When choosing an iron alloy for a casting, a metallurgist considers the budget, required mechanical properties, and post-casting steps like machining and heat treatment. These requirements decide what alloy to use. These days, the biggest choice is between ductile iron vs. cast iron.

Technically, “cast iron” suggests iron alloys that are cast in a foundry. Ductile iron is one such alloy. However, ductile is a relative newcomer on the scene, with unique mechanical properties that set it apart from the other iron alloys. This difference means that “cast iron” usually specifies gray or white iron alloys that have been part of metalsmithing for centuries.

What types of iron are cast in the foundry?

  • Ductile Cast Iron: This type of iron was developed in 1948 and has become a very important alloy for ferrous foundries. It is much less brittle than other cast irons.
  • Malleable Cast Iron: Before the invention of ductile iron, this alloy was more popular. It is a white cast iron that is heat treated for a very long period. Conditions in the heat treater must be 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: This alloy is cooled more quickly than other cast irons, producing a molecule called cementite in its lattice. It is a brittle alloy but 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.