Heat treatment of grey iron casting can not change the shape of the graphite and eliminate the harmful effect of flake graphite. It can only be used to eliminate casting stress of grey iron castings and stable the size castings. Besides , it can remove the chilling organization, reduce the hardness to change its processing performance and increase the surface hardness and wear resistance.
For example, the grey iron casting with complicated shape has uneven wall thickness, so it is easy to generate internal stress during the casting process which will lead to the cracking formation of grey iron casting.
The purpose of aging treatment is to eliminate the casting stress. Aging treatment can be divided into natural aging and artificial aging treatment. The method of natural ageing is topalce the grey iron casting in the open-air for a few months, six months or longer. Theynaturally and slowly, the casting will begin to deform and gradually eliminate the casting stress.
The method of artificial aging is low temperature annealing which increases the temperature of casting at a relatively slow rate (60-100℃/ h) to 520-520℃. After heat preservation for a period of time, the grey iron casting should be cooling at a slow speed (20-30℃/h) with the furnaceto 150-200 ℃. At last the casting should be take out of the furnace for air cooling. At this point, the grey iron casting stress is basically eliminated.