JPH0815114A - Thermal fatigue evaluation method for metallic material - Google Patents

Abstract

PURPOSE:To shorten the time required for evaluation and test by simplifying the method for evaluating the thermal fatigue of a metallic material. CONSTITUTION:A high frequency heating coil 3 is conducted to heat only the rapid heating/cooling part of a test piece 1 located immediately under the coil 3. The rapid heating/cooling part is formed above a part to be evaluated substantially concentrically thereto. Heating is sustained for 5sec and it is interrupted when the temperature at the rapid heating/cooling part reaches about 700 deg.C. Subsequently, it is cooled naturally for 7sec to lower the temperature at the rapid heating/cooling part down to about 300 deg.C. The evaluating part of the test piece 1 is then cooled with cooing water fed through a cooling water pipe 4. Cooling water supply is interrupted upon elapse of 6sec. The temperature at the rapid heating/cooling part is thereby lowered down to about 60 deg.C. Finally the test piece 1 is cooled with the air blown through an air blow piping 5 for 10sec and heated again by conducting the high frequency heating coil 3. The series of operation is repeated thus effecting the thermal fatigue test.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating the degree of thermal fatigue of metallic materials.

[0002]

2. Description of the Related Art Thermal fatigue is a failure phenomenon that is caused by repeated thermal stress caused by restraint of thermal strain. The thermal strain is obtained by multiplying the temperature change of the object by the thermal expansion coefficient, because the temperature of the object changes due to the heat transfer between the heat medium outside the object and the surface of the object and the heat conduction transferred from the surface of the object to the inside. The thermal strain thus generated is constrained in various forms to generate thermal stress, and thermal fatigue occurs by repeating this.

With respect to the thermal fatigue that occurs in this way, a thermal fatigue test for investigating the thermal fatigue degree of a material is conducted by various methods. The thermal fatigue test means heating and cooling the test piece 1
This is a test in which a cycle test is repeated to examine the degree of thermal fatigue due to thermal strain associated with thermal expansion and thermal contraction of the test piece, which can be roughly divided into two types. One is an external restraint type thermal fatigue test, which heats and cools a test piece uniformly and restrains its expansion and contraction from the outside by the rigidity of a testing machine. The other is an internal restraint type thermal fatigue test, which restrains thermal strain inside the object by a combination of different materials having different thermal expansion coefficients in the object.

A commonly used method for evaluating the degree of thermal fatigue of a test piece subjected to a thermal fatigue test is an evaluation method that focuses on cracks that occur during the thermal fatigue test.
That is, the length of a crack is defined, and a crack is generated when a crack longer than that is generated, and the number of test cycles at the time of crack generation is used as an index of thermal fatigue. An example will be described with reference to FIG. FIG. 8 is a graph showing the relationship between the number of cracks and the number of test cycles, where the horizontal axis represents the number of test cycles with heating and cooling as one cycle, and the vertical axis represents the number of cracks. The test was performed on four materials, and each material was the same material and subjected to different heat treatments. The crack length defining the occurrence of cracks is 1 mm.
From this graph, the material A was tested at 2000 test cycles.
It can be seen that there are cracks in the other materials but no cracks in the other materials. Therefore, it can be seen that the material A is the weakest of the four materials with respect to thermal stress. Further, the number of test cycles is 3000 times, 4000 times, 5000 times
Examining the number of cracks generated for each material over time reveals that the material most resistant to heat is D, then C, B, and the weakest is A. By making comparisons in this way, it is possible to evaluate the strength of materials against thermal fatigue. Based on this data, selection of materials, judgment of superiority or inferiority of different materials, heat treatment conditions applied to materials, surface treatment The conditions and the blending ratio of the components were evaluated.

[0005]

However, in the conventional evaluation method, the thermal fatigue test must be performed until cracks occur in the test piece, and it not only takes a lot of time,
There is a possibility that the evaluation result may vary depending on the experience of the measurer who measures the crack, and there is a problem that the evaluation takes time.

[0006]

The technical means taken in the invention of claim 1 to solve the above technical problems are as follows:
In the method of evaluating the degree of thermal fatigue of a metal material, a thermal fatigue test is performed by repeating rapid heating and cooling of the evaluation site of the test piece,
The test is terminated before cracks due to thermal fatigue occur in the test piece, and after the test is finished, rapid heating of the test piece, surface roughness is measured in a direction straddling the rapid cooling portion, rapid heating, forming in the rapid cooling portion. That is, the method for evaluating the thermal fatigue level of a metal material, which evaluates the thermal fatigue level of a material based on the height of the formed convex portion, is used.

