JPS6126721A - Method for evaluating hardenability of steel by x-ray diffraction - Google Patents

Abstract

PURPOSE:To measure rapidly the hardenability of steel on a nondestructive basis by obtaining a diffraction line showing the relation between the diffraction angle and the intensity of X-rays, and measuring the sharpness of a peak from a Gaussian curve parameter closely resembling the peak. CONSTITUTION:The diffraction line showing the relation between the diffuraction angle and the intensity of X-rays is obtained by irradiating X-rays on hardened steel. The sharpness of the peak of a sharp diffraction line characteristic of incompletely hardened steel is measured by using the parameter of a Gaussian curve (a constant, hereinafter referred to as GCP) resembling closely the peak of the diffraction line. The GCP at this time is compared with the previously obtained GCP of completely hardened steel contg. the same amt. of C. The incompleteness of hardening or the area ratios of troostite structure and pearlite structure to martensite structue are thus measured. The completeness of hardening of steel or hardenability can be evaluated by this method on a nondestructive basis without depending on the measurement of hardness, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for evaluating the hardenability of steel by X-ray diffraction, which evaluates the completeness of hardening or hardenability of steel by X-ray diffraction.

Conventional technology Generally speaking, if the quenching is incomplete when steel is quenched, sufficient strength and toughness cannot be obtained by subsequent tempering. It is important to harden it. In order to judge the completeness of such hardening, hardness measurement using a hardness meter and microstructural observation using a microscope are most widely used.

Problems to be Solved by the Invention However, these methods require cutting out a test piece from the member. Therefore, it not only requires a lot of effort for evaluation, but also has drawbacks such as damage to components.

This invention was made for the purpose of improving the above-mentioned drawbacks.

Means and Effects for Solving the Problems When the steel structure becomes martensite through complete quenching, the X-ray diffraction lines become significantly broadened. However, if the quenching is incomplete and the structure contains ferrite, barley, or troostite structure, the diffraction peaks become sharp. Here, the diffraction line refers to the diffraction angle X as shown in Figure 1.
This is a curve showing the relationship between y and X-ray intensity y. The diffraction angle X is
The direction of the X-rays incident on the object to be measured and the X-rays diffracted from the object
It is the supplementary angle of the angle formed by the direction of the line. In FIG. 1, a base line AB connecting points A and B at both ends of the base of the diffraction line is called the background of the diffraction line (hereinafter referred to as BG).

In general, incompletely hardened steel has a mixed structure of martensitic structure showing broad diffraction lines and troostite (structure consisting of ferrite and fine cementite) showing sharp diffraction lines.
Or, if there is no quenching at all, no martensitic structure appears and a mixed structure of ferrite (and cementite) appears.
perlite, loosetite, sorbite).

Figure 2 is a diffraction line measured using CrKα characteristic X-rays for a completely hardened steel (steel whose structure is entirely martensite) made by water-quenching mechanical structural carbon ``@545C'', which shows the characteristic martensitic structure. Figure 3 shows water-quenched fully hardened steel 545c at -500°C.
This is a diffraction line of a specimen with a uniform sorbite structure tempered by .

Pearlite, troostite, and sorbite are all mixed structures of ferrite and cementite, and among these, troostite and sorbite are structures in which fine cementite is dispersed in a ferrite phase. While martensitic structures have broad diffraction lines as shown in FIG. 2, pearlite, troostite, and sorbite structures all have sharp diffraction lines as shown in FIG. 3, which are characteristic of ferrite phases.

L/ In other words, the diffraction line of incompletely hardened steel consisting of martensite structure and troostite structure is a diffraction line obtained by superimposing the diffraction lines of Fig. 2 and Fig. The diffraction peaks are sharper than the broad diffraction lines of hardened steel.

Next, a method of determining a Gaussian curve parameter for measuring the sharpness of this diffraction line peak will be explained.

Generally, it has been revealed that the vicinity of the peak of a diffraction line can be approximated by a Gaussian curve expressed by the following equation.

y-Cgxp(-eLx+6z)(l!, (')Q)
(+) However, C9α and b are constants. Furthermore, as is well known in statistics, the normal probability density function f (-) representing a probability distribution is expressed by the following equation.

However, μ and σ are the average value and standard deviation of f (−). It is a well-known fact that the spread of the normal probability density function i<x> is expressed by its standard deviation σ.

