How do you Temper Steel?

Tempering is a heat treatment process applied to steels to achieve a balance of hardness, strength, and toughness. It involves heating the steel to a temperature below its critical point after it has been hardened (quenched). This process reduces brittleness and relieves internal stress without significantly compromising hardness. Here’s a step-by-step guide on how to temper steel:

Step-by-Step Process of Tempering Steel

Preparation:

• Ensure that the steel has been properly hardened. This usually involves heating it to a high temperature (above the critical point) and then rapidly cooling it, typically in water or oil.

Heating:

• Place the hardened steel in a furnace or heat it using a controlled source.
• Heat the steel to the desired tempering temperature, which usually ranges between 150°C and 650°C (302°F and 1202°F). The specific temperature depends on the desired properties and the type of steel.

Holding:

• Maintain the steel at the tempering temperature for a set period. The time required can vary but typically ranges from 1 to 2 hours. This allows the steel to stabilize and ensures uniformity in the tempering process.

Cooling:

• After holding, allow the steel to cool gradually to room temperature. This can be done in air or in the furnace by turning it off and letting it cool down slowly.

Considerations in Tempering Steel

Temperature Selection:

• Lower Temperatures (150°C to 300°C / 302°F to 572°F): Produce a harder and more brittle steel. Used for applications requiring high hardness and wear resistance.
• Medium Temperatures (300°C to 500°C / 572°F to 932°F): Achieve a good balance between hardness and toughness. Suitable for most structural applications.
• Higher Temperatures (500°C to 650°C / 932°F to 1202°F): Result in tougher, less hard steel, ideal for components subjected to impact or shock.

Steel Composition:

• Different types of steel require different tempering temperatures. Alloying elements like carbon, chromium, and vanadium can influence the appropriate temperature and duration.

Quenching Medium:

• The medium used for quenching (water, oil, air) can affect the hardness and tempering results. The choice of medium depends on the steel composition and desired properties.

Purpose of Tempering:

• Reduce brittleness and relieve internal stresses induced by hardening.
• Adjust mechanical properties to suit specific applications, such as cutting tools, springs, or structural components.

Example: Tempering a High Carbon Steel Blade

Hardening:

• Heat the blade to around 800°C (1472°F) until it becomes austenitic (non-magnetic), then quench it in oil to harden.

Tempering:

• Heat the hardened blade to around 200°C to 250°C (392°F to 482°F) for about 1-2 hours to reduce brittleness and increase toughness.

Cooling:

• Allow the blade to cool slowly in air.

Tempering is an essential step in the heat treatment of steel, allowing for the customization of its properties to meet the demands of various applications.