Microstructural Stability During Creep Exposure of 9Cr-1Mo Steel Treated at Different Normalization Temperatures
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Microstructural Stability During Creep Exposure of 9Cr-1Mo Steel Treated at Different Normalization Temperatures Arya Chatterjee
, P. Modak, K. Barat, D. Chakrabarti, and R. Mitra
(Submitted September 26, 2018; in revised form April 6, 2019) A modified 9Cr-1Mo steel has been exposed to three separate austenitization temperatures, i.e., at 950, 1025, and 1100 °C for normalization. After subsequent tempering at 750 °C, the normalized and tempered samples were creep-tested at temperatures of 550, 600, and 650 °C. The creep strength of the investigated samples was evaluated in terms of minimum creep strain rate and time to rupture. The effects of microstructure, precipitate and boundary misorientation on the creep behavior of the samples have been studied with TEM and EBSD analyses. Further, the evolution of crystallographic texture after creep tests has also been studied. The presence of an intermediate size of martensitic microstructural units (i.e., prioraustenite grain, martensitic packets, etc.) and combination of fine coherent and incoherent Nb(C,N) precipitates has provided superior creep strength for the samples normalized at 1025 °C, when these are subsequently subjected to low-temperature (i.e., 550 °C) and high-temperature (i.e., 650 °C) creep tests, as compared to other conditions of normalizing heat treatment.
Keywords
creep strength, martensitic microstructure, modified 9Cr-1Mo steel, precipitates, texture evolution
1. Introduction Modified 9Cr-1Mo steel, having tempered martensitic structure and containing microalloying elements such as V and Nb, is widely used in ultra-supercritical power plants or in fuel tubes of fast-breeder reactors, where the material is subjected to elevated temperature (up to600 °C) for long duration (Ref 1). Precipitation of fine and stable MX-type microalloy carbides or carbonitrides along the martensitic lath boundaries and also within the laths during the tempering treatment provides creep resistance to this air-hardened steel (Ref 2, 3). The MX precipitates prevent lath coarsening and thereby resist structural softening upon prolonged thermal exposure (Ref 4, 5). Previous studies on creep behavior of Cr-Mo steels extensively dealt with the mechanisms of creep deformation depending upon different operating temperature and stress regimes (Ref 6-17). Evolution and stability of martensitic microstructure (Ref 18, 19), different types of carbide particles (M23C6 or MX) (Ref 20-23), the intermetallic phases (like Laves phase) (Ref 17, 24-26) and the nature and distribution of Arya Chatterjee, School of Engineering, Brown University, Providence, RI 02912, USA; P. Modak, D. Chakrabarti, and R. Mitra, Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India; and K. Barat, National Aerospace Laboratories (NAL-CSIR), Bangalore, 560017, India. Contact e-mails: [email protected] and [email protected].
3076—Volume 28(5) May 2019
grain boundaries (Ref 27) were
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