Steels that have undergo plastic deformation consist of pearlites which are irregularly shaped and relatively large, but varying in size. Normalizing is a heat treatment used on steel so as to refine its crystal structure and produces a more uniform and desired grain size distribution. Fine grained pearlites are tougher than coarse grained ones. Normalization eliminates internal stresses, strains and improves the mechanical properties of the steel, such as improving its toughness and machinability. A better ductility can also be obtained without compromising the hardness and strength.
Normalizing is accomplished by heating the steel to a temperature above the transformation range and into the range of complete austenite. This is dependent on the composition of the steel as indicated by the iron-carbon diagram shown below. The usual normalizing temperature ranges from 815°C to 980°C (1500°F to 1800°F), depending on the steel involved.
After sufficient time is given for complete transformation to austenite, i.e. austenitizing of the steel, the alloy is air-cooled to a temperature substantially below the transformation range. The air-cooling avoids excessive proeutectoid segregation. The cooling rate is usually in the range of 500 to 1000 °C/h (900 to 1800 °F/h).
The final microstructure consists of fine pearlite and an absence of massive proeutectoid ferrite. Normalizing is commonly specified for plates of pressure vessel quality hence to ASTM standards above 1 ½ inch in thickness.
1. Lawrence H. Van Vlack, Elements of Material Science & Engineering, 4th Edition, Addison Wesley, 1980, p 416-417
2. Leong Huat Handbook: Structural Steel and Related Products
3. William D. Callister, Jr, Material Science and Engineering: An Introduction, 3rd Edition, John Wiley & Sons, Inc., 1994, p 326-327