Effect of heat treatment on microstructure and hydrogen embrittlement of additively manufactured and cast 18Ni300 maraging steel

The microstructure is significantly different between cast and additively manufactured 18Ni300 maraging steel due to the different manufacturing principles. The resulting differences in hydrogen embrittlement susceptibility need to be clarified. Electron back scatter diffraction and hydrogen pre-charged tensile test were used to carry out the comparative studies. The results demonstrate that the average grain size of the additively manufactured steel is significantly smaller and the dislocation density is higher than the cast 18Ni300 maraging steel. After heat treatment, the grain size of the additively manufactured sample becomes larger, meanwhile the dislocation density and the fraction of low-angle grain boundaries decrease. The strength and plasticity of additively manufactured steel are superior to those of cast steel due to the microstructure characteristics of fine grain, high dislocation density and good element uniformity. With the increase of hydrogen charging current density, the hydrogen embrittlement fracture tendency of cast and additively manufactured 18Ni300 steel increases significantly. Solution treatment has a significant effect on improving the hydrogen embrittlement resistance of 18Ni300 steel. After further aging treatment, the hydrogen embrittlement resistance for both cast and additively manufactured sample becomes very poor, which reflects the close correlation between the hydrogen embrittlement susceptibility and strength.