With heat treatment, the hardness and abrasion resistance of the materials increase. This, of course, is associated with a decrease in toughness and – a prolonged relative length.
Chipping and non-chipping tools should be hard and resistant to abrasion. Structural steels under abrasion should be hardened.
Hardening takes place in three steps:
Warm up to hardening temperature,
Maintenance at hardening temperature and
Quenching (rapid heat discharge of the work piece)
Warm up to hardening temperature:
To prevent tensions and high temperature differences, the work piece is first heated slowly and uniformly up to a temperature of 600-700 ° C. Then, the hardening temperature, which is specified for each steel, is heated at a high speed, which results in a fine grain structure. In the long run, the surface of the work piece burns and flake, and its structure becomes coarse grains. This phenomenon is prevented by using a protective gas or heating it in a salt bath.
Hardness temperature: It depends on the type of steel as well as the shape and dimensions of the work piece. For thin and complex parts, the lower limit is applied, and for large and simple parts, the upper limit of the hardness temperature recommended by the manufacturer is applied.
In non-alloy steels, it is recommended that the work piece be heated to about 30-60 ° C above the G-S-K line in the iron-carbon diagram. At this temperature, components of the perlite structure change to austenite. The austenitic structure is a necessary condition for hardening. If the hardening temperature is low, the steel is soft, and if the temperature is high, the steel becomes coarse grains.
Structural evolution in heating:
In the austenitic structure, the iron atom lies in the center of the cube’s face (an atomic cube network in the side) and the carbon atom in the center of the cube. At the temperature below the G-S-K line, the carbon atom outside the iron network is chemically bonded (Fe3C).
Maintenance at hardening temperature:
To ensure that the cross-sectional structure completely changes to austenite, the hardness should be kept at a temperature of about 10 minutes for each 10 mm cross-sectional thickness. In the dimensions above 60 mm, the maintenance time T is obtained by the following thumbnail formula:
T is duration of keeping to minD is the diameter or thickness of the work piece to mm
The steel should be cooled very quickly to increase the hardness of the work piece. Each steel has a critical cooling rate. The heat of the work piece must be repelled so quickly that it can safely exceed the critical cooling velocity. If the cooling velocity is very low, the uniformity of hardness at all parts of the work piece is not guaranteed. Depending on the requirements, different cooling materials are used.
A very high cooling rate should be avoided so that the tensions stay as low as possible and hardening crack does not go down.
In quenching with liquids, make sure that the work piece is quickly immersed in the fluid and stir continuously and well. This prevent the air inhale insulation on the work piece to prevent the hardening process. The hollow parts should be immersed so that the air contained in it can reach the top. Asymmetric parts are often immersed vertically and with additional charge in the coolant fluid. In this way, the thin and thick parts of the work piece cool almost simultaneously in the bath.
Structural evolution when quenching:
In a quiet cooling of the austenitic structure, the carbon atom is driven by an iron atom from the center of the cube network. The carbon atom makes a very hard iron carbide bond and in the form of a bar and outside the network. In a quick cooling (for example, a convection in cold water), the carbon atom does not have enough time to get out of the center of the cube and form a carbide of iron, the carbon atom stays closed in the network. This will result a network under tension.
This internal tension is due to the increasing hardening. This delicate form of needle structure is named Martennsit after its discoverer, namely, Martens.
The time-temperature evolution diagram (CCT) for different steels can be used to determine the structure, structure and hardness of HRC in relation to temperature and time.
In quenching mode in oil, the most part of the structure is the baynite structure, which does not increase the stiffness. In air cooling, perlite is often produced.