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HOT isostatic pressing (HIP) is a component manufacturing technique that employs the use of high temperature and isostatically controlled pressure to consolidate metal alloy powder of desired chemistry into bulk metal under an inert (usually argon) atmosphere.[1–3] A good analogy to this is when a child forms a snowball with their hands.[4] The most significant advantage of HIP over more conventional forging and casting lies within its ability to produce components with increasingly complex geometries, commonly referred to as near-net shape, since the consolidation of metal powder occurs within a prefabricated vessel, the shape of which governs the geometry of the HIP’d component.[5,6] This increased design freedom allows the direct production of, for example, pipes with several T-piece junctions, nozzles, and elbows, without the need for additional machining or welding of subsequent components. The potential to reduce welded joints from component manufacture is beneficial on several levels: the ability to avoid welding issues such as the risk of hot cracking, problems associated with having several A.J. COOPER and R.J. SMITH are with the School of Materials, University of Manchester, Manchester M13 9PL, United Kingdom. Contact email: [email protected] A.H. SHERRY is with the School of Materials, University of Manchester, and also with the National Nuclear Laboratory, Warrington WA3 6AE, United Kingdom. Manuscript submitted August 24, 2016. Article published online February 24, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A

metallurgical zones (fusion line, heat-affected zone) in a single component, and weld residual stresses, which can provide the driving force for crack growth. Not having to perform the welding procedure in the first place also reduces the manufacturing time and, therefore, costs. In addition, the homogeneous and isotropic microstructure and smaller grain size produced by HIP lends itself better to nondestructive examination techniques, since the smaller grains interfere less with the ultrasonic inspection.[7] In terms of mechanical properties, HIP typically produces material with a higher yield stress and ultimate tensile strength compared to equivalently graded forged material, which is generally attributed to the smaller grain size and isotropic nature of the HIP microstructure.[7] However, the authors recently showed that HIP’d steels exhibit a slight reduction in impact toughness over a substantial temperature range.[8–10] This has been attributed to the observation that HIP’d austenitic steel typically contains oxygen concentrations over an order of magnitude higher than those of ‘‘chemically equivalent’’ graded forged material. This significant increase in oxygen is thought to arise as a result of powder surface oxidation during one, or several, of the many HIP stages, beginning with gas atomization of the initial powder, during handling and storage of the metal powder, to filling of the canister. Despite the HIP procedure being performed under an inert atmosphere, the several stage