Abstract
It is known that austenite grain coarsening during carburizing is likely to occur in cold-forged steel parts. We estimate that carbide morphology of spheroidizing annealed microstructure prior to cold forging have an influence on the austenite grain coarsening behavior strongly through the ferrite recrystallization during heating to carburizing temperature. In this study, the formation of austenite grain during heating for carburizing of the cold-upset carburizing steel containing higher Cr than conventional JIS SCR420 was investigated. The carburizing steel with 2.1 mass% Cr showed relatively uniform distribution of spheroidized carbides after spheroidizing prior to cold working. It contained much less lamellar carbides than JIS SCR420 with almost 1 mass% Cr. This spheroidizing annealed microstructure is likely to deform more uniformly during cold working than lamellar carbide bearing microstructure. Consequently, both recrystallized ferrite grain and subsequent austenite grain were formed relatively coarse and uniform during heating for carburizing. Because of its higher austenite grain coarsening temperature, the carburinzing steel with 2.1 mass%Cr is suitable for the combined processes of cold forging and carburizing. In this steel, the formation of spherical carbide at ferrite and austenite interface occurs preferentially during spheroidizing and as a result, few lamellar carbides are observed.
