A Microstructural Study on the Observed Differences in Charpy Impact Behavior Between Hot Isostatically Pressed and Forg
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HOT Isostatic Pressing, or ‘HIPing’, is an increasingly attractive approach for manufacture, by which high quality metal powder of required chemistry is poured into a form, and subjected to elevated temperatures [typically 1223 K to 1433 K (950 C to 1160 C)][1] and isostatically controlled pressures (100 MPa)[1] under an inert atmosphere.[2] The merits of HIP’d steel over conventional steel are well documented[1,3–7]; HIP’d materials typically display an increased yield strength, ultimate tensile strength, and enhanced ductility over their forged counterparts.[4] The HIP’d materials typically exhibit a finer grain structure than conventionally forged materials.[8] The application of high pressure and an inert gas atmosphere allows the sintering of internal voids[9] and significantly, the isostatically applied pressure ensures the grain microstructure is isotropic in geometry and in ADAM J. COOPER, Postdoctoral Research Associate, is with the School of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, UK Contact e-mail: adam.cooper@manchester. ac.uk NORMAN I. COOPER, Head of Materials Technology, and ANDREW BELL, Principle Metallurgist, are with BAE Systems, Bridge Road, Barrow-in-Furness, LA14 1AF, UK. ANDREW H. SHERRY, Chief Scientist, is with the National Nuclear Laboratory, Chadwick House, Birchwood Park, Warrington, WA3 6AE, UK. JEAN DHERS, Manager of Areva Research and Development, is with the AREVA European R&D, Areva, France. Manuscript submitted May 27, 2015. Article published online September 3, 2015 5126—VOLUME 46A, NOVEMBER 2015
properties, exhibiting no texture or anisotropy as observed in some forgings and in rolled plates. This fine and isotropic grain structure is deemed the fundamental reasoning behind the observed enhanced mechanical properties associated with HIP’d materials.[3] Furthermore, the finer grain structure induces a greater concentration of grain boundaries per unit volume, at which any inclusions and/or impurities are able to reside.[4] A significant advantage of HIPping over conventional manufacture routes, at least from an engineering perspective, lies in the potential for near-net shape manufacture[5,8,10] where it is possible to manufacture components with geometries of greater complexity than those achievable from forging. The design freedom is ultimately governed and limited by the ability to produce a capsule from which the HIPped component is molded. In addition, the finer grain size of the HIP’d material enhances non-destructive analysis, such as ultrasonic ‘time of flight’. Finally, HIP’d NNS components can be manufactured using as much as 80 to 90 pct of the original starting material, in contrast to a comparatively poor throughput of 10 to 30 pct from forged materials, where a large proportion of material is unrecovered during machining.[9] The efficiency of material use and the potential to reduce the number of welds by HIP has the potential to both drive down manufacturing costs and improve service life for engineering components such as those METALLU
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