JPH0466863A - Residual stress measuring method by steel working - Google Patents

Abstract

PURPOSE:To make a correct measurement possible even in the working where a stress condition becomes isotropic, by applying change values of a permeability before and after working of a steel having a retained austenitic texture, to the relative relation coming to a reference to find residual stress. CONSTITUTION:Permeabilities before working (including the portion other than a worked portion where a portion of a steel 1 is worked) and after working of the steel 1 having a retained austenitic texture are measured. A sensor 21 consisting of a magnetic core 210 consisting of a laminated silicon steel plate, etc., an exciting coil 211 to which alternating current is applied, and a detection coil 212 wound round the magnetic core 210 and outputting the permeability muas a voltage value V; is used in a detection of the permeability. A calculation of a residual stress can be carried out as follows; the relative relation between a residual stress value being a reference previously found by an X-ray stress measuring device, etc. and a change value of the permeability mu (the voltage value V), is found. The residual stress corresponding to an individual change value (let the permeability before the working be mu0 (the voltage V0) and it after the working be mu (the voltage Vx), then the change value DELTAV is given by DELTAV=V0-VX) is found from such a relative relation.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for measuring residual stress when a steel material used for parts such as gears and axles is subjected to processing such as shot peening that leaves residual stress. This measurement method can be used as a measure of the hardness, fatigue strength, etc. of processed steel materials.

[Prior Art] Japanese Patent Application Laid-Open No. 18-43954 discloses a method for non-destructive sorting of materials (Iill materials). This sorting method takes advantage of the fact that there is a certain relationship between material hardness and coercive force, sets the hardness into desired stages to be sorted, and sets the coercive force corresponding to the hardness at each sorting limit. The material is saturated and excited, and the coercive force corresponding to the hardness of the selection limit is demagnetized, and the positive and negative magnetic flux densities in each demagnetized state are detected. Alternatively, the desired level of hardness is selected depending on the degree of negative number.

A method for measuring residual stress in an induction hardened shaft using magnetism is also known. This measurement method uses two sensors in which an excitation coil and a detection coil are wound around a U-shaped magnetic core made of laminated silicon steel plates. The residual stress of the axle is determined by comparing the stress value with the output obtained by contacting each sensor with the axle in the axial and circumferential directions. Furthermore, a method is also known in which output detection in the axial and circumferential directions is simultaneously performed using a magnetic anisotropic sensor in which the above two sensors are combined at right angles to each other (Non-destructive Testing No. 3).
(Vol. 5, No. 2 (62, 2, 20)) [Problems to be Solved by the Invention] However, all of the above-mentioned conventional techniques cannot cope with changes in coercive force caused by tension and compression in the axial and circumferential directions. In other words, it takes advantage of the fact that there is a proportional relationship between the principal stress difference in the perpendicular direction and the difference in magnetic permeability in the principal stress direction, and its application range is in the case of stress state or anisotropy. It was limited to measurements. For this reason, with this technique, when shot peening is performed in which the stress state is isotropic, no difference in magnetic permeability occurs in the perpendicular direction, making it impossible to accurately measure residual stress. .

Furthermore, in the method disclosed in JP-A-48-43954, the steel material is first magnetically saturated using a direct current, and then the magnetic field is increased in the opposite direction to the magnetic field when magnetic saturation is reached. Measure the coercive force at which the magnetization becomes O. For this reason, this method requires a magnetic field to reach magnetic saturation,
It is necessary to apply a magnetic field that changes the magnetization to O to the steel material being measured, and it is difficult to apply it to a production line because of poor operability.

Furthermore, in the method described in the above publication, due to the opposite effect of the "magnetostrictive effect" in which distortion occurs when a magnetic material is magnetized, distortion occurs in the steel material when the steel material reaches magnetic saturation. The residual stress of the steel material changes accordingly. For this reason, the correlation between the measured coercive force and the residual stress, which is one element of the hardness of the steel material, is broken, and there have been cases where it is insufficient to accurately measure the hardness of the steel material.

