about 2500 Hz, ultrasonic impact amplitude of not less than about 0.2 mm, average statistical length of ultrasonic impacts of not less than about 1 ms, ultrasonic vibration carrier frequency of about 15 kHz and above, ultrasonic vibration amplitude during impact of not less than about 15 μm depending on the temperature and grade of the metal being treated and not less than about 30 μm when cold metal is treated, pressing force on the tool against a treated surface of not less than about 5 kg for manual treatment and not less than about 10 kg during mechanized treatment. (M) Aggressive environment — stress corrosion (treatment before and during)

[00139] Resistance of a weld joint to stress corrosion damage or failures under fluctuating loading defines the reliability and life of a loaded structure with a long operational cycle. Main pipelines and offshore platforms are examples of such structures. Their protection against stress corrosion is very costly.

[00140] Treatment to provide new properties in accordance with the invention solves this problem. Described below are the main parameters of ultrasonic impact treatment effect on a metal surface in aggressive

environment under stressed conditions or fluctuating loading:

— a roughness which is not less than 5 μm at a sampling length of 0.8 mm and waviness which is not less than 15 βxa at a sampling length of 2.5 mm,

— compressive stresses in the area of ultrasonic and impulse deformation which are not less than the material yield strength,

— depth of plastic deformation and introduced residual compressive stresses which are not less than 1.5 mm, and

— amorphous microstructure modification with the formation of a white layer depending on the material properties which is not less than 50 βm.

[00141] Since surface and material properties are transformed, stress corrosion resistance of the joint is increased at least by a factor of 2 ultimate corrosion- fatigue strength increased by at least 1.3 times and the life increased by at least 7 times under various loading in a corrosive environment as compared to the joint in an untreated condition. It is significant that these properties pertain equally to newly welded joints and welded joints in operation.

[00142] The results and properties of welded joints made of steel with high carbon content and subjected to ultrasonic impact treatment are shown in FIGURE 21. It is shown in FIGURE 21 that following the irregular corrosion, which is typical to occur on the surface of any material, the stable process occurs, wherein the corrosion rate of the layer treated by ultrasonic impact treatment in accordance with the process is a minimum of 4 times less than that of the as-welded metal based on the experimental data. A minimum equivalent time during

which the carbon steel treated by ultrasonic impact treatment in accordance with the invention resists stress corrosion in sea water is 10 years.

[00143] Parameters of ultrasonic impact treatment in accordance with the invention that ensure the results presented above include: ultrasonic impact frequency of up to about 500 Hz, ultrasonic impact amplitude of not less than about 0.5 mm, average duration of ultrasonic impacts of not less than about 1 ms, ultrasonic vibration carrier frequency of about 15 kHz and above, ultrasonic vibration amplitude during impact of not less than about 20 βm, and pressing force on the tool against a treated surface of not less than about 5 kg.

(N) Holes in welded joints

[00144] The practice of welded structure operation is associated to a certain extent with the need to use holes as a crack arrest means in an area near or within a welded joint. Damage in such joints may develop not only from the crack stopped by such holes, but also from the holes themselves. The reason is in the surface tearing produced during making of the holes, which become stress concentration areas in operation which in turn cause fatigue.

[00145] To obtain a reliable welded joint with crack arrest holes, ultrasonic impact treatment in accordance with the invention is first applied to both crack sides and then to the hole. A hole is treated where the metal is damaged during the making of the hole at the entrance and exit regions, but not less than 1/5 of the hole depth from the damaged side. Residual compressive stresses, not less than the material yield strength, are formed in the layer subjected to ultrasonic and impulse plastic deformation. It is noted that the indenter shape in this

case is chosen to provide free access to the damaged portions of the hole.

[00146] A schematic diagram of a welded joint with holes and the results of the treatment are shown in FIGURES 22a and 22b. FIGURE 22a illustrates a crack between two holes in a weld 150 prepared using conventional tip drilling which results in known associated stresses. FIGURE 22b illustrates a crack between two holes in a weld 151 prepared with conventional tip drilling followed by ultrasonic impact treatment with an impact tool 152. Associated stresses which result from the tip drilling are altered due to formation of the compressive stress area 153. FIGURE 22b also illustrates the needle indenter 154 of the ultrasonic impact tool 152 and the manner of treating the holes 155 and edges of holes 156 to result in the tearing of material in the holes at the end of the cracks. It is shown that tensile stresses in the hole area after drilling thereof are replaced by compressive stresses and possible tensile stresses are displaced into the region of the structure where operational stress concentration and hence fatigue crack initiation is unlikely to occur.

