The application of dispersion strengthening of martensite in the design of high-strength steels

Abstract

With some plain carbon and alloy steels, structures consisting of a matrix of lath martensite strengthened by fine dispersed particles were obtained by unconventional methods of heat-treatment. It was proved that this type of structure is very effective in increasing both strength and ductility. The two-fold role of dispersion particles present in a matrix of low-carbon lath martensite was explained according to Cheng and Thomas: during the transformation the carbides are contributing to multiplication of dislocations in the martensitic phase, and in the final matrix they act by the mechanism of dispersion hardening. Since the practical applicability of unconventional methods (including ausforming) is known to be limited, a new approach to achieving the proper microstructural features and thereby better combination of strength and ductility with martensitic UHS steel was proposed. It consists in designing low- or medium-alloyed hyper-eutectoid steels in which a matrix of lath martensite and fine dispersed carbidic phase is attainable by means of conventional treatment. The practical applicability of this approach was proved by the results of both laboratory and practical tests of a new contact fatigue resistant steel the design of which was based on the principles of Hyper-Eutectoid Lath Martensitic (HELM) steels. Besides, the application of the same approach in designing low-alloyed medium-carbon structural steels was explored as a possibility.