Heat treatment is a controlled process that changes the physical and mechanical properties of metals (and sometimes other materials) by heating and cooling them in a specific manner. The goal is to increase the metal's hardness, strength, toughness, ductility, and resistance to wear and corrosion. There are four types of heat treatment: annealing, normalizing, quenching, and tempering.

 

Annealing

• Procedure: Heat the metal above its recrystallization point (for steel, the full annealing temperature is approximately 816-927°C), maintain a warm temperature ("soaking") until it becomes uniform, and then cool it slowly (typically through furnace cooling) to prevent hardening.
• Purpose: Annealing can soften the metal, reduce hardness, increase plasticity, facilitate cutting, eliminate internal stress, and prevent deformation or cracking. Annealing can also refine the grains and improve the uniformity of the structure.
• Application: Annealing can prepare cold-worked or welded parts for subsequent processing, making the metal softer for forming, drawing, or machining. It is often used in the preparation of automotive body panels, forging billets, and wire drawing, among other applications.

 

Normalizing

• Procedure: Heat the steel to its critical value (such as above the steel's Ac₃or Ac₅₀), keep it warm, and then cool it in still air. Air cooling is faster, and the grains are finer.
• Purpose: Normalizing refines the grain size and produces a more uniform and tougher microstructure than annealing. Normalizing usually produces a mixture of fine pearlite (and some martensite or bainite), resulting in higher strength and hardness (but slightly lower ductility) than full annealing.
• Application: Improve the structure of castings and forgings; eliminate stress after hot working; general heat treatment of structural steel. Suitable for automobiles (chassis, gears), tracks, and mechanical parts.

 

Quenching

• Procedure: The metal (steel at about 815-950°C, above its critical temperature) is austenitized, held to homogeneity, and then rapidly quenched in a cooling medium. Quenching media include brine, water, polymer solution, oil, or forced air cooling.
• Purpose: Quenching maximizes strength and hardness by forming martensite. When steel is rapidly cooled from the austenite phase, it transforms into very hard but brittle martensite, which gives it high hardness and wear resistance.
• Application: Cutting tools, dies, blades, bearings, gears, shafts, springs, and other parts that require high hardness must be used with tempering; otherwise, they are prone to cracking.

 

Tempering

• Procedure: Heating the quenched (martensitic) steel to below the critical temperature (usually 392-1,112°F), holding as needed (minutes to hours), and cooling (usually in air).
• Purpose: Tempering is performed after quenching to reduce brittleness and impart toughness. Untempered martensite is extremely hard but brittle, so tempering provides a balance of hardness and ductility. Controlled tempering produces fine carbide precipitation and relieves residual stresses.
• Application: Tempering is a necessary process after quenching to prevent brittle fracture of parts.