JP5495033B2 - Detection method of plastic strain in stainless steel - Google Patents

Description

  The present invention relates to a technique for detecting plastic strain applied to stainless steel using an electrochemical technique.

In order to evaluate damage and ensure soundness of mechanical parts and structures, it is necessary to establish a method for detecting plastic strain applied to metal materials. Various studies using magnetism, ultrasonic waves, X-rays, hardness, etc. have been attempted at the laboratory level as methods for detecting plastic strains [Non-Patent Documents 1 to 4].
However, it is not always established from the viewpoint of accuracy and quantitativeness, and it may be unclear how the observed event is related in principle to plastic deformation.
On the other hand, there is an electrochemical method (anode characteristics / electrochemical etching) as a measurement method with a clear principle of damage measurement against aging of metal materials [Non-Patent Documents 5 to 7].

Kurita, M., Miyagawa, M., Chiaki, K. and Sakamoto, I .; Transactions of the Japan Society of Mechanical Engineers, Series A, Vol.52, No.482, (1986), pp.2429-2435 Kobayashi, M .; Journal of JSNDI, Vol.32, No.11, (1983), pp.887-894 Kurita. M .; Journal of Japan Society for Technology of Plasticity, Vol.26, No.295 (1985), pp.869-875 Majima, T .; Transactions of the Japan Society of Mechanical Engineers, Series A, Vol.57, No.534 (1991), pp.398-403 Watanabe, Y. and Shoji, T .; Metallurgical Transactions A, Vol.22A (1991), pp.2097-2106 Yutaka Watanabe and Tetsuo Shoji; Proceedings of the 1995 ASME / JSME Pressure Vessels and PipingConference, ASME, PVP-Vol.315 (1995), pp.397-405 Saito, Y., Shoji, T. and Watanabe, Y .; Transactions of the Japan Society of Mechanical Engineers, Series A, Vol.57, No.538 (1991), pp.1442-1448

  In order to evaluate damage and ensure soundness of mechanical parts and structures, it is necessary to establish a method for detecting plastic strain applied to metal materials. However, there are cases where it is unclear from the viewpoint of accuracy and quantitativeness, and how the observed event is related in principle to plastic deformation. There is also a problem that the method is not an established method.

The present inventor has intensively studied to solve the above problems. That is, by etching the material under appropriate conditions, the metal structure that has undergone local structural or structural changes due to plastic deformation can be selectively dissolved. With regard to the evaluation of plastic strain applied to materials, it is possible to evaluate based on etching marks by finding etching conditions that preferentially dissolve the site where the microscopic structure or component changes according to the plastic strain. Is expected to be. Further, since the change in the dissolution behavior of the material means the change in the anode characteristics, there is a possibility that the plastic strain can be detected as a change in the anode polarization curve.
Therefore, the development of a technique for detecting plastic strain using an electrochemical technique was advanced, and the present invention was successfully completed.

The present invention provides the following.
[1] A stainless steel plastic strain characterized by using a nitric acid solution as a solution component and subjecting the stainless steel material to be subjected to electrochemical measurement under constant potential conditions to detect the plastic strain of the stainless steel material. Detection method.
[2] The electrochemical measurement is performed by etching the stainless steel material to be tested so that a linear etching mark appears on the crystal grains. The plastic strain of the stainless steel according to the above [1] Detection method.
[3] Etching the test stainless steel material to reveal linear etching marks on the crystal grains, obtaining the etching mark density, and detecting the plastic strain of the stainless steel material using the etching mark density as an index. The method for detecting plastic strain of stainless steel as described in [2] above.
[4] The method for detecting plastic strain of stainless steel as described in [2] or [3] above, wherein the test stainless steel material is etched in a nitric acid solution at a potential of −400 mV or less.

It is possible to detect plastic strain of stainless steel by electrochemical method. There is provided a technique for detecting plastic strain from etching marks that appear when a stainless steel material is etched under potential control. Since the plastic strain applied to the metal material can be detected by the technique of the present invention, it is useful for damage evaluation and soundness assurance of mechanical parts and structures.
Other objects, features, excellence and aspects of the present invention will be apparent to those skilled in the art from the following description. However, it is understood that the description of the present specification, including the following description and the description of specific examples and the like, show preferred embodiments of the present invention and are presented only for explanation. I want. Various changes and / or modifications (or modifications) within the spirit and scope of the present invention disclosed herein will occur to those skilled in the art based on the following description and knowledge from other parts of the present specification. Will be readily apparent. All references cited in this specification (including references) are cited for illustrative purposes and should be construed in their entirety as part of this specification. Is.

