Most manhole frames, catch basin frames, and grates on Ontario streets are grey iron. Some tenders call for ductile iron, or a ductile cover in a grey frame. Both are "cast iron" and look the same on the road, but they behave differently under load. Here is the difference, with numbers from the ASTM standards your spec cites.
Same chemistry, different graphite
Grey and ductile iron carry roughly the same carbon. What changes is the shape the carbon takes as the casting solidifies. In grey iron the graphite forms interconnected flakes. In ductile iron the melt is treated with magnesium (a residual of roughly 0.03 to 0.06%, per the Metal Casting Institute) and the graphite solidifies as spheroids, or nodules. ASTM A536 defines ductile iron as cast iron "with the graphite substantially spheroidal in shape."
Flake graphite acts like a network of internal notches, so grey iron cracks before it bends. Nodular graphite leaves the matrix continuous, so ductile iron yields before it fails.
Strength numbers from the standards
Grey iron is graded under ASTM A48 by minimum tensile strength in a separately cast test bar. Class 35 means 35 ksi (241 MPa). The letter after the class (A, B, C, S) is the test bar size, not a different iron; Class 35B is the baseline in AASHTO M306. Grey iron has no measurable yield strength, and its elongation is so low it is rarely measured or specified.
Ductile iron is graded under ASTM A536 with a three-part number: tensile, yield, elongation. Grades 65-45-12 and 80-55-06 are tested as-cast; 60-40-18 needs a full ferritizing anneal.
| Grade | Standard | Min. tensile | Min. yield | Min. elongation |
|---|---|---|---|---|
| Class 35 grey iron | ASTM A48 | 35 ksi (241 MPa) | Not measurable | Not specified |
| 60-40-18 ductile | ASTM A536 | 60 ksi (414 MPa) | 40 ksi (276 MPa) | 18% |
| 65-45-12 ductile | ASTM A536 | 65 ksi (448 MPa) | 45 ksi (310 MPa) | 12% |
| 80-55-06 ductile | ASTM A536 | 80 ksi (552 MPa) | 55 ksi (379 MPa) | 6% |
A48 test-bar values describe the iron poured, not the strength at every point in a casting. Material certs and proof-load documentation for any DomCast SKU are confirmed per SKU at quote.
Impact and ductility
Penticton Foundry puts ductile iron at a minimum of 7 foot-pounds of impact resistance against 2 for grey iron. DIPRA states ductile iron's impact strength and elongation are "many times greater" than grey iron's, and its tensile strength approximately double. A grey cover hit hard enough fractures; a ductile cover of the same design dents and stays in one piece.
Corrosion
Neither needs a coating to survive in a road. DIPRA reports that lab and field testing show ductile iron's corrosion resistance is equal to or greater than grey iron's, and NPCA says Class 35B grey iron has the best corrosion and wear resistance along with high compressive strength, which is why it makes good load-bearing street hardware. Pick on load behaviour, not corrosion.
Weight for a given rating
With roughly twice the tensile strength, a ductile cover can pass the same proof load with a thinner section, so foundries commonly quote ductile covers as lighter than grey covers of the same type. Grey iron gets there through mass, which is also why a heavy grey lid sits quietly in its frame.
Where each is typically specified
- Grey iron (A48 Class 35B) is the default for standard streets. AASHTO M306 names it as the baseline, and Neenah Foundry calls it the more economical choice and the standard material across most municipal applications. In Ontario, OPSS.MUNI 1850 governs frames, grates, and covers and allows either grey iron to ASTM A48M Class 30B or ductile iron to ASTM A536 Grade 65-45-12, so check which your municipality names per drawing. Toronto TS 407 and OPSS.MUNI 407 both send frames and covers to OPSS 1850 and the OPSD 401.010 to 401.060 (maintenance hole frames with covers) and 400.010 to 400.120 (catch basin frames with grates) series.
- Ductile iron (A536) is specified above H-20: airports, ports, industrial yards, truck routes, and anywhere impact loads are expected. NPCA says it is usually used where loads exceed H-20, and notes a common cost-effective mix of a ductile lid or grate in a grey iron frame, since the frame is mostly in compression, where grey iron is strong.
Cost
Grey iron is the cheaper casting for a given pattern. Ductile adds the magnesium treatment and tighter process control, and some grades add heat treatment. For a residential street with a standard manhole cover at H-20, grey iron has done the job for a century. For a transfer station apron or a bus loop, the premium is small next to one broken cover.
Side by side
| Property | Grey iron | Ductile iron |
|---|---|---|
| Graphite form | Flake | Spheroidal (nodular) |
| Governing standard | ASTM A48 (AASHTO M105) | ASTM A536 |
| Typical municipal grade | Class 35B (AASHTO M306); Class 30B (OPSS.MUNI 1850) | 65-45-12 (OPSS.MUNI 1850) |
| Yield strength | Not measurable | 45 ksi (310 MPa) for 65-45-12; 55 ksi (379 MPa) for 80-55-06 |
| Elongation | Not specified | 6 to 12% (as-cast grades) |
| Failure mode | Fractures | Deforms first |
| Corrosion in service | Good | Equal or better |
| Weight at equal rating | Heavier | Lighter |
| Cost | Lower | Higher |
| Typical use | Standard streets, H-20 | Above H-20, impact, airports, ports |
How to write it into a tender
Name the standard and grade per line item, not just "cast iron." Grey: "ASTM A48 Class 35B, proof-load tested per AASHTO M306" (or Class 30B where OPSS.MUNI 1850 applies). Ductile: "ASTM A536 Grade 65-45-12" with the same proof-load clause. DomCast Metals is a Canadian-owned supplier of municipal castings such as OPSD-pattern 401.010 maintenance hole frames and covers and the 400.010 dished overflow catch basin; material grade and certification for any SKU are confirmed at quote. See also catch basins.
Frequently asked questions
Is ductile iron always better than grey iron for a manhole cover?
No. It is stronger and tougher, but grey iron Class 35B is the AASHTO M306 baseline and has served standard streets for a century. Ductile earns its premium above H-20 or where a fractured cover is a safety problem.
Does ductile iron rust less than grey iron?
Not meaningfully. DIPRA reports ductile iron's corrosion resistance is equal to or greater than grey iron's. Choose on load and impact behaviour, not corrosion.
What do the numbers in ASTM A536 65-45-12 mean?
Minimum tensile 65 ksi (448 MPa), minimum yield 45 ksi (310 MPa), minimum elongation 12%. The same pattern applies to 60-40-18 and 80-55-06.
What does the B in ASTM A48 Class 35B mean?
The number is minimum tensile strength in ksi (35 ksi, 241 MPa). The letter is the size of the separately cast test bar, not a different iron.
Can I mix a ductile cover with a grey iron frame?
Yes. NPCA lists it as a common cost-effective combination. Specify each part separately and confirm fit per SKU at quote.
Which Ontario standard covers frames and grates?
OPSS.MUNI 1850, which allows grey iron to ASTM A48M Class 30B or ductile iron to ASTM A536 Grade 65-45-12. Check your municipality's own standard for which material it names per drawing.
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