Tailoring the tempered microstructure of a novel martensitic heat resistant steel G115 through prior cold deformation and its effect on mechanical properties

In this paper, the influences of prior cold deformation and tempering time on the precipitation behavior, sub-structure evolution, and mechanical properties of a novel G115 martensitic heat resistant steel were systematically investigated. It was found that the deformation reduction and tempering time had significant influences on the precipitation behavior of secondary strengthening particles, the recovery of dislocations, and the formation of subgrains, which played a crucial role in determining the tensile properties and micro-hardness. When the deformation reduction was lower than 20%, the increase of reduction could induce more dislocations or defects, which provided additional nucleation sites for M23C6 particles and obstructed the motion of dislocations and sub-boundaries, thus enhancing the tensile strength. However, when the deformation reduction was higher than 20%, the excessive reduction could accelerate the coarsening of M(23)C(6 )particles by providing more diffusion channels, thus weakening the pinning ability for boundaries and leading to substructure instability. For the G115 steel with a deformation reduction of 20%, an optimal combination of mechanical properties with an average hardness of 283.4 +/- 8 HV, a tensile strength of 1102 +/- 6 MPa, a yield strength of 783.6 +/- 31 MPa, and a considerable ductility of 19 +/- 2% could be achieved after tempering for 60 min.