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Grain Size Control - The grain size is a very important factor in considering the properties of polycrystalline materials because they affect the area and the length of the grain boundaries. Fine grained materials generally exhibit greater yield stresses than coarse grained materials at low temperatures whereas at high temperatures grain boundaries become weak and sliding occurs.
The basic principle of grain size control is, In steel, grain refinement occurs only upon heating within or above the critical temperature range. Once an austenitic grain size has been established during heating, it cannot be refined during cooling, regardless of the cooling rate*.It can be refined, however, by subsequent reheating.
In the case of pure metals and non-heat-treatable alloys, suitable grain refinement can be brought about by peening the weld, whilst still hot, and so bring about recrystallization.
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