Hardening and tempering

Hardening and tempering

toplotna obdelavaHARDENING AND TEMPERING = HARDENING + TEMPERING

The process is carried out in furnaces with dimensions Φ1000×2000 mm.

In most cases, products are once again heated up to 200°C after hardening and are then slowly cooled down to room temperature. This is necessary during the hardening of alloy steel in the martensitic phase. The purpose of such heating is to reduce thermal stress. The stress may be so high that many products are rendered useless (due to cracks, deformities, and destruction). The effect of internal stress is significantly reduced by means of immediate tempering. This type of heat treatment is called steel tempering.

Heat treatment during which products are tempered after hardening at temperatures over 100°C is called hardening and tempering. The purpose of this process is to achieve satisfactory strength and toughness of products. This is, therefore, a two-phase process, consisting of hardening and tempering.  It is carried out for steel with carbon content of at least 0.3 to 0.6%. In most cases, these are steel alloys with Cr, Mo, Co, V, W, Ti, and others. The alloy components play different roles. They increase hardness by forming solid and stable carbides, they affect the size of crystal grains, and they improve toughness. Hardening and tempering is especially important in high-speed steel.

According to temperature, tempering may be:

  • low - up to 200°C – martensitic phase;
  • medium -  in bainite and fine-pearlite (troostite) phase;
  • high - in sorbite or pearlite phase;
  • very high - in special types of steel.

Specifying hardening and tempering temperatures is not appropriate because these temperatures may differ significantly for different types of steel with different chemical composition. It is better to specify structures which are formed after the tempering of a specific type of steel. During tempering, the martensitic structure of steel may be changed into bainite, fine pearlite (troostite), sorbite, or pearlite. At a select tempering temperature, a structure with known properties is formed. As the tempering temperature increases, the internal stress keeps reducing, the hardness also reduces, and the toughness increases.

 
1. Tempering in the martensitic phase
. During tempering up to 200°C, the structure is still martensitic. It is used after the hardening and cementation of carbon steel. During this type of tempering, the following occurs:

  • after lengthy annealing, carbon is partly extracted from the martensite and bonds into fine-grain cementite.
  • the formed martensite is called tempered martensite.
  • internal stress is partly reduced
  • internal stress is partly reduced

2. Tempering in the bainite phase is practiced more commonly. It significantly reduces internal stress and the bainite structure is the cause for the increased strength and toughness. In this phase, the hardness decreases by an insignificant amount. The process is performed in carbon low-alloy tool steel.

3. Tempering in the fine-pearlite (troostite) and sorbite phase takes place at temperatures between 350 and 500°C. According to the chemical composition of steel and the selected temperature, martensite transforms into fine pearlite or sorbite. After this type of tempering, the hardness significantly reduces and toughness significantly increases. Internal stress is almost entirely eliminated, thus significantly improving the strength of the products. The tempering in this phase is suitable for alloy steels and products that must have good strength and toughness.

4. High-temperature tempering

is carried out at temperatures of up to 700°C. This type of tempering is used with dynamically strained products which must have very good strength, hardness, and toughness. In this case, steel forms alloys with V, Nb, Ti, Co, W, Mo, Cr, and others. At such tempering temperatures, alloy components with released carbon form very strong and stable carbides and/or nitrides. It is very important that the extracted carbides and nitrides are very small and equally distributed over the entire structure. This is only possible when the carbides form with the help of the carbon which has been compulsorily dissolved in martensite. During normal cooling (without hardening and tempering), the formed carbides would be significantly more course-grained. In this type of steel, the carbon content is also adjusted to the alloy component content. Some types of steel reach their optimal properties after multiple tempering at various temperatures. During use, these products will maintain these properties, up to the temperatures at which the formed carbides begin to decompose. The diagram shows that the hardness begins to significantly decline during tempering in the fine-pearlite (troostite) stage (300 to 400°C). Toughness begins to increase as early as the bainite phase. In alloy steel, these changes occur in a different manner. At temperatures between 200 and 300°C, the hardness slightly decreases, and it once again begins to increase at high temperatures and reaches its maximum value at temperatures between 500 and 800°C. The optimal tempering temperatures depend on their chemical composition.

Types of heating:

  • cooking in hot water
  • in heated oil
  • in salt furnaces
  • in furnaces

Tempering temperatures after hardening (°C)

  • 100 to 200 - dynamically unstrained construction elements from carbon steel after hardening and cementation.
  • 150 to 300 - carbon and low-alloy tool steel
  • 500 to 580 - high-speed steel
  • 530 to 670 - special steel for hardening and tempering
  • up to 700 - die steel

Tempering time is from 1 to 3 hours. For measuring tools, the tempering time is up to 24 hours. This way, better dimensional stability may be achieved.