Steel Hardening and Tempering: Essential Metallurgical Processes

Factors Influencing Steel Hardening

The effectiveness of the hardening process depends on several critical variables:

• Piece Size: Larger components require increased warming and cooling cycle durations.
• Chemical Composition: Alloying elements significantly influence the tempering response.
• Grain Size: Primarily affects the critical velocity; coarser grains generally increase hardenability.
• Cooling Medium: The ideal medium achieves a cooling rate slightly higher than the critical quenching rate. Common media include air, oil, water, lead baths, mercury baths, molten salt, and water-soluble polymers.

Quenching Methods

Standard quenching techniques include:

• Total or Normal Quenching
• Martempering: Step-wise martensitic transformation.
• Austempering: Phased bainitic hardening.
• Interrupted and Sub-zero Quenching

The Tempering Process

Tempering involves heating steel after hardening to a temperature below the critical point, followed by controlled cooling. Rapid cooling is used to achieve high toughness, while slow cooling minimizes thermal stresses and distortions.

For complex requirements, double tempering is often employed: the first stage uses rapid cooling, followed by a second stage with slow cooling to 300°C.

Objectives of Tempering

• Minimize steel fragility.
• Reduce internal processing stresses.
• Modify mechanical properties: reduces ultimate tensile strength, yield strength, and hardness while increasing ductility, elongation, and resilience.

Factors Influencing Annealing

The success of annealing is determined by:

• Temperature: Impacts mechanical characteristics.
• Time: Beyond a specific limit, changes become uneconomical; increasing temperature is often more effective.
• Cooling Rate: It is prudent to avoid rapid cooling.
• Dimensions: Size is fundamental; a standard recommendation is 1 to 2 hours of treatment per 25mm of thickness or diameter.