Heat treatment and three methods

Heat treatment

Heat treatment is the controlled heating of steel to a definite temperature, soaking, and cooling at a set rate to alter its structure and properties without changing shape.

Why it is necessary

• To get the required hardness, strength, toughness and ductility.
• To remove internal stresses and refine the grain.
• To improve machinability and wear resistance, and to harden surfaces.

Three methods

1. Annealing Steel is heated to about 30-50 C above the upper critical temperature, soaked, and cooled very slowly in the furnace. It gives soft, ductile steel with a coarse pearlite and ferrite structure, relieves stresses and improves machinability.

2. Normalizing Steel is heated about 40-50 C above the upper critical temperature, soaked briefly and cooled in still air. Faster cooling gives a finer grain, higher strength and toughness than annealing. Used for forgings, castings and structural steel.

3. Quenching (hardening) and tempering Quenching is the rapid cooling of steel heated above the upper critical temperature (30-50 C above), by dipping in water, brine or oil.

• Rapid cooling prevents formation of soft pearlite, and austenite changes into martensite, giving very high hardness and strength.
• The steel becomes brittle and has residual stress, so it is tempered afterwards.
• Cooling rate: brine > water > oil > air. High-carbon steel is hardened by quenching; low-carbon steel does not harden much.

After quenching, steel is tempered by reheating below 723 C (150-650 C) to reduce brittleness.