Since the Gaussian curve in Equation (1) has the same type as the normal probability density function in Equation (2), it can be obtained by setting the coefficients of x2 in Equations (1) and 2〇) to be equal. Therefore, 1/J τ obtained from the coefficient α of the Gaussian curve fitted to the peak of the diffraction line corresponds to the standard deviation σ representing the spread 9 of the function of Fuki (2), so this l/v”27 can represent the spread of a diffraction line approximated by a Gaussian curve.This value is named the Gaussian curve parameter (hereinafter abbreviated as GCP) and is expressed by α as follows.

Next, how to obtain GCPα in equation (4) will be explained.

Generally, in order to obtain a correct diffraction X-ray intensity, it is necessary to correct the measured value y of the X-ray intensity by the background (JG) and LPA factors. However, since it is clear that the LPA factor correction has almost no effect on the value of α in equation (4), it can be omitted. BG correction refers to converting diffraction lines into EG
It is to measure based on the strong intensity, that is, to use y-y instead of the actual measurement value y as the X-ray intensity.

However, the G calculated from the actual measured value y by omitting this BG correction
Even if CP is used, the spread9 of the diffraction line can be evaluated, so from a practical standpoint that measurement time and calculation time can be shortened, we will mainly discuss GCP obtained without EG correction, but GCP obtained without BG correction will be discussed here. It goes without saying that the GCP in this case can also be obtained in a similar manner.

Generally, it has been revealed that if the maximum X-ray intensity of a steel diffraction line is yw, the vicinity of the peak of a diffraction line of at least 0.8yy can be well approximated by a 4 Gauss curve. now,
0,8 measured with a constant step width C as shown in Figure 4.
By applying a Gaussian curve to the points on '/mm The Gaussian curve parameter (GCP) α that represents α can be calculated using the following equation.

However, lrb represents the natural logarithm, Tz"=l2tS-n"+1.

GCP when EG correction is performed is the value y−y obtained by subtracting the BG intensity y6 from y instead of y in equation (5).

It can be found by substituting .

Example Next, an actual measurement example of incompletely hardened steel will be explained.

FIG. 5 shows the shape of the test piece used.

Figure 6 shows a 545c test piece with the shape shown in Figure 5.
This is an optical microscopic micrograph taken when the specimen was heated to 0°C and then cooled while standing still in water.
It has a mixed structure of white part) and troostite (black part). FIG. 7 shows the diffraction lines of this test piece. When we compare this with the diffraction lines of heat-treated steel with a uniform structure shown in Figures 2 and 3, we find that the diffraction line peaks are unusually sharp compared to the wide base of the diffraction lines. be.

FIG. 8 shows a test piece 545C having the shape also shown in FIG.
This is an optical micrograph taken when the sample was heated to 850°C and then cooled in oil.・Similar to the test piece shown in Figure 6, this test piece also consists of a mixed structure of martensite (white part) and troostite (black part), and the diffraction lines are as shown in Figure 9. , showing sharp diffraction line peaks.

FIG. 10 is a diagram explaining the reason why such abnormal diffraction lines are obtained in incompletely hardened steel, and schematically shows the diffraction lines corresponding to the microstructure photograph. The depression in the center of this microstructure photograph is an indentation when the micro Vickers hardness Hv was measured. The structure of the incompletely hardened steel shown in Figure 10 (C) is a mixed structure of martensite and troostite, and its diffraction line is a superposition of the sharp diffraction line of troostite and the broad diffraction line of martensite. It can be seen that the profile is sharp near the peak and wide at the hem.

Therefore, GCP can measure the sharpness of the peak of the diffraction line.
α takes a smaller value for incompletely hardened steel than for completely hardened steel. If the GCP of fully hardened steel (steel whose structure is entirely martensite) is determined in advance for steels with various carbon contents (0%), If the ratio r between the GCP of the filled steel (this is set as α) and the GCP of the fully hardened steel (this is set as 4) is calculated from the following formula % formula % (6), then from the value of r, the quenching It is possible to know the imperfections or the area ratio occupied by martensite and troostite structures.

Figure 11 shows the relationship between 0% and micro-Vickers hardness Hv when 15 types of steel with different carbon contents (0%) are water-quenched and then subjected to sub-zero treatment and completely quenched. . This figure also shows 545 carbon steel for machine structures.
The white circles indicate the hardness of C and carbon tool steel SK3 when they are incompletely hardened by changing the cooling rate.

