From cluster to casting
- Knockout. The shell is removed by pneumatic vibration, hammering or high-pressure water jets. Residual ceramic in holes and recesses is removed by blasting; for cores and inaccessible passages, chemical leaching in caustic solutions is used.
- Cut-off. Castings are separated by abrasive wheels, band saws, plasma or laser cutting, or by breaking at notched gates. Hardenable steels may need to be cut while warm or after an annealing treatment to avoid cracking.
- Gate grinding. Gate stubs are ground to an agreed witness height by belt grinders or robotic cells. The permitted gate witness (for example flush, or a defined projection or recess) should appear on the drawing.
Heat treatment
Heat treatment gives the casting its specified microstructure and properties and is therefore a controlled special process with calibrated furnaces and recorded cycles.
| Treatment | Purpose | Typical alloys |
|---|---|---|
| Annealing, normalising | Homogenise and refine the as-cast structure, relieve stress | Carbon and low-alloy steels |
| Quenching and tempering | Strength and toughness | Low-alloy and martensitic stainless steels |
| Solution annealing with rapid cooling | Dissolve carbides and intermetallic phases, restore corrosion resistance | Austenitic and duplex stainless steels, Ni-Cr-Mo alloys |
| Solution treatment and ageing | Precipitation hardening | 17-4 PH type steels, nickel superalloys, Al-Si-Mg alloys |
| Case hardening, nitriding | Hard, wear-resistant surface | Low-carbon alloy steels |
| Hot isostatic pressing | Close internal porosity | Titanium, superalloys, fatigue-critical parts |
Points that require attention are distortion (parts are supported or fixtured, and straightened afterwards if agreed), surface decarburisation and scale (protective atmosphere or vacuum, or machining allowance), delay between furnace and quench, and traceability of test pieces to the furnace load.
Cleaning and surface finishing
Blasting with steel shot, stainless media, glass beads or ceramic grit removes scale and gives a uniform appearance. Stainless castings are often pickled and passivated to remove surface contamination and restore the passive layer. Vibratory finishing, electropolishing and mechanical polishing reduce roughness where function or hygiene demands it. Coatings such as zinc, phosphate, paint or powder coating are applied to carbon steels as specified. See surface finish.
Straightening and sizing
Thin or long castings may be straightened in presses or sized in coining dies to meet flatness and form tolerances. This is a normal operation for some part families, but it should be agreed because it introduces local plastic deformation.
Weld repair
Many material specifications permit repair welding of castings under defined conditions: qualified procedures and welders, limits on the extent of repair, post-weld heat treatment and re-inspection. Whether repairs are allowed, and whether they must be recorded or approved in advance, is a contractual matter to settle at the enquiry stage.
Machining
Functional features are machined after heat treatment, located from datum targets on the casting that were agreed during design. Combining casting and machining in one responsibility avoids disputes about allowance and datum errors; see capabilities.