The twice softening of martensitic matrix in Q–P–T steels and its effect on ductility

Abstract

When a Si-containing steel is subjected to quenching-partitioning-tempering (Q-P-T) process, it will become a martensitic steel with considerable volume of retained austenite, which is attributed to the partitioning of carbon from martensite to retained austenite leading to carbon-depletion in martensitic matrix and carbon-enrichment in the retained austenite. The softening of the martensitic matrix caused by carbon-depletion markedly enhances its deformation ability and raises the ductility of Q-P-T steels. During deformation dislocation absorption by retained austenite (DARA) effect will occur, that is, the dislocations in martensitic matrix will move into nearby retained austenite through the martensite-austenite interface and be absorbed by the retained austenite. This second softening of the martensitic matrix also enhances its deformation ability and effectively raises the ductility. The effect of DARA on ductility during deformation is more effective than the transformation-induced plasticity (TRIP) effect caused by martensitic transformation of retained austenite, because during the TRIP effect the twin-type martensite produced by strain-induced martensitic transformation of retained austenite is very detrimental to ductility. KEYWORDS Q-P-T process; ductility; softening of martensite; retained austenite; DARA effect

Variational approach and experimental investigation of the phase-field model predicting brittle fracture

“…These microstructural changes are able to orient the crack path with various ways under mechanical loading as it was observed in Figure 11 for the tensile test. When the