A study of the effect of microstructure on tensile property has been carried out on 0.25%C plain carbon steel (S25C) with duplex ferritic-martensitic microstructures. The variation in size of the microduplex structures makes a considerable difference in fracture strength σmax of the material at room temperature. At σmax stress level, a large crack formed by coalesence of n unit microcracks starts to extend unstably through the martensite at its tip. This fracture process suggests that the tensile strength of the present material can be estimated by the following equation.
σmax=√6wm/n(2df+wm)σym
