Hydrogen Embrittlement Characteristics of Low Carbon Steels After Heat and Surface Treatments

The hydrogen embrittlement (HE) characteristics of the low carbon steels: Fe-xC-1.5Mn-0.03Si (0.05C, 0.1C, and 0.15C) with ultimate tensile strength (UTS) of ~ 1200 MPa, were examined experimentally after heating and surface treatment processes. For this investigation, the low carbon steels with different microstructural characteristics, including phase structures, precipitates, and internal strain, were employed. The high internal strain was seen in the Fe-0.15C steels after cold rolling (CR), water quenching (WQ), and bake hardening (BH) process, while the low strain was obtained in the steel samples after the annealing process. The high strain for CR and WQ (and BH) is caused by severe plastic deformation and the phase transformation of the martensite structure, respectively. Carbide particles were precipitated in the WQ and BH samples, and the amount of carbide was correlated with their carbon content. HE occurred only in the CR-0.15C and WQ-0.15C samples although the low UTS of ~ 1200 MPa was used, where the fracture strain decreased significantly. HE was attributed to the high internal strain and high carbon content which led to the hydrogen trapping site. A multiple regression analysis was also carried out to estimate the extent of HE of the low carbon steels.