SG iron vs grey cast iron comes down to one main question: does your part need toughness, or does it need damping and easy machining? SG iron (also called ductile iron) bends a little before it breaks and handles shock well. Grey cast iron is brittle but absorbs vibration, machines easily and conducts heat well.
Both are iron castings made with similar raw materials. The big difference is the shape of the graphite inside the metal. This guide explains that difference in simple words and helps engineers and buyers pick the right material for a drawing.
What Is Grey Cast Iron?
Grey cast iron contains carbon in the form of graphite flakes. When you break a piece, the fracture looks grey, which gives the material its name.
The flakes act like tiny cracks inside the metal. That makes grey iron weaker in tension and brittle under impact. But the same flakes give it some very useful properties:
- Very good vibration damping
- Easy machining, with short chips
- Good heat conduction
- Good wear resistance on sliding surfaces
- Good castability for complex shapes
Common uses include machine beds and bases, brake drums and discs, pump and valve bodies, gearbox housings and cylinder blocks.
What Is SG Iron?
SG stands for spheroidal graphite. In SG iron, a small treatment (usually with magnesium) makes the graphite form tiny round balls, or nodules, instead of flakes.
Round nodules do not act like cracks. So the metal can stretch a little before it breaks. This gives SG iron:
- Higher tensile strength than grey iron
- Real ductility (it bends before it breaks)
- Better impact and shock resistance
- Better fatigue strength under repeated load
Common uses include crankshafts, gears, suspension and steering parts, pipe fittings, valve parts, pump parts under pressure and heavy-duty brackets.
SG Iron vs Grey Cast Iron: Quick Comparison
| Property | Grey cast iron | SG iron |
|---|---|---|
| Graphite shape | Flakes | Round nodules |
| Tensile strength | Lower | Higher |
| Ductility | Very low, brittle | Good, bends before breaking |
| Impact resistance | Poor | Good |
| Vibration damping | Excellent | Moderate |
| Machinability | Excellent | Good |
| Heat conduction | Very good | Good |
| Typical parts | Machine beds, housings, brake parts | Gears, crankshafts, pressure parts |
Grades and Standards to Know
Both materials are bought by grade. The grade tells the minimum strength and, for SG iron, the minimum elongation.
- Grey cast iron in India is covered by IS 210. Grades are written like FG 200 or FG 260, where the number is the minimum tensile strength in MPa. The international match is ISO 185.
- SG iron in India is covered by IS 1865. Grades are written like SG 400/15 or SG 500/7. The first number is the minimum tensile strength in MPa, and the second is the minimum elongation in percent. The international match is ISO 1083.
Always write the standard and grade on your drawing. This avoids confusion between the buyer, the foundry and the inspector.
Does Heat Treatment Change the Choice?
Heat treatment can change the properties of both irons. Grey iron parts are often stress relieved so they stay stable during machining. SG iron can be annealed for more ductility, or normalised and tempered for higher strength and wear resistance. Some SG iron parts are austempered to reach even higher strength. If your part needs a special property, mention it on the drawing so the foundry can plan the right route from the start.
How to Choose: Five Simple Questions
1. Will the part take shock or impact?
If yes, choose SG iron. Grey iron can crack under sudden load.
2. Does the part need to absorb vibration?
If yes, grey iron is often better. This is why many machine tool beds use grey iron.
3. Is the part under pressure or safety-critical?
Pressure parts and safety parts usually need SG iron, because it gives warning by bending before failure.
4. How much machining is needed?
Grey iron machines very easily. SG iron machines well too, but tool wear can be higher. For parts with heavy machining, factor this into your plan.
5. What about weight?
Because SG iron is stronger, you can sometimes use thinner sections. This can reduce the weight of the part for the same strength.
Testing That Proves the Grade
A grade on paper means little without testing. Typical checks for iron castings include:
- Chemical analysis of the melt
- Tensile test on a test bar cast with the parts
- Hardness test on the casting
- Microstructure check to confirm graphite shape (flakes or nodules) and nodularity in SG iron
- Dimensional inspection against the drawing
Alloynex runs in-house metallurgical testing to check grade and quality before parts are released.
Common Mistakes When Choosing
- Using grey iron for a part that takes shock, only because it machines easily
- Specifying “cast iron” on the drawing without a standard or grade
- Ignoring section thickness, which changes the final properties
- Skipping microstructure checks on SG iron parts
How Alloynex Can Help
Alloynex makes grey iron, SG iron, steel and alloy steel castings to the buyer’s drawing and grade. We review your drawing, grade and tolerances before work starts, and we support machining and heat treatment as your part needs. Our work runs under ISO 9001, ISO 14001 and ISO 45001 systems. See our full casting services, browse our casting projects or read about Alloynex.
FAQs
Is SG iron stronger than grey cast iron?
Yes. SG iron has higher tensile strength and good ductility, while grey iron is brittle under tension and impact.
Why is grey cast iron still used so widely?
It damps vibration very well, machines easily, conducts heat well and casts complex shapes with ease. These qualities suit machine beds, housings and brake parts.
Is ductile iron the same as SG iron?
Yes. Ductile iron, nodular iron and SG iron are different names for the same family of cast iron with round graphite nodules.
Which standard covers SG iron castings in India?
IS 1865 covers SG iron castings in India. ISO 1083 is the international standard.
Can SG iron replace steel castings?
In some parts it can, with a weight and machining gain. The choice depends on the load, temperature and the strength needed. A technical review of the drawing is the best way to decide.
Get a Technical Review
Not sure which iron fits your part? Send your drawing for a technical review, or request a metallurgy consultation. Email info@alloynex.com with your drawing, grade and quantity, or use our contact page.