Aging refers to the heat treatment process when the tempering temperature is lower than 100 ℃. The newly formed martensite of high chromium cast iron is an unstable ordered structure (hexagonal structure), which will decompose into cubic structure during aging. The carbon atoms in the martensite of high chromium cast iron diffuse and migrate again due to unstable decomposition, forming high-carbon region and low-carbon region, resulting in ε- /η- Fe2C carbide precipitates.
In recent years, most high chromium cast iron martensitic industrial steels will face the redistribution of carbon atoms in martensite at room temperature. This is because martensite of high chromium cast iron is formed through non diffusion transformation during austenite quenching or machining. For Fe-C alloy, the solubility of C in austenite of high chromium cast iron is 100 times that of martensite. Therefore, the newly formed martensite, especially at the moment of martensite formation, is in a supersaturated state. At this time, C in martensite of high chromium cast iron can diffuse and migrate, and aggregate to defects (such as dislocation and interface) due to instability decomposition to form clusters or precipitate carbides. Lu et al. Found that after natural aging treatment of fe-15ni-1c alloy, η- Fe2C carbide precipitates from martensite. But there was no research η- Effect of Fe2C carbide precipitation on properties such as hardness of fe-15ni-1c alloy.
The experiment shows that the matrix structure of high chromium cast iron is martensite after destabilization treatment at 900 ℃ – 1000 ℃. This microstructure provides an experimental basis for studying the microstructure changes of high chromium cast iron at room temperature. However, few people have studied the change of microstructure of high chromium cast iron at room temperature after destabilization treatment. Therefore, in this chapter, we mainly study the effect of natural aging on the microstructure of high chromium cast iron, and further study the effect of microstructure change on hardness.
The effect of natural aging on the matrix structure and hardness of high chromium cast iron was studied. The results show that the martensitic matrix will precipitate after natural aging treatment of quenched high chromium cast iron without tempering η- M2C carbide, resulting in the increase of macro hardness of high chromium cast iron by 2 ~ 3 HRC and the decrease of micro hardness of matrix structure. The exploratory test shows that the macro hardness of high chromium cast iron reaches the highest, 66.4 HRC, after water quenching at 950 ℃ and artificial aging treatment at 80 ℃ for 3h.