In the past, if you wanted to accurately measure the residual stress of a steel material after shot peening using conventional techniques,
This measurement had to be carried out individually using a linear stress measuring device, and the necessity of electrolytically polishing the surface of each vA material made it impossible to use the steel material after the measurement, and in order to obtain accurate measured values. There was a problem in that it took a long time to measure because the number of measurements needed to be increased.

The present invention has been made in view of the above-mentioned conventional problems, and even when machining steel materials in which the stress state is isotropic, the present invention does not destroy the steel materials and has good operability and accuracy. The purpose of this study is to provide a method for measuring residual stress that can be used to measure residual stress.

[Means for Solving the Problems] As a result of intensive research into changes in the properties of steel materials when processed to generate residual stress, the inventors discovered the following measurement method.

That is, the method for measuring residual stress due to processing of steel materials of the present invention includes a pre-processing magnetic permeability detection step of detecting the magnetic permeability of a steel material with retained austenite structure before processing, and a step of detecting the magnetic permeability of the steel material after processing. This method is characterized by comprising a post-processing magnetic permeability detection step, and a calculation step of calculating residual stress from the detected change value between the pre-processing magnetic permeability and the post-processing magnetic permeability.

Here, before processing, in the case where a part of the steel material is processed, it includes a part other than the part to be processed.

Magnetic permeability is detected using a magnetic core made of laminated silicon steel plates, an excitation coil wound around this magnetic core to which an alternating current is applied, and a detection coil wound around the magnetic core and outputting the permeability as a voltage value. This can be done using a sensor consisting of.

To calculate the residual stress, first find the relative relationship between the reference residual stress value obtained using an X-ray stress measuring device, etc. and the change value of magnetic permeability, and then calculate the residual stress corresponding to each change value from this relative relationship. This can be done by asking for.

[Function] The present inventor has discovered that the retained austenite structure of the steel material transforms into a deformation-induced martenzyme structure due to processing, which causes a large change in magnetic permeability, and that the value of change in magnetic permeability is proportional to the residual stress. I found out that there is a relationship.

In the past, we first detected the permeability of the steel material with residual austenana 11 ~ structure before processing, then detected the permeability of the steel material after processing, and calculated the detected permeability before processing and after processing. By determining the change value from the magnetic permeability, the residual stress of the steel material after processing can be calculated.

[Example] Hereinafter, an example embodying the present invention will be described with reference to the drawings.

(Steel Material) In this embodiment, as shown in FIG. 1, a vehicle drive pinion 1 is used as the steel material. This drive pinion 1 has an umbrella part 11 and a shaft part 12. The drive pinion 1 before processing is made of carburized steel and consists of about 80% martensite structure and about 20% retained austenite structure.

(Measuring device) The measuring device 2 used is also shown in FIG. This measuring device 2 includes an umbrella sensor 21 that is in contact with the umbrella 11 of the drive pinion 1, a shaft sensor 22 that is in contact with the shaft 12 of the drive pinion 1, and the umbrella sensor 21 and the shaft. It consists of a sensor 22 and a measuring device 23 connected to it.

As shown in FIG. 2, the umbrella sensor 21 includes a U-shaped magnetic core 210 made of a laminated silicon steel plate, an excitation coil 211 wound around the magnetic core 210, and to which an alternating current is applied.
It consists of a detection coil 212 that is wound around a magnetic core 210 and outputs a voltage value by electromagnetic induction. The magnetic core 210 is the third
As shown in the figure, each tip is formed symmetrically so that it almost definitely comes into contact with the tooth groove of the umbrella part 11 of the drive pinion 1, and the side wall of each tip is designed to concentrate magnetic flux to each tip. A coating layer 210a made of ceramic or the like is formed to improve wear resistance.

The shaft sensor 22 shown in FIG. 1 is basically the same as the umbrella sensor 21 shown in FIGS. The difference is that it is formed symmetrically to ensure secure contact.

As shown in FIG. 2, the measuring instrument 23 includes an oscillator 231,
An amplifier 232 is connected to the oscillator 231 and applies an alternating current to the excitation coil 211; an amplifier 233 is connected to the detection coil 212 and amplifies the voltage value;
, a microcomputer 235 connected to the input/output device 234 , and a display device 236 connected to the input/output device 234 .

