JPS58177456A - Method for diffusing al - Google Patents

Abstract

PURPOSE:To form a diffused Al layer with low hardness, superior workability and superior strength at high temp., by diffusing Al at a specified temp. in the surface of each of parts made of high alloy steel used in an atmosphere of hot combusting gas and by carrying out after-heat-treatment at the same temp. CONSTITUTION:Al is diffused at 1,000-1,250 deg.C in the surface of each of parts made of stainless steel or higher alloy steel used in an atmosphere of hot combustion gas such as parts of a boiler or a heat exchanger to form an Al-enriched layer having >=10mum thickness. Afterheat-treatment is then carried out by heating at the same temp. of 1,000-1,250 deg.C in a nonoxidizing atmosphere. A surface treated layer having lower surface hardness than a material subjected to conventional Al diffusion is formed. The layer has superior strength at high temp. as well as supeiror workability.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an AI diffusion infiltration method, a novel AI diffusion infiltration method that combines the formation of a %KAj enriched layer and a post-heat treatment.

Heat-resistant materials used at high temperatures are often used in a combustion gas atmosphere, and problems include gas corrosion and oxidation, corrosion due to sulfidation or molten salts, and steam oxidation due to the atmosphere.

Specific uses of such materials include various boilers, gasifiers, heat exchangers, heating furnace tubes, reaction tubes, heat-resistant structural materials, and the like. However, since high-temperature strength is specified for these uses, it must also have this performance.

By the way, the OAj diffusion infiltration method, which is known as one of the surface treatment methods for improving the heat resistance of steel products, is effective in improving the heat resistance of the product being treated, partly because Aj itself has high heat resistance. It is expected that this will be an effective method for improvement. However, conventional AI diffusion infiltration treatment
It was carried out at a temperature between 1000° C. and used as is after the diffusion infiltration treatment. Therefore, the AI diffusion treated material had extremely high diffusion layer surface hardness and poor workability. Therefore, at present, the AI diffusion infiltration method has not been used for materials used in applications such as boiler tubes that require high-temperature strength or workability.

Focusing on the excellent features of AI diffusion and penetration treatment, the present inventors conducted various studies with the aim of applying it to lira tube materials, etc., and found that the Aj diffusion infiltration treatment at 1250°C followed by post-heat treatment at 1000-1250°C reduces surface hardness and improves workability.
The present invention was completed by discovering that high-temperature strength can also be sufficiently ensured. Therefore, the gist of the present invention is to apply Aj diffusion permeation to a treated product, preferably a treated product made of a high-alloy material of stainless steel or higher. Processing from 1000 to
After forming an Aj-enriched layer with a thickness of 10 μm or more at 1250°C, a post-heat treatment is performed at 1000 to 1250°C to reduce surface hardness and improve workability.
The AI diffusion infiltration method also ensures sufficient high-temperature strength.

That is, according to the present invention, the diffusion infiltration treatment is performed until an AI-enriched layer with a thickness of 10^m or more is obtained, but if the thickness of the AI-enriched layer is less than 10 μm, the desired sufficient corrosion resistance cannot be obtained. do not have. Furthermore, if the processing temperature is less than 1000° C., it takes a long time to obtain an AI-enriched layer of the required thickness, and the quality of the material inevitably deteriorates. On the other hand, if the treatment is performed at a temperature exceeding 1250°C, the diffusion treatment layer becomes extremely thick and porous, so that sufficient workability cannot be obtained even after post-heat treatment. 1 refers to a processed product made of steel, Fe-Ni-Cr alloy, or Ni-Cr alloy, and when used in applications requiring particularly high high-temperature strength, it is a high-alloy material (higher than stainless steel). For example, TP 347 H%Al toy 800) is preferred, but is not limited thereto.

AI diffusion infiltration treatment can be performed using similar treatment operations to conventional methods, but is not limited thereto. For example, the post-heat treatment can be carried out by a powder pack method using a powder mixture in which a small amount (e.g., 1 to 2%) of ammonium chloride is added to a homogeneous mixture of AI or Fe-Al alloy powder and alumina powder. After the AI diffusion treatment, the product to be treated is heated to 1000 to 1250° C. in a non-oxidizing atmosphere, either once cooled to room temperature or continuously. This temperature is generally held for about 3 minutes, followed by water cooling to complete the post-heat treatment. If the temperature of the post-heat treatment is less than 1000°C, the surface hardness of the Aj-enriched layer will not be sufficiently reduced;
When post-heat treatment is performed at a temperature exceeding 0°C, the Aj of the Aj-enriched layer
The concentration decreases rapidly, making it impossible to ensure sufficient high-temperature corrosion resistance.

Next, the present invention will be further explained by examples.

EXAMPLES Test pieces of each alloy shown in Table 1 were subjected to AJ diffusion and penetration treatment in the manner shown in Table 1, and surface hardness measurements and flatness tests were performed on the treated test pieces obtained. The results are summarized in Table 2◎ (Note) *Aj diffusion and penetration treatment conditions: Method: Powder pack treatment Raw materials: Aj: Aj, 0. = 30X:
70X (1 quantity ratio) Atmosphere gas 2 I (processing time in 1 air flow: 1 hour (Note)) El: Hardness at a position 20 μm from the surface *2:
Test results for pipe material with a diameter of 45 cm and a thickness of 6 cm - *4: Untreated ridge material

Claims (1)

[Claims] Table 1flK of product to be treated An Aj diffusion infiltration method characterized by diffusing and infiltrating A1 at 1000 to 1250°C to form an A4 enriched layer with a thickness of 10 μm or more, followed by post-heat treatment at 1000 to 1250°C.