The structure photograph of the test piece after anodic polarization measurement (HNO ₃ ) is shown. (a) 30% plastic strain imparting material. (b) Test solution (HNO ₃ ) treatment material. [1 1 1] plane in fcc crystal structure is shown. The anodic polarization curve of HNO ₃ for the specimen is shown. It shows the cathodic polarization curves of HNO ₃ for specimens. The relationship between the strain measured with stainless steel and the etching mark density is shown. (a)-(c) shows a surface photograph of a test piece (30% plastic strain imparting material) after anodic polarization measurement, and (d) shows a locus of potential during anodic polarization measurement. The surface photograph of the test piece (HNO ₃ solution, 30% plastic strain imparting material) after anodic polarization measurement is shown. FIG. 7 shows a locus of potential at the time of anodic polarization measurement (a photograph of the surface of the test piece (HNO ₃ solution, 30% plastic strain imparting material) after anodic polarization measurement is shown in FIG. 7)). (a)-(c) shows a surface photograph of a test piece (HNO ₃ solution, 30% plastic strain imparting material) after cathodic polarization measurement, and (e) shows a locus of potential during cathodic polarization measurement.

  In the present invention, a technique for detecting plastic strain using an electrochemical technique has been successfully developed. The detection technology is based on a search for conditions for detecting plastic strain by performing electrochemical measurements on austenitic stainless steel previously applied with various levels of plastic strain using the solution components and potential as parameters. Based on the findings.

In the present invention, by etching stainless steel (metal material such as Fe-Cr-Ni ternary alloy) such as austenitic stainless steel under potential control, etching marks, that is, local deformation caused by plastic deformation. The present invention provides a technique for detecting a plastic strain by observing the etched traces that appear when a metal structure that has undergone a structural or structural change is selectively dissolved.
Etching marks can be revealed by etching a material at a specific solution component / potential. Examples of the solution component include an aqueous solution of a mineral acid. Especially as a mineral acid, nitric acid can be used conveniently. In a preferred embodiment, the solution component is an aqueous nitric acid solution, typically 0.8-1.5N HNO ₃ aqueous solution, and more preferably 1N HNO ₃ aqueous solution.

The potential can be selected so that a predetermined etching mark can appear, and in one embodiment, for example, a potential of −400 mV _SCE or less can be mentioned. In one more preferred embodiment, the potential includes -600 mV _SCE and lower. In etching, the potential can typically be a constant potential (constant potential). The holding period of the constant potential can be selected so that a predetermined etching mark can appear, and is not particularly limited, and an appropriate time can be selected. In a more preferred embodiment it is about 8-22 minutes, and in a more preferred embodiment it is approximately 20 minutes.

In one specific embodiment, the conditions under which the etching marks of the stainless steel material are etched are 1N.
HNO ₃ , -600mV _SCE . In one more preferred embodiment, etching is performed at that potential for about 20 minutes. It is also possible to quantify after constant potential etching. In quantification, the surface of the test material is observed with a microscope, for example, and the number of etched traces per unit area is counted to obtain the “etching trace density”. The degree of plastic strain applied to the test material was measured based on the relationship between the strain and the etching mark density obtained by measuring the standard materials to which the respective amounts of plastic strain were previously applied. It can be determined from “etching mark density”. Here, the “etching mark density” is defined as the number of etching marks etched per unit area when the surface of the test material is observed with a microscope.

As described above, by the plastic strain detection technique of the electrochemical method of the present invention, it is possible to recognize a one-to-one correlation between the plastic strain and the etching mark density, and it is also possible to use the correlation. Is within the scope of the idea. According to the plastic strain detection method of the electrochemical method of the present invention, it has a clear detection sensitivity at least at 1% plastic strain.
In the above, the surface of the etched metal sample is observed using a detector such as a microscope equipped with a CCD camera and, in some cases, a system such as a computer linked to an image processing program. It is also possible. The present invention also includes a detection technique linked with such an image processing system.
The present invention will be described in detail with reference to the following examples, which are provided merely for the purpose of illustrating the present invention and for reference to specific embodiments thereof. These exemplifications are for explaining specific specific embodiments of the present invention, but are not intended to limit or limit the scope of the invention disclosed in the present application. In the present invention, it should be understood that various embodiments based on the idea of the present specification are possible. All examples were performed or can be performed using standard techniques, except as otherwise described in detail, and are well known and routine to those skilled in the art. .

