Aging Annealing Process of Gray Cast Iron

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During the cooling process of the casting, the different cooling speeds of various parts cause inconsistent shrinkage and form internal stress. This internal stress can be relieved by the deformation of the casting, but this process is generally slow. Therefore, castings need to be aging treatment after they are formed, especially for some large, complex or high-precision castings (such as bed, frame, etc.). There are generally two methods for aging treatment, namely natural aging and artificial aging. Natural aging is to place the formed castings at room temperature for a long time to eliminate the internal stress. This method takes a long time (half a year or even more than a year). In order to shorten the aging time, in most cases, artificial aging (immediate annealing) is used to eliminate the internal stress of the casting. The principle is to reheat the casting to 530~620℃ and keep it warm (2~6h) for a long time. Plastic deformation reduces stress, and then it is slowly cooled in the furnace to below 200°C and air-cooled from the furnace. After aging annealing, more than 90% of the internal stress can be eliminated. The typical aging annealing process curve is shown in Figure 1.

Fig 1 Typical aging annealing process curve

The higher the aging annealing temperature, the more significant the residual stress of the casting is eliminated, and the better the dimensional stability of the casting; but as the aging temperature increases, the mechanical properties of the casting after aging will decrease. Therefore, the highest heating for aging annealing of the casting should be set reasonably temperature. Generally, it can be selected as follows:

T/℃=480+0.4σ_b/MPa

The holding time is generally calculated as 25 mm per hour of heat penetration castings. The heating rate is generally controlled below 80°C/h, and complex parts are controlled below 20°C/h. The cooling rate should be controlled below 30°C/h, and air cooled after 200°C.

After the surface of the casting is cut, the original stress field is destroyed, which will cause the redistribution of the stress of the casting, so the aging annealing is best performed after the rough machining. For precision parts with particularly high requirements, aging annealing can be carried out twice after the casting is formed and rough processed.

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