JP4192230B2 - Method and apparatus for nondestructive evaluation of cast iron - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a nondestructive evaluation method and a nondestructive evaluation apparatus for cast iron that can accurately measure determination of a ferrite base or pearlite base or identification of a ferrite / pearlite ratio using eddy current.
The term “nondestructive evaluation” used in this specification is not limited to nondestructive inspection or flaw detection, which is usually used to detect the presence of a flaw or a defect, but also to quantify the structure unique to cast iron. Used to measure percentage.
Therefore, “non-destructive evaluation” is used in a concept including non-destructive inspection or flaw detection, and can be replaced with the term “non-destructive inspection or flaw detection” for the purpose of inspection or flaw detection only.
[0002]
[Prior art]
Cast iron is widely used as a structural material for engines and parts for automobiles (gasoline vehicles), diesel engines for ships or vehicles, industrial machinery, and the like.
Cast iron is largely classified into two types, gray cast iron (flaky graphite cast iron) and spheroidal graphite cast iron, based on the shape of graphite inside. The former contains a large number of flake graphite inside, which acts as an internal notch, and is generally a low-reliability material with low strength and varying strength. However, it is a general material that is widely used as a casting material because it has good castability and balanced industrial properties.
On the other hand, the latter is obtained by spheroidizing flake graphite and exhibits high strength and high toughness.
[0003]
The type of base used as the matrix of cast iron ranges from perfect pearlite to complete ferrite, and the approximate ratio of the ground can be changed depending on the production conditions.
One ferrite matrix is excellent in ductility and improves the toughness of the cast iron material, but has the property of reducing the strength. On the other hand, pearlite bases are hard and have the opposite properties of ferrite that they increase the strength of cast iron material but reduce toughness.
Along with the graphite shape of the spherical graphite or flake graphite, the area ratio of ferrite matrix and pearlite matrix in cast iron is an important parameter that determines the mechanical properties of cast iron.
In cast iron, there is a case where a large cementite Fe ₃ C structure is generated at a location that is further rapidly cooled in the production process. This cementite structure is the hardest, brittle and easy to break in the cast iron structure. Therefore, in general, cast iron for structural members having high strength is treated as a defective product if it has a chilled structure.
[0004]
In general, if the manufacturing process is the same, almost the same product can be produced. However, in the case of a cast product, there is considerable variation due to its manufacturing characteristics. For this reason, it is important to inspect the material and the presence / absence of defects in the cast product while strictly controlling the process.
However, there is currently no comprehensive nondestructive evaluation of cast materials and defects, and the current situation is that they rely on destructive testing.
From the above, it is necessary to establish a simple and reliable non-destructive evaluation method in order to truly realize the manufacturing method improvement, quality assurance, and in-service inspection of cast products.
Under these circumstances, technological developments for casting defect inspection and material evaluation have been carried out centering on ultrasonic cast iron evaluation, and it has become possible to evaluate the graphite spheroidization ratio related to tensile strength.
However, the ferrite / pearlite ratio related to strength and toughness and the evaluation of the presence or absence of a chilled structure have not been solved at all. Moreover, in the defect detection by an ultrasonic test, the dead zone is within a depth of 5 mm near the surface, which is also a problem.
As an electromagnetic method developed in Germany, there is one that estimates the hardness of a steel material by magnetic measurement based on a harmonic method. However, this system is complicated, and the information that can be acquired is limited to materials related to magnetism.
[0005]
As described above, a nondestructive evaluation method regarding the ferrite / pearlite ratio and the presence or absence of a chilled structure has not been established. To investigate the material and mechanical properties, microscopic observation by cutting and polishing, indentation hardness test, tensile Tests etc. must be carried out. These tests are destructive tests such as cutting the product or leaving an impression on the product.
Therefore, the current inspection technology not only requires a lot of cost and labor, but also has the problem that all products cannot be inspected in principle, and the establishment of a nondestructive evaluation method is awaited. .
On the other hand, if the ferrite / pearlite ratio of the product and the distribution of the chilled structure can be easily grasped non-destructively, the product result can be easily reflected in the design. At present, it is very difficult to control or improve the material in the casting manufacturing process because the material evaluation cannot be performed easily.
Simple nondestructive evaluation facilitates feedback to the manufacturing process, which has been difficult until now, and can optimize the manufacturing method. For example, it leads to higher performance of products such as weight reduction of cast iron.
