The postweld heat-treatment response of simulated coarse-grained heat-affected zones in a new ferritic steel

The tempering behavior of simulated coarse-grained (CG) heat-affected zones (HAZs) in two ferritic alloy steels, 2.25Cr-1Mo and HCM2S, was investigated. The hardness of HCM2S was found to be stable at longer times and higher temperatures than the 2.25Cr-1Mo steel, even though the "aswelded" hardnesses were approximately equal. Both materials reached a peak secondary hardness after tempering for 5 hours at 575 degreesC. The increase in hardness of the 2.25Cr-1Mo steel was due to precipitation of Fe-rich M3C carbides within the prior-austenite grains, whereas the secondary hardening in HCM2S was due to a fine dispersion of intragranular, W-rich carbides. The HCM2S steel retained its hardness at longer times and higher temperatures than 2.25Cr-1Mo steel, because of the precipitation of intragranular, W-rich carbides and V-rich MC carbides that stabilized the lath structure. This study shows that HCM2S should not be heat treated in the same way as 2.25Cr-1Mo steel and also provides a basis for defining the postweld heat treatment (PWHT) of HCM2S.