JP2936657B2 - Measurement method of heat treatment hardened layer depth - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for measuring the depth of a hardened layer of a heat-hardened product made of an alloy steel or carbon steel such as an induction hardened bearing, a hub unit, and a CVJ.

[Conventional technology]

In general, in the case of heat-hardened products made of alloys such as induction hardened bearings, hub units, and CVJ or carbon steel, it is important to guarantee the depth of the heat-treated hardened layer in order to ensure sufficient properties such as life and wear resistance.

Therefore, it is necessary to accurately and nondestructively measure the depth of the heat-treated hardened layer.
There is a non-destructive method for measuring the quenched depth described in Japanese Patent Publication No. 2435, JP-B-45-28274.

The measurement method disclosed in this conventional example creates a calibration curve of quench hardening depth-coercive force from a plurality of samples having different quench hardening depths using a linear relationship between the quench hardened layer depth and the coercive force. . Then, the coercive force of the heat-cured product is measured, and the quenching depth is measured from the measured coercive force and the calibration curve.

[Problems to be solved by the invention]

However, as a matter of realization, we prepared a large number of heat-hardened products with different quench hardened layer depths of the same melted products as mass-produced products,
In addition, it is very difficult to measure the coercive force of each cured product and create a calibration curve. In addition, when the chemical composition of the heat-cured product is different, the quenchability varies with the chemical composition. As a result, the relationship between the coercive force and the quench depth also varies, and the calibration curve also varies. As a result, for each heat-treated cured product with a different component composition,
There is a problem that an enormous amount of calibration curve between the coercive force and the hardened layer depth must be prepared in advance by preparing heat-hardened products having different heat-hardened layer depths.

Therefore, the present invention provides a method capable of measuring the depth of the heat-treated hardened layer by preparing one calibration curve in advance without preparing such a huge amount of calibration curve in advance. It is the purpose.

[Means for solving the problem]

In order to achieve the above object and solve the above-mentioned problems, the present invention measures the coercive force of a heat-treated hardened product made of alloy steel or carbon steel, and determines the heat-treated hardened layer depth from the coercive force. In the method of measuring the ideal critical diameter
Calibration curves of the coercive force and the heat treatment hardened layer depth were prepared in advance for heat-treated hardened products of 1.7 or more.
The coercive force and the heat treatment hardened layer depth of the non-defective heat-treated hardened product selected from the heat-treated hardened products to be measured whose ideal critical diameter is 1.7 or more are measured, and the coercive force and the heat-treated hardened layer are measured. A correction calibration curve is prepared by correcting the coercive force of the heat treatment hardened layer of the calibration curve at a depth of 0 from the measured value of the depth, and the heat treatment hardened layer depth of the heat treated cured product to be measured based on the corrected calibration curve Is obtained.

[Action]

The present inventors have conducted intensive studies and found that the ideal critical diameter is 1.7
Calibration curves of the coercive force and the depth of the heat-treated hardened layer are prepared in advance for the heat-treated hardened products described above, and then, among the heat-treated hardened products to be measured having a crystal grain size number of 6 or more and an ideal critical diameter of 1.7 or more. The coercive force and the heat treatment hardened layer depth of the non-defective heat-treated cured product selected from the above are measured, and the coercive force at the heat treatment hardened layer amount depth 0 point of the calibration curve is determined from the measured values of the coercive force and the heat treated hardened layer depth. By creating a corrected calibration curve, it has been found that the depth of the heat-treated hardened layer of the heat-cured product to be measured can be measured based on the corrected calibration curve. The details are described below.

It has been known that when the hardenability of carbon steel or alloy steel is poor, the correlation between the coercive force and the hardened layer depth is also poor. The reason is as follows.

Poor hardenability makes it difficult for hardness to enter. This hardness is due to altensite. This Altensite,
Coercive force is large and hard. Therefore, the relationship with the coercive force of the cured layer of the heat-cured product is affected by the volume ratio of martensite in the measurement object. If the hardenability is poor, the martensite volume ratio in the measured product volume tends to decrease, and the coercive force tends to decrease. As it is, when measuring the quench hardened layer depth of mass-produced products made from JIS standard structural carbon steel or alloy steel with coercive force, the hardenability of the measured products, that is, all the heat treated hardened products with different chemical components It is necessary to prepare a calibration curve in advance.