(Measurement method 1) In this example, shot peening (hereinafter referred to as
It's simply called a shot. ) was applied to the drive pinion 1, and the residual stress was measured using the measuring device 2.

(Pre-processing magnetic permeability detection step) As shown in FIG. 1, first, the cap sensor 21 is brought into contact with the cap 11 of the drive pinion 1 before the shot i. As a result, as shown in FIG. 2, the umbrella portion 11 of the drive pinion 1 generates magnetic flux within the range of depth t. Thereby, the measuring device 23 detects the voltage value of the umbrella sensor 21 (voltage value before processing; Vo). This pre-processing voltage value (Vo) has the following correlation with the pre-processing magnetic permeability (μ0).

That is, 15k is applied to the excitation coil 211 of the umbrella sensor 21.
By applying an alternating current of Hz, the umbrella portion 11 of the drive pinion 1 is heated by 1 from the surface as shown in FIG.
Magnetic flux (Φ) is generated at a depth of about 00 μm. The amount of generated magnetic flux (Φ) is determined by the magnetic permeability (μ) at depth t.
determined by

The magnetic permeability (μ) and magnetic resistance (Rm) have the following relationship.

Rm=f(1/μm5)ds-・-(1) formula % formula %) As can be seen from this formula (1), magnetic resistance (Rm) and magnetic permeability (μ) are in an inversely proportional relationship, The smaller the magnetic permeability (μ), the higher the magnetic resistance (Rm). This magnetic resistance (
Rm) and magnetic flux (Φ) have the following relationship.

Φ=Vm/Rm・・・・・・・・・・・・・・・・・・
......Formula (2) (where, Vm; magnetomotive force of the sensor (
Vm=I-N) Here, ■; excitation current; ) is larger, the amount generated increases. Therefore, a constant magnetomotive force (Vm
), when the drive pinion 1 is excited, the magnetic resistance (Rm) is determined by the magnetic permeability (μ) within the depth t, and finally the state of generation of the magnetic flux (Φ) is determined based on equation (2).

The voltage value (V) induced in the detection coil 212 by this magnetic flux (Φ) is determined by the following relationship.

■=-dΦ/dt・・・・・・・・・・・・・・・・・・
......Under the correlation of equation (3) or higher, measuring instrument 2
The pre-processing voltage value (VO) is displayed on the display device 236 of No. 3. On the other hand, the maximum residual compressive stress (hereinafter referred to as residual stress (R5o) of the drive pinion 1 with this pre-processing voltage value (Vo)
). > is determined using an X-ray stress measuring device, etc.

(Post-processing magnetic permeability detection step) Next, a drive pinion 1 with a predetermined shot applied to the cap portion 11 is prepared, and a cap sensor 21 is attached to the cap portion 11 of the drive pinion 1 as shown in FIG. come into contact with As a result, the measuring instrument 23 detects the voltage value of the umbrella sensor 21 (voltage value after processing), similar to the above (pre-processing permeability detection step).
Vl) is detected. This post-processing voltage value (Vl) also has a correlation with the post-processing magnetic permeability (μm), similar to the relationship between the pre-processing voltage value (Vo) and the pre-processing magnetic permeability (μ0).

In this way, the post-processing voltage value (Vl) is displayed on the display device 236. The residual stress (R8I) of the drive pinion 1 having this post-processing voltage value (vl) is also determined using an X-ray stress measuring device or the like.

Then, at the stage of multiple shots, the above (post-processing magnetic permeability detection process) is performed in the same way, and each post-processing voltage value (V2
~V4) of each residual stress (R5) of the drive pinion 1
2 to R54) are determined using an X-ray stress measuring device or the like.

(Clamping process) The electric rF value after each force U in the drive pinion 1 (vo
~V4) and each residual stress (R52~R54) is shown in FIG. This is input from the input/output device 234.

Thereafter, every time the umbrella sensor 21 comes into contact with a new drive pinion 1, the measuring device 23 detects the post-processing voltage value (VX), and the post-processing voltage value (VX) and the pre-processing voltage value (
The amount of change (V.

VX-ΔV) is determined, and the residual stress (R5x) is calculated from this amount of change (ΔV) and the relationship shown in FIG. 4 above.

