Heat Treatment of Steel

Heat treatment is a process that involves the heating and cooling of metals of alloys in their solid-state to enhance specific desirable properties without changing the composition. The process is carried out to change the grain size, relieve stress, modify the structure and prepare the material after hot or cold working.

Other reasons for heat treatment of metals like steel include;

• To improve machinability

• Improve electrical and magnetic properties

• Increase resistance to corrosion, wear and heat and others.

There are various types of heat-treatment processes. The ideal method depends on the type of metal and the desired properties after the heat treatment process. The four primary heat treatment methods for stell include;

Annealing

Annealing is a heat treatment process that relieves stress and softens steel to improve grain structure and increase ductility.

Without the necessary pre-heating stage, processes like welding can leave the metal with uneven temperatures and some molten spots next to areas at room temperature. Such results can weaken steel, and as these areas around the weld get cooler, internal stresses develop around the brittle and hard spots. Annealing helps to fix this problem by relieving internal stress.

Annealing Process

The annealing process must be conducted carefully and procedurally to produce the highest level of ductility for steel and other ferrous metals. First, you must heat the metal slowly to the appropriate temperature, followed by soaking and then allowing it to cool by turning off the furnace and letting the furnace and the steel cool together or by burying it in an insulating material.

How long the metal soaks depends on its type and its mass. If it’s a low carbon steel, it requires the highest possible annealing temperature. As the carbon content increases, the annealing temperature decreases.

Normalising

Normalising is another steel heat treatment process that helps to relieve internal stress. This process removes internal stress caused by machining, forging, forming, casting, heat treatment or casting. Uncontrolled stress can cause the metal to fail. Normalising before any hardening can help ensure the success of the project.

Normalising Vs Annealing

Normalising and annealing sound like two processes that achieve the same results. However, there are several differences between the two.

Normalising only applies to ferrous materials like steel. Also, when normalising, the metal is typically heated to a higher temperature than when annealing. The metal is also air-cooled after removal from the furnace. Generally, normalised steel is stronger than annealed steel. Both options produce metals with high strength and ductility, but normalisation produces tougher steel than annealed steel. Normalisation is more recommended than annealing if the metal part in question is required to withstand impact or have maximum toughness to resist internal stresses.

Because normalised steel is air cool, the mass of the metal is critical in determining the cooling rate and the part's resulting hardness level. When normalising, small and thinner parts cool faster and become harder than thicker pieces. But with the furnace cooling of the annealing process, both thick and thin parts have comparable hardness.