ASTM A48 sorts gray iron by one number: the class equals the minimum tensile strength in ksi, so Class 20 = 20,000 psi (138 MPa) and Class 40 = 40,000 psi (276 MPa) on a separately cast test bar [S2][S4]. That 2:1 jump in tensile floor is achieved by changing the matrix from ferrite to pearlite and tightening the carbon equivalent [S1][S5].
The standard lists ten classes (20, 25, 30, 35, 40, 45, 50, 55, 60), each with A/B/C/S test-bar size variants, but the chemistry ranges themselves are not specified by ASTM A48; the foundry controls strength through composition, cooling rate, and casting geometry [S2][S5]. For engineers who want a side-by-side of gray iron against other structural materials, the cast iron reference page gives a wider alloy context.
What the Class Number Actually Means
ASTM A48 classifies gray iron by the minimum tensile strength of a separately cast test bar, expressed in ksi, so the class number is the strength floor, not a typical or nominal value [S2][S6]. Class 20 sets a 20,000 psi (138 MPa) minimum, Class 40 sets 40,000 psi (276 MPa) on a B-bar, and tensile specimens cut from continuously cast bars must also reach 40,000 psi to meet Class 40b [S6].
Each class is offered in A, B, C, and S test-bar geometries; the letter picks the bar diameter (A = 0.88 in, B = 1.20 in, C = 2.00 in) and the foundry selects whichever bar matches the casting section thickness being qualified [S2]. Foundries therefore write "Class 40B" on a cert to flag both the strength target and the bar size that proved it.
Mechanical Properties: Class 20 vs Class 40 at a Glance
Class 20 typical UTS sits at 20,000 psi (138 MPa) with a Brinell hardness near 160, a compressive strength of roughly 83 ksi (572 MPa), and density of 0.258 lb/in³ (7.15 g/cm³) [S1]. Class 40 typical UTS rises to 40,000 psi (276 MPa), Brinell hardness ranges 183 to 285, compressive strength climbs to about 150 ksi (1,034 MPa), with the same 0.258 lb/in³ density and identical 27 Btu/hr·ft·°F (46 W/m·K) thermal conductivity [S5].
Yield strength also widens: Class 20 yield is roughly 14.2 ksi (98 MPa) versus a substantially higher yield for Class 40 driven by the pearlitic matrix, while elongation for both classes stays in the brittle gray-iron regime typical of flake-graphite material [S3]. A foundry test bar for Class 20 typically shows 3-4 times higher compressive than tensile strength, a ratio that compresses (not literally) once pearlite dominates in Class 40 because both numbers rise together [S1].
Chemistry and Microstructure: Why the Strength Gap Exists
Class 20 chemistry typically lands at 3.25 to 3.50% C, 0.5 to 0.9% Mn, 1.8 to 2.3% Si, with P and S capped at 0.12% each; the high carbon equivalent drives a ferrite-rich matrix with coarse Type A graphite flakes [S1]. Class 40 tightens C to 2.7 to 3.5%, raises Mn to 0.5 to 0.95% and Si to 1.8 to 2.5%, which drops the carbon equivalent and stabilizes a pearlitic matrix with finer graphite [S5].
That pearlite-versus-ferrite swap is the mechanism: ferrite is soft and ductile, pearlite is a lamellar ferrite-cementite structure that resists dislocation motion, so the same graphite-flake population delivers nearly twice the tensile load-bearing capacity [S4][S5]. The trade-off is real, however: Class 40 sacrifices some vibration damping compared with Class 20 or 30, and it gains a higher modulus of elasticity in the process [S5].
Decision Matrix: When to Pick Each Class
Use Class 20 when the part sees static load, needs maximum damping, or has to be machined aggressively at minimum cost: pump bodies, valve bodies, handwheels, covers, frames, pulleys, and pipe fittings are textbook fits [S1]. The 160 BHB cap also makes Class 20 the most machinable gray iron on the chart, which matters when a casting is covered in deep pockets or tapped holes [S3].
Use Class 40 when wear, heat-treatment response, or higher cyclic loading enters the picture: brake drums and rotors, clutch plates, cylinder liners, gears, hydraulic spools and manifolds, continuous-caster rolls, and pump liners all lean on the 183 to 285 BHB hardness band and 40,000 psi tensile floor [S4][S5]. If your flow meter body, industrial valve body, or pressure transmitter housing sees pulsation or abrasive service, Class 40 is the safer call even at higher per-pound cost.
For a broader alloy comparison on similar load paths, the A356-T6 squeeze casting tensile and elongation reference gives a useful aluminum-side benchmark, while 42CrMo4 vs 34CrNiMo6 shaft and gear spec covers the through-hardened steel alternative for the same gears Class 40 often replaces.
Casting and Machining Behavior
Class 20 flows into thin sections and complex geometries because of its high carbon equivalent and low chilling tendency, which is why protective covers, frames, and pump housings are routinely cast in this grade [S1]. Class 40 can be cast to similar shapes but demands closer inoculation and cooling control to keep the pearlite fraction up; expect a foundry to qualify the heat with a chilled-wedge or Brinell check before signing the MTC [S5].
Machinability index is a hidden cost: Class 20 at ~160 BHB cuts cleanly with carbide tooling and short chip break, while Class 40 at 200+ BHB needs slower surface feet, coated inserts, and rigid setups [S3][S5]. Buyers who flip a Class 20 print to Class 40 purely for "more strength" without re-quoting machining hours routinely overrun their cost plan.
Cross-Standard Equivalents Buyers Should Know
Class 20 lines up with ISO 185/JL/150, SAE J431, and EN-GJL-150; Class 40 lines up with ISO 185/JL/275, SAE J431 G4000, and EN-GJL-250 or EN-GJL-300 [S1][S5]. On a global RFQ, the EN-GJL or ISO 185 designation is often what gets stamped on the European foundry's paperwork, so matching the class number across standards prevents accidental under-spec on import jobs.
Mechanical and tribological context outside ASTM A48 is covered on the pressure sensor and tensile testing machine reference pages, which explain why foundries still cut separate B-bar coupons rather than testing the casting itself.
Limitations and Common Pitfalls
The "minimum" wording is the most-misread clause: a Class 40 test bar at exactly 40,000 psi passes, but the same heat poured into a 4 in thick section can come in at 32,000 psi because cooling rate drives strength as much as chemistry does [S5]. Section size, pouring temperature, and inoculant choice all shift the result, which is why the A48 standard requires the strength to be re-qualified on the bar size (A/B/C/S) that matches the heaviest section of the casting [S2].
Gray iron also stays brittle at every class, with fracture following the graphite flakes, so impact-loaded or fatigue-critical parts (hammer mills, crusher jaws, high-cycle pump impellers) belong in ductile iron or steel regardless of class number [S1][S4]. Finally, do not request "Class 40" without a hardness range, Brinell target, or microstructure callout if your application actually needs the wear resistance, because two foundries can both hit 40,000 psi tensile with very different pearlite fractions and very different field life.