Cast Iron vs Ductile Iron — Grey Iron and SG Iron Compared, Grade by Grade

The Short Answer

Choose grey iron for parts that sit, damp and carry heat; choose ductile iron for parts that are pulled, bent, struck or under pressure. EN-GJL-250 (the old GG-25), the grey iron most drawings name, has a minimum tensile strength of 250 MPa and breaks without bending; EN-GJS-400-15 (the old GGG-40), the most common ductile grade, reaches 400 MPa and stretches 15 % first. Grey iron costs less to cast, fills thin walls, machines to short chips and damps vibration, which is why machine beds, brake discs, pulleys and many pump and gearbox housings are grey. Ductile iron is stiffer (169 against 103–118 GPa) and takes shock, which is why valve bodies, pipe fittings, hubs, brackets and many crankshafts are ductile.

Grey vs Ductile Iron — the Numbers

The grey iron and the ductile iron that most drawings name, side by side. Minimums are from EN 1561:2011 and EN 1563:2018; modulus, conductivity and density are the informative values of the same standards.

Grey Iron Grades — EN 1561 With Old DIN, JIS, GB and ASTM Names

EN 1561 names grey iron by its minimum tensile strength, and the maximum of every grade is the minimum plus 100 MPa: EN-GJL-250 is 250–350 MPa. Since the 2011 edition the value belongs to a sample cast to the size of the wall — for EN-GJL-250, 250 MPa up to 50 mm, 220 MPa for 50–100 mm and 200 MPa for 100–200 mm; drawings from before 2011 relate the grade to a 30 mm test bar. The German names GG-10 to GG-35 (DIN 1691, replaced in 1997) are the same grades under their old names.

EN 1561 also specifies grey iron by hardness instead of strength: EN-GJL-HB155 to EN-GJL-HB255, where the number is the maximum Brinell hardness in walls up to 50 mm. That is the grade to name when machinability or wear matters more than strength, for example on a series of housings machined on one line.

Ductile (SG) Iron Grades — EN 1563 With Old DIN, JIS, GB and ASTM Names

EN 1563 names ductile iron by minimum tensile strength and elongation: EN-GJS-400-15 is 400 MPa and 15 % in walls up to 30 mm. Grades ending -LT guarantee Charpy impact energy at −20 °C or −40 °C, grades ending -RT at room temperature. Since 2011 the standard also has three solution-strengthened ferritic grades, EN-GJS-450-18, 500-14 and 600-10: silicon in the ferrite instead of pearlite gives them more proof strength and elongation for the same tensile strength, and a narrower hardness band that machines evenly.

Two details that trip up a crosswalk: JIS FCD450-10 has a 0.2 % proof minimum of 280 MPa where EN-GJS-450-10 has 310 MPa, so an EN casting meets a JIS drawing but not the other way round; and ISO 1083 has 500-10 and 550-5 grades that have no EN counterpart.

ASTM on the Drawing — Which Grade Meets It

Crosswalk tables pair grades by name, and most pairs are not equal on paper. A grade meets another only when every minimum — tensile strength, proof strength, elongation — is at or above it. The two tables check each usual pair in walls up to 50 mm (grey) or 30 mm (ductile), with ASTM values converted from ksi.

Of the five usual grey pairs, none holds both ways: EN-GJL-150, 250 and 350 meet A48 Class 20, 35 and 50, while Class 30 and Class 45 meet EN-GJL-200 and 300. The metric companion standard A48M uses the EN numbers directly — A48M Class 250 is 250 MPa, like EN-GJL-250.

The other way round, A536 60-40-18 meets EN-GJS-400-18 and 400-15, and 120-90-02 meets EN-GJS-800-2; 65-45-12 misses EN-GJS-450-10 by 2 MPa of tensile strength, which is 65 ksi rounded. For an ASTM drawing the EN 1563 solution-strengthened grades are the ones that fit, and nothing in EN 1563 meets 100-70-03 or 120-90-02 — those are poured to A536 itself.

The Drawing Says GG25, GGG40, Class 30 or 65-45-12 — What the Casting Is Poured In

No stockholder table has this column. A drawing arrives with one country’s names and the casting is poured where another name is on the ladle. The rule behind every row: the grade that will be poured is named on the quote before the pattern or printed mould is made, and where it is not the grade on the drawing, the quote says which minimum changes.

How to Tell Grey Iron From Ductile Iron

How can you tell grey iron from ductile iron?

The certain test is the graphite shape: a polished cross-section under a microscope at about 100× shows flakes in grey iron and spheres in ductile iron. Without a lab, four signs point the same way. A broken edge of grey iron is dark grey and granular with no bending, while a ferritic ductile iron bends or tears before it breaks. Drilling grey iron gives short fragments and black graphite dust; ductile iron gives chips that hold together. Struck with a hammer, grey iron gives a dull note and ductile iron rings. Cast marks read GG, GJL, HT, FC or CI on grey iron and GGG, GJS, QT, FCD, DI or DUCTILE on ductile iron.

Is ductile iron stronger than cast iron?

Yes in tension and bending: the most common ductile grade, EN-GJS-400-15, has a minimum tensile strength of 400 MPa against 250 MPa for the most common grey grade, EN-GJL-250, and stretches 15 % before it breaks against under 1 %. In compression the gap narrows, because grey iron carries about three times its tensile strength in compression. Grey iron also damps vibration and conducts heat better, so the stronger iron is not the better one for a machine bed or a brake disc.

Can a cracked grey iron part be replaced in ductile iron?

Yes: in most cases the same geometry can be cast in ductile iron, and a part that cracked from shock, over-tightened bolts or frost comes back stronger — EN-GJS-400-15 takes 400 MPa and stretches 15 % where EN-GJL-250 took 250 MPa and cracked. Three things change and go on the quote: the part damps vibration less, it conducts about a fifth less heat (36.2 against 46.5 W/(m·K) at 300 °C), and it is stiffer. Machine bases, brake drums and discs, and parts chosen for damping stay in grey iron.

Which Parts Are Grey Iron and Which Are Ductile

One Piece: Cast It, Machine It From Iron Bar, or Machine It From Steel?

A pattern costs more than one casting, so for one or two parts there are three routes, and the quote names the one used.

Machined from continuous-cast iron bar. Grey EN-GJL-250 and ductile EN-GJS-400-15, 500-7 and 600-3 are sold as round and flat bar, so a small housing, bush, gear blank or bracket is machined in the grade the casting would have had, without casting porosity.
Cast in a 3D-printed sand mould. No pattern: the mould is printed from the 3D model, for parts too large or too hollow for bar — see the routes on the cast parts page and the casting tolerance chart for the allowance on machined faces.
Machined from steel or another metal. When the part does not need iron’s damping or wear behaviour: C45 or S355 steel, 6061 aluminium for weight, or 304 (1.4301) or 316L (1.4404) stainless where water or chemicals corrode iron. The geometry is copied, not the production method.

Where the Catalogue Stops

Related pages: custom cast parts by name, valve bodies, casting tolerance chart (ISO 8062-3), pump replacement parts, material grade equivalents and C45 / 1045 steel equivalents.

Catalogue pillow blocks are EN-GJL-250; a remade one in small numbers is steel, 304 stainless or 6061 machined from block — sizes and what it keeps on bearing housing replacement.

Cast Iron vs Ductile Iron FAQ