Fracture performance and fracture toughness tests have been carried out to clarify fracture behaviors of welded joint of low yield-ratio HT80 (LY HT80) made by using TMCP process followed by Q'-quenching and tempering. Two kinds of HT80 steel, QT type conventional HT80 and CR-Q'-T type low yield-ratio HTRO which have the same chcmical composition, are prepared. Three point bending COD test, and narrow and wide plate tensile tests have been conducted for welded joints with a fatigued pre-crack along fusion line; through-thickness type notch or surface notch.
There is a considerable difference of fracture performance in welded joint between of low yield-ratio HT80 and of conventional HT80, regardless fracture toughness of crack tip region in both welded joints is almost the same. Fracture resistance such as critical COD of LY HT80 welds apparently increases due to remarkable plastic deformation in the vicinity of crack tip, and fracture ductitity of LY HT80 welds becomes larger than that of conventional HT80 welds at the temperature range at which general yielding fracture initiates. An appropriate decrease in yield strength and yield-ratio of HT80 steel is a proper treatment for obtaining sufficiently large fracture performance at the temperature range for actural use of HT80 steels.
