Heat Treatment of Steel & Effect on Hardness

Our experience in dealing with special steels has led to the observation that while melting and forging processes play a role in the final quality and performance of the steel, the correct heat treatment process is equally, if not more, vital.

Eutectoid temperature for austenite for steel ranges between 723°C to 727°C, depending on the alloy composition.

At this temperature, the body-centric cubic (BCC) ferrite transitions to face-centric cubic (FCC).

The solubility of interstitial carbon is reversible & the final microstructure is dictated by the cooling rates.

Therefore basics of heat treatment involve -

• Heating steel to a certain temperature

• Heating at the temperature for a certain time

• Controlled removal of heat from the steel causes microstructure transformation

General carbon steel retains austenite only at high temperatures; the addition of manganese & nickel suppresses the eutectoid reaction (one solid into two solids when cooling) & maintains austenite at room temperature.

For example, austenitic stainless steel (300 series) contains nickel for this very purpose.

How does steel get hard after quenching?

• Steel heated above the critical temperature causes an austenitic structure

• Carbon atoms get dissolved within its structure

• When cooled faster than the critical cooling rate, there is no time for the carbon atoms to diffuse out of the softer phase

• Steel undergoes a diffusionless transformation to martensite

• This microstructure is highly strained and distorted because of the trapped carbon atoms in the lattice

• This distorted lattice resists deformation, thus increasing the hardness.

Therefore, the hardness of the steel can be manipulated with the help of a correct heat treatment process.

This, in turn, affects all the mechanical properties of the steel, which makes it essential as the steel must achieve the TDC ranges for the desired performance.