Malleable or ductile? What actually changes in an iron fitting
Both materials are cast iron. Both go on the same pipe. The difference decides whether a fitting deforms or shatters when a fitter puts a two-foot wrench on it.
Why threaded fittings are malleable
Malleable iron is cast white, then annealed for several days. The annealing turns the brittle carbide into rosettes of temper carbon, and the metal stops behaving like glass. That is exactly what a threaded fitting needs: the thread root is the thinnest section in the whole assembly, and it is the place where a brittle material cracks.
Black-heart malleable iron to EN 10242 (KTH330-08 in the Chinese standard) deforms before it breaks. Over-tighten a malleable elbow and the thread strips or the body bulges — annoying, but the run does not fail.
Where ductile iron takes over
Ductile iron gets its nodular graphite from a magnesium treatment in the ladle. It is stronger in tension, tougher in thick sections, and far easier to cast at large diameter. Grooved couplings, valve bodies, hydrant bodies and OEM castings are almost all ductile — QT450-10, or EN-GJS-450-10 in the European designation.
A grooved fitting does not need a thin, malleable wall. The joint is made on a machined groove, not on a thread, so the wall can stay thick and the material can stay strong.
What you actually notice on site
- Over-tightening. A malleable fitting yields and warns you. A ductile threaded fitting is rare for exactly this reason: it is stronger, but the thread root is still thin, and the failure is more sudden.
- Impact. Drop a ductile coupling on concrete and it usually survives. A thin malleable fitting may crack at the band.
- Galvanising. Both can be hot-dip galvanised. Malleable takes the zinc without stress cracking at the thread; ductile valve bodies are usually epoxy coated instead, because zinc adds thickness inside a bore that has to stay accurate.
- Cutting and welding. Ductile can be welded with the right procedure. Malleable should not be welded at all.
Side by side
| Grade | Malleable KTH330-08 / EN 10242 · Ductile QT450-10 / EN-GJS-450-10 |
| Tensile strength | Malleable ≥ 330 MPa · Ductile ≥ 450 MPa |
| Elongation | Malleable ≥ 8 % · Ductile ≥ 10 % |
| Typical products | Malleable: threaded elbows, tees, sockets, nipples 1/2″–4″ · Ductile: grooved couplings, valve bodies, hydrants, OEM castings |
| Wall thickness | Malleable: thin sections, threaded roots · Ductile: thick sections, machined faces |
| Failure mode | Malleable: yields and deforms · Ductile: yields at higher load |
| Welding | Malleable: not recommended · Ductile: possible with procedure |
What to write on the specification
- For threaded fittings, name the standard, not just the material: EN 10242, ISO 49, GB/T 3287. It fixes the thread, the wall and the test pressure together.
- For ductile parts, name the grade (QT450-10) and the inspection you expect — a material certificate per heat, or a dimensional report per batch.
- Say whether the finish is hot-dip galvanised, electro-galvanised or epoxy. On potable water, the lining matters more than the outside coating.
Where we fit
We cast both: black-heart malleable threaded fittings from 1/2″ to 4″, and ductile iron grooved fittings, valve bodies and OEM castings from QT450-10. If you are unsure which one a drawing should call for, send it and we will say which of our lines suits it.