The effects of grain boundary phosphorus segregation and heat treatment on toughness of 9% Ni steel and its welded joint.

Effects of phosphorus (P) and heat treatment on toughness have been investigated on a quenchedand tempered 9 '/•Ni steel base metal and its welded joint simulated by a thermal cycle simulator.The mechanismof toughness improvement by the diminution of Pcomponentwas examinedby mechanical tests, microstructure observation and computer simulation of the grain boundary segregation.The critical CTOD value and Charpy absorbed energy in the base meta[ showedremarkable deterioration as increasing P content, whenthe cooling rate after tempering was lower than I x 10-2 K/s.Simulated bondsshowedreduction of toughness whenthe P Ievel was higher than 0.008 •/• and/orthe cooling rate after post weld heat treatment (PWHT) was lower than I x 10-2 K/s.TheCharpy absorbed energy of base metal was higher than that of PWHT welded joint whenthey showedthe same critical CTODvalue.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 Pwas estimated by computer simulation, based on Guttmann-McLean theory.Thecomputedamount 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 thatwhen the grainboundary segregation of Pwas lower than 0.1 , high toughness level was obtained in the base metal and the simulated bond.The ditference of toughness betweenthe 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 Ievel lowerthan 0.005 "/• showsexcellenttoughness in the base metal and the welded joint.