Steel Gray Iron
also Grey Iron, Gray Cast Iron
Gray Iron is a flake-graphite iron–carbon–silicon casting alloy supplied to ASTM A48 Class 20–60.
Values for the As Cast condition
Gray Iron tempers and conditions
Gray Iron is supplied in 14 conditions, soft to hard. As Cast is the default FabDigit quotes; the values on this page are for it.
- Class 20
- Class 25
- SR
- Class 30
- As Cast
- Annealed
- Class 35
- Flame/Induction Hardened
- Class 40
- Class 45
- Class 50
- Class 55
- N&T
- Class 60
Compare all 14 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
ASTM A48 Class 20 | Lowest-strength, highest-graphite grade — best damping, thermal conductivity and machinability; counterweights, low-stress covers, stove parts. | 110 | 152 | 0.6 | 156 HB | 1.6 % IACS |
ASTM A48 Class 25 | Light-duty general castings where a small strength margin over Class 20 is wanted. | 130 | 179 | 0.5 | 174 HB | 1.7 % IACS |
Stress relieved | Standard for machine tool beds, surface plates and precision housings — removes casting residual stress with almost no loss of strength. | 160 | 215 | 0.5 | 180 HB | 1.7 % IACS |
ASTM A48 Class 30 | The workhorse grade — machine bases, gearboxes, pump and valve bodies, engine blocks. | 160 | 214 | 0.5 | 187 HB | 1.7 % IACS |
As CastDefault as-cast (equivalent to ASTM A48 Class 30 / GB HT200) | Standard stocked condition — general machine castings, housings, pump bodies; no heat treatment beyond shakeout. | 165 | 220 | 0.5 | 187 HB | 1.7 % IACS |
ferritizing/full anneal (700–760 °C, slow cool) | Pick when maximum machinability and dimensional stability matter more than strength (e.g. hard-edged chilled areas, complex machined bodies). | 180 | 0.8 | 150 HB | 50 W/m·K | |
ASTM A48 Class 35 | Medium-duty pressure and wear castings — hydraulic bodies, cylinder liners, flywheels. | 185 | 252 | 0.5 | 199 HB | 1.8 % IACS |
Flame or induction surface hardened (+ low temper) | Choose for slideways, gears, cam surfaces and cylinder bores needing a hard wear skin on a machinable core. | 190 | 250 | 0.3 | 52 HRC | 1.6 % IACS |
ASTM A48 Class 40 | Higher-strength pearlitic grade for heavily loaded frames, presses, gear housings and brake components. | 215 | 293 | 0.4 | 17 HRC | 1.9 % IACS |
ASTM A48 Class 45 | Intermediate high-strength grade, usually alloyed with Cr/Cu/Mo for heavy sections. | 240 | 330 | 0.4 | 21 HRC | 2 % IACS |
ASTM A48 Class 50 | Alloyed, fine-flake pearlitic iron for high-load, wear-critical parts; harder to machine and less damping. | 265 | 362 | 0.3 | 26 HRC | 2 % IACS |
ASTM A48 Class 55 | Near-maximum-strength gray iron; specified only where flake-graphite damping is still required at high stress. | 290 | 400 | 0.3 | 30 HRC | 1.6 % IACS |
Normalized and tempered | Use to raise strength, hardness and wear resistance of a pearlitic gray iron and to homogenise thick/thin section properties. | 300 | 0.4 | 22 HRC | 42 W/m·K | |
ASTM A48 Class 60 | Highest ASTM A48 class — very fine type-A flakes in an alloyed pearlite matrix; brittle, difficult to machine, section-sensitive. | 310 | 431 | 0.3 | 32 HRC | 1.6 % IACS |
Supply forms, finishes and certifications
Ways Gray Iron is supplied — form, process route, surface or paperwork — without a datasheet of their own. The property values above apply.
| Option | What it is | Type |
|---|---|---|
Continuous Cast Bar | Continuous cast bar stock | Supply form |
What is Gray Iron?
