The chemical heat treatment of steel castings refers to placing the castings in an active medium at a certain temperature for heat preservation, so that one or several chemical elements can penetrate the surface. Chemical heat treatment can change the chemical composition, metallographic structure and mechanical properties of the surface of the casting. Commonly used chemical heat treatment processes include carburizing, nitriding, carbonitriding, boronizing and metalizing. When performing chemical heat treatment on castings, the shape, size, surface condition, and surface heat treatment of the casting should be considered comprehensively.
Carburizing refers to heating and insulating the casting in a carburizing medium, and then infiltrating carbon atoms into the surface. The main purpose of carburizing is to increase the carbon content on the surface of the casting, while forming a certain carbon content gradient in the casting. The carbon content of carburizing steel is generally 0.1%-0.25% to ensure that the core of the casting has sufficient toughness and strength.
The surface hardness of the carburized layer is generally 56HRC-63HRC. The metallographic structure of the carburized layer is fine needle martensite + a small amount of retained austenite and uniformly distributed granular carbides. Network carbides are not allowed, and the volume fraction of retained austenite generally does not exceed 15%-20%.
The core hardness of the casting after carburizing is generally 30HRC-45HRC. The core metallographic structure should be low-carbon martensite or lower bainite. It is not allowed to have massive or precipitated ferrite along the grain boundary.
In actual production, there are three common carburizing methods: solid carburizing, liquid carburizing and gas carburizing.
Nitriding refers to a heat treatment process that infiltrates nitrogen atoms into the surface of the casting. Nitriding is generally performed below Ac1 temperature, and its main purpose is to improve the hardness, wear resistance, fatigue strength, seizure resistance and atmospheric corrosion resistance of the casting surface. Nitriding of steel castings is generally carried out at 480°C-580°C. Castings containing aluminum, chromium, titanium, molybdenum, and tungsten, such as low alloy steel, stainless steel, and hot mold tool steel, are suitable for nitriding.
In order to ensure that the core of the casting has the necessary mechanical properties and metallographic structure, and to reduce the deformation after nitriding, pre-treatment before nitriding is required. For structural steel, quenching and tempering treatment is required before nitriding in order to obtain a uniform and fine tempered sorbite structure; for castings that are easily distorted during nitriding treatment, stress relief annealing treatment is also required after quenching and tempering; for stainless steel and Heat-resistant steel castings can generally be quenched and tempered to improve the structure and strength; for austenitic stainless steel, solution heat treatment can be used.
Post time: Jul-21-2021