APPLICATION OF LOCAL STRESS CRITERION APP ROACH TO TOUGHENING BEHAVIOR DUE TO NICKEL ADDITION IN LO SV CARBON-STEELS

It is well known that nickel is one of the most effective alloy elemen ts to improve the fracture toughness of low carbon steels. Although se veral mechanisms of toughening due to nickel addition were proposed. i nfluential and persuasive mechanisms have not been established yet. In the present work, the fracture toughness of steels with nickel additi on from 0 to 4 mass % were investigated for two series of steels; one for the steels subjected to the same heat treatment and the other for the steels with the same bainitic microstructure. Improvement of the f racture toughness with nickel addition was observed for the steels sub jected to the same heat treatment. For the steels with same bainitic m icrostructure, however, the fracture toughness did not improve with ni ckel addition. The authors have previously formulated fracture toughne ss of low carbon steels in terms oi tensile properties on the basis of the statistical local fracture criterion. They showed in the previous work that fracture toughness of the steels is expressed as a function of yield stress and cleavage fracture stress. The effect of nickel ad dition on fracture toughness was analyzed through the change of yield stress and cleavage fracture stress. Decrease of the temperature depen dency on yield stress with nickel addition was observed for the both s eries oi steels, indicating the tendency of toughening. Whether the fr acture toughness was improved or not by nickel addition depends on the variation of the cleavage fracture stress with nickel addition.