FIELD OF INVENTION

The invention generally relates to a method for evaluating and improving material quality. Particularly, the invention utilizes metal magnetic memory to evaluate the condition of the material and ultrasonic peening to improve the quality of the material.

BACKGROUND OF THE INVENTION

Fatigue in metals is a challenge that is commonly faced by engineers and solutions have constantly been developed to extend the fatigue lives of metals and to reduce fatigue failure that may lead to fracture, crack which eventually damage metallic components. The element that causes fatigue in the metallic component is mainly stress concentration, or more specifically, residual stress that remains in the metallic component after the original cause of stress is removed. If the residual stress remains unattended, fracture or crack would grow from repeat and fluctuated loadings until it is unable to support the stress and eventually collapses. Fatigue in metals appear commonly in different structures and applications, for example bridges, axles or shafts in machinery or vehicles, towers, cranes, pressure vessels and pipelines in cyclic operation or more particularly weldments in these structures. To reduce damages in these structures and applications, as well as to save cost for maintenance and repair, numerous researches and solutions have been developed to combat fatigue failures.

Fatigue tends to grow easier in weldments due to pre-existing flaws which would promote crack formations. A number of techniques and methods have been known to be used to improve the fatigue lives of weldments, they include weld geometry improvement such as burr grinding, disc grinding, TIG dressing and plasma dressing, and residual stress improvement such as hammer peening, ultrasonic hammer peening, shot peening, vibratory stress relief and thermal stress relief. Among these methods, ultrasonic peening is considered to be the most effective due to its capability in reducing tensile stresses, introducing compressive stresses and removing surface defects. These techniques are commonly used in a vast scope of industries such as aerospace, energy, automotive, transportation, marine structure and shipping industry.

It is also important to note that ultrasonic peening is a preventive approach instead of corrective, in order to minimize maintenance cost and maximize benefits including increasing the fatigue life of welded steel structures. Whilst ultrasonic peening helps maintain the quality of the welded steel structures, it is also important to take note of the stress-strain conditions of the welded steel. In common practice, stress-strain conditions are measured before the peening treatment, so that they can adjust the peening accordingly to maximize the benefits of the treatment.

KR 20110016687 A is a Korean patent application that discloses a method and apparatus for forming compressive stress in a welded portion and its periphery by using an ultrasonic peening process to remove tensile stress remaining in a welded portion in welding and repairing a large metal structure. More specifically, the method is capable of removing residual tensile stress in the welded portion of the structure to improve resistance. In particular, the residual stress is measured and evaluated before and after the forming of compressive stress on the surface of the welded part based on the principles of X-ray diffraction. Whilst X-ray diffraction is a common practice used in non-destructive testing equipment, the scanning and imaging of X-ray diffraction is easily influenced, which make it difficult to determine the stress-strain condition of the metal structure.

US 9486894 B2 is a patent document that discloses a method where evaluation is conducted to determine if the conditions of the region shot-peened are desirable. In the evaluation step, compressive residual stress and surface roughness of the region shot-peened is measured. It further explains that the peening step may be conducted after a number of evaluations, so that the surface of the water-cooling hole can be shot-peened under more desirable peening conditions. The patent document also explains that it uses a residual stress measuring device to measure compressive residual stress at a predetermined region of the mould. Specifically, the device is an X-ray diffraction residual stress measuring device. Similar to ultrasonic peening, shot peening is one of the preventive approaches used to increase fatigue strength and corrosion resistance of the subject material. However, unlike some other stress relief technologies, shot peening is unable to decrease residual deformation and residual weld stress.

