Recovery and recrystallization in modified 9Cr-1Mo steel weldments after post-weld heat treatment

The Post Weld Heat Treatment (PWHT) of modified 9Cr-1Mo steel is indispensable to the as-welded joints in fabricating high-temperature structural components of steam power plants. The microstructural evolution and the relationship between the formed precipitates and the recovery and recrystallization of welded joints after PWHT have been studied in details. It is found that the recovery and recrystallization occurring at above 730 °C/2 h led to fragmentation of martensite laths and thus formation of equiaxed grains. Only M23C6 precipitates were observed in the weldments according to the X-ray diffraction results of slag received by electrolytic corrosion. The variation of the recovery and recrystallization behaviors was largely influenced by the distribution of the M23C6 precipitates in the microstructure of the modified 9Cr–1Mo steel weldments after PWHT.

► Micro-hardness decrease with the increment of PWHT holding temperature and time.
► Growth rate of intergranular precipitates decreases with extending of holding time.
► Recovery and recrystallization occur during PWHT.
► Location and size of M23C6 precipitates affect the nucleation of recrystallization.