Ductile iron grades are sets of minimum mechanical properties defined by two standards: ASTM A536 in North America, with five general-use grades from 60-40-18 to 120-90-02, and EN 1563 in Europe, with a series running from EN-GJS-350-22 to EN-GJS-900-2. In both systems the name is the specification.
What moves a grade up either list is the matrix around the graphite nodules: ferrite at the ductile end, pearlite through the middle, a heat-treated structure at the top. Neither standard fixes the chemistry — the foundry sets the matrix by alloying or heat treatment and proves the result on a test bar.
North America Ductile Iron Grades
ASTM A536 lists five general-use grades, 60-40-18 through 120-90-02, each named by its minimum tensile strength in ksi, minimum yield strength in ksi at 0.2 % offset, and minimum elongation in percent. The ksi figures are the requirement; the MPa column is a conversion the standard prints for information only.
| ASTM A536 Grade | Min. Tensile Strength | Min. Yield Strength | Min. Elongation | Typical Structure | General Characteristics |
|---|---|---|---|---|---|
| 60-40-18 | 60 ksi / 414 MPa | 40 ksi / 276 MPa | 18% | Ferritic | Very ductile, tough, easily machined |
| 65-45-12 | 65 ksi / 448 MPa | 45 ksi / 310 MPa | 12% | Ferritic / ferritic-pearlitic | Good general-purpose grade |
| 80-55-06 | 80 ksi / 552 MPa | 55 ksi / 379 MPa | 6% | Mainly pearlitic | Higher strength and wear resistance |
| 100-70-03 | 100 ksi / 689 MPa | 70 ksi / 483 MPa | 3% | Heat-treated | High strength and hardness |
| 120-90-02 | 120 ksi / 827 MPa | 90 ksi / 621 MPa | 2% | Heat-treated | Very high strength, lowest ductility |
60-40-18 and 65-45-12 are both ferritic but are not reached the same way: A536 expects 60-40-18 to need a full ferritizing anneal, while 65-45-12 is normally met as-cast. The anneal is an extra furnace cycle, so 65-45-12 is the cheaper ferritic grade whenever 12 % elongation is enough. 80-55-06 is a pearlite-and-ferrite mix, also as-cast, with the pearlite brought in by alloying; how far it leans toward pearlite varies from one foundry to the next.
“Heat-treated” against 100-70-03 and 120-90-02 names the route, not the matrix. 100-70-03 is essentially all pearlite, usually reached by normalizing and tempering; 120-90-02 is quenched and tempered, so its matrix is tempered martensite. A536 accepts quench-and-temper, normalize-and-temper, or isothermal treatment for either grade and leaves the choice to the foundry.
A536 subordinates chemistry to mechanical properties, and hardness, composition, microstructure and pressure tightness are requirements only when the order specifies them. A wear part that needs a hardness band does not get one from the grade callout; the band has to be written on the order.
European Ductile Iron Grades
EN 1563 names each grade EN-GJS followed by minimum tensile strength in MPa and minimum elongation in percent — EN-GJS-500-7 is 500 MPa and 7 % — with the designation based on a 25 mm cast sample and the printed values applying to relevant wall thicknesses up to 30 mm.
| EN Grade | Min. Tensile Strength | Typical Character |
|---|---|---|
| EN-GJS-350-22 | 350 MPa | Extremely ductile |
| EN-GJS-400-18 | 400 MPa | Ferritic, tough |
| EN-GJS-400-15 | 400 MPa | Common ferritic grade |
| EN-GJS-450-10 | 450 MPa | Strength/ductility balance |
| EN-GJS-500-7 | 500 MPa | General engineering |
| EN-GJS-600-3 | 600 MPa | Higher-strength pearlitic |
| EN-GJS-700-2 | 700 MPa | High strength |
| EN-GJS-800-2 | 800 MPa | Very high strength |
Unlike A536, EN 1563 prints its own derating for thicker walls. EN-GJS-500-7 drops to 450 MPa tensile at 30–60 mm and 420 MPa at 60–200 mm, its 0.2 % proof strength going from 320 MPa to 300 and then 290, and its elongation from 7 % to 5 % in the heaviest band.
Three grades carry no standard minimums past 30 mm at all: EN-GJS-450-10, 800-2 and 900-2 (the grade above this table) are left to agreement between purchaser and foundry for anything thicker, as every grade is above 200 mm. On a heavy housing, specifying one of them means negotiating the acceptance numbers, not reading them off the standard.
The 350-22 and 400-18 grades also come as -LT and -RT variants with a Charpy requirement. The test runs only if the purchaser specifies it, and EN 1563 itself notes that impact energy’s relevance to brittle fracture in a loaded casting is under reassessment — an -LT callout buys a verified ferritic structure, not a fracture guarantee.
The table also omits EN 1563’s second group, the solid-solution-strengthened ferritic grades EN-GJS-450-18, 500-14 and 600-10, which use higher silicon in place of pearlite. At the same 500 MPa tensile, 500-14 gives 400 MPa proof strength against 320 for 500-7, and 14 % elongation against 7 %, with less hardness scatter and easier machining. They turn up on newer European drawings, and A536 has no counterpart.
