The difference between ductile iron and gray iron is the shape of the graphite: ductile iron carries it as spheroids, gray iron as flakes, and every property that separates them — yield strength, elongation, impact, damping, machinability, price — follows from that one fact. Ductile iron is the material for a part that has to survive a load, gray iron for a part that has to hold still, soak up vibration or heat, and machine cheaply.
What Is Ductile Iron
Ductile iron is cast iron whose graphite has been forced into spheroids by a magnesium treatment of the melt just before pouring. ASTM A536, the specification for general-purpose ductile iron castings, defines it as cast iron with the graphite “substantially spheroidal in shape.” The base iron keeps gray iron’s melting point, fluidity and machinability; the rounded nodule does not notch the matrix the way a flake does, so the iron yields and stretches before it breaks.
Because the iron yields and stretches, A536 can promise three numbers for every grade, and the grade name is the promise: 65-45-12 means 65 ksi minimum tensile, 45 ksi minimum yield at 0.2 % offset, and 12 % minimum elongation. The general-purpose grades run from 60-40-18, a ferritic iron with 18 % elongation, through 65-45-12 and 80-55-06, both reachable as-cast, to 100-70-03 and 120-90-02, which give up elongation for strength and need heat treatment to get there.
What Is Gray Iron
Gray iron is cast iron with the graphite in its natural flake form, and ASTM A48, the specification for gray iron castings, classes it by one number: the minimum tensile strength of a separately cast test bar, in ksi, from Class 20 to Class 60. There is no yield strength and no elongation anywhere in A48.
Yield and elongation are missing because flakes make them meaningless. Every flake is a sharp-ended slit in the matrix; under tension the stress concentrates at the flake tips, a crack starts at the longest flake and runs, and the bar breaks with elongation close to zero, before any yield point you could design against. In compression the flakes are simply closed up, which is why gray iron carries far more that way than it ever will in tension.
The flakes that ruin tension are also where gray iron’s advantages come from. Connected flakes carry heat through the casting, dissipate vibration energy at every graphite-matrix interface, and break the chip short at the tool. All three are strongest in the low classes: a higher class is reached by refining the flakes shorter, and conductivity and damping fall with flake length. Class 20 or 30 is the quiet, heat-carrying iron; Class 50 or 60 has given some of that back for strength.
Ductile Iron vs. Gray Iron
Ductile iron wins every row that is about carrying load; gray iron wins the rows about damping, heat flow, machining and price, and the two tie on wear.
| Property | Ductile Iron | Gray Iron |
|---|---|---|
| Graphite shape | Spherical/nodular | Flake-shaped |
| Tensile strength | High | Moderate |
| Yield strength | Clearly defined | Usually not a primary design property |
| Elongation | Moderate to high | Very low |
| Impact resistance | Excellent | Poor |
| Fatigue resistance | Better | Lower |
| Vibration damping | Moderate | Excellent |
| Machinability | Good | Excellent |
| Wear resistance | Good | Good |
| Thermal conductivity | Moderate | Higher |
| Pressure-tightness | Generally better | Good with proper casting quality |
| Typical cost | Slightly higher | Lower |
| Common applications | Housings, gears, suspension parts, pipes | Machine bases, engine blocks, brake components |
At the pairing most housings get quoted in, Class 30 against 65-45-12, the ASTM minimums are 30 ksi tensile with nothing else promised against 65 ksi tensile, 45 ksi yield and 12 % elongation. A Class 60 gray iron matches 60-40-18 on tensile; what gray iron cannot supply at any class is a yield to design to and elongation to absorb a shock.
Damping is the widest gap in the table. Measured at similar composition, gray iron’s relative damping capacity is 1.0 against 0.14 for ductile iron; the nodules do not have the flake-matrix interfaces that absorb the energy.
Pressure-tightness is the row to read carefully. Nodules do not give a leak path through the wall the way a connected flake network can — that is the “generally better” — but ductile iron shrinks more on solidification, and a housing risered short leaks through shrink porosity whatever the grade says.
Which Should You Choose
Choose ductile iron when the part carries a load it must not fail under — shock, cyclic, bending, internal pressure — and gray iron when the part’s job is mass, damping, heat flow or cheap machining and nothing asks it to yield. Most parts on a heavy-equipment drawing sort themselves that way: gearbox and bearing housings, pump and valve bodies, hydraulic manifolds and anything that gets struck or pressurized go ductile; machine bases, counterweights, brake and engine components go gray.
If your calculation needs a yield strength, the choice is already made, because gray iron has none you can put on paper.
The hard case is a part that wants gray iron’s damping and more strength than Class 30 gives. Climbing to Class 50 or 60 buys tensile by shortening the flakes, which is exactly what spends the damping, so at that point 65-45-12 gives the strength plus a yield and you accept moderate damping. Which property you can afford to lose decides it, not the tensile number.
The ductile premium is the magnesium treatment and the tighter melt control it demands, plus the anneal or quench-and-temper where the grade needs one. Ductile iron also needs risering that gray iron’s graphite expansion often lets it do without, so fewer of the pounds poured end up in the part. Against what a cracked housing costs, the premium is small.
Conclusion
The whole difference is the graphite, and the two specs encode it: A536 promises tensile, yield and elongation for every ductile iron grade; A48 promises a test-bar tensile for gray iron and nothing more. Pick by what the part has to do — survive a load, or hold still and dissipate — and write the callout for that.