Gray Iron is a flake-graphite iron–carbon–silicon casting alloy supplied to ASTM A48 Class 20–60. The graphite flakes give high vibration damping, 46 W/m·K conductivity, easy machining and good casting fluidity at low cost — hence its use for machine tool bases, engine blocks, pump housings, pipe fittings and brake discs to about 400 °C. The same flakes are stress raisers: elongation is near 0.5%, tensile strength 220 MPa at 187 HB in the default class and far below compressive strength, and impact and tensile fatigue capacity are limited. Higher classes have finer graphite and higher hardness; use ductile iron where ductility is required. Gray Iron has a density of 7.15 g/cm³ (0.258 lb/in³), lighter than 99% of steel grades. It melts at 1,150°C (2,102 °F). Gray Iron has a yield strength of 165 MPa (23.9 ksi) and a tensile strength of 220 MPa (31.9 ksi) — weaker than 99% of steel grades. Elongation at break is 0.5% and the elastic modulus is 103 GPa (14.9 Msi).
What is Gray Iron used for?
- Machine tool bases and columns
- Engine cylinder blocks and heads
- Pump and valve housings
- Brake discs and drums
- Pipe and pressure fittings
- Gearbox and compressor housings
- Manhole covers
- Machine counterweights
Advantages
- Low embodied carbon — 1.5 kg CO₂/kg, better than 100% of steel grades
- Easy to machine — 85/100, better than 98% of steel grades
Limitations
- Low stiffness — 103 GPa (14.9 Msi), worse than 100% of steel grades
- Limited ductility — 0.5%, worse than 100% of steel grades
- Low fracture toughness — 15 MPa·√m (13.7 ksi·√in), worse than 100% of steel grades
- Low electrical conductivity — 1.7 % IACS, worse than 100% of steel grades
- Low strength — 165 MPa (23.9 ksi), worse than 99% of steel grades
Gray Iron properties
Typical room-temperature values for Gray Iron-As Cast; bars are the position among steel grades.
Physical3
Gray Iron has a density of 7.15 g/cm³ (0.258 lb/in³), lighter than 99% of steel grades. It melts at 1,150°C (2,102 °F).
Mechanical16
Gray Iron has a yield strength of 165 MPa (23.9 ksi) and a tensile strength of 220 MPa (31.9 ksi) — weaker than 99% of steel grades. Elongation at break is 0.5% and the elastic modulus is 103 GPa (14.9 Msi).
Thermal6
Gray Iron is rated for continuous service to 400°C (752 °F). It conducts heat at 46 W/m·K (26.6 BTU/hr·ft·°F), better than 71% of steel grades. Thermal expansion is 11.5 µm/m·K (6.39 µin/in·°F).
Electrical3
Gray Iron conducts electricity at 1.7 % IACS, worse than 100% of steel grades.
Chemical & Environmental3
Corrosion resistance is poor (18/100), better than 83% of steel grades.
Sustainability4
Producing a kilogram of Gray Iron takes about 18 MJ of energy and emits 1.5 kg of CO₂ — less than 100% of steel grades. Typical recycled content is 90%.
Manufacturability8
Gray Iron's machinability is excellent (85/100, better than 98% of steel grades); weldability poor (25/100); formability not recommended (3/100).
Common Calculations5
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the steel family
Yield strength against density for every steel grade FabDigit runs. Gray Iron is highlighted; hover a dot for its name, or open the interactive family chart.
Working with Gray Iron
Is Gray Iron easy to machine?
Excellent dry machining with carbide or ceramic tools — graphite breaks chips and lubricates; watch abrasive casting skin, chilled edges and free graphite dust.
Can Gray Iron be welded?
Poor fusion weldability: repair only, with 200–350 °C preheat, NiFe/Ni electrodes, short stringer beads and peening, or cold braze-welding to avoid a hard martensitic HAZ.
Can Gray Iron be formed, bent or molded?
Formed only by casting — outstanding fluidity, low solidification shrinkage and near-net section fidelity in green sand, shell or permanent-mould; no cold forming possible.
Manufacturability
The process limits FabDigit applies to Gray Iron. Every Gray Iron part is priced by an engineer; design inside these and the quote comes back without a redesign loop.
