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

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sostatic pressing (HIP) is a component manufacturing technique, which employs the use of high temperature, and isostatically controlled pressure to consolidate metal alloy power of desired chemistry into bulk metal under an inert (usually argon) atmosphere.[1] The advantages of HIP are well documented,[1–4] the most significant being within HIP’s ability to produce near-net shape components; components with exceedingly complex geometries thus eliminating the need for subsequent machining/welding procedures on the manufactured component. This may not only reduced the costs associated with the overall manufacture process, but through the elimination of welded joints, produces components of homogenous metallurgy; omitting common issues associated with welding of components; hot cracking, different metallurgical zones, induced residual stresses, etc. This is clearly an advantage for components which will be subjected to high stress conditions throughout their lifetime. The degree of metallurgical

A.J. COOPER and W.J. BRAYSHAW are with the School of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, UK. Contact e-mail: [email protected] A.H. SHERRY is with the School of Materials, University of Manchester and also with National Nuclear Laboratory, Birchwood Park, Warrington, WA3 6AE, UK. Manuscript submitted October 13, 2017.


homogeneity which HIP produces results in no grain directionality, like that commonly seen in forgings, due to the isostatically controlled pressure and temperature and therefore HIP materials display isotropic mechanical properties. Finally, HIP produces material with a comparatively smaller grain size than that of forgings and castings, which not only improves the yield strength and ultimate tensile strength, also lends itself to easier inspection view non-destructive examination techniques. Because of HIP’s ability to increase design freedom, there have been increased efforts to demonstrate that components produced by HIP have equivalent or better material properties than those of equivalently graded forged materials. 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. Thus, it was shown that the impact toughness was governed by the concentration of oxygen remaining in the austenite matrix. The authors’ previously reported work has demonstrated that