Ductile Iron
Also: ductile cast iron, nodular iron, spheroidal graphite iron, SG iron, ASTM A536 65-45-12, 65-45-12, 60-40-18, 80-55-06, EN-GJS-450-10, GGG-40, GGG-50, D4512, DI, Dura-Bar 65-45-12 (continuous cast bar), ADI (austempered ductile iron)
Ductile iron is tough cast iron: 448 MPa (65 ksi) tensile, 12% elongation, a true yield strength, cast and machined like gray iron. Values for A536 65-45-12.
Sourcing ductile iron parts or stock?
- Metal Casting companies that work with cast iron · 34
- Machine shops that work with cast iron · 33
- All U.S. manufacturers working with cast iron · 97
At a glance
| Density | 7.1 g/cm³ (0.256 lb/in³) |
|---|---|
| Tensile strength | 448 MPa (65 ksi) min; typically 485 MPa (70 ksi) |
| Yield strength | 310 MPa (45 ksi) min (0.2% offset); typically 345 MPa (50 ksi) |
| Hardness | 156-217 HB (typically 170-200 HB) |
| Elongation | 12% min; typically 15-18% |
| Modulus of elasticity | 165-170 GPa (24-25 Msi) |
| Thermal | Melts at about 1150-1200 °C (2100-2190 °F); thermal conductivity ~36 W/m·K; damping well below gray iron but above steel |
| Composition | Fe, 3.2-4.0% C (as spheroidal graphite nodules), 1.8-2.8% Si, 0.1-0.6% Mn, 0.03-0.06% Mg (residual), up to 0.05% P, up to 0.02% S; Cu or Ni added for pearlitic grades |
Typical values, As cast, ASTM A536 grade 65-45-12 (ferritic-pearlitic). 60-40-18 annealed ferritic: 414 MPa (60 ksi) tensile, 276 MPa (40 ksi) yield, 18%; 80-55-06 pearlitic: 552 MPa (80 ksi), 379 MPa (55 ksi), 6%; 100-70-03 normalized: 690 MPa (100 ksi), 483 MPa (70 ksi), 3%. Confirm against the mill certificate and the purchase spec.
- MachinabilityExcellent
- The ferritic grades (60-40-18, 65-45-12) machine faster than 1020 steel with short chips and no gumminess; 80-55-06 machines like 1045, and 100-70-03 and ADI need carbide and slower speeds. Cast skin and any surface carbides are abrasive, so carbide for the first pass. Continuous-cast bar is uniform and free of skin.
- WeldabilityPoor
- Better than gray iron but still repair-grade: the heat-affected zone forms carbides and martensite. Nickel-iron rod (ENiFe-CI), preheat of 300 °C (570 °F), short beads and post-heat, or braze welding. Design ductile iron parts to be bolted, not welded; the grades with a steel-like matrix can be joined to steel in production only with qualified procedures.
- FormabilityNot recommended
- Cast to shape; even the 18% elongation grade is not bent or formed in practice. Straightening of shafts and rails is done cold with care on ferritic grades only.
- Corrosion resistanceFair
- Rusts like carbon steel; paint, e-coat, powder coat or galvanize for outdoor service. Ductile iron water pipe relies on cement lining and polyethylene encasement rather than the iron itself.
Ductile iron is cast iron whose graphite is in spheres instead of flakes, produced by treating the melt with magnesium just before pouring. The nodules do not act as crack starters, so the iron behaves like a medium-carbon steel with graphite in it: it has a yield strength, elongation from 2% to 18% depending on grade, real toughness and fatigue strength, and it still casts into complex shapes and machines like gray iron. It has displaced steel castings and forgings in crankshafts, suspension parts, gears, hydraulic bodies and heavy machinery over the past fifty years largely on cost.
ASTM A536 names the grades by tensile-yield-elongation: 60-40-18 is fully ferritic, the most ductile, and the pressure-retaining grade (ASTM A395); 65-45-12 is mostly ferritic with some pearlite, the general-purpose grade and the one most continuous-cast bar is made to; 80-55-06 is pearlitic for strength and wear; 100-70-03 is normalized and 120-90-02 quenched and tempered. Austempered ductile iron (ADI, ASTM A897) takes the same castings to 900-1600 MPa (130-230 ksi) with better wear resistance than through-hardened steel at the same hardness.
