The invention relates to an apparatus for measuring hot hardness of materials compris- ing a main frame and a displacement gauge connected to the frame by spring elements, and the displacement gauge is provided by a core rod biased against the frame by a mounting spring, and a method for measuring hot hardness of materials, comprising the steps of: determining a value of initial penetration (hk) of a measuring tip having known geometry in a test piece at room temperature; then said test piece is heated, while the measuring tip is abutted against said test piece by a biasing force, and determining a value of indentation made by the measuring tip in the test piece.

Although the apparatus and method of the present invention are described in conjunction with measuring of hot hardness of metals in this description, measuring hot hardness of other, e.g. non-metallic and/or non-crystalline materials can be performed adequately by the apparatus and method according to the invention as well.

Manufacturing of workpieces made of various metals and metal alloys can be realized by so called shaping operations including casting, machining or plastic forming. While on casting operation the total mass of the metal is melted by heat and then poured into a mold, in which the metal solidifies by formation and growth of crystal particles, on machining opera- tion removing excess material by cutting is done.

In plastic forming a permanent deformation is created by imparting mechanical energy to the workpiece. However, plastic deformation significantly affects mechanical properties of the metals depending on the degree of forming, since ductility is reducing during forming, but strength increases at the same time, because crystal lattice of the crystallites (crystal grains) thus shaped is also deformed and in addition the number of lattice defects (dislocation density) increases. While shaping the crystal grains both the distance of atoms set in each atomic site and their energy level proportionally increase. Since the dislocations can displace in response to an increasing mechanical stress as the dislocation density increases, the resistance against shaping is also increases, the metal workpiece becomes harder, internal stresses are generated, which may lead to a break of the workpiece when loaded.

Therefore, the removal or relieve of the stress accumulated in the workpiece due to the effect of forming is essential, that can be accomplished by a heat treatment of the cold shaped workpiece or by hot forming the material of the workpiece at a temperature above the temper- ature of recrystallization. Recrystallization initiated by means of heat input takes place above the temperature limit of recrystallization, where diffusion processes occur in the workpiece, which affect removing internal stress and attaining to balanced level of energy. Consequently, hardness, strength as well as dislocation density of the workpiece decrease intensively and at length approach again the equilibrium values existed before shaping, while new crystal seeds are shaped, from which new crystallites are formed, that is the material of the workpiece re- crystallizes in solid state. By increasing further the temperature larger crystallites are generated through the fusion of the new crystallites (grain coarsening), which affects lower energy levels to be reached, however, adversely affects the mechanical properties of the material, re- ducing the tensile strength and toughness.

Therefore, as unfavorable mechanical properties of the plastically formed workpiece can be improved by re-crystallization, examination of recrystallization process has a fundamental importance. By investigating recrystallization the characteristic temperature range thereof should be defined, and transformation kinetics must be known as well for optimizing industrial heat treatment processes.

However, prior art methods for analyzing recrystallization processes require expensive equipment as well as room temperature examination of several samples previously heat- treated or thermally formed at various temperatures then cooled down. For example, the nature of recrystallization can be determined by plotting a yield curve by heat treating at differ- ent temperatures a plurality of samples formed analogously and made of the same material, then plotting yield curves of cooled samples by means of a tension tester. Thus, using these stress-strain (e.g. elongation) functions, a temperature being definitely above the recrystallization threshold temperature can approximately be determined on the one hand, and the other hand the nature of recrystallization (continuous, intermittent or partial, etc.) becomes deduci- ble.

