Steel normalization is often considered from two points of view - thermal and microstructural.
Thermally and classically, steel normalization - this is the heating of steel to the austenitic state, followed by cooling in calm air. Sometimes, operations with accelerated air cooling are also referred to as normalization..
The place of the normalization temperature on the iron-carbon phase diagram is shown in the figure 1.
From the point of view of the microstructure, the normalized structure is considered perlite for steel with carbon content 0,8 %, and for steels with a lower carbon content - hypereutectoid steels - a mixture of pearlite and ferrite.
The normalization operation is used for most steels and, including steel castings. Very often, welded steel seams are normalized to refine the structure of the steel in the welded area..
Steel normalization target
Steel normalization targets can be different: eg, how to increase, and to reduce strength and hardness depending on the thermal and mechanical history of the product.
Normalization targets are often overlapped or even confused with annealing, thermal hardening and stress relief tempering. Normalization is applied, eg, to improve the machinability of the part by cutting, grinding grain, homogenization of the grain structure or reduction of residual stresses. Comparison of temperature-time cycles for normalizing and annealing is shown in the figure. 2.
Picture 2 ─ Comparison of temperature-time cycles of normalization and complete annealing. Slower cooling during annealing leads to a higher ferrite-pearlite transformation temperature and a coarser microstructure, than with normalization.
For steel castings, normalization is used to homogenize their dendritic structure., reduction of residual stresses and greater susceptibility to subsequent thermal hardening.
Products, obtained by pressure treatment, can be normalized to reduce banding after rolling or uneven grain after forging.
Normalization followed by tempering is used instead of conventional hardening, when products have a complex shape or abrupt changes in cross-section. Do it, to avoid cracking, warpage and excessive thermal stress.
Cooling rate of steel during normalization
Cooling rate during normalization is usually not critical. but, when the product has large differences in cross-sectional dimensions, take measures to reduce thermal stresses, to avoid warping.
Holding at normalization temperature
The role of the exposure time at the normalization temperature is only that, to ensure homogenization of the austenitic structure prior to cooling. One hour of exposure for each 25 mm section thickness is the norm.
Cooling rate during normalization significantly affects the amount of perlite, its dimensions and thickness of pearlite plates. The higher the cooling rate, the more pearlite is formed, and its plates become thinner and closer to each other. An increase in the proportion of pearlite in the structure and its grinding give an increase in the strength and hardness of steel. Lower cooling rates mean less strong and harder steel.
After, how the products were uniformly cooled in their cross section below the lower critical point Ar1, they can be cooled in water or oil to reduce the overall cooling time.