[00147] Parameters of ultrasonic impact treatment in accordance with the invention that ensure the results presented above for a widest range of metals include: ultrasonic impact frequency of up to about 500 Hz, ultrasonic impact amplitude of not less than about 0.5 mm, average duration of ultrasonic impacts of not less than about 1 ms, ultrasonic vibration carrier frequency of 15 kHz and above, ultrasonic vibration amplitude during impact of not less than about 30 μm, pressing force on the tool against a treated surface of not less than about 5 kg.

(0) Brackets

[00148] Weld joints of brackets with a radius cutout where a bracket plane intersects the main weld are a typical welded joint that is extensively used in the fabrication of welded structures. The most dangerous components of such a structure are the weld ends in the cutout area and the weld toe line when the bracket is welded to a panel. Dimensional accuracy in such a joint also presents a very significant problem.

[00149] Ultrasonic impact treatment of the weld along the bracket and weld end in a radius cutout when within the parameters of the invention results in a weld joint that meets dimensional accuracy requirements with a minimum increase in fatigue resistance of 1.3 times that of an untreated joint.

[00150] A schematic representation of a bracket welded joint prior to and after ultrasonic impact treatment are shown in FIGURES 23a and 23b. The bracket panels 160 have cracks 161 in the areas of bracket welding in the absence of ultrasonic impact treatment. The bracket plane intersects the main weld wherein a connection with the panel is made by longitudinal fillet welds relative to the bracket end in a radius cutout. FIGURE 23b shows a bracket treated by ultrasonic impact to provide treatment zones 162. Ultrasonic impact treatment of the weld along the bracket and at the weld end in the radius cutout insures that the welded joint meets dimensional accuracy requirements and results in a minimum increase in fatigue resistance of 1.3 times as compared to the same properties in an untreated bracket structure.

[00151] When the weld end in the cutout area is treated by ultrasonic impact treatment in accordance with the invention special tool heads are used to provide an access for the indenter to this area.

[00152] Parameters of ultrasonic impact treatment in accordance with the process of the invention which ensure the results presented above for a widest range of metals include: ultrasonic impact frequency of up to about 300 Hz, ultrasonic impact amplitude of not less than about 0.5 mm, average duration of ultrasonic impacts of not less than about 1 ms, ultrasonic vibration carrier frequency of about 15 kHz and above, ultrasonic vibration amplitude during impact of not less than about 30 μm, pressing force on the tool against the treated surface of not less than about 3 kg.

(P) Welded joints prone to martensite formation [00153] When residual welding deformation should be minimized, intense forced cooling of a welded joint immediately following the welding process is used in some specific cases. This causes a well-known hardening effect, especially in carbon steels, that is accompanied by expelling martensite and the formation of a joint having limited ductility. Martensite decomposition is achieved by additional forced heating of the joint and soaking of the joint for a long time within a narrow specified temperature range. This procedure has a large energy consumption, is complex as regards achieving the conditions of heating and soaking within the narrow temperature range and is characterized by insufficient consistency of results.

[00154] Ultrasonic impact treatment of this type of joint within the parameters of the invention at a distance from the heating arc corresponding to the temperature of martensite decomposition and its replacement by sorbite or tempered martensite, changes the welded joint structure in a temperature range which is a minimum of 1.5 times greater than the bottom boundary of this range, while the range itself is a

minimum 2 times greater than that required in welding to reduce the likelihood of martensite formation under the above-mentioned conditions in the absence of ultrasonic impact treatment. As this takes place, the martensite decomposition time is reduced by at least 10 times. This produces a weld joint with a radically increased process temperature range of martensite decomposition, while the average temperature of the range is reduced relative to standard conditions required to solve this problem.

[00155] A diagram of supercooled austenite (martensite) decomposition is shown in FIGURE 24 for an exemplary- sample of steel 12XH3. Lines 1 indicate martensitic transformation at a temperature Tl for a sample not subjected to ultrasonic treatment. A sample, as indicated by lines 2, subjected to ultrasonic impact treatment according to the invention has martensitic transformation at temperature T2. Tl > T2. It is shown in FIGURE 24 that the martensite decomposition process during standard heat treatment can occur within the temperature range from 495° to 430 ⁰ C for a minimum of 3 hours. During ultrasonic impact treatment in accordance with the invention the same process can last for 3-4 min. within the temperature range of 260° to 39O ⁰ C.

