JPH0551800A - Method for revealing grain boundary of steel material sample - Google Patents

Abstract

PURPOSE:To brightly reveal grain boundary of martensite structure. CONSTITUTION:A surface to be inspected of a steel material having martensite structure is dipped into an aqueous nitric acid of 40-70wt.% conc. and by supplying current with anode of the steel material sample at 10-500mA/dm<2> current density for 0.5-2hours current supplying time, grain boundary of martensite structure is selectively etched. As carbide of grain boundary is selectively etched and grain boundary is drastically brightly revealed. As the result, precision of measured value of crystal grain size is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for revealing crystal grain boundaries of a steel material sample. More specifically, in a stainless steel having a martensite structure, the crystal grain boundaries of the martensite structure are clearly expressed, The present invention relates to a method for developing grain boundaries of steel, which contributes to improvement in the accuracy of grain size measurement values.

[0002]

2. Description of the Related Art With respect to steel materials that have been subjected to heat treatments such as quenching, quenching and tempering, the grain structure is measured by observing the metal structure of the steels with a microscope. This work is generally performed as follows. That is, first,
The steel material after the heat treatment is cut and sampled, and the surface to be inspected of the sample is polished to a perfect flat surface without scratches, and then the surface to be inspected is mirror-finished.

Then, the surface to be inspected is subjected to etching treatment by a chemical or electrochemical method to expose crystal grain boundaries, which are then observed with a microscope. This etching process is performed by selecting an appropriate method in relation to the target sample, but in any case, the crystal grains and the crystal grains are selectively corroded by selectively corroding the components of the crystal grain boundaries or the crystal grains themselves. The purpose is to make the grain boundaries appear by differentiating the boundaries.

By the way, among the various steel materials, when a crystal grain boundary appears in a steel material having a martensitic structure which is one of the quenched steel structures, it is usually used as an etchant.
Picric acid, hydrochloric acid, alcohol, cupric chloride, hydrochloric acid, etc. are used. However, even if it processes with these etchants, a clear grain boundary cannot be revealed. This is because in the case of martensite, carbide is precipitated in the crystal grain boundaries, and at the same time solid solution carbon is present in the crystal grains. This is because the corrosion of the solid solution carbon proceeds at the same time and the intergranular corrosion cannot be distinguished due to the intragranular corrosion.

Therefore, in measuring the grain size of the martensite structure, an operator estimates the grain boundaries from the state of the structure observed under a microscope, and assigns the grain size number specified by JIS based on that. Is the current situation. That is, the measured value of the crystal grain size depends largely on the skill of the operator, and therefore the reliability of accuracy is low and it is difficult to standardize the measurement.

Further, as a method of revealing the grain boundaries of the martensite structure, the sample surface is faced upward, oxidized in a tubular furnace for a required time, the surface is mirror-finished again, and then etched with a predetermined etchant. The oxidation method of observing with a microscope is also applied. However, this oxidation method not only requires a high degree of skill and complicated operations, but also has a problem that the crystal grain boundaries after etching are not always clear.

[0007]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems in the grain boundary revealing method of the martensite structure, and makes it possible to reveal the crystal grain boundaries extremely clearly,
Therefore, it is an object of the present invention to provide a method for revealing crystal grain boundaries of a steel material, which enables an improvement in the accuracy of the measured value of crystal grain size.

[0008]

In order to achieve the above-mentioned object, in the present invention, the test surface of a steel material sample having a martensitic structure is immersed in an aqueous nitric acid solution having a concentration of 40 to 70% by weight to obtain the steel material. Using the sample as an anode, current density 10
~500mA / dm ^2, by applying a current under conditions of energization time 0.5-2 hours, the grain boundaries of the steel samples, characterized by selectively galvanic corrosion of grain boundaries of the martensitic structure A revealing method is provided.

According to the method of the present invention, a martensite structure is formed by a steel type in which a martensite structure is precipitated in the matrix by heat treatment such as quenching, quenching and tempering, that is, carbon is dissolved in the crystal grains to form a martensite structure. However, the present invention can be applied to steel types having a structure in which carbide is precipitated at the grain boundaries. Specifically, the present invention can be applied to heat-resistant stainless steel containing 12% or more of Cr, precipitation hardening stainless steel in which two phases of a ferrite structure and a martensite structure are mixed.

In the method of the present invention, first, the test surface of the sample is mirror-finished in the same manner as in the prior art, and then the test surface is 40 to
Immerse in a 70% by weight aqueous nitric acid solution. As a result, a passive film is formed on the surface of the test surface. Then, using the sample as an anode and the insoluble electrode disposed in the nitric acid aqueous solution as a cathode, a direct current of 10 to 500 mA / dm ² was applied to 0.5.
Energize for ~ 2 hours.

