Heat treatment of metals & its types

Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. Heat treatment is sometimes done inadvertently due to manufacturing processes that either heat or cool the metal such as welding or forming.

Heat Treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability, restore ductility after a cold working operation. Thus it is a very enabling manufacturing process that can not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics.

Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the commercial use of steels exceeds that of any other material. Steels are heat treated for one of the following reasons:

 

1.Softening

 2.Hardening 

3.Material Modification

 

These three changes are one of the most important characteristics that are modified in the materials to make them more rust-resistant and more hard:

 

1-Softening: Softening is done to reduce strength or hardness, remove residual stresses, improve toughness, restore ductility, refine grain size or change the electromagnetic properties of the steel.

Restoring ductility or removing residual stresses is a necessary operation when a large amount of cold working is to be performed, such as in a cold-rolling operation or wire drawing. 

2-Hardening: Hardening of metals is done to increase the strength and wear properties. One of the prerequisites for hardening is sufficient carbon and alloy content. If there is sufficient Carbon content then the metal can be directly hardened. Otherwise the surface of the part has to be Carbon enriched using some diffusion treatment hardening techniques.

3-Material Modification: Heat treatment is used to modify properties of materials in addition to hardening and softening. These processes modify the behavior of the metals in a beneficial manner to maximize service life, e.g., stress relieving, or strength properties and cryogenic treatment.

Metals have been boon to engineering. Engineers, in turn, have learned how to improve many metals and tailor their properties to the task at hand. Here are some of the heat treating processes metalworkers rely on to ensure the metals they turn out have the properties customers need.

Annealing. Heating and slowly cooling metal to remove stress, make the metal softer, refine the structure, or change its ductility.

Carburizing. Adding carbon to the surface of iron-based alloys by heating the metal to below its melting point (usually 1,560 and 1,740°F) and putting it in contact with carbon-rich solids, liquids, or gasses. This increases the metal’s strength and makes the surface harder and more abrasion-resistant, but also decreases its toughness. Carburizing is usually done on finished parts.

hardening. This hardens an iron alloy’s surface by first carburizing and then quenching it to fix the process, so that the center is relatively soft. Thus, the part consists of a hardened case around a soft core.

Cyanide hardening. This is a variant on case hardening that brings the metal surface in contact with molten cyanide salt. The part is then quenched.

Decarburization. This involves the removal of carbon from the surface of steel, either by applying heat or through the normal aging process of oxidation.

Drawing (tempering):Metal is reheated after being hardened, then held at a specific temperature, and finally quenched. This process reduces hardness and increases toughness.

Nitriding:This is a hardening process whereby nitrogen is added to the metal surface through contact with ammonia gas. It produces case hardening without quenching. 

Precipitation hardening:This is a hardening process (also known as age hardening) in which certain metals are held at elevated temperatures without quenching. The process increases the yield strength of malleable materials, including most structural alloys of aluminum, magnesium, nickel, titanium, and some stainless steels. In superalloys, it can improve high-temperature strength.

Quenching:Steel is rapidly cooled by immersing it into oil or water, fixing its structure in a hardened state.

Spheroidizing (anneal):This describes any process of heating and cooling steel for the purpose of increasing the amount of rounded or globular carbide in the metal. It softens the steel and improves its ductility.

Cold working:While not a heat treating process, it is used to change a metal’s characteristics. This is accomplished by deforming the metal at room temperature without fracturing it. This changes its shape, giving the metal more tensile strength and better machinability.