How to Choose a Ductile Iron Grade
Choose the lowest grade whose yield strength and elongation clear the design at the casting’s actual section thickness, then check three things the grade name does not tell you: how hot or cold the part runs, whether the load cycles, and whether the drawing was written to the other standard. Lowest, because each step up the list trades ductility and machinability for strength, and the heavy sections of a sand casting are the first to fall short of a test-bar number.
Start from yield and elongation, not tensile
Tensile strength is the number in the name, but a housing or bracket is designed against yield, and elongation is its tolerance for the variation any casting carries. The ferritic grades — 60-40-18 and 65-45-12, or EN-GJS-400-18 through 450-10 — give the highest toughness and the easiest machining; they are the default for housings, flanges and anything that sees shock or cold. The pearlitic grades — 80-55-06 and 100-70-03, or EN-GJS-500-7 through 700-2 — are where wear resistance and higher strength come from, at a cost in machinability that is steepest at 100-70-03.
Derate for section thickness
The grade minimums are test-bar values, and A536 says outright that no precise relationship can be stated between a separately cast test specimen and the casting, or between two locations in the same casting. Under A536 the purchaser chooses the coupon: a 3 in. Y-block represents sections 1 1/2 in. (38 mm) and over, a 1 in. block 1/2 to 1 1/2 in., a 1/2 in. block anything thinner — and if the drawing is silent, the foundry chooses.
On the ductile iron castings we pour, the controlling section is rarely the nominal wall; it is the flange, the boss, or the section under the riser that cools slowest, and that is the thickness the coupon should match. A 1 in. Y-block on a gearbox housing whose flanges run past 1 1/2 in. passes easily and proves nothing about the flange. Write the coupon size next to the grade.
There are two honest ways to cover a heavy section: specify the coupon or wall thickness that matches it and let the foundry alloy to meet the grade there, or design to the derated numbers EN prints for that thickness. Going up a grade by name alone is a blunt instrument — it raises the number the coupon must show, pays the machinability cost on every section, and still leaves the heavy section unmeasured.
Temperature limits
For pressure-retaining parts that run hot — valve bodies, flanges, pump casings — the callout moves from A536 to ASTM A395, which rates all its grades to 450 °F and only 60-40-18 from 450 to 650 °F. Below zero, ASTM A874 covers ferritic ductile iron for service down to −40 °F and requires a ferritic structure with no massive carbides; the EN route is an -LT grade, 400-18-LT at −20 °C or 350-22-LT at −40 °C. In both directions the answer is ferrite — a pearlitic grade is the wrong choice for cold service whatever its strength.
Cyclic loads
A536 itself cautions that ductile iron quenched to martensite and tempered may have substantially lower fatigue strength than as-cast iron of the same hardness. For a flywheel, a crank, or anything under reversing load, that favors a pearlitic grade reached as-cast or by normalizing — 80-55-06, 100-70-03, EN-GJS-600-3 or 700-2 — over a quench-and-tempered 120-90-02, even where the quenched grade’s static numbers look safer.
Reading a grade from the other standard
There is no official ASTM-to-EN equivalence, and the nearest neighbors do not line up on yield or elongation.
| ASTM A536 grade (min. tensile / yield / elongation) | Nearest EN 1563 grade (min. Rm / Rp0.2 / A) | Where the neighbor falls short |
|---|---|---|
| 60-40-18 — 414 / 276 MPa / 18 % | EN-GJS-400-18 — 400 / 250 MPa / 18 % | 14 MPa under on tensile, 26 MPa under on yield |
| 65-45-12 — 448 / 310 MPa / 12 % | EN-GJS-450-10 — 450 / 310 MPa / 10 % | Tensile and yield match; elongation two points lower |
| 80-55-06 — 552 / 379 MPa / 6 % | EN-GJS-500-7 — 500 / 320 MPa / 7 %, or EN-GJS-600-3 — 600 / 370 MPa / 3 % | 500-7 is under on both strengths; 600-3 is 9 MPa under on yield with half the elongation |
| 100-70-03 — 689 / 483 MPa / 3 % | EN-GJS-700-2 — 700 / 420 MPa / 2 %, or EN-GJS-800-2 — 800 / 480 MPa / 2 % | 700-2 is 63 MPa under on yield; 800-2 is 3 MPa under; both a point under on elongation |
| 120-90-02 — 827 / 621 MPa / 2 % | EN-GJS-900-2 — 900 / 600 MPa / 2 % | 21 MPa under on yield |
Write the grade the drawing calls for, in the standard it calls for. Quoting EN-GJS-400-18 as 60-40-18 asks the foundry for 26 MPa of yield the European designer never needed, plus the anneal that comes with it. Where a translation is unavoidable, put both designations on the drawing and state which one governs acceptance.
Conclusion
A ductile iron grade is a promise about a test bar: five A536 grades and the EN-GJS series each name a minimum tensile, yield and elongation, and the matrix — ferrite, pearlite, or a heat-treated structure — is what delivers it. Choose the lowest grade that clears yield and elongation at the real section, move to A395, A874 or an EN -LT grade when temperature takes over, keep quench-and-tempered grades off fatigue-loaded parts, and never translate between the standards without checking yield. Then write the coupon size or relevant wall thickness next to the grade — without it, the number on the drawing describes a bar the foundry chose.