The advantages are:
*Decrease in yield strength, therefore it is easier to work and uses less energy or force
*Increase in ductility
*Elevated temperatures increase diffusion which can remove or reduce chemical inhomogeneities
*Pores may reduce in size or close completely during deformation
*In steel, the weak, ductile, face-centered-cubic austenite microstructure is deformed instead of the strong body-centered-cubic ferrite microstructure found at lower temperatures
Usually the initial workpiece that is hot worked was originally cast. The microstructure of cast items does not optimize the engineering properties, from a microstructure standpoint. Hot working improves the engineering properties of the workpiece because it replaces the microstructure with one that has fine spherical shaped grains. These grains increase the strength, ductility, and toughness of the material.
The engineering properties can also be improved by reorienting the inclusions (impurities). In the cast state the inclusions are randomly oriented, which, when intersecting the surface, can be a propagation point for cracks. When the material is hot worked the inclusions tend to flow with the contour of the surface, creating stringers. As a whole the strings create a flow structure, where the properties are anisotropic (different based on direction). With the stringers oriented parallel to the surface it strengthens the workpiece, especially with respect to fracturing. The stringers act as "crack-arrestors" because the crack will want to propagate through the stringer and not along it.
The disadvantages are:
*Undesirable reactions between the metal and the surrounding atmosphere (scaling or rapid oxidation of the workpiece)
*Less precise tolerances due to thermal contraction and warping from uneven cooling
*Grain structure may vary throughout the metal for various reasons
*Requires a heating unit of some kind such as a gas or diesel furnace or an induction heater, which can be very expensive