Microstructures and High-Temperature Mechanical Properties of a Martensitic Heat-Resistant Stainless Steel 403Nb Process
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TRODUCTION
THE requirement for improving the thermal efficiency of fossil-fired power plants is strongly driven by new environmental regulations, energy-saving requirements, and commercial demands.[1] Utilization of heatresistant steels with high performance will undoubtedly play the most important role in increasing the thermal efficiency of fossil-fired power plants. Generally, there are two ways for obtaining high performance heatresistant steels; one way is to use new steel grades with superior mechanical properties at elevated temperatures by multi-elemental alloying to increase the working temperature such that the thermal efficiency could be increased, while the other is achieved by utilizing traditional steels available via further deliberate control of the microstructures without changing their chemical compositions which could be realized by special processing techniques. Among several types of heat-resistant steels, martensitic steels containing 9 to 12 pct Cr (in wt pct) are widely used as structural materials in the LIQING CHEN, FUXIAN ZHU, and XIANGHUA LIU, Professors, and YANG ZHAO, Post-Doctorial Researcher, are with the State Key Laboratory of Rolling and Automation, Northeastern University, 3-11 Wenhua Road, Shenyang 110819, P.R. China. Contact e-mail: [email protected] ZHOUYU ZENG, Scientific Researcher, is with the Research Institute, Nanjing Iron & Steel Co., Ltd., Nanjing 210035, P.R. China. Manuscript submitted January 27, 2013. Article published online November 7, 2013 1498—VOLUME 45A, MARCH 2014
fossil-fired power industry, e.g., main steam pipes or rotors of turbine, owing to their good corrosion and oxidation resistance as well as excellent anti-creep properties at elevated temperatures.[2–4] Also, they are candidates to replace austenitic stainless steels because they show a lower coefficient of thermal expansion and a higher thermal conductivity than austenitic stainless steels.[4] In processing 9 to 12 pct Cr martensitic steels, the subsequent heat treatment is usually subjected to normalizing and tempering prior to service in order to obtain a tempered lath martensitic microstructure consisting of lath and block martensite, where a high density of dislocations and fine precipitated carbides and/or carbonitrides exist along the lath, block boundaries, and within the matrix.[5] It is now clear that there are four main types of strengthening mechanisms in these steels, i.e., solid solution hardening, precipitation or dispersion hardening, dislocation hardening, and boundary or sub-grain boundary hardening.[6] Among these, dislocation hardening and sub-grain boundary hardening have the most dominant effect on the hightemperature creep behavior of 9 to 12 pct Cr steels.[7] During creep, however, dislocation density is easily lowered and sub-grains are sharply coarsened, which will seriously deteriorate the creep strength.[8,9] In order to improve the creep strength, it is usually required that the precipitates of MX carbonitrides and M23C6 carbides be dispersed along boundaries giving rise to excel
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