21. Metallurgical Treatment

• We can perform operations that do not effect any visible or geometrical properties of a part, but will change the materials properties.

21.1 Heat Treating

• Main processes include,

Annealing: The metal is heated enough to recrystallize and held at the temperature for hours normally. Parts are cooled slowly.

Stress Relieving: The part is held for hours at a lower temperature than annealing so that the metal properties remain the same, but residual stress is reduced. The part is also cooled slowly.

Quench Hardening: The part is heated to Austenetizing range and then cooled rapidly using oil, water, etc. This created a very hard metal structure. Parts can be hardened in locations using inductive coils for location specific heating.

Tempering: After a hardening process (such as quenching) the part may be heated to beneath the Austenite range. This can be used to soften the part to a desired hrdness.

Carburizing: Soft steel parts can be heated for hours while coated in carbon. Carbon is absorbed into the surface and then the part is quenched. This gives a hard outside case, but the inside remains more ductile.

Carbon Nitriding: Similar to carburizing, except nitrogen is also absorbed in the surface. These parts have better surface wear properties.

Age Hardening: These parts are heated, quenched and then held at relatively low temperatures so that grains precipitate. The process is stopped using refrigeration. The result is parts with higher strength.

21.2 Ion Nitriding

• In a vacuum, a part is heated and bombarded with nitrogen (or other) ions.

• The nitrogen ions penetrate the surface a few atoms deep.

• A common use is titanium on steel for hardening tools.

21.3 Problems

What is the purpose of heat treating?

How can steels be softened? How can steels be hardened?

How can aluminum be hardened?

How does case hardening work?

What techniques can be used to harden specific surfaces?

List and describe 6 different applications of heat treating.

Explain how the following changes the metals; a) recrystalization, b) smaller broken up grains, c) increase of carbon.

What is the difference between annealing and tempering?