JPS61250109A - Heat treatment of iron-base parts - Google Patents

Abstract

PURPOSE:To enhance the toughness of iron-base parts and to improve the surface characteristics by heating the parts to the austenite stabilization temp. above the A1 transformation point, infiltrating or diffusing a metallic or nonmetallic element into the surfaces of the parts and rapidly cooling the parts to the bainite formation temp. CONSTITUTION:Iron-base parts are heated to the austenite stabilization temp. above the A1 transformation point, and surface treatment for infiltrating or diffusing a metallic or nonmetallic element into the surfaces of the parts is carried out to provide characteristics such as heat and corrosion resistances to the surfaces of the parts. The treated parts are rapidly cooled to the bainite formation temp. and properly held to enhance the toughness of the parts.

Description

[Detailed Description of the Invention] Purpose of the Invention] (Industrial Application Field) This invention relates to iron-based parts (including when they are called products).
The present invention relates to a heat treatment method for iron-based parts, which is used to improve the toughness of iron-based parts and to improve surface properties. 2 (Prior art) Conventional methods for toughening iron-based parts include adding alloying elements, heat treatment, or surface treatment. There is an austempering heat treatment in which the material is rapidly cooled and maintained at a bainitic temperature from a stabilization temperature (for example, described in Steel Handbook (published by Maruzen), 3rd edition, Vol. 6, page 488).

(Problem to be solved by the invention) However, in such conventional austempering heat treatment, although it is expected that the structure of the heat-treated part will be changed to bainite and its toughness will be greatly improved, It has been difficult to impart properties such as strength and corrosion resistance to such parts. That is, in order to impart the heat-resistant and corrosion-resistant properties to conventional austempered heat-treated parts, there was no suitable method other than using a rather expensive steel material that already had excellent heat and corrosion resistance properties.

Therefore, in order to improve toughness and impart heat resistance and corrosion resistance, it was necessary to perform both conventional austempering heat treatment and heat-resistant/corrosion-resistant surface treatment. The target surface treatment requires heating the part to a considerably high temperature, so if the surface treatment is performed after austempering,
There have been problems in that the effectiveness of austempering is likely to be lost during the surface treatment process, and the effectiveness of the surface treatment is also likely to be lost when austenitization occurs after one surface treatment.

Therefore, it has been desired to develop a heat treatment method for iron-based parts that can provide excellent toughness through heat treatment and at the same time impart properties such as heat resistance and corrosion resistance to the surface of the part.

This invention was made by focusing on such conventional problems, and it is a heat treatment method for iron-based parts that can improve the toughness of the iron-based parts and improve the surface properties at the same time. It is intended to provide a method.

[Structure of the Invention] (Means for Solving the Problems) A heat treatment method for iron-based parts according to the present invention provides a heat treatment method for iron-based parts.
While heated to an austenite stabilization temperature equal to or higher than the I transformation point, a surface treatment is performed to infiltrate and diffuse a metallic element or a nonmetallic element into the surface of the component, thereby imparting properties such as heat resistance and corrosion resistance to the surface of the component. After that, the toughness of the part is improved by rapidly cooling it to a bainitic temperature and holding it appropriately.

In the method for heat treatment of iron-based parts according to the present invention, first, the iron-based part, which is the part to be heat-treated, is heated to an austenite stabilization temperature higher than the A1 transformation point.

Next, the above-mentioned iron parts are A! Metallic elements or nonmetallic elements are permeated and diffused into the surface of the component while heated to an austenite stabilization temperature higher than the transformation point. In this case, when trying to obtain excellent heat resistance and corrosion resistance on the surface of iron-based parts by infiltrating and diffusing metallic elements or non-metallic elements into the surface of the part, aluminum is infiltrated and diffused into the surface of the part. These include calorizing treatment, a surface treatment method that allows chromium to penetrate and diffuse, chromizing treatment, a surface treatment method that penetrates and diffuses chromium, and siliconizing treatment, a surface treatment method that penetrates and diffuses silicon. In such cases, surface treatment is performed using conventional methods.

Of these, to take calorizing treatment as an example, a small amount of ammonium chloride is added as a reaction accelerator to aluminum powder or aluminum-iron alloy powder, and the process is performed in a neutral state in a sealed container together with the iron-based parts that are to be heat-treated. This is carried out by heating and maintaining the temperature at a temperature equal to or higher than the A1 transformation point in an atmosphere for several hours or more.

Next, the iron-based parts subjected to the above-mentioned surface treatment are taken out from the closed container, and immediately transferred to a bainitic furnace and rapidly cooled to a bainitic temperature. Incidentally, the conditions for the bainitic treatment after the surface treatment can be determined as appropriate according to the transformation characteristics of the component material, etc., similarly to general austempering heat treatment.

The bainite furnace used in the heat treatment method of the present invention can be the one used as a bainite furnace in general austempering heat treatment.

