Hot-Working Operations

Abstract

Hot working is the initial step in the mechanical working of most metals and alloys. Not only does hot-working result in a decrease in the energy required to deform the metal and an increased ability to flow without cracking, but the rapid diffusion at hot-working temperatures aids in decreasing the chemical inhomogeneities of the cast-ingot structure. Blowholes and porosity are eliminated by the welding together of these cavities, and the coarse columnar grains of the casting are broken down and refined into smaller equiaxed recrystallized grains. These changes in structure from hot working improves ductility and toughness over the cast state.

However, there are certain disadvantages to hot-working. First, because high temperatures are usually involved, surface reactions between the metal and the furnace atmosphere become a problem. Ordinarily hot working is done in air, oxidation results, and a considerable amount of metal may be lost. Second, surface decarburization of hot-worked steel can be a serious problem, and frequently extensive surface finishing is required to remove the decarburized layer. Rolled-in oxide makes it difficult to produce good surface finishes on hot-rolled products, and because allowance must be made for expansion and contraction, the dimensional tolerances for hot-worked mill products are greater than for cold-worked products. Further, the structure and properties of hot-worked metals are generally not so uniform over the cross section as in metals which have been cold worked and annealed. Since the deformation is always greater in the surface layers, the metal will have a finer recrystallized grain size in this region.

This way, this chapter five is dedicated to the hot working of the steel, and it is complemented with several exercises, problems, and case-studies related with the industrial practice.