〔experimental method〕
The sample material was processed into a flat plate test piece after solution treatment (1150 ° C., 30 min), and was given plastic strain by tension. The applied plastic strains are 1, 3, 10, 20, and 30%, respectively. After applying plastic strain, it was processed into a test piece for electrochemical measurement, and subjected to anode / cathode polarization measurement and constant potential etching test. Two types of test specimens for electrochemical measurements were prepared, with the test surface perpendicular to the tensile axis and parallel to the tensile axis. A saturated gypsum electrode (SCE) is used as the reference electrode, and the potential is expressed in terms of SCE.
The specimen was SUS316L austenitic stainless steel. Table 1 shows the chemical composition.
Shown in

1N HNO ₃ , H ₂ SO ₄ , and HCl were used as the anodic polarization measurement solution. After performing N ₂ deaeration at a solution temperature of 35 ° C. and performing a cathode treatment, the potential was swept in a noble direction from the natural potential. The potential sweep rate is 30 mV / min. For the cathodic polarization measurement, 1N HNO _{3 from} which the clearest etching mark was obtained in the anodic polarization measurement was used. After degassing with N ₂ , the potential sweep started from the natural potential to the base direction. From these measurements, the potential at which etching marks peculiar to HNO ₃ appeared was searched, and constant potential etching was performed at that potential (−600 mV _SCE ) for 20 minutes. A metal microscope was used to observe the surface structure, and plastic strain was quantified.

[Experimental results and discussion]
After the polarization measurement, countless linear etching traces were observed in the plastic strain imparting material, although the degree was different depending on the solution, in addition to the grain boundary. Among these, HNO ₃ showed the most obvious etching marks. FIG. 1 shows a structure photograph of the test piece after anodic polarization measurement with HNO ₃ . In the solution material, there were very few etching marks as seen in the plastic strain imparting material. The etching marks on the individual crystal grains were almost linear, and no penetrating through a plurality of grains was confirmed. The test surface of the sample in FIG. 1 is taken to be perpendicular to the tensile axis. Etching marks have various angles, but there are at most three types of etching marks in one grain. Plastic deformation of metallic materials is due to the generation and growth of crystal plane slips or twins. SUS316L used for this test
In the fcc structure as shown above, it is known that both the slip plane and the twin plane are four {111} planes (FIG. 2). The observed etching marks have different directions for each crystal grain, and are considered to be closely related to the crystal orientation, such as being directed to a specific direction in the same grain. On the other hand, as can be seen in FIG. 1, there is a clear difference in height on the sample surface after anodic polarization measurement with HNO ₃ for each crystal grain. This is a fact that suggests that the dissolution rate of the material strongly depends on the crystal orientation in the electrochemical test conditions. Furthermore, as can be seen in FIG. 1, the groove-like etching marks that appear characteristically in the plastic strain imparting material are more distinct on the convex crystal grains (that is, the crystal grains that have been oriented at a low dissolution rate). It was appearing. Based on these facts, the formation mechanism of the groove-like etching marks that appear characteristically in the plastic strain imparting material can be understood as follows. The portion with relatively fast azimuth was formed in a linear band on the surface with low melting rate, so that the portion preferentially melted and appeared as etching grooves with straight grooves. Let's go. It is understood that the band-like portions having different orientations formed only on the test piece to which plastic strain is applied are deformed twins, that is, the groove-like etching marks are obtained by etching the deformed twins.

3 and 4 show an anodic polarization curve and a cathodic polarization curve of 1 N HNO ₃ , respectively. In both cases, no change in the behavior of the polarization curve accompanying the dissolution behavior of the material was confirmed. For this reason, it was impossible to determine in which potential region the etching mark appeared. Therefore, the potential sweeping was stopped halfway, and a test was conducted to confirm whether or not etching marks appeared on the surface, and a potential suitable for forming the groove-shaped etching marks was searched. As a result, it was found that etching marks appeared near -600mV _SCE . In the anodic polarization measurement, sweeping was not performed in this potential range, but the potential was maintained at −600 mV for 10 min with HNO ₃ during the cathode treatment. This is probably because the etching marks were observed after the anodic polarization measurement.