[0007]

According to the above technical means, the evaluation portion is raised before the crack is generated in the evaluation portion of the thermal fatigue test, and the convex portion is formed on the surface. Further, the height of the convex portion increases almost linearly until a crack occurs at the evaluation site. Therefore, when evaluating the thermal fatigue degree of a metal material, it is not necessary to perform a thermal fatigue test until a crack occurs as in the conventional case, and the heat of the material is determined based on the height of the convex portion at a certain number of test cycles. The degree of fatigue can be evaluated.

[0008]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a schematic view of the thermal fatigue testing machine, FIG. 2 is an enlarged perspective view of the vicinity of the high frequency heating coil of the thermal fatigue testing machine, and FIG. 3 is an enlarged front view of the vicinity of the high frequency heating coil of the thermal fatigue testing machine. Show.

The jig 2 for fixing the test piece is provided with a cylindrical hole in the upper part thereof so that the test piece 1 can be inserted therein, and the cylindrical test piece 1 is put in the hole. The shape of the test piece 1 is shown in FIG. The test piece 1 used in this example has a diameter of 65 mm and a height of 60.
It is a cylindrical test piece of mm. Jig 2 for fixing the test piece
A circular flat surface that faces upward when the test piece 1 is inserted into the heat fatigue test evaluation site (hereinafter referred to as an evaluation site).
1a. Further, the test piece 1 is fixed at two places on its side surface with the tightening bolts 6. In this example, SKD61, which is a kind of steel material, was used as the material of the test piece.

The high frequency heating coil 3 is provided on the evaluation portion 1a of the test piece 1 so as to face it.

The tip of the cooling water pipe 4 is provided so that the cooling water supplied from a cooling water tank (not shown) falls on the evaluation portion 1a of the test piece 1 and the test piece 1 can be efficiently cooled. It is arranged toward one evaluation site 1a.

In the air blow pipe 5, the air supplied from an air supply source (not shown) is the evaluation site 1 of the test piece 1.
The tip of the test piece 1 is arranged toward the evaluation portion 1a of the test piece 1 so that the test piece 1 can be efficiently air-cooled.

In the thermal fatigue tester having such a structure,
The high frequency heating coil 3 is energized. In this embodiment, the output frequency is 100 KHz and the output electric energy is 22 KV.
A. Then, only the portion 1b of the evaluation portion 1a of the test piece 1 immediately below the high-frequency heating coil 3 (hereinafter, referred to as a rapid heating / quenching portion) is heated. This rapid heating and cooling section 1b
In the case of the present embodiment, as shown in FIG. 4, it is formed on a concentric circle having a center substantially at the same position as the center of the evaluation part 1a. Continue heating for 5 seconds, and the temperature of the rapid heating and cooling section 1b will be about 7
The heating is stopped when the temperature reaches 00 ° C. Then, it is naturally cooled for 7 seconds, and the temperature of the rapid heating and quenching section is set to about 300 ° C. After cooling naturally, cooling water is jetted from the tip of the cooling water pipe 4 to cool the evaluation site 1a of the test piece 1. After cooling with water for 6 seconds, stop the cooling water. Here, the temperature of the rapid heating and cooling section is about 6
It drops to 0 ° C. After that, air is blown from the tip of the air blow pipe 5 from the air supply source, and the evaluation site 1a of the test piece 1 is air blown and air-cooled. After performing air blowing for 10 seconds, the high frequency heating coil 3 is again energized to heat the test piece 1. A thermal fatigue test is performed by repeating these series of operations. Note that FIG.
The temperature change pattern of the rapid heating and quenching part 1b of the test piece 1 in this example is shown in FIG.

