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UBC Theses and Dissertations

Strengthening mechanisms in some heat-treated low alloy steels Malik, Lalit Mohan


The microstructures and substructures of several heat-treated low alloy steels have been studied using x-ray line profile analysis, electron microscopy and electron diffraction. Yield strengths of the steels at 20°C have been determined after different heat treatments, and have been related to structure in terms of theories for various strengthening mechanisms. For tempered low-carbon and medium-carbon martensites, dislocation substructures originating with the martensite transformation were found to be responsible for a significant part of the strength, in contrast with the findings of the previous investigators. The balance of that part of the yield strength which varied with tempering temperature in these materials was attributed to dispersed cementite particles. When the dislocation substructure took the form of subgrains, as in heavily tempered specimens, the data were consistent with a Langford-Cohen model of subgrain strengthening, in which strengthening is related to the reciprocal of subgrain size. An age-hardening copper-bearing steel was also studied in the aged, and in the cold worked-and-aged conditions. In the peak-aged state, almost all the copper was present as coherent clusters, which produced strengthening associated with the necessity to shear them and produce new interfaces. When the steel was overaged, incoherent precipitates were present, but their contribution is shown to be insignificant in view of the large ratio of their spacing to their diameter. In cold-worked-and-aged specimens, the substructure contribution to strength was significant, but was essentially independent of aging time.

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