Enhanced hydrogen embrittlement resistance of directed energy deposition 316 L by post-manufacturing heat treatment

This study investigates the hydrogen embrittlement resistance of directed energy deposition additive manufactured 316 L stainless steels with heat treatments. The hydrogen embrittlement sensitivity has been examined through microstructural investigation correlated with grain orientation and dislocation density. The synergistic effect of heat-treatment and hydrogen on strain distribution and instability evolution behavior was revealed. The heat treatment reduced the sub-grain structure and dislocation density, resulting in a more homogeneous microstructure with trace of recrystallization. Hydrogen uptake behavior was closely affected by the microstructure changes and thus hydrogen embrittlement sensitivity was reduced by increasing annealing temperature. AMed samples after heat treatment have a similar hydrogen embrittlement index with conventional manufactured samples. In the heat treated 1200 degrees C specimens, more strain and instability hotspots were initiated, becoming uniformly distributed and developed. In contrast, the AMed samples initiated fewer strain accumulation sites, which developed into stress/strain concentration areas, leading to wider secondary cracks and ultimate failure.