A further possibility is to accomplish scanning electron-microscopy and X-ray diffractometry on the samples to study the process of recrystallization, relative orientation of transformed crystal grains and formation of boundaries of grains and phases as well. For example, the alteration of dislocation density can be observed by X-ray diffractometry even in the course of recrystallization. (Gubicza, L. Balogh, R.J. Hellmig, Y. Estrin, T. Ungar, Dislocation structure and crystallite size in severely deformed copper by X-ray peak profile analysis, Materials Science and Engineering A, 400-401 (2005) 334-338.) Recrystallization and other transformation processes can also be examined by measuring hot hardness of a workpiece or a test sample, since the hardness is changing considerably during any structure transformation. Using this method the sample is heated up to various temperatures then the transformation is halted by sudden cooling and the hardness of the sample is measured on room temperature by a well known hardness measurement method - like HV Vickers, HB Brinell or HR Rockwell. This way a sequence of hot hardness values can be obtained that is suitable to determine both initial and end temperatures of the transformation. A drawback of this method in the one hand is that neither the initial nor the end temperatures of the transformation cannot be determined by sufficient accuracy, and in the other hand it is time consuming and laborious. To simplify this method an apparatus is provided by the patent publication document, which includes a device for measuring the micro hardness of a heated sample, the device contains a vertical penetrator column provided with a measuring element for penetrating into the sample and the device is provided by a counterweight balancing the weight of the column, and means for applying controlled load to the column. The column is held by electromagnets against the loading force of the means for applying controlled load to the column, and the measuring element is mounted on the lower end of the measuring column. When the current of the electromagnet was turned off the measuring element would indent into the sample placed beneath the column. The value according to the depth of indenta- tion can be transmitted by a capacitive measurement device. The test may be repeated subsequently. The drawback of this solution, however, is that although the apparatus is provided by a cooling device for preventing the measuring column from heat motion, measuring aberrations related to the heat motion cannot be eliminated perfectly due to the complex structure of the apparatus and the measuring column as well, moreover, the accuracy of any measurement is affected largely not only by mass forces resultant from vertically arranged structural parts but also by oscillations acting in the vertical plane. A further drawback is that the apparatus is applicable neither to continuous measurement of hot hardness nor to continuous observation of transformations entailing alteration of hot hardness, e.g. recrystallization processes.

Consequently, prior art solutions above do not provide any direct information to be obtained continuously from processes like recrystallization or similar phenomena even by using extremely expensive devices. The process of recrystallization and recrystallized proportion of texture of the crystalline material cannot be observed continuously. We have found, however, that if a transformation process intensively affecting the hot hardness is taking place in a sample while heating, a huge alteration in expansion velocity occurs on the thermal expansion curve plotted by a device far harder than the sample and biased against the sample for indenting, because the linear thermal expansion of the sample and the displacement of the device depending on the alteration of hardness of the sample are summed. Consequently, it is thus possible to obtain information directly and continuously about the hot hardness and thus the progress of processes involving alteration of hot hardness as well by recording a thermal expansion curve of a sample.

Therefore, an object of this invention is to provide an apparatus and a method for measuring hot hardness of materials, which is suitable for continuous observation of alteration of hot hardness as a function of temperature change, that is it is adapted for measuring hot hardness continuously, and the progress of processes involving alteration of hot hardness e.g. recrystallization can be observed continuously as a function of temperature, and due to its horizontally arranged structural parts the accuracy of any measurement would be affected nei- ther by mass forces nor by oscillations acting in the vertical plane.

Above object can be achieved by providing an apparatus for measuring hot hardness of materials comprising a main frame and a displacement gauge connected to the frame by spring elements, and the displacement gauge is provided by a core rod biased against the frame by a mounting spring, and a measuring frame having an inner surface for fixing a test piece is mounted on the frame, and a measuring rod is attached to the core rod, and a measuring tip is connected to the measuring rod, and a furnace surrounding at least the inner surface is arranged around the measuring frame, and a thermocouple for measuring the temperature of the test piece is provided, wherein the measuring rod and the measuring frame are made of the same material.

The measuring rod and the measuring frame are made of quartz glass.

The core rod, the measuring rod and the measuring tip are aligned in the same horizontal axis (t).

The apparatus is provided by a PID type control circuit containing the furnace and the thermocouple.

The displacement gauge is an inductive type displacement gauge.