Then, the present inventor has arrived at the present invention as a result of intensive studies on how the change in hardenability affects the relationship between the coercive force and the depth of the hardened layer of the heat-treated cured product.

First, the present inventors used a known Di value (ideal critical diameter) as a method for quantitatively expressing the hardenability of structural carbon steel and alloy steel. This Di value is the ideal diameter (inch) of a round bar that becomes a 50% martensite structure to the deep part of a steel round bar when cooled at an ideal quenching rate. It can be obtained as follows.

The Di value is determined by the chemical component and the crystal grain size, and the calculation formula thereof is as shown in the following equation (1) when the crystal grain size is 7.

_{Di = D 0 · fSi · fMn} · fNi · fCr · fMo ...... (1) where, D ₀ · FSI ...... is a hardenability multiples determined by containing element concentration (wt%), the first of the following The values are as shown in the table.

Therefore, the outer ring groove is determined from the upper and lower values of the steel composition of the JIS-S35C, 45C, 53C, which are structural carbon steels having different Di values and having a crystal grain size of 6 or more (see Table 1 below). Hub units having different quench hardened layer depths were prepared, and the relationship between the quench hardened layer depth (mm) and the coercive force [AT (ampere turn)] was examined. The result is shown in FIG. First
As can be seen from the figure, the relationship between the hardened layer depth and the coercive force at the upper and lower limits of the chemical composition of each carbon steel is significantly different.

If the Di value is different, the way of burning is different,
As shown in the figure, the change in hardness in the depth direction from the surface of the induction hardened portion is different for the same S53C when the Di value is 1.4 and 2.5, respectively. From this, it can be seen that if the hardenability is poor (especially when the Di value is 1.5 or more), the hardness is hard to enter. As a result, the martensite volume ratio differs, and the relationship between the coercive force and the hardened layer depth is also affected by the change in the Di value.

Under such circumstances, the present inventors have studied the influence of the change in the Di value on the calibration destination of the coercive force and the hardened layer depth.

Generally, there is a relationship between the quenched layer depth and the coercive force as shown in the following equation (2).

y = Ax + B (2) where x: depth of quenched hardened layer (mm, the same applies hereinafter), y: coercive force (AT, the same applies hereinafter), A: slope of calibration curve (rate of change), B: The y-intercept of the calibration curve is shown.

The coercive force is a relative value that varies depending on the shape and interval of the measured object, the shape of the yoke, and the contact state between the yoke and the measured object at the time of measurement, in addition to the depth of the quenched hardened layer. Since the factors can be kept constant, the influence on the quench hardened layer depth can be neglected. Therefore, the content of each component was changed within the standards of JIS-S53C, and the relationship between the Di value and A and B in the above formula (2) was studied diligently. Of these, the relationship between A, which is the slope of the calibration curve of the quenching depth-the coercive force, and the Di value was obtained as shown in FIG. As can be seen from FIG. 3, the value of the slope A is substantially constant when the Di value is 1.7 or more. Then, a relationship as shown in FIG. 4 was obtained for the y-intercept B and the Di value, and it was found that the rate of change of B greatly decreased when the Di value was 1.7 or more.

