JP2004325190A - Method of making austenitic grain boundary of steel emerged - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of making an austenitic grain boundary emerged clearly relative to a quenching material of a high alloy steel including many kinds of alloy elements, which is difficult to be emerged by the conventional corrosive liquid. <P>SOLUTION: This method of making the austenitic grain boundary of a steel emerged is characterized by having a polishing process for slicing a sample from the object steel and polishing the sample surface, a nitriding process for applying nitriding to the polished sample surface, a re-polishing process for re-polishing the nitrided sample surface, and a grain boundary emergence process for applying corrosion processing for corroding preferentially the grain boundary on the re-polished surface. The nitriding is preferably applied in an ammonia atmosphere for 30 minutes to 10 hours at a temperature of 450-700°C. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing austenite grain boundaries in steel, and more particularly, to a method for producing austenite grain boundaries in a high-alloy steel that has been subjected to quenching.
[0002]
[Prior art]
A variety of high alloy steels have been developed with recent advances in materials technology. Since these high alloy steels contain various kinds of alloying elements, it is often difficult to clearly show austenite crystal grain boundaries with a conventional corrosive liquid. For this reason, proposals have been made on a corrosive liquid and a sample preparation method for clearly showing austenite crystal grain boundaries of a high alloy steel. For example, there has been proposed a corrosion liquid in which nitric acid, hydrochloric acid, and sulfuric acid are added to picric acid, and further, ferrous chloride, hydrogen peroxide, a surfactant, and the like are added (see Patent Document 1).
[0003]
[Patent Document 1]
JP-A-7-325080
[Problems to be solved by the invention]
However, in the corrosive liquid of Patent Document 1, it is possible to show clear grain boundaries for steel having a Cr content of 0.05% by weight or more, but more alloy elements such as Ni and Mo are used. In a high alloy steel containing 0.2% by weight or more, a clear grain boundary could not always be obtained even with this corrosion liquid.
[0005]
Therefore, an object of the present invention is to provide a method for clearly showing austenite grain boundaries in a hardened material of a high alloy steel containing various types of alloy elements, which has been difficult to appear with a conventional corrosion liquid. That is.
[0006]
[Means for Solving the Problems]
The present inventors have repeated research on a method for producing austenite grain boundaries of a high alloy steel after quenching, and have come to realize that a sample before corrosion treatment is subjected to nitriding. By subjecting the sample to nitriding treatment, nitrogen diffuses deeply along the grain boundaries of austenite crystal grains to form nitrides with additional elements such as C and Cr precipitated at the grain boundaries. Therefore, it has been found that the subsequent corrosion treatment causes a large difference in the degree of corrosion between the inside of the austenite crystal grains and the grain boundary, and the grain boundary can clearly appear. The present invention has been completed through intensive experiments based on this finding.
[0007]
That is, the method of presenting the austenite grain boundary of steel according to the present invention includes a polishing step of cutting out a sample from a target steel and polishing the sample surface, a nitriding step of performing a nitriding process on the polished sample surface, The method comprises a re-polishing step of re-polishing the sample surface that has been made, and a grain boundary appearing step of performing a corrosion treatment on the re-polished surface to preferentially corrode the grain boundaries.
[0008]
Here, the nitriding treatment is preferably performed in an ammonia atmosphere at 450 to 700 ° C. for 30 minutes to 10 hours.
[0009]
In addition, in the grain boundary appearance step after the re-polishing, it is preferable to perform electrolytic etching in a 5 to 30% chromic acid solution.
[0010]
Further, in order to make the austenite grain boundaries appear more clearly, it is preferable to repeat the repolishing step after the nitriding treatment and the grain boundary appearance step a plurality of times.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
The method for revealing austenite grain boundaries of steel according to the present invention includes a polishing step of cutting out a sample and polishing the surface of the sample, a nitriding step of nitriding the polished sample, and re-polishing the nitrided sample surface. The method comprises a re-polishing step and a grain boundary appearance step of performing a corrosion treatment on the re-polished surface to preferentially corrode grain boundaries.
[0012]
In the surface polishing step of the cut sample, it is desirable that the surface of the sample is mirror-finished by ordinary buffing or the like. This is to make it easier for nitrogen to enter the austenite crystal grain boundaries and grains in the sample during the subsequent nitriding treatment. After buffing, it is preferable to wash the sample surface with pure water or alcohol and dry it.