The object of the present invention is also achieved by a method for measuring hot hardness of materials, comprising the steps of: determining a value of initial penetration of a measuring tip having known geometry in a test piece at room temperature; then said test piece is heated, while the measuring tip is abutted against said test piece by a biasing force, and determining a value of indentation made by the measuring tip in the test piece, and recording thermal expansion curves of said measuring tip and said test piece as a function of tempera- ture by heating said measuring tip and said test piece separately at different times, and abutting measuring tip against said test piece applying said biasing force, applying continuous load to the measuring tip by said biasing force, while simultaneously heating said measuring tip and said test piece and determining continuously a relative displacement of the measuring tip and said test piece as a function of temperature, determining the value of indentation as a function of temperature by deducting the relative displacement as a function of temperature of the measuring tip and said test piece from the sum of thermal expansion curves, determining an area of indentation on the surface of said test piece made by the measuring tip by using the sum of indentation and initial penetration and calculating the value of hot hardness using this value of area:

HIT =

A(H + h _k )

Determining the value of initial penetration of the measuring tip comprises measuring Brinell or Vickers hardness of the test piece at room temperature by the same load as the biasing force.

The invention will be disclosed in details by referring to the drawing as attached. In the drawing

Fig. l is a schematic view of the apparatus according to the present invention, Fig. 2 shows a preferred geometrical form of the measuring tip,

Fig. 3 presents a temperature-expansion curve and factors affecting the displacement,

Fig. 4 shows a typical temperature-expansion curve,

Fig. 5 shows the way of determination of indentation value by means of the curve as shown in Fig 4,

Fig. 6 shows a correlation between Vickers hardness of samples measured on

25 °C and indentation of measuring tip,

Fig. 7 depicts a metallographic section of a sample as cooled from 90 °C containing cold worked texture, and Fig. 8 depicts a metallographic section of a sample as cooled from 180 °C containing an all but full recrystallized texture.

In Fig. 1 a schematic view of the apparatus for measuring alterations of hot hardness according to the present invention is shown. A test piece 5 can be placed in the measuring frame 1 fixed to the frame V of the apparatus B. As it is shown in the Figure, test piece 5 can be abutted against an inner surface 1 1 formed on the measuring frame 1 to be fixed thereon. A measuring tip 4, e.g. a needle, to be fitted to the test piece 5 and clamped in a collet 3 is arranged preferably in the measuring axis t of the apparatus B, for measuring hot hardness as indentation. The collet 3 is mounted on an end 21 of a measuring rod 2 adapted to transmit linear displacements. A core rod 91 of an inductive type displacement gauge 9 is attached to an end 22 of the measuring rod 2. The core rod 91, measuring rod 2 and the measuring tip 4 are preferably arranged in the same measuring axis t. A displacement gauge 9 operating by different technical principle e.g. a capacitive one can also be used instead of the inductive type displacement gauge 9. The measuring rod 2 and the measuring frame 1 are made of the same material, preferably of a material having a low coefficient of expansion and a very low factor of thermal conductivity, e.g. of quartz glass in this embodiment. This very low thermal conductivity is for decreasing a quantity of heat transferred towards the displacement gauge 9, while a low coefficient of expansion for moderating measurement errors due to the dimensional changes caused by thermal expansion. A unit consisting of the measuring rod 2, collet 3 and measuring tip 4 is biased against the inner surface 1 1 by springs 6 fixed to the frame V in the one hand and connected to the core rod 91 in the other. A test coil 92 of the inductive type displacement gauge 9 is supported by springs 12 fixed to the frame V and its position along the axis t can be adjusted for calibration against the force of springs 12 by means of a screw spindle 10 arranged on the frame V and fitted to a control member 93 of the test coil 92. Measuring frame 1 can be heated e.g. by means of a resistance heated furnace 8, preferably tube furnace, having a PID control loop 7, arranged around the measuring frame 1 , but at least in the area of the inner surface 1 1. The temperature of the test piece 5 can be measured by means of a thermocouple forming a part of the control loop 7 and welded to the test piece 5 preferably by spark discharge. If the welding of the thermocouple onto the test piece 5 comes up against any difficulty (e.g. in the case of a test piece 5 made of copper) the thermocouple might be connected to the collet 3 as well. In the latter case one must apply a slower heating speed to obtain sufficient thermal compensation. An advantageous geometric form of the measuring tip 4 is shown in Fig. 2, where an approximate dimension Y of the measuring tip 4 parallel to the axis t and an approximate dimension X perpendicular to the axis t are calculated. Experimentally, the inventors applied the tip of quenched steel sewing needle as a measuring tip 4 having a carbon content of 1 mass%. The measuring tip 4 was fixed in the collet 3 by a shrink-on-joint and then heat treated for three hours on 320 °C (annealed) in order to prevent its hardness from decreasing in the 20 - 300 °C range of measurement thus avoid a potential measurement error. However, a harder material (e.g. diamond) can also be used instead of a steel measuring tip 4 allowing increasing the temperature limit above 300 °C.

