Influence of Mo content on susceptibility of medium-carbon martensitic high-strength steels to hydrogen embrittlement: single and double Q&T

On a global scale there is a tendency to substitute fossil fuels with cleaner and renewable sources of energy. In recent years, hydrogen appears to be a promising alternative to achieve energy transition. In this context, the new infrastructure for the hydrogen transportation and storage should be considered. Exposure of the high-strength steels to hydrogen can result in deterioration of the mechanical properties by the mechanisms of hydrogen embrittlement (HE). To prevent this phenomenon, the pipeline steels, e.g. martensitic steels can be modified by various means. One of the possible ways is to change the microstructural features by addition of carbide forming elements such as Mo and controlling the microstructure by heat treatment. This paper aims to investigate the role of different Mo content as well as different heat treatment procedures on susceptibility of two martensitic steels to HE. Hydrogen trapping behavior and permeation were investigated by means of electrochemical permeation test and thermal desorption spectroscopy (TDS). Slow Strain rate tests (SSRT) of electrochemically charged steels were performed to elucidate mechanical performance. The carbide distribution and microstructure of tested steels were observed using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results implied that change of heat treatment leads to the control of Mo carbides size and dispersion and positively effects on HE susceptibility. Tempered martensitic steel with higher Mo content and modified heat treatment was found to be less susceptible to HE.