1. Thermal Hardening
In thermal hardening a hard layer is formed at the surface of plain carbon and low alloy steels
of medium carbon content (0.3 - 0.6%) by heating the steel till it reaches the austenistising
temperature (about 850oC, depending upon carbon content) then rapidly quenching it to
form a martensitic structure.
The depth of hardening can be controlled by the rate of heating, rapid heating for a 5 - 20 seconds with an induction coil will limit the hardened zone to material adjacent to the surface. The more rapid the cooling the greater will be the depth of hardening (up to the limit of the austenitised zone) - the thinner the section the easier it is to cool rapidly. However the faster the quench the greater the danger of distortion or cracking. Increasing alloying content also gives greater depth of hardening and will allow a less severe quench to be used.
The depth of hardening is normally in the range of 1 - 5 mm.
2. Carburising
Carburising is a process in which carbon (up to 0.8 - 0.9%) is diffused into the surface of a steel component
which is subsequently hardened by quenching (and then often tempered).
The process is carried out in a controlled gaseous atmosphere, or a molten salt bath in the temperature range of 850 - 950oC. The process is applicable to a wider range of steels than thermal hardening, so a wider range of properties of the surface and of the core can be obtained. Again increased alloying content permits the use of a less severe quench, reducing the danger of distortion and cracking. Some distortion normally occurs and must either be allowed for (not always possible) or a grinding allowance provided.
Although case depths can be up to several mm, most applications use less than 2 mm.
3. Carbonitriding
Carbonitriding is a variation on carburising in which nitrogen is diffused into the steel along with
carbon. In the days of cyanide based molten salt carburising baths, this could readily be
achieved by lowering the temperature of the bath to about 750oC. Appropriate mixtures
of gases tend to be used currently.
Advantages of this process include improved wear and fatigue resistance of processed components.
4. Nitriding and Nitrocarburising
In nitriding, nitrogen is diffused into the surface of steel components by heating them to about 520oC
in an atmosphere of ammonia or exposing them to a low pressure nitrogen + hydrogen atmosphere
while they are subjected to a glow discharge. To obtain a high surface hardness (above 750 HV)
elements that form hard nitrides, eg Al, Cr, Mo, V, must be present in the steel.
Due to the lower temperature than that used for carburising, the process times are much longer (20 + hours) and this factor combined with the fact that there is no quench means that danger of distortion is greatly reduced.
Nitrocarburising is a variation on nitriding, normally carried out between 570 and 585oC in either a molten salt bath or gaseous atmosphere. A treatment time of 90 - 120 minutes gives a shallow layer, about 20 micro m, and although this is not particularly hard, it provides excellent wear and fatigue resistance.
There are a number of proprietary processes: Tufftride, Tennifer, Nitrotec. In some variations sulphides are added to the salt bath, giving even better wear resistance. Two processes are Sulfinuz and Sursulf.
Sulf BT is low temperature (200oC) salt bath process that adds only sulphur to the surfaces of ferrous components, this provides excellent wear resistance.
5. Boriding (boronising)
In this process boron is diffused into the surface of a plain carbon or low alloy steel at
approximately 950oC to form a layer of iron borides about 100 micro m thick with a
hardness in the range 1800 - 2100 Hv. The process can also be used on some other alloys eg
cobalt, nickel and titanium alloys. Processing time is a few hours and is carried out
with the components packed in granules.
6. Metalliding
The most important process in this category is probably the one developed by Toyota, Japan,
known as the Toyota Diffusion (or TD) process in which carbide forming elements such as
vanadium and niobium are diffused into steels from a salt bath at about 1000oC.
Carbide layers 5 - 12 micro m of very high hardness, about 3000 Hv, are produced.
The term metalliding is also used to describe processes in which coatings are electroplated from molten salt baths.
David Plane and David Grieve, 5th March 2003.
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