Therefore, the quenching characteristic value of structural carbon steel and alloy steel
If the value is 1.7 or more, the value of A is almost constant, and the value of B only slightly changes. Therefore, using a heat-cured product having a Di value of 1.7 or more, the calibration curve of the hardened layer depth-coercive force is obtained. If created beforehand, the depth of the heat-treated hardened layer can be determined from the coercive force of the heat-treated cured product based on this calibration curve. In addition,
At the start of the measurement, the coercive force (y ₁ ) is measured for a non-defective product having a known quenched hard layer depth (x ₁ ) of which Di value is 1.7 or more, and the quenched hard layer depth (x ₁ ) , The intercept B is corrected based on the coercive force (y ₁ ) (B = y ₁ −Ax ₁ ), and based on the corrected calibration curve (y = Ax + B ₁ ), the hardened layer depth is determined from the coercive force of the measured product. can be further accurately determined is (y sections after B ₁ represents corrected). At the end of the measurement, the coercive force of a non-defective product may be measured to correct B. Therefore, it is not necessary to prepare heat treatment hardened products having different quenching hardened layer depths for each measured value of Di value, and to prepare an enormous amount of calibrations for each coercive force. Prepare
Thereby, or by simply correcting this with a non-defective product before measurement, the quenched hardened layer depth of the measured product can be accurately determined from the coercive force.

By the way, the size of the crystal grains of the heat-cured product at the time of preparing the calibration curve needs to be 6 or more in particle size number. When the crystal grains are large, the hardenability is improved, but the metal structure after quenching tends to be unstable.In order to measure the stable coercive force, the hardened structure of the measured product must have a stable metal structure. It is necessary to be. Therefore, the crystal grain size is 6
It was necessary to be above. When calculated only from chemical components, the metal structure (martensite, tolstite, ferrite) is different because the grain size is different even if the Di value is the same.
In some cases, the hardness distribution and the coercive force value were different.

When a calibration curve is created in advance as described above, the Di value must be 1.7
It is necessary to use a heat-hardened product having a crystal grain size number of 6 or more, and when accurately measuring the quench hardened layer depth, the product to be measured also has a Di value of 1.7 or more and a crystal grain size number of It is desirable that the product is a heat-treated cured product of 6 or more.

〔Example〕

 Next, examples of the present invention will be described.

Structural carbon steel JIS-S35C, S45 having the composition shown in Table 2 below
C and S53C were melted, and the hub unit was machined using the upper and lower limit materials of the chemical composition standard for each steel. Next, induction hardening was performed on the outer ring groove of the hub unit. In the induction hardening, the depth of the hardened hardened layer was changed by changing the frequency of the electric current in the range of 1 to 10 kHz using an MG type (motor-driven type) for deep hardening.

Then, using a heat treatment hardened layer depth measuring device described later, a calibration curve of hardened layer depth-coercive force was created. In addition, the crystal grain of each ingot was prepared so as to have a crystal grain number of 7.

Next, a heat treatment hardened layer depth measuring device used in the present embodiment will be described.

FIG. 5 is a partial cross-sectional view of the device, showing a state in which the coercive force of the hub unit outer ring groove is measured using this device. FIG. 6 is a cross-sectional view taken along the line VI-VI in FIG. It is.

In the drawing, reference numeral 1 denotes a hub unit, 2 denotes a non-quenched portion, and 3 denotes a quench hardened layer 3 formed by subjecting an outer ring groove of the hub unit 1 to induction hardening.

This quenching hardened layer depth measuring device is provided with a magnetic core 4 having a pair of protruding pieces 20 each of which abuts an outer ring groove, and the magnetic core 4 is inserted into a hub unit via a spacer 5. The magnetic core is wound with an exciting coil 6 for magnetizing the core.

A variable DC source 9 for supplying a positive or negative DC current to the coil 6 and an ammeter 8 are connected to the coil 6 in the yoke 13.

Further, a Hall element 7 is provided on the magnetic iron core, and the Hall element 7 is connected to a variable DC power supply 11 and an ammeter 10 for exciting the Hall element. In addition, 12
Is a Hall element output voltage measuring voltmeter.

Next, an operation of measuring the coercive force of the hardened layer of the outer ring groove of the hub unit using this device will be described.

