Effect of reduction pretreatment process on evolution of micro-porosity in 42CrMo billet

Reduction pretreatment (RP) process, which is implemented after the billet is completely solidified, is an alternative process to further improve billet quality. Finite element method and experiment were used to investigate the effect of the RP process on the internal quality of 42CrMo billet. An ultrasonic testing technology was used to study the influence of the RP process on the micro-porosities. Through a three-dimensional finite element model, the mechanism of the RP process on the porosity closure was studied. The experimental results show that the RP process eliminates the macro-porosities of the billet center. The number of micro-porosities in the billets is effectively reduced, and the increase in deformation contributes to the reduction in micro-porosities. Compared with isothermal rolling, the RP process leads to a larger equivalent strain and hydrostatic integration in the billet center because of the temperature gradient, and the effect of the temperature gradient is greater with the increase in deformation. The closure effect of micro-porosity is related not only to the stress state, but also to the distribution of micro-porosity. The increase in three-dimensional compressive stress is beneficial to the porosity closure. If the micro-porosities distributed densely, excessive stress could induce micro-porosity coalescence, which has the risk of inducing cracks.