Gray Iron vs. Ductile Iron: How OEM Engineers Choose
Updated: Sep 21
Most casting conversations start with a print and a material callout that was made years ago by someone who is no longer at the company. When an OEM engineer finally revisits that callout, the decision usually comes down to one question: gray iron or ductile iron.
Both are cast irons. Both machine well. Both are poured every week in foundries across the country. They behave very differently under load, and the reason comes down to the shape of the graphite inside them.
Flake vs. Nodular Graphite: Why Gray and Ductile Iron Behave Differently
Gray iron carries graphite in flakes. Those flakes interrupt the metal matrix. They absorb vibration, which gives gray iron its damping ability, and they help chips break cleanly during machining. They also act as internal stress risers, which is why gray iron has almost no elongation.
Ductile iron carries graphite in nodules. The change is produced by treating the molten iron with magnesium before pouring, which causes the graphite to solidify as spheroids rather than flakes. The rounded shape leaves the surrounding matrix continuous, so the material yields and stretches before it fails.
That one structural difference is what separates a machine base from a crankshaft.
How Gray Iron and Ductile Iron Are Specified
This is where a lot of material discussions go sideways. Saying "cast iron" or even "ductile iron" on a print is not a specification. The governing standards are.
ASTM A48 Gray Iron Classes and What the Class Number Means
Gray iron is classified by class number, and the class number is the minimum tensile strength in thousands of psi measured on a separately cast test bar.
Class | Minimum tensile strength |
20 | 20,000 psi |
25 | 25,000 psi |
30 | 30,000 psi |
35 | 35,000 psi |
40 | 40,000 psi |
45 | 45,000 psi |
50 | 50,000 psi |
55 | 55,000 psi |
60 | 60,000 psi |
The letter that sometimes follows the class number, as in A48-30B, identifies the size of the test bar used to qualify the iron. The test bar sizes exist so results can be matched to the wall thickness and cooling rate of the actual casting. If your print calls out a class without a letter, that is worth a conversation.
Gray iron's compressive strength typically runs three to four times its tensile strength, which is why it performs well in parts loaded in compression even though its tensile numbers look modest.
Valve, flange, and pipe fitting castings are often specified to ASTM A126 instead, which is a gray iron standard written specifically for pressure service.
ASTM A536 Ductile Iron Grades Explained
Ductile iron grades use three numbers: minimum tensile strength, minimum yield strength, and minimum elongation.
Grade | Tensile | Yield | Elongation |
60-40-18 | 60 ksi | 40 ksi | 18% |
65-45-12 | 65 ksi | 45 ksi | 12% |
80-55-06 | 80 ksi | 55 ksi | 6% |
100-70-03 | 100 ksi | 70 ksi | 3% |
120-90-02 | 120 ksi | 90 ksi | 2% |
As strength climbs, ductility drops. Grade 65-45-12 is the most commonly specified because it balances the two. The right grade is the one that meets the application without over-specifying strength.
ASTM A536 also includes special application grades, and pressure-retaining parts intended for elevated temperature service are often specified to ASTM A395 instead.
Gray Iron vs. Ductile Iron Comparison Chart
Gray iron | Ductile iron | |
Graphite form | Flakes | Nodules (spheroids) |
Governing spec | ASTM A48, A126 | ASTM A536, A395 |
Typical tensile range | 20 to 60 ksi | 60 to 120 ksi |
Elongation | Essentially none | 2 to 18 percent |
Failure mode | Brittle, little warning | Yields before fracture |
Impact resistance | Low | Good to excellent |
Fatigue resistance | Lower | Higher |
Vibration damping | Excellent | Moderate |
Thermal conductivity | Higher | Lower |
Machinability | Excellent | Good |
Relative cost | Lower | Higher |
Neither material is better. They are built for different jobs.
When to Use Gray Iron and When to Use Ductile Iron
Gray iron belongs in machinery bases, housings, engine blocks, brake components, gear housings, and valve bodies. It fits anywhere rigidity, vibration damping, thermal conductivity, and machinability matter more than impact resistance. In practice that puts it in compressor components, valve bodies, and industrial equipment where the part is bolted down and loaded steadily.
Ductile iron belongs in gears, axles, crankshafts, pressure-containing components, structural supports, and heavy equipment parts. It fits anywhere the part sees shock loading, cyclic stress, or internal pressure. That covers much of our aggregate and mining work, much of pump work, and the components in steel mill straighteners and mill guides that take repeated impact.
Gray Iron vs. Ductile Iron Cost Comparison
Gray iron generally costs less. It requires no magnesium treatment, it is more forgiving to pour, and it machines faster, which shows up in the machining line item as much as the casting line item.
That does not automatically make it the cheaper decision. If a gray iron part fails in service on equipment that costs thousands of dollars an hour to shut down, the material saving is irrelevant. The reverse is also true. Specifying ductile iron on a part that only ever sees compressive load adds cost with no return.
The right comparison is total cost across the part's service life: casting cost, machining cost, expected replacement interval, and what a failure actually costs the end user.
How Section Thickness Affects Cast Iron Properties
Section thickness changes both materials.
Heavy sections cool slowly, and slow cooling produces coarser graphite and lower strength than a thin section poured from the same ladle. This is why ASTM A48 defines multiple test bar sizes in the first place. A class 40 result on a small test bar does not guarantee class 40 properties in a three-inch wall.
The practical consequence for a designer is that wildly uneven wall thickness produces a part with wildly uneven properties. Evening out sections is not only a castability issue. It is a material properties issue.
How to Choose Between Gray Iron and Ductile Iron
Does the part see impact or fatigue loading, or is the load steady and compressive?
Is it pressure containing, and at what temperature?
Does vibration or noise damping affect how the assembly performs?
How thick are the heaviest sections, and how uneven are they?
What does a failure in service actually cost?
When to Re-Specify a Cast Iron Material Callout
Prints get reused long after the application changes. If duty cycle, operating pressure, or assembly weight has moved since the part was designed, the material deserves a fresh look. The same is true when a fabricated or machined-from-bar part is converted to a casting. That conversion is the right moment to choose the material on purpose rather than inherit it.
Trumbull Foundry & Alloy pours both gray and ductile iron in low-to-medium production runs, with ferrous castings from 1 to 3,000 pounds and heat sizes as low as 100 pounds. Chemistry is verified in house by optical emission spectrometer.
Get Help Choosing the Right Iron Casting Material
Send Adam Danch your print, application details, expected loading, and annual volume, and Trumbull's team will walk through the material options with you.
Adam Danch, Vice President of Sales
Written in partnership with Jessica Creative Strategies