After the thermal fatigue test, the test piece 1 is taken out and the surface roughness of the evaluation site 1a is measured. The surface roughness is measured in the direction from the center of the circular evaluation portion 1a to the periphery and across the rapid heating / cooling portion 1b. The measurement result is shown in FIG. FIG. 6a shows the surface roughness measured after a thermal fatigue test of 10 cycles of heating and cooling. When you see this, the rapid heating and quenching part rises to form a convex part,
It can be seen that the height rises by about μm. Also, FIG. 6b shows
The surface roughness was measured after a thermal fatigue test of 100 cycles of heating and cooling. From this, it can be seen that the swelling of the rapid heating and quenching portion is further increased and is raised by about 7 μm from the surroundings. Further, FIG. 6c shows the surface roughness measured after the thermal fatigue test of 1500 cycles of heating and cooling, in which the rapidly heated and quenched portion is raised by about 15 μm from the surroundings. FIG. 7 is a graph showing the height of the protrusion formed on the vertical axis and the number of test cycles on the horizontal axis, and showing the change in height of the protrusion with respect to the number of test cycles. In FIG. 7, the graph of (a) shows the test piece without heat treatment and surface treatment, the graph of (b) shows the test piece only with heat treatment and no surface treatment, and the graph of (c) shows the test piece. Both are heat-treated and surface-treated. From this, it can be seen that the height of the convex portion sharply increases for about 10 cycles from the start of the test. However, thereafter, the height of the convex portion increases almost linearly in proportion to the number of test cycles. Eventually, even if the number of test cycles is increased, the height of the convex portion remains unchanged, and finally cracks occur. Also, comparing the three types of materials, the number of test cycles until cracking occurs is shortest in case (a), then in case (b), and longest in condition (c). I understand. Further, comparing the heights of the convex portions at the same number of test cycles (for example, 500 times), (a)
Is the highest, then (b), and (c)
It can be seen that the condition of is the lowest. As described above, it is understood that the same result can be obtained by comparing the number of test cycles until the occurrence of cracks and also by comparing the heights of the convex portions at a certain number of test cycles. Therefore, when evaluating the thermal fatigue degree of heat treatment of a certain material, the one whose surface treatment state has been changed, or the one whose compounding ratio has been changed, even if the thermal fatigue test is not performed until crack initiation, the convex portion at a certain number of test cycles Compare the heights of.

[0016]

The invention of claim 1 has the following effects.

In the method of evaluating the degree of thermal fatigue of a metal material, after the thermal fatigue test, the height of the convex portion formed by being raised at the test evaluation site of the test piece is measured and compared to determine the thermal fatigue of the material. Evaluate the degree. This allows the thermal fatigue test to be completed before the crack occurs in the conventional thermal fatigue test piece, whereas the thermal fatigue test can be completed before the crack occurs, and the test time can be shortened.

Further, in the past, judgment of whether or not a crack had occurred in a test piece, measurement of the number of cracks, and the like required the experience of the measurer, and the measurement also took a considerable amount of time. Then, since only the raised portion of the test piece is measured with a surface roughness meter or the like, the measurement can be easily performed without the experience of a measurer.
Therefore, there is no difference in the evaluation result due to the experience of the measurer, and the time required for the evaluation can be shortened.

[Brief description of drawings]

FIG. 1 is a perspective view of a thermal fatigue tester according to this embodiment.

FIG. 2 is an enlarged perspective view of the vicinity of a high frequency heating coil of the thermal fatigue tester according to the present embodiment.

FIG. 3 is an enlarged front view of the vicinity of a high frequency heating coil of the thermal fatigue tester according to the present embodiment.

FIG. 4 is a perspective view of a test piece according to the present embodiment.

FIG. 5 is a heating / cooling pattern diagram of the test piece in this example.

FIG. 6 is a graph showing the measured surface roughness of the test piece after the thermal fatigue test according to the present embodiment.

FIG. 7 is a graph showing the relationship between the number of thermal fatigue test cycles and the height of the convex portion formed by being raised in this example.

FIG. 8 is a graph showing the relationship between the number of thermal fatigue test cycles and the number of cracks that occurred in the prior art.

[Explanation of symbols] 1 test piece 2 test piece fixing jig 3 high frequency heating coil 4 cooling water piping 5 air blow piping 6 tightening bolt 1a evaluation part 1b rapid heating and quenching section

Claims (1)

[Claims] 1. A method for evaluating the degree of thermal fatigue of a metal material, wherein a thermal fatigue test is performed by repeating rapid heating and rapid cooling of an evaluation site of a test piece, and the test is performed before cracks due to thermal fatigue occur in the test piece. After the end of the test, rapid heating of the test piece, surface roughness is measured in the direction across the quenching portion, rapid heating, thermal fatigue of the material based on the height of the convex portion formed in the quenching portion Evaluation method of thermal fatigue of metal materials.