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Annealing of casting

In the case of steel castings full annealing is the only way for completely effacing the coarse grains and Widmanstätten structure, with its associated brittleness.

The following properties of a 0,3 % carbon steel illustrate the improvement obtained by annealing:

Double annealing consists of heating the steel to a temperature considerably over the A3 point, cooling rapidly to a temperature below the lower critical range and then immediately reheating to a point just over the upper critical point (Ac3), followed by slow cooling.

This method is particularly useful for castings. The high temperature treatment effaces the strains, coalesces the sulphide films in the ferrite which embrittle the steel and produces homogenity by rapid diffusion. The quick cooling prevents the coarse deposition of the ferrite in the large grains, but tends to harden the metal. The second heating refines the coarse grains and leaves the steel in a softened condition.

To soften high carbon and air-hardening steels, in order to allow machining operation to be carried out, they are heated just below the lower change point (650-700°C), causing. the cementite to balls-up into rounded masses (i.e. spheroidising anneal). When the cementite is in this condition high carbon steels can be cold drawn; but too high a temperature causes pearlite to be reformed, with consequent high resistance to deformation.

It should be remembered, however, that coarse laminated cementite spheroidises extremely slowly, and the above treatment is therefore carried out on a "hardened" material, obtained by suitable cooling from above A3 or after cold working. A short cycle anneal consists of heating just above Ac1, cooling below Ar1 and then raising temperature just below Ac1 for 8 hours. Although the softest condition is obtained when the large globules of cementite are embedded in the ferrite, a smooth machined surface is difficult to obtain due to tearing. Groups of large globules cause failure of sharp-edged cutting tools.

During the subsequent hardening operations, the time required to dissolve fine spheroidised cementite is less than for the lamellar type. This property is being used in hardening thin sections, such as safety razor blades and needles, in order to reduce decarburisation.