Influence of Mo carbides and two-stage tempering methodology on the susceptibility of medium carbon martensitic steel to hydrogen embrittlement

Abstract

The objective of the present study was to enhance the hydrogen embrittlement (HE) resistance of the quenched and tempered martensitic steel via the interplay of heat treatment variance and precipitation of nanosized carbides. For this purpose, one-stage tempering and two-stage tempering methodologies were implemented, and steel was alloyed with Mo to instigate the precipitation of Mo carbides. The results revealed that two-stage tempered steel exhibited superior resistance to HE, as a result of reduced dislocation density and higher quantity of Mo2C. To discern the role and trapping behaviour of Mo2C carbides, Thermal Desorption Spectroscopy (TDS) combined with electrochemical hydrogen charging was utilized. Precipitated nanosized Mo2C exhibited the ability to trap hydrogen. On the contrary, an increase of dislocations and higher diffusible hydrogen content in one-stage tempered steel promoted deterioration of mechanical properties which was investigated by Slow Strain Rate Test (SSRT) and fracture surface morphology analysis. In addition, the effective diffusion coefficient for one-stage tempered steel was lower, as dislocations served as additional trap sites.