Exploring the origins of surface cracks in forged ingots: Impurity element influence and temperature-induced formation of detrimental phases in case hardening steel

Abstract

Surface hot shortness is a critical defect affecting the performance and reliability of steel components. This study investigates the factors influencing surface hot shortness in DIN 16MnCr5 steel, with a particular focus on the role of impurity elements and temperature. Experimental analysis reveals that copper and zinc, as impurity elements, play a significant role in the formation of detrimental phases and the initiation of surface cracks. Copper enrichment occurs due to its lower affinity to react with oxygen, resulting in its aggregation at the oxide layer/iron interface. Zinc contamination contributes to the formation of intermetallic phases and the initiation of hot shortness, likely due to its low melting point and reactivity. Furthermore, the impact of temperature during hot tensile testing on the ductility and formation of detrimental phases is investigated. Below 950 °C, fractures primarily occur due to proeutectoid ferrite formation at grain boundaries, with limited formation of Cu-rich phases. However, temperatures above 950 °C exhibit increased Cu-rich phase formation, leading to decreased ductility and the formation of hot cracks. The critical temperature range of 1000 °C shows minimum ductility and the highest content of Cu-rich compounds.Grain boundary penetration of Cu-rich melt is identified as a crucial mechanism in the nucleation and propagation of cracks during surface hot shortness. Exceeding a certain copper concentration along grain boundaries leads to the formation of a molten phase at the crack tip, facilitating wetting of the grain boundaries and the subsequent formation of surface cracks. Understanding the influence of impurity elements, such as copper and zinc, as well as the effect of temperature on surface hot shortness is essential for mitigating this defect and improving the performance of steel components. The findings of this study contribute valuable insights toward the development of effective strategies for preventing and controlling surface hot shortness in DIN 16MnCr5 steel.