5 Things You Wish You Knew About Heat Treated Steels

Almost everyone is familiar with the fact that heat plays an important part in steels, be it carbon steel, alloy steel, tool & die steel or stainless steel. Heat treatment is the controlled treatment of steel by heating and cooling process for obtaining desired properties for improved life of the end product and or machinability etc.

Product life is improved as this process changes the mechanical/physical properties of steel. Normally, they are heated and cooled to appropriate temperatures and then cooled maintaining the heating and the cooling rate which produces the desired mechanical results. It is indeed a very important part of the steel manufacturing process. The most common type of heat treatment is associated with metals, including alloy steel, but the process can also be used to make glass, aluminium, and many other materials.

Different Types Of Heat Treatments

There are also several types of heat treatment processes, including:

Annealing

This involves heating steel up to a suitable temperature but allowing it to cool off very slowly until it gets to room temperature. It is mainly a softening process. Annealing is done to facilitate machining and cold-working.

Normalizing

When steel is normalized it is heated and held at a particular temperature for a certain amount of time depending on the cross section before cooling it through normal air. The result is a microstructure at the heated temperature is retained, which can be pearlite+ferrite. It provides uniformity in structure, relieves stresses and produces greater uniformity in grain size.

Stress Relieving

Heating to a suitable temperature, holding long enough to reduce residual stresses and then cooling slowly enough to minimize the development of new residual stresses. It is not intended to modify mechanical properties.

Hardening

Increasing hardness by suitable treatment usually involves heating and cooling to turn the microstructure in austenite at higher temperature and then suitably cooling by water, oil & polymer or air to produce a high hardness phase called martensite. There are different types of hardening e.g Quench hardening, induction hardening, flame hardening, precipitation hardening as well as case hardening through case carburising, nitriding to obtain wear resistant surface along with tough core for better load absorbing capability.

Surface Hardening

With this process, the surface of a particular component is hard enough not to be disturbed by erosion and general wear while the ductility and toughness are maintained in order to withstand shock and impact loading.

Tempering

During hardening, the martensite phase is formed which is a very hard phase in steel but has insufficient toughness. Tempering of steels by heating for a certain time at below critical temperature is therefore introduced to exchange some of the strength for greater ductility. Some steels require double or triple tempering.

Carburization

In this process, carbon content of surface area is increased by providing a carbonaceous atmosphere to the hot component that needs to be carburised towards obtaining a hard surface and tough core.

5 Things You Need To Know When Buying Heat Treated Steels

1. Consider Your Processing Requirements

   In case steel has to undergo further heating like hot forging etc, then buying heat treated steel will be a 100% loss. Hence it is beneficial to buy a heat treated product only in case you have to go for direct machining.

2. Testing Stage Matters

   In case it has to be direct machined, then it is important to check that at what stage of the product the mechanical/physical properties will be tested, if it is in the raw material stage or after pre machining because if you buy heat treated steels which require higher hardness than the raw material it might result in additional machining costs on the CNC. In this case it is better to heat treat after pre machining.

3. Compare Hardness Requirements

   In case the hardness requirement is same or lesser in heat treated steels than the raw material hardness then it is beneficial to buy raw material that is heat treated and ready to use.

4. Look Beyond Hardness

   Hardness is usually an indicative measure, it is best to measure the other mechanical properties like tensile, yield, impact etc.

5. Uniformity Is Key

   Uniformity in properties is key to product life, as it is easier to achieve properties at the ends of the long solid bars or on the surface than the centre, therefore the guarantee of uniformity affects the pricing of each supplier and can be guaranteed only based on how good their setup is.