Steels owe their dominance of the field of engineering materials to their ability to respond to heat treatment and provide appropriate properties at economic cost on a production scale.
The consistent and reproducible nature of the properties developed by heat treatment.
Why Heat Treat?
Steels can be softened, hardened and have their surface properties altered by heat treatment.
The Softening Processes
Annealing and normalising, reduce hardness, refine grain size and improve machinability. Their principal uses are therefore to make further processing operations easier or possible.
The Hardening Processes
Hardening (quenching) and tempering, develop the appropriate bulk and surface properties. Their principal use is to render the part fit for final use or purpose.
The Thermochemical Processes
Carburising, nitriding and boronising, are used to develop specific surface properties, again to make the part fit for final use or purpose.
All steels are alloys of iron and carbon, while other alloying elements are added to confer particular properties. The manipulation of heat treatment response is a prime reason for adding alloying elements to steels.
An appreciation of the thermal behaviour, with the accompanying microstructural changes, is fundamental to the understanding of heat treatment and the mechanical properties so generated.
These curves describe the decomposition of austenite into ferrite and cementite or martensite with time and temperature. They are the scientific basis for modern heat treatment and exist for all commercially available steels.
Figure 1 shows an idealised TTT Curve. In this figure, A represents austenite, F represents ferrite, C represents cementite. M represents martensite. As is the austenite/ferrite transformation temperature and Ms is the martensite start transformation temperature.
Figure 1. An idealised TTT curve for a plain carbon steel.
Cooling at different rates from point X, i.e. above the As temperature will develop very different microstructures, and therefore properties, in the steel.
When the steel is cooled rapidly, following Curve 1, to get below the 'nose temperature' of 520°C in less than approximately one second, it will begin to transform at the Ms temperature to martensite. The steel is said to be hardened by this process.
Martensite is a strong, hard, but brittle structure. After tempering, which increases toughness and reduces brittleness, it has widespread use throughout engineering.
Conversely if the steel cools very slowly (Curve 2) then the austenite transforms to ferrite and cementite and a much softer structure will result.
In summary, the rate of cooling from the austenite phase is the main determinant of final structure and properties.
All steels have TTT Curves of essentially the same shape.
Alloying elements influence the As and Ms temperatures significantly and move the position of the 'nose' to the right. This will allow slower cooling rates to 'miss the nose' and still permit transformation to martensite. In metallurgical terms this is described as increased hardenability.
Increased hardenability has two important practical effects. Less severe quenches can be used to achieve martensite, and therefore hardening. The risks of quench cracking and distortion are consequently reduced.
As the centre of a section will always cool more slowly than the edge it allows thicker sections to through harden. With sufficient alloying element content the 'nose' can move so far to the right that an air cool will permit transformation to martensite. Such steels, and many are tool steels, are described as air hardening.
Certain alloying elements, eg Nickel, Manganese and Nitrogen individually and collectively and in sufficient quantity, depress the As temperature below room temperature making the steel austenitic (hence not hardenable or indeed magnetic) at ambient temperatures.