[0006]
[Problems to be solved by the invention]
The present invention accurately measures determination of ferrite base or pearlite base or identification of ferrite / pearlite ratio using eddy current, and solves the weak points of conventional nondestructive evaluation, and nondestructive evaluation method and nondestructive of cast iron It is an object to provide an evaluation device.
[0007]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present inventors have conducted intensive research. As a result, by detecting eddy current signals of eddy currents, the determination of the ferrite base or the pearlite base or the identification of the ferrite / pearlite ratio can be further performed. The knowledge that it can measure accurately was obtained.
The present invention is based on this finding,
1. Judgment of ferrite base or pearlite base or identification of ferrite / pearlite ratio based on the difference in electromagnetic properties of ferrite and pearlite phases of cast iron that is inspected, impedance change due to eddy current, induced electromotive force or magnetic flux density, respectively When detecting non-destructive evaluation of cast iron using the eddy current , the coil excitation frequency is adjusted between 10 kHz and 1 MHz. As a result, eddy current is induced in the cast iron that is the object to be inspected, the induced eddy current is detected, a calibration curve is obtained based on this eddy current signal, and a new object to be inspected is compared with this calibration curve. Casting characterized in that it determines the ferrite base or pearlite base of a cast iron or identifies the ferrite / pearlite ratio Nondestructive Evaluation Method 3 of cast iron using the eddy current of the 1, wherein the measuring of a plurality of frequencies using a non-destructive evaluation methods 2.2 or more probes. A calibration curve obtained by examining the correlation between the ferrite / pearlite ratio and the eddy current signal of a number of cast irons is stored as a data base, and the eddy current signals due to eddy currents of individual cast irons are measured and the stored data is stored. 3. Nondestructive evaluation of cast iron using eddy current according to 1 or 2 above, wherein the presence or amount of chilled structure of cast iron or the presence or amount of ferrite / pearlite is determined from a comparison with a calibration curve of the base Method 4. From the correlation between the pearlite ratio and the hardness, tensile strength, or toughness, the hardness, tensile strength, or toughness of the cast iron is obtained. Destructive evaluation method 5. While measuring the spheroidization rate which is the shape of graphite in cast iron on the one hand and measuring the ferrite / pearlite rate on the other hand, the tensile strength, hardness or toughness of the cast iron is obtained on the other hand The nondestructive evaluation method of cast iron in any one of 1-4 is provided.
[0008]
The present invention also provides: A cast iron nondestructive evaluation apparatus using eddy currents used in the cast iron nondestructive evaluation method described in any one of 1 to 5 above, wherein the probe is arranged on the cast iron as an object to be inspected so that lift-off adjustment is possible. 7. Non-destructive evaluation apparatus for cast iron using eddy current characterized by: Non-destructive evaluation of cast iron using eddy current according to claim 6, comprising two or more probes and measuring at a plurality of frequencies Apparatus 8. The non-destructive evaluation apparatus for cast iron using eddy current as described in 7 above, wherein two or more probes are arranged in tandem with respect to an object to be inspected.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, an eddy current is induced in cast iron as an object to be inspected by passing a high-frequency alternating current through the probe. The electromagnetic information of the eddy current can be acquired as an eddy current signal such as a coil impedance (change) signal, an induced electromotive force, and a magnetic flux density.
For example, the impedance Z of the coil is expressed as L = R + JωL as a complex number by the DC resistance R, the inductance L, and the AC frequencies ω.
This electromagnetic information includes the shape, conductivity, and permeability of the object to be inspected, and the conductivity and permeability are obtained by changing the distance between the probe and the object to be inspected and the coil excitation frequency. be able to. Since the electromagnetic properties of the ferrite phase and pearlite phase are different, the ferrite / pearlite ratio of cast iron can be known.
Specifically, a probe is placed close to an object to be inspected of cast iron, and an eddy current is induced in the object to be inspected by flowing a high-frequency alternating current through the probe. Detect as a signal.
[0010]
By changing the distance between the object to be inspected and the probe or the coil excitation frequency, the conductivity and the magnetic permeability can be obtained more accurately.
Conventionally, there is an eddy current flaw detector that uses eddy currents in nondestructive inspection of metals. When a defect of a test product enters the test coil, the balance of the bridge is lost, and the impedance of the coil is changed according to the defect. There is a known technique for searching for scratches using the output.
