Isothermal quenching and boronizing isothermal quenching of gray cast iron

Gray cast iron is an Fe-C-Si alloy containing graphite phase in pulse distribution. The annual output of castings in the world reaches 60 million tons, of which about 70% are gray cast iron. This material has been widely used because its production cost is lower than that of steel. The lower production cost comes from its lower heat treatment temperature, low melting point and good fluidity. In addition, gray cast iron also has the advantages of organic processability and good shock absorption.

In order to expand the application range of gray cast iron, isothermal quenching can be used to improve its mechanical properties and wear resistance. This process is carried out in two steps. Firstly, the material is heated to 850 ~ 950 ℃ and fully insulated to change its structure into austenite with saturated carbon. The second step is to rapidly cool the material to 230 ~ 400 ℃ and hold it for a certain time to obtain the structure of acicular ferrite and high carbon retained austenite, that is, the so-called austenitic ferrite. Hsu et al. Found that this structure can also improve the fatigue strength of gray cast iron.

The wear resistance and / or corrosion resistance of gray cast iron can further expand its potential application range. Therefore, boronizing treatment of gray cast iron is proposed. Compared with other strengthening treatment technologies, the boronized layer has higher hardness, lower friction coefficient and better corrosion resistance.

Liquid boronizing can form metal boride layers on various surfaces by diffusion and deposition of boron atoms in high temperature salt bath. In this process, the material is usually heated in a molten salt bath at 700 ~ 1000 ℃ for 1 ~ 8 h. In this way, boron can diffuse into the surface of the treated material, so as to obtain 40 ~ 270 μ M thick boronized layer.

According to the diffusion potential of boron, two phases of Feb and or Fe2B may be formed on the surface of the casting. The boron potential is low and only Fe2B layer is formed; When the boron potential is high, a mixture of these two phases is formed, which is a bad structure. Because the Feb layer is very hard, it will weaken the interface between the infiltrated layer and the matrix. Therefore, in industrial applications, it is desirable to obtain a single Fe2B layer, which can be achieved by controlling the boronizing time and temperature. The purpose of heat treatment and chemical heat treatment of gray cast iron is to improve its mechanical properties and expand its application range.

By analyzing the microstructure of isothermal quenched samples, it is found that the austenite ferrite obtained at lower isothermal temperature (240 ℃) is also fine, resulting in higher hardness and adhesion wear resistance than those quenched at 300 ℃ and 360 ℃. The surface layer of the boronized sample has high hardness, so it also has good adhesion and wear resistance. The hardness and adhesion wear resistance obtained by Boronizing for 2 h and 4 h are the same. However, the boronizing time is long and the boronizing layer is thick.