Quenching of a hot casting shortly after heat treatment

It is obviously desirable to achieve the required metal properties in the as-cast form to save further treatments being necessary, but this is not always possible because of variations in section thickness, etc. TTie heat treatment of the castings can eliminate carbides in thin sections, produce more consistent matrix structures and for a given stmcture, often improve its 

Stress relief consists of heating the castings at a rate of 50 - 100 °C/h to 600 °C (taking care not to exceed 610 °C), followed by soaking them for a minimum of one hour, plus an extra hour for every 25 mm of section thickness in the thickest section, and then cooling them at a rate of 50 - 100 °C/h or less. The castings must be adequately supported in the furnace so that they are not subjected to stress. 

Thin section castings may contain carbides in the as-cast structure. These can be eliminated by soaking the castings at 900 - 925 °C for 3 to 5 hours. 

Annealing involves soaking the castings at 900 - 925 °C for 3 - 5 hours, followed by a slow cooling at around 20- 35 °C/h through the critical temperature range (about 800 - 710 C), and finally furnace cooling at, say 50 - 100 °C/h to 200 °C. 

For normalising, a soaking of the castings above the critical temperature followed by air cooling is necessary. Again a soaking temperature of 900 - 925 C is usually used, to ensure that the carbides are broken down. Forced air cooling is used to form pearlite. The type of heat treatment furnace available and the size of the load determines the process cycle that is possible. It may be necessary to adjust the metal composition with tin or copper to help the formation of fully pearlitic structures. 

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Figure 2.52 Quenching of a hot casting shortly after heat treatment [237, HUT, 2003]...

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