You may wonder why the same metal material has different mechanical and physical properties. It is probably because it has been heat-treated, and annealing is a standard process.
Annealing Definition
Annealing is a heat treatment process used in metallurgy to change a material’s physical and sometimes chemical properties, especially a metal. The annealing process involves heating the metal to a specific temperature, maintaining it for some time, and then slowly cooling it. The primary purposes of annealing are to eliminate internal stresses, increase flexibility, improve toughness, refine the grain structure, and restore elasticity to the material after cold working.
This specific temperature is generally based on the critical temperature.
The critical temperature of steel is the temperature at which it undergoes a phase transition between two phases. When steel is heated above the critical temperature, which is approximately 1335°F (724°C), it recrystallizes into austenite. There are two essential temperatures to consider:
1. Lower critical temperature (Ac1): This is the temperature at which austenite starts to form from ferrite.
2. Upper critical temperature (Ac3): This is the temperature at which the complete transformation from ferrite to austenite occurs.
Temperature ranges for annealing of plain carbon steels
Common Types, Processes, and Applications of Annealing
Normalizing
- Process: The metal is heated to a temperature above its critical point (usually around 30-50°C above the upper critical temperature) and then allowed to cool in still air.
- Purpose: Refines the grain structure, improves mechanical properties such as toughness, and reduces internal stresses.
- Applications: Commonly used for carbon steels and alloy steels that require enhanced mechanical properties.
Full Annealing
- Process: The metal is heated above its upper critical temperature and then cooled very slowly in the furnace to room temperature.
- Purpose: Produces a soft and ductile microstructure with coarse grains, improving machinability and making the metal easier to work with.
- Applications: Used for low to medium carbon steels that require extensive forming or machining.
Spheroidizing Annealing
- Process: The metal is heated to a temperature just below the eutectoid temperature and held there for an extended period, followed by slow cooling. Sometimes, cycling between temperatures slightly above and below the eutectoid temperature is used.
- Purpose: Produces a spheroidized or globular form of carbides within a ferritic matrix, which minimizes hardness and makes the metal more ductile and easier to machine.
- Applications: Commonly applied to high-carbon steels to prepare them for machining and further heat treatment processes.
Process Annealing
- Process: The metal is heated below the lower critical temperature (typically between 550-650°C) and then cooled slowly.
- Purpose: Reduces hardness and increases ductility, particularly for metals that have been cold-worked, to restore their workability without significantly altering the microstructure.
- Applications: Often used for low-carbon steels and other alloys that require frequent cold working.
| Type | Heating Temperature | Cooling Method | Purpose | Applications |
| Normalizing | Above upper critical temperature | Air cooling | Refines grain structure, improves toughness | Carbon and alloy steels needing improved properties |
| Full Annealing | Above upper critical temperature | Furnace cooling | Produces soft, ductile structure for machining | Low to medium carbon steels |
| Process Annealing | Below lower critical temperature | Slow cooling | Reduces hardness, restores ductility after cold working | Low-carbon steels and other cold-worked metals |
| Spheroidizing Annealing | Just below eutectoid temperature (or cyclic) | Slow cooling | Produces spheroidized carbides for machinability | High-carbon steels for improved machinability |
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