[00156] Parameters of ultrasonic impact treatment in accordance with the invention that ensure the results presented above for a widest range of metals include: ultrasonic impact frequency of up to about 800 Hz, ultrasonic impact amplitude of not less than about 0.5 mm, average duration of ultrasonic impacts of not less than about 1 ms, ultrasonic vibration carrier frequency of about 15 kHz and above, ultrasonic vibration amplitude during impact of not less than about 30 _/ xm, pressing

force on the tool against a treated surface of not less than about 10 kg.

[00157] This produces a weld joint with a radically- increased process temperature range of martensite decomposition, while the average temperature of the range is reduced relative to a standard conditions required to solve this problem within a period of the actual flow- line automatic or computer-aided production of welded structures.

(Q) Welded joints with protective and/or hardening coating

[00158] The maintenance of welded joints is associated in many respects with the need for their protection or hardening by using various metallic or nonmetallic coatings. In such a case, the use of any type of mechanical operation, including the known methods of plastic deformation of the weld, near-weld area and weld toe, is limited by the coating integrity required.

[00159] Treating with ultrasonic impact in accordance with the invention solves the above problem and makes it possible to produce welded joints with specified new properties since the ultrasonic impact treatment can be conducted over the coating. In this case, the integrity and improvement in properties of protective or hardening coatings are obtained along with specified properties in the welded joint.

[00160] An example of such a welded joint is shown in FIGURES 25a, 25b and 25c. FIGURE 25a illustrates a weld before coating and ultrasonic impact treatment. FIGURE 25b illustrates the same weld after a coating 170 is applied and before ultrasonic impact treatment of the coated weld. In FIGURE 25c, the coated weld is shown following ultrasonic impact treatment. The groove and stress raiser modification in the weld is denoted by 171

over the coating 170. In the weld joint of FIGURE 25c, the radius is a minimum of 0.5 mm, the width is up to 10 mm, the depth is up to 2 mm, and the coating thickness is 0.15 mm when the web thickness is 4 mm. It is shown in FIGURES 25a-25c that ultrasonic impact treatment in accordance with the invention makes possible the process of producing a welded joint with specified properties due to the use of special coating in the following order: fabrication of a joint by welding, application of the protective or hardening coating, and ultrasonic impact treatment in accordance with the invention. [00161] To maintain the coating integrity, the conditions of ultrasonic impact treatment in accordance with the invention are selected so that the contact pressure on the coated surface and pressure gradients in the ultrasonic impact treatment area are not greater than the breaking strength of the coating.

[00162] Parameters of ultrasonic impact treatment in accordance with the invention that ensure the results presented above for a widest range of metals include: ultrasonic impact frequency of up to about 1500 Hz, ultrasonic impact amplitude of not less than about 1 mm, average duration of ultrasonic impacts of not less than about 1 ms, ultrasonic vibration carrier frequency of not less than about 20 kHz, ultrasonic vibration amplitude during impact of not greater than about 30 μm, contact pressure and stress gradient at the boundary between individual ultrasonic impact treatment tool marks of not greater than the coating breaking strength, pressing force on the tool against a treated surface of not less than about 3 kg. (R) Welded structures

[00163] The above described welded joints, and processes for obtaining the joints, make possible the

creation of welded structures that meet high quality and reliability requirements. A structural representation is schematically shown in FIGURE 26 to illustrate various welded joints 180 obtainable under the invention. Such structures in aggregate or in any combination of elements, details, joints and materials may include: panels, cylindrical elements with continuous or varying bevel angle that are welded perpendicularly or at an angle to the panel, flat structural elements, webs, brackets, corner joints, lap joints, etc. The quality and reliability of the welded joints are improved by provision of improved properties in the joints through ultrasonic impact treatment of the joints in accordance with the invention.

[00164] As will be apparent to one skilled in the art, various modification can be made within the scope of the aforesaid description. Such modifications being within the ability of one skilled in the art form a part of the present invention and are embraced by the appended claims.