Investigation on Internal Crack Defects in Medium Carbon Steel by Soft Reduction

Abstract

This paper investigated the effect of soft reduction amount increasing from 1-4mm with a cooling time of 120s and 180s to reduce the internal crack defects by analyzing the microstructure, crack morphology, extent and nature of grain boundary ferrite, centre macrosegregation, and fracture surfaces in medium carbon steel. It was found that the large extent of grain boundary ferrite with coarse morphology weakens the bonding force of the PAGB and makes it brittle with the initiation of micro-cracks. The sever centre carbon macrosegregation of 1.3 takes place at cooling time of 180s with different reduction amounts, which becomes homogeneous at cooling time of 120s. The results show that the length of inter-granular and trans-granular cracks reaches 20mm with the carbon macrosegregation index of 1.3. The carbon macrosegregation index of 1.3 with a small fraction of 15% is enough to deteriorate the centre macrostructure of the ingot. The cleavage facet size of fracture surfaces is remarkably decreased below 100um at soft reduction parameters of 2mm/120s, 3mm/120s, and 4mm/120s with micro-cracks less than 100um, which reveals that the microstructure of specimens with these soft reduction parameters is more resistant to the cracks propagation.

Keywords:
Medium carbon steel; soft reduction; internal crack defects; grain boundary ferrite; carbon macrosegregation