| Abstract: | This study investigated the effect of multi-step heat treatment on the microstructure,mechanical properties and fracture behavior of thick 15Co-12Ni secondary hardening steel.As-quenched sample was found to have elongated prior austenite grain (PAG) and coarse lenticular martensitic structure.On the other hand,heat-treated sample was observed to have fine lenticular martensitic structure due to fine PAG size and a lot of nano-sized carbides.Also,after heat treatment,nano-scale reverted austenite film was formed at the martensite interfaces.The heat-treated sample showed 2.47 GPa superior tensile strength and superior elongation of about 12 ％.The high strength was mainly due to fine block size and high number density of nano-sized carbides.The average value of plane strain fracture toughness (KIC) was 29.3 MPa √m,which indicated a good fracture toughness even with the high tensile strength.The tensile fracture surface was observed to have ductile fracture mode (cup-and-cone) and the formation of about ～1 μm ultra-fine dimples.In addition to this,nano-sized carbides were observed within the dimples.The findings suggested that the nano-sized carbide had a positive effect not only on the strength but also on the ductility of the alloy.The fractured surface after toughness test,also showed ductile fracture mode with a lot of dimples.Based on the above results,correlation among microstructural evolution,deformation and fracture mechanisms along the heat-treatment was also discussed. |
