Steel Gray Iron Class 55
Grade
also Grey Iron, Gray Cast Iron
Class 55 = aSTM A48 Class 55
Near-maximum-strength gray iron; specified only where flake-graphite damping is still required at high stress.
Values for the Class 55 condition
Other Gray Iron tempers and conditions
Gray Iron is listed in 14 conditions; see the grade overview at Gray Iron.
- 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 |
Class 55This page 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 |
What is Gray Iron Class 55?
Gray Iron Class 55 is Gray Iron in the Class 55 grade — near-maximum-strength gray iron; specified only where flake-graphite damping is still required at high stress. Gray Iron Class 55 has a yield strength of 290 MPa (42.1 ksi) and a tensile strength of 400 MPa (58 ksi) — weaker than 88% of steel grades. Elongation at break is 0.3% and the elastic modulus is 145 GPa (21 Msi). Gray Iron Class 55 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). As Cast is the default condition FabDigit quotes; Class 55 is available on request or by drawing note.
Advantages
- Low embodied carbon — 1.5 kg CO₂/kg, better than 100% of steel grades
Limitations
- Limited ductility — 0.3%, worse than 100% of steel grades
- Low electrical conductivity — 1.6 % IACS, worse than 100% of steel grades
- Limited formability — 2/100, worse than 100% of steel grades
- Low fracture toughness — 18 MPa·√m (16.4 ksi·√in), worse than 99% of steel grades
- Hard to polish — 35/100, worse than 99% of steel grades
Gray Iron Class 55 properties
Typical room-temperature values for Gray Iron Class 55; 22 are specific to this condition. Other listed values are Gray Iron reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
Gray Iron Class 55 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 Class 55 has a yield strength of 290 MPa (42.1 ksi) and a tensile strength of 400 MPa (58 ksi) — weaker than 88% of steel grades. Elongation at break is 0.3% and the elastic modulus is 145 GPa (21 Msi).
Thermal6
Gray Iron Class 55 is rated for continuous service to 425°C (797 °F). It conducts heat at 37 W/m·K (21.4 BTU/hr·ft·°F), worse than 71% of steel grades. Thermal expansion is 11.5 µm/m·K (6.39 µin/in·°F).
Electrical3
Gray Iron Class 55 conducts electricity at 1.6 % IACS, worse than 100% of steel grades.
Chemical & Environmental3
Corrosion resistance is poor (20/100), better than 87% of steel grades.
Sustainability4
Producing a kilogram of Gray Iron Class 55 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 Class 55's machinability is fair (50/100, worse than 53% of steel grades); weldability poor (15/100); formability not recommended (2/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with Gray Iron Class 55
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.
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.
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 | — |
Gray Iron Class 55 — frequently asked
What is Gray Iron Class 55 used for?
Gray Iron Class 55 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 Class 55?
Gray Iron Class 55 has a typical yield strength of 290 MPa (42.1 ksi) and a tensile strength of 400 MPa (58 ksi) — weaker than 88% of steel grades. Strength varies by condition: see the 14 listed tempers.
Is Gray Iron Class 55 easy to machine?
Reasonably — machinability is rated fair (50/100, worse than 53% 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 Class 55 be welded?
Not readily — weldability is rated poor (15/100). 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 Class 55?
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 Class 55 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 Class 55?
Gray Iron Class 55 is rated for continuous use to about 425°C (797 °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.
Can FabDigit make parts in Gray Iron Class 55?
Yes — Gray Iron Class 55 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. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.