Typical austempering heat treatment stages

Unalloyed ductile irons may be austempered in sections of up to about 8 mm thickness. Thicker section castings require the addition of Mo or Ni to increase the hardenability. 

Austenitic or duplex stanless steels undergo a solution annealing and water quenching heat-treatment. Additionnal heat-treatments such as for stress relief or post weld treatment are also used to eliminate the internal stresses of the material. 

Castings with changes of section, or of a complex sh e are likely to develop internal stresses. These can then cause dimensional changes upon machining. To stabilise the casting and to remove internal stresses, castings are heated to a temperature of 200 °C for 5 hours, followed by slow cooling in the furnace. 

The controlled precipitation of alloying constituents is promoted by heating the casting to a temperature of between 150 C and 200 °C. The casting strength and hardness are increased. The process is therefore also referred to as structural hardening. Each alloy has an optimal heat treatment cycle. 

Some casting alloys increase in strength and hardness while left standing at room temperature. The process can take several weeks but can be speeded up by heating above room temperature and then sustaining this heat over time. 

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Figure 2.53 Typical austempering heat treatment stages [174, Brown, 2000]...

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