Gray cast iron and ductile iron differ by the kind and shape of carbon inside them. So, which one fits your project? Before we hunt for answers, let’s walk through some of the main differences between these two cast irons.
What is Gray Cast Iron?
Gray iron, or grey cast iron, is a type of cast iron with a graphitic microstructure. It gets its name from the gray color of the fracture it forms, which comes from the graphite inside. It is the most common cast iron and the most widely used cast material by weight.
People use it for housings where the stiffness of the part matters more than its tensile strength, like internal combustion engine cylinder blocks, pump housings, valve bodies, electrical boxes, and decorative castings. Makers also take advantage of grey cast iron’s high thermal conductivity and specific heat capacity to make cast iron cookware and disc brake rotors.
Its once very common use on brakes in freight trains has dropped a lot in the European Union because of noise pollution concerns. For example, Deutsche Bahn replaced grey iron brakes on 53,000 of its freight cars (85% of their fleet) with newer, quieter models by 2019, partly to follow a law that took effect in December 2020.
What is Ductile Cast Iron?
Ductile iron, also called ductile cast iron, nodular cast iron, spheroidal graphite iron, spheroidal graphite cast iron, and SG iron, is a graphite-rich cast iron that Keith Millis discovered in 1943. Most cast irons break easily and do not handle pulling forces well, but ductile iron resists impact and fatigue much better because its graphite forms round nodules.
Ductile iron is not just one material. It belongs to a family of materials. Makers can produce many different properties by controlling the microstructure. The one thing that defines this whole group is the shape of the graphite. In ductile irons, the graphite looks like little nodules. In grey iron, the graphite looks like thin flakes.
What types of iron are cast in the foundry?
- Ductile Cast Iron: Engineers developed this type of iron in 1948, and it has become a very important alloy for ferrous foundries. It bends much more easily than other cast irons and breaks far less often.
- Malleable Cast Iron: Before ductile iron came along, this alloy was more popular. It starts as a white cast iron, and workers heat treat it for a very long time. The heat treater must keep conditions very controlled. When the malleable iron is finished, it is a lot less brittle. Malleable is still used for small, thin castings where ductile is less successful.
- White Cast Iron: Workers cool this alloy more quickly than other cast irons, which produces a molecule called cementite in its lattice. It breaks easily, but it has excellent hardness and abrasion resistance. It’s often used in bearings and other high-friction applications.
- Gray Cast Iron: Most standard “cast iron” items are made from gray cast iron, so when people use the term, this is likely the alloy that is meant. Unlike white cast iron, gray iron has a graphitic microstructure. It has excellent vibration damping capacity and great machinability.
White and gray cast irons create very similar-looking castings. Their coloring is only evident when they fracture.
Differences Between Ductile Iron and Gray Iron Castings
1. Formation Process.
You make both types of iron by adding carbon to hot molten iron. But when you make Ductile Iron, you also add magnesium.hrcak.
2. Graphite Structure.
Graphite in Grey Iron forms a rod-like or flaky pattern. In Ductile Iron, the graphite forms nodules or spheres.
3. Ductility.
Ductility means the material stretches more before it breaks when you pull on it. The magnesium in ductile iron makes the graphite form round nodules instead of flakes, which gives it higher strength and ductility than gray iron. For example, you can easily get 18% elongation with ASTM A395 and A536 grades 60-40-18 material.
4. Tensile and Yield Strength.
Gray and ductile iron definitely differ in tensile and yield strength. Ductile iron has a minimum tensile strength of 60,000 psi and a minimum yield strength of 40,000 psi.
Gray iron comes in many grades under the ASTM A48 standard. Gray iron does not have a measurable yield strength, and its tensile strength ranges from 20,000 psi to 60,000 psi.
5. Impact Resistance.
Impact strength, also called toughness, measures how well a metal resists breaking when it takes a hit. Ductile iron resists impacts much better, and it can handle at least 7 foot-pounds of impact (compared to just 2 pounds for gray iron).
This means you can use ductile iron in critical applications that involve impact, but gray iron has limits that keep it out of certain jobs.
6. Thermal Conductivity.
Ductile iron has lower thermal conductivity than gray iron. The graphite phase in gray iron gives it very high thermal conductivity because heat travels through the graphite flakes. The isolated spheres of graphite in ductile iron lower its thermal conductivity a lot; in fact, it is not much more conductive than steel.
As the amount of carbon increases, the thermal conductivity increases. This means lower-strength gray irons will have higher thermal conductivity. The design engineer needs to keep this in mind if the main reason for choosing gray iron is for thermal conductivity.
Gray iron, with its high thermal conductivity, is a great choice for parts that undergo thermal shock, like brake drums.
7. Vibration Damping.
Internal friction is how materials absorb vibrational energy. Gray iron dampens vibrations more effectively than ductile iron because gray iron shows non-elastic behavior at very low stresses.
A higher damping capacity boosts fatigue resistance because the time when stress is at or above the fatigue limit gets shorter. This boost in fatigue resistance is the most beneficial part of higher damping capacity. The other major benefit is the reduction of vibrations and noise that can come from components of machinery.