Fettling, finishing and heat treatment of castings
Domain 10 - Fettling, finishing and heat treatment of castings
This domain tests how castings are cleaned, how gates and risers are removed, and how heat treatment relieves stress and develops properties.
Fettling and finishing
Fettling removes gates, risers, flash, sand and cores. Methods include chipping, grinding, cutting, tumbling and shot blasting. The sequence matters: remove feeders before heavy cleaning, avoid cutting into the casting, and blend ground surfaces. Finishing may include machining, painting, and dimensional and surface inspection. Cores are removed by shaking, rapping or chemical leaching, depending on the core binder.
Heat treatment
- Stress relieving: heat to about 550–650 °C, hold, and cool slowly; reduces residual stress from casting and fettling.
- Annealing: heat to 850–950 °C, hold, and furnace-cool; softens and improves machinability.
- Normalising: heat and air-cool; refines grain and improves strength.
- Hardening and tempering: for steel castings, quench and temper to achieve strength and toughness.
- Malleableising: for white cast iron, a long anneal breaks up cementite into graphite.
Worked example
A large grey-iron machine bed is rough-machined and then stress-relieved at 600 °C, held for one hour per 25 mm of section, and cooled slowly in the furnace. If it were quenched, thermal gradients would add new stresses and could crack the casting. Stress relief is done before final machining so that distortion occurs before the final cuts.
Traps
Do not confuse annealing with normalising; normalising uses air cooling. Quenching a complex casting can cause cracks. Remove cores before heat treatment where possible, because trapped core sand affects heating. Over-grinding can reduce wall thickness. Heat-treatment temperatures for cast iron differ from those for steel.