What is tempering?

Short Answer:

Tempering is a heat treatment process used to reduce the brittleness and increase the toughness of hardened steel. After hardening, steel becomes very hard but also brittle. To overcome this problem, the hardened steel is reheated to a temperature below its critical point, held for some time, and then cooled. This process helps in balancing hardness and toughness.

Tempering relieves internal stresses caused during hardening and improves the ductility and impact strength of the metal. The temperature used for tempering depends on the required properties of the material. Higher tempering temperatures reduce hardness but increase toughness, making the steel more useful in engineering applications.

Detailed Explanation :

Tempering

Tempering is an essential heat treatment process carried out after hardening to obtain desired mechanical properties in steel. When steel is hardened, it becomes very hard due to the formation of a hard structure known as martensite. However, this martensitic structure also makes the steel brittle and prone to cracking or breaking under shock or impact. Therefore, tempering is done to remove these unwanted brittleness and to make the steel stronger, tougher, and more stable in structure.

In this process, the hardened steel is reheated to a temperature below the critical temperature, generally between 150°C and 650°C, depending on the desired mechanical properties. After heating, it is held at that temperature for a certain period of time and then cooled, usually in air. The exact temperature and holding time are chosen based on the composition of the steel and the mechanical properties required.

Purpose of Tempering

The main purposes of tempering are:

  1. To reduce the brittleness developed during hardening.
  2. To relieve internal stresses created by quenching.
  3. To improve ductility and toughness.
  4. To increase shock resistance and fatigue strength.
  5. To make the steel structure more stable for further machining or service.

Tempering balances the hardness and toughness, which is necessary for components that experience dynamic or impact loads. For example, tools, springs, and shafts are commonly tempered after hardening to ensure they do not fail during operation.

Process of Tempering

  1. Heating: The hardened steel is slowly heated to the required tempering temperature. This temperature is always kept below the lower critical point (about 723°C).
  2. Soaking or Holding: The steel is maintained at that temperature for a sufficient time so that uniform temperature is achieved throughout the section.
  3. Cooling: After soaking, the steel is cooled down slowly, generally in still air. The cooling rate affects the final mechanical properties, but it is not as critical as during hardening.

Temperature Range and Effects

The tempering temperature determines the final properties of the steel:

  • Low-temperature tempering (150°C–250°C): Produces high hardness and wear resistance but lower toughness. Used for cutting tools and knives.
  • Medium-temperature tempering (350°C–450°C): Produces a balance between hardness and toughness. Used for springs and chisels.
  • High-temperature tempering (500°C–650°C): Produces high toughness and ductility but lower hardness. Used for structural parts, shafts, and machine components.

At low temperatures, only internal stresses are relieved, but at higher temperatures, structural changes occur that improve ductility. The internal structure changes from martensite to tempered martensite or sorbite, depending on the temperature used.

Microstructural Changes During Tempering

During tempering, the unstable martensite formed during hardening starts to decompose into more stable phases. At first, small particles of iron carbide (Fe₃C) begin to form within the martensite structure. As the temperature increases, the carbides grow, and the martensite transforms into tempered martensite or sorbite. These microstructural changes reduce hardness slightly but greatly enhance ductility and toughness.

The transformation also helps relieve internal stresses that were generated due to rapid cooling during hardening. This makes the steel more dimensionally stable and less prone to cracking during service.

Applications of Tempering

Tempering is widely used for components that need both strength and toughness. Some common applications include:

  • Cutting tools, dies, and punches
  • Automobile and aircraft parts
  • Machine tools and gears
  • Shafts, springs, and connecting rods
  • Structural components in engineering machinery

Tempering allows engineers to achieve the exact balance of properties required for different applications. For example, a hammer must be hard enough to resist wear but also tough enough not to shatter on impact—tempering provides this balance.

Conclusion:

Tempering is a vital process in heat treatment that enhances the usefulness of hardened steel. It reduces brittleness, relieves internal stresses, and improves toughness and ductility while maintaining adequate hardness. By carefully controlling the tempering temperature and time, steels can be made suitable for a wide range of applications, from cutting tools to machine parts. Thus, tempering helps in achieving an ideal combination of mechanical properties needed for practical engineering use.