Creep Rupture Strength and Microstructure of Low C-10Cr-2Mo Heat-Resisting Steels with V and Nb
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1. I N T R O D U C T I O N D U R I N G the 1950's and 1960's steam temperatures and pressures in large boilers increased to the point where in 1967 a boiler operating at 538 ~ and at a supercritical pressure of 24.1 MPa was installed. T o reach still higher temperatures and pressures, highgrade austenitic heat resisting steels had to be used in larger quantities. In view of these facts, Japan and other countries developed relatively economical steels that have higher elevated-temperature strength than the conventional 2 1 / 4 C r - l M o and 9Cr-lMo steels. 1-3 For boiler components such as superheater tubes which have supercritical-pressure and high temperatures requirements, high Cr content steel is specified to ensure sufficient oxidation resistance. In practice, austenitic stainless steels such as AISI 304, 321,347 are used in such applications, but they are prone to stress corrosion cracking. Steels less prone to this kind of trouble include AN-31 (16Cr- 13Ni- 1.3Mo- 1.2Nb-0.7V) developed in West Germany, 17-14CuMo (16Cr-14Ni-2.5Mo0.45Nb-0.1Ti-3Cu) developed in the United States, and Esshete 1250 (15Cr- 1.0Ni- 1Mo- 1Nb-0.25V) developed by Swinden Laboratories in Britain. It has already been reported that the 0.05C-10Cr2Mo-V-Nb ferritic heat resisting steels have good hightemperature strength and ductility. 4 In this experiment we show that the 10Cr-2Mo-0.1V-0.05Nb heat-resisting steels are an improvement on T A F steel (0.2C-10.5Cr1.5Mo-0.2V-0.15Nb-0.04B-0.02N) 5 which has tempered T. FUJITA is Professor, K. ASAKURA is Research Associate, and T. SAWADA is Graduate Student of Department of Materials Science. Faculty of Engineering, The University of Tokyo, Tokyo 113, Japan. T. TAKAMATSU is General Manager and Y. OTOGURO is Chief Researcher of the 4th Laboratory, Product Research and Development Laboratories, Nippon Steel Corp., Fuchinobe, Sagamihara 229, Japan. Manuscript submitted July 9, 1980. M E T A L L U R G I C A L TRANSACTIONS A
martensite structures. They have a mixed structure of 6 ferrite and tempered martensite and their creep rupture strength at 600 ~ is approximately equal to that of AISI 304. For this series of heat resisting steels, the effects of V + Nb, Mo, 4 Ni, and Co (Ref. 6) and Cr (Ref. 7) are reported. The effects of V and Nb have already been reported for the 12 pct Cr steels whose carbon content ranges from 0.15 to 0.25 pct. 8 In the present study V and N b have been added separately and in combination to the low C-10Cr-2Mo steels to determine the effects of V and Nb on the creep rupture strength and microstructure of the steels and thus derive the optimum composition. 2. STEELS U S E D A N D EXPERIMENTAL METHODS 2.1 Steels and Heat Treatments The chemical compositions of steels used in this experiment are shown in Table I. The base composition of these steels was set at 0.05 pct C to assure weldability, 10 pct Cr to obtain oxidation resistance and hightemperature strength in the vicinity of 600 to 650 ~ and 2 pct Mo to provide solution strengthening and stabilize carbides. The content
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