[0013]
The nitriding step is a step of injecting nitrogen into the austenite crystal grains of the sample and into the grain boundaries to form nitrides with precipitates and the like. Although there is no particular limitation on the nitriding method, gas nitriding with ammonia gas is preferable in consideration of safety and ease of handling. Here, the nitriding temperature is preferably in the range of 450 to 700 ° C. If the nitriding temperature is lower than 450 ° C., the penetration of nitrogen into the grain boundaries is not sufficient, so that a small amount of nitride precipitates and clear crystal grain boundaries cannot be obtained. On the other hand, if the temperature exceeds 700 ° C., the crystal grains may grow, and it may not be possible to observe the necessary crystal grains that should be originally observed. More preferably, it is 500 to 650 ° C.
[0014]
The nitriding time is preferably 30 minutes to 10 hours. If the treatment time is less than 30 minutes, formation of a nitride at a grain boundary is insufficient, so that it may be difficult to make a clear grain boundary appear. On the other hand, even if the nitriding treatment is performed for 10 hours or more, the effect is saturated and the working efficiency is reduced, which is not appropriate. More preferably, it is 1 to 5 hours.
[0015]
Note that the residual ammonia concentration in the nitriding treatment is suitably 20% or more. This is because if the residual ammonia concentration is 20% or less, the nitriding ability may be insufficient, and the precipitation of nitride may be insufficient.
[0016]
The repolishing step is a step for removing a nitride layer formed on the surface of the processed product by nitriding. This nitride layer is an iron nitride layer generally called a white layer, and this layer is formed on the outermost surface of the processed product by the nitriding treatment regardless of the grain boundaries or the inside of the grains. If a nitride layer is present, it is difficult to reveal grain boundaries even if corrosion is performed. Therefore, the nitride layer formed on the outermost surface of the sample is removed by performing repolishing. For the re-polishing, for example, it is desirable to perform a mirror finishing process with a surface roughness of about 0.01 Ra using an abrasive such as a diamond paste.
[0017]
As a corrosion method in the grain boundary appearance step, it is preferable to perform electrolytic etching in a 5 to 30% by weight aqueous solution of chromic acid. If the chromic acid concentration of the chromic acid aqueous solution is less than 5% by weight, a clear grain boundary cannot be produced. On the other hand, if it exceeds 30% by weight, it becomes difficult to adjust the degree of corrosion, so that it is not appropriate. More preferably, it is 8 to 15% by weight. The electrolytic etching may be performed by a usual method. For example, current: 0.2 to 1 mA, voltage: 0.5 to 20 V, time: 1 to 10 minutes, and temperature: 10 to 30 ° C.
[0018]
By repeating the repolishing step after the nitriding treatment and the grain boundary appearance step a plurality of times (for example, two to three times), a clearer grain boundary can be obtained. In particular, in the case of tempered high-alloy steel, the amount of precipitates formed in the grains is large, so after removing the nitrides in the grains as much as possible and confirming that the grain boundaries are clear, It is preferable to perform the re-polishing step and the grain boundary appearance step a plurality of times.
[0019]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples.
(Test material)
A test material was a material obtained by quenching and tempering a high alloy steel containing 0.2% by weight, Cr: 0.5%, Mo: 0.25%, and Ni: 1.8% by weight. And The shape of the test material was a round bar having a diameter of 10 mm and a length of 20 mm. The heat treatment conditions were as follows: quenching: 900 ° C. × 1 hour, oil quenching and tempering: 600 ° C. × 2 hours.
(Sample preparation)
Three specimens for observation were cut out from the above test materials, and the observation surfaces were polished. Polishing was performed by buff polishing using diamond paste as an abrasive to obtain mirror samples 1, 2, and 3.
[0020]
The mirror-finished sample 1 was subjected to a nitriding treatment to provide an example of the present invention, and samples 2 and 3 were subjected to corrosion treatment with different corrosion liquids without nitriding treatment as comparative examples to try to reveal austenite grain boundaries.
