Martensite in steel: strength and structure

This paper reviews the strengthening mechanisms associated with the various components of martensitic microstructures in steels and other ferrous allo ys. The first section examines the experiments and strengthening theories a ssociated with Fe-Ni and Fe-Ni-C alloys, in which the martensite, because o f subzero M-s temperatures, can be evaluated with carbon atoms trapped in o ctahedral interstitial sites. The evaluation of strengthening in these allo ys has been limited to interpreting yield strength of unaged, untempered ma rtensite in terms of interstitial solid solution strengthening. The second section reviews strengthening of martensitic Fe-C alloys and low-alloy carb on steels with above-room-temperature M-s temperatures. In these alloys, it is impossible to prevent C diffusion during quenching, and strengthening o f martensite becomes dependent on static and dynamic strain aging due to ca rbon atom interaction with dislocation substructure. In all alloys the domi nant strengthening component of martensitic microstructures is the matrix o f martensitic crystals, either in lath or plate morphology, but secondary e ffects due to other microstructural components such as carbides and retaine d austenite are also discussed. (C) 1999 Elsevier Science S.A. All rights r eserved.