Blake Albritton, CTO of The Lawton Standard Co. and industrial engineer gives us a quick lesson in metallurgy. Follow along as he explains what ductile iron is and how it is made.
On behalf of the Lawton family, welcome to “Four-minute metallurgy,” where we will attempt to explain metallurgical or foundry engineering concepts in four minutes or less. I’m Blake Allbritton, episode one.
What is ductile?
Ductile first emerged in the 1940s as an advancement in iron casting technology that provided improved strength, fatigue resistance, impact resistance, and of course, ductility over previously available iron castings.
But, at the microstructural level, ductile iron is a solid solution of iron, carbon, and silicone in the form of ferrite, pearlite, and graphite nodules. What is ferrite? Ferrite simply is iron ferrite as solidified in ductile iron castings, is a soft, low-strength phase that appears in halos around graphite nodules in two dimensions or three dimensions and encapsulates the graphite nodules.
What about pearlite?
Pearlite is a mixture of ferrite and cementite (iron carbide) that forms during solidification in a process driven by a localized carbon concentration and diffusion as solidification occurs. To learn more about how and why pearlite forms, check out the other series, 40-minute metallurgy.
Pearlite is a hard, high-strength phase controllable by alloy addition that can respond to heat treatment. Graphite is carbon that is rejected from the solution and forced to solidify in the form of soft spheroidal nodules. So what is ductile? Relative to prior iron-casting alloys, ductal iron is capable of higher strength, greater elongation, greater impact resistance, greater fatigue resistance, and a better strength-to-weight ratio. It changed the game.
This graphic is a differential etch transverse section of an essential specimen showing the morphology at the fracture surface. The arrows point to places where the matrix of ferrite and pearlite ripped. Around the graphite nodules, no nodules were ruptured or transected. This is one of the mechanisms by which the microstructure of the material controls its mechanical properties.
Commercially in the US, most ductile irons are specified by ASTM A 5 36 and sold by grade names containing the three main identifying characteristics, tensile strength, yield, strength, and elongation. The lower the strength, the higher the elongation, and the higher the strength, the lower the elongation. 60 40 18 really just says 60,000 PSI tensile 40,000 PSI yield, 18% elongation minimum required.
The image here is pretty self-explanatory. These five grades are not exhaustive. There are many other grades of ductile iron, but these five grades are generally available as casts without requiring heat treatment to achieve the required mechanical properties.
Back to microstructure, at the low end, 60 40 18 is going to have mostly ferrite with maybe a little bit of pearlite around graphite nodules. And the 120 90 02 will likely have zero ferrites with an all-pearlite matrix around the graphite nodules.
How is it made?
Carefully sourced raw materials are weighed by grade and charged into a coreless induction furnace where a high-frequency magnetic current is introduced, exciting and vibrating atoms within individual pieces of charge material, which creates enough heat to cause the materials to de-solidify.
The current in the high-frequency magnetic field introduces a strong stirring action and helps to homogenize the melt bath. This melted iron is called base iron. The nonmetallic impurities float to the surface and are skimmed off. The base iron is then poured into a ladle containing magnesium. The reaction with magnesium is what causes the graphite to solidify in spheroids instead of flakes. The reaction is highly energetic. Once treated, the iron, which is now ductile iron, is poured into sand molds made from highly engineered patterns based on customer models or drawings. These ductile iron castings are clean and ready to ship for use or for final machining.
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