(Example)
The polished sample 1 was subjected to a nitriding treatment at 650 ° C. for 4 hours in an ammonia gas atmosphere whose flow rate was adjusted so that the residual ammonia concentration was 75 to 85% by volume. Next, the observation surface nitrided with a diamond paste (manufactured by Marumoto Struers) was polished again to obtain a mirror finish with a surface roughness of 0.001 Ra. After the repolishing, electrolytic etching was performed in a 10% aqueous chromic acid solution using an etching apparatus manufactured by Marumoto Struers. The electrolytic etching conditions were as follows: current: 0.5 A, voltage: 4 V, temperature: 20 ° C., time: 4 minutes. Note that repolishing and electrolytic etching were repeated twice under the same conditions. FIG. 1 shows a microscopic observation photograph of the grain boundaries that have appeared.
[0021]
FIG. 1 shows that a clear grain boundary was obtained.
(Comparative Example 1)
The sample 2 after the mirror polishing was subjected to a corrosion treatment with a corrosion liquid 1 at 70 ° C. for 2 minutes.
[0022]
Corrosion liquid 1 is obtained by adding picric acid: 6 g, hydrochloric acid: 4 ml, and sodium laurylbenzenesulfonate: 9 g as a surfactant to 300 ml of water. The corrosive liquid 1 is a corrosive liquid that is preferably used for producing austenite grain boundaries of, for example, carbon steel and chromium steel.
[0023]
The obtained microscopic observation photograph is shown in FIG. It can be seen that the grain boundaries cannot be clearly distinguished because the degree of corrosion between the precipitates in the grains and the precipitates at the grain boundaries is almost the same.
(Comparative Example 2)
The sample 3 after the mirror polishing was subjected to a corrosion treatment with a corrosive liquid 2 at room temperature for 25 minutes.
[0024]
The composition of the etchant 2 was the following composition disclosed in Patent Document 1.
[0025]
Specifically, saturated picric acid aqueous solution: 1100 ml, 61% nitric acid: 5 ml, 36% hydrochloric acid: 11 ml, 97% sulfuric acid: 0.6 ml, 30% hydrogen peroxide solution: 0.5 ml, sodium dodecylbenzenesulfonate: 100 mg, glycerin : 20 ml and ferrous chloride: 30 mg were added to make the total amount of corrosive liquid 1137 ml.
[0026]
The obtained microscopic observation photograph is shown in FIG. As in Comparative Example 1, since the degree of corrosion between the precipitates in the grains and the precipitates in the grain boundaries is almost the same, it can be seen that the grain boundaries cannot be clearly distinguished.
[0027]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the austenitic grain boundary of the high alloy steel after the quenching process which was difficult to observe conventionally can be uniformly and clearly revealed without unevenness. Therefore, it is possible to evaluate a high alloy steel based on crystal grain size, which has a great effect on research and development and quality control of a high alloy material, and has an extremely large industrial value. In particular, it is suitably applied to a high alloy steel such as a maraging steel, an SNCM steel, or a Cr-Mo-V steel.
[Brief description of the drawings]
FIG. 1 is a metallurgical micrograph showing a grain boundary observed by a method for producing an austenite grain boundary according to the present invention.
FIG. 2 is a metallurgical micrograph showing a grain boundary that appeared in a corrosion liquid 1 without being subjected to nitriding treatment.
FIG. 3 is a metallurgical micrograph showing a grain boundary that appears in a corrosive liquid 2 without being subjected to a nitriding treatment.

Claims (4)

In a method of revealing austenite grain boundaries of steel,
A polishing step of cutting a sample from the steel and polishing the sample surface;
A nitriding step of subjecting the polished sample surface to a nitriding treatment;
A re-polishing step of re-polishing the nitrided sample surface,
A grain boundary appearance step of subjecting the polished surface to a corrosion treatment that preferentially corrodes the grain boundaries;
A method for producing austenite grain boundaries in steel, characterized by having: 2. The method of claim 1, wherein the nitriding step is a step of performing a treatment at 450 to 700 ° C. for 30 minutes to 10 hours in an ammonia atmosphere. 3. 3. The method according to claim 1, wherein the grain boundary appearance step is a step of performing electrolytic etching in a 5 to 30% chromic acid aqueous solution. 4. The method according to claim 1, wherein the re-polishing step and the grain boundary appearance step are repeated a plurality of times after the nitriding treatment.

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