Effect of heat treatment on creep properties and microstructures of HAZ for 9%Cr ferritic steel

Abstract

Type IV creep cracking in the heat affected zone (HAZ) of welded joint has been one of the serious problems in 9 to 12%Cr ferritic steels during long time service. We have already shown that the specimens heated to peak temperature close to Ac3 do not have clear lath martensitic microstructure and that the coarsening of precipitates, M23C6, in these specimens is observed after post weld head treatment (PWHT). In this study, a coarse grained steel is produced by the higher temperature normalizing heat treatment. The average grain size of coarse grained steel was 250 μm and that of the Ac3 simulated HAZ specimens was 8μm, compared to 30μm and 6μm respectively for a normal grain size steel. The creep life, microstructural changes and distribution of precipitates of simulated HAZ specimens for coarse grained steel are investigated and compared with those for the normal grain size steel. The creep rupture time of the simulated HAZ specimens for coarse grained steel is found to be longer than that for the normal grain size steel, because the regions of large precipitates in the coarse grained steel, which are considered to have low creep resistance, are relatively smaller than those for normal grain size steel.