Microstructure evolution and martensitic transformation behaviors of 9Cr-1.8W-0.3Mo ferritic heat-resistant steel during
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un Ning School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, People’s Republic of China
Zhixia Qiao School of Mechanical Engineering, Tianjin University of Commerce, Tianjin 300134, People’s Republic of China (Received 11 May 2013; accepted 30 August 2013)
The advanced quenching and partitioning (Q&P) heat treatment has been applied to 9Cr–1.8W–0.3Mo heat resistant steel. The phase transformation during Q&P is measured by a high-resolution differential dilatometer by which the accurate information can be obtained. The transmission electron microscope examination was conducted to study the microstructure evolution after Q&P, and the refined carbon-enriched martensite laths, which were produced during the second martensitic transformation, were observed. The thermodynamics of carbon partitioning was described by a paraequilibrium model according to which the partitioning of carbon from martensite into austenite can be proved. A kinetic model for the second martensitic transformation was developed with the parameters discussed in details. The retardation of onset and end temperature of the second martensitic transformation can be ascribed to the austenite stabilization caused by carbon enrichment.
I. INTRODUCTION
9–12%Cr ferritic heat resistant steels have been considered as the main materials for power plants operating under supercritical or ultra-supercritical conditions due to their good high-temperature endurance, creep resistance properties, excellent heat conductivity, low thermal expansion coefficient, and high performance–cost ratio.1–3 For these steels, quenching and tempering treatment are usually used during which tempered martensite without residual stress can be achieved,4 and the dispersive carbides or carbonitrides play a significant role in strengthening phase.5,6 On account of new environmental regulations, energy saving requirements, and commercial demands, it is necessary to elevate the working temperature of power plants to improve thermal efficiency.7 Thus, developing an advanced heat treatment is an alternative approach to meet these requirements. The new concept of quenching and partitioning (Q&P) heat treatment initially proposed by Edmonds et al.8 consists of two steps: first, the steel is quenched from full austenite microstructure to a temperature [quenching tema)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.267 J. Mater. Res., Vol. 28, No. 20, Oct 28, 2013
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perature (QT)] between the starting temperature of martensitic transformation (Ms) and the finishing temperature of martensitic transformation (Mf) so that a certain amount of untransformed austenite remains within martensite; then, the steel is held at a constant temperature [partitioning temperature (PT)] that is either equal to the quenched temperature (a case called one-step Q&P) or higher than the quenched temperature (a case called two-step Q&P) 9 for carbon atoms to migrate from as-quenched mar
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