Cast Iron
Cast iron is iron with 2–4 % carbon and 1–3 % silicon — so much carbon that it solidifies as graphite in an iron matrix instead of dissolving, which is why it pours at about 1,200 °C into thin sections, machines with short chips and damps vibration better than any steel. Graphite shape decides the grade: flakes give Gray Iron (ASTM A48 Class 30, about 200 MPa tensile, brittle, best damping); spheroids give Ductile Iron (ASTM A536 65-45-12: 450 MPa tensile, 12 % elongation, steel-like toughness); austempered ductile becomes ADI (ASTM A897 Grade 3, 1,200 MPa); and high-chromium white irons (ASTM A532) have no free graphite and reach about 60 HRC for abrasion.
Popular cast iron grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| Gray Iron | 165 | 220 | 7.15 | Excellent | Poor | Machine bases, housings, brake discs; best damping |
| Ductile Iron | 340 | 480 | 7.1 | Good | Fair | Shock-loaded brackets, hubs, gear cases, fittings |
| A897 Grade 5 | 1,200 | 1,500 | 7.1 | Poor | Not recommended | Hardest ADI grade |
| A897 Grade 3 | 810 | 1,120 | 7.1 | Poor | Not recommended | Steel strength at casting cost |
| A897 Grade 1 | 700 | 950 | 7.1 | Fair | Poor | Steel-like strength, cast cost |
| A536 120-90-02 | 660 | 900 | 7.1 | Fair | Poor | Strongest standard ductile iron |
| A536 100-70-03 | 520 | 760 | 7.1 | Good | Poor | High-strength pearlitic ductile iron |
All 26 cast iron grades
- A47 32510
- A47 M3210
- A48 Class 20
- A48 Class 30
- A48 Class 40
- A220 45008
- A220 M5003
- A436 Type 1
- A439 Type D-2
- A532 Class I Type A
- A532 Class III Type A
- A536 60-40-18
- A536 65-45-12
- A536 80-55-06
- A536 100-70-03
- A536 120-90-02
- A842 Grade 300
- A842 Grade 400
- A842 Grade 450
- A897 Grade 1
- A897 Grade 3
- A897 Grade 5
- Ductile Iron
- EN-GJMW-400-5
- Gray Iron
- Ingot Iron
Choose cast iron for
Machine tool bases and columns, gearbox and pump housings, engine blocks and heads, brake discs and drums, manhole and municipal castings, counterweights, pipe fittings and any complex shape in quantity where casting cost, machinability and vibration damping outweigh strength per kilogram.
Look elsewhere when
The part must be welded into a fabrication or will see weld repair in service (cast steel WCB or ZG270-500), impact at low temperature is severe or the wall is under about 3 mm (ductile can manage, gray iron cannot), tensile or fatigue strength above about 500 MPa is needed without going to ADI (cast or forged 4140 / 42CrMo4), or weight matters (cast A356-T6 aluminum housings at a third of the density).
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Cast iron by application
- Machine bases and housings
- Gray Iron ASTM A48 Class 30 or ASTM A48 Class 40 — damping, dimensional stability after a stress-relief anneal, and free-cutting machinability; Ductile Iron ASTM A536 65-45-12 where a lug or flange could be struck or over-torqued.
- Shock and bending loads
- Ductile Iron ASTM A536 65-45-12 as the default, ASTM A536 60-40-18 for cold-service toughness, ASTM A536 80-55-06 or ASTM A536 100-70-03 for crankshafts, gears and heavy suspension parts. ASTM A897 Grade 1 or ASTM A897 Grade 3 ADI where fatigue strength must approach forged steel.
- Heat and corrosion
- Ni-Resist: ASTM A436 Type 1 gray and ASTM A439 Type D-2 ductile for exhaust manifolds, turbocharger housings, seawater and caustic pumps; compacted graphite ASTM A842 Grade 400 for heavy-duty diesel blocks.
- Abrasion
- ASTM A532 Class I Type A (Ni-Hard) and ASTM A532 Class III Type A (25 % Cr white iron) at about 53–65 HRC for slurry pump casings, mill liners and crusher parts; cast to shape and finished by grinding only.
Working with cast iron
- Machining
- Gray iron is one of the easiest metals to machine — short, powdery chips, no built-up edge, dry cutting with carbide or ceramic at high speed; it is abrasive, so tool wear rather than cutting force sets tool life. Ductile iron cuts more like a medium-carbon steel with longer chips. Skin and hard spots (chill, sand) demand a first cut of 2–3 mm; ADI and white irons are ground or machined with CBN.
- Welding and repair
- Cast iron is not a fabrication material. Gray iron can be repair-welded with nickel electrodes (ENi-CI, ENiFe-CI) after preheating to 300–600 °C and cooling slowly, or braze-repaired; ductile iron is repaired the same way with more risk of losing ductility in the heat-affected zone. Design joints as bolted flanges, and choose cast steel where welding is part of the plan.
