Gray Cast Iron
Also: gray iron, grey cast iron, ASTM A48 Class 30, ASTM A48 Class 40, flake graphite iron, G3000, G3500, EN-GJL-250, GG25, Dura-Bar G2 (continuous cast bar), CI, Class 30 gray iron
Gray iron is the cheapest metal to cast and machine: high damping, strong in compression, good wear, but brittle and not weldable. Values for A48 Class 30/40.
Sourcing gray 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 | 7.15 g/cm³ (0.258 lb/in³) |
|---|---|
| Tensile strength | Class 30: 207 MPa (30 ksi) min; Class 40: 276 MPa (40 ksi) min (compressive strength about 3-4x tensile: 750-965 MPa / 109-140 ksi) |
| Yield strength | n/a (no yield point; the stress-strain curve is non-linear from zero, and gray iron is designed to tensile class and compressive strength) |
| Hardness | Class 30: 180-220 HB; Class 40: 200-250 HB |
| Elongation | Under 1% (essentially nil) |
| Modulus of elasticity | Class 30: 90-113 GPa (13-16 Msi); Class 40: 110-138 GPa (16-20 Msi); non-linear and rising with class |
| Thermal | Melts at about 1150-1250 °C (2100-2280 °F); thermal conductivity ~46-52 W/m·K, the highest of the common ferrous materials; damping capacity many times that of steel |
| Composition | Fe, 2.5-4.0% C (mostly as graphite flakes), 1.0-3.0% Si, 0.2-1.0% Mn, up to 0.15% P, up to 0.15% S; Class 40 typically 3.1-3.3% C, 1.9-2.2% Si |
Typical values, As cast, ASTM A48 Class 30 (general purpose) and Class 40 (pearlitic, higher strength and wear resistance), test bar B. Properties fall in heavy sections and rise in thin ones. Confirm against the mill certificate and the purchase spec.
- MachinabilityExcellent
- Graphite lubricates the cut and breaks the chip, so it machines dry, fast and with short chips; the standard for machinability is often quoted against it. Use carbide, ceramic or CBN for the first pass through the cast skin and chilled corners, and expect abrasive dust rather than swarf. Class 40 machines a little slower than Class 30.
- WeldabilityPoor
- Fusion welding forms brittle white iron and martensite in the heat-affected zone and cracks. Repair only, with nickel or nickel-iron rod (ENi-CI, ENiFe-CI), preheat of 300 °C (570 °F) or more, short beads and peening; braze welding is often the safer repair. Never design a gray iron part to be welded.
- FormabilityNot recommended
- Nil ductility; it is cast to shape and machined. Any attempt to bend or straighten it cracks it.
- Corrosion resistanceFair
- Rusts like carbon steel, though the graphite-rich skin and the rust layer slow it somewhat; fine indoors with oil or paint, poor outdoors bare or in chlorides. High-silicon and nickel-alloyed irons exist for acid and seawater service.
Gray cast iron is iron with 2.5-4% carbon, most of it present as flakes of graphite in a pearlitic or ferritic matrix; the fracture surface looks gray because of the graphite, hence the name. The flakes make it brittle in tension and give it essentially no ductility, but they also make it cheap to cast into intricate shapes, easy to machine with short chips and self-lubricating surfaces, very strong in compression, and extraordinarily good at damping vibration. That is why machine tool bases, engine blocks, brake rotors, pump housings and gear cases have been gray iron for a century and mostly still are.
It is specified by tensile class under ASTM A48: Class 30 has 207 MPa (30 ksi) minimum tensile and is the general-purpose grade; Class 40 has 276 MPa (40 ksi) minimum, a pearlitic matrix, and is the choice for wear surfaces, ways and heavier duty housings. There is no yield strength, because the stress-strain curve is curved from the origin, and the modulus itself rises with class. Strength also depends on section thickness: thin sections cool fast and are stronger and harder, heavy sections coarser and weaker, so the class is tied to a test bar size and the foundry needs to know the section.
Machine shops also buy gray iron as continuous-cast bar (Dura-Bar and equivalents), which is Class 30-40 iron with a fine, sound structure in rounds, squares and rectangles, machined like any bar stock into bushings, gears, hydraulic manifolds and fixture parts. The limits to keep in mind are brittleness, no fusion welding except careful repair with nickel rod, ordinary rusting, and a hard skin and chilled corners from the mold that need carbide tooling for the first cut.
Heat treatment
Usually used as cast. Stress relieve machine bases and precision housings at about 540-600 °C (1000-1100 °F) with slow cooling before finish machining, which removes casting stresses that would otherwise let the part move. Anneal (ferritize) at about 700-760 °C (1300-1400 °F) to improve machinability at some loss of strength. Flame or induction harden pearlitic Class 40 ways and wear surfaces to about 50-55 HRC; through hardening is unusual. Austempered and other specialty treatments are the province of ductile iron.
Tempers and conditions
- As cast (Class 30, Class 40)
- Stress relieved
- Annealed (ferritic, for machinability)
- Flame- or induction-hardened wear surfaces
Stock forms
- Sand castings (green sand, no-bake, shell)
- Continuous-cast bar: rounds, squares, rectangles and tubes (Dura-Bar type)
- Permanent mold castings (limited)
- Centrifugally cast tube and liners
Relative cost
Tier 1 of 5. The cheapest engineering metal per pound as castings, and continuous-cast bar costs about the same as 1018 bar while machining faster. Pattern or tooling cost is the entry fee for castings; bar stock needs none.
