Influence of martensite content and morphology on the toughness and fatigue behavior of high-martensite dual-phase steels
A series of high-martensite dual-phase (HMDP) steels exhibiting a 0.3 to 0.8 volume fraction of martensite (Vm), produced by intermediate quenching (IQ) of a vanadium and boron-containing microalloyed steel, have been studied for toughness and fatigue behavior to supplement the contents of a recent report by the present authors on the unusual tensile behavior of these steels. The studies included assessment of the quasi-static and dynamic fracture toughness and fatigue-crack growth (FCG) behavior of the developed steels. The experimental results show that the quasi-static fracturetoughness (KICV) increases with increasing Vm in the range between Vm=0.3 and 0.6 and then decreases, whereas the dynamic fracture-toughness parameters (KID, KD, and JID) exhibit a significant increase in their magnitudes for steels containing 0.45 to 0.60 Vm before achieving a saturation plateau. Both the quasi-static and dynamic fracture-toughness values exhibit the best range of toughnesses for specimens containing approximately equal amounts of precipitate-free ferrite and martensite in a refined microstructural state. The magnitudes of the fatigue threshold in HMDP steels, for Vm between 0.55 and 0.60, appear to be superior to those of structural steels of a similar strength level. The Paris-law exponents (m) for the developed HMDP steels increase with increasing Vm, with an attendant decrease in the pre-exponential factor (C).
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