Ductile iron pipe is cast iron whose graphite has been made spheroidal by treating the melt with magnesium, giving it strength and ductility close to steel together with cast iron's resistance to corrosion. Spun in a centrifugal casting machine, lined with cement mortar and coated with zinc, it is a standard pipe for water mains, sewerage and fire mains.
Grey cast iron, the material of the old water mains, is brittle because its carbon forms flakes of graphite that act as cracks. Adding a small amount of magnesium to the molten iron makes the graphite form spheres instead, and the result — ductile iron, or spheroidal graphite (SG) iron — has a tensile strength of at least 420 MPa and elongation of 10% or more. Pipes made of it can take ground movement, traffic loads, water hammer and rough handling without breaking.
Pipes are made by centrifugal casting. Molten iron is poured into a spinning water-cooled steel mould, which throws it against the mould wall to form the pipe with its socket; the pipe is then annealed to give the ductile structure. Inside, a cement-mortar lining (ISO 4179) is spun on to protect the iron and keep the water smooth-flowing; outside, a coating of metallic zinc (or zinc-aluminium) with a bitumen or epoxy finishing layer protects against soil corrosion. In aggressive soils the pipe is also sleeved in polyethylene.
Pipe is specified by DN and class. The outside diameter for each DN is the same across EN 545, EN 598 and ISO 2531 (see the diameter table), so pipes and fittings from different makers fit the same gaskets. The traditional thickness classes (K9 for most water mains) have been joined in EN 545 and ISO 2531 by pressure classes such as C25, C30 and C40, which give the allowable operating pressure in bar; American practice uses AWWA C151 pressure classes. Push-on joints with an EPDM gasket are the norm; restrained joints are used at bends and on slopes instead of thrust blocks.
The GCC's water systems — transmission mains from desalination plants, city distribution networks, treated sewage effluent (TSE) and irrigation lines — make it a large market for ductile iron pipe, which competes there with steel, GRP and HDPE. Most is imported, from India, China, Europe and elsewhere. Saline groundwater and sabkha soils along the coast are highly corrosive, so GCC specifications commonly call for zinc-aluminium coatings or heavier zinc, polyethylene sleeving, and sulphate-resisting cement linings.
Ductile iron pipe is ordered by DN, class, joint type, lining, coating and quantity in metres or pieces, with fittings (bends, tees, reducers, flanged spigots) and gaskets to the same standard.
Full dimensions: Ductile iron pipe outside diameters and K9 wall thickness (ISO 2531 / EN 545)
| Region | Standard | What it covers |
|---|---|---|
| Europe | EN 545 CEN-CENELEC |
Ductile iron pipes, fittings, accessories and their joints for water pipelines — requirements and test methods |
| Europe | EN 598 CEN-CENELEC |
Ductile iron pipes, fittings, accessories and their joints for sewerage applications |
| International | ISO 2531 ISO |
Ductile iron pipes, fittings, accessories and their joints for water applications (DN 40–2600) |
| International | ISO 7186 ISO |
Ductile iron products for sewerage applications |
| International | ISO 4179 ISO |
Ductile iron pipes and fittings for pressure and non-pressure pipelines — cement mortar lining |
| International | ISO 8179-1 / -2 ISO |
Ductile iron pipes — external coatings: metallic zinc with finishing layer; zinc-rich paint |
| USA | AWWA C151/A21.51 AWWA |
Ductile-iron pipe, centrifugally cast |
| USA | AWWA C150/A21.50 AWWA |
Thickness design of ductile-iron pipe |
| USA | AWWA C104/A21.4 AWWA |
Cement-mortar lining for ductile-iron pipe and fittings |
| USA | AWWA C105/A21.5 AWWA |
Polyethylene encasement for ductile-iron pipe systems |
| USA | AWWA C111/A21.11 AWWA |
Rubber-gasket joints for ductile-iron pressure pipe and fittings |
| India | IS 8329 BIS |
Centrifugally cast (spun) ductile iron pressure pipes for water, gas and sewage |
| China | GB/T 13295 SAC |
Ductile iron pipes, fittings and accessories for water or gas applications |
Grade equivalents and the standards bodies: Steel grades & standards
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