In that case, the martensite structure in which carbon is solid-solved remains protected by the passivation film and the corrosion of the crystal grains does not proceed. However, although the reason is not clear, the grain boundary is not clear. Then, the passive film is broken and the carbide is electrolytically corroded. Due to the remarkable difference in corrosion progress between the crystal grain boundaries and the crystal grains, the crystal grain boundaries appear clearly.

In this series of processes, the nitric acid aqueous solution used is usually concentrated nitric acid having a nitric acid concentration of 60 to 70% by weight dissolved in water. The nitric acid aqueous solution obtained has a nitric acid concentration of 40% by weight. When it is less than%, the passivation film exhibiting the above-mentioned function is not formed on the surface to be inspected, and 70
When the concentration is higher than the weight%, harmful nitrogen dioxide is generated and workability is deteriorated. Therefore, the nitric acid concentration of the nitric acid aqueous solution is set to 40 to 70 weight% as described above.

The current density during energization is 10 mA / dm ²
When it is lower than 500 mA / dm ² , it is impossible to selectively electrolytically corrode carbides at the grain boundaries, and when it is higher than 500 mA / dm ² , corrosion in the crystal grains also progresses. The grain boundary cannot be clearly displayed. Furthermore, when the energization time is shorter than 0.5 hours, the selective electrolytic corrosion of the crystal grain boundaries does not proceed sufficiently, and when it exceeds 2 hours, not only the crystal grain boundaries but also the corrosion inside the crystal grains progresses. Also, no clear appearance of grain boundaries occurs.

FIG. 1 shows the influence of the current density and the conduction time on the appearance of grain boundaries. FIG. 1 shows Cr 12% by weight,
Sample 1 in which a stainless steel material having a composition of 2.5 wt% Ni, 1.5 wt% Mo, 0.3 wt% V, and the balance Fe was heated at 950 ° C. for 1 hour and then air-cooled and quenched, and Cr
13 wt%, Ni 8 wt%, Mo 2.2 wt%, Al1.1
A stainless steel material having a composition of wt% and the balance Fe (AM
(Steel grade corresponding to S5627C) was heated at 925 ° C. for 1 hour, then oil-cooled and hardened, further heated at 500 ° C. for 4 hours and then air-cooled twice. Result of judging the sharpness of grain boundary appearance by observing the surface to be inspected of each sample after treatment with a microscope at a magnification of 400 times in an aqueous nitric acid solution while changing the current density and the energization time. Is.

In the figure, a region A shows a region where crystal grain boundaries appear clearly in Sample 1, a region B shows a region where crystal grain boundaries appear clearly in Sample 2, and a region C shows grain boundaries. The area where internal corrosion occurs is shown.

[0016]

【Example】

Example 1 A steel material composed of Cr: 12% by weight, Ni: 2.5% by weight, Mo: 1.5% by weight, V: 0.3% by weight and the balance: Fe was heated at 950 ° C. for 1 hour and then air-cooled. As a quenching material, a sample was taken from this quenching material and the test surface was mirror-finished.

The sample was immersed in a 45% by weight nitric acid aqueous solution (room temperature) and subjected to an electric current treatment at a current density of 10 mA / dm ² for 1 hour. The processed sample is washed with water and the surface to be inspected is magnified 400 times.
It was observed under a microscope at a magnification of 2. The micrograph is shown in FIG.
For comparison, 1 g of picric acid and 5 ml of hydrochloric acid were used for comparison.
Etchant 1 prepared with 100 ml of alcohol and 5 ml of cupric chloride, 100 ml of hydrochloric acid and 10 of alcohol
Etchant 2 prepared with 0 ml; picric acid 4 g
And 10 g of surfactant (sodium dodecylbenzene sulfonate, manufactured by Kanto Chemical Co., Ltd.) and 100 ml of water prepared for 10 seconds, 10 seconds, and 1 hour respectively, and then taken out to reveal the crystal grain boundaries. The state was microscopically observed at a magnification of 400 times. Comparative example 1 using the etchant 1 and comparative example 2 using the etchant 2
The case of using the etchant 3 was set as Comparative Example 3, and the micrographs in each case are shown in FIGS. 3 to 5.

