Heat Treatment Process

Heat Treatment Process

• Definition: Heat treatment is the process of heating metal without letting it reach its molten stage, and then cooling the metal in a controlled way to select desired mechanical properties.
• Purpose: To make the metal stronger or more malleable, more resistant to abrasion, or more ductile.

Stages of Heat Treatment

• Heating Stage: Ensure uniform temperature, heat slowly, and choose heating rate according to metal conductivity, condition, size, and cross-section.
• Soaking Stage: Keep metal at desired temperature until desired internal structure takes shape.
• Cooling Stage: Cool metal back to room temperature using a cooling medium (gas, liquid, solid, or a combination).

History of Heat Treatment

• 6000 BC: Humans learned to recover metal from ores.
• 1200 BC: Iron Age began, and metalworking advances allowed for new technology and machinery.

Properties of Metals Influenced by Heat Treatment

• Mechanical Properties: ductility, corrosion resistance, hardness, machinability

Types of Heat Treatment

Annealing

• Definition: Process that changes physical and sometimes chemical properties of a material.
• Process: Heat metal to appropriate temperature, hold, and then slowly cool.
• Purpose: Soften metal, increase ductility and machinability, and reduce hardness.
• Types of Annealing: full annealing, partial annealing, subcritical annealing.

Normalizing

• Definition: Process used to relieve internal stresses caused by processes like welding, casting, or quenching.
• Process: Heat material to pre-decided elevated temperature, hold, and then cool freely in air.
• Purpose: Remove structural irregularities, improve ductility, reduce hardness, and relieve internal stresses.

Hardening

• Definition: Process that improves mechanical properties and increases hardness.
• Process: Use tempering or stress relieving to improve dimensional stability and toughness.
• Case Hardening: Harden outer layer of material while keeping inside soft.
• Methods of Case Hardening: carburizing, nitriding, cyaniding.

Tempering

• Definition: Process that develops required combination of hardness, strength, and toughness.
• Process: Reheat steel to relatively low temperature to precipitate and spheroidize carbides.
• Purpose: Reduce brittleness and increase strength of metal.

Quenching

• Definition: Process that rapidly cools heated metal to achieve desired properties.
• Media for Quenching: air, oil, water, brine.

Time-Temperature-Transformation (TTT) Diagram

• Definition: Plot of temperature against log of time for a steel alloy of definite composition.
• Purpose: Determine when transformation begins and ends for an isothermal heat treatment.

Continuous Cooling Diagram (CTT)

• Definition: Diagram used to represent phase changes in material as it is cooled at different rates.
• Purpose: Determine phase changes in steel during continuous cooling.

Benefits of Heat-Treating Metals

• Increased strength and ductility
• Increased resistance to corrosion
• Increased malleability and workability during manufacturing
• Improved electrical and magnetic properties