Effect of Oxygen Content Upon the Microstructural and Mechanical Properties of Type 316L Austenitic Stainless Steel Manufactured by Hot Isostatic Pressing

Abstract

Although hot isostatic pressing (HIP) has been shown to demonstrate significant advances over more conventional manufacture routes, it is important to appreciate and quantify the detrimental effects of oxygen involvement during the HIP manufacture process on the microstructural and material properties of the resulting component. This paper quantifies the effects of oxygen content on the microstructure and Charpy impact properties of HIP'd austenitic stainless steel, through combination of detailed metallographic examination and mechanical testing on HIP'd Type 316L steel containing different concentrations (100 to 190 ppm) of oxygen. Micron-scale pores were visible in the microstructure of the HIP'd materials postmetallographic preparation, which result from the removal of nonmetallic oxide inclusions during metallographic preparation. The area fraction of the resulting pores is shown to correlate with the oxygen concentration which influences the Charpy impact toughness over the temperature range of 77 K to 573 K (À196°C to 300°C), and demonstrates the influence of oxygen involved during the HIP manufacture process on Charpy toughness. The same test procedures and microstructural analyses were performed on commercially available forged 316L. This showed comparatively fewer inclusions and exhibited higher Charpy impact toughness over the tested temperature range.

Effect of Temperature on the Fracture Toughness of Hot Isostatically Pressed 304L Stainless Steel

“…The reduction in toughness is expressed as a ratio between F304L and HIP304L at each respective test temperature. The data herein show that previous observations of Charpy impact toughness behavior [5][6][7][8] are translatable to J-R fracture toughness testing. HIP304L, containing 120 ppm oxygen in the bulk material, exhibits a reduction in fracture toughness (J 0.2BL ) by approximately 40 pct at ambient and 300°C.…”

“…However, the authors have recently shown that the fracture behavior is subtly different between equivalently graded HIP and forged austenitic stainless steel, with HIP 304L and 316L exhibiting a reduction in impact toughness [5,6] as well as HIP 304L exhibiting a reduction in J-integral fracture toughness at ambient temperature. [7,8] This difference in fracture behavior was attributed to the presence of a comparatively large volume fraction of non-metallic oxide inclusions in the HIP microstructure, which lower the energy required to cause fracture via an unzipping effect, whereby ductile void growth is unable to occur on the same scale as in forged stainless steel, resulting in premature microvoid coalescence with neighboring voids.…”

“…[5][6][7] This Charpy impact toughness behavior was also found to extend to J-R fracture toughness at ambient temperature [7,8] ; for completeness, these data have also been included here. The mechanism by which Charpy impact and fracture toughness is reduced, is governed by the volume fraction and distribution of non-metallic oxide inclusions that are present in the austenite microstructure.…”

“…This reduction in fracture toughness is more pronounced at À 140°C, where a reduction of approximately 50 pct is observed. The mechanism by which fracture toughness is reduced is the same mechanism by which oxygen concentration affects Charpy impact toughness of HIP stainless steel [5][6][7] : non-metallic oxide inclusions act as initiation sites for the nucleation, growth, and coalescence of voids during failure. It is interesting to note here that previous studies on Charpy behavior were unable to explicitly reveal quantitative data at elevated temperature, because Charpy test pieces did not fail completely as a result of excessive plastic deformation of the test pieces, [6,7] and because of this, HIP and Forged variants of stainless steel exhibited comparable Charpy toughness when tested at 300°C.…”

“…The mechanism by which fracture toughness is reduced is the same mechanism by which oxygen concentration affects Charpy impact toughness of HIP stainless steel [5][6][7] : non-metallic oxide inclusions act as initiation sites for the nucleation, growth, and coalescence of voids during failure. It is interesting to note here that previous studies on Charpy behavior were unable to explicitly reveal quantitative data at elevated temperature, because Charpy test pieces did not fail completely as a result of excessive plastic deformation of the test pieces, [6,7] and because of this, HIP and Forged variants of stainless steel exhibited comparable Charpy toughness when tested at 300°C. We have shown here, however, that under high-constraint testing conditions, where the plastic zone is localized to a relatively small region ahead of the crack tip, that a comparable level of reduction in toughness is observed at elevated temperature testing as it is at ambient and cryogenic temperature testing.…”