What is Heat Treatment?

Although most people don’t know what heat treatment is, it’s actually an essential part of the manufacturing process. That’s because heat treating allows a metal piece to be improved in order for the material to better withstand wear and tear. Heat treatment involves heating a metal or alloy to a specific temperature and then cooling it to harden the material. 

Heat treatment can be used at different stages in the manufacturing process to change certain properties of that metal or alloy. For example, you might use heat treatment to make it stronger, harder, more durable, or more ductile, depending on what the material needs in order to perform properly. 

·       Hardening

Hardening is a metalworking process used to increase the hardness of a metal. A harder metal will have a higher resistance to plastic deformation than a less hard metal.  Heat treatment is the use of heat to modify the properties of a material, especially in metallurgy. It is a type of industrial process involved in altering the chemical and physical properties of metals and metal alloys. There are four major types of heat treatment methods as annealing, tempering, hardening and normalizing. 

There are many ways to metals perform and react to precision machining. One of these methods is heat treating. Heat treating can be applied to the part before to make the material more machinable, or the components may be machined before the final hardening and heating stages. Heat treating can affect a number of different aspects of the metal including strength, hardness, toughness, machinability, formability, ductility, and elasticity

Common objectives of heat treatment are to:

• Increase strength
• Increase hardness
• Improve toughness
• Improve machining
• Improve formability
• Increase ductility
• Improve elasticity

·       Annealing

Annealing involves heating steel to a high temperature, then cooling it slowly to room temperature, so that the resulting microstructure is ductile and tough. In practice, a workpiece is heated to an appropriate temperature, then kept at that temperature for a short time, and finally cooled by shutting it off the furnace with the workpiece inside. To reduce energy requirements, steel is annealed before cold forming.

During the annealing process, the metal is heated to a specific temperature where recrystallization can occur. At this stage, any defects caused by deformation of the metal are repaired. The metal is held at that temperature for a fixed period, then cooled down to room temperature.

Annealing is commonly used to:

• Soften a metal for cold working
• Improve machinability
• Enhance electrical conductivity.

·   Tempering

Tempering is used to increase the toughness of iron alloys, particularly steel. Untampered steel is very hard but is too brittle for most applications. Tempering is commonly done after hardening to reduce excess hardness.

Tempering involves heating the metal to a precise temperature below the critical point, and is often done in air, vacuum or inert atmospheres.The temperature is adjusted depending on the amount of hardness that needs to be reduced. While it varies depending on the metal type, generally, low temperatures will reduce brittleness while maintaining most of the hardness, while higher temperatures reduce hardness which increases elasticity and plasticity, but causes some yield and tensile strength to be lost.

Tempering is used to:

• Hardness
• Ductility
• Toughness
• Strength
• Structural stability

·       Case Hardening

The outside of the material is hardened while the inside remains soft. Since hardening can cause materials to become brittle, case hardening is used for materials that require flexibility while maintaining a durable wear layer. Parts that are subject to high pressures and sharp impacts are still commonly case-hardened. Examples include firing pins and rifle bolt faces, or engine camshafts. In these cases, the surfaces requiring the hardness may be hardened selectively, leaving the bulk of the part in its original tough state.

·       Normalization

Similar to annealing, this process makes the steel more tough and ductile by heating the material to critical temperatures and keeping it at this temperature until transformation occurs. Normalizing is high-temperature austenitizing heating cycle followed by cooling in still or agitated air that is performed for a variety of reasons but primarily is performed to homogenize the microstructure and remove any segregation or non-uniformities that may exist at the microscopic level.

Normalizing is used to:

• Improve machinability.
• Improve dimensional stability.
• Modify and/or refine the grain structure.
• Produce a homogeneous microstructure.
• Reduce banding.

Advantages of induction heat treatment

• Localized areas can be heat treated
• Very short surface heat-up times
• Steel can be pre-heat treated to obtain prior core hardness values
• Very minimal surface decarburization
• Very minimal surface oxidation
• Slight deformation (bending); this can occur due to internal residual machining stresses
• Straightening can be carried out on a deformed bar/shaft; however, care must be exercised
• Increased fatigue strength