By Dave Clinton,
President of Darton Sleeves
Basically there is no standard chemistry for ductile iron, instead there is an ASTM 536-84 SAE developed standard for "mechanical properties." This means that any foundry can, and does, cast material with various concoctions of chemistry to equal a particular mechanical property. Mechanical properties describe the basic strength and resilience of a material in engineering terms such as tensile, yield, elongation, and hardness. From an engineering standpoint, these callouts basically determine how a material will work in a given environment or operational circumstance. For instance, gun barrels are made of high-strength steel or stainless steel and usually contain high levels of chromium to combat the heat and prevent wear.
Cast iron typically has tensile strength in the area of 30,000 psi, hardness of 210BHN(Brinell), and no elongation to speak of. Ductile iron typically will be 100,000 psi tensile, 70,000 yield, and have three percent elongation. Ductile, or nodular iron was invented by GM in the '40s primarily for suspension parts. The use of ductile has grown exponentially, and in the '60s foundries began centrifugal casting of ductile for round parts such as cylinder liners.
Darton began intensive research into the chemistry of ductile in the late '80s when we were selected to manufacture cylinders for a certified aircraft engine by Orenda Aerospace. This challenge involved making a sleeve that would withstand 75-percent horsepower for 1,500 hours on an engine test stand on a continuous basis. In the end, the engines were disassembled and all the parts were expected to still have service life. The testing lasted for 18 months and 20 engines were involved. The engines were certified by the FAA and the Canadian Department of Transport.
The chemistry aspect of ductile is akin to "mom's" apple pie; it is not just the ingredients but when and how the ingredients are added and mixed in. In the case of Darton ductile DDI 2007A material, our process has resulted in the strongest, and most dense ductile iron ever produced without induction hardening. Typically, ductile iron density under a microscope at 100x will reveal 175-200 nodules per sq millimeter. Darton DDI 2007A is 500-600, which represents a superior wear surface. For instance, the majority of Top Fuel Teams use Darton sleeves and, we're now providing a sleeve, which routinely will last 5-7 runs-an unheard of prospect just two years ago.