Normalizing—the threshold of heat treatment

The necessary first step in the process of steel heat treatment involves “normalising”, aiming to provide the homogeneity microstructure during carburization. In order to reduce the distortion of the parts as a whole,  homogenization happening in the carburizing cycle of the finished gear component is not desirable. A separate normalization reduces the problems associated with separation of Pearlite and ferrite. When homogeneity happens in the ferrite area, it doesn’t offer the same  transformation of hardness and stress levels as in the pearlitic zone, and therefore creates more distortion later.

Normalizing in the heat treatment process involves heating above the operating temperature of the workpiece, then usually air cools outside the furnace to release the residual stress in gear blank, and thus helps maintain the dimensional stability. Usually normalization will be considered in both the temperature and microstructure. In terms of temperature,  normalization means firstly autensited and then cooled in the still air or slightly agitated air. From the micro-structure point of view, the microstructural area with 0.8% carbon is the Pearlite area, while ferrite is the one with lower carbon percentage. Workpiece after normalization is machinable but harder then the one after annealing. Moreover, normalizing will also release the residual stress generating from steel manufacturing process and  forging process, leading to the plausible distortion in later carburization.

A good normalizing includes holding “at temperature” in rough materials for at least two hours, or one hour for each one inch section thickness.  Normalizing temperature should be the same or even higher than the carburization temperature.