Cast iron grades listed for gray and ductile iron generally cover 125–360 HB, while austempered ductile iron extends to 555 HB [S1][S2].
Across ASTM A48 gray iron, ASTM A536 ductile iron, and ASTM A897 austempered ductile iron, Brinell hardness is a grade-specific property rather than a universal acceptance criterion [S1][S2].
Gray Iron Hardness by Tensile Grade
ASTM A48 grades are classified by minimum tensile strength, and the associated hardness rises with the grade. The commonly published ranges are 125–205 HB for Class 150, 150–230 HB for Class 200, 180–250 HB for Class 250, 200–275 HB for Class 300, and 220–290 HB for Class 350 [S1].
These grades are not interchangeable by hardness alone. Class 150 carries a 150 MPa minimum tensile strength, while Class 350 carries a 350 MPa minimum; typical uses include pipes and covers at the lower end, and gears, bearings, cams, and precision machine components at the upper end [S1][S3].
SAE J431 grades use a tensile-strength-to-hardness relationship. Published ranges include 170–229 HB for G2500, 187–241 HB for G3000, 207–255 HB for G3500, and 217–269 HB for G4000, with corresponding minimum tensile strengths of 25,000, 30,000, 35,000, and 40,000 psi [S3].
One independent ASTM A48 comparison lists Class 30 at 187–241 HB, Class 35 at 207–255 HB, and Class 40 at 217–269 HB, illustrating the same upward progression under a different class-numbering sequence [S2].
Ductile Iron Hardness by ASTM Grade
Ductile iron grade designations encode three mechanical requirements: tensile strength, yield strength, and elongation. ASTM A536 Grade 60-40-18, for example, specifies 60,000 psi tensile strength, 40,000 psi yield strength, and 18% elongation over a 2 in or 50 mm gauge [S5].
The typical hardness range for Grade 60-40-18 is 170 HB maximum, while published ranges for stronger grades include 156–217 HB for 65-45-12, 187–255 HB for 80-55-06, and 241–302 HB for 100-70-03 [S2].
ISO 1083 and ASTM A536 cross-reference data provide another comparison. Grade 400-18 and 400-15 are associated with 130–180 HB, Grade 450-10 with 160–210 HB, Grade 500-7 with 170–230 HB, Grade 600-3 with 190–270 HB, and Grade 700-2 with 225–305 HB [S1].
The ductile iron designation is therefore not enough to select a hardness value. Section size, chemistry, cooling rate, and heat treatment can shift measured hardness, so a purchase specification should identify the exact grade and the required test location rather than rely on a generic material name [S1][S5].
Austempered Ductile Iron Extends the Range
Austempered ductile iron reaches 269–555 HB across ASTM A897 grades, materially exceeding the typical 125–360 HB envelope cited for common gray and ductile iron [S1][S2]. Grade 850/550/10 covers 269–321 HB, while 1050/700/7 covers 302–363 HB [S2][S5].
The higher range is associated with increasing strength requirements: Grade 1200/850/4 covers 341–444 HB, Grade 1400/1100/1 covers 388–477 HB, and Grade 1600/1300 covers 444–555 HB. Their minimum tensile strengths progress from 850 MPa to 1,600 MPa, while minimum elongation falls from 10% to 4%, 1%, and then no listed elongation requirement [S5].
This comparison makes the engineering trade-off explicit: hardness and tensile strength rise, but ductility falls. Grade 850/550/10 combines 269–321 HB with 10% minimum elongation, whereas Grade 1400/1100/1 combines 388–477 HB with 1% minimum elongation [S5].
Austempered ductile iron is therefore suited to demanding wear and load applications, but its higher hardness should not be treated as a general substitute for conventional ductile iron where impact resistance or elongation governs the design [S2][S5].
Gray, Ductile, and Austempered Iron Compared
Gray iron generally provides moderate hardness with high castability and damping capacity, while ductile iron trades some casting simplicity for substantially higher tensile and yield strength. Austempered ductile iron extends the wear-resistant end of the range through controlled heat treatment [S2][S4].
The comparison also shows why grade codes cannot be converted directly into one universal hardness band. The same ASTM A48 class can appear with different published ranges, and the same ASTM A536 grade can be affected by section size and heat-treatment requirements [S1][S2].
Where Brinell Limits Belong in a Specification
Brinell hardness is a practical inspection requirement when abrasion resistance or local hardness is important, but it is not automatically a required inspection item in every international cast iron specification [S1]. A purchase order can add hardness requirements so the foundry controls chemistry and processing for the intended service condition [S1].
For ductile iron, the contract may also specify heat treatment, chemical composition, and microstructure alongside hardness [S5]. Ferrite ductile iron for high-pressure valves, flanges, fittings, and pumps is listed at 143–187 HB using a 3,000 kgf force and 10 mm ball, with heat-treatment requirements included in the purchase order [S5].
For austempered ductile iron, the published Brinell values are described as typical hardness ranges, not a substitute for the grade's tensile, yield, and elongation requirements [S5]. This distinction matters because a sample can satisfy a hardness band while failing the mechanical-property requirements.
Engineers should also verify whether the governing designation is ASTM A48, ASTM A536, ASTM A897, ISO 1083, ISO 185, or SAE J431. These systems organize grades differently, so mixing their grade numbers into one hardness chart can produce an incorrect procurement specification [S1][S3].
Metallurgy, Sectioning, and Test Control
Cast iron normally contains 2–4% carbon and 1–3% silicon, with manganese and impurities such as sulfur and phosphorus also present. Its heterogeneous solidification and section-dependent cooling behavior make hardness a property of the casting as produced, not merely of its nominal grade [S4].
Ductile iron contains globular graphite, and austempered heat treatment narrows property variation between different parts of the same casting or from the same furnace hot metal [S5]. By contrast, gray iron uses flake graphite, and its strength and hardness relationship differs substantially from ductile iron [S3][S5].
A controlled comparison therefore requires the same test force and indenter designation. The ferrite ductile iron value of 143–187 HB is specified with a 3,000 kgf force and 10 mm ball, while some higher-hardness tables use 3,000 kgf notation without providing equivalent detail for every grade [S5].
Inspection planning should state the grade, hardness range, test force, indenter diameter, sampling plan, and inspected locations. For abrasion-resistant components, hardness should be tied to the working surface and service wear mechanism rather than selected from a general grade table [S1].
Specification Decision by Service Requirement
Choose ASTM A48 gray iron when castability, damping behavior, and moderate strength are dominant, then select the class from the calculated load rather than from hardness alone. Published examples include Class 30 for pump bodies, gearboxes, and housings, and Class 40 for diesel engine castings, liners, and heavy components [S2].
Choose ASTM A536 ductile iron when higher tensile and yield strength, fatigue resistance, or pressure-containing integrity is required. Grade 60-40-18 is associated with valve and pump bodies, 65-45-12 with shock and fatigue-loaded machinery, and 80-55-06 with crankshafts, gears, and rollers [S2].
Choose austempered ductile iron when wear resistance and high strength justify the reduction in elongation. Grade 125-80-10 covers 269–321 HB, Grade 150-100-07 covers 302–363 HB, Grade 175-125-07 covers 341–444 HB, and Grade 200-155-01 covers 388–477 HB [S2].
Do not use hardness as the sole acceptance criterion for fatigue-critical, pressure-retaining, or impact-loaded components. Combine the grade-specific tensile, yield, elongation, heat-treatment, and microstructure requirements with the Brinell range [S5].
For a new purchase, the traceable specification package should identify the exact iron grade and applicable designation, followed by the agreed Brinell range, test method, sampling locations, and heat-treatment condition.