Creep Deformation Behavior and Alloy Design Philosophy of Creep-Resistant Tempered Martensitic 9Cr Steel

摘要

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 rate ε~min times the increase in creep rate by strain din~ε /d~ε in the acceleration region as tr= 1.5/[(~εmin)(dln~ε/d~ε)]. The parameters ~εminand din s /d~ε are closely correlated with the time to minimum creep rate tm and the strain to minimum creep rate s m, 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 M_(23)C_6 carbides along boundaries near prior austenite grain boundaries during creep. The enhancement of boundary and sub-boundary hardening increases the tm and decreases the ^min, which improves the creep life. The boundary and sub-boundary hardening is significantly reduced in fine-grained region of heat-affected-zone (HAZ) of conventional steel P92 welded joints, promoting Type IV fracture. 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 Type Ⅳ fracture.