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 2 conditions, two or more such cycles may be combined or used in succession to achieve the desired results. The success of any annealing operation depends on the proper choice and control of the thermal cycle and based on metallurgical principles. NORMALIZING In the simplest concept, normalizing involves heating to achieve complete austenitization of the steel, followed by cooling in still air. Advanced practices may involve managed cooling rates, and normalizing typically involves cooling to 400°C in approximately 1 min. The heating should be at a slow enough rate to minimize temperature gradients and thermal stresses. Time-at-temperature is usually not specified for a normalization process, owing to the rapid transformation to austenite. Nonetheless, it is quite important for enough time to be allowed for the achievement of a uniform temperature throughout the workpiece. The details of normalizing treatments applied to three typical production parts are given in Table 1, which also lists the reasons for normalizing and gives some of the mechanical properties obtained in the normalized and tempered condition. Normalizing of steel is a heat treating process that is often considered from both a thermal processing and a microstructural standpoint. In terms of thermal processing, normalizing is defined as heating of a ferrous alloy to a suitable temperature above the transformation range and then cooling it in air to a temperature substantially belowthe transformationrange. Goodnormalizingpractice requires that: • The piece be uniformly heated to a temperature high enough to cause complete transformation of ferrite to austenite • It remain at this temperature long enough to achieve uniform temperature throughout its mass • It bepermitted tocool in still air or ina controlledmanner (such as a tunnel with cooling fans) to produce desired microstructure. Normalizing also is described in terms of micro- structure. Normalizing is used to convert a heterogeneous structure, such as develops from a high-austenitizing- temperature treatment (e.g., during hot forging), to a finer and more uniform structure. The microstructures in Fig. 2 illustrate the effect. The annealed microstructure consists of coarse primary ferrite grains that have formed on the boundaries of large austenite grains. When the steel with this microstructure is reaustenitized in a lower temperature range, smaller austenite grains form, and a finer structure results upon air cooling (Fig. 2b). In addition to the refinement of the prior austenite grains, there is a reduction in the size of the primary ferrite grains. This is due to the effect of the temperature of formation on the nucleation rate of these crystals. In the higher temperature range, the lower the transformation 10 TABLE 1 — TYPICAL APPLICATIONS OF NORMALIZING AND TEMPERING OF STEEL COMPONENTS[6] Part Steel Heat treatment Properties after treatment Reason for normalizing Cast 50 mm valve body, 19 to 25 mm in section thickness Ni-Cr-Mo Full annealed at 955°C, normalized at 870°C, tempered at 665°C Tensile strength, 620 MPa; 0.2 % yield strength, 415 MPa; elongation in 50 mm, 20%; reduction in area, 40% To meet mechanicalproperty requirements Forged flange 4137 Normalized at 870°C, tempered at 570°C Hardness, 200 to 225 HB To refine grain size and obtain required hardness Valve-bonnet forging 4140 Normalized at 870°C and tempered Hardness, 220 to 240 HB To obtain uniform structure, improved machinability, and required hardness Fig. 2 — Microstructures showing the refinement of primary ferrite grains by normalizing of a 0.5%C steel. (a) Air cooled from hot working range (e.g., 1200°C). (b) Normalized after treatment in (a)[3]. (a) (b)
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