ESSC & DUPLEX 2019

Additive manufacturing (AM) of austenitic stainless steel has been shown to be capable of producing samples with a unique combination of high strength and ductility. This increase in yield strength without the conventional ductility trade-off is usually attributed to the non-equilibrium, hierarchical microstructure produced by AM, particularly the cellular subgrain structures and dislocation network1. However, little attention is paid to the influence of the nitrogen content, which is known to generally increase the strength without severely reducing the ductility. The austenitic stainless steel powders for AM are commonly gas-atomised in nitrogen as this is an economical method for producing powders with a high degree of sphericality. Furthermore, most AM processes use nitrogen as a protective (but not inert) atmosphere during the printing of stainless steel. Both the atomisation and AM processes therefore have the potential for adding nitrogen to stainless steel, which is measured in the powder as well as the printed parts. This research focuses on characterisation of the hierarchical nature of AM-manufactured austenitic stainless steel using electron microscopy, electron backscatter diffraction, X-ray diffraction and hardness measurement. Special attention is paid to the nitrogen content and its influence on the microstructure and mechanical performance. The enhanced nitrogen content is also taken into account when investigating the effect of various heat treatments and their effect on microstructure evolution. Conventionally, austenitisation is carried out in a hydrogen atmosphere, which can result in a loss of nitrogen and associated beneficial properties. Instead, an “active” austenitisation with a low pressure of N2 is utilised in order to keep the enhanced nitrogen content from the as-printed material and retain the mechanical properties. Furthermore, the effect of hardening treatments such as solution nitriding and low temperature nitriding on the microstructure is investigated.