In the sense of utilizing the impedance change of the probe, it is similar in principle to the present invention. However, it has not been known at all that this can be used as a means for determining the ferrite base or pearlite base of cast iron or identifying the ferrite / pearlite ratio.
[0011]
In the present invention, the results of FIG. 1 can be obtained by measuring the pearlite ratio and eddy current signal of various compositions, and the correlation between the eddy current signal and the pearlite ratio was obtained.
Furthermore, when the pearlite ratio was changed with a cast iron sample having the same composition, the result of FIG. 2 could be obtained.
As a result, it was confirmed that a calibrated curve in which the eddy current signal (coil impedance signal, etc.) increases in proportion to the pearlite rate is obtained in cast iron having a constant composition.
Since this calibration curve depends on the probe shape, frequency, lift-off, etc., it is necessary to obtain the calibration curve in accordance with the conditions to be measured.
The lift-off curve is obtained by changing the lift-off between the object to be inspected and the coil, and the reliability can be improved by measuring this together.
[0012]
The frequency of the alternating current used for excitation is usually between 10 kHz and 1 MHz and is measured at a plurality of frequencies. If necessary, it can be used outside the above frequency range. In the evaluation, the reliability of the ferrite / pearlite ratio estimated by measuring at a plurality of frequencies and obtaining a plurality of calibration curves can be improved.
Thus, the eddy current signal of the target cast iron is measured, and calibration curves can be obtained from various angles to estimate the pearlite rate.
A calibration curve for the correlation between ferrite / pearlite ratio and eddy current signal of a number of reliable cast irons tested is stored as a data base.
On the other hand, an eddy current signal such as impedance due to eddy current of each cast iron which is the object to be inspected is newly measured, and a calibration curve is obtained. This is a stored calibration curve based on the database. From the comparison, the presence / absence or amount of the ferrite / pearlite can be determined with higher accuracy.
[0013]
As shown in FIG. 3, in cast iron, the correlation between the eddy current signal and the Vickers hardness is known, and the hardness can be obtained from the pearlite rate from the correlation between the eddy current signal and the pearlite rate in FIG. Similarly, the tensile strength and toughness can be known from the correlation with the pearlite ratio.
On the other hand, the tensile strength, hardness, or toughness of cast iron can be determined by measuring the spheroidization ratio, which is a graphite shape in cast iron, on the one hand and measuring the ferrite / pearlite ratio on the other hand.
As described above, mechanical properties such as hardness, tensile strength and toughness of cast iron can be obtained comprehensively by the nondestructive evaluation method of the present invention.
5 and 6 show a series of algorithms for determining the pearlite ratio and a technique for comprehensively obtaining mechanical properties such as the spheroidal graphite ratio, cast iron hardness, tensile strength, and toughness.
[0014]
The nondestructive evaluation apparatus for cast iron using eddy current according to the present invention arranges a probe on cast iron as an object to be inspected so that lift-off adjustment is possible. Usually, one probe can be measured, but two or more probes can be measured at a plurality of frequencies.
Furthermore, these two or more probes can be arranged in tandem with respect to the object to be inspected. FIG. 4 shows an example of the structure of the apparatus, and shows a differential coil that brings a coil carrying a high-frequency alternating current close to a conductive test specimen from above.
When an electric current flows through the coil 1 in FIG. 4, an induced current (eddy current) 3 is generated inside the cast iron 2 that is the object to be inspected due to the phenomenon of electromagnetic induction. Since an induced current flows through the coil 1 due to the eddy current 3 flowing through the cast iron 2, an eddy current signal such as the impedance of the current flowing through the coil is measured.
The difference between the signal 4 detected from the side closer to the material and the environmental noise 6 picked up by the coil 5 far from the material is taken as an eddy current signal of the target cast iron.
[0015]
Depending on the chemical composition and ferrite / pearlite ratio of each cast iron, a specific eddy current signal can be detected, whereby the cast iron material can be evaluated.
As a method of measuring an eddy current signal, there is a method using an absolute value type probe using one coil. When two or more probes are used as described above, an excitation coil and a detection coil or There is a method of using one or a plurality of magnetic sensors in combination.
These probes can be appropriately designed and adopted according to the inspection or measurement of the shape, size (size), composition, structure, etc. of the cast iron product to be evaluated for nondestructive evaluation or evaluation conditions. The present invention encompasses all of these.
