Effects of Cold Work on Abnormal Grain Growth During Simulated Carburizing of 4121 Steel Containing Nb

Precision cold-forging processes are used to produce near-net-shape parts that are then carburized. During carburization thermal cycles, abnormal grain growth (AGG) after cold-forging is known to develop microstructures which limit fatigue. In the present study, a small 0.04 wt pct Nb addition was made to a low-alloyed AISI 4121 steel containing 0.3 wt pct Mo. Subcritically annealed specimens were cold rolled at selected reduction ratios up to 50 pct, heated according to a simulated gas carburizing cycle at 930 ^oC, and water quenched to produce a final martensitic microstructure. The number density of abnormally grown grains increased rapidly as the cold rolling reduction ratio increased from 0 to 10 pct to show a substantial occurrence of AGG.  With a further increase in reduction ratio, the extent of AGG decreased and was absent in samples subjected to the maximum reduction ratio of 50 pct. The evolution of fine (Nb,Mo)(C,N) precipitates at various stages of processing was characterized by thermodynamic calculations and electron microscopy and compared to the occurrence of abnormal austenite grain growth. The significance of these results to control of AGG and thus optimize fatigue performance in commercially-produced cold-forged and carburized components is discussed.