High Temperature Strengthening in 12Cr-W-Mo Steels by Controlling the Formation of Delta Ferrite
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INTRODUCTION
IN recent years, the persistent eruptible global energy consumption and environmental pollution have put forward an urgent requirement for high efficient utilization of the fossil energy. It has been certified that as the steam parameters are increased from 803 K (530 C)/ 18 MPa to the supercritical conditions of 873 K (600 C)/30 MPa, the efficiency of fuels will be increased from 30 to 45 pct, which will then result in the reduction of the emission of equivalent CO2 by 30 pct.[1,2] Therefore, it has become the mainstream for the future power stations to possess large capacity and to be run under the USC conditions. And then the revolution in steam parameters of the power plants inevitably requests an innovation of new heat-resistant steels (HRSs) with higher performance and more safety consideration.[3] The ferritic/martensitic type of steels with 9 to 12 pct Cr represents one of the important branches of HRSs. This kind of HRS was initially exploited in 1912 by Krupp and Mannesmann Ltd. in Germany,[4,5] and now have evolved into a series of compositions through alloying with 1 to 5 pct (W, Mo), ~0.1 pct C, V, Nb, Ta, B, and N, etc. Compared with 18Cr-8Ni type of austenitic steels, ferritic/martensitic HRSs are much less expensive, and exhibit higher thermal fatigue resistance, higher thermal conductivity, and lower expansion coefficients.[6,7] Moreover, this kind of steels also shows comprehensive mechanical properties when heat treated SHUSHEN WANG and DEYE LIN, Ph.D. Candidates, LI CHANG, Master, XIAOHUA CHEN, Assistant Researcher, and XIDONG HUI, Professor, are with the State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: xdhui@ ustb.edu.cn Manuscript submitted October 26, 2013. Article published online July 1, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
under the conditions of normalizing and high temperature tempering. Nowadays, the 9 to 12 pct Cr ferritic/ martensitic HRSs have become the popular steels for petrochemical and chemical plants, gas turbine engineering, aircraft and aerospace industries, electrical power plants, and nuclear fission and fusion reactor components.[8] T/P122 ferritic/martensitic HRSs, which mainly contains 11.2 pct Cr, 1.99 pct W, 0.56 pct Mo, 0.21V pct, 0.98Cu, 0.36Mn, 0.44Si, 0.11C, and Fe bal.,[9] were developed in 1990s by increasing the content of Cr and W, and adding Cu[10–13] as compared with T/P92 steel. The microstructure of T/P122 steels is composed of martensitic matrix, partial ferrite phase, and some precipitates. This kind of steels possesses better oxidation resistance and higher strength than the 9Cr type of HRSs, and has been applied for higher temperature components of the superheaters of the USC power plants.[13,14] The duration rupture strengths of T/P122 steel predicted at 873 K (600 C)/105 hours and 923 K (650 C)/105 hours reach about 120 and 60 MPa,[15,16] respectively. It has been found that after 3 years’ service, the microstructure, the amount of fer
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