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The process of heating steel to above its critical range and then allowing it to cool slowly.

a. Process of sub critical annealing is carried out on cold worked low carbon steel to relieve internal stress and to soften the material. The steel is heated to about 50°C below lower critical temperature and then allowed to cool.

b. Full annealing is employed on steel casting and hot worked steels to obtain grain refinement and ductility. The steel is heated to about 50°C above upper critical temperature and then allowed to cool slowly in the furnace.

c. Normalising differs from full annealing in that the metal is allowed to cool in still air. Tensile strength and impact values are higher than the figures obtained by annealing.

d. Spheroidising annealing is used for softening high carbon steel to facilitate machining. The steel is heated to just below lower critical temperature. Due to surface tension effects, the cementite assumes globular form while the remainder reverts to ferrite. After shaping, the pearlite-cementite structure can be restored by heat treatment. The heat treatment ranges for plain carbon steels are shown in figure 3.

e. Case-Hardening - Where an article requires to have a hard wear-resisting surface, together with a tough core, it is necessary to employ a low carbon steel and case harden the surface to obtain this dual structure.

The carbon content at the surface is enriched by surrounding the components in materials rich in carbon, such as a mixture of charcoal and barium carbonate; the pieces to be carbonized are placed into boxes with the carbonizing materials and sealed. The boxes are then placed in a furnace and brought to a temperature around 900°C and held at that temperature for a time dependant on the depth of case required. After six to eight hours, a low carbon steel may have 0.9% carbon to a depth of about 0.040%.

A part which has been carbonized at such a temperature will have a coarse grain structure which must be refined. This is achieved by reheating the parts above the critical point and quenching in oil. This treatment results in refining the structure of the core. If it is desired, the case can be tempered at 150°C. The case is then hardened, i.e. heated to 770° then quenched in water. It can then be tempered, i.e. heated to 200°C depending on hardness required.

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