Investigation of microstructural changes in a ferritic steel caused by high temperature fatigue
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I.
INTRODUCTION
A ferritic steel, Fe9CrlMo modified by the addition of small amounts of the strong carbide formers V and Nb, has been proposed for applications which involve combinations 6f cyclic and static loads at elevated temperatures. This steel was developed as part of the Advanced Alloy Program at Oak Ridge National Laboratory (ORNL) I1'2'31 and is intended for use in power generation applications. Ferritic steels containing 9 wt pct or more of chromium have found considerable acceptance for high temperature use in corrosive environments. Modified Fe9CrlMo steel has a number of attractive features, e.g., high rupture strength at ambient and elevated temperatures, good weldability, low thermal expansion, and resistance to radiation-induced void swelling. 1~'21 An extensive effort to characterize this material has been carried out at O R N L 11-5] and elsewhere. 16'7'81 It is, of course, essential that a candidate material for power generation applications exhibit good microstructural stability over very long periods of exposure to high temperature service conditions. Otherwise a loss in mechanical strength is likely to occur. The present research has been concerned with an investigation of microstructural changes in modified Fe9CrlMo produced by high temperatures, with and without accompanying deformation, and with correlating these changes with the observed mechanical behavior. Fatigue tests have been run under a number of mechanical and environmental conditions. Resulting changes in dislocation structures, carbide distributions, and the state of the surface were studied by electron and optical microscopy. Results of this investigation are presented and discussed in the following sections.
S. KIM is Assistant Professor, Department of Marine Engineering, College of Engineering, Yeungnam University, Gyongsan 632, Korea. J. R. WEERTMAN is Professor and Chairman, Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208. Manuscript submitted July 25, 1986.
METALLURGICALTRANSACTIONS A
II.
E X P E R I M E N T A L DETAILS
One-inch thick plates of modified Fe9CrlMo were obtained from the Advanced Alloy Program at ORNL. The plates had been hot rolled from an electroslag remelted and argon-oxygen-decarburized ingot. Details of the steelmaking process are given in Reference 3. The chemical composition of the material is listed in Table I. Heat treatment consists of normalization for one hour at 1038 °C, air cool, tempering at 760 °C for one hour, air cool. The resultant microstructure is fully tempered martensite. Carbides lie along prior austenite grain boundaries and lath boundaries while smaller carbides are contained in the matrix. Both M23C 6 and MC carbides are present. I4'5'61 A detailed microstructural analysis of the normalized and tempered condition and tensile properties from room temperature to 760 °C have been documented elsewhere, t1'2'4'51
Table I. Chemical Composition of Modified Fe9CrlMo (Wt Pct) Element C Mn P Si Ni Cr Mo V Nb Ti Co Cu AI W N Fe
Fe9Crl Mo 0.
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