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Metal Casting Technologies : March 2006
METAL Casting Technologies March 2006 47 ANNEALING The annealing of steel is performed for either of two specific purposes. The first is to "soften" the steel for subsequent operations like machining, stamping, bending, etc. The second is to produce a microstructure that will improve performance in drilling, broaching, and other machining and cutting operations. Metallurgical developments have shown that, while these properties are primarily dependent on the hardness of steel, they are also influenced to a marked degree by the microstructure. Full Annealing. The most common annealing procedure is the "full anneal." In this process, the steel is heated to above the critical temperature range, held there for a sufficient length of time to allow for the dissolution of carbon and alloying elements to take place, and then cooled very slowly, either by definitely controlling the cooling rate of the furnace or by shutting down the furnace and allowing the furnace and the load to cool down together. This treatment is illustrated in Figure 2. While it is simple to perform and is reliable for most steels, it generally results in a lamellar microstructure of pearlite. Further, it is time consuming, so that more modern refinements such as "isothermal" or "cycle" annealing have been developed to obtain the desired microstructure and hardness in shorter times. Isothermal or Cycle Annealing. This is done by cooling a furnace charge rapidly from the soaking temperature range to a predetermined temperature selected from the topmost portion of the S-curve and holding for the time indicated to produce complete transformation. Figure 3 is a schematic diagram showing this process that contrasts with the conventional "full annealing" treatment in that a more uniform microstructure results and the desired hardness can be obtained with a shorter time cycle. In one instance, the annealing cycle for wrenches was cut from 16 hours to 8 hours by changing to an isothermal annealing treatment. The rapid heat transfer and the uniformity of heat distribution necessary to perform this process satisfactorily are best obtained with furnaces using forced-convection heating and cooling methods. Spheroidize Annealing. This is done to agglomerate the plate-like carbides into "Spheroids" in the steel. This type of microstructure is desirable for certain cold- working operations and for machining the higher carbon alloy steels. It is obtained by holding the steel at the lower critical temperature (Ac1 = temperature at which austenite begins to form upon heating), either slightly above or slightly below, for a sufficient length of time to spheroidize the carbides, and then cooling slowly to room temperature. Stress-relief Annealing. This heat treatment is performed after certain operations such as deep- drawing, machine-straightening, flame-cutting, welding, or machining to "relieve the stresses" set up by these operations. As soon as the steel is heated above room temperature, these stresses begin to relive themselves, however, it is advisable to heat up slowly to at least 540˚ C (1,000˚ F) to assure complete relief of stresses. Soak at this temperature, as usual, and cool down to room temperature slowly. HARDENING The hardening of steels is carried out by quenching or the rapid cooling of steel from above its critical temperature range to some temperature below the Ms temperature (temperature below which martensite is formed). The quenching medium may be oil, water or brine, depending on the cooling rate desired (oil is slowest and brine is fastest), as determined by the chemical composition of the steel and the size and type of section. The steel as- quenched will be much harder than as-cast, as-rolled, Fig. 1. Normalizing Fig. 2. Full Annealing Fig. 3. Isothermal Annealing