Influence of martensite content and morphology on the toughness and fatigue behavior of high-martensite dual-phase steels

Abstract

A series of high-martensite dual-phase (HMDP) steels exhibiting a 0.3 to 0.8 volume fraction of martensite (V m ), 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 (K ICV ) increases with increasing V m in the range between V m ϭ 0.3 and 0.6 and then decreases, whereas the dynamic fracture-toughness parameters (K ID , K D , and J ID ) exhibit a significant increase in their magnitudes for steels containing 0.45 to 0.60 V m 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 V m between 0.55 and 0.60, appear to be superior to those of structural steels of a similar strength level. The Parislaw exponents (m) for the developed HMDP steels increase with increasing V m , with an attendant decrease in the pre-exponential factor (C ).

Effects of Austenitising Conditions on the Microstructures and Mechanical Properties of Martensitic Steel with Dual Matrix Structure

“…Recently, it has been observed that dual phase steel with finer microstructure exhibits higher ductility [6]. On comparison, the result of the ductility obtained in this investigation appears to agree with previous finding.…”

“…It is widely reported [6,11] that these precipitates are formed during the reheating process to the ICA temperature. The amount of carbide precipitates …”

“…This, however, might be done at the expense of ductility and toughness and for this reason such are inappropriate for undergoing various deformation processes [5,6]. Recently, the degradation of ductility and impact toughness of DP steels has been receiving attention and this phenomenon has been attributed to the formation of coarse martensite phases as the intercritical annealing temperature increases [3,5,6].…”

“…This, however, might be done at the expense of ductility and toughness and for this reason such are inappropriate for undergoing various deformation processes [5,6]. Recently, the degradation of ductility and impact toughness of DP steels has been receiving attention and this phenomenon has been attributed to the formation of coarse martensite phases as the intercritical annealing temperature increases [3,5,6]. It has been shown that the ductility and fracture toughness can satisfactorily be optimized by developing microstructures with fine distribution of ferrite and martensite in DP steel either by introducing new alloy design or adopting an appropriate heat treatment procedure [3,6,7].…”

“…Recently, the degradation of ductility and impact toughness of DP steels has been receiving attention and this phenomenon has been attributed to the formation of coarse martensite phases as the intercritical annealing temperature increases [3,5,6]. It has been shown that the ductility and fracture toughness can satisfactorily be optimized by developing microstructures with fine distribution of ferrite and martensite in DP steel either by introducing new alloy design or adopting an appropriate heat treatment procedure [3,6,7]. On the basis of alloying design, the new chemistries are low in carbon and rich in hardenability enhancing element such as…”