The difference comes down to carbon content and the shape of the graphite inside the metal. Grey iron has flake graphite and is brittle, SG iron has round nodules and bends before it breaks, and steel has little carbon and is the toughest of the three.
Each one suits different parts. Choosing the wrong metal is one of the most common reasons a casting fails in service or costs more than it should.
Grey iron
Grey iron, also called grey cast iron or flake graphite iron, contains graphite in thin flakes. The flakes make it easy to cast and easy to machine, and they absorb vibration well. They also act as tiny cracks, so grey iron has low tensile strength and almost no elongation.
It suits machine beds, pump casings, motor frames, brake drums, pulleys and housings that carry compressive loads. It is a poor choice for parts that take shock, bending or impact.
SG iron
SG iron, also called ductile iron or spheroidal graphite iron, is made by treating molten iron with magnesium. The graphite then forms round nodules instead of flakes, which gives much higher strength and real elongation.
It is used for valve bodies, gear blanks, crankshafts, hubs, pipe fittings and many parts that once were steel. It needs tighter control than grey iron: the magnesium treatment, inoculation and cooling must all be right, or the nodules do not form properly. That is why nodularity is checked on SG iron and not on grey iron.
SG iron also shrinks differently from grey iron during solidification, so a pattern made for one cannot simply be poured in the other. If you switch a part from grey iron to SG iron, expect the foundry to revise the gating and feeding and to make fresh samples.
Steel castings
Steel castings have far less carbon and no free graphite. They are tough, can be welded and repaired, and can be heat treated to a wide range of properties. Alloy and stainless grades add wear, heat or corrosion resistance.
Steel is harder to cast. It melts at a higher temperature, shrinks more as it cools and needs larger risers to feed the casting. Expect more fettling, more risk of shrinkage defects and, often, radiography or ultrasonic testing on critical parts. Small, detailed steel parts are often better made by investment casting.
How to choose
Use these rules of thumb as a starting point, then confirm with your design engineer:
- Choose grey iron for stiff, heavy, vibration-damping parts with mainly compressive loads.
- Choose SG iron when you need strength and some ductility but still want the castability and cost position of iron.
- Choose steel when the part takes impact, must be welded, works at low temperature, or must meet a pressure code that calls for steel.
- Choose stainless or alloy steel when corrosion, heat or abrasion is the main problem.
Next step
The grade number on your drawing matters as much as the metal family. Our guide to FG 260, SG 500/7 and EN-GJS grades explains how to read them. If you are unsure which metal fits your part, book a free sourcing review.