Microstructural Evolution Induced By Hot Upsetting Of Steel Ingots

Sammanfattning

Internal defects such as porosity, segregation, and non-metallic inclusions can reduce the quality and performance of forged steel products. Increasing deformation through upsetting prior to cogging may improve internal quality by promoting closure of discontinuities and modifying defect morphology. The aim of this study was to investigate the effect of upsetting prior to cogging on internal defects and microstructural features in steel ingots. The study was conducted in collaboration with Ovako, where two experimental trials were performed using industrial steel ingots subjected either to conventional cogging or to upsetting followed by cogging. The effects of upsetting were evaluated through characterization of internal quality using ultrasonic testing, porosity assessment, carbide segregation evaluation, and nonmetallic inclusion analysis. The results indicated that upsetting increased accumulated deformation, particularly in the tapered bottom regions of the ingots. Increased deformation was associated with improved internal quality, shown by reduced ultrasonic indications and lower values of the largest detected pore size compared with reference bars. However, higher upsetting degrees also increased the tendency for surface cracking. Carbide segregation and non-metallic inclusion severity remained generally low for both upset and reference bars, limiting the ability to identify clear effects of upsetting. Nevertheless, differences in the morphology of oversized globular oxide inclusions suggested possible deformation-related changes at the inclusion–matrix interface. Overall, the study suggests that upsetting prior to cogging can contribute to improved internal quality in steel ingots, although optimization is required to balance defect closure and the risk of surface cracking.