Iron is where GW started in 1932, and it is still most of what we ship. Prints for iron castings carry grade callouts that read like part numbers — Class 30, 65-45-12, ASTM A48, A536 — and each one tells the foundry something specific. This is a working translation.
Grey iron
The graphite in grey iron forms as flakes. That gives it three properties buyers pay for: it damps vibration better than any other common engineering metal, it is thermally stable, and it machines easily. It is comparatively brittle in tension, which is why it goes into parts that are loaded in compression or need to sit still.
Grey iron is graded by tensile strength under ASTM A48. Class 30 is the everyday grade for bases, housings, and covers. Class 40 adds strength for gearbox housings and pump bodies. Class 60 is the top of the range. A higher class generally means a finer graphite structure, a harder part, and more attention to section thickness in the mould.
Ductile iron
Treat the same iron with magnesium in the ladle and the graphite forms as nodules. Ductile iron behaves much more like steel: it bends before it breaks, and it survives impact. It is graded under ASTM A536 by three numbers — tensile strength, yield strength, and elongation. 65-45-12 is the common general-purpose grade; 60-40-18 gives more ductility, 80-55-06 more strength. Ductile iron is the usual answer when a buyer wants to move a part off a steel weldment or a forging.
Alloyed and specialty irons
Nickel, chromium, molybdenum, and copper additions push iron toward wear resistance, heat resistance, or corrosion resistance. Austempered ductile iron, heat-treated after casting, reaches strengths that compete with forged steel. These grades are specified less often and poured by fewer foundries — part of our job is knowing which ones.
What the grade asks of the foundry
A grade is a promise about mechanical properties, and the foundry keeps it through chemistry, inoculation, and cooling rate. Section thickness matters: a thick section cools slowly and a thin one fast, and a single part can miss grade in one place and make it in another. That is why GW's supplier process includes test bars poured with the job, material certifications to your requirement, and first-article validation before production release.
Which grade?
Yours. If your print carries a spec, GW meets it. If you are moving a part from another material and are not certain which grade fits, ask — you will get an honest conversation about the load case and the trade-offs before you commit, not an excuse after.