Steel is a commodity used in almost every facet of our society. From the beams in our bridges to the bodies of our cars, it is used in construction, manufacturing, transport, packaging and many more industries to such an extent that we produce and consume over 1 billion metric tons of the metal every single year.
Not all steel is the same however. In fact, different compositions of steel have very different mechanical properties. Moreover, we can treat steel in a number of ways in order to manipulate these properties and make the metal fit for a specific purpose. Using different techniques we can alter the ductility, hardness, strength or resistance to impact of steel without altering its thermal, electrical or elastic properties.
All heat treatments are involved in a trade-off between the steel’s ductility and its strength. By increasing the ductility of a particular steel part, you decrease its strength and vice versa. As such, different heat treatments are suitable for different applications and it is imperative that one characteristic is not overly sacrificed for another.
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The majority of heat treatments for steel begin by heating the steel to a level at which the iron elements within undergo a phase change. At this higher temperature, iron enters what is known as the austenite phase, where it is able to absorb more carbon. The steel is then cooled, or quenched, at a rate that allows carbon to diffuse. The quenching rate dictates how quickly the carbon diffuses; a swift rate produces finer grains whilst a slower rate produces coarser grains.
A typical quenching process of carbon steel involves heating the metal up before rapidly cooling it in water or oil to its critical temperature. This critical temperature is dictated by the carbon content of the steel and is typically lower for a lower percentage of carbon content. This process produces a steel part that is saturated with carbon. It is extremely brittle (usually too brittle for any practical use) but has up for four times the fold hardness of non-quenched steel.
Full annealing is a heat treatment used to soften brittle steels. It is achieved by heating the metal to around 50°C (depending on its carbon content) and holding it at that temperature for a period before allowing it to cool slowly, under the control of a furnace. Its internal structure becomes what is known as coarse pearlite, meaning the steel is soft and ductile.
Normalising is a similar process that also softens brittle steels. The steel is heated to around 60°C, held there for a period and allowed to cool at room temperature under natural convection, rather than under the control of a furnace. This produces a fine pearlite structure, again meaning the metal is more ductile.
Apart from the cost, the major difference between full annealing and normalising is the uniformness of the steel’s internal structure. As full annealing utilises a furnace to control the cooling process the ductility of the workpiece is uniform, meaning machinability is consistent across the part. Whilst it is more expensive, it is far more reliable and hence the more popular process in manufacturing.
The most ductile form of steel is produced by a process known as spheroidization. It involves heating the steel up to around 700°C for over 30 or more hours or a process of cyclic heating to produce an internal structure of spherical cementite within the ferrite or pearlite. It is used to prepare steel for manufacturing or prior to hardening.
There are also other forms annealing. Process annealing involves heating steel to slightly lower than its critical temperature (usually just below 700°C), long enough to allow re-crystallisation of the ferrite phase, before cooling in air. There is no phase change as in other heat treatments and so the grain structure’s size and distribution is the only thing that changes. It is usually used to treat steel with a low carbon content that has been work-hardened, before undergoing further processing or cold working.
Short-cycle annealing involves either of the aforementioned annealing processes, but over much shorter periods of time. A steel part is heated, held and cooled in somewhere around a 5 hour cycle in order to make the part more malleable before further processing.