The Study On The Microstructure And Properties Of 9-12%Cr Ferritic/martensitic Heat Resistant Steels

Microstructure and properties of five 9-12%Cr ferritic/martensitic heat resistant steels have been studied by using the scanning electron microscope (SEM) equipped with energy dispersive spectrum analysis (EDS), transmission electron microscope (TEM), X-ray diffraction (XRD) and tensile creep test in this paper. The conclusions are made as follows:After normalizing at 1100℃for 1h and then tempering at 750℃/700℃for 1h, the microstructure of all steels studied is lath martensitic. M₂₃C₆-type carbide precipitates along grain boundaries and lath boundaries while nano-sized MX carbonitride precipitates within lath. The amount of M₂₃C₆ carbide reduces with the decrease of carbon content. And there are few M₂₃C₆-type carbide particles in ultra-low carbon steels. Two kinds of nano-sized MX phase distribute densely and homogenously in the matrix of the ultra low carbon steels. The larger nano-sized MX precipitate with the size of 30～50nm is rich in Nb and Ti, while the smaller one with the size of about 10nm is rich in V. Small amount of M₃B₂-type boride are found in the steel with 1.5wt.%Mo. Large TiC particles with a size of about 2μm distribute in grain boundaries and dense nano-sized TiC particles in martensitic lath boundaries are found in the steel with 0.14wt.% Ti .Under condition of high stress at 650℃, high chromium steels with high density nano-sized MX precipitates have longer creep rupture life. However, creep rupture life increases lowly with the decrease of creep stress indicating that the strength degradation at high temperature is fast. Creep crack tends to initiate at the interfaces between coarse TiC particles and the matrix and causes a decrease in the creep property of the steel with 0.14wt.% Ti.During long time aging at 650℃after normalizing at 1100℃for 1h and tempering at 700℃/750℃for 1h, MX phase has transformed into Z-phase, which has a tetragonal structure and is prone to coarsen. Z-phase formation occurs earlier in high nitrogen steels. The formation of Z-phase consumes the nano-sized MX carbonitrides because the composition of both MX carbonitrides and Z-phase are all consisted of the same elements of V and Nb, resulting in decrease of the strengthening effect of MX precipitates. During long time aging and creep deformation at 650℃, large Fe2Mo-type (in steels without W) and Fe₂W-type (in steel with W) Laves phase precipites along lath and grain boundaries. In steels with high tungsten, Laves phase precipitates gradually leading to the increase of dispersion strengthening effect and hardness of the steels before 500h aging. After 500h aging, the hardness of the steels decreases because of the coarsening of Laves phase and the formation of large Z-phase.