A fractographic investigation of thermal embrittlement in cast duplex stainless steel
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I.
INTRODUCTION
D U P L E X stainless steels are Fe-Cr-Ni alloys that have a microstructure consisting of two primary phases: austenire (gamma) and ferrite (alpha), Metallurgical overviews of duplex stainless steels are provided in the articles by Solomon and Devine. ~,2 These steels are of particular interest to the nuclear industry because they exhibit high strength and fracture toughness, high resistance to pitting and crevice corrosion, and less susceptibility to sensitization and intergranular corrosion than austenitic stainless steel. However, the use of duplex stainless steels by utilities is as yet negligible because of concerns related to thermal embrittlement, that is, the long-term performance when the materials are subjected to temperatures as low as 250 ~ for an extended period. Thermal-embrittlement characteristics of duplex stainless steels have been investigated by many researchers. 2-s The well-recognized 475 ~ embrittlement is associated with the separation of the ferrite into an iron-rich alpha phase and a chromium-rich alpha-prime phase (the lowest-temperature decomposition product of ferrite in a duplex stainless steel). ~,z,4,5The concern is that this precipitation of the alpha prime that gives rise to the phenomenon of 475 ~ embrittlement may also degrade the toughness after long exposures to temperatures as low as 250 ~ ~'6making duplex stainless steels unreliable for certain nuclear power applications. Because of a lack of appropriate experimental data, the embrittlement behavior at low temperatures has been extrapolated from higher-temperature results. Grobner 4 and Solomon ~found that an Arrhenius rate equation with the Cr diffusion activation energy (48 kcal/mol) could be used to describe low-temperature (475 ~ embrittlement; however, Trautwein and Gysel 6 noted that an activation energy of only 24 kcal/mot would correlate their embrittlement data in the temperature range of 300 to 400 ~ for cast duplex stainless steels and that the rate of embrittlement was 10 times faster than that assumed up to now. It is apparent that significant questions still exist regarding the nature and kinetics of the embrittlement mechanisms of alpha prime. T. KOBAYASHI,formerlywLthSRI International.MenloPark, CA, is with the Departmentof MechanicalEngineering,NagoyaInstituteof Technology, Goklso-cho,Showa-ku,Nagoya, Japan 466. D. A. SHOCKEYis Director, Department of Metallurgy and Fracture Mechanics, SRI International, Menlo Park, CA 94025. Manuscript submittedJuly 22, 1986. METALLURGICAL
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A
To investigate the thermal embrittlement characteristics of cast duplex stainless steel, the mechanical properties (specifically, the fracture toughness) of a specified alloy in both the as-cast and aged conditions were characterized. 9 The specific aging treatment was 3t2 hours at 400 ~ which was assumed equivalent to 1.7 years at the nuclearcomponent operating temperature of 280 ~ The test results indicated that the as-cast material possessed a high fracture toughness [J~c = 2.49 MJ/m z (14,200 in-lb/i
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