Cast iron materials: malleable and grey cast iron

What the short name says

In the European system, cast irons are named following the pattern EN-GJ…: G for cast, J for iron (the I was already taken). The third letter gives the graphite form or the process, followed by the properties. EN-GJMW-400-5 reads as follows: M for malleable, W for white: whiteheart malleablised, meaning annealed in a decarburising atmosphere until the carbon has left the surface zone. 400 is the minimum tensile strength in N/mm², 5 the minimum elongation at fracture in per cent (reference: GF malleable cast iron technical catalogue 2015). The old material number 0.8040 means the same material; drawings from the DIN era still show GTW-40.

EN-GJL-250 is simpler: L for lamellar – cast iron with lamellar graphite, the classic grey cast iron – and 250 for the minimum tensile strength. There is no elongation figure because grey iron has practically none: the graphite flakes act like internal notches. Old number 0.6025, old name GG-25.

Key elements according to the designation system EN 1560; the two individual values 400 N/mm² and 5 % with reference GF malleable cast iron technical catalogue 2015. Further properties are given in the manufacturer documentation.

Why the fitting is malleablised

A raw iron casting with a lot of carbon is hard and brittle like glass – drop it and it breaks. During malleablising, the part lies for days at high temperature in an atmosphere that draws the carbon out of the surface zone. What remains is a material that behaves like mild steel: you can cut a thread into it, strike it with a hammer, tighten it with a pipe wrench without anything cracking. That is exactly what a fitting needs that is screwed onto a thread on site. Grey iron could not do this – which is why fittings are made of malleable iron and bodies of grey iron, never the other way round.

The standard designation of a fitting therefore reads on two levels: “Elbow EN 10242 – A1 – ½ – Zn – A” gives standard, design, thread size, galvanising and manufacturer’s mark – the material EN-GJMW-400-5 is contained in the standard itself (manufacturer information GF). Operating limits (pressure per medium and temperature) are given by the manufacturer in its catalogue; for steam and for the Pressure Equipment Directive, the manufacturer’s approval applies, not the material name.

Source note: process and standard designation according to GF malleable cast iron technical catalogue 2015; operating limits as manufacturer information (ibid.). The classification fitting vs. body is our statement about our range.

Examples of where the material is found in our range

Source note: stock levels according to our ERP survey (as of 10/2026: malleable iron fittings in stock); scope of range according to the GF list 2026; material per item according to the manufacturer’s catalogue.

Identifying the part – in this order

1. Paperwork and stamp first. A malleable iron fitting carries the manufacturer’s mark and the thread size cast in, often also the type number; the standard is given in the catalogue. Reading aid: Reading markings.

2. The casting skin. Mould seam, gate remnants and the slightly rough skin reveal the casting; a steel fitting (forged or made from tube) is smooth. Galvanised malleable iron fittings are matt grey with spangle, black ones dark grey to black.

3. Magnet. Sticks – as with steel. The magnet separates cast iron from copper and austenite, but not from steel.

4. Fracture. If you have a defective part: malleable iron breaks bright and tough with visible deformation, grey iron grey and smooth without deformation – hence the name. This is the most reliable difference, but only on a broken part.

Limit: identifying the material on the part indicates the probable family – not the batch, heat treatment or suitability for pressure and medium, and it does not replace a manufacturer’s certificate.

What the certificate must show

Malleable iron fittings are series parts with a manufacturer’s mark and are supplied without a per-batch 3.1. Their proof is the manufacturer’s declaration of compliance 2.1 or test report 2.2 on request, together with conformity to EN 10242 and, for gas or drinking water use, the DVGW registration. For projects with pressure equipment documentation we supply the manufacturer’s declaration; the proof for the individual elbow is this declaration together with the manufacturer’s mark: Inspection documents to EN 10204 · Certificates and documentation.

The typical mix-ups

“Malleable iron is steel.” No – it is cast, not rolled, and has considerably more carbon. It behaves like steel, but weldability is limited to thin walls and manufacturer approval. The matching tube is steel (EN 10255) – same threads, same galvanising, two materials.

“Grey iron fittings.” In the installation trade, anyone saying “cast fitting” means malleable iron. Grey iron sits in bodies that are neither struck nor screwed.

“GTW-40 is something different from EN-GJMW-400-5.” No – the same material, old and new name. Old drawings say GTW, the catalogue says EN-GJMW.

“Galvanised means rustproof.” No – the zinc layer protects as long as it is intact; at the thread flank and the cut, the fitting is iron. For stainless threaded fittings: Stainless steel threaded fittings.

Further reading

The world of materials: Carbon steels and heat-resistant steels · Copper materials · Austenitic stainless steels · Materials overview.

In the knowledge base: Malleable iron fittings · Threaded tubes EN 10255 · Joining without welding · Flanges · Reading markings.