JPH0792140A - Method for evaluating fatigue strength of steel member - Google Patents

Abstract

PURPOSE:To provide a method for evaluating the fatigue strength of steel member in which the accuracy of evaluation can be enhanced by suppressing or avoiding the disturbance factors in the vicinity of the surface of steel member. CONSTITUTION:Before a steel member 1 is subjected to shot-blasting, exciting current of frequency f1 is fed to the exciting coil 211 of a sensor 21 to establish a magnetic loop M connecting the machining face 11 of the steel member 1 and a core 210 and then an induced output voltage V01 is determined. The exciting frequency is then switched to f2 and an induced output voltage V02 is determined. Subsequently, a difference V0=alphaXV01-V02is determined. After the steel member 1 is subjected to shot-blasting, an output voltage V11 induced by the exciting current of frequency f1 is determined. Similarly, an output voltage V12 induced by the exciting current of frequency f2 is determined. Subsequently, a difference V1=alphaXV11-V12is determined, where alpha is a correction coefficient. Increase of residual compressive stress and fatigue strength are grasped from (V1-V0) and (V1/V0).

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 fatigue strength of steel materials. This evaluation method can be applied, for example, when evaluating the fatigue strength in the case where shot peening treatment is performed on steel materials used for parts such as gears and axles.

[0002]

2. Description of the Related Art Conventionally, as a nondestructive screening method for steel materials,
An apparatus is known that utilizes the fact that there is a certain relationship between the hardness of steel and the coercive force (Japanese Patent Laid-Open No. 48-43954). In this device, the coercive force corresponding to each hardness is obtained in advance, the direct current is used to magnetically saturate the steel material, and then the magnetic field is increased in the direction opposite to the magnetic field when magnetic saturation is reached. Thus, the coercive force at which the magnetization becomes 0 is measured, and the hardness is evaluated from the coercive force. However, this method requires a magnetic field to reach magnetic saturation,
It was necessary to apply a magnetic field to magnetize the steel material, which was inferior in operability and was difficult to apply to the production line.

Generally, in terms of improving the fatigue strength of steel, a predetermined depth (for example, 100 μm) from the outermost surface of the steel is used.
It is said that it is preferable that the peak value of the compressive residual stress exists in the region m). Therefore, even in an actual production line, the quality of fatigue strength of a steel material is evaluated based on the peak value of the compressive residual stress. The applicant is
When retained austenite, which is a paramagnetic material in steel, undergoes work-induced transformation to martensite, which is a ferromagnetic material, the permeability of the steel changes and volume expansion occurs due to the transformation. Focusing on the correlation between the value and the compressive residual stress, in recent years, the following non-destructive evaluation methods for fatigue strength of steel materials have been developed. That is, this method
Permeability detection step before processing to detect the magnetic permeability of the steel material with residual austenite before processing, post-processing magnetic permeability detection step to detect the magnetic permeability of the steel material after processing, detected magnetic permeability before processing and after processing A measuring method comprising a calculation step of calculating the compressive residual stress from the change value with the magnetic permeability (1991).
Annual patent application 312312).

According to this method, a magnetic core applied to the surface of the steel material, an exciting coil wound around the magnetic core to form a magnetic path connecting the surface of the steel material and the magnetic core, and an exciting coil wound around the magnetic core. A sensor having a detection coil that outputs a voltage signal according to the compressive residual stress of the steel material by electromagnetic induction associated with the exciting current is used. In this method, when a high-frequency alternating current is applied to the exciting coil as the exciting current in the state where the magnetic core of the sensor is applied to the surface of the steel, the magnetic flux in the magnetic path changes with time, and therefore the magnetic induction of the steel causes Eddy current is generated in the tissue. Further, a high frequency magnetic field is generated by this eddy current.
This is measured by the detection coil as a change in impedance and thus as a change in output voltage value. Here, since the magnetic permeability differs depending on the structure of the steel material (such as martensite which is a ferromagnetic material and austenite which is a paramagnetic material), the output voltage value corresponding to the signal of the detection coil depends on the structure of the steel material. Change. Since the structural ratio of the steel material changes depending on the degree of work-induced transformation, the degree of compressive residual stress can be grasped from the output voltage value of the detection coil.

[0005]

The present invention has been made as part of such development, and an object of the present invention is to provide a method for evaluating the fatigue strength of a steel material which can further improve the accuracy of evaluation.

