The Physical Metallurgy of Steel
The advantage of steel as an engineering material is its versatility and, to a large extent, this arises from the fact that its properties can be controlled and changed by heat treatment. Thus, if steel is to be formed into some intricate shape, it can be made very soft and ductile by a particular heat treatment; if, on the other hand, it is to resist wear, it can be made very hard by a different heat treatment.
The characteristics provided by these finishing processes are inherent in the chemical composition of the steel and its physical condition. Grain structure is also important.
To examine the micro-structure, a sample is first polished to a very high finish, and then lightly etched in acid or some other reagent, so that the particular grains are revealed.
The sample is then viewed under a microscope, 50 to 500 magnifications being common for routine examinations.
Steels at temperatures above the critical range (727°C) have austenite as their main microconstituent.
The relative amounts of ferrite, cementite and pearlite in steel at room temperature depend on its carbon content, and the rate of cooling from the critical temperature range.
Austenite is basically a “solid solution” of carbon in iron. The carbon and iron are intimately mixed, as they would be in a liquid, but both remain in a solid state. Austenite is non-magnetic, so steel loses its magnetism at temperatures above the critical range.
On cooling below the critical temperature range, the austenite decomposes to form “ferrite”, “cementite”, “pearlite” or some combination of these. The relative amounts of the constituents in a particular steel will influence its properties.
Ferrite is almost pure iron and is very soft and ductile. Steels with a large percentage of ferrite in their structure will have relatively low strength characteristics, but will be extremely ductile.
Pearlite consists of alternate lamellae (or plates) of ferrite and cementite. It contains approximately 0.85% carbon. Pearlite is harder than ferrite but softer than cementite. Thus the amount of pearlite in the structure influences the toughness of the steel, as it combines the hardness of the cementite with the ductility of the ferrite.
Cementite is iron carbide (Fe3C) containing approximately 6.7% carbon. It is very hard and brittle, and reduces the ductility of steel. It may occur as free cementite or as part of the pearlite constituent.
When sufficient alloying elements are present, possible to retain the austenitic structure at normal temperatures. This produces non-magnetic steels with a high tensile strength and no loss of ductility. Austenitic grades of stainless steel are examples of this category.
There are three methods of heat treatment in common use:
quenching and tempering.
The difference between the heat treatment processes is the rate of cooling. Annealing involves a slow, controlled cooling; normalising involves a faster cooling in still air, and quenching involves very rapid cooling usually in a liquid bath.
Annealing is a general term describing several procedures. The process is used mainly to relieve stresses set up in the steel due to its original cooling or to its subsequent hot and cold working.
Manufacturers usually anneal to soften the steel prepare it for further machining, cold working or heat treating operations. Annealing may also be used to develop a particular micro structure in the steel. Full annealing consists of heating the steel above its critical range, holding at that tempera for an hour or two and then cooling at a slow, controlled rate. This results in a coarse, pearlitic structure which is soft, ductile and without strain. For low carbon and medium carbon grades of steel, coarse pearlite is the best structure for machinability.
When this property is particularly required, the steel is held for prolonged periods at a temperature just below the critical range and then cooled slowly to developed “spheroids” or globules of cementite in a ferrite matrix.
Stress relief annealing is used to remove the stresses set up during cold working particularly in low carbon steels. It is also used to relieve internal stresses set up by welding. The steel is heated to temperatures below the critical range and held there for up to three or four hours to equalise the temperatures throughout, and then cooled slowly. This reduces internal stress and increases ductility. This type of annealing is not as effective in close control of hardness and microstructure.
For applications requiring a bright smooth surface, inert gas atmosphere is used in the heating furnaces to prevent oxidation scale. This is known as “bright annealing”.
Normalising, though not an annealing treatment, is a related process. The steel is heated to a temperature above the critical range, held for a short time, then allowed to cool in still air. This results in a refined grain size and a more uniform structure, thus removing any undesirable effects of hot or cold working. Normalised structures are tougher but less ductile than annealed structures. Normalising is also used to condition steel for further heat treatment.
Quenching and tempering
Quenching and tempering usually consists of three successive operations:
a) heating the steel above the critical temperature range and holding at that temperature to develop a uniform austenitic structure,
b) hardening the steel by quenching it in oil, water, brine or caustic bath; and
c) tempering the steel by reheating to a selected point below the critical range to obtain the desired combination of hardness and ductility.
The rapid cooling action during quenching prevents the austenite structure from developing normally into ferrite, cementite or pearlite. Instead, a new micro-constituent called martensite is formed. The long needle-like grains of martensite are the hardest and most brittle form of steel, with an extremely high tensile strength but practically no ductility.
By carefully selecting the temperature for reheating, the martensite is reformed into a finer, tougher form called “tempered martensite”. This tempering action can produce any desired combination of hardness and ductility possible for that steel.
Annealing results in a low degree of hardness, tensile strength and toughness, but gives outstanding ductility. Normalising results in medium hardness, tensile strength and toughness, but also retains fairly good ductility. Quenching and tempering is used to give the greatest hardness and tensile strength.