In other words, as a bainite furnace, a salt bath furnace or a fluidized bed furnace is used, and the selection of these furnaces depends on the cooling capacity of the parts and the hardenability of the parts when cooling from the surface treatment temperature to the predetermined bainitic temperature. This is done by taking into account the following. However, in the case of a salt bath furnace, care must be taken as corrosion problems will remain if the salt bath agent is not removed by thorough cleaning after heat treatment.

By the way, a surface treatment layer is formed on the surface of the iron-based component by the above surface treatment, but at least the inside of the component is in an austenitized state, so the bainite structure is changed by bainite formation due to subsequent rapid cooling. The toughness of the precipitated parts is significantly improved.

(Example 1) In this Example 1, spheroidal graphite cast iron having the composition shown in Table 1 was used as the raw material, and the impact test piece shown in Fig. 1 ('L = 55 m,
W=10 layer angle, D=2 fat, R=is■)
1 was used as a sample.

Table 1 In this Example 1, in order to impart heat resistance and corrosion resistance to the sample, a chromizing treatment was performed to infiltrate and diffuse chromium into the surface of the sample.

First, in materials with a high carbon content such as cast iron, the diffusion of chromium is slow, so before the chromizing treatment, the sample was heated in the air at 900°C for 2 hours, and then air-cooled. decarburization was carried out.

After this, the sample was heated to 950°C in a container filled with a mixed powder of 55% chromium powder and 45% alumina powder to which 1% ammonium chloride was added as a reaction accelerator.
After the chromizing treatment, the above sample was immediately heated to 350°C.
The sample was placed in a salt bath maintained at a temperature of 100 mL, and subjected to a bainitic treatment for 4 hours, and then placed in water to be rapidly cooled. Note that a sodium nitrite bath was used as the salt bath.

(Comparative Example 1) For comparison, the same sample as in Example 1 was used, and the sample was heated at 950°C in a reducing atmosphere furnace.
After being austenitized for 6 hours, the sample was placed in a salt bath maintained at 350° C. for 4 hours of bainite treatment, and then a normal austenite heat treatment in which it was rapidly cooled in water was carried out.

(Evaluation Example 1) In this Evaluation Example 1, the sample of Example 1 and the sample of Comparative Example 1 were evaluated. It was conducted using a chemical test. Among these, the oxidation resistance test was performed by heating the impact test piece for 200 hours while flowing hydrogen gas into the container, and measuring the oxide scale thickness and oxidation weight gain after heating. went.

In addition, in order to evaluate the corrosion resistance of the sample of Example 1 and the sample of Comparative Example 1, each was immersed in 1000 cc of a 33% acetic acid solution at room temperature for 1 month (720 hours), and the corrosion loss was measured. The above evaluation results are summarized in Table 2.

950° C. in an electric tube furnace at atmospheric pressure.

As is clear from the results shown in Table 2, when the heat treatment of Example 1 of the present invention was performed, the excellent toughness due to the bainitic treatment was maintained.

Surface properties such as heat resistance and corrosion resistance can be significantly improved compared to those of comparative examples.

(Example 2) In this Example 2, an impact test piece having exactly the same material and shape as that used in Example 1 was used as a sample.

In this Example 2, in order to impart heat resistance and corrosion resistance to this sample, a colorizing treatment was performed to penetrate and diffuse aluminum.

Therefore, aluminum-iron alloy (Ai-50%; Fe-
The sample was placed in a container filled with powder prepared by adding 1% ammonium chloride as a reaction accelerator to the powder (50%), and heated to 950 ° C. while flowing hydrogen gas into the container.
Calorizing treatment was performed for 6 hours.

A small Buddha was placed in a salt bath prepared with a tooth top, ζ0, and a bainite, and after 4 hours of bainitic treatment, it was placed in water and rapidly cooled.

(Evaluation Example 2) In this Evaluation Example 2, the sample of Example 2 and the sample of Comparative Example 1 were subjected to the impact value, oxidation resistance test and Corrosion resistance tests were conducted and each was evaluated. The evaluation results are summarized in Table 3.

As is clear from the results shown in Table 3, when the heat treatment of Example 2 of the present invention was performed, the toughness achieved by normal austempering heat treatment was maintained.

At the same time, it was confirmed that it had significantly superior surface properties such as heat resistance and corrosion resistance compared to Comparative Example 1.

[Effects of the Invention] As explained above, according to the heat treatment method for iron-based parts of the present invention, the surface of the iron-based parts is heated to an austenite stabilization temperature of A1 transformation point or higher. After infiltrating and diffusing metallic or non-metallic elements into the steel, the steel parts are rapidly cooled to the bainitic temperature, so that the iron-based parts after heat treatment are significantly improved in toughness due to bainite precipitation due to austempering treatment, and toughness is significantly improved due to surface treatment. This brings about a very excellent effect in that it is possible to simultaneously improve the surface properties.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the shape of Charpy impact test pieces used in Examples and Comparative Examples of the present invention.

Claims (1)

[Claims] (1) After heating the iron-based part to an austenite stabilization temperature higher than the A_1 transformation point, infiltrating and diffusing metallic elements or non-metallic elements into the surface of the iron-based part, and then rapidly cooling it to the bainitic temperature. Characteristic heat treatment method for iron parts.