Achieving the method for measuring hot hardness of materials continuously according to the invention by means of the apparatus according to the invention as disclosed above the measurement of hot hardness of a test piece 5 takes place by continuously indenting the measuring tip 4 into the test piece 5. The measuring frame 1 made of quartz glass is heated by means of a furnace 8 provided with a PID control loop 7. Due to this heating the temperature of both measuring tip 4 and test piece 5 increases continuously and both expand in some extent. This expansion is transmitted to the inductive displacement gauge 9 by the rod 2 made of quartz glass. Calibration of the displacement can be accomplished by means of the screw spindle 10 and the temperature of the test piece 5 is measured by thermocouple 13. The temperature of the test piece 5 and measuring tip 4 can be adjusted according to a predetermined heating program, by means of the PID control loop 7 attached to the furnace 8.

Two different processes take place while heating. In the one hand, a thermal expansion process increasing a displacement signal AL provided by the displacement gauge 9 can be observed. In the other hand, if the hardness of the test piece 5 decreases while heating, the measuring tip 4 penetrates more and more into the test piece 5, that is the aggregated length of the test piece 5 and the measuring tip 4 measured along the axis t decreases. Direction of this penetration is opposite to the direction of thermal expansion, that is the displacement signal AL provided by the displacement gauge 9 is decreased due to this effect. A value of the signal AL is affected by three displacements occurring simultaneously along the axis t as shown in Fig. 3:

- the thermal expansion ALs _z of the whole measuring tip 4 along the axis t, which increases the value of signal AL, - the thermal expansion AL _P of the test piece 5 along the axis t, which also increases the value of signal AL, and

- the extent of indentation H of measuring tip 4 along the axis t due to the decrease of the hot hardness, which decreases the value of signal AL.

With the method for continuous measurement of hot hardness of materials according to the invention an initial hardness state biased by the force of the spring 6 would be determined as a first step that is the hardness of the test piece 5 at room temperature. This step can be achieved by placing the test piece 5 onto the surface 11 of the apparatus B and plotting the force-path diagram of the biasing at 25 °C, where the initial hardness is the indentation depth h _k created by the biasing force of the spring 6. A contact surface area A between the test piece 5 and any measuring tip 4 having a known geometry, that is the area A of the indentation, can be determined as a function of the indentation depth h _k , from which the hardness HV is computable as a ratio of the biasing force F _e and the area A:

F

HV =— .

A

Initial hardness can be determined out of the apparatus B as well, namely by measuring the hardness using Vickers or Brinell methods. In order to eliminate an error caused by the dependency of the hardness on the loading force it is advantageous to measure the Brinell HB or Vickers hardness by a force equal to the biasing force F _e . This way an area Ak of indentation and an indentation depth h _k belonging to a given measuring tip 4 shown e.g. in Fig. 2 can be calculated in aware of measured hardness HB or HV. Thus, if the HV _25°C Vickers hardness of the test piece 5 is known, then the initial area A _k of indentation belonging to the measuring tip 4 with a biasing force F _e applied by the spring 6 at 25 °C can be computed by the equation:

