Abstract
Long-term creep rupture properties and microstructure of 12% Cr heat resisting steels were investigated. It has been found that solid solution hardening by an addition of Mo is very effective on long-term creep rupture strength, but precipitation or dispersion hardening by an individual addition of V or Nb is little effective. Especially, the presence of excessive amounts of undissolved NbC leads to the coarsening of MX precipitates, and a marked drop in long-term creep rupture strength. The combined addition of V+Nb with Mo and B leads to superior 30 000h creep rupture strength. Microstructural differences between long-term crept specimens at 600°C and short-term crept specimens at 650°C were discussed by considering the parameter, T(C+log t). Long-term specimens at 600°C exhibit higher dislocation density substructure caused by intensive precipitation reactions which include both fresh precipitation within lath and agglomeration at boundaries, and exhibit intergranular fracture probably caused by stress concentration around the agglomerates, whereas short-term specimens at 650°C exhibit intragranular fracture. It is considered that the deviation of long-term creep rupture strength from the extrapolated value is caused by differences in precipitation hardening level and fracture mode.
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