Iron, when used metaphorically, refers to certain traits of the metal iron. Used as an adjective and sometimes as a noun, it refers to something stern, harsh, unyielding, inflexible, rigid, sturdy, strong, robust. It is sometimes used for something technological (iron lung) or not technologically advanced (iron bomb).
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This list is categorized superficially, rather than by the actual meanings of the expressions, to stress the metaphorical usage.