The hardness Hv in FIG. 11 is the average value of values measured at seven points for one test piece. FIG. 11 shows that the hardness of incompletely hardened steel is naturally smaller than the hardness of fully hardened steel with the same 0%.

Figure 12 shows 0% calculated using the same test piece as in Figure 11.
and GCPα. Figure 12 shows the same tendency as Figure 11, with GCP of incompletely hardened steel (white circles).
has a smaller value than the GCP (black circle) of fully hardened steel.

When the r of these incompletely hardened steels was calculated from equation (6), it was r-0.23 to 0.90 for 545c and r-0.23 to 0.90 for SK3.
-0.16 to OJ4. Figure 12 shows the results obtained by the method of X-ray diffraction using GCP proposed in the present invention.
This shows that it is possible to evaluate the incompleteness of quenching.

Figure 13 shows the relationship between hardness Hv and GCPα of 545C specimens that were completely quenched and then tempered at various temperatures (indicated by black circles) and those that were incompletely quenched (indicated by white circles). It is. This figure shows that GCPα increases with increasing hardness Hv for all test specimens, and that for the same hardness, the GCP of incompletely hardened steel is the same as that of completely quenched and tempered specimens. This shows that the value is small compared to the above.

Effects of the Invention As described above, the present invention produces diffraction lines showing the relationship between diffraction angle and X-ray intensity by irradiating X-rays on hardened steel to be evaluated using an X-ray diffraction device or an X-ray stress measurement device. The sharpness of the sharp diffraction line peak, which is characteristic of incompletely quenched steel, is measured using the parameters of a Gaussian curve that approximates the vicinity of the peak of this diffraction line, and the obtained GCP is Complete ′ with the same carbon content determined in advance
Incompleteness of quenching of incompletely quenched steel by comparing with GCP of quenched steel or completeness of quenching of steel by measuring the area ratio of troostite structure or pearlite structure to martensitic structure Unlike conventional hardness measurement or microscopic structure observation methods, this method allows measurement of the actual object, and there is no need to cut out a test piece from the component, making non-destructive measurement possible. You will be able to do it easily and quickly.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention; Fig. 1 is a schematic diagram of diffraction lines showing the relationship between the diffraction angle Diffraction diagram of steel 545c, Fig. 3 is a diffraction diagram of 545C obtained by water quenching and tempering at 500°C, Fig. 4 is an explanatory diagram of the method of approximating the diffraction peak with a Gaussian curve, Fig. 5
Figure (W) # (6) is a plane and side view showing the shape of the specimen, and Figure 6 is incomplete quenching @ cooled while stationary in water.
The metallographic diagram of 545C, Figure 7 is the diffraction diagram of incompletely quenched @545C cooled in Y-state while standing still in water, and Figure 8 is the metallographic diagram of incompletely quenched steel 545c cooled in oil. 9th
The figure shows the metal diffraction diagram of incompletely hardened steel 5h5C cooled in oil. Figure 10 (cL), (6) I (C) shows incompletely hardened steel with a mixed structure of martensite and troostite structure. Fig. 11 is a diagram showing the relationship between carbon content and micro-Vickers hardness of fully quenched steel and incompletely quenched steel, Fig. 12 is a diagram showing the relationship between carbon content and micro-Vickers hardness of fully quenched steel and A diagram showing the relationship between carbon content and GCPα of incompletely hardened steel, and FIG. 13 is a diagram showing the relationship between hardness Hv and GCPα of 545C hardened and tempered steel and incompletely hardened steel. . Fig. 4 Diffraction angle χ Fig. 5 (b Fig. 6

Claims (1)

[Claims] By irradiating the hardened steel to be evaluated with X-rays using an X-ray diffraction device or an X-ray stress measuring device, a diffraction line indicating the relationship between the diffraction angle and the Parameters (constants: hereinafter G) of the approximated Gaussian curve
G
By comparing the CP with the predetermined GCP of fully hardened steel with the same carbon content, it is possible to determine the imperfection of quenching in incompletely hardened steel or the presence of troostite or pearlite structure in contrast to martensitic structure. A method for evaluating the hardenability of steel by X-ray diffraction, which comprises evaluating the completeness of hardening or hardenability of steel by measuring the area ratio.