The matrix is set by the foundry through chemistry (silicon, copper, nickel) and cooling rate, so section thickness changes the as-cast grade the same way it does for gray iron, and the foundry needs to know the critical section. Ductile iron machines well in the ferritic grades and slower in the pearlitic ones, is more forgiving of casting defects than steel, welds only as a careful repair, and rusts like steel; it is not a substitute for gray iron where damping matters, because the nodules damp far less than flakes.
Heat treatment
The as-cast matrix is set by chemistry and cooling rate. Anneal (ferritize) at about 900 °C (1650 °F) then slow cool through 700 °C (1300 °F) for 60-40-18 ductility and machinability. Normalize at about 900 °C (1650 °F) with air cooling for a pearlitic 100-70-03 matrix. Quench in oil from about 870-900 °C (1600-1650 °F) and temper for 120-90-02. Austemper (ADI, ASTM A897) by austenitizing at 870-950 °C (1600-1750 °F) and quenching into a salt bath at 250-400 °C (480-750 °F) for an ausferrite matrix at 900-1600 MPa (130-230 ksi). Induction or flame harden pearlitic grades on gear teeth and journals to about 55 HRC. Stress relieve at about 510-565 °C (950-1050 °F) before finish machining precision castings.
Tempers and conditions
- 65-45-12 (as cast, ferritic-pearlitic)
- 60-40-18 (annealed ferritic)
- 80-55-06 (as cast pearlitic)
- 100-70-03 (normalized pearlitic)
- 120-90-02 (quenched and tempered)
- ADI grades (ASTM A897, austempered)
- Stress relieved
Stock forms
- Sand castings (green sand, no-bake, shell, lost foam)
- Continuous-cast bar: rounds, squares and rectangles (65-45-12 and 80-55-06)
- Centrifugally cast pipe and tube
- Investment castings (limited)
Relative cost
Tier 1 of 5. A little above gray iron per pound because of the magnesium treatment and tighter process control, and a fraction of a steel casting or forging for the same shape. Continuous-cast 65-45-12 bar costs about the same as 1045 bar and machines faster.
Typical applications
- Crankshafts, camshafts and connecting rods
- Steering knuckles, control arms and suspension components
- Differential cases, gears and gear blanks (ADI for high load)
- Hydraulic valve bodies, manifolds and pump housings
- Wind turbine hubs and heavy machinery frames
- Pressure-retaining valve and fitting bodies (60-40-18, ASTM A395)
- Water and sewer pipe and fittings
- Agricultural and construction equipment castings
Process compatibility
How ductile iron takes each process, and what to specify. Each process links to its own guide and to the shops that do it.
| Process | Rating | Notes |
|---|---|---|
| Sand Casting | ExcellentExcellent | The primary route: casts as easily as gray iron into sections from about 5 mm (0.2") up, with the foundry controlling the matrix through chemistry and cooling. Say which grade and the critical section. |
| CNC Machining | ExcellentExcellent | Ferritic grades machine faster than mild steel with short chips; pearlitic grades like 1045. Carbide for the cast skin; continuous-cast bar needs no special care. |
| CNC Turning | ExcellentExcellent | Turned journals, bushings and gear blanks from castings or continuous-cast bar; expect fine graphite dust rather than stringy chips. |
| Gear Cutting | GoodGood | Hobbed and shaped in 65-45-12 and 80-55-06; ADI gears rival carburized steel in pitting resistance and run quieter. Cut before austempering when possible. |
| Heat Treating | ExcellentExcellent | The matrix is adjustable from ferritic to austempered: anneal, normalize, quench and temper, or ADI per ASTM A897 for 900-1600 MPa (130-230 ksi). |
| Induction Hardening | GoodGood | Pearlitic grades (80-55-06 and up) induction or flame harden to about 55 HRC on gear teeth, journals and cam lobes; ferritic 65-45-12 responds poorly. |
| Stress Relieving | GoodGood | 510-565 °C (950-1050 °F) before finish machining on precision castings and thick sections; less critical than for gray iron but worth it on housings that must stay flat. |
| Welding | PoorPoor | Repair-grade only, with nickel-iron rod, preheat and post-heat, or braze welding; production welding to steel needs a qualified procedure and a ferritic grade. Bolt instead. |
| E-Coating (Electrocoating) | ExcellentExcellent | The automotive standard for suspension and chassis castings after shot blasting; even coverage on complex shapes. |
| Powder Coating | GoodGood | Common on housings and fittings; bake to outgas porosity before coating, and shot blast the skin for adhesion. |
| Hot-Dip Galvanizing | GoodGood | Hot-dip galvanized for pipe fittings, poles and outdoor hardware after shot blasting; silicon in the iron gives a thicker, duller grey coating than on steel. |
| Forging | Not recommendedNot recommended | Not forgeable; the graphite nodules and cast structure will not hot work. Forged parts are steel; the comparison is ductile iron casting versus steel forging. |
| Laser Cutting | Not recommendedNot recommended | Cast to shape and not stocked as sheet; a laser leaves a cracked, chilled edge on cast iron. Machine or saw it. |
Design and sourcing tips
- Specify the grade and spec: "ASTM A536 65-45-12" for general parts, "60-40-18 per ASTM A395" for pressure-retaining and low-temperature parts, "80-55-06" for wear and strength, ADI per ASTM A897 for gears and high-load parts.