As is clear from these micrographs,
According to the method of the present invention, the crystal grain boundaries are clearly exposed,
In the conventional method, the appearance of the crystal grain boundaries is extremely unclear due to the progress of corrosion in the crystal grains. Example 2 Except that the steel material was a tempered / tempered material which was heated at 950 ° C. for 1 hour, air-cooled and hardened, and then tempered twice by air-cooling after heating at 650 ° C. for 2 hours. Performs electrolytic corrosion in the same manner as in Example 1,
The test surface was observed under a microscope. The micrograph is shown in FIG.

In addition, the etchant 1 used in Example 1
The sample was treated with etchant 2 and etchant 3, and the case with etchant 1 was used as Comparative Example 4, the case with etchant 2 was used as Comparative Example 5, and the case with etchant 3 was used as Comparative Example 6, and the micrographs of each case are shown in FIGS.
Shown as FIG. As is clear from these micrographs, according to the method of the present invention, the crystal grain boundaries of the quenched and tempered material are clearly exposed.

Examples 3 and 4 Steel material consisting of Cr: 13% by weight, Ni: 8.0% by weight, Mo: 2.2% by weight, Al: 1.1% by weight, and balance: Fe.
After heating at 5 ° C. for 1 hour, it was oil-cooled to obtain a hardened material, and a sample was taken from this hardened material and its test surface was mirror-finished. The sample was immersed in a 45 wt% nitric acid aqueous solution (room temperature) and subjected to energization treatment at a current density of 50 mA / dm ² for 0.5 hours and 1 hour.

Each sample after the treatment was washed with water, and the surface to be inspected was observed with a microscope at a magnification of 400 times. The photomicrographs are shown in FIG. 10 for the energization time of 0.5 hours and in FIG. 11 for the one hour time. Examples 5-8 As samples, the composition is the same as the steel types of Examples 3 and 4,
A quenching / tempering material was prepared in which it was heated at 925 ° C. for 1 hour, then oil-cooled for quenching, and further heated at 500 ° C. for 4 hours and then air-cooled twice for tempering.

This sample was etched under the conditions shown in Table 1.

[0023]

[Table 1] A micrograph (magnification of 400 times) of each obtained sample is shown in FIG.
2 (Example 5), FIG. 13 (Example 6), FIG. 14 (Example 7) and FIG. 15 (Example 8).

[0024]

As is clear from the above description, according to the method of the present invention, the crystal grain boundaries of the martensite structure can be made to appear extremely clearly. Therefore, the accuracy of the measured value of the crystal grain size is remarkably improved, and its industrial value is great.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a current density and a conduction time when a grain boundary appears during electrolytic corrosion according to the method of the present invention.

FIG. 2 is a micrograph showing a metallographic structure of a test surface of a quenched steel sample treated by the method of the present invention.

FIG. 3 is a micrograph showing a metallographic structure of a surface to be inspected of a quenched steel sample treated by a comparative example method.

FIG. 4 is a micrograph showing a metal structure of a surface to be inspected of a hardened steel sample treated by another comparative example method.

FIG. 5 is a micrograph showing a metal structure of a surface to be inspected of a hardened steel sample treated by a method of another comparative example.

FIG. 6 is a micrograph showing a metal structure of a surface to be inspected of a quenched and tempered steel sample treated by the method of the present invention.

FIG. 7 is a micrograph showing a metal structure of a test surface of a quenched / tempered steel sample treated by a comparative example method.

FIG. 8 is a micrograph showing a metallographic structure of a test surface of a quenched / tempered steel sample treated by another comparative example method.

FIG. 9 is a micrograph showing a metal structure of a test surface of a quenched / tempered steel sample treated by still another comparative example method.

FIG. 10 is a micrograph showing a metal structure of a surface to be inspected of a hardened steel sample treated by the method of the present invention.

FIG. 11 is a photomicrograph showing the metal structure of the surface to be inspected of the hardened steel sample treated by another method of the present invention.

FIG. 12 is a micrograph showing a metal structure of a surface to be inspected of a quenched and tempered steel sample treated by the method of Example 5.

FIG. 13 is a micrograph showing a metal structure of a test surface of a quenched / tempered steel sample treated by the method of Example 6;

FIG. 14 is a micrograph showing the metal structure of the surface to be inspected of another quenched steel sample treated by the method of Example 7.

FIG. 15 is a micrograph showing a metal structure of a test surface of a quenched / tempered steel sample treated by the method of Example 8.

Claims (1)

[Claims] 1. A test surface of a steel material sample having a martensitic structure is immersed in a nitric acid aqueous solution having a concentration of 40 to 70% by weight,
Using the steel material sample as an anode, a current density of 10 to 500 mA
/ Dm ² , current flow for 0.5 to 2 hours, the grain boundary of the martensitic structure is selectively electrolytically corroded, and the grain boundary of steel material appears. Method.