Heat Treatments in Metallurgy: Processes and Techniques
Heat Treatments in Metallurgy
Heat treatments are metallurgical processes aimed at regenerating or modifying the crystalline structure of metals by heating and cooling them at strictly controlled temperatures and times.
Annealing
Annealing consists of heating the metal to temperatures above its critical point and then cooling it slowly. This process eliminates internal stress and regenerates the metal’s structure, which may have been affected by previous improper cooling. However, it leaves the material extremely soft and ductile. Different types of annealing are used to achieve specific properties:
Types of Annealing
- Regeneration or Complete Austenitizing: Slow, uniform heating brings the steel to the austenite phase, where it remains until the entire structure transforms. This annealing softens steel and homogenizes its structure.
- Subcritical Annealing: Used to eliminate internal stress and increase ductility. It involves heating the steel to temperatures near, but below, its critical point. Three subcategories exist:
- Softening: Optimizes steel for machining.
- Recrystallization: Enhances ductility for further processing.
- Globular: Reduces hardness and creates a globular cementite structure.
- Globular or Incomplete Austenitization: Heating the steel to a temperature between the upper and lower critical points to achieve a globular cementite structure.
- Double Annealing: Performing a full austenitization annealing followed by a subcritical annealing without allowing the metal to cool completely. This results in very low hardness values.
Normalizing
Normalizing is similar to full austenitizing annealing, but with a slightly higher cooling rate and heating temperature.
Tempering
Tempering involves heating the entire steel mass to the austenitic structure, followed by slow cooling at different temperatures to obtain various structures like sorbite, troostite, and martensite. Factors influencing tempering include:
- Steel Type: Each steel has an ideal cooling rate, which generally decreases with increasing alloy content.
- Structure Condition: Larger grain size requires a lower critical cooling speed.
- Thermal Conductivity and Piece Size: These factors affect the cooling rate at different depths, influencing the extent and uniformity of the temper.
Cooling Media for Tempering
The cooling process has three stages:
- Heat loss by conduction and radiation from the gaseous mass.
- Cooling by vapor transport, influenced by the bath’s viscosity and agitation.
- Cooling by conduction and convection in the liquid, affected by thermal conductivity and stirring.
Different coolants have specific characteristics:
- Water: Its low boiling point extends the first stage, slowing down the process.
- Oil: Its volatilization point, inflammation, and combustion properties are important considerations.
- Mercury: Expensive and used only for special parts requiring high hardness.
- Lead and Basic Salts: Promote isothermal transformation, benefiting the process and steel quality.
Patenting
Used for high-strength wires, patenting is an isothermal annealing process. It involves heating the wire to complete austenitization and then cooling it in a molten lead bath to transform austenite into fine-grained sorbite, which is ideal for drawing.
Austempering
Steel is heated to the austenitizing temperature and then cooled in a molten salt bath until it transforms into bainite. This structure provides higher strength, ductility, toughness, and resistance to bending.
Martempering
This treatment eliminates stress cracks and ensures uniform transformation before further processing.
Tempering (Post-Quenching)
This subcritical annealing is performed after quenching to eliminate internal stress and improve ductility and toughness, which may have been compromised during martensite formation. The improvement depends on the annealing temperature and treatment duration.
Treatments with Varying Composition
These treatments are used to achieve a combination of tenacity, hardness, and toughness in machine parts. Low carbon content provides toughness, while high carbon or alloy content provides hardness and wear resistance. Different treatments can achieve this combination:
- Cementation: Increases surface carbon content by exposing the metal to high temperatures in contact with carburizing agents.
- Carbonitriding: Similar to cementation, but involves simultaneous absorption of carbon and nitrogen. Nitrogen enhances the structure by decreasing the critical cooling rate and tempering temperature.
- Sulfinization: Improves wear resistance without increasing hardness by introducing sulfur from salt mixtures.