CNC machining
| Machinability | Excellent85 / 100 |
|---|---|
| ISO material group | P.2 |
| Cutting speed (rough / finish) | 180 / 210 m/min |
| Minimum wall | 0.8 mm |
| Minimum internal radius | 1 mm |
| Drill depth limit | 5× diameter |
| Best surface (machined / polished) | Ra 1.6 / — |
| Hardenable after machining | No |
Gray Iron chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| Ctotal carbon; mostly flake graphite | 2.8 – 3.6 | 3.25 |
| Sigraphitizer; lowered in higher-strength classes | 1.6 – 2.6 | 2.1 |
| Mn | 0.4 – 1 | 0.7 |
| Psteadite; higher in some plumbing irons | ≤ 0.15 | 0.08 |
| S | ≤ 0.15 | 0.09 |
| Croptional, pearlite stabiliser in Class 40–60 | ≤ 0.4 | 0.1 |
| Cuoptional strengthener | ≤ 1 | 0.2 |
| Mooptional, heavy-section high-strength irons | ≤ 0.6 | 0 |
| Fe | balance |
What surface finishes work on Gray Iron?
Shot-blast then prime/paint, phosphate, enamel or electroless nickel; graphite flakes limit mirror polish, and porosity may need impregnation before plating.
Surface treatments FabDigit runs on steel parts — pick them in the quote configurator or call them out on the drawing. Each links to its datasheet in the surface-finish library (thickness, specs, call-out, material verdicts).
Plating
CVD Coating
Black / Phosphate
Thermal Spray
Equivalent designations
Standards and trade names that resolve to Steel Gray Iron in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- Trade names
- Grey IronGray Cast IronGrey Cast IronCast Iron
How much does Gray Iron cost?
Gray Iron is a standard-cost steel ($$), cheaper than about 81% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Gray Iron — frequently asked
What is Gray Iron used for?
Gray Iron is typically used for machine tool bases and columns, engine cylinder blocks and heads, pump and valve housings, brake discs and drums, pipe and pressure fittings and gearbox and compressor housings. In short: best vibration dampening.
What is the yield strength of Gray Iron?
Gray Iron has a typical yield strength of 165 MPa (23.9 ksi) and a tensile strength of 220 MPa (31.9 ksi) — weaker than 99% of steel grades. Strength varies by condition: see the 14 listed tempers.
Is Gray Iron easy to machine?
Yes — machinability is rated excellent (85/100, better than 98% of steel grades). Excellent dry machining with carbide or ceramic tools — graphite breaks chips and lubricates; watch abrasive casting skin, chilled edges and free graphite dust.
Can Gray Iron be welded?
Not readily — weldability is rated poor (25/100). Poor fusion weldability: repair only, with 200–350 °C preheat, NiFe/Ni electrodes, short stringer beads and peening, or cold braze-welding to avoid a hard martensitic HAZ.
What surface finishes work on Gray Iron?
Shot-blast then prime/paint, phosphate, enamel or electroless nickel; graphite flakes limit mirror polish, and porosity may need impregnation before plating.
Can Gray Iron be formed, bent or molded?
Formed only by casting — outstanding fluidity, low solidification shrinkage and near-net section fidelity in green sand, shell or permanent-mould; no cold forming possible.
What is the maximum service temperature of Gray Iron?
Gray Iron is rated for continuous use to about 400°C (752 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between Gray Iron As Cast and Gray Iron-SR?
As Cast is the default condition — standard stocked condition — general machine castings, housings, pump bodies; no heat treatment beyond shakeout. SR: standard for machine tool beds, surface plates and precision housings — removes casting residual stress with almost no loss of strength. Yield strength is 165 MPa in As Cast versus 160 MPa in SR.
How much does Gray Iron cost?
Gray Iron is a standard-cost steel ($$), cheaper than about 81% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Can FabDigit make parts in Gray Iron?
Yes — Gray Iron is available for CNC machining and casting & forging; every part is priced by an engineer rather than instantly. Upload a STEP file to get a price and a DFM check.
Sources
- ASTM A48/A48M — Standard Specification for Gray Iron Castings — ASTM International (2022)
- GB/T 9439 (2023)
- EN 1561 — Founding: Grey cast irons — CEN (2011)
- ASM Specialty Handbook: Cast Irons — ASM International (1996)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels and High-Performance Alloys — ASM International (1990)
- Continuous cast gray iron bar (G2/G1) property data — Charter/Dura-Bar (2021)
Property data is compiled from published supplier and standards handbooks and normalised for comparison; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.