Influence of austenite and ferrite stabilizers on the microstructure and related mechanical properties of carburized steels

Carburization is a thermo-chemical treatment generally employed to enhance the surface (wear) properties of low carbon steels. The recent carburization studies also focus considerable positive impact on bulk properties like tensile strength and toughness. In view of these observations, the present study focuses on mechanical properties and microstructure of carburized steels. Accordingly, the commercially available three types of case hardenable steels like plain carbon (EN 3), alloy steels with only ferrite stabilizer (20MnCr5) and with both ferrite (Cr) and austenite (Ni) stabilizers (EN 353) were initially normalized to standardize the room temperature structure before carburizing and machined to ASTM standards to prepare the specimens. The machined specimens were gas carburized using carburizing furnace for 2.5 mm case depth and furnace cooled. Tensile and hardness tests were conducted before and after carburization. The plain carbon steel displayed slight reduction in tensile strength and the steels with alloying elements increased the tensile strength considerably. It was also found that Ni and Cr restrict the grain growth and increase the strength of steel even in furnace cooled condition. Microstructure analysis of carburized steels revealed markable impact on the type and distribution of room temperature phases. The carbon content in the case was nearly 0.8 wt. % after carburization and hardness increase in the surface ranges from 130 to 170% as that of its original hardness. The combined effect of Ni and Cr also improves hardenability.