ASTM A536 organises ductile iron (spheroidal graphite iron) into five standard grades, each labelled with a three-number triplet: minimum tensile strength in ksi, minimum yield strength in ksi, and minimum elongation in percent [S1][S3][S5]. The most common triplets encountered in castings orders are 60-40-18, 65-45-12, 80-55-06, 100-70-03 and 120-90-02 [S3][S6][S9].
A536 is a property-based specification, not a chemistry-based one; the standard deliberately subordinates chemical composition to the mechanical properties in Table 1 [S3][S4]. That is why two foundries can ship the same grade on different melt chemistries and both pass, and why a buyer specifies the triplet, not a recipe. The current ASTM designation for the standard is A536-84(2019)e1, reapproved in 2019 with an editorial change [S1].
How the three numbers decode
The first number is the minimum tensile strength in ksi (1000 psi units), the second is the minimum 0.2% offset yield strength in ksi, and the third is the minimum elongation over the standard gauge length per Test Methods E8 [S3][S5]. So 80-55-06 means 80 ksi minimum tensile, 55 ksi minimum yield, and 6% minimum elongation; 120-90-02 means 120 ksi / 90 ksi / 2% [S3][S6].
The trade-off is mechanical, not metallurgical magic: higher strength grades carry less ductility because the matrix shifts from ferrite toward pearlite and toward tempered martensite at the top end [S4][S5]. Penticton Foundry's grade notes capture the gradient directly, with 60-40-18 marketed for maximum ductility, 120-90-02 for very high strength and wear resistance, and 80-55-06 for moderate ductility plus impact resistance [S2].
The five standard grades at a glance
ASTM A536 Table 1 lists the five general-use grades with the following minimum properties (ksi for strength, % for elongation) [S3][S6]:
60-40-18: 60 ksi tensile, 40 ksi yield, 18% elongation. Fully ferritic, normally requires a full ferritising anneal per Section 5 of A536 [S3]. It is the grade picked for low-temperature toughness and maximum ductility, and is the conventional entry point for pressure-containing components in cast iron bodies [S2][S3].
65-45-12: 65 ksi / 45 ksi / 12%. Industry sources describe it as the workhorse grade when converting a mild steel fabrication to a ductile iron casting [S2].
80-55-06: 80 ksi / 55 ksi / 6%. Partially pearlitic; Penticton Foundry places its mechanicals as comparable to low-alloy steels [S8]. Common in gearboxes, pumps, brakes, and wind-energy castings where higher strength is needed without losing useful elongation [S2][S5].
100-70-03 (also called 100-70-02 by some stock-bar suppliers): 100 ksi / 70 ksi / 2-3% elongation. Pearlitic; the most commonly poured fully pearlitic grade in Penticton's book, achievable by alloying alone for most castings without mandatory quench and temper [S2][S4].
120-90-02: 120 ksi / 90 ksi / 2%. The strongest of the A536 set, generally requires quench-and-temper, normalise-and-temper, or an isothermal heat treatment per Section 5 of the standard [S3].
Matrix, heat treatment and the chemistry that sits behind the numbers
Grade is controlled by adjusting the ferrite-to-pearlite ratio around Type I and II nodular graphite (classified per ASTM A247), not by adding exotic alloys [S4][S3]. Magnesium treatment during pour converts the graphite from flake to spheroid, and that nodular shape is what gives A536 its combination of strength and elongation versus gray iron [S2][S5].
Typical ductile-iron chemistry sits around 3.50-3.90% C and roughly 2-3% Si for ferritic grades, with Mn, Mg and trace inoculants adjusted to hit the target matrix; Dura-Bar publishes these ranges for its 65-45-12 stock [S4]. Section 5 of A536 is explicit that 60-40-18 normally needs a full ferritising anneal, that 100-70-03 and 120-90-02 generally need a quench-and-temper or equivalent, and that the other two grades (65-45-12 and 80-55-06) can be met either as-cast or by heat treatment [S3]. A warning worth flagging on shop prints: ductile iron quenched to martensite and tempered can have substantially lower fatigue strength than as-cast material of the same hardness, so a 120-90-02 specified for a fatigue-loaded part deserves a fatigue review, not just a hardness check [S3].
How to pick a grade for a real part
For parts that are converted steel fabrications and need to machine easily with good chip break, start at 65-45-12; it is the lowest-strength grade that still gives steel-like static strength and the best machinability of the family [S2][S4]. For low-temperature or impact-loaded service, drop to 60-40-18; Penticton specifically calls out low-temperature toughness for that grade [S2].
For structural castings in pumps, gearboxes, valve bodies and brake components where higher strength without sacrificing too much ductility matters, 80-55-06 is the workhorse and pairs well with copper, nickel or chromium additions when corrosion resistance is also on the spec [S2][S5]. A directly comparable high-chromium white iron reference covers the alternative when abrasive wear, not toughness, drives the choice. For highly loaded gears, transmission housings and high-strength machine components where elongation can drop to 2-3%, step up to 100-70-03 [S5]. Reserve 120-90-02 for the highest-stress pearlitic/martensitic applications and confirm the heat-treat route with the foundry up front, since the as-cast route is rarely available for that grade [S3].
Ordering language and what A536 actually requires you to call out
Section 3 of A536 lists what belongs on a purchase order: the ASTM designation (A536), the grade (one of the five triplets in Table 1, or a Table 2 grade for special uses such as pipe and fittings), any special properties required, sample count if different from default, certification if required, and any special preparation for delivery [S3]. Mechanicals are verified on separately cast test coupons (1-in. Y-block, 1-in. keel block, or modified keel block), not on the casting itself, and A536 explicitly disclaims any precise quantitative relationship between coupon and casting properties [S3]. Continuous-cast bar stock suppliers such as Dura-Bar guarantee properties in the actual bar, which is a separate commercial model from the foundry-coupon route [S4].
For industrial valve bodies, pump housings and similar pressure-retaining castings, the most common triplet on drawings in North America remains 65-45-12 for general service and 60-40-18 for cold-temperature or impact-rated service, with 80-55-06 and 100-70-03 specified where the design stress justifies the loss of elongation [S2][S3][S9].
Track the 2026-09-21 publication window of A536-84(2019)e1 as your baseline; the next ASTM Committee A04 ballot cycle is the natural trigger to re-check whether Table 1 has been amended, and any drawing note that still references "A536 grade 60-45-10" or other off-spec triplets is a red flag worth correcting against Table 1 before it reaches the foundry [S1][S3].
Spec-level background on the components involved: pressure transmitter.