[0006]

[Means for Solving the Problems] By the way, assuming that the microstructure in the depth direction of the steel material is uniform, the magnetic flux density penetrating the steel material is uniform at the outermost surface due to the "skin effect" caused by the high-frequency alternating current. The magnetic flux density that is the largest and penetrates from the outermost surface to the inside basically decreases exponentially toward the inside as shown by the general characteristic line E (t) in FIG. t means the depth from the surface of the steel material. The distribution of the compressive residual stress in the depth direction of the steel material subjected to shot peening is as shown by the stress distribution characteristic line K (t) in FIG. 2, and the peak value Kp of the compressive residual stress in the region of a certain depth from the outermost surface is shown. With. Therefore, considering the above-mentioned general characteristic line E (t) and stress distribution characteristic line K (t), the magnetic flux density distribution of the steel material having the peak value Kp of the compressive residual stress is basically F ( The characteristic is as shown by t). Here, F (t) = E (t) × K (t). The output voltage value V based on the signal of the detection coil in the above device is basically an integration of F (t), and V = ∫F (t) d
It is indicated by t (integration range 0 to ∞).

Since the outermost surface of the steel material comes into direct contact with the quenching liquid and shot particles, it is easily affected by the quenching conditions and shot projection conditions, and is likely to fluctuate. Moreover, as can be understood from FIG. 1, since the magnetic flux density on the outermost surface of the steel material is the largest due to the “skin effect”, the condition of the outermost surface of the steel material greatly affects the output voltage value V when the “skin effect” is also taken into consideration. . on the other hand,
As described above, the evaluation of the fatigue strength of the steel material is basically determined not by the outermost surface of the steel material but by the magnitude of the peak value of the compressive residual stress in a predetermined depth region from the outermost surface. Therefore, the condition of the outermost surface of the steel material becomes a disturbance factor in the evaluation accuracy, affects the evaluation of the fatigue strength of the steel material, and there is a limit in improving the evaluation accuracy.

For example, the compressive residual stress distribution of steel material A is shown in FIG.
Is a characteristic line KA shown by a solid line, the compressive residual stress distribution of the steel material B is a characteristic line KB shown by a solid line in FIG. 3, and the magnitude of the residual stress peak value at a predetermined depth is Kp.
And the same. As described above, the magnetic flux density distribution of the steel material A is represented by the characteristic line FA (t) even though the steel material A and the steel material B both have the same residual stress peak value Kp. The distribution is shown by the characteristic line FB (t). Therefore, the output voltage value VA of the steel material A and the output voltage value VB of the steel material A have different values by the shaded area ΔV in FIG. Therefore, although the steel material A and the steel material B have the same residual stress peak value Kp at a predetermined depth, the evaluation of the steel material may be different, which is not preferable. VA = ∫FA (t) dt and VB = ∫
This is because it can be represented by FB (t) dt.

Therefore, the present inventor has been keenly aware of the above situation.
I proceeded with the development. And the disturbance of the outermost surface of the steel material
In order to avoid the cause, we found that the following method is preferable.
It was That is, the frequency of the exciting current of the exciting coil is f₁And f
₂And two, and select the first frequency f ₁Is the exciting current
Magnetic flux density on the outermost surface and the first frequency f₂Exciting electric
And the magnetic flux density on the outermost surface is substantially equal
Set to respond. Also, the exciting current is set to the first frequency f₁
In case of
The pressure value Vα is calculated, and the exciting current is set to the second frequency f.₂If
The second output voltage value Vβ by the signal of the detection coil at
Then, the difference between Vα and Vβ is calculated, and based on the value of this difference
Therefore, if the fatigue strength of the steel material is judged, the evaluation accuracy will be improved.
It was found that this was the case, and was confirmed by a test.

That is, the method for evaluating the fatigue strength of a steel material according to the present invention uses a steel material having a peak value of the compressive residual stress contributing to the fatigue strength in a region of a predetermined depth from the outermost surface,
A magnetic core applied to the surface of the steel material, and an exciting coil wound around the magnetic core to form a magnetic path connecting the surface of the steel material and the magnetic core,
Using a sensor that has a detection coil that is wound around a magnetic core and outputs a voltage signal according to the compressive residual stress of the steel material by electromagnetic induction caused by the excitation current of the excitation coil, The magnitudes of the first frequency and the second frequency are selected so that the magnetic flux density on the outermost surface of is substantially equal, and the magnitudes of the first frequency and the second frequency are set according to the signal detected by the detection coil with the frequency of the exciting current being the first frequency Whether the fatigue strength of the steel material is good or bad is determined based on the difference between the first output voltage value and the second output voltage value according to the signal detected by the detection coil in the state where the frequency of the exciting current of the exciting coil is set to the second frequency. It is characterized by doing.

