Almost always used attributively as
铁打的 + noun:
铁打的纪律 (iron discipline),
铁打的事实 (rock-solid fact),
铁打的身体 (iron constitution). Comes from a famous saying:
铁打的营盘流水的兵 - 'an iron camp, soldiers like flowing water', i.e. the institution endures while individuals come and go.