F

4 =— HV— (ΐ· ⁾ where: HV ₂₅ «c corresponds to the Vickers hardness at 25 °C, F _e is the biasing force applied by the spring 6 on placing the test piece 5, and the area A _k of indentation is the area of the indentation created on the test piece 5 measured on the surface of the test piece 5. Knowing the area A _k of indentation created by the measuring tip 4 an initial indentation depth hk then the value of initial hardness can be expressed:

h _k = f(A _k ) (2.) Then plotting the thermal expansion diagram ALp, ALs _z both of test piece 5 and measuring tip 4 respectively, as a function of temperature T according to Fig. 3, so that e.g. heating the test piece 5 without the measuring tip 4 then the measuring tip 4 without the test piece 5 in the apparatus B. Then deducting an expansion diagram M obtained by heating together the test piece 5 and the measuring tip 4 placed in the apparatus B in a way as shown in Fig. 1 from the sum ALp+ALsz of expansion diagrams ALp, ALs _z thus obtained, that is the relative displacement AL of the test piece 5 and the measuring tip 4 as a function of the temperature, which difference is a value of indentation H belonging to each temperatures. Adding this value to the value of initial indentation hk a value of whole indentation H+hk of the measuring tip 4 into the test piece 5 can be obtained at any temperature. Knowing this value of whole indentation H+hk of the measuring tip 4 the area A created by the measuring tip 4 can be calculated, from which a value of hot hardness HIT can be expressed at any temperature, if the loading force is known:

HIT = ^ (3.)

A(H + h _k )

where: A(H + h _k ) \s a surface area of indentation belonging to the sum of the initial indentation hk and an indentation H measured at T temperature.

Summing up above, hot hardness HIT of materials can be measured continuously by determining at room temperature, e.g. at 25 °C, the initial indentation depth hk of a measuring tip into the test piece 5 having known geometry. Then heating the test piece 5 and the measur- ing tip 4 together, while the measuring tip 4 is abutted against said test piece 5 by a biasing force F _e , and determining continuously a relative displacement AL of the measuring tip 4 and said test piece 5 as a function of temperature. Then determining the value of indentation H as a function of temperature by deducting the relative displacement AL of the measuring tip 4 and said test piece 5 from the sum AL _P + ALs _z of thermal expansion curves AL _P , AL _Sz deter- mined in the first step of the method according to the invention, and determining an area A of indentation on the surface of said test piece 5 made by the measuring tip 4 by using the sum of indentation H and initial penetration hk, and calculating the value of hot hardness HIT using this value of area A:

F

HIT = ^e - .

A(H + h _k ) Since the hot hardness HIT changes largely with transformation of the texture like re- crystallization, method and apparatus according to the invention allow examination of transformations also with continuous heating, based on a continuous measurement of hardness.

Example

Hot hardness HIT has been measuring continuously by means of the apparatus B and method according to the invention, while test piece 5 and the measuring tip 4 were heated. Experimental results have been depicted by plotting a temperature-expansion curve as a function of the displacement signal provided by the inductive displacement gauge 9 and a thermal signal provided by the thermocouple 13, as shown in Fig. 4. If a transformation accompanied by any alteration of hardness, e.g. recrystallization takes place in the test piece 5 while heated, a considerable change in expansion rate would be expected in the temperature-expansion curve.

Test pieces 5 made of electrolyte copper cold rolled by 80% reduction (at 20 °C) were used to measure hot hardness continuously. A strip rolled to 1,5 mm thick were cut for form- ing test pieces with a diameter of 5 mm. For measurement of hot hardness a test piece 5 was placed between the measuring tip 4 and the inner surface 11 of the frame 1 , as shown in Fig. 1. Measuring tip 4 has been biased against the test piece 5 by a force F _e of 760 mN provided by the spring 6. The whole indentation H+h _k of the measuring tip 4 into the test piece 5 was produced by this load in the course of alteration of hot hardness. At the beginning an initial indentation h _k was created, as for all measurement methods in measuring hardness based on depth measurement. The indentation H of the measuring tip 4 into the test piece 5 was calculated from this initial indentation h _k depth. The quartz glass measuring frame 1 was heated together with the test piece 5 up to 300 °C by a heating rate of 10 °C/min. A heating rate of 10 °C/min involves relatively low rate of temperature change allowing complete heat balance of the test piece 5, measuring frame 1 and measuring tip 5. Several measurements have been achieved with electrolyte copper test pieces 5 by above circumstances. A linear change of expansion as a function of temperature can be observed up to 80 °C in an initial section Kl of heating curve F in Fig. 4. This expansion equal to the sum of expansions ALp+ALs _z of measuring tip 4 and test piece 5, deducting an indentation H therefrom, all belonging to the actual temperature, and adds up to 0.25 μηι at 80 °C in this example.

