Low-ductility fracture (LDF) occurred occasionally in the heat affected zone (HAZ) when the welded constructions of Cr-Mo steels were subjected to the long time services at high temperature. The LDF in 21/4Cr-lMo steel was reproduced in the laboratory by the creep rupture test in the temperature range of 775 to 875 K. The tests were made on synthetic HAZ specimens of the original and SR-treated conditions. SR treatment was carried out at 975 K for 2 hours. The ductility shown by the reduction in area of the specimen was discussed in relation to the time to fracture and the fracure mode.
Two different types of LDF were recognized in HAZ specimen in the original condition. The first type of LDF appeared in the short term range of rupture producing a typical intergranular fracture (the fracture of type I). The second type appeared in the long term range of rupture, longer than a critical time period, tf, producing another type of intergranular fracture (type II). In this type of fracture, fractures occurred along grain boundaries accompanying a significant deformation of ferrite grains. A small quantity of ductile fracture of type III was usually accompanied to those fracture types.
SR-treated specimen fractured with ductile manner in the short term range of rupture, but it lost the ductility when the time exceeded a critical time period, tsr, which was approximately the same as the time tf. The fracture of type II appeared in this time range. Those results informed that, (1) the LDF in short term range, which produced the type I fracture, disappeared after SR treatment, (2) the LDF in long term range, which produced the type II fracture, revived even after the SR treatment.