Further to the problems disclosed in the prior art documents, there is therefore a need for the present invention to provide a method utilizing technologies such as ultrasonic peening and metal magnetic memory that are capable of addressing the drawbacks of the prior arts during the evaluation and improvement of the quality of the subject material. SUMMARY

The present invention discloses a method for evaluating and improving quality of an object, the method comprising the steps of measuring residual stress of the object to obtain stress concentration zones of the object, assessing the stress concentration zones to obtain stress-strain state of the object, treating the object with peening based on the stress-strain state of the object, measuring residual stress of the object after the peening to obtain stress concentration zones of the object, and assessing the stress concentration zones after the peening to obtain the stress- strain state of the object, characterized in that the steps of assessing the stress concentration zones and obtaining the stress-strain state of the object before and after the peening is based on metal magnetic memory in order to evaluate the quality of the object, and the step of treating the object with peening is based on ultrasonic peening in order to improve the quality of the object.

It is an object of the present invention to provide a method that is capable of extending material life of the object exposed to fatigue failures, thereby deferring the incurrence of cost for asset replacement.

It is another object of the present invention to provide a method that is capable of reducing unplanned operational downtime since area of the object with cracks is detected easily and conveniently.

It is still an object of the present invention to provide a method that is capable of preventing crack formation and growth at stress-prone area of the object. It is further an object of the present invention to provide a method that requires minimal energy to operate a handheld metal magnetic memory equipment and a handheld ultrasonic peening equipment.

It is still further an object of the present invention to provide a method that utilizes handheld metal magnetic memory equipment and handheld ultrasonic peening equipment which are small and versatile for use in remote and difficult-to-reach areas.

BRIEF DESCRIPTION OF DRAWINGS The invention will now be described with reference to the drawings wherein:

Figure 1 is a flow chart illustrating the fundamentals of the present invention.

Figure 2 illustrates an example of data on stress concentration zones that is particularly marked with high stress and strain states.

Figure 3 illustrates an example of data on stress concentration zones where the previously marked high stress and strain states have been eliminated.

Figure 4 illustrates an ultrasonic power generator and a hand tool with a piezoelectric transducer and a striker.

Figure 5 illustrates surface conditions of the object before and after the use of the methods disclosed in the present invention.

DETAILED DESCRIPTION

The following description is presented to enable a person of ordinary skill in the art to make and use the various embodiments. Descriptions of specific devices, techniques, and applications are provided only as samples. Various modifications to the examples described herein will be readily apparent to those of ordinary skill in the art, and the general principals defined herein may be applied to other examples and applications without departing from the scope of the various embodiments. Thus, the various embodiments are not intended to be limited to the examples described herein and shown, but are to be accorded the scope consistent with the claims.

As previously mentioned, stress concentration and residual stress are the main reasons to have caused fatigue failure in ferritic objects. Especially for welded joints of these ferritic objects, residual stresses primarily develop due to differentials in weld thermal cycles experienced by the welded ferritic object and region closed to fusion boundary, such as heat affected zone. With the aforementioned concerns, there is therefore a need for the present invention to provide a method that focuses on measurement and evaluation, as well as treatment and improvement of residual stresses. Specifically, the method utilizes and combines two non-destructive testing technologies, i.e. metal magnetic memory and ultrasonic peening. More specifically, the present invention discloses a method for evaluating and improving quality of an object that comprises the steps of measuring residual stress of the object to obtain stress concentration zones of the object, assessing the stress concentration zones to obtain stress-strain state of the object, treating the object with peening based on the stress-strain state of the object, measuring residual stress of the object after the peening to obtain stress concentration zones of the object, and assessing the stress concentration zones after the peening to obtain the stress-strain state of the object, characterized in that the steps of assessing the stress concentration zones and obtaining the stress-strain state of the object before and after the peening is based on metal magnetic memory in order to evaluate the quality of the object, and the step of treating the object with peening is based on ultrasonic peening in order to improve the quality of the object.

Figure 1 discloses a flow chart illustrating the fundamentals of the present invention. The general concept of the method disclosed in the present invention involves stress measurement and material surface treatment. Specifically, stress measurements are conducted before the material surface treatment to ensure that the treatment is conducted on surface areas with high stress and strain states and stress measurements are conducted after the treatment to ensure that the stress concentration zones in the treated area are eliminated or has been minimized.

