Mechanistic studies on type 300 stainless steels manufactured by Hot Isostatic Pressing: The impact of oxygen involvement on fracture behaviour

With near-net shape technology becoming an increasingly desirable route toward component manufacture due to its ability to create components with increasingly complex geometries, minimizing the number of potential welded joints, as well as reducing machining time and associated costs, it is important to demonstrate that components fabricated via Hot Isostatic Pressing (HIP) are able to perform to similar standards as those set by equivalent forged materials. Hot Isostatically Pressed (HIP'd) materials are typically accredited with displaying enhanced yield strengths, ultimate tensile strengths, and ductility over their forged counterparts. 

In this paper we explore the effects of oxygen, which resides in the austenite matrix during the HIP manufacture process, on a material's fracture toughness properties. We quantify the influence of different concentrations of oxygen on both the microstructural and mechanical properties of HIP'd 304L and 316L stainless steel, highlighting the benefit of reducing the oxygen as much as possible. Various mechanical tests have been performed on materials containing a range of oxygen contents (between 100 ppm and 200 ppm) and over a large temperature range, including J1C fracture toughness testing, instrumented Charpy, and tensile, and the mechanistic involvement of residual oxygen on the results is discussed. The effects of oxygen become more apparent at cryogenic temperatures, whereas the fracture behavior of HIP'd and forged variants of 304L and 316L at elevated temperatures appears to be comparable.