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Boron metallurgy of advanced ferritic power plant steels and welded joints
Affiliation:Fujio ABE National Institute for Materials Science,Tsukuba 305 -0047,Japan
Abstract:This paper describes the alloy design philosophy for the improvement of long-term creep strength of tempered martensitic 9Cr steel,including welded joints.The creep life tr is inversely proportional to the minimum creep rateεmin times the increase in creep rate by strain dlnε/dεin the acceleration region as tr = 1.5/[(εmin) (dlnε/dε)].The parametersεmin and dlnε/dεare closely correlated with the time to minimum creep rate tm and the strain to minimum creep rateεm,which characterize the creep deformation behavior in the transient region.The boundary and sub-boundary hardening is shown to be the most important strengthening mechanism in creep of 9Cr steel and is enhanced by fine dispersions of precipitates along boundaries.The addition of boron reduces the coarsening rate of M23C6 carbides along boundaries near prior austenite grain boundaries during creep.The enhancement of boundary and sub-boundary hardening increases the tm and decreases theεmin,which improves the creep life.The boundary and sub-boundary hardening is significantly reduced in fine-grained region of heataffected -zone(HAZ) of conventional steel P92 welded joints,promoting TypeⅣfracture.In NIMS 9Cr boron steel welded joints,the distribution of carbonitrides along boundaries are substantially the same between the HAZ and base metal,suppressing the TypeⅣfracture.
Keywords:ferritic steel  creep strength  boron  welded joint  grain boundary hardening
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