Heat treatment is a process involving heating of a metal/alloy at a specific rate, holding it at a temperature for a period of time and cooling it at a specific rate. Heat treatment is performed to obtain a desired microstructure and achieve certain physical, mechanical, magnetic or electrical properties.
Components produced by forging are heat treated to obtain desired microstructure and specified properties.
Objectives of heat treatment:
The major objectives are
-To increase strength, hardness and wear resistance
-To increase ductility and toughness.
-To obtain fine grain size by recrystallization annealing, full annealing or normalising
-To remove residual internal stresses formed due to cold working by stress relief annealing
-To improve machinability of forged components
-To improve cutting properties of tool steels
-To improve surface hardening, corrosion resistance, high temperature resistance by precipitation hardening
-To improve electrical properties
-To improve magnetic properties by phase transformation, of the forged components
There are various types of heat treatment processes which are performed separately or as a combination to get the desired properties and microstructure after forging.
Annealing – Heating of the alloy above critical temperature and allowing to cool in the furnace itself after switching off the furnace is called annealing. This increases the ductility and toughness but reduces the hardness.
Normalising – Heating of alloy above critical temperature, soaking it at that temperature and cooling it in air is called normalising. This increases the strength and hardness. This process is performed on forging components that are to be machined as normalising improves the machinability of forged components.
Stress Relieving – Forging, rolling, cold working, non-uniform cooling cause internal stresses in a metal. To remove or reduce these internal stresses created in a metal, stress relieving is done by heating a metal below the lower critical temperature, followed by uniform cooling.
Quenching – Heating of the alloy above critical temperature, holding it at that temperature for a specified period of time and cooling it in water, oil, salt bath or organic solvent, such that it cools rapidly is called quenching or hardening. This increases the hardness but deteriorates the toughness and increases the brittleness, making it unsuitable for machining. So, forged components are tempered after hardening.
Tempering – Heating of alloy below the lower critical temperature (e.g., for steel, around 300ᵒC-450ᵒC) soaking it for a specified period of time and cooling it in air is called tempering. This reduces the brittleness and increases the ductility and toughness. Thus, making the forged components suitable for machinability and increasing their shock absorbance.
Precipitation Hardening – Strength and hardness of some metals can be improved by formation of second phase particles dispersed in the parent matrix. These new phase particles are called precipitates. These precipitates must be coherent to the parent matrix. These precipitates act as barriers to the dislocation motion, thus increasing the stresses required to overcome these barriers eventually hardening and strengthening the alloy.
Case Hardening – Some components require higher hardness at surfaces than at cores. For such components surface hardening is done which hardens only the surface/ case. This is done to improve the wear and tear resistance of the component. Usually automobile gears produced by forging are case hardened.
The above are the common heat treatment processes performed on forgings. The temperature and type of process may vary according to the component, composition and properties required.