Microstructural changes by controlling austenitizing and tempering conditions on the J-R fracture resistance of SA508 Gr. 1A low alloy steels

The microstructure, tensile properties, and J-R fracture resistance of SA508 Gr.1A low alloy steels were evaluated for different heat-treatment conditions, and the relationship between them was analyzed to investigate how the resulting microstructures affected ductile crack propagation behavior. Heat-treatment was carried out by varying the austenitizing temperature and tempering time, which changed the sizes of grains and precipitates. The SA508 Gr.1A microstructure was determined to be composed of ferrite and tempered bainite. The fraction of tempered bainite and grain size increased as austenitizing temperature increased. The overall microstructure was not changed by varying tempering time, but precipitate coarseness increased as tempering increased. When the austenitizing temperature was increased, yield strength increased slightly due to the formation of bainite. Even though its grain size increased, J-R fracture resistance decreased. When tempering time was increased, strength decreased, and J-R fracture resistance increased despite the formation of coarse carbides. The grain boundaries act as obstacles to crack propagation, and therefore, the number of grains that form in the plastic zone at the crack tip is closely related to the crack propagation resistance. The J-R fracture resistance increases as the number of effective grains in the plastic zone increases, because the effective grain boundaries act as strain energy absorption sites against plastic deformation. They exhibited a linear relationship regardless of the formation of coarse carbides.