Steel Tempering: Enhancing Hardened Steel Properties

Posted on Apr 7, 2025 in Electromechanical Engineering

Tempering is a heat treatment following the hardening of steel. Its purpose is to reduce internal tensions within the piece, tensions created by strain hardening or cold working. It improves mechanical properties by reducing brittleness and slightly decreasing hardness. This effect is more pronounced at higher tempering temperatures. Tempering also aims to eliminate internal or residual stresses left in the steel due to the tempering, laminating, or forging process.

Types of Tempering

There are three different types of Tempering:

  • Low temperature or stress relieving
  • High temperature or bonus
  • Stabilization

General Features of Annealing

Annealing is a treatment given after quenching to soften the steel and eliminate internal tensions. The heating temperature is between 150 and 500 ºC (must be less than AC1, because exceeding this temperature would reverse the prior process). Cooling can be done in air or oil.

Stages of Tempering

Tempering is done in three stages:

  1. Heating to a temperature below the critical point.
  2. Maintaining the temperature to equalize it throughout the piece.
  3. Cooling at a variable speed. It is important, but should not be too quick.

Heating

Heating is usually done in salt furnaces. For carbon steel construction, the tempering temperature is between 450 to 600 °C, while for tool steels, the tempering temperature is 200 to 350 °C. At this stage, martensite, which leads to the tempering process, expels the excess carbon. Maintaining the temperature during tempering is crucial; duration at low temperatures is greater than at higher temperatures to allow sufficient time for the temperature to become homogeneous throughout the piece.

Cooling

The tempering cooling rate has no influence on the material as long as the temperatures reached do not exceed those that determine the fragility of the material. In this case, the pieces are cooled directly with water. If the tempering is carried out at temperatures higher than those of fragility, they should be cooled in a hot oil bath to about 150 °C and then water, or simply outdoors.

Tempering Fast Steel

Tempering temperature is made 500 to 600 °C in a bath of molten lead or salt. The heating must be slow, maintenance of heating will be at least half an hour, then allowed to cool the air.

Low Temperature or Stress Relieving

Purpose: Reduce tension within the hardened material without significantly reducing the hardness (steel tools).

Procedure:

  1. Select the right steel.
  2. Select the heating temperature.
  3. Determine the initial hardness.
  4. Heat the specimen to 200 °C to 300 °C, maintaining a constant temperature (t = k) “X” min (depending on the thickness of the piece).
  5. Remove the part from the oven and cool.
  6. Clean the piece (remove scales).
  7. Determine the final hardness.

Tempering Rapid Steels

Procedure:

  1. Select the right steel.
  2. Select the heating temperature.
  3. Initial determination of hardness.
  4. Heat the piece from 550 °C to 630 °C, maintaining a constant temperature (t = k) “X” min (depending on the thickness of the piece).
  5. Remove from oven and cool.
  6. Clean the piece (removing the scales).
  7. Determine the final hardness.

High Temperature or Hardening

Purpose: Increase the toughness of hardened steels.

Recommended Materials: Hardening steels (0.25 to 0.75% C).

Procedure:

  1. Select the right steel.
  2. Select the heating temperature.
  3. Initial determination of hardness.
  4. Heat the piece from 580 °C to 630 °C, maintaining a constant temperature (t = k) “X” min (depending on the thickness of the piece).
  5. Remove the piece from the oven and cool slowly, preferably in air.
  6. Clean the piece (remove scales).
  7. Determine the final hardness.

Stabilization

Purpose: To eliminate internal stress of mild steel for dimensional stability.

Select the right steel.

Procedure:

  1. Select the heating temperature.
  2. Initial determination of hardness.
  3. Heat the piece to 150 °C, keeping the temperature constant (t k = approx. 6 to 8 Hrs.).
  4. Remove the piece from the oven and cool slowly, preferably in air.
  5. Clean the piece (removing the scales).
  6. Determine the final hardness.

Procedure:

  1. Select the right steel.
  2. Select the heating temperature.
  3. Initial determination of hardness.
  4. Heat the piece to 150 °C, keeping the temperature constant (k t = approx. 6 to 8 Hrs.).
  5. Remove the piece from the oven and cool slowly, preferably in air.
  6. Clean the piece (removing the scales).
  7. Determine the final hardness.