- Heat treatment
- Stress-relieve gray iron bases at 550–600 °C before finish machining to stop later movement. Ductile iron is annealed (60-40-18), normalized (80-55-06 / 100-70-03), quenched and tempered (120-90-02), or austempered at 250–400 °C to make ADI (ASTM A897). Flame or induction hardening of pearlitic ductile to about 55 HRC is standard for cam and gear surfaces.
- Tolerances and design
- As-cast tolerance is about ±0.5 mm per 100 mm in sand (ISO 8062 CT8–CT10), so machine every locating face and add 1–3 mm of allowance. Keep walls above 3 mm for ductile and 5 mm for gray, and avoid abrupt section changes that chill white. Gray iron carries roughly three times more in compression than in tension — design it to be compressed.
Other steel series
- Low-Carbon and Mild Steel
- Medium and High-Carbon Steel
- Free-Machining Steel
- Alloy Steel
- Case-Hardening Steel
- Tool Steel
- Mold Steel
- Spring and Bearing Steel
- Structural and HSLA Steel
- Wear Plate and Advanced High-Strength Steel
- Pressure Vessel and Chromium-Molybdenum Steel
- Electrical Steel and Soft Magnetic Alloys
- Aerospace and Maraging Steel
- Cast Steel
- Steel Welding Consumables
Cast Iron — frequently asked
What is cast iron?
Cast iron is iron with 2–4 % carbon and 1–3 % silicon — so much carbon that it solidifies as graphite in an iron matrix instead of dissolving, which is why it pours at about 1,200 °C into thin sections, machines with short chips and damps vibration better than any steel. Graphite shape decides the grade: flakes give Gray Iron (ASTM A48 Class 30, about 200 MPa tensile, brittle, best damping); spheroids give Ductile Iron (ASTM A536 65-45-12: 450 MPa tensile, 12 % elongation, steel-like toughness); austempered ductile becomes ADI (ASTM A897 Grade 3, 1,200 MPa); and high-chromium white irons (ASTM A532) have no free graphite and reach about 60 HRC for abrasion.
Which grade of cast iron should I use?
Gray Iron (ASTM A48 Class 30 or SAE G3500) is the default for machine bases, housings, brake discs and anything loaded in compression that benefits from damping; ASTM A48 Class 40 when a stiffer, stronger gray casting is needed. Ductile Iron in ASTM A536 65-45-12 is the general-purpose grade for brackets, hubs, gear cases and pipe fittings that see shock or bending; ASTM A536 60-40-18 (ferritic) for the highest ductility and low-temperature toughness, ASTM A536 80-55-06 and ASTM A536 100-70-03 (pearlitic) for crankshafts, gears and heavier loads, ASTM A536 120-90-02 quenched and tempered where hardness matters. ASTM A897 Grade 1 to Grade 3 austempered ductile iron replaces forged steel in gears, suspension arms and track components. Ni-Resist ASTM A436 Type 1 and ASTM A439 Type D-2 resist heat and corrosion in exhaust and seawater pumps; ASTM A532 Class III Type A high-chromium white iron is for slurry pumps and crusher liners. Compacted graphite iron (ASTM A842 Grade 400, ASTM A842 Grade 450) is the diesel-block material between gray and ductile.
Which grade of cast iron is the strongest?
A897 Grade 5 has the highest yield strength in the series at 1,200 MPa; A897 Grade 3 follows at 810 MPa. Values are for each grade's default condition.
Which grade of cast iron is easiest to machine?
A48 Class 20 (machinability excellent, 90/100), then A47 32510 (85/100).
Is cast iron heat-treatable?
Some grades are. Check the conditions listed on each grade page.
Gray iron or ductile iron?
Gray Iron is cheaper, machines faster, damps vibration better and holds its shape after a stress-relief anneal — the right answer for bases, housings and anything that is not struck or bent. Ductile Iron costs more per kilogram and damps less, but at 450 MPa tensile and 12 % elongation (65-45-12) it takes shock, over-torqued bolts and bending that would crack Class 30 gray iron (about 200 MPa, near-zero elongation). If the part has a lug, arm or thin flange that could be hit, or carries a fatigue load, go ductile.
Can cast iron be welded?
Only as a repair, and only with care. Gray iron and ductile iron form brittle white iron and martensite in the fusion zone unless preheated to 300–600 °C, welded with high-nickel electrodes in short beads, peened and cooled slowly under insulation; even then the joint is weaker than the casting and ductile iron loses ductility in the heat-affected zone. Brazing and cold metal-stitching are the alternatives for cracked castings. For a part that must be welded in production, specify cast steel (WCB, ZG270-500) or a steel fabrication instead.