Heat Treatments in Metallurgy: A Comprehensive Guide
Heat Treatments in Metallurgy
Heat treatments are metallurgical processes that regenerate or modify the crystalline structure of metals. This is achieved by subjecting the metal to controlled heating and cooling cycles at specific temperatures and durations.
Annealing
Annealing involves heating the metal above its critical temperature and then slowly cooling it. This process eliminates internal stresses, regenerates the metal’s structure, and enhances ductility. Various annealing types include:
Types of Annealing
- Regeneration or Complete Austenitizing: Heats steel to the austenite phase, holding it until the entire structure transforms. This softens the steel and homogenizes its structure.
- Subcritical Annealing: Heats steel to just below its critical point to relieve internal stresses and improve ductility. Three sub-types exist:
- Softening: Optimizes steel for machining.
- Against Embrittlement: Enhances ductility for further processing.
- Globular: Reduces hardness by achieving a globular cementite structure.
- Globular or Incomplete Austenitization: Heats steel between the upper and lower critical points to achieve a globular cementite structure.
- Double Annealing: Combines full austenitization and subcritical annealing for very low hardness.
Normalizing
Normalizing is similar to full austenitizing annealing but with a slightly higher cooling rate and heating temperature.
Tempering
Tempering involves heating steel to the austenitic phase and then carefully controlling its cooling to achieve desired properties. This process creates structures like sorbite, troostite, and martensite. Factors influencing tempering include:
- Steel type and its ideal cooling rate.
- Initial structure’s grain size.
- Thermal conductivity and size of the workpiece.
Cooling Media for Tempering
Different cooling media impact the tempering process:
- Water: Rapid cooling due to low boiling point.
- Oil: Offers a slower cooling rate.
- Mercury: Expensive but provides rapid cooling for high hardness.
- Lead and Basic Salts: Facilitate isothermal transformation for enhanced quality.
Other Heat Treatments
- Patenting: Used for high-strength wires, involving isothermal annealing in a molten lead bath to create a fine-grained sorbite structure.
- Austempering: Involves cooling in a molten salt bath to form bainite, increasing strength, toughness, and resistance to deformation.
- Martempering: Minimizes stress cracks and ensures uniform transformation before further processing.
Tempering (Post-Tempering)
Tempering after hardening relieves internal stresses and improves ductility and toughness. The degree of improvement depends on the annealing temperature and duration.
Treatments with Varying Composition
These treatments, often used for machine parts, achieve a balance of toughness, hardness, and wear resistance. Examples include:
- Cementation: Increases surface carbon content by heating the metal in contact with carbon-rich substances.
- Carbonitriding: Similar to cementation but incorporates both carbon and nitrogen for enhanced properties.
- Sulfinization: Improves wear resistance without increasing hardness by introducing sulfur to the surface.