By developing the appropriate combination of microstructure phases, cast iron can obtain the required properties in any specific application. These microstructure characteristics can be achieved by using the appropriate heat treatment of solid phase transformation. Common heat treatment processes, such as annealing, normalizing, hardening, tempering and surface treatment, can obtain microstructure characteristics with required properties to improve the mechanical and corrosion properties of the alloy.
The effect of cryogenic treatment on microstructure, hardness and wear resistance of 16cr1mo1cu cast iron after instability treatment was studied by Jun Wang. The results show that cryogenic treatment can effectively reduce the retained austenite after instability heat treatment, but it can not make the retained austenite transform completely. Cryogenic treatment can improve the hardness and wear resistance of high chromium cast iron. In the process of instability treatment and low temperature treatment, the amount of secondary carbide M23C6 precipitated is higher than that of air cooling. Compared with air cooling, the fine secondary carbides precipitated during cryogenic treatment after destabilization heat treatment are the main reasons for the increase of hardness and wear resistance.
The effect of high temperature heat treatment on the precipitation of M7C3 / M23C6 eutectic carbides was studied by K wieczerzak. It is found that the rapidly solidified hypoeutectic chromium alloy can obtain eutectic carbides with high thermal stability and high volume fraction. The crystal structure of carbides in the alloy changes from Bcc to FCC when the temperature is higher than 1100 ℃.
In chromium based alloys, the required strengthening carbide phase depends largely on the solidification conditions, so non-equilibrium solidification and metastable solidification may occur. Pearce and Elwell found evidence for the transformation of carbides from M7C3 to M23C6 in fe-31cr-2.4c alloy, which occurred in eutectic carbides during normal hardening process. A. In the alloy with 18 wt.% Cr, more than 3.5 wt.% C and 3 ~ 8 wt.% w, the formation of eutectic m3c is inhibited by the extremely fast cooling rate, resulting in the structure containing very fine austenite and mixture of M7C3 and M23C6, and the transformation of carbide type occurs, namely m3c → M7C3 → M23C6.