The Anti-Galling Properties of Cast Iron

What is Galling

• Galling is when two metal surfaces come in contact and leave material on one or both pieces in the form of raised metal streaks or bumps. In order to reduce this, the metals used in the pieces must have some sort of resistance to this type of wear and tear. They can also be coated. Cast iron contains carbon, which improves its resistance.

Effects of Carbon

• The carbon in cast iron forms into various types of graphite depending on how the cast iron is made. The metal must cool and solidify after it is cast, and the speed at which it does so encourages different carbon formations. Cooling too quickly makes unstable ferrites or iron carbides. Cooling at a slower rate results in more stable graphite. Graphite is also somewhat self-lubricating, which helps prevent galling, so this form of carbon is more desirable for cast iron. The shape of the graphite is important as well. Flaked graphite slides more smoothly over other surfaces, reducing galling further, while lumpy or nodular graphite does not.

Effects of Silicon

• Many cast irons also contain silicon, which influences the way the carbon forms into graphite. High amounts of silicon encourages a flaked shape, rather than nodes or lumps. Silicon also encourages an oxide layer to form on the outside of the cast iron, which protects the iron surfaces from rubbing directly against each other.

Most Anti-galling Cast Iron

• The cast iron with the best anti-galling property is gray iron. This cast iron is high in graphite and silicon. It can also have copper and nickel, which function similarly to silicon in encouraging graphite formation. Gray iron's graphite is also flake-shaped. The high amount of graphite means that there are more areas that are self-lubricating, reducing the need for extra oils and coatings on the cast iron parts.

Least Anti-galling Cast Iron

• The type of cast iron that is least resistant to galling is malleable cast iron. Malleable cast iron has very little graphite compared to other forms of cast iron. What graphite there is forms into lumps called temper carbon. Because of the low amount of graphite and its irregular shape, malleable cast iron can cause streaks of galling when it rubs against another metal. However, the galling is limited and often does not progress beyond the original streak.