{"id":8928,"date":"2024-11-19T16:28:22","date_gmt":"2024-11-19T16:28:22","guid":{"rendered":"https:\/\/www.h-metal.ro\/blog\/?p=8928"},"modified":"2024-11-19T16:39:07","modified_gmt":"2024-11-19T16:39:07","slug":"steel-tempering","status":"publish","type":"post","link":"https:\/\/www.h-metal.ro\/blog\/en\/steel-tempering\/","title":{"rendered":"Tempering of steel"},"content":{"rendered":"

Steel tempering <\/strong>is a thermal treatment that consists in heating it above the temperature<\/strong> of the phase transformations, maintaining <\/strong>it at this temperature to complete all the phase transformations and cooling<\/strong> it at high speed.<\/p>\n\n\n\n

In the quenching treatment, besides the cooling speed factor, the alloying elements also play an important role: nickel, chromium, molybdenum, silicon, tungsten, vanadium.<\/p>\n\n\n\n

This process is carried out in order to obtain the desired physical-chemical properties: high hardness<\/strong> or resistance to abrasion.<\/p>\n\n\n\n

The heating temperature for tempering is chosen depending on the need to put the components in solution.<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

The carbon content influences the tempering temperature. The time interval in which the steel <\/a>piece is subjected to the tempering temperature is necessary to be high enough, to favor the complete dissolution of the carbon in the austenite.<\/p>\n\n\n\n

Cooling media for steel tempering<\/h2>\n\n\n\n

Tempering is performed by immersing the metal object in different cooling media:<\/p>\n\n\n\n

– water and aqueous solutions that contain either active substances (sodium chloride, sodium hydroxide), or substances that reduce the cooling capacity (silicates, glycerin);<\/p>\n\n\n\n

– vegetable or mineral oils;<\/p>\n\n\n\n

– baths of salts or molten metals, brought to a prescribed temperature;<\/p>\n\n\n\n

– gases (usually air, exceptionally hydrogen).<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

There are two main requirements that must be met by the steel tempering media:<\/p>\n\n\n\n

1. To help the penetration of tempering as deep as possible inside the part, if possible on its entire section.<\/p>\n\n\n\n

2. During the tempering process, no internal stresses should occur that could lead to deformation or cracking of the product.<\/p>\n\n\n\n

Types of steel hardening<\/h3>\n\n\n\n

Classic tempering: in one environment<\/em><\/p>\n\n\n\n

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.<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

Disadvantages: thermal stresses (between areas with different sections) structural (martensite has the maximum volume).<\/p>\n\n\n\n

Special tempering procedures: for the (partial) elimination of the disadvantages of classical tempering<\/p>\n\n\n\n

Interrupted quenching (2 mediums: water \u2013 oil)<\/em><\/p>\n\n\n\n

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.<\/p>\n\n\n\n

Tempering in steps (maintenance to equalize the temperature)<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

The martensitic transformation occurs at a slow speed, in air, which leads to a strong decrease in the internal stresses in the part.<\/p>\n\n\n\n

Isothermal tempering (to obtain a bainitic structure)<\/em><\/p>\n\n\n\n

The steel is kept in the bath until the end of the isothermal transformation of the austenite.<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

The duration of the maintenance is established with the help of the austenite isothermal transformation diagrams.<\/p>\n\n\n\n

Hardenability is the property of a steel to be hardened in depth and is characterized by two dimensions:<\/h4>\n\n\n\n

Superficial tempering<\/strong><\/p>\n\n\n\n

Only for the exterior of the parts that becomes hard and resistant; the interior remains tenacious.<\/p>\n\n\n\n

1. The tempering capacity<\/strong>, expressed by the maximum hardness that can be achieved by tempering, in the case of the respective alloy.<\/p>\n\n\n\n

2. The depth of tempering<\/strong>, which represents the ability of the material to form a tempered layer, with a martensitic or semi-martensitic structure, at a certain depth.<\/p>\n\n\n\n

 Hardenability depends on the composition of the steel, the heating temperature for tempering and the tempering environment.<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

The highest hardness after tempering is achieved by the steel with the highest carbon content.<\/p>\n\n\n\n

Since hardened steel becomes brittle, it is necessary to apply tempering <\/strong>treatment, otherwise we have a hard steel piece, but at the same time very fragile.<\/p>\n\n\n\n

Similar articles:<\/p>\n\n\n\n

The history of steel<\/a><\/p>\n\n\n\n

Steel ductility<\/a><\/p>\n\n\n\n

Tempered steel<\/a><\/p>\n

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