Heat Treatment

HEAT TREATMENT

Heat treatment is the process of heating and cooling metals in a controlled way, in order to change their properties and characteristics.

The various heating processes fall into the following categories:

Hardening processes - These processes are intended to produce a hardened structure. Hardening increases wear resistance and material strength, but often makes the structure of the metal brittle.

Softening processes - These processes are intended primarily to soften the material, or remove stresses built up as the metal is worked.

Toughening processes - These processes are intended to produce a structure which improves strength and ductility.

Case-hardening process - These processes are employed to produce a 'case' or surface layer which is harder than the interior core of the metal.

Hardening Processes

The only method of hardening most metals (other than some steels) is by work hardening. Work hardening takes place when a metal has been deformed (by hammering, bending, pressing or rolling for instance). The internal stresses that are set up as a material is worked have to be relieved by a process called annealing.

Iron, like all metals, will organize itself into a uniform geometric atomic structure. At normal room temperature some metals such as tungsten take on what is called a ‘body centered cubic’ structure. This consists of a cuboid shape with an atom at each corner, and one more in the center of the cube.

Other metals arrange themselves in what is called a ‘face centered cubic’ structure. This consists of a cuboid shape with an atom at each corner, and one more in the center of each face of the cube.

Steel is basically iron with some carbon mixed in, though modern alloys have various other metals and substances as well. When steel is heated to the critical temperature (about 1400 degrees F), the iron will change to face centered, and the carbon atoms will migrate into the central position formerly occupied by an iron atom. This form of red-hot steel is called austentite.

If you let this cool slowly, the iron atoms migrate back into the cube and force the carbon back out, resulting in soft steel called pearlite. If the sample was formerly hard, this softening process is called annealing.

If you cool (quench) the sample suddenly by immersing it in oil or water, the carbon atoms are trapped, and the result is a very hard, brittle steel. The structure is now a body centered tetragonal form called martensite.

Surface Hardening

For many engineering purposes it is desirable for parts to have a hard surface to resist wear and abrasion and the inner portion remains soft and tough to sustain impact loading.

This depth of the hardened surface is normally from 0.0001 mm to a few mm depending on applications. Carbon is diffused into the surface of a low-carbon steel so that, when the piece is quenched from a high temperature, considerable hardness is obtained on the surface whilst the interior remains tough.

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Toughening Processes

After quenching the steel is hard, brittle and internally stressed.

Before use, it is usually necessary to reduce these stresses and increase toughness by 'tempering'.

There will also be a reduction in hardness and the selection of tempering temperature dictates the final properties.

If we heat polished steel through a range 150 ⁰C to just above 300 ⁰C it progressively changes colour from faint yellow through straw, brown, blue and finally near black.

We can use this colour change to indicate the temperature of the metal and so control the tempering process. Each stage of the colour change indicates a percentage of the carbide compound remaining in the steel, and therefore it's hardness. The objective of tempering is to reduce the hardness to the point required and then stop the carbide breaking down any further by immediately quenching in cold water.

Useful colours are:

	Pale straw - 230 ⁰C - hardness - Suitable for machining mild steels. Low resistance to fracture.
	Dark straw - 240 ⁰C - hardness 700-750 Vickers - Suitable for home made taps, scribers and the like.
	Purple-blue - 275 ⁰C - hardness 650-700 Vickers - Suitable for chisels and punches.
	Dark blue - 290 ⁰C - hardness 640-690 Vickers - suitable for springs.

Softening Processes

Annealing

If a steel bar is cooled slowly in a furnace from a temperature above its upper critical temperature to a temperature below the lower critical temperature, the structure of the steel will become ferrite and cementite again. This steel consists of a somewhat coarser grain structure that is low in strength, more ductile and soft. This process of heat treatment is called 'Annealing'.

Normalizing

If instead of cooling in furnace as described above, the steel is taken out from the furnace and cooled in still air, it is termed 'Normalizing'. A normalized steel bar possesses higher strength and toughness than its annealed counterpart.