The normalizing process for steel differs from the annealing process for steel in terms of heating temperature and cooling rate. In the normalizing process, steel is heated to a much higher temperature and then removed from the furnace for air cooling. Compared to annealing, the heating temperature is lower and the cooling rate in the furnace is much lower. Because of the faster cooling rate during normalizing, the steel has higher strength and hardness compared to steel that has been annealed
There is no significant difference between annealing and normalizing on the ductility of mild steel. The tensile strength and yield point of normalized steels are higher than those of annealed steels, except in the case of mild steels.
As in the case of annealing, normalizing also leads to the formation of ferrite, carburite and lamellar pearlite. However, in normalizing, the austenite transformation occurs at a much lower temperature compared to annealing due to the higher cooling rate. Due to it, the transformation product pearlite is finer and the interlayer distance between two neighboring carburite plates is smaller.
The main difference between fully annealed and normalized parts is that the softness (and machinability) of a fully annealed part is uniform throughout the part because the entire part is exposed to controlled furnace cooling. With standardized parts, the cooling is not uniform, depending on the geometry of the part, resulting in non-uniform material properties throughout the part.
Normalizing eliminates internal stresses caused by cold working, and grain growth is limited by the relatively high cooling rate, so the mechanical properties (strength and hardness) of normalized steel are superior to those of annealed steel.
The machined surface quality of normalized parts is also superior to that of annealed parts. This effect is caused by the increased ductility of annealed steel, which favors the formation of tears in the machined surface.
