Effect of Microalloy Precipitates on the Microstructure and Texture of Hot-Deformed Modified 9Cr-1Mo Steel
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THE modified 9Cr-1Mo steel emerged as a suitable material for in-core applications in fast breeder reactors, owing to their high-void swelling resistance under irradiation backed by high-temperature strength, high thermal conductivity, and low thermal expansion.[1,2] Modified 9Cr-1Mo steel is made superior to conventional 9Cr-1Mo steel in terms of creep properties by controlled addition of V and Nb, which form finely dispersed and stable MX precipitates (M stands for Nb and/or V, and X represents C and/or N) during tempering treatment of air-hardened martensitic structure. Fine MX precipitates preferentially form along the martensitic lath boundaries and stabilize the lath structure. Thus, lath coarsening and the resultant structural softening upon prolonged thermal exposure are prevented, which improve the elevated temperature properties (especially creep and fatigue) of modified 9Cr-1Mo steel.[3–6] Now, in order to be used as either steam pipes for ultra-supercritical (USC) power plants or fuel tubes in fast breeder nuclear reactors, heat-resistant modified 9Cr-1Mo steel is processed through various hot-working techniques like forging, rolling, and extrusion. Therefore, several studies have been ARYA CHATTERJEE, A. DUTTA, MD BASIRUDDIN SK, R. MITRA, and D. CHAKRABARTI are with the Indian Institute of Technology Kharagpur, Kharagpur, WB, 721302, India. Contact e-mail: [email protected], [email protected] A.K. BHADURI is with the Indira Gandhi Centre for Atomic Research, Kalpakkam, TN, 603102, India. Manuscript submitted December 20, 2016. Article published online March 8, 2017 2410—VOLUME 48A, MAY 2017
carried out on the hot-deformation behavior of 9Cr-1Mo steel.[7–16] Hot-deformation simulation experiments have been conducted using different techniques such as, uniaxial compression, plane-strain compression, and hot-torsion.[7–9,13–16] The effect of processing parameters such as strain (e), strain rate (_e), and temperature (T) on the hot-flow behavior, microstructural softening mechanism (recovery, recrystallization etc.), microstructural instability, and defect generation has been studied.[9,13,15] Based on the results of such studies, deformation mechanism maps and constitutive models have been proposed for the representation of high-temperature deformation of both conventional and modified 9Cr-1Mo steel.[8,10,15] Earlier studies on hot deformation of modified 9Cr-1Mo steel, however, hardly investigated the following aspects:
Effect of microalloy precipitates especially Nb precipitates, on the microstructural refinement and softening. Effect of processing parameters and microalloy precipitates on the evolution of crystallographic texture. The present study is undertaken to address the above aspects. Equiaxed grain structure and microalloy precipitates are certainly expected to influence the elevated temperature properties such as tensile property, creep, and creep-fatigue interaction during the service condition of modified 9Cr-1Mo steel. However, this aspect is beyond the scope of the present ma