- Give the foundry the critical section thickness and the location of machined and tested surfaces; the matrix, and therefore the grade, changes with cooling rate.
- Use ductile iron where a gray iron part sees impact, tension or fatigue; keep gray iron where damping and cost matter more than toughness, such as machine bases.
- Design to be bolted, not welded, and treat welding as a repair; if the assembly must be welded, make it from steel.
- For small quantities of shafts, gears, bushings and manifolds, specify continuous-cast 65-45-12 or 80-55-06 bar instead of a pattern and casting.
- When an ADI or quench-and-temper grade is planned, machine before heat treatment where tolerances allow and finish-grind afterward; the austemper cycle causes slight growth and distortion.
Frequently asked questions
- What does 65-45-12 mean for ductile iron?
- It is the ASTM A536 grade designation: 65 ksi (448 MPa) minimum tensile strength, 45 ksi (310 MPa) minimum yield strength and 12% minimum elongation. The other common grades read the same way: 60-40-18 (ferritic, most ductile), 80-55-06 (pearlitic), 100-70-03 (normalized) and 120-90-02 (quenched and tempered).
- Is ductile iron stronger than steel?
- It overlaps with medium-carbon steel. Grade 65-45-12 is close to hot-rolled 1045 in strength with somewhat less ductility and toughness; 100-70-03 approaches quenched-and-tempered steel; austempered ductile iron reaches 900-1600 MPa (130-230 ksi), above most heat-treated carbon steels, with better wear resistance. Its stiffness is lower (about 168 versus 200 GPa) and its fatigue and impact strength are below wrought steel of the same tensile strength.
- Can ductile iron be welded?
- Only as a repair with the right procedure: nickel-iron electrodes, a preheat of about 300 °C (570 °F), short beads, peening and slow cooling, or braze welding. The heat-affected zone forms brittle carbides and martensite otherwise. Parts should be designed to be bolted or made from steel if they must be welded in production.
- What is the difference between ductile iron and gray iron?
- The shape of the graphite. Gray iron has flakes, which make it brittle, cheap, highly damping and very machinable. Ductile iron has spheres, produced by a magnesium treatment, which give it a yield strength, elongation from 2% to 18% and toughness near steel at a small cost premium. Gray iron for bases, housings and brakes; ductile iron for crankshafts, gears, suspension parts and anything that sees shock.
- What is austempered ductile iron (ADI)?
- Ductile iron given a special heat treatment: austenitized, then quenched into a salt bath at 250-400 °C (480-750 °F) and held, which produces an ausferrite matrix. ADI grades under ASTM A897 reach 900-1600 MPa (130-230 ksi) tensile with useful ductility and excellent wear and fatigue resistance, at about 10% less weight than steel. It is used for gears, crankshafts, suspension parts and ground-engaging tools, replacing forged and carburized steel.
Processes that commonly use it
Related materials
Where to verify these values
- ASTM A536 and ASTM A395 (grade minimums and test requirements)
- ASTM A897 / A897M (austempered ductile iron grades)
- ASM Handbook, Volume 1 (Properties and Selection: Irons, Steels) and ASM Specialty Handbook: Cast Irons
- Ductile Iron Society, Ductile Iron Data for Design Engineers
