Heat Treatment Of Metals
Heat Treatment can precisely be described as 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.
Additionally, 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 especially lumber. Most steels are heat treated for one of the following reasons:
3. Material Transmogrification
Common Heat Treatments
Softening is used as a common heat treatment for many reasons, such are, 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 wiredrawing. Annealing â€” full Process, spheroid zing, normalizing and tempering â€” aus tempering, mar tempering are the principal ways by which steel is softened.
Quenched steel though strong has many disadvantages and inappropriate for use in most applications. Tempering alleviates such a problem. For most steels, tempering involves heating to between 250 and 500 °C, holding that temperature (soaking) for an appropriate amount of time, then cooling slowly over an appropriate length of time (minutes or hours). This heat treatment results in higher toughness and ductility, without sacrificing all of the hardness and tensile strength gained from rapid quenching. Tempering balances the amount of hard marten site with ductile ferrite and pearlite. In some applications, different areas of a single object are given different heat treatments. This is called differential hardening. It is common in high quality knives and swords.
Annealing is a technique used to recover cold work and relax stresses within a metal. This process results in soft ductile metal. When cooled in a furnace it is called a full anneal heat treatment. When an annealed part is removed from the furnace and allowed to cool in air, it is called a “normalizing” heat treatment. During annealing, small grains re-crystallize to form larger grains. Diagram (a) below shows high stress within the metal property. Diagram (b) illustrates the stress reduction after annealing process.
Normalization is an annealing process in which a metal is cooled in air-cool to room temperature after heating. This process is typically confined to harden able steel. It is used to refine grains which have been deformed through cold work, and can improve ductility and toughness of the steel. It involves heating the steel to just above its upper critical point. It is soaked for a short period then cooled in air. Small grains are formed which give a much harder and tougher metal with normal tensile strength and not the maximum softness achieved by annealing.
Hardening of steels is done to increase the strength and wear properties. One of the pre-requisites for hardening is sufficient carbon and alloy content. If there is sufficient Carbon content then the steel can be directly hardened. Otherwise the surface of the part has to be Carbon enriched using some diffusion treatment hardening techniques.
Case hardening is a simple method of hardening steel. Is a simple process during which soft, low carbon iron or steel is heated in the intimate presence of a carbon donating (carburizing) material. This can be charcoal, cyanide, acetylene gas or any of many other materials. The carbon, is infused into the surface layer of the soft iron, and turns it into a HIGH CARBON STEEL. This when quenched from a suitable temperature, results in a thin, surface CASING of glass hard, wear resistant material. It is the process of choice for such things as automotive transmission gears, and guns, etc.
Carburization (often referred to as carburizing) is the name of the process by which carbon is introduced into a metal. The carbon diffused into the surface is intended to make the surface harder and more abrasion resistant. Carburization of steel generally involves a heat treatment of the metallic surface using a gaseous, liquid, solid or plasma source of carbon. The process depends primarily upon ambient gas composition and furnace temperature, which must be carefully controlled, as the heat may also impact the microstructure of the rest of the material. For applications where great control over gas composition is desired, carburization may take place under very low pressures in a vacuum chamber.
The process of carburization works via the implantation of carbon atoms in to the surface layers of a metal. As metals are made up of atoms bound tightly into a metallic crystalline lattice, the implanted carbon atoms force their way into the crystal structure of the metal and either remain in solution or react with the host metal to form ceramic carbides. In oxy-acetylene welding, a carburizing flame is one with little oxygen, which produces a sooty, lower-temperature flame. It is often used to anneal metal, making it more malleable and flexible during the welding process.