Fracture toughness behavior of ex-service 2-1/4Cr-1Mo steels from a 22-year-old fossil power plant
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I.
INTRODUCTION
R E M A I N I N G life prediction has recently become an issue in the power generation industry, tl-91 The interest in the area of remaining life prediction arises from the necessity to avoid costly forced outages, safety considerations, and the need to extend the operating life of structural components beyond the original design life. u-a1 In fossil fuel power generation systems, many structural components, such as steam pipes and headers, high-pressure steam turbine rotors, and casings, operate at high temperatures. While there is an industrial need for the development of a life prediction methodology for these components, in order to be accurate, this methodology must incorporate the appropriate elevatedtemperature failure mechanisms. Recently, a remaining crack growth life prediction methodology for high-temperature structural components was developed u'8'9~ in which the final life of the component was determined by fracture toughness. Therefore, fracture toughness properties in ex-service materials are of considerable importance. Usually, fracture toughness data were obtained on virgin materials which have not experienced in-service operation. Little work has been conducted to characterize fracture toughness behavior of in-service or ex-service materials. Because of the long-term, elevatedrtemperature operation, the fracture characteristics of an in-service (or ex-service) material may be significantly different from that of a corresponding virgin material, t6,~~ Thus, the consideration of service history is important in selecting fracture toughness values for life prediction analyses. Potentially, failure of elevated-temperature components can result from creep or creep-fatigue crack growth associated with pre-existing fabrication defects or incluP.K. LIAW, Fellow Engineer, and M.G. BURKE, Senior Engineer, are with Department of Materials Evaluation, Westinghouse Science and Technology Center, Pittsburgh, PA 15235. A. SAXENA, Professor, is with School of Materials Engineering, Georgia Institute of Technology, Atlanta, GA 30332. J.D. LANDES, Professor, is with Department of Engineering Science and Mechanics, The University of Tennessee, Knoxville, TN 37996-2030. Manuscript submitted August 8, 1989. METALLURGICALTRANSACTIONSA
sions or service-initiated defects. Also, high-temperature components are prone to cracking along the heat-affected zone (HAZ) areas where weld defects can be present, as shown by higher crack growth rates and lower fracture resistance along HAZ regions.iS'9] Thus, the fracture behavior in the HAZ regions needs to be emphasized for life prediction. In this investigation, the fracture toughness behavior of a 2-1/4Cr-lMo steel weldment and corresponding base metal samples from ex-service superheater headers are characterized. Extensive microstructural characterization was performed to further understand the fracture characteristics of these steel weldments. II.
EXPERIMENTAL PROCEDURE
A. Material The chemical compositions of the 2-1/4Cr-lMo steel investigated in this stud
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