Effects of Various Carbide Forming Elements on Hydrogen Embrittlement

Abstract

Precipitation of various carbides can strongly influence hydrogen-embrittlement (HE) phenomenon as well as to mechanical properties. This study reveals the role of Cr/Mo/V carbides on the hydrogen embrittlement in the tempered martensitic steel while eliminating other factors, such as chemical composition of other elements, mechanical strength, and dislocation density. In case of V carbide, the HE resistance was best at 0.2 wt.% V and decreased with the further V addition because of large size undissolved V carbides. V carbides of the plate-type acted as strong hydrogen-trapping sites, which generally enhanced HE resistance in cathodic charging environment. However, excessive V content led to formation of large undissolved carbides that gave rise to brittle fracture and decrease HE resistance. Cr and Mo carbides also trapped hydrogen, but their trapping ability is lower than V carbide because of their morphology and hydrogen affinity. In particular, Cr carbide cannot contribute to improve the HE resistance because of low hydrogen trapping ability and lager size than other two carbides. Mo-added steel, V-added steel and carbide free steels were conducted with the cyclic corrosion test (CCT). There was no the dramatic reduction of the notch fracture stress of these specimens in the CCT. However, internal hydrogen sharply increased in V-added steel as number of cycle increased. This is due to that most hydrogen trapped at plate-type V as stable state. This hydrogen did not influence on the notch fracture stress in CCT, but more research is needed with respect the hydrogen trapped at stable trapping sites such as plate-type V carbide. In conclusion, Mo carbide is most effective in improving the strength and hydrogen embrittlement resistance because of high NFS at same hydrogen content in pre-charged samples and low hydrogen penetration in CCT.