In metallurgy, hot working refers to processes where

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

s are plastically deformed above their recrystallization temperature. Being above the recrystallization temperature allows the material to recrystallize during deformation. This is important because recrystallization keeps the materials from

strain hardening In materials science, work hardening, also known as strain hardening, is the strengthening of a metal or polymer by plastic deformation. Work hardening may be desirable, undesirable, or inconsequential, depending on the context. This strength ...

, which ultimately keeps the

yield strength In materials science and engineering, the yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and ...

and

hardness In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...

low and

ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile str ...

high.Degarmo, p. 373. This contrasts with

cold working In metallurgy, cold forming or cold working is any metalworking process in which metal is shaped below its recrystallization temperature, usually at the ambient temperature. Such processes are contrasted with hot working techniques like hot r ...

. Many kinds of working, including

rolling Rolling is a type of motion that combines rotation (commonly, of an axially symmetric object) and translation of that object with respect to a surface (either one or the other moves), such that, if ideal conditions exist, the two are in contact ...

, forging,

extrusion Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex ...

, and drawing, can be done with hot metal.

Temperature

The lower limit of the hot working temperature is determined by its recrystallization temperature. As a guideline, the lower limit of the hot working temperature of a material is 60% its melting temperature (on an absolute temperature scale). The upper limit for hot working is determined by various factors, such as: excessive oxidation, grain growth, or an undesirable phase transformation. In practice materials are usually heated to the upper limit first to keep forming forces as low as possible and to maximize the amount of time available to hot work the workpiece. The most important aspect of any hot working process is controlling the temperature of the workpiece. 90% of the energy imparted into the workpiece is converted into heat. Therefore, if the deformation process is quick enough the temperature of the workpiece should rise, however, this does not usually happen in practice. Most of the heat is lost through the surface of the workpiece into the cooler tooling. This causes temperature gradients in the workpiece, usually due to non-uniform cross-sections where the thinner sections are cooler than the thicker sections. Ultimately, this can lead to cracking in the cooler, less ductile surfaces. One way to minimize the problem is to heat the tooling. The hotter the tooling the less heat lost to it, but as the tooling temperature rises, the tool life decreases. Therefore the tooling temperature must be compromised; commonly, hot working tooling is heated to 500–850 °F (325–450 °C).Degarmo, p. 374.

Advantages and disadvantages

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 Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K ...

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 Cast may refer to: Music * Cast (band), an English alternative rock band * Cast (Mexican band), a progressive Mexican rock band * The Cast, a Scottish musical duo: Mairi Campbell and Dave Francis * ''Cast'', a 2012 album by Trespassers William ...

. 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 A grain is a small, hard, dry fruit (caryopsis) – with or without an attached hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and legumes ...

. 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 Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...

. 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

Processes

Rolling Rolling is a type of motion that combines rotation (commonly, of an axially symmetric object) and translation of that object with respect to a surface (either one or the other moves), such that, if ideal conditions exist, the two are in contact ...

** Hot rolling * Hot spinning *

Extrusion Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex ...

* Forging * Drawing *

Rotary piercing Rotary piercing is a hot working metalworking process for forming thick-walled seamless tubing. There are two types: the Mannesmann process, invented in the 1880s, and the Stiefel process, developed two decades later. Mannesmann process A heated ...

References

Notes

Bibliography

*. {{Authority control Metalworking Metallurgical processes