In situ synthesis and strengthening of ultra high-carbon martensitic stainless steels in addition of LaB6

A powder metallurgy route which is characterized by the vacuum solid state sintering is utilized to fabricate Fe-20Cr-3C and Fe-20Cr-3C-2Mo-3V-2Co ultra high-carbon martensitic stainless steels. A sintering window from 1190 °C to 1210 °C is chosen to obtain high density samples and avoid liquid phase simultaneously. The densification process is promoted by solubility effect during formation of M7C3 and grain boundary diffusion. TEM analysis demonstrates three types of reaction products: the La(BO2)3 phase, the La2O3 phase and the La(Fe0.5V0.5)O3 phase, after adding 0.1 wt% LaB6. The impurity elements such as S and O are absorbed following the LaB6 addition to form non-gaseous products, which promotes densification by decreasing the sintering resistance and thus contributes to the improvement of bending strength and hardness. Both of the electrochemical and immersion results lead to the fact that the corrosion resistance of the steels are improved by 0.1 wt% LaB6 addition, in 3.5 wt% NaCl solution. However, the corrosion resistance of Fe-20Cr-3C-0.1LaB6 still can not compete with that of 316L stainless steel. Nevertheless, Fe-20Cr-3C-2Mo-3V-2Co and Fe-20Cr-3C-2Mo-3V-2Co-0.1LaB6 after quenching at high temperature present comparable corrosion resistance with that of 316L in 3.5 wt% NaCl solution.