cast iron vs cast-iron

cast iron

adj

  • Inflexible or without exception. 

  • Made of cast iron. 

  • Durable; tough; resilient. 

noun

  • A hard and brittle, but strong, alloy of iron, carbon, and silicon, formed by casting in a mould. 

cast-iron

How often have the words cast iron and cast-iron occurred in a corpus of books? (source: Google Ngram Viewer )