SECONDARY HARDENING OF HOMOGENEOUS T24 WELDS AFTER SHORT-TERM ANNEALING AT 450 and 530 °C

Abstract

The T24 steel grade has been frequently used for the construction of membrane walls in modern power plant boilers. This steel grade has been developed with the intention of welding without post weld heat treatment. However, commissioning of new units of power plants into service revealed many defects in homogeneous T24 welds. Several remedial measures have been proposed. One of them represents annealing of boilers before putting them into service at 450 °C. This paper deals with characterization of microhardness and microstructure of homogeneous peripheral butt T24 welds after annealing at 450 °C for 48 hours and subsequent annealing at 530 °C for 10 hours. The results revealed that annealing at 450 °C led to a further increase of hardness in overheated parts of heat affected zones of homogeneous T24 welds. Subsequent short-term annealing at temperature of 530 °C, which corresponded to the typical temperature of boilers commissioning, caused a significant effect of secondary hardening in overheated parts of heat affected zones of homogeneous T24 welds. TEM investigations were carried out in order to explain this effect.