A comparative study of microstructure and hydrogen embrittlement of selective laser melted and wrought 17–4 PH stainless steel

Abstract

In this study, the microstructure and hydrogen embrittlement of 17–4 PH stainless steel produced by Selective Laser Melting (SLM) were investigated. The microstructure of SLM-ed 17–4 PH stainless steel was found to be fully ferritic, in contrast to the wrought martensitic steel. This finding was correlated to the high cooling and heating rates of the SLM process that suppressed the austenite formation and retained the delta ferrite to room temperature. The SLM-ed steel shows grains elongated in the building direction and its grain size is higher than the prior austenitic grain size of the wrought steel. The two steels present nanoscale copper precipitation after ageing 4 h at 580 °C. The yield strength of the SLM-ed steel was found lower by only 10% with respect to the wrought steel. The hydrogen embrittlement was evaluated by performing slow strain rate tensile tests under cathodic charging after ageing 4 h at 580 °C. It was found that SLM-ed 17–4 PH steel was more susceptible to hydrogen embrittlement compared to its wrought counterpart. This was attributed to the difference in microstructures, more specifically grain size. The crack initiation and propagation was much easier in the ferritic SLM-ed steel than in the martensitic wrought steel because of the higher grain size. The fracture in both steels was due to a significant subcritical crack growth followed by fast overload fracture of the remaining ligament. The fracture surface of the wrought steel showed a brittle intergranular fracture mode close to the surface and a ductile mode at the center. The brittle intergranular fracture mode was associated with the slow subcritical crack growth, while the ductile mode was due to the final fast overload fracture. On the other hand, in the SLM-ed steel, both the subcritical crack growth and the final fast overload fracture were obtained by transgranular cleavage. This shows that under hydrogen the martensitic wrought steel is prone to brittle intergranular fracture in contrast to the ferritic SLM-ed steel which is subject to brittle transgranular cleavage. The same tendency is obtained under air when notched specimens are used. This propensity of the martensitic steel to fracture along prior austenite grain boundaries can be interpreted in terms of the easiest fracture path.