Tempering Process Heat Treatment

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Heat treatment tempering process is used to improve the toughness and reduce the hardness of ferrous metals, such as steel. A molten salt bath is one of the methods employed for tempering ferrous metals. The specific formula for a molten salt bath varies depending on the type of steel being tempered and the desired tempering temperature.

The most common salt used in tempering baths is a eutectic mixture of sodium nitrate (NaNO3) and potassium nitrate (KNO3), commonly known as a “salt bath.” The eutectic mixture has a lower melting point, making it suitable for tempering operations at different temperatures. The composition of the salt bath can vary depending on the desired tempering temperature, but a typical mixture might contain approximately 60-70% sodium nitrate and 30-40% potassium nitrate by weight.

Here’s a general guideline for preparing a molten salt bath for tempering ferrous metals:


  • Sodium Nitrate (NaNO3)
  • Potassium Nitrate (KNO3)

Tempering process in heat treatment

  1. Heat the mixture gradually using a furnace or suitable heating equipment until it reaches the desired tempering temperature. The specific temperature will depend on the steel type and tempering requirements.
  2. Immerse the preheated ferrous metal object into the molten salt bath. Ensure the entire piece is submerged to achieve uniform tempering.
  3. Allow the metal to remain in the molten salt bath for the required duration to achieve the desired tempering effect. The duration depends on the steel type and thickness.
  4. Remove the metal from the salt bath and quench it in water or oil to cool it rapidly and halt the tempering process.
  5. Clean the metal after quenching to remove any residual salt.

Product making guide of heat treatment

Safety precautions must be followed when working with molten salts. Wear appropriate personal protective equipment (PPE), work in a well-ventilated area, and follow safety guidelines for handling hot materials.

It’s essential to consult with material engineers or metallurgists for precise tempering specifications based on the specific type of ferrous metal and its intended use. The tempering process can significantly impact the mechanical properties of the metal, so accurate temperature control and timing are critical to achieve the desired results.