Effect of prior microstructure on microstructure and mechanical properties of modified 9Cr–1Mo steel weld joints

Strength of modified 9Cr–lMo steel, which is used in the normalized and tempered condition, increases with increase in solutionising temperature with a corresponding reduction in percentage elongation. There is an increase in the prior-austenite grain size with increase in solutionising temperature. The heat-affected zone (HAZ) of the weld joints prepared from this material that has been subjected to solutionising at a higher temperature (1150–1200 °C for 1 h) is found to be stronger than that of the material subjected to conventional solutionising treatment (1050–1070 °C for 1 h). Tensile tests conducted on these weld joints showed that there is about 25% increase in yield strength over the weld joints made from the material subjected to normal solutionising treatment. A larger prior-austenite grain size, with correspondingly increased martensite lath size, in the base metal significantly alters the tempering behaviour of the base material and HAZ. The inter-critical HAZ (ICHAZ) microstructure of the weld joint is also modified because of the modification in the base material microstructure. Improvement in the strength of the ICHAZ by increasing the solutionising heat treatment temperature is attributed to the absence of the typical fine grained microstructure found in this material subjected to normal solutionising heat treatment.