In the following configuration, when a current is applied to the exciting coil 6, the exciting force generates a magnetic flux 14 in the quench hardened layer of the magnetic iron core 4 and the outer ring groove of the hub unit as the object to be measured. At this time, the magnetomotive force is selected so that the measured object starts to saturate or saturates. Next, the output of the Hall element 7 is monitored while the current of the exciting coil 6 decreases the current value of the power supply 9 due to the negative value. If the exciting current of the Hall element is kept constant while monitoring the ammeter 1, the output of the Hall element is almost proportional to the magnetic flux density at the place where the Hall element is placed, and the sensitivity is large if the exciting current is increased. Therefore, if the exciting current when the output of the Hall element becomes zero is read by the ammeter 8, the value of the magnetomotive force proportional to the value of the coercive force of the magnetic hysteresis curve can be obtained. From this magnetomotive force, the coercive force of the quench-hardened layer can be determined. In preparing the calibration curve, the depth at which martensite was present under a microscope was taken as the quench hardened layer depth.

With this measuring device, the coercive force of the outer ring quench hardened layer of the hub unit prepared at the upper and lower limits (maximum and minimum Di values) of each material in Table 1 was measured, and the coercive force and hardened layer depth were measured. When a calibration curve was prepared, characteristics as shown in FIG. 1 were obtained.

As can be seen from the characteristics shown in FIG. 1, the coercive force greatly changes at the same quenched layer depth at the upper and lower limits of the chemical composition of the material (the maximum and minimum values of the Di value). As a result, it is difficult to measure the quench hardened layer depth without using the respective calibration curves.

Therefore, when a calibration curve of coercive force-hardened layer depth was prepared for the chemical component composition of S53C having a Di value of 1.69 to 1.99, characteristics as shown in FIG. 7 were obtained. In FIG. 7,
Is a calibration curve when the Di value is 1.95, is a calibration curve when the Di value is 1.80, is a calibration curve when the Di value is 1.75, and is a calibration curve when the Di value is 1.69.

As understood from the characteristics of FIG. 7, the Di value is 1.7
In the above steels, the change rate (gradient) of the calibration curve of the coercive force-quenching hardened layer depth is almost the same, and the values of the intercepts of this straight line are slightly different. Therefore, it is not necessary to prepare heat treatment hardened products with different quench hardened layer depths for each Di value measurement product, and create a huge amount of measured coercive force for each. It is expected that the depth of the quenched hardened layer can be accurately measured from the coercive force only by correcting the y-intercept with a non-defective product before measurement.

Therefore, the following experiment was conducted in order to examine the difference between the quench hardened layer depth obtained by correcting the calibration curve created in advance and the actual quench hardened layer depth.

First, a plurality of hub units were made of S53C steel having a Di value of 1.7, and the induction hardening was performed so that the hardened layer depth of the outer ring grooves was different. Then, a first calibration curve between the coercive force and the quench hardened layer depth was created using the plurality of hub units.

y = A ₁ · x + B ₁ ... First calibration curve Here, y is the coercive force, x is the depth of the quenched hardened layer, A ₁ is the slope (change rate) of the calibration curve, and B ₁ is y of the calibration curve. Section.

On the other hand, a plurality of hub units were formed using S53C steel having a Di value of 2.0, and induction hardening was performed so that the hardened layer depth of the outer ring groove was different.

Then, the following second calibration curve of the coercive force and the quench hardened layer depth was created.

y = A ₂ × x + B ₂ ... Second calibration curve Here, A ₂ is the slope (change rate) of the calibration curve as in the case of A ₁ ,
Y intercept of B ₂ is the B ₁ Similarly, a calibration curve.

Next, the coercive force y ₁ and the quench hardened layer depth of a good product (the coercive force and the quench hardened layer depth are known) of the hub unit to be measured.
by substituting x ₁ to the first calibration curve to correct the B _1. B _ST corrected can be determined by _{B ST = y 1 -A 1 ·} x 1. Therefore, the corrected calibration curve is expressed as follows.

y = A ₁ · x + B _ST Calibration curve after correction Then, for a plurality of hub units to be measured, the quenched layer depth was obtained from the coercive force based on the second calibration curve. The quenched layer depth was determined from the coercive force based on the corrected calibration curve, and the quenched layer depth of the former was compared with the quenched layer depth of the latter. As a result, the quenching depth of each of the hub units could be measured with high accuracy of ± 0.2 mm.

Next, the influence of the crystal grain size on the coercive force was examined.

