Microstructure and Properties of Fe-2Cr-Mo-0.12C (Wt Pct)-Tempered Steel Plate at Different Normalizing Temperature Afte
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INTRODUCTION
FE-2CR-MO-0.12C (wt pct) steel has been considered as a candidate material for pressure vessels and superheater tubes because of its excellent high-temperature strength, good weldability, and good resistance to creep.[1,2] The steel derives its strength from tempered bainite, which is stabilized by M23C6 and M7C3[3,4] carbides and a bainite phase transformation that induces high dislocation density. After long-term service aging, the M6C becomes the predominant carbide at the expense of other carbides and is the main carbide for precipitation strengthening.[5] Stable precipitates such as M23C6, M7C3 and M6C play an important role in the creep resistance and toughness of Cr-Mo pressure vessel steel.
CHANGSHENG LI, and XINGYANG TU are with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China. Contact e-mail: [email protected] QIWEN WANG is with the Ansteel Beijing Research Institute, Ansteel Group, Beijing, 102211, China. Contact e-mail: [email protected] SONGBO HUO is with the Nanjing Iron and Steel Co., Ltd., Nanjing 210035, China. Manuscript submitted October 14, 2019; accepted on October 5, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Research on the evolution process of microstructures in Fe-2Cr-Mo-0.12C (wt pct) steel during heat treatment has drawn great attention; however, most of the work has focused on the tempering[6] and high-temperature failure.[7] However, the slow cooling rate after austenitizing heat treatment may induce the pro-eutectoid ferrite, which significantly influences the mechanical properties. The formation of pro-eutectoid ferrite could be inhibited by improving the hardenability of steel through higher normalization temperature. Thomson[8] reported that M6C changes at a slower rate in a mixed microstructure of bainite and ferrite than that in a full bainite microstructure. Kucharova[9] reported that the coarsening of precipitates and the change in morphologies were the main causes of the degradation of creep strength when exposed to service temperatures. Generally, Fe-2Cr-Mo-0.12C (wt pct) bainite steel is used under normalized and tempered conditions. The heat treatment process has a significant effect on the microstructure type and precipitation behavior. The major purpose of normalization is to make the microstructure homogeneous and coarse precipitates re-dissolve in hot rolling. The major purpose of tempering is to make the carbides, such as M2C, M7C3, and M23C6, to improve the high-temperature strengthening and creep properties by precipitation strength.[10–13] To dissolve more alloy elements such as manganese,
chromium, and molybdenum, the normalizing temperature should be elevated, and then the precipitates can be re-precipitated adequately.[14] However, elevating the normalizing temperature can lead to the coarsening of austenite grains, resulting in a lower toughness[15] Moreover, the decrease in the strength and toughness of the steel with service time has been attributed to a decrease in dislocation density an
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