Effect of hydrogen on mechanical properties and fracture of martensitic carbon steel under quenched and tempered conditions

The change in the mechanical properties of tempered high strength carbon steel (AISI 4130) due to hydrogen embrittlement (HE) has been investigated using slow strain rate tensile tests (SSRT). The heat treatment processes consisted of austenitization at 873 °C for about 45 min before quenching in salt bath and tempering at temperatures between 350 °C to 550 °C for about 1 h. The SSRT tests were performed on both hydrogen pre-charged and uncharged samples. Vickers hardness test was carried out to study the variation in hardness with the tempering temperature. Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) were performed for Microstructural characterizations and fracture surface analysis. The fracture surface analysis shows a transition of failure mechanism from brittle intergranular cracking at the lowest tempering temperature to ductile micro void coalescence at the highest temperature. TEM analysis reveals formation of carbide precipitates of the samples which were tempered. The AISI 4130 was subjected to HE for hydrogen charging when the tempering temperatures were 430 °C and lower, corresponding to a hardness value of 450 HV and greater.