Steel Hardening Heat Treatment Methods

Posted on Mar 10, 2025 in Electromechanical Engineering

Tempering Steel

Tempering is a heat treatment process used to produce hardened steel surfaces.

Factors Influencing Hardness Achieved by Tempering

  • Cooling Rate: The speed of cooling.
  • Carbon Content:
    • Low carbon: Less than 0.20%
    • Medium carbon: 0.20% to 0.40%
    • High carbon: 0.40% to 1.7% (2.0%)
  • Alloy Percentage:
    • Low alloy: Less than 8% alloy content.
    • High alloy: More than 8% alloy content.
  • Size of the Piece: Larger pieces cool more slowly.

Procedure for Tempering

  1. Select the type of steel.
  2. Determine the appropriate heating temperature.
  3. Select the cooling medium.
  4. Determine the initial hardness.
  5. Heat the piece 25°C to 50°C above Ac3 (900°C) for hypoeutectoid steels, or 25°C to 50°C above Ac1 (723°C) for hypereutectoid steels.
  6. Maintain the piece at a constant temperature in the oven.
  7. Remove the piece from the oven and cool it rapidly, agitating it in the cooling medium (e.g., forming slow circles).
  8. Clean the piece.
  9. Determine the final hardness.

Types of Surface Hardening

  • Carburizing
  • Cyaniding
  • Nitriding
  • Induction Hardening
  • Flame Hardening
  • Metallizing

Carburizing

Procedure: Heat the piece to red-hot, keeping it in contact with charcoal or coke, and then cool it slowly.

Obtained thickness: Approximately 0.8 to 1.5 mm depth of the hardened layer.

Low carbon steel (around 0.5% C) is transformed into high carbon steel (0.39 to 1.2% C).

Cyaniding

This process combines carbon and nitrogen absorption to harden low carbon steels that do not typically respond to heat treatment.

Procedure:

  1. Dip the metal in sodium cyanide (NaCN) salt at 840°C.
  2. Maintain at temperature until red-white hot.
  3. Cool in oil or water.

Obtained thicknesses: Approximately 0.13 to 0.5 mm.

This process is highly toxic.

Nitriding

This process involves heating the steel piece to 500°C in contact with ammonia (NH3). The nitrogen forms very hard nitrides.

Hardness of approximately 900-1100 HB and thicknesses of approximately 0.025 to 0.13 mm are achieved.

Induction Hardening

This process is used for hardening small parts “almost instantaneously” using an induced current through a copper tube ring, through which cooling water circulates.

The procedure is suitable for extremely rapid heating of medium-sized pieces such as bars, chisels, screwdrivers, etc.

Flame Hardening

This process allows heat treatment (hardening) of bulky items that cannot be hardened by other procedures. It uses a heat-generating unit (oxyacetylene) and a cooling system (water shower).

The process can be performed in three ways:

  • Part in motion, stationary unit.
  • Part stationary, moving unit.
  • Moving part and unit (both moving in opposite directions).

Hardened thicknesses of about 1.6 mm can be obtained.

The forward speed of the flame (thermal-cooling unit) is approximately 50 to 200 mm/min.

Metallizing

This process involves coating a metal part by melting a wire or powder, generally with an oxyacetylene torch. The molten material is deposited on the surface to harden it. To obtain a uniform surface, the process is often done on a lathe for cylindrical surfaces. In some cases, correction (with a tool) is applied for regular and uniform surfaces, especially when the coating is made with very hard metals such as Co, W, Tu, Mo, etc.