FIG. 5 shows the relationship between the residual stress measured by the measuring device 23 and the reduction rate of retained austenite. Fifth
The figure confirms that there is a proportional relationship between the reduction rate of retained austenite and residual stress. As shown in Figures 6 and 7, the retained austenite structure undergoes volume expansion due to deformation-induced martensitic transformation when shot, and the retained austenite structure undergoes plastic deformation (procedure). This is because it is thought that residual stress is applied to the umbrella portion 11 due to these.

Moreover, the relationship between the reduction rate of retained austenite and the voltage value of the cap sensor 21 (voltage value before machining and voltage value after machining) is shown in FIG. From FIG. 8, it can be seen that as the reduction rate of retained austenite increases, the voltage value increases. This is because the austenite structure is paramagnetic and the martensite structure is ferromagnetic, so the shot transforms into a retained austenite structure or a deformation-induced martensite structure, resulting in a large change in magnetic permeability and a decrease in magnetic resistance. This is because the amount of magnetic flux generated increases and the voltage value increases.

Furthermore, the relationship between the residual stress measured by the measuring instrument 23 and the voltage value is shown in FIG. From FIG. 9, it can be seen that there is a proportional relationship between residual stress and voltage value. Therefore, by detecting the voltage value of 1-live pinion 1 before the shot, then detecting the voltage value after the shot, and finding the amount of change in these voltage values, the residual stress of the drive pinion 1 after the shot can be calculated. I know it's going to shut me out. In addition, at this time, it is not necessary to apply a magnetic field to reach magnetic saturation using a direct current and a magnetic field to change the magnetization to O to the drive pinion 1, and there is no need to apply distortion that causes an error to the drive pinion 1 due to the magnetostrictive effect. do not have. Therefore, according to this measurement method, the drive pinion 1 after measurement
In addition to being able to use the
And it can be done accurately.

Note that FIG. 10 shows the relationship between the amount of retained austenite before shot and the residual stress. As can be seen from Figure 10,
Depending on the amount of retained austenite before shot, there is a region where the residual stress is maximum. In this embodiment, the applicable range is the amount of retained austenite in which this residual stress is maximum. Moreover, as shown in FIG. 11, if this application range is expressed by the amount of arc height used as a guideline for shot conditions, it is in the range of 0.6 to 0.88, which is a relatively high strength region.

(Use of measurement method 1 1) The above measurement method 1 can be applied to a production line.

In this case, for example, as shown in FIG. 12, the post-processing voltage value (Vx) after the shot of each drive pinion 1 conveyed on the line is detected, and the pre-processing voltage value (\10) determined previously is detected. The amount of change (
Find Vo Vx-△V). Then, each amount of change (Δ
The microcomputer 235 determines whether V) is within a predetermined range.
to select the drive pinions 1 on the line. The relationship between residual stress and voltage value at this time is shown in FIG. This shows whether drive pinions 1 having a predetermined fatigue strength etc. can be selected with sufficient accuracy.

(Use of measurement method 1 2) If the amount of retained austenite varies in each drive pinion 1, for example, as shown in FIG. Detect the pre-processing voltage value (Vy) and the post-processing voltage value (Vz) after the shot, and calculate each change amount (Vy) from each pre-processing voltage value (Vy) and each post-processing voltage value (Vz).
Find Vy - VZ = △V). Then, each amount of change (△
The microcomputer 235 determines whether or not V) is within a predetermined range.
Then, the drive pinions 1 on the line are sorted. The relationship between residual stress and voltage value at this time is shown in FIG. This makes it possible to ignore variations in the amount of retained austenite in each drive pinion 1, so that it is possible to select drive pinions 1 having a predetermined fatigue strength, etc. with even greater precision.

(Measurement method 2) In the measurement method 1 described above, the voltage values before and after processing were measured only by the umbrella sensor 21, but the voltage value of the portion where the shot was applied by the umbrella sensor 21 (shot voltage value:
It is also possible to measure the residual stress by measuring a) and then measuring the voltage value (unshot voltage value: Vb) of the portion that is not subjected to shots using the shaft sensor 22. In this case, the shot voltage value (Va) and the unshot voltage value (■
The rate of change (Va /Vb = r (%)) with respect to b) is determined, and the residual stress is calculated from this rate of change (r). At this time, the relationship between residual stress and rate of change is shown in FIG.

