Tempering of steel
Steel tempering is a thermal treatment that consists in heating it above the temperature of the phase transformations, maintaining it at this temperature to complete all the phase transformations and cooling it at high speed.
In the quenching treatment, besides the cooling speed factor, the alloying elements also play an important role: nickel, chromium, molybdenum, silicon, tungsten, vanadium.
This process is carried out in order to obtain the desired physical-chemical properties: high hardness or resistance to abrasion.
The heating temperature for tempering is chosen depending on the need to put the components in solution.
The cooling is carried out at a speed higher than the critical tempering speed of the respective steel. In order to obtain this structure, carbon steels and alloy steels in which carbon exceeds 0.15 – 0.20% are tempered.
The carbon content influences the tempering temperature. The time interval in which the steel piece is subjected to the tempering temperature is necessary to be high enough, to favor the complete dissolution of the carbon in the austenite.
Cooling media for steel tempering
Tempering is performed by immersing the metal object in different cooling media:
– water and aqueous solutions that contain either active substances (sodium chloride, sodium hydroxide), or substances that reduce the cooling capacity (silicates, glycerin);
– vegetable or mineral oils;
– baths of salts or molten metals, brought to a prescribed temperature;
– gases (usually air, exceptionally hydrogen).
The environment is chosen according to the composition of the steel. It must be stated that certain steels do not allow tempering in any environment, there is a risk of not obtaining the desired hardness because they either cool down too quickly, or they don’t cool down fast enough.
There are two main requirements that must be met by the steel tempering media:
1. To help the penetration of tempering as deep as possible inside the part, if possible on its entire section.
2. During the tempering process, no internal stresses should occur that could lead to deformation or cracking of the product.
Types of steel hardening
Classic tempering: in one environment
It consists in cooling the product in a single medium, which can be water, oil or even air. The heated piece is immersed in the liquid cooling medium until it cools completely.
For carbon steel parts with sections larger than 5 mm, the most used medium is water. While, for parts with sections smaller than 5 mm, or from alloy steels – the medium used is oil.
Disadvantages: thermal stresses (between areas with different sections) structural (martensite has the maximum volume).
Special tempering procedures: for the (partial) elimination of the disadvantages of classical tempering
Interrupted quenching (2 mediums: water – oil)
To reduce the internal tensions, tempering in two environments is applied. The piece is first cooled in water to 300-400 C, and then in oil to ambient temperature.
Tempering in steps (maintenance to equalize the temperature)
Through this method, the part is quickly cooled in a salt bath with a temperature slightly higher (by 30-50 C) than the martensitic transformation temperature Ms, maintaining at this temperature until the temperature is uniform in the entire section of the part and the final cooling in air.
The martensitic transformation occurs at a slow speed, in air, which leads to a strong decrease in the internal stresses in the part.
Isothermal tempering (to obtain a bainitic structure)
The steel is kept in the bath until the end of the isothermal transformation of the austenite.
The temperature of the salt bath is usually 250-350 C. As a result of the isothermal tempering, a bainitic structure with a hardness of 45-55 HRC is obtained, keeping plasticity and toughness at high values.
The duration of the maintenance is established with the help of the austenite isothermal transformation diagrams.
Hardenability is the property of a steel to be hardened in depth and is characterized by two dimensions:
Superficial tempering
Only for the exterior of the parts that becomes hard and resistant; the interior remains tenacious.
1. The tempering capacity, expressed by the maximum hardness that can be achieved by tempering, in the case of the respective alloy.
2. The depth of tempering, which represents the ability of the material to form a tempered layer, with a martensitic or semi-martensitic structure, at a certain depth.
Hardenability depends on the composition of the steel, the heating temperature for tempering and the tempering environment.
An optimal tempering structure is influenced both by the applied processes, the heating and cooling media, as well as by the duration, speed, heating temperature and cooling speed.
The highest hardness after tempering is achieved by the steel with the highest carbon content.
Since hardened steel becomes brittle, it is necessary to apply tempering treatment, otherwise we have a hard steel piece, but at the same time very fragile.
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