FIG. 8 shows that in the case where the Di value of S53C = 1.8, 20 hub units each having a grain size of 3, 6, 7 were induction hardened to a quenching depth of 2.5 mm (target value), 3 shows the results of an investigation of the variation in the coercive force value of the groove portion with respect to FIG.

According to the results of FIG. 8, it was confirmed that the variation range of the upper and lower coercive forces was stable within 1 AT for those having a crystal grain size of 6 or more.

The reason for this is that the diffusion time becomes shorter as heating and cooling become faster, but as the crystal grains become smaller, the concentration distribution of the metal structure (martensite, tolstite, ferrite) tends to become more uniform, and the crystal grain size is 6 or more. Then, the coercive force value of the quenched structure is also stabilized.

In this embodiment, the case where the present invention is applied to a hub unit made of structural carbon steel has been described. However, the present invention is not limited to this, and other alloy steels, particularly high carbon chromium bearing steel, carburized bearing steel, and high-temperature shaft The present invention can be applied to heat-treated and hardened products made using high-speed steel, stainless steel for rolling bearings, steel for punched cages, and structural carbon steel for machined cages.

The heat treatment is not limited to induction quenching, but may include flame quenching, subbing quenching, carburizing quenching, carbonitriding quenching, and the like.

〔The invention's effect〕

As described above, according to the method for measuring the heat treatment hardening depth according to the present invention, the coercive force and the heat treatment hardening layer are obtained for a predetermined heat treatment hardening product having a crystal grain size number of 6 or more and an ideal critical diameter of 1.7 or more. Since a calibration curve with depth is created in advance, and the depth of the heat-treated hardened layer of the heat-treated cured product to be measured is determined from this calibration curve, the depth of the heat-treated hardened layer can be determined without creating a huge amount of calibration curve in advance. Can be measured. Further, by creating a corrected calibration curve corrected in accordance with the non-defective heat-treated cured product to be measured in the calibration curve, by obtaining the heat treatment hardened layer depth of the heat-treated cured product based on the corrected calibration curve,
The heat treatment hardened layer depth can be measured more accurately.

[Brief description of the drawings]

FIG. 1 is a calibration curve between the quench hardened layer depth and the coercive force, FIG. 2 is a characteristic diagram showing the relationship between the subsurface depth and the hardness, and FIG. 3 is the Di value and the coercive force-quenching hardening. FIG. 4 is a characteristic diagram showing the relationship between the inclination (change rate) in the calibration curve of the layer depth, FIG. 4 is a characteristic diagram showing the relationship between the y-intercept and the Di value in this calibration curve, and FIG.
The figure is a partial cross-sectional view of a quench hardened layer depth measuring device, FIG. 6 is a VI-VI cross-sectional view of FIG. 5, and FIG. 7 is a calibration curve of quench hardened layer depth and coercive force. FIG. 8 is a characteristic diagram showing the relationship between the coercive force and the grain size number. In the figure, 1 is a hub unit, 3 is a hardened layer, 4 is a magnetic core,
Reference numeral 6 denotes an exciting coil, and reference numeral 7 denotes a Hall element.

Claims (1)

(57) [Claims] 1. A method for measuring the coercive force of a heat treated hardened product made of an alloy steel or a carbon steel, and determining the heat treated hardened layer depth from the coercive force, wherein the ideal critical diameter is 1.7 or more. Calibration curves of the coercive force and the depth of the heat-treated hardened layer are created in advance for the heat-treated hardened product, and then the crystal grain size number is 6 or more, and selected from the heat-treated hardened products to be measured with an ideal critical diameter of 1.7 or more. The corrected calibration obtained by measuring the coercive force and the depth of the heat-treated hardened layer of the non-defective heat-treated cured product, and correcting the coercive force of the heat-treated hardened layer at the zero point of the calibration curve from the measured values of the coercive force and the depth of the heat-treated hardened layer. Create a line,
A method for measuring the depth of a heat-treated hardened layer, wherein the depth of the heat-treated hardened layer of the heat-treated hardened product to be measured is obtained based on the corrected calibration curve.