From FIG. 16, it can be seen that there is a proportional relationship between the residual stress and the rate of change, and as a result, it is possible to sufficiently cope with fluctuations in the amount of retained austenite in each drive pinion 1.
It can be seen that drive pinions 1 having a predetermined fatigue strength etc. can be selected with high accuracy.

In addition, in the above embodiment, drive pinions were selected based on the amount of change or rate of change in voltage value, or according to the measurement method of the present invention, a shot or the like that imparts desired fatigue strength etc. is determined based on the amount of change or rate of change in the voltage value. It is also possible to control the processing conditions.

Further, in the above embodiment, the residual stress was measured when the drive pinion was shot, but it goes without saying that the present invention can also be applied to steel materials such as axles.

[Effects of the Invention] As detailed above, in the method for measuring residual stress due to processing of steel materials of the present invention, the LA permeability of the steel material with retained austenite structure before processing is detected, and then! After machining the steel material, the permeability is detected and the residual stress is calculated from the change value between the detected magnetic permeability before machining and the magnetic permeability after machining. Even if a certain processing is performed, the steel material will not be destroyed,
Residual stress can be measured accurately and with good operability.

Therefore, by measuring the residual stress of steel materials using the measurement method of the present invention, it is possible to use the steel materials after measurement, and also to perform 100% automatic inspection, which in turn makes it possible to feedback control the processing conditions. quality is guaranteed.

[Brief explanation of the drawing]

1 to 11 relate to measurement method 1, which is an embodiment of the present invention, in which FIG. 1 is a front view of the measuring device, FIG. 2 is a block diagram of the measuring device, and FIG. 3 is an umbrella sensor of the measuring device. Figure 4 is a graph showing the relationship between residual stress and voltage value, Figure 5 is a graph showing the relationship between residual austenite reduction rate and residual stress, Figures 6 and 7 are An explanatory diagram showing the effect of the measurement method of the example, FIG. 8 is a graph showing the relationship between the reduction rate of retained austenite and the voltage value, FIG. 9 is a graph showing the relationship between the residual stress and the voltage value, and FIG. 10 Ha shorts l~
The graph shown above is a graph showing the relationship between the amount of retained austenite and residual stress, and FIG. 11 is a graph showing the relationship between arc height and residual stress. FIG. 12 and FIG. 13 relate to the measurement method 10 used], FIG. 12 is an explanatory diagram showing the process, and FIG. 13 is a graph showing the relationship between residual stress and voltage value. 14 and 15 relate to the use 2 of measurement method 1,
FIG. 14 is an explanatory diagram showing the process, and FIG. 15 is a graph showing the relationship between residual stress and voltage value. FIG. 16 is a graph showing the relationship between residual stress and rate of change according to measurement method 2. DESCRIPTION OF SYMBOLS 1... Drive pinion (steel material) 11... Umbrella part 12... Shaft part 2... Measuring device 21... Umbrella part sensor 22... Shaft part sensor 23... Measuring instrument patent applicant Toyota Motor Corporation Agent Patent Attorney Okawa Disciple 1 Diagram 1...Drive pinion 1],... Umbrella part 2... Measuring device 22... Shaft sensor (steel material) 12... Shaft 21 ... Umbrella sensor 23 ... Measuring instrument Fig. 3 Fig. 6 Carburized composition remaining W Yamarato Fig. 4 Remaining formula η Field stress (k≠field 2) Retained austenite reduction rate (%) Fig. 8 Figure 9 Residual stress (kg/mm2) Figure 10 Figure 11 0.4 0.6 0.8 Arc height (mm) Figure 16 Residual stress (kg/mm2)

Claims (1)

[Claims] (1) A pre-processing magnetic permeability detection step for detecting the magnetic permeability of a steel material with a retained austenite structure before processing; a post-processing magnetic permeability detection step for detecting the magnetic permeability of the steel material after processing; and a detected pre-processing magnetic permeability detection step. A method for measuring residual stress due to processing of steel material, comprising a calculation step of calculating residual stress from a change value between magnetic permeability and post-processing magnetic permeability.