As disclosed in the present invention, stress measurements before and after the treatment include the steps of measuring residual stress of the object to obtain stress concentration zones of the object and assessing the stress concentration zones to obtain stress-strain state of the object. In a preferred embodiment of the present invention, these steps are conducted based on metal magnetic memory, where magnetic signals are obtained and magnetic anomalies are analysed. Before conducting any treatment on the object, residual stress on the surface of the object is measured using a handheld metal magnetic memory equipment. The residual stress, in magnetic signals, are obtained and compiled into a group of data that are viewed as stress concentration zones. Generally, stress concentration zones are illustrated in a chart where it indicates locations where stresses are concentrated. The measurement that is first made on the particular area of the object is collected and saved as a sample data. The magnetic signals that are obtained from subsequent measurements are compared to the sample data as a basis to determine the conditions of the object. Thereafter, the stress concentration zones are assessed in order to obtain the stress-strain state of the object. In the assessment of the stress concentration zones, areas with high stress and strain states are marked so that treatments are made to these marked areas. Figure 2 illustrates an example of data on stress concentration zones that is particularly marked with high stress and strain states.

In a preferred embodiment of the present invention, the handheld metal magnetic memory equipment that is used herein comprises preferably a tester of stress concentrations, TSC, where it is capable of recording distribution of earth magnetic fields that is parallel to the inspected object, and a magnetic scanning device capable of obtaining natural magnetic fields emitted by the ferritic material in the object. By the operation principle, each TSC comprises multi-channel flux-gate magnetometers can be connected to a series of scanning devices. It should also be obvious to the skilled addressee to choose the type and number of channel as well as the type of scanning device according to the material and conditions of the object.

After conducting the measurements and assessments, the next step in the method as disclosed in the present invention is to treat the object with peening based on the stress-strain states of the object. Peening is a process of surface modification that is commonly used to improve material properties by the application of mechanical force. There are different types of peening available in the market, for example hammering and blasting - shot and laser. Hammering is a conventional way of improving surface properties of an object as it applies blunt forces and induces compressive and tensile stresses. More advanced and advantageous solutions are shot peening and laser peening, which their functions are explained through their names. Shot peening involves the propulsion of cast iron particles under high pressure onto the object's surface to be treated. Laser peening involves the use of high-energy beams to generate shock waves on the metal surface. Besides the aforementioned peening methods, other stress relief technologies include Tungsten Inert Gas - TIG dressing, grinding and post weld heat treatment - PWHT. However, the peening method used in the present invention is preferably ultrasonic peening, where it applies ultrasonic energy onto the object's surface using an electro-mechanical ultrasonic transducer. As compared to the other available methods, ultrasonic peening is more advantageous as its tool can be easily and conveniently operated and it provides easy working conditions with minimum vibrations. More technically, ultrasonic peening is much more efficient compared to the existing methods as it increases fatigue strength and corrosion resistance, and at the same time decreases residual deformation and residual weld stress. The comparison table below illustrates the abilities of different stress relief technologies.

Table 1

In another preferred embodiment of the present invention, the handheld ultrasonic peening equipment comprises an ultrasonic power generator and a hand tool with a piezoelectric transducer and a striker, where the hand tool is able to hold different types of working heads on the striker to cater different applications. The types of working heads that may be used in the present invention include single pin working head, two or three pins in-line working head and multi-striker. Specifically, the ultrasonic power generator provides power of 400 to 600 watts and oscillation amplitude ranging from 10 to 40 pm operating at 20 kHz that creates cyclic acceleration of around 50,000 g, g being 9.8 m/s ² . Figure 4 illustrates the hand tool connected to the ultrasonic power generator. In a working condition, the following procedures are taken to conduct the peening treatment towards the object. Firstly, the length of the area obtained from the stress concentration zones is marked and measured on the object according to pre determined identification for reporting purposes. Surface preparation is then performed on the marked area of the object. Surface preparation is needed when there is coating on the metal. In order to conduct the peening treatment towards the object, the striker has to be in direct contact to the surface of the object without any coating. In other words, surface preparation is to remove coating on the object. Thereafter, the surface area is performed with ultrasonic peening using the aforementioned handheld equipment. During the peening process, strikers on the equipment face the surface of the marked area of the object by the range of about 80 to 90°, depending on the accessibility. The average speed of the peening treatment is set at about 0.5 m/min per treatment cycle without any application of external force. The treatment is applied onto the marked surface area of the object at least three to four cycles, which it is subjects 50% of stress relief at the marked surface area.

