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 1 Annealing is a generic term usually defined as a treatment that consists of heating to and holding at a suitable temperature followed by cooling at an appropriate rate, primarily for the softening of metals. Normalizing is a type of annealing specific to ferrous alloys. The functions of normalizing may overlap with (or be confused with) those of annealing, hardening, and stress relieving. Improved machinability, grain-structure refinement, homogenization, and modification of residual stresses are among the reasons normalizing is done. Homogenization of castings by normalizing may be done to break up or refine the dendritic structure and facilitate a more even response to subsequent hardening. Similarly, for wrought products, normalization can help reduce banded grain structure due to hot rolling, as well as large grain size or mixed large and small grain size due to forging practice. ANNEALING Generally, annealing is done by heating in furnaces, although at times, annealing by induction heating is also done when rapid heating is an effective method (such as annealing of wire after drawing). In plain carbon steels, annealing produces a ferritepearlite microstructure. Steels may be annealed to facilitate cold working or machining, to improve mechanical or electrical properties, or to promote dimensional stability. The iron-carbon binary phase diagram (Fig. 1) can be used to better understand annealing processes[1]. Although no annealing process ever achieves true equilibrium conditions, it can closely parallel these conditions. In defining the various types of annealing, the transformation temperatures or critical temperatures are usually used. The critical temperatures define the onset and completion of the transformation to or fromaustenite. The equilibrium critical temperatures depicted on the binary iron-carbon phase diagram (Fig. 1) are A1 and A3 for hypoeutectoid steel and A1 and Acm for the hypereutectoid steel [1]. It must be noted that due to the nonequilibrium effect, the critical cooling temperatures Ar1, Ar3, and Arcm (denoted with a suffix “r” for the French word refroidissement meaning cooling) are lower, whereas the critical heating temperatures Ac1, Ac3, and Accm (denoted with a suffix “c” for the French word chauffage) are higher than HEAT TREAT BASICS: ANNEALING AND NORMALIZING Annealing and normalizing both involve heating metal to a temperature and cooling back to room temperature and are di erentiated by the metals involved and rate of cooling. the corresponding equilibrium temperatures. Various alloying elements markedly affect these critical temperatures. For example, chromium raises the eutectoid temperature, A1, and manganese lowers it. The upper and lower critical temperatures can be calculated using the actual chemical composition of the steel[2]. In practice, annealing cycles are classified based on the specific purpose and the temperature to which the steel is heated and the method of cooling used. The maximum temperature may be below the lower critical temperature, A1 (subcritical annealing); above A1 but below the upper critical temperature, A3 in hypoeutectoid steels, or Acm in hypereutectoid steels (intercritical annealing); or above A3 (full annealing), which has been illustrated in Fig. 1. Because some austenite is present at temperatures above A1, cooling practice through transformation is a crucial factor in achieving desired microstructure and properties. Accordingly, steels heated above A1 are subjected either to slow continuous cooling or to isothermal treatment at some temperature below A1 at which transformation to the desired microstructure can occur in a reasonable amount of time. Under certain 9 Fig. 1 — Iron-carbon binary phase diagramwith superimposed full annealing, process annealing, and spheroidizing treatments[1].
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