NORMALIZING OF STEEL is a heat-treating process that is often considered from both thermal and microstructural standpoints. In the thermal sense, normalizing is an austenitizing heating cycle followed by cooling in still or slightly agitated air. Typically, the work is heated to a temperature about 55 ¡ãC (100 ¡ãF) above the upper critical line of the iron-iron carbide phase diagram, as shown in Fig. 1; that is, above Ac3 for hypoeutectoid steels and above Acm for hypereutectoid steels. To be properly classed as a normalizing treatment, the heating portion of the process must produce a homogeneous austenitic phase (face-centered cubic, or fcc, crystal structure) prior to cooling. Figure 2 compares the time-temperature cycle of normalizing to that of normalizing to that of full annealing.
Fig. 1 Partial iron-iron carbide phase diagram showing typical normalizing range for plain carbon steels
Fig. 2 Comparison of time-temperature cycles for normalizing and full annealing. The slower cooling of annealing results in higher temperature transformation to ferrite and pearlite and coarser microstructures than does normalizing. Source: Ref 1
Normalizing is also frequently thought of in terms of microstructure. The areas of the microstructure that contain about 0.8% C are pearlitic (lamellae of ferrite and iron carbide). The areas that are low in carbon are ferritic (body-centered cubic, or bcc, crystal structure). In hypereutectoid steels, proeutectoid iron carbide first forms along austenite grain boundaries. This transformation continues until the carbon level in the austenite reaches approximately 0.8%, at which time a eutectoid reaction begins as indicated by the formation of pearlite. Air-hardening steels are excluded from the class of normalized steels because they do not exhibit the normal pearlitic microstructure that characterizes normalized steels.