After an indentation of little extent expansion-heating curve F loses its linearity in the section K2 above 80 °C and an increasing indentation can be observed as the temperature be- comes higher. After a maximum Max reached near 140 °C the extent of indentation gets more intensive in a section K3, then slackens in a section K4 over 200 °C. Consequently, the transformation, recrystallization in this case, involving considerable change of hot hardness took place in sections K2 and K3, and this fact has also been proved by polished metallographic sections of test samples previously heated up to 90 °C (Fig. 7) as well as 180 °C (Fig. 8), respectively, and cooled down to room temperature along a direction marked by an arrow L.

The elastic force of the spring 6 applied was 1.42 N/mm. The length of the spring 6 changed less than 4 μπι. During this change of length the force biasing the measuring tip 4 to the test piece 5 (that is the load applied during measurement of hardness) decreased by 5.7 mN, which is less than 1 % of the initial biasing force of 760 mN. Moreover, the force of the spring 6 changes only 1.5 mN in the section K3 during recrystallization that is less than a change of 0.3 %. Therefore, we can say that the measurement took place by a quasi constant load.

Indentation H occurring in the course of measurement can be calculated by means of an expansion curve M of Fig. 4 and demonstrated in Fig. 5. The value of indentation H is compared to the sum ALp+ALs ₂ . The value of indentation H can be obtained as a difference between the cumulative expansion curve ALp+ALs _z of measuring tip 4 and test piece 5 and measurement curve M. When a more intensive decrease of the hot hardness occurs in the section K2 than in K, the measuring tip 4 biased by the spring 6 penetrates into test piece 5 more deeply and the measured curve differs more and more from the line of sum ALp+ALsz in the section K2. The difference between the line of the sum ALp+ALsz and the actual expansion curve M involves the value of indentation H. The softer state the test piece 5 gets, the higher the difference that is the extent of indentation H. The value of indentation H is assumed to be zero in a state after the formation of initial indentation h _k caused by the biasing force of the spring 6 and the measuring tip 4 in the cold rolled test piece 5, it value has been calculated from this zero value according to Fig. 5.

Relation between the indentation H and Vickers hardness at 25 °C belonging thereto has been established by the following test series. Cold rolled electrolyte copper test pieces 5 have been heated up to different temperatures by a heating rate of 10 °C/min, then quenched (by spraying water) from the selected temperature. After quenching a Vickers hardness HV5,2 was measured at 25 °C with a load of 5.2 kg. Interrupted heat treatments caused test pieces 5 recrystallized at different rates. A relationship between the Vickers hardness of each test piece 5 measured at 25 °C and indentation H of the measuring tip 4 has been found as depicted in Fig. 6. Vickers hardness decreases from 1 16 to 59 HV5,2 in the course of recrystallization of electrolyte copper test pieces 5. Indentation H denoting hot hardness of test pieces 5 has been increased from Ιμιη to 3...3.2 μιη in the course of recrystallization, that is in the stages Kl and K2. Since the value of indentation H increases with the temperature, but the Vickers hardness at 25°C does not change, recrystallization terminates when indentation H reaches the values of 3..3.2 μιυ. This fact is also proven by polished metallographic sample shown in Fig. 8.

The main advantage of the apparatus and method for measuring hot hardness of mate- rials according to the invention is that it is suitable for continuous measurement of alteration of hot hardness of different materials, it is adapted for measuring hot hardness continuously, and the progress of processes involving alteration of hot hardness e.g. recrystallization can be observed continuously as a function of temperature. Due to its horizontally arranged structural parts the accuracy of any measurement would be affected neither by mass forces of measuring tip 4, collet 3, core rod 91, measuring rod 2, spring 6 and spring elements 12 nor by oscillations acting in the vertical plane.