[0011]

When a high-frequency alternating current is passed through the exciting coil as an exciting current, the magnetic flux passing through the magnetic core and the steel material changes with time. Therefore, electromagnetic induction causes an eddy current in the structure of the steel material. Further, a high frequency magnetic field is generated by this eddy current.
This is detected by the detection coil as a change in impedance and thus a change in output voltage value. Here, since the magnetic permeability varies depending on the structure of the steel material (such as martensite and austenite) and the output voltage value of the detection coil changes depending on the structure of the steel material, the compressive residual stress is based on the output voltage value of the detection coil. The degree of is understood.

When the frequency of the exciting current flowing through the exciting coil of the present invention is f and its period is t, the period t becomes smaller when the frequency f is larger because of the relation of t = 1 / f. When the period t becomes small in this way, the temporal displacement of the magnetic flux in the magnetic path increases, so that the output voltage value of the detection coil tends to be output high due to the electromagnetic induction phenomenon that cancels the temporal variation of the magnetic flux. . Therefore, it is preferable to correct the output voltage value of the detection coil according to the frequency of the exciting current.
Therefore, in the method of the present invention, it is preferable that the relationship between the frequency of the exciting current and the output voltage value is measured in advance, and the output voltage value of the detection coil is corrected by the correction unit according to the frequency based on the relationship. The correction unit can be configured using, for example, a memory such as a RAM or a ROM that stores the above relationship.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. (Steel) Steel 1 used in this example
Is before carburizing and shot peening of carburized steel, and has a martensite structure of about 60 to 70% and a retained austenite structure of about 20 to 30% in area ratio. When this steel material 1 is shot peened, the martensite structure increases to about 80%. The stress distribution of this steel material 1 is basically the stress distribution characteristic line K of FIG.
It has a form as shown in (t) and has a peak value Kp.

(Measuring Device) The measuring device 2 is shown in FIG. The measuring device 2 includes a sensor 21 that is in contact with the machined surface 11 of the steel material 1, and a measuring instrument 23 connected to the sensor 21. The sensor 21 includes a U-shaped magnetic core 210 formed by stacking silicon steel plates in the thickness direction, and an exciting coil 211 wound around the magnetic core 210 and supplied with a high-frequency alternating current.
And a detection coil 212 wound around the magnetic core 210 and outputting a voltage value by an electromagnetic induction phenomenon based on a magnetic field change. The magnetic core 210 has a tip end portion on which the processed surface 11 of the steel material 11 is processed.
Is formed so as to surely abut.

As shown in FIG. 4, the measuring instrument 23 includes an oscillator 231 and a frequency variable timing circuit 238 for changing the frequency of a high frequency alternating current oscillated from the oscillator 231.
An amplifier 232 that is connected to the oscillator 231 and applies an alternating current to the exciting coil 211; an amplifier 233 that amplifies the output signal of the detection coil 212; a correction circuit 234 that corrects the output signal amplified by the amplifier 233; The arithmetic unit 235 is connected to the circuit 234, the display device 236 is connected to the arithmetic unit 235, and the memory 237 is connected to the correction circuit 234. Here, the correction circuit 234 and the memory 237 form a correction unit.

(Selection of Frequency) In this method, the frequency of the exciting current supplied to the exciting coil 211 is selected as the first frequency f ₁ and the second frequency f ₂ . The selection is as follows. That is, in general, the magnetic flux density penetrating from the surface of the steel material 1 to the inside decreases exponentially continuously toward the inside as shown by the general characteristic line E (t) in FIG.
In terms of handling, it is preferable to determine the magnetic flux penetration depth with a fixed value. Therefore, as can be understood from FIG. 1, the magnetic flux penetration depth δ as an eigenvalue is generally 1 / where the magnetic flux density is 1 when the magnetic flux density on the outermost surface of the steel material 1 is 1.
The depth is defined as ε (ε: natural logarithm, ε = 2.7). Also in this example, this fixed value is used as the magnetic flux penetration depth δ.
I am trying.

