Metal normalisation is a heat treatment aimed at improving the mechanical properties and microstructure of metals. It involves heating the metal above its recrystallisation temperature followed by cooling in air. The aim of normalisation is to remove residual stresses, reduce grain size, increase ductility and toughness and homogenise the chemical composition of the metal.
Normalising is usually used for ferrous alloys such as steel and cast iron, but can also be applied to non-ferrous metals such as copper and aluminium. The normalising process varies depending on the type and grade of metal, but usually consists of three stages:
1. Heating: The metal is heated to a temperature equal to a temperature above the recrystallisation point, which is the temperature at which new grains form in the metal. The heating time depends on the thickness and shape of the metal, but must be long enough to ensure that the entire metal reaches the desired temperature.
2. Soaking: The metal is held at the heating temperature for a period of time to allow new grains to form and impurities or decompositions in the metal to dissolve. The soaking time also depends on the size and composition of the metal, but should be sufficient to achieve a uniform microstructure throughout the metal.
3. Cooling: The metal is removed from the furnace and cooled in air at medium speed. The cooling rate affects the final properties and microstructure of the metal as it determines the size and distribution of the grains. A faster cooling rate results in finer grains and higher strength, while a slower cooling rate results in coarser grains and lower strength.
The benefits of normalisation include:
- Improving the machinability and weldability of metals by reducing hardness and brittleness.
- Improving the mechanical properties of metals by increasing strength, ductility, toughness and fatigue resistance.
- Homogenise the chemical composition of metals by eliminating any variation or segregation.
- To improve the grain size and shape of metals by forming uniform and equiaxed grains.
- To remove residual stresses in metals by allowing the metals to relax and contract uniformly.
Normalising is a common heat treatment for metals subjected to forging, rolling, extrusion or machining. It can also be used as a pre-treatment for other heat treatments such as annealing, hardening or tempering. Normalising can improve the quality and performance of metals in a variety of applications and industries.