For quantification of plastic strain, 1N HNO ₃ -600 mV _SCE , 20 min constant potential etching was used.
For quantification, a test piece taken so that its test surface was parallel to the tensile axis was used. This is because when observing with an actual structure, the observable surface is often a surface parallel to the tensile axis. The surface of the test piece was observed with a metallographic microscope, and the number of groove-like etching traces per unit area was counted and defined as “etching trace density”.
The results are shown in FIG. A clear one-to-one correlation was observed between strain and etching mark density. Etching traces were confirmed even slightly in the solution material that was not pre-strained. This may be due to the influence of cutting or surface polishing during sample preparation. However, a clear difference from the pre-strain of 1% was confirmed, indicating that at least 1% of the plastic strain has a sensitivity to detect clearly.

After cathodic treatment of the specimen with -700 to -600mV _SCE , the current density from natural potential is 1μA / cm ²
The potential was swept in the noble direction until the above was reached. FIGS. 6A to 6C show a surface photograph of the test piece after the anodic polarization measurement, and FIG. 6D shows a locus of the potential at the time of anodic polarization measurement. In HNO ₃ , etching marks were clearly observed. Slight etching marks were observed with H ₂ SO ₄ and HCl, but the number and clarity clearly did not reach HNO ₃ .

After the cathode treatment, the potential was swept in the noble direction from the natural potential. In order to find the potential at which the groove-shaped etching traces were formed, the sweep was stopped at various potentials, and the presence or absence of the etching traces was confirmed. FIGS. 7A to 7D show a surface photograph of the test piece after the anodic polarization measurement, and FIG. 8E shows a locus of potential at the time of anodic polarization measurement. Groove-like etching trace was observed at all in the range of 100 mV _SCE is active state area on the polarization curve to 1000 mV _SCE peracetic passive zone. However, groove-like etching marks were confirmed even in the test piece in which the potential was not swept only by the cathode treatment. That is, the formation of the groove-like etching mark is during the cathode treatment. As a test, the potential was swept to 1000 mV _SCE without cathodic treatment. Groove etching was formed, but not as clearly as after the cathode treatment. The effective etching potential is considered to be on the cathode side from the natural potential.

The potential was swept from the natural potential toward the base direction. In order to search for potentials at which etching marks appear, sweeping was stopped at various potentials, and the presence or absence of etching marks was confirmed. FIGS. 9A to 9C show a surface photograph of the test piece after the cathodic polarization measurement, and FIG. 9E shows a locus of potential during the cathodic polarization measurement. Etching marks are not confirmed up to -300mV _SCE, and etching marks are confirmed at a potential of -400mV _SCE or less. It is considered that etching marks are formed at a potential of −400 mV _SCE or less.

We have established a technology to detect plastic strain by revealing etching marks by etching austenitic stainless steel with plastic strain under potential control. The etching trace was etched at a specific solution component / potential, and the conditions under which the etching trace was etched were 1N HNO ₃ and −600 mV _SCE . In quantification after constant potential etching, a one-to-one correlation was observed between plastic strain and etching mark density. At least 1% plastic strain was shown to have a clear detection sensitivity, indicating the applicability of electrochemical techniques to plastic strain detection.

The detection technology of the present invention can be easily performed in a short time, without requiring excessive equipment, for example, in the field, and is extremely practical and effective. It is possible to detect the presence or absence and the degree. Therefore, the plastic strain applied to a metal material such as stainless steel can be detected and used for damage evaluation and soundness assurance of important mechanical parts and structures.
It will be apparent that the invention may be practiced otherwise than as particularly described in the foregoing description and examples. Many modifications and variations of the present invention are possible in light of the above teachings, and thus are within the scope of the claims appended hereto.

Claims (4)

A method for detecting plastic strain in stainless steel, comprising using a nitric acid solution as a solution component, subjecting the test stainless steel material to electrochemical measurement under constant potential conditions, and detecting the plastic strain of the stainless steel material . 2. The method for detecting plastic strain in stainless steel according to claim 1, wherein the electrochemical measurement is performed by etching the test stainless steel material to cause linear etching marks to appear on the crystal grains. The test stainless steel material is etched to reveal linear etching marks on crystal grains, the etching mark density is obtained, and the plastic strain of the stainless steel material is detected using the etching mark density as an index. 2. A method for detecting plastic strain of stainless steel according to 2. 4. The method for detecting plastic strain in stainless steel according to claim 2, wherein the test stainless steel material is etched in a nitric acid solution at a potential of -400 mV or less.