The magnetic flux penetration depth δ is generally calculated by the following equation (a). That is, when the excitation frequency is f, the permeability of the steel surface layer is μ, and the electrical conductivity of the steel surface layer is σ, δ = [1 / (square root of π · f · μ · σ)] ... (a) Then, as shown in FIG. 5, in the region near the depth exhibiting the peak value Kp of the compressive residual stress of the steel material 1, the magnetic flux penetration depths δ ₁ and δ ₂ are selected, and based on the formula (a), the first frequency f ₁ of the magnetic flux penetration depth [delta] ₁ is obtained, selecting the second frequency f ₂ to the magnetic flux penetration depth [delta] ₂ is obtained. In this example, specifically, the first frequency f ₁ is 15 KHz and the second frequency f ₁ is
₂ was 30 KHz.

Further, in FIG. 5, a general characteristic line E ₁ shows a general magnetic flux permeation distribution when the first frequency f ₁ is adopted when a steel material having a uniform structure in the depth direction is used. The characteristic line E ₂ shows a general magnetic flux permeation distribution when the second frequency f ₂ is adopted. As can be seen from the left ends of the general characteristic lines E ₁ and E ₂ in FIG.
Is set so that the magnetic flux density on the outermost surface of is substantially equal (assuming 1 on the vertical axis in FIG. 5). The reason is,
This is for reducing or avoiding disturbance factors near the outermost surface of the steel material 1.

Further, in FIG. 5, the characteristic line F₁Is a compression residue
Using the steel material 1 having the peak stress value Kp, the frequency f₁To
The magnetic flux density distribution when an exciting current is used is shown. Characteristic
Line F ₂Is a steel material 1 having a peak value Kp and a frequency f
₂Shows the magnetic flux density distribution when is the exciting current. (Permeability detection step before processing) As shown in FIG.
The sensor 21 is brought into contact with the machined surface 11 of the steel material 1 before turning.
As a result, the machined surface 11 of the steel material 1 has a depth h._nWithin the range of
Generates magnetic flux. In the measuring instrument 23, the signal of the detection coil 212 is received.
Output voltage value V₀₁Is detected and stored in the memory 237.
Remembered.

That is, the exciting coil 211 of the sensor 21.
When a high-frequency alternating exciting current (frequency f ₁ ) is applied to, a loop-shaped magnetic path M that connects the machined surface 11 of the steel material 1 and the magnetic core 210 is formed. At this time, as shown in FIG. 1, the depth h from the surface of the processed surface 11 to about 100 μm
A magnetic flux (Φ) is generated at n. Since the magnetic permeability of the magnetic core 210 is known, the generation amount of the magnetic flux (Φ) is basically determined by the magnitude of the magnetic permeability (μ) at the depth hn of the processed surface 11.

Next, the frequency variable timing circuit 238 switches the excitation frequency from f ₁ to f ₂ , and similarly the output voltage value V ₀₂ at the frequency f ₂ is obtained. And memory 2
V ₀₁ is called from 37, and the difference is set as the difference value V ₀ in the steel material 1 before shot peening. That is, the difference value V ₀ is represented by (V ₀ = α × V ₀₁ −V ₀₂ ). Where α
Is a correction coefficient, and the "skin effect" varies depending on the frequency of the exciting current, and the higher the exciting current frequency, the higher the output voltage value of the detection coil 212 tends to be output.
It is for correcting it.

The magnitude of the difference value is shown by the shaded area in FIG. As shown by the shaded area in FIG. 5, the shaded area is small near the outermost surface in the depth direction of the steel material and is zero or close to zero, but near the peak value Kp of the compressive residual stress. Is larger than near the surface. Therefore, if the difference value V ₀ is used as the evaluation means, it is advantageous to reduce or avoid the influence in the vicinity of the outermost surface of the steel material 1 which is likely to cause a disturbance during the evaluation. Therefore, the accuracy of fatigue strength evaluation is improved and reliability is increased.

