Ductile iron is specified by numbers, and the numbers are not arbitrary. 65-45-12 reads as three properties the casting has to meet: 65,000 psi tensile strength, 45,000 psi yield strength, and 12 percent elongation before it breaks. Every ductile iron grade under ASTM A536 is written the same way. Here is what buyers see most, and what each grade asks of the foundry.
The four common grades
- 60-40-18. The most ductile of the standard grades. A ferritic matrix, high elongation, the best impact resistance at low temperature. Goes into pressure parts, waterworks hardware, and anything that must not fracture.
- 65-45-12. The general-purpose grade. Mostly ferritic with some pearlite; a balance of strength, ductility, and machinability. The default for housings, brackets, hubs, and most machine parts unless the print says otherwise.
- 80-55-06. A pearlitic matrix for strength and wear resistance, at the cost of elongation. Gears, crankshafts, heavy-duty brackets, and parts that see abrasion.
- 100-70-03. Fully pearlitic or heat-treated; the strongest of the as-cast grades. Reserved for high-stress components where a forging was the alternative.
Beyond those sit austempered ductile iron grades, heat-treated after casting to strengths that compete with forged steel, and poured by comparatively few foundries.
What the grade asks of the foundry
Grade is achieved through chemistry, the magnesium treatment that nodularises the graphite, inoculation, and cooling rate. A ferritic grade wants slower cooling and lower alloy; a pearlitic grade wants copper or tin additions and faster cooling. Section thickness fights all of this: a heavy section cools slowly regardless of grade, and a thin one fast. A foundry that pours 65-45-12 well in a one-inch wall may not make grade in a four-inch one without adjusting its practice.
That is why test bars are poured with the job, why material certifications report actual results rather than the target, and why GW validates first articles before a production release — for every grade, at every foundry in the network.
Choosing the grade
Yours to choose; ours to meet. If you are moving a part to ductile iron for the first time and want to talk through the load case before the print is issued, the conversation is free and it is worth having before the pattern is cut.