Abstract
In 18% Ni maraging steel whose strength level is from 2.0 GPa up to 2.4 GPa, the relation between plane strain fracture toughness, KIC, and tensile strength, σB. is experimentally given by
KIC∝σB n n 4.
Crack growth of the present alloy is controlled by micro-cleavage and their subsequent ductile coalescence. Increase of the strength level brings about increase of areal fraction of micro-cleavages and the resultant decrease of KIC. Solution treatment under the condition of unrecrystallized austenite (unrecrystallized solution treatment) results in strengthening and toughening due to remarkable refinement of martensite structure, particularly the block dimension. In the alloy whose strength level is beyond 2.4 GPa, however, unrecrystallized solution treatment dose not cause toughening. The reason behind is that the critical grain size which is necessary for ductile fracture has to become much smaller than that which is realized in usual unrecrystallized solution treatment.