(Processing Permeability Permeability Detection Step) Next, the machined surface 11 of the steel material 1 is subjected to a predetermined shot peening treatment. As a result, the residual austenite undergoes work-induced transformation to increase the amount of martensite, which is a ferromagnetic material. In this state,
As shown in FIG. 5, the sensor 21 is brought into contact with the machined surface 11 of the steel material 1, and the exciting coil 211 is supplied with an exciting current which is a high-frequency alternating current of the frequency f ₁ . As a result, the measuring instrument 23
Then, the output voltage value V ₁₁ based on the first frequency f ₁ is obtained as in the above-described pre-processing magnetic permeability detection step. Further, as described above, the frequency of the exciting current is switched from f ₁ to f ₂ by the frequency variable timing circuit 238, and the second frequency f is similarly set.
An output voltage value V ₁₂ based on ₂ is obtained. Then, the difference is defined as a difference value V ₁ in the steel material 1 after shot peening. The difference value V ₁ is represented by (V ₁ = α × V ₁₁ −V ₁₂ ). α is a correction coefficient as described above.

(Calculation step) The relationship between the difference (V ₁ -V ₀ ) between the difference values before and after shot peening and the peak value of the compressive residual stress measured by the X-ray residual stress measuring device is basically as shown in the figure. As shown in 6, there is a correlation between the two. Further, the peak value of the compressive residual stress and the fatigue strength improvement rate have a correlation as shown in FIG. Therefore, FIG. 6 and FIG.
In consideration of the above, the fatigue strength improvement rate can be grasped from (V ₁ −V ₀ ).

In this embodiment, since the correlation characteristic shown in FIG. 6 and the correlation characteristic shown in FIG. 7 are stored in the memory 237, the difference value V ₀ in the steel material 1 before shot peening and the difference in the steel material 1 after shot peening. If the value V ₁ is obtained, the form of the compressive residual stress of the steel material 1 when the steel material 1 is shot peened can be quickly grasped, and the evaluation of the fatigue strength of the steel material 1 can be determined nondestructively and quickly.

(Other Examples) The method of the present invention is not limited to the above-mentioned embodiments, and can be appropriately selected as needed within the scope not departing from the gist. For example, steel is not limited to shot peened steel.

[0027]

According to the method for evaluating the fatigue strength of a steel material of the present invention, when evaluating the quality of the fatigue strength by the peak value of the compressive residual stress, it is possible to determine the vicinity of the outermost surface of the steel material which is likely to be a disturbance factor in the evaluation. It is advantageous to avoid the influence. Therefore, the accuracy of fatigue strength evaluation is increased, and the reliability is improved.

[Brief description of drawings]

FIG. 1 is a graph showing a general form of magnetic flux penetration depth in a steel material.

FIG. 2 is a graph showing a distribution of compressive residual stress and a distribution of magnetic flux density in a depth direction of a steel material when a shot peened steel material is used.

FIG. 3 is a graph showing the distribution of compressive residual stress and the distribution of magnetic flux density for each product.

FIG. 4 is a configuration diagram of a measuring device.

FIG. 5 is a graph showing a difference value.

FIG. 6 is a graph showing the relationship between (V ₁ −V ₀ ) and the peak value of compressive residual stress.

FIG. 7 is a graph showing the relationship between the peak value of compressive residual stress and the fatigue strength improvement rate.

[Explanation of symbols]

In the figure, 1 is a steel material, 11 is a machined surface, 2 is a measuring device, 21 is a sensor, 23 is a measuring instrument, 210 is a magnetic core, 211 is an exciting coil, 212 is a detecting coil, 231 is an oscillator, 238 is a frequency variable timing. Circuit 234 is a correction circuit 235
Indicates an arithmetic unit.

Claims (1)

[Claims] 1. A steel material having a peak value of compressive residual stress that contributes to fatigue strength in a region of a predetermined depth from the outermost surface, a magnetic core applied to the surface of the steel material, and a magnetic core wound around the magnetic core. An exciting coil that forms a magnetic path connecting the surface of the steel material and the magnetic core, and a voltage signal according to the compressive residual stress of the steel material by electromagnetic induction that is wound around the magnetic core and is accompanied by the exciting current of the exciting coil. A sensor having a detecting coil for controlling the frequency of the exciting current of the exciting coil of the sensor so that the magnetic flux density on the outermost surface of the steel material is substantially equalized.
The frequency and the magnitude of the second frequency are selected, and the first output voltage value corresponding to the signal detected by the detection coil in the state where the frequency of the excitation current is set to the first frequency, and the frequency of the excitation current of the excitation coil. Of the fatigue strength of the steel material is determined based on the difference between the second output voltage value according to the signal detected by the detection coil in the state where the second frequency is Method.