Typical applications
- Machine tool bases, columns, tables and ways
- Engine blocks, cylinder heads and liners
- Brake rotors and drums
- Pump housings, valve bodies and impellers
- Gearboxes, motor frames and bearing housings
- Flywheels, counterweights and press frames
- Hydraulic manifolds, bushings and gears from continuous-cast bar
- Cookware, stove parts and municipal castings
Process compatibility
How gray 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 process gray iron exists for: cheap patterns, excellent fluidity into thin and intricate sections, and almost no shrinkage cracking. Minimum wall about 4-6 mm (0.16-0.25"). |
| CNC Machining | ExcellentExcellent | Fast, dry machining with short chips; carbide for the cast skin, ceramic or CBN for production. Continuous-cast bar machines like a very forgiving free-machining steel. |
| CNC Milling | ExcellentExcellent | Milled faces come out flat and stable, especially after a stress relief; tolerate the abrasive dust with good chip evacuation and covered ways. |
| CNC Grinding | ExcellentExcellent | Grinds cleanly to fine finishes for ways, seal faces and brake rotors; the graphite gives a slightly open, self-lubricating surface. |
| Induction Hardening | GoodGood | Pearlitic Class 40 (about 0.6% combined carbon) flame or induction hardens to 50-55 HRC on ways and wear surfaces; ferritic or Class 30 iron does not respond well. |
| Stress Relieving | ExcellentExcellent | Standard for machine bases and precision housings: 540-600 °C (1000-1100 °F), slow cool, then finish machine. Cheap insurance against parts that move after machining. |
| Welding | PoorPoor | Repair only, with nickel or nickel-iron rod, preheat and peening, or braze welding; cracks in the heat-affected zone are the norm. Bolt or dowel assemblies instead. Can cast iron be welded? |
| Powder Coating | GoodGood | Common on housings and municipal castings; bake the casting first to outgas porosity, or the coating blisters. |
| E-Coating (Electrocoating) | GoodGood | The automotive finish for brake and chassis castings; good coverage of complex shapes once the surface is shot blasted and clean. |
| Electroless Nickel Plating | FairFair | Plates for corrosion and wear on hydraulic parts, but porosity and graphite at the surface make adhesion and coverage variable; seal and specify a thickness with a porosity test. |
| Laser Cutting | Not recommendedNot recommended | Cast to shape, not stocked as sheet; laser cutting cast iron leaves a cracked, chilled edge. Machine or saw it. |
| Forging | Not recommendedNot recommended | Nil ductility at any temperature; it cannot be forged or formed. Forged parts are steel; cast-and-machined parts are iron. |
Design and sourcing tips
- Specify the class and the spec: "ASTM A48 Class 30" for general housings, "Class 40" for wear surfaces and ways; tell the foundry the critical section thickness because properties change with it.
- Design for compression and stiffness, not tension: keep tensile stresses low, avoid sharp internal corners and abrupt section changes, and use ribs rather than thick walls.
- Never design a gray iron part to be welded; bolt, dowel or shrink-fit assemblies, and treat welding as a last-resort repair.
- Call out a stress relief before finish machining on any base, housing or plate that must hold flatness or alignment.
- For bushings, gears, manifolds and fixture parts in small quantities, specify continuous-cast bar (Class 40 equivalent) instead of a casting; it is sound, fine-grained and needs no pattern.
- Add machining stock of 2-3 mm (0.08-0.12") on cast surfaces to get under the skin and chilled corners, and expect carbide tooling for the first pass.
Frequently asked questions
- Why does gray cast iron have no yield strength?
- Because the graphite flakes act as internal notches, the stress-strain curve of gray iron bends from the origin with no straight elastic region and no distinct yield point, and it fractures at under 1% strain. It is specified and designed by tensile class and by compressive strength, which is three to four times the tensile value. Ductile iron, with graphite nodules instead of flakes, does have a yield strength.
- Can gray cast iron be welded?
- Only as a repair, and carefully. The heat-affected zone forms brittle white iron and martensite that cracks on cooling. Repair welds use nickel or nickel-iron electrodes, a preheat of 300 °C (570 °F) or higher, short beads and peening, followed by slow cooling; braze welding avoids most of the problem. Parts should never be designed to be welded.
- What is the difference between Class 30 and Class 40 gray iron?
- The class is the minimum tensile strength in ksi from a standard test bar: 207 MPa (30 ksi) for Class 30, 276 MPa (40 ksi) for Class 40. Class 40 has a fully pearlitic matrix, is harder (200-250 HB), wears better and can be flame hardened, but machines a little slower and needs more care to cast without chill. Class 30 is the general-purpose grade for housings and bases.
- Is gray cast iron easy to machine?
- Yes, among the easiest of all metals. The graphite lubricates the tool and breaks the chip into short segments, so it is machined dry at high speed with carbide, ceramic or CBN tooling. The cast skin and chilled corners are hard and abrasive, so the first pass needs carbide and enough stock to get under the skin.
- What is the difference between gray iron and ductile iron?
- Both are cast irons with similar carbon content. In gray iron the graphite is in flakes, which makes it brittle, cheap, highly damping and very machinable. In ductile iron a magnesium treatment rounds the graphite into nodules, giving it a real yield strength, 2-18% elongation and toughness closer to steel, at slightly higher cost. Use gray iron for bases, housings and brake parts; ductile iron for crankshafts, gears and parts that see shock.
Processes that commonly use it
Related materials
Where to verify these values
- ASTM A48 / A48M (tensile class minimums and test bar sizes)
- SAE J431 (automotive gray iron grades and hardness)
- ASM Handbook, Volume 1 (Properties and Selection: Irons, Steels) and ASM Specialty Handbook: Cast Irons
- Continuous-cast bar producer data sheets (Dura-Bar and equivalents)
