VACUUM HEAT TREATMENT

It is the heat treatment of steels (Primarily Alloy Steels) in a “vacuum” atmosphere. Vacuum Heat Treatment is a series of Processes namely:- Annealing, Stress Relieving, Hardening, Tempering.

ANNEALING
It is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the re-crystallization temperature and maintaining a suitable temperature, and then cooling. Annealing is used to induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties.

Stress Relieving

Machining induces stresses in parts. The bigger and more complex the part, the more the stresses. These stresses can cause distortions in the part long term. If the parts are clamped in service, then cracking could occur. Also hole locations can change causing them to go out of tolerance. For these reasons, stress relieving is often necessary.

Hardening and Quenching

Hardening is a process in which steel parts are heated(at a controlled rate) till the austenitic crystal phase is attained and is then quickly cooled(quenched) by introducing a suitable gas. The rate of quenching dependent on the Time-Temperature-Transformation (TTT) and Continuous Cooling Transformation CCT diagrams. An acceptable rate, for example is 28 °C per minute.

Tempering

Tempering is a process done subsequent to quench hardening. Quench-hardened parts are often too brittle. This brittleness is caused by a predominance of Martensite. This brittleness is removed by tempering. Tempering results in a desired combination of hardness, ductility, toughness, strength, and structural stability. Tempering is done immediately after quench hardening.

The primary advantages of heat treatment in vacuum are :-

  1. No oxidation of the surface of steel components and hence no scaling
  2. Minimal distortion.
  3. Zero Emissions – Environmental and Personnel friendly
  4. Uniform Temperatures across the work zone. (Tolerance of ± 5 °C)

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