Effects of Size and Distribution of Spheroidized Cementite on Void Initiation in the Punched Surface of Medium-Carbon Steel

In this study, we investigated the effects of the size and distribution of spheroidized cementite on the characteristics of a punched surface as well as the effect of stress triaxiality on the void initiation at the interface between the cementite (θ) and ferrite matrix under shearing deformation. Punching and interrupted punching tests were conducted with two types of annealed medium-carbon steels, θL and θS, which contained large and small cementite particles at low and high densities, respectively. The microstructural deformation in the interrupted punching test was simulated by the finite-element (FE) method. Based on the scanning electron microscopy (SEM) images of the microstructures and microstructural FE models, the stress triaxiality around cementite was identified as an important factor for void initiation. Moreover, the microstructural FE models indicated that a large number of cementite particles reduce the stress triaxiality around them. This mechanism explains why the number of voids in an area away from the surface of θS is smaller than that in an area away from the surface of θL. In contrast, the number of voids near the punched surface was larger in θS than that in θL, owing to the high stress triaxiality caused by the large shearing deformation. The stress triaxiality tended to change the critical equivalent plastic strain for void initiation.