Yield Strength and Microstructural Scale: A Continuum Study of Pearlitic Versus Spheroidized Steel

A continuum mechanics approach is applied to the microstructure of pearlitic and spheroidized steels and compared with experiment as in earlier studies of WC-Co and aluminum-silicon. Success in the prediction of the yield strengths of pearlitic steels from the yield strengths of spheroidized steels requires little more than the conventional starting points in metallurgy and mechanics. Major conclusions which emerge are the same for all these very different crystal and microstructures. The flow stress of the ductile matrix of structural metals is determined primarily by microstructural features not visible in the optical microscope. The most rigid of particles or pearlite clumps, microns in size or larger, have negligible influence on yield or flow strength in volume fractions up to 1/10 and little influence up to 1/3. Yet, solute atoms, which raise the yield strength of pure metals significantly cannot account for the far higher yield strength of structural metals. Therefore, until very high volume fractions of very strong inclusions are present, the dominant control of plastic deformation must come from submicron particles in the ductile matrix. The microstructure visible in the optical microscope seems relevant here only to the extent that it reflects the invisible.