HOT FORMING (OR HOT-WORKING) OF METALS
When metals are formed or worked at temperatures above their recrystallization temperature but below their absolute melting temperature, they are called being hot worked. At that stage, they behave as perfectly plastic materials. With increasing temperature, generally the yield strength and rate of strain hardening progressively reduce and ductility increases. Hot-working may also be defined as metal working at a temperature above which no strain hardening takes place. In other words, hot-working refers to deformation carried out under the conditions of temperature and strain rate such that recovery processes occur substantially during the declaration process, so that large strains may be achieved with essentially no strain-hardening. The strain in hot-working is large (ε=2 to 4) compared with tension or creep test. Not only less energy is required in hot-working the metal, the blow holes and porosity are also eliminated by the welding together of these cavities. The course columnar grains of the cast ingots are broken down and refined into smaller equiaxed recrystallized grains. This results in increase in ductility and toughness of the hot worked metal. The four major hot-working processes are schematically
When a metal is hot worked, say by rolling it passes through the opposing rollers and is thus reduced in its section thickness. The original large grain structure of the hot metal being rolled is elongated while passing through the rollers and later broke up into fragments in the deformation zone, and the fragments of the crystals thus formed become the nuclei for the formation of new smaller crystals and hence a fine uniform grained structure is produced in the hot rolled portion of the metal.
The hot rolling thus refines the gran structure. The temperature range (upper temperature and lower temperature) for hot-wang is discussed in the following The metals that can be hot worked include carbon steels (low, medium and high alloy steels, stainless steel, copper and its alloys such as brasses, bronzes, aluminium and magnesium alloys and titanium alloys
Temperature Range for Hot-working
As already mentioned, hot-working is carried out at a temperature above the recrystallization temperature but below the absolute melting temperature of the metal. Generally for hot-working. the ratio of absolute hot-working temperature (7) to the absolute melting point (Tm) of the metal being worked is kept greater than 0.6. In most cases, the metal is beated to such a temperature (below its sous temperature that after completion of hot working its temperature will remain a little higher than its recrystallization temperature. The following temperature range for hot-working is given only as a general guide
- For ferrous metals (steels)-930 10 1370°C
- For copper,brasses and bronzes-590 to 930-C
- For aluminium and magnesium alloys-345 so 480°C
The upper temperature limit for hot-working is determined by the temperature at which either melting or excessive oxidation occurs. Usually the maximum working temperature is limited to SPC below the melting point of the metal. This is to safeguard against the segregated regions of lower-melting point material present in the metal, as only a very little amount of a grain-boundary film of a lower-melting constituent may make the metal crumble into pieces when it is deformed. Such a condition is called hot shortness or burning
The lower temperature limit for hot-working of a metal is the lowest temperature at which the rate of recrystallization is rapid enough for eliminating strain hardening in the time when the metal is at that temperature. The lower hot-working temperature will depend upon amount of deformation and the time that the metal is at temperature. A metal which is capable of being rapidly deformed and cooled rapidly from temperature, will require a higher hot working temperature for the same degree of deformation than will metal slowly deformed and slowly cooled.
Advantages and Disadvantages of Hot-working
- Large deformations (or changes in size and shape) in the metal are easily, more rapidly and economically produced as the metal is hot and in plastic state and thus needs less power in deforming. Hot-working is mainly preferred where heavy reduction in size or heavy deformation is required and work-hardening (for increase in strength) is not the main requirement.
- Hot-working gives high production volumes of products of desired shapes and sizes (examples: 1-beams, channels, plates, rods, etc.) from the cast ingots received from the steel plants. These hot worked or wrought products find direct use in the market, thus bringing saving in time, material and machining costs by avoiding further forming or shaping of the wrought products.
- Hot-working of ingots (or metal in any form) improves mechanical properties of the metal by refining the grain structure, minimizing porosity (ingots have porosity being a cast product), developing directional flow lines of grains and breaking up and distributing unavoidable inclusions or impurities in the metal.
- Hot rolling is an effective way to reduce grain size in metals, for improved strength, ductility and impact resistance.
- Most hot worked products become the raw material for the secondary processes used to produce finished items by cold forming, cutting, drawing, bending, machining or welding.
- Rapid oxidation or scaling of hot metal occurs.
- Loss of carbon from steel surface during hot rolling results in loss of strength. The rolled product may thus give rise to fatigue crack during service.
- Close dimensional tolerances are not generally achieved in hot worked parts.
- Hot rolling and other hot-working operations like hot forging involve large tooling. costs on plants and their maintenance, although this is compensated by the high production volumes.
See More: Metal Forming:(Processes)
See More: Sheet metal process