Effects of phosphorus (P) and heat treatment on toughness have been investigated on a quenched and tempered 9% Ni steel base metal and its welded joint simulated by a thermal cycle simulator. The mechanism of toughness improvement by the diminution of P component was examined by mechanical tests, microstructure observation and computer simulation of the grain boundary segregation. The critical CTOD value and Charpy absorbed energy in the base metal showed remarkable deterioration as increasing P content, when the cooling rate after tempering was lower than 1×10^-2 K/s. Simulated bonds showed reduction of toughness when the P level was higher than 0.008% and/or the cooling rate after post weld heat treatment (PWHT) was lower than 1×10^-2 K/s. The Charpy absorbed energy of base metal was higher than that of PWHT welded joint when they showed the same critical CTOD value. Intergranular ductile fracture was occurred in the base metal, while intergranular brittle fracture in PWHT welded joint was observed.
The effect of cooling rate following the tempering on the grain boundary segregation of P was estimated by computer simulation, based on Guttmann-McLean theory. The computed amount of P segregation in grain boundary was closely related to the toughness both for a base metal and a simulated bond. It was also suggested that when the grain boundary segregation of P was lower than 0.1, high toughness level was obtained in the base metal and the simulated bond. The difference of toughness between the base metal and the simulated bond was attributed to the coarsening of grain size by the welding thermal cycle. The 9% Ni steel with P level lower than 0.005% shows excellent toughness in the base metal and the welded joint.