(1) Destabilization treatment: destabilization treatment (also known as destabilization treatment) is a heat treatment process in which high chromium cast iron is discharged from the furnace and cooled to room temperature after holding at 800 ℃ – 1100 ℃ for 1 – 6h. The destabilization treatment is heated at a temperature higher than the A1 critical point to precipitate the secondary carbide from the austenitic matrix. The secondary carbonization precipitation will reduce the content of alloying elements (C, Cr, Mn) in the original austenite, resulting in the increase of martensitic transformation point (MS), so as to transform the austenite to martensite during cooling and obtain martensitic matrix structure. With the increase of Cr content, the precipitation order of secondary carbides is to precipitate m3c carbides first, then M7C3 carbides, and finally M23C6 carbides. The morphology varies from rod M7C3 and m3c to fibrous M23C6. It is found that the hardness of high chromium cast iron will first increase and then decrease with the extension of holding time after holding high chromium cast iron at 1000 ℃ for 0.5h-2h water quenching treatment.
(2) Subcritical treatment: subcritical treatment refers to the heat treatment process in which high chromium cast iron is insulated at 300 ℃ – 700 ℃ (i.e. lower than A1 point) for 2-6h and cooled to room temperature. Subcritical treatment is to reduce the content of austenite (original or residual austenite) and obtain martensitic matrix structure by forming secondary carbides. Compared with destabilization treatment, subcritical treatment can avoid gradual deformation and fracture and make the precipitated carbide finer due to the lower heat treatment temperature. However, direct subcritical treatment can not significantly improve the hardness of cast iron. Therefore, in order to avoid deformation and crack of cast iron and obtain high hardness cast iron, the heat treatment process of subcritical treatment + destabilization treatment is often used to obtain excellent mechanical properties.
(3) Cryogenic treatment: cryogenic treatment can also be called ultra-low temperature treatment. It is generally a process of quenching the workpiece into liquid nitrogen for heat preservation for a period of time before it is cooled to the greenhouse. Cryogenic treatment can improve the toughness and wear resistance of high chromium cast iron. This is because cryogenic treatment will cause the lattice shrinkage of low-temperature martensitic iron and increase the driving force of carbon precipitation. After cryogenic treatment, the sample needs to continue tempering and raise the temperature. In this process, carbon atoms will gather at the defects in martensite, and finally precipitate in the form of ultrafine carbides. The segregation of vacancy and carbon atoms becomes the boundary of the new sub unit of martensite, which further refines the martensite structure. In the process of cryogenic treatment, under the condition of thermodynamics, the residual austenite in the microstructure will continue to transform to martensite. When the transformation temperature exceeds the end temperature MF of martensite transformation, the transformation of residual austenite to martensite will stop. Therefore, a large amount of retained austenite in the structure of high chromium cast iron obtained after quenching treatment will continue to undergo phase transformation at the cryogenic temperature of – 196 ℃ and transform into martensite structure with higher hardness, so as to further improve the hardness and wear resistance of martensite matrix, so as to improve the properties of high chromium cast iron.