Heat Treatment of Steel
Many changes occur when steel is subjected to heat. There are different heat treatment processes which are listed below:
Normalizing
Heating to a suitable temperature, between 800-930 degrees Celsius, dependent on steel specification, holding at temperature followed by cooling in still air. Relieves internal stresses, refines the grain structure and improves mechanical properties.
Annealing
Heating and holding at a suitable temperature and cooling slowly in the furnace with the object of softening the steel, improving machinability and cold working properties.
Stress Relieving
Usually carried out after rough machining or cold work to remove stresses. It is usually carried out at a temperature range of 550-650 degrees Celsius, followed by cooling in air.
Hardening
Heating to a temperature above the critical range, holding for sufficient time at that temperature followed by quenching in a suitable medium such as water or oil.
Tempering
Carried out immediately after hardening to relieve stresses, remove brittleness and reduce hardness to the required range. Usually carried out between 150 – 650 degrees Celsius, and cooling in air.
Nitriding
A process which produces a very hard case by the absorption of nitrogen into the surface of the steel. Depending on the specification, hardness figures up to 1100 VPN can be attained, whilst minimizing distortion of the workpiece.
Carburising
The diffusion of carbon into the surface of a steel that is low in carbon by heating in a solid, liquid, or gaseous medium, containing carbon at a temperature around 900 degrees Celsius. Allows a hard case to be produced on low carbon steels to achieve.
Induction Hardening
A surface hardening process where a component is heated by electrical induction followed by immediate quenching. The surface hardness will depend on the carbon content of the steel. For ideal results this is usually in the range 0.40%-0.45%C.
Influence of Elements in Steel
Steel is essentially an alloy of carbon and iron. It also contains other elements, some of which are retained from the steelmaking process, other constituents are added to produce specific properties. The more common elements are listed below:
Carbon (C)
Carbon is arguably the most important element in steel. It is the principal factor in the hardenability of steel, controlling both the hardness and strength of the material.
Manganese (Mn)
Its presence has three main effects, it is a mild de-oxidant acting as a cleanser taking the sulphur and oxygen out of the melt into the slag. It increases hardenability and tensile strength but decreases ductility. It combines with sulphur to form manganese sulphides, essential in free cutting steels.
Silicon (Si)
In most commercial steels it is present in a range of 0.05/0.35% and acts as a powerful deoxidizing agent. It is present in higher contents in Silicon-Manganese Spring Steels and Acid and heat resisting steels.
Sulphur (S)
It is a non-metal normally regarded as an impurity and has an adverse effect on impact properties if a steel is high in sulphur but low in manganese. The welding properties of steels with high sulphur are poor. Free cutting steels have sulphur deliberately added to improve machinability, usually up to a maximum of 0.35%.
Lead (Pb)
The addition of about 0.25% lead improves machinability. It also reduces fatigue strength, ductility and toughness, although this only becomes serious in the transverse direction and at high tensile levels.
Selenium & Tellurium (Se,Te)
These elements are added to certain steels to improve machinability. In free machining stainless steels a selenium content of 0.15/0.25% is typical. Tellurium levels of 0.03/0.05% are added to leaded free cutting steels to further improve machinability.
Phosphorus (P)
Although it increases the tensile strength of steel and improves machinability it is usually regarded as an undesirable impurity because of its embrittling effect. Most steels do not exceed 0.05% phosphorus.
Nickel (Ni)
When added to carbon steel in amounts up to 5% it increases the tensile strength, toughness and hardenability without loss of ductility. Often used in combination with other alloying elements, especially chromium and molybdenum. Stainless steels contain between 8% and 14% nickel.
Chromium (Cr)
Increases hardenability and with high carbon improves resistance to abrasion and wear. An essential element in stainless steels and heat resistant steels where contents of up to 30% may be present.
Molybdenum (Mo)
Increases hardenability and reduces the risk of temper brittleness in low alloy steels. It is added to stainless steels to increase their resistance to corrosion and is also used in high speed tool steels.
Tungsten (W)
Is used as the main element in high speed tool steels. After heat treatment the steel maintains its hardness at high temperature making it particularly suitable for cutting tools.