JPS60177174A - Surface treatment method for iron-based sintered parts - Google Patents

Abstract

PURPOSE:To improve remarkably the wear resistance and corrosion resistance of iron-base sintered parts by heat treating the parts in an atmosphere of steam to form oxide films on the surfaces of the parts before surface nitriding and by carrying out surface nitriding by heating in an atmosphere of gaseous ammonia. CONSTITUTION:Iron-base sintered parts are put in a heating furnace kept at 500-570 deg.C, and steam is introduced into the furnace for 30-90min under 5- 500mm. H2O vapor pressure to form oxide films on the surfaces of the parts before surface nitriding. The introduction of steam into the furnace is then stopped, the inside of the furnace is kept at 500-600 deg.C, and gaseous ammonia is introduced into the furnace under 5-100mm. H2O pressure to carry out the surface nitriding of the iron-base sintered parts.

Description

[Detailed description of the invention] In this invention, nitriding does not progress to the inside of the component.

This paper relates to a surface treatment method for iron-based sintered parts using gas nitriding.

Conventionally, nitriding treatment has been performed to improve the wear resistance and corrosion resistance of iron-based mechanical parts. The nitriding process is roughly divided into the following methods: gas nitriding, salt bath nitriding, and ion nitriding. Among these, the gas nitriding method requires relatively simple equipment. A large number of parts can be processed at once. Unlike salt bath nitriding, gas nitriding is generally used for molten products because it does not require harmful substances such as cyanide. At that time, the gas nitriding method is 500 to 6
Since it is carried out at a low temperature of 00℃, compared to hardening etc.
This is a surface hardening method that causes extremely little deformation. However, the application of gas nitriding to sintered parts has not been put to practical use. This is because sintered parts are porous due to their manufacturing method, so when gas nitriding is applied, the inside of the part is nitrided in a network along the continuous pores communicating with the surface. This is because the parts have large distortions due to the increased nitridation, and the iron-based sintered part itself also becomes brittle, reducing its toughness and making it unusable.

It is therefore an object of the invention to overcome the above-mentioned drawbacks and to
The aim is to establish a gas nitriding method suitable for sintered parts.

The inventors have made this the result of intensive research.

■ When an iron-based sintered part is subjected to steam treatment, a film of triiron tetroxide (hereinafter also referred to as an oxide film) is formed on the part's surface, which seals the pores on the surface of the sintered body ( At the same time, an oxide film is also formed around the continuous pores inside the part near the surface), which isolates the internal pores, and as a result, the steam-treated part becomes 30 Ky
In other words, it is possible to maintain airtightness even under a gas pressure of /la.

Ammonia gas in the next step can be prevented from entering the inside of the part; It was discovered that when heated inside, reduction and nitridation of triiron tetroxide occurs, resulting in an iron-based sintered part having a nitride layer only on the surface of one part.

This invention was invented based on the above knowledge,
When surface nitriding iron-based sintered parts, the iron-based sintered parts are first heat-treated in a steam atmosphere.

This method is characterized in that after forming an oxide film on the surface of the component, it is heated in an atmosphere surrounded by ammonia gas to nitride one surface. What is the part surface area here?

It refers not only to the surface of the component but also to the internal cavity walls near the surface.

The present invention will be described in detail below.

First, the steam treatment in the first step can be carried out under any conditions as long as it forms an oxide film that prevents the ammonia gas from entering the interior in the second step. In a heating furnace at ℃, the vapor pressure is 5 to 4.
This is done by introducing 00 g of water vapor for 30 to 90 minutes. and.

In order to prevent ammonia gas from entering the interior in the second step, the average thickness of the oxide film must be the same as that formed on the surface of one part, and that formed on the walls of the internal pores near the surface of one part. The thickness (not including the material) is 9, for example, 5μ
m or more is preferable.

Next, the nitriding treatment in step @2 is performed in the same furnace as the one that was used for the steam treatment in step 1, and steam is introduced into the furnace. This can be done by stopping the steam treatment and starting the introduction of ammonia gas, or by introducing the part into another heating furnace and introducing ammonia gas after the part has cooled in the heating furnace where the steam treatment has already been performed. You may go. The nitriding treatment can be carried out under any conditions as long as the oxide film is reduced and nitrided and an iron-based sintered part having a nitride layer on only one surface is obtained, but the temperature in the heating furnace is 500℃. -600℃
is preferred. Also, the pressure of ammonia gas is 5 to 10 tons.
) m water column is preferred. The nitriding time varies depending on the heating temperature and the ammonia gas pressure, but the nitriding time is 30
The average thickness of the nitride layer formed under the above conditions, preferably -120 minutes, is about 5-30 .mu.m.

Hereinafter, the structure and effects of the present invention will be explained in detail using Examples and Comparative Examples.

Examples and comparative examples The iron-based sintered ring and iron-based sintered body shown below.

Batch type steam treatment furnace, temperature 570'C, steam pressure 401113 water columns, treatment time 30 minutes or 60 minutes, respectively, average thickness 5μm or 8
After forming an oxide film of μm, it was placed in ammonia gas (pressure: 5 mm water column) in a batch-type 9-treatment furnace.

The nitriding treatment was carried out at a temperature of 570° C. for the treatment time shown in the table.

For comparison, the above iron-based sintered ring and iron-based sintered body were subjected to surface treatment in the same manner as above, except that they were not subjected to steam treatment.

The roundness of the obtained surface-treated iron-based sintered ring and the impact value of the surface-treated iron-based sintered body were measured.

The values are shown in Tables 1 and 2, respectively.

Furthermore, regarding surface-treated iron-based sinter/g.

The average thickness of each formed nitride layer was calculated from an optical microscopic photograph of the structure of a sample etched with a trivalent nital etchant. The results are shown in Table 3.

Iron-based sintered ring and iron-based sintered body: Both have an apparent density of 6-
417cd Fe-1.5wt% Cu-41.5wt 1■
They each have an outer diameter of 72 m and an inner diameter of 55 m.

A ring with a height of 10 m and a rectangular parallelepiped with dimensions of 10 strips x 10 strips x 55 strips were both sintered at about 114L1°C in a reducing atmosphere, and the outer diameter of the sintered ring was mechanically etched.

Roundness (aligned to 1,020WIL Table 1) Roundness of ring outer diameter after surface treatment (roundness) Table 2 Impact value after surface treatment (K9/m) Table 3 Formed nitride The average thickness of the layer is μm) From the above results, the first order can be understood.

If the parts are nitrided without steam treatment, a nitride layer is formed inside the parts as shown in Table 3, and as is clear from Table 1, the T4e roundness is significantly deteriorated. On the other hand, according to the method of the present invention in which steam treatment is performed before nitriding, nitriding does not progress to the inside of the part (see Table 3), so even sintered parts can be prevented from deforming (see Table 1). reference). Similarly, as shown in Table 2, there is an effect of greatly preventing a decrease in impact value.

As described above, the present invention has great industrial significance in that it has made it possible to perform gas 9ization treatment on sintered parts.

Applicant: Mitsubishi Kinko Co., Ltd. Agent: Tomi 1) Kazu, 1 other person

Claims (1)

[Claims] When performing surface nitriding on iron-based sintered parts, first heat-treat the iron-based sintered part in a steam atmosphere to form an oxide film on the surface of the part, then heat it in an ammonia gas atmosphere to completely nitride the surface. A surface treatment method for iron-based sintered parts.