When cold worked steel pressure, the hardness inevitably increases, and the plasticity falls. At a certain point, further cold working of the steel becomes impossible and it requires annealing to restore its plasticity.. Such annealing is called technological. In most cases, quite a low, subcritical annealing, which is the cheapest. Temperature range, which are used in process annealing is shown by the shaded area in the simplified iron-carbon phase diagram of the figure below..
Technological annealing before stamping
Technological annealing is often used then, when parts need to be exposed, eg, stamping. Hot rolled high carbon steels also often require annealing to prevent cracking and softening prior to such operations., how cutting, felling or straightening. Technological annealing usually involves heating to a temperature below Ac1, holding and then cooling - usually in air. Usually heating to a temperature of 10-20 ° C below Ac1 gives the best combination of microstructure, hardness and mechanical properties. Temperature control is only necessary for, to prevent heating above Ac1, which could lead to undesirable changes in the microstructure.
When the sole purpose of annealing is to soften the steel in operations such as cold sawing or cold chopping, then the temperature is kept much lower than Ac1 without much of her control
Technological annealing for steel cutting
To ensure high quality machining of steel by cutting, its microstructure and hardness are of great importance.. Optimum microstructure types for various steels, processed by cutting, presented in the table.
Annealing of forgings
Forgings are often annealed to facilitate subsequent operations - usually machining or cold forming. The annealing method is determined by the type and quantity of the intended machining or cold forming, as well as the type of steel.
Annealing bars and wires
A significant volume of rods and wires is subjected to various types of annealing to reduce their hardness and prepare for subsequent machining or cold forming. To prepare for cold forming mild steel (to 0,20 % carbon) short low annealing is usually sufficient. Higher carbon steels, as well as alloy steels, require maximum plasticity to perform the spheroidization operation.
Annealing steel plates
Steel plates are sometimes annealed to facilitate shaping or machining. Plates are usually annealed with low annealing, while long exposures are avoided. The problem is maintaining the flatness of large slabs.
Steel pipes are often machined or shaped, eg, flexible. In most cases, low annealing with short exposure is used to lower the hardness of pipes.. High-carbon steel pipes are spheroidized before machining.. Pipes, which are made on pipe mills, rarely annealed and are mainly used in hot-rolled, normalized or hardened condition.
1. Gulyaev A. P. Metallurgy, 1986.
2. The Heater’s Guide: Practices and Procedures for Irons and Steels, AMS International, 1995.