A study of strengthening mechanisms in tempered martensite from a medium carbon steel

Abstract

Previous studies of strengthening mechanisms in quenched and tempered steels have been based almost entirely on microscopy. The density and distribution of residual dislocations in these materials have generally been poorly characterized and there is doubt as to the contribution of substructure to strength. In the present work, a 0.42 pct C steel has been hardened and tempered for 1 hr at seven temperatures in the range 250° to 550°C. The structures have been studied by X-ray diffraction line profile analysis as well as by electron microscopy. The measured 0.2 pct offset yield strength, over the whole tempering temperature range, could be represented by $$\sigma _{0.2{\text{ = }}} \sigma _0 {\text{ + }}\frac{{k_{\text{1}} }}{D}{\text{ ln }}\frac{D}{{2b}}{\text{ + }}k_{\text{2}} {\text{ }}\rho ^{1/2} $$ whereD is the mean planar spacing of carbide particles,ρ is the density of residual dislocations in the as-tempered structure, and b is the Burger’s vector of a dislocation in iron. The stress ct0 was found to be about 35 kgm mm-2 for the steel used and probably includes an appreciable contribution from incremental interstitial solid solution strengthening. The constantsk 1 andk 2 have values which are consistent with both theory and previous experimental work with the Orowan mechanism and strain-hardening, respectively. The dislocation substructure contribution accounted for more than 35 pct of the yield strength of the steel tempered at temperatures up to 400°C.