The Effects of Carbon Profile and Quenching Condition on Dimensional Change of Carburized Steel Rings

摘要

Quenching distortion of carburizing steel has been investigated with ring specimens of AISI 5120 equivalent steel. Cylindrical ring specimens of the same outer diameter with different inner diameters have been carburized and hardened under various carburizing and quenching conditions. Ring diameter decreases as the total case depth and the ratio to half wall-thickness increases. On the other hand, after the ratio comes to 1, the diameter increases as the core carbon concentration increases. Under the same carburizing condition, diameters of rings increase as the cooling intensity increases. In a carburized steel part, cross-sectional carbon profile determines the martensitic transformation (starting and finishing) temperature curves, while the quenching condition determines the actual temperature gradient across the section. Consequently, the transformational expansion and thermal shrinkage domains during phase trasnformation are determined. Until additional carbon diffuses to the core, increase in case depth extends thermal shrinkage area. On the other hand, further carburization results in slower carbon profile and strong cooling intensity increases slop of actual temperature distribution, hence they extend the transformational expansion area. This study well gives the conceptual idea which is easy to understand why and how carburizing steel can change in certain dimensions after quenching including shrinkage phenomenon.