[0016]
【The invention's effect】
The present invention has an excellent effect that it is possible to accurately and easily obtain determination of a ferrite base or pearlite base of cast iron or identification of a ferrite / pearlite ratio, which has been conventionally impossible, by nondestructive evaluation.
Traditionally, destructive testing has been done by cutting the product or leaving indentations on the product, but this can be fundamentally improved and not only saves a lot of money and labor, but also 100% inspection of the actual product is fundamental. Furthermore, since the ferrite / pearlite ratio of the product can be easily grasped by nondestructive evaluation, the product result can be easily reflected in the design.
This makes it possible to easily evaluate the material, to control or improve the material in the casting manufacturing process, that is, to facilitate feedback to the manufacturing process, and to optimize the manufacturing method, for example, It has an excellent effect of improving the performance of products such as weight reduction of cast iron.
[Brief description of the drawings]
FIG. 1 is a diagram showing a relationship between pearlite ratios of a plurality of compositions and eddy current signals.
FIG. 2 is a diagram showing a relationship between a pearlite rate and an eddy current signal.
FIG. 3 is a diagram showing a correlation between a pearlite ratio and Vickers hardness.
FIG. 4 is an explanatory view showing an example of an apparatus for measuring a ferrite / pearlite ratio and a distribution of a chilled structure due to an eddy current using a differential coil.
FIG. 5 is a process explanatory diagram of pearlite ratio measurement using an eddy current signal.
FIG. 6 is a view showing an evaluation algorithm for cast iron by specifying a pearlite ratio by an eddy current signal, specifying a graphite shape by ultrasonic waves, and specifying a tensile strength, hardness, and toughness by these.
[Explanation of symbols]
1 Coil 2 Cast iron 3 Inductive current (eddy current)
4 Detected signal 5 Coil far from material 6 Environmental noise

Claims (8)

Judgment of ferrite base or pearlite base or identification of ferrite / pearlite ratio based on the difference in electromagnetic properties of ferrite and pearlite phases of cast iron that is inspected, impedance change due to eddy current, induced electromotive force or magnetic flux density, respectively When detecting non-destructive evaluation of cast iron using the eddy current , the coil excitation frequency is adjusted between 10 kHz and 1 MHz. As a result, eddy current is induced in the cast iron that is the object to be inspected, the induced eddy current is detected, a calibration curve is obtained based on this eddy current signal, and a new object to be inspected is compared with this calibration curve. Casting characterized in that it determines the ferrite base or pearlite base of a cast iron or identifies the ferrite / pearlite ratio Non-destructive evaluation method.   2. The nondestructive evaluation method for cast iron using eddy current according to claim 1, wherein measurement is performed at a plurality of frequencies using two or more probes. A calibration curve obtained by examining the correlation between the ferrite / pearlite ratio and the eddy current signal of a number of cast irons is stored as a data base, and the eddy current signals due to eddy currents of individual cast irons are measured and the stored data is stored. The nondestructive of cast iron using eddy current according to claim 1 or 2, wherein the presence or amount of chilled structure of cast iron or the presence or amount of ferrite / pearlite is determined from comparison with a calibration curve of a base Evaluation methods.   The hardness of the cast iron, the tensile strength or the toughness is determined from the correlation between the pearlite ratio and the hardness, the tensile strength or the toughness. The cast iron using eddy current according to any one of claims 1 to 3, Nondestructive evaluation method.   A method for determining the tensile strength, hardness, or toughness of cast iron by measuring the spheroidization ratio which is a graphite shape in cast iron on the one hand and measuring the ferrite / pearlite ratio on the other hand. The nondestructive evaluation method of cast iron in any one of claim | item 1-4. A non-destructive evaluation apparatus for cast iron using eddy currents used in the non-destructive evaluation method for cast iron according to any one of claims 1 to 5, wherein the probe is disposed on the cast iron as an object to be inspected so that lift-off adjustment is possible. A nondestructive evaluation system for cast iron using eddy currents.   The nondestructive evaluation apparatus for cast iron using eddy current according to claim 6, comprising two or more probes and measuring at a plurality of frequencies.   The nondestructive evaluation apparatus for cast iron using eddy current according to claim 7, wherein two or more probes are arranged in tandem with respect to the object to be inspected.

Images