United States Capitol dome: Characterization of cast and wrought materials

Restoration of the cast iron dome at the United States Capitol is needed because moisture is leaking into interior areas of the building due to corrosion damage. Microstructure, composition, and tensile properties of cast and wrought samples from the dome are discussed in this report. The cast iron skin of the dome is a ferrite–pearlite gray iron with strength consistent with the carbon and silicon content. By current compositional and strength requirements the gray iron alloy comes close to meeting requirements for a class 20 gray iron. The microstructure shows good morphology and distribution of Types A and B graphite flakes that are appropriate for the intended service of the castings. In 20–20 hindsight, the decisions made concerning the composition of the iron, and the details of the molding and casting conditions were all quite good. Fatigue data indicate that good performance should be expected to continue for the dome for many years to come.