Steel Gray Iron · grade
Steel Gray Iron Class 20
Properties of the Class 20 condition, compared with the other Gray Iron tempers.
Lowest-strength, highest-graphite grade — best damping, thermal conductivity and machinability; counterweights, low-stress covers, stove parts.
What is Gray Iron Class 20?
Gray Iron Class 20 is Gray Iron in the Class 20 grade — lowest-strength, highest-graphite grade — best damping, thermal conductivity and machinability; counterweights, low-stress covers, stove parts. Gray Iron Class 20 has a yield strength of 165 MPa (23.9 ksi) and a tensile strength of 152 MPa (22 ksi) — weaker than 99% of steel grades. Elongation at break is 0.6% and the elastic modulus is 80 GPa (11.6 Msi). Gray Iron Class 20 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 20 is available on request or by drawing note.
Advantages
- Low embodied carbon — 1.5 kg CO₂/kg, better than 100% of steel grades
- Conducts heat well — 55 W/m·K (31.8 BTU/hr·ft·°F), better than 99% of steel grades
- Easy to machine — 85/100, better than 98% of steel grades
Limitations
- Low stiffness — 80 GPa (11.6 Msi), worse than 100% of steel grades
- Limited ductility — 0.6%, 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
- Limited formability — 3/100, worse than 100% of steel grades
Gray Iron Class 20 properties
Typical room-temperature values for Gray Iron Class 20 — 9 properties are specific to this condition; the rest are grade-level values shared by every Gray Iron temper. Each bar shows where the value sits among the steel grades in FabDigit's library — further right is higher.
Physical3
Gray Iron Class 20 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).
Mechanical15
Gray Iron Class 20 has a yield strength of 165 MPa (23.9 ksi) and a tensile strength of 152 MPa (22 ksi) — weaker than 99% of steel grades. Elongation at break is 0.6% and the elastic modulus is 80 GPa (11.6 Msi).
Thermal6
Gray Iron Class 20 is rated for continuous service to 400°C (752 °F). It conducts heat at 55 W/m·K (31.8 BTU/hr·ft·°F), better than 99% of steel grades. Thermal expansion is 11.5 µm/m·K (6.39 µin/in·°F).
Electrical3
Gray Iron Class 20 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 Class 20 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 20's machinability is excellent (85/100, better than 98% of steel grades); weldability poor (25/100); formability not recommended (3/100).
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Other Gray Iron tempers and conditions
Gray Iron is also supplied in 14 other conditions. The full side-by-side table is on the Gray Iron overview.
- As CastDefaultStandard stocked condition — general machine castings, housings, pump bodies; no heat treatment beyond shakeout.220 MPa tensile
- Class 60Highest ASTM A48 class — very fine type-A flakes in an alloyed pearlite matrix; brittle, difficult to machine, section-sensitive.431 MPa tensile
- Class 50Alloyed, fine-flake pearlitic iron for high-load, wear-critical parts; harder to machine and less damping.362 MPa tensile
- N&TUse to raise strength, hardness and wear resistance of a pearlitic gray iron and to homogenise thick/thin section properties.300 MPa tensile
- Class 40Higher-strength pearlitic grade for heavily loaded frames, presses, gear housings and brake components.293 MPa tensile
- Class 35Medium-duty pressure and wear castings — hydraulic bodies, cylinder liners, flywheels.252 MPa tensile
- Class 30The workhorse grade — machine bases, gearboxes, pump and valve bodies, engine blocks.214 MPa tensile
- AnnealedPick when maximum machinability and dimensional stability matter more than strength (e.g. hard-edged chilled areas, complex machined bodies).180 MPa tensile
- Class 25Light-duty general castings where a small strength margin over Class 20 is wanted.179 MPa tensile
- Class 20Lowest-strength, highest-graphite grade — best damping, thermal conductivity and machinability; counterweights, low-stress covers, stove parts.152 MPa tensile
Working with Gray Iron Class 20
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 20 — frequently asked
What is Gray Iron Class 20 used for?
Gray Iron Class 20 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 20?
Gray Iron Class 20 has a typical yield strength of 165 MPa (23.9 ksi) and a tensile strength of 152 MPa (22 ksi) — weaker than 99% of steel grades. Strength varies by condition: see the 15 listed tempers.
Is Gray Iron Class 20 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 Class 20 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 Class 20?
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 20 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 20?
Gray Iron Class 20 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.
Can FabDigit make parts in Gray Iron Class 20?
Yes — Gray Iron Class 20 is available for CNC machining and casting & forging with instant online pricing. 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.