Hydrogen pre-charging effects on the notch tensile properties and fracture behaviour of heat-affected zones of thermally aged welds between T24 and T92 creep-resistant steels

The effects of thermal exposure at 600 degrees C and subsequent electrolytic hydrogen charging at ambient temperature were studied in terms of their influence on notch-tensile properties and fracture behaviour of individual heat-affected zones (HAZ) of Ni-based transition weldments between T24 and T92 creep-resistant steels. The experimental welded joints were prepared by gas tungsten arc welding using Inconel-type filler metal Nirod 600. Post-weld heat treatment (PWHT) was carried out at 720 degrees C for 2 h, followed by passive cooling within tempering furnace. Subsequent long-term ageing of two series of prepared T24/T92 dissimilar welds at 600 degrees C for 1000 and 5000 h, respectively, led to gradual changes in both T24 and T92 HAZ microstructures, which were mainly related to their additional tempering accompanied by the coarsening of original precipitates as well as newly-formed secondary phases. The observed thermally-induced microstructural changes together with subsequently applied hydrogen charging effects resulted in complex variations in tensile properties and fracture behaviour of individual HAZ regions related to the pre-existing differences in their initial tempering grade due to the used PWHT procedure.