Abstract
In order to strengthen and toughen 280kg/mmm2 grade maraging steels, the effects of solution and aging treatments on the microstructure, tensile properties and fracture toughness have been investigated, and metallurgical factors affecting the strength-toughnesss balance have been discussed.
Tensile ductility is strongly dependent upon prior γ grain size. Unstable fracture under low stresses occurs due to the decrease in ductility above a critical grain size, which depends on the strength level and fracture toughness of the steel. On the other hand, the fracture toughness is independent upon prior γ grain size but strongly dependent upon undissolved precipitates. The presence of undissolved precipitates can greatly lower the fracture toughness. Therefore, a simultaneous improvement of ductility and toughness would be obtained in the fine grained martensite structure without undissolved precipitates. Three special treatments, which consist of complete solution treatment of undissolved precipitates and grain refinement, have been proposed by utilizing the difference of metallurgical factors affecting the ductility and toughness respectively. Simultaneous improvements of the ductility and toughness for the higher strength level of 13 Ni-15 Co-10 Mo steel are obtained by a special thermomechanical treatment.
