Effect of Process Parameters on Microstructure and Hardness of Oxide Dispersion Strengthened 18Cr Ferritic Steel
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INTRODUCTION
OXIDE dispersion strengthened (ODS) steels are being considered as structural materials for steam and gas turbines by virtue of their excellent high-temperature creep resistance and as fuel clad tubes in fast breeder reactors due to low propensity for irradiation-induced void swelling.[1–10] In general, ODS steels are produced by mechanical milling of pre-alloyed steel powders with nano-yttria and subsequent consolidation by either hot isostatic pressing or hot upset forging of cans containing milled powders. Hot extrusion is invariably employed to achieve a product with high density which is free from prior particle boundaries. Many of these alloys for the
M. NAGINI, Senior Research Fellow, is with the International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad 500005, India, and also Ph.D. Scholar with the School of Engineering Sciences and Technology (SEST), University of Hyderabad, Gachibowli, Hyderabad 500046, India. R. VIJAY, Scientist-F and Team Leader, M. RAMAKRISHNA, Scientist-D, A.V. REDDY, Technical Adviser, and G. SUNDARARAJAN, Distinguished Emeritus Scientist, are with the International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI). Contact e-mail: [email protected] KOTESWARARAO V. RAJULAPATI, Assistant Professor, is with the School of Engineering Sciences and Technology (SEST), University of Hyderabad. K. BHANU SANKARA RAO, formerly Dean with the School of Engineering Sciences and Technology (SEST), University of Hyderabad, is now Ministry of Steel (Govt. of India) Chair Professor with the Mahatma Gandhi Institute of Technology, Hyderabad 500075, India. Manuscript submitted February 23, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
above-mentioned applications are based on Fe-(9-18 pct)Cr-2W-0.2Ti matrix with yttria as dispersoids. Addition of Ti to the yttria-containing steel has been reported to form uniformly distributed nano-sized complex oxides of the type Y2TiO5 or/and Y2Ti2O7 during hot consolidation.[1,8,11–16] These dispersoids act as obstacles to dislocation motion and grain boundary sliding imparting excellent high-temperature mechanical properties. Even though ODS-9Cr ferritic-martensitic steels exhibit nearly equiaxed grain structure, they suffer from corrosion and oxidation. The resistance to corrosion and oxidation of ODS steels can be enhanced significantly by increasing the Cr content from 9 to 18 pct without causing much concern from r phase embrittlement.[17] The microstructural evolution in ODS steels is very complex and is dependent on several variables employed during high-energy ball milling,[18,19] upset forging,[20,21] and hot extrusion.[22,23] The present study is aimed at developing a comprehensive understanding of evolving microstructure during various stages associated with manufacturing of ODS-18Cr steel. A special attention has been paid toward determining the effect of mechanical milling duration on the grain size and hardness, and size, shape, and structure of dispersoids in milled powders, u
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