| Abstract: | The effect of microstructure on strength and fatigue properties has been investigated in two medium carbon alloy steels (BS 817M40 and BS 835M30) by developing dual-phase, ferritic-martensitic microstructures. Hardness-strength relationships and fatigue resistance at comparatively high strength levels were investigated by producing various microstructures. Conventional quenching and tempering, intercritical annealing and step quenching were used to vary the proportion, morphology and distribution of the ferrite and martensite phases. The results of the present study show that both hardness and strength increase with increasing proportion of martensite and/or hardness of the second phase. The relationship between hardness or strength and martensite percent is not in good agreement with a simple “law of mixtures” but is compatible with a more rapid strength increase at high martensite contents. The dual phase microstructures from the present study show superior near threshold ΔKTH values than normal tempered martensite. The results also show a high degree of correlation between Paris equation m values and fracture toughness KIC, showing that for high m values KIC is low and vice versa. The present experiments show that although crack initiation resistance in dual-phase steels is excellent crack propagation rates are higher than in quenched and tempered microstructures for a given ΔK. |
