FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A R C H 2 0 2 3 5 3 temperature, the higher the nucleation rate. In addition, the faster cooling allows less primary ferrite to form, so that more pearlite is present. Also, because the pearlite forms in a lower temperature range, it will be finer and hence harder. With appropriate processing that produces a larger percent of finer pearlite, a normalized steel would be appreciably harder than the same steel in the annealed condition. However, the purpose of normalizing varies considerably. Normalization may increase or decrease the strength and hardness of a given steel in a given product form, depending on the thermal and mechanical history of the product. For normalizing, austenitizing is carried out in a temperature range slightly higher than that normally used for hardening for water quenching, to ensure a homogeneous austenite. Typically, the work is heated to a temperature approximately 55°C above the upper critical line of the iron-iron carbide phase diagram, as shown in Fig. 3; 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 prior to cooling. Figure 4 compares the time-temperature cycle of normalizing to that of full annealing. Stainless steels are not generally normalized. This is the case even with martensitic stainless steels, because the high degree of hardenability makes it difficult to avoid martensite formation with air cooling. Similarly, normalization is generally not undertaken with most tool steels. ~HTPro Note: This heat treat basics article was derived from Annealing of Steel[5] and Normalizing of Steel[6], articles from ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and Processes. References 1. G. Krauss, Normalizing, Annealing and Spheroidizing Treatments, Steels: Processing, Structure, and Performance, ASM International, p 251-262, 2005. 2. K.W. Andrews, Empirical Formulae for the Calculation of Some Transformation Temperatures, J. Iron Steel Inst., Vol 203, p 721, 1965. 3. C.R. Brooks, Principles of the Heat Treatment of Plain Carbon and Low-Alloy Steels, ASM International, 1996. 4. G. Krauss, Steels: Heat Treatment and Processing Principles, ASM International, 1990. 5. S.S. Sahay, Annealing of Steel, Steel Heat Treating Fundamentals and Processes, Vol 4A, ASM Handbook, p 289-304, ASM International, 2013, https://doi. org/10.31399/asm.hb.v04a.a0005787. 6. Normalizing of Steel, Steel Heat Treating Fundamentals and Processes, Vol 4A, ASM Handbook, p 280-288, ASM International, 2013, https://doi. org/10.31399/asm.hb.v04a.a0005783. 11 Fig. 3 — Partial iron-iron carbide phase diagram showing typical normalizing range for plain carbon steels. Fig. 4 — 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[4].
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