Upon treating the surface of the object with peening, debris created from the peening are removed. The residual stress of the object after the peening is then measured using the aforementioned handheld metal magnetic memory equipment in order to obtain stress concentration zones of the object. Thereafter, the stress concentration zones are assessed to obtain stress-strain state of the object. The steps of measuring the residual stress and assessing the stress concentration zones are reviews of the quality of the peening. In other words, the surface area of the object that are peened are reviewed to determine if the surface area has a preferred or optimum stress-strain state. If the surface area of the object, upon being assessed, does not have the preferred or optimum stress-strain state, the steps of treating the object with peening, measuring residual stress of the object after the peening and assessing the stress concentration zones of the object after the peening are repeated until the preferred or optimum stress-strain state of the object is obtained. The preferred or optimum stress-strain state relies mainly on the material type of the object and the application of the object. At the end of the evaluation and improvement processes, a report containing inspection methods, tools, data and analyses is generated for further review and reporting purposes.

Figure 3 illustrates an example of data on stress concentration zones where the previously marked high stress and strain states have been eliminated. The method disclosed in the present invention therefore aims to improve and rehabilitate conditions of the material of an object, particularly that the object is of metallic or ferritic material. The improvement and rehabilitation of the object, particularly the steps as disclosed in the present invention, refines the material grain structure and eventually increases the material life span. Figure 5 illustrates surface conditions of the object before and after the use of the methods disclosed in the present invention. As shown in figure (i), it illustrates the surface of the object as of its original condition, or in other words, condition of the surface of the object before the peening treatment. Figures (ii) and (iii) illustrate the surface of the object after the peening treatment. Another point to take note is that the figures are taken with 8x of magnification. Further, the surface roughness of the surface of the object before and after the peening conditions A and B are also measured. Particularly, in its original condition, the surface roughness is 6.6 ± 2.1 Ra(p). After the treatments under the conditions A and B, the surface roughness is 4.4 ± 0.4 Ra(p) and 2.9 ± 0.4 Ra(p) respectively. It is generally understood that the higher the deviation, the rougher the surface is. From the figures (ii) and (iii), as well as the surface roughness of these surfaces, it is obvious that the application of the method disclosed in the present invention is capable of refining grain structure of the material of the object. Together with the review of the surface of the object after the application of the method of the present invention, it is therefore proven that the method is capable of increasing fatigue strength, increasing corrosion resistance, decreasing residual deformation and decreasing residual weld stress. With the aforementioned advantages, the material life of the object is extended, thereby deferring the incurrence of cost for asset replacement. Since material life is extended, the occurrence of unplanned operational downtime is also expected to be reduced. With the use of the method disclosed in the present invention, the area of the object with cracks can also be detected easily and conveniently. Even if cracks are detected, the use of the method is capable of preventing growth of the cracks or formation of new cracks at stress-prone area of the object. The apparatuses used in the method of the present invention also require minimal energy to operate and they are small-sized handheld tools that can be easily transported and used in remote and hard-to-reach areas.

Although only certain exemplary embodiments have been described in detailed above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Additionally, aspects of embodiments disclosed above can be combined in other combinations to form additional embodiments. Accordingly, all such modifications are intended to be included within the scope of this invention.