JP2002030333A - Nitride layer and austenitic iron-based metal having the same on the surface - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new nitride excellent in wear resistance and corrosion resistance and to provide austenitic stainless steel or heat resistant steel having the same. SOLUTION: This nitride layer is formed on an austenitic iron-based metal and, as precipitates, contains chromium carbonitride, the main phase of the nitride layer preferably has a face-centered cubic structure, and the nitride has a compositional ratio of Me4N (Me: metallic elements such as Fe, Cr, Ni and Mo).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a nitride excellent in wear resistance and corrosion resistance, and an austenitic stainless steel and a heat-resistant steel having the same.

[0002]

2. Description of the Related Art Austenitic stainless steel and heat-resistant steel are used for various devices and machine parts, and are particularly frequently used in parts where severe corrosion resistance is required. For example, in an engine valve, since corrosion resistance and wear resistance due to exhaust gas are required as well as heat resistance, austenitic heat-resistant steel subjected to a surface hardening treatment is used.

By the way, when such a surface hardening treatment is performed, it is required that the hardening be performed without impairing the corrosion resistance of the surface. A nitriding treatment is generally used as a surface hardening method. However, austenitic stainless steels and austenitic heat-resistant steels are known as steels that have been difficult to nitridate for a long time, and various types of nitriding by ion nitriding have recently become possible. However, in order to perform nitriding treatment while maintaining corrosion resistance, a film such as Fe4N must be formed in a layer on the surface of austenitic stainless steel or heat-resistant austenitic steel by ion nitriding at a low temperature range of about 400 ° C. ,
The formation of iron nitride is extremely slow and not practical. Further, when such a layered nitride is exposed to a high temperature, its corrosion resistance is rapidly reduced, and it is considered that practical use is also difficult in this aspect.

[0004]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a novel nitride having excellent wear resistance and corrosion resistance, and an austenitic stainless steel and heat resistant steel having the same.

[0005]

According to the present invention, there is provided a nitride layer formed on an austenitic iron-based metal, wherein the nitride layer contains chromium carbonitride as a precipitate. Preferably, the main phase of the nitride layer has a face-centered cubic structure, and Me ₄ N (Me: Fe, C
a metal element such as r, Ni, Mo, etc.).

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Austenitic heat-resistant steel has M
There is austenitic steel containing a large amount of n.
For example, SUH35 and SUH36. Since these do not contain a large amount of Ni in practical use, they have a great advantage in terms of cost. However, these heat-resistant steels have a C of about 0.1.
It contains 35 to 0.60% by weight and precipitates CrC in austenite. Conventionally, it has been considered that these steels cannot provide high corrosion resistance due to the precipitates. Therefore, it is considered that Cr nitride also impairs the corrosion resistance of steel, and it has been determined that conventional ion nitriding and gas nitriding are performed at a low temperature so as not to generate CrN.

However, both iron nitride and chromium nitride are ceramics having high corrosion resistance, and have high corrosion resistance unless the metal surface of the substrate is exposed. The hardness is 800 HV or more, which is sufficient for practical use.

[0008] The nitride layer of the present invention is a nitride layer formed on an austenitic iron-based metal, wherein fine crystal grains of chromium carbonitride are precipitated in the nitride layer. Preferably, the main phase of the nitride layer has a face-centered cubic structure, and has a composition ratio of Me ₄ N (Me: a metal element such as Fe, Cr, Ni, or Mo).

In the nitride layer of the present invention, the size of the chromium carbonitride crystal grains is preferably 10 μm or less.
If the ratio is outside this range, there is a high possibility that practically sufficient corrosion resistance will not be obtained.

[0010] Even if the austenitic heat-resistant steel having nitride according to the present invention is subjected to a salt spray test for 24 hours, it does not rust.

[0011]

EXAMPLES Examples of the present invention are shown in 1-4. Note that the present invention is not limited to the following examples (particularly, ion nitriding).

(Example 1) A sample of SUH35 was prepared with a diameter of 1
installed in an ion nitriding furnace with a height of 1 m and a holding temperature of 400
° C, holding time 3 hours, applied voltage 900V, holding pressure 0.
Ion nitriding was performed at 70 × 10 ² Pa.

(Example 2) A sample of SUH35 was prepared with a diameter of 1
m in a 1 m high ion nitriding furnace, with a holding temperature of 400
° C, holding time 3 hours, applied voltage 900V, holding pressure 1
Ion nitriding was performed at 3.3 × 10 ² Pa.

(Example 3) A sample of SUH36 was prepared with a diameter of 1
installed in an ion nitriding furnace with a height of 1 m and a holding temperature of 600
° C, holding time 3 hours, applied voltage 900V, holding pressure 0.7
Ion nitriding was performed at 0 × 10 ² Pa.

(Example 4) A sample of SUH36 was prepared with a diameter of 1
installed in an ion nitriding furnace with a height of 1 m and a holding temperature of 400
° C, holding time 3 hours, applied voltage 900V, holding pressure 1
Ion nitriding was performed at 3.3 × 10 ² Pa.

With respect to the samples obtained in Examples 1 to 4, the presence or absence of CrCN and the surface hardness as a result of spraying with salt water according to JIS Z 2371 were examined by EPMA surface analysis for the surface nitrided layer. Table 1 shows the obtained results. In addition,
The main phase of the nitride had a face-centered cubic structure, and had a composition ratio of Fe ₄ N.

Table 1 | Surface hardness HV0.1 | Salt spray test (after 24 hours) | Presence or absence of CrCN Example 1 | 1000 | No rust | Yes Example 2 | 1100 | No rust | Yes Example 3 | 1200 | No rust | Yes Example 4 | 1200 | No rust | Yes It was also found that each precipitated chromium carbonitride was uniformly dispersed as fine crystals of 10 microns or less.

(Conventional example) The same procedure as in Example 1 was carried out except that a commercially available austenitic material whose surface was subjected to nitriding treatment was used. As a result of spraying, the surface hardness was investigated. As a result, the carbonitrided layer could not be confirmed,
Rust was observed. The main phase of the nitride had a face-centered cubic structure, and had a composition ratio of Fe ₄ N.

[0019]

The nitride of the present invention is contained in a nitride layer formed on the surface of austenitic stainless steel or corrosion-resistant steel.
Carbon nitrides are precipitated and dispersed, and the carbon nitrides are precipitated and dispersed, so that high wear resistance and corrosion resistance can be exhibited. The present invention is more effective when the main phase of the nitride layer has a face-centered cubic structure and the composition ratio of Me ₄ N (Me: a metal element such as Fe, Cr, Ni, and Mo).

Claims (5)

[Claims] 1. A nitride layer formed on an austenitic iron-based metal, wherein the nitride layer contains chromium carbonitride as a precipitate. 2. The nitride layer according to claim 1, wherein the chromium carbonitride is uniformly dispersed in the nitride layer. 3. The chromium carbonitride in the nitrided layer has a particle size of 10
3. The nitride layer according to claim 1, which has a thickness of not more than μm. 4. The nitride main phase has a face-centered cubic structure and a composition ratio of Me ₄ N (Me: a metal element such as Fe, Cr, Ni, and Mo). ~ 3
A nitride layer according to any of the preceding claims. 5. An austenitic iron-based metal having the nitride layer according to claim 1 on its surface.