MICROSTRUCTURE-FRACTURE TOUGHNESS CORRELATION IN WELD JOINTS OF CR-MOSTEEL

The strength-toughness-microstructure relationship in relation to the micromechanics of a fracture process has been investigated in the weld joints of two alloys: 0.5 Mo and 2.25 Cr-1 Mo steels. These alloys ar e extensively used to fabricate super-heater tubes, boilers, piping, g as lines, etc., by welding. The applications require high temperature and pressure to be maintained during service. The crack initiation tou ghness and tearing resistance were evaluated using crack tip opening d isplacement/J-integral parameters at different temperatures. Quantitat ive analysis of micro-structure and fracture surfaces was used to stud y the micromechanics of fracture process in the heat-affected zone (HA Z) of the alloys. Molybdenum steel exhibited a higher percentage of fe rrite and lower martensite content, while the other steel showed align ed carbide as the major constituent. The higher hardness and strength values in the HAZ and welding zone (WZ) of Cr-Mo steel, compared to mo lybdenum steel, may be attributed to the higher amount of martensite p hase in the alloy. The higher initiation toughness at 200 degrees C in both the alloys was reflected in the larger dimple size, compared to the size observed at room temperature. A tendency for void sheet forma tion was noticed in both alloys. Acicular ferrite and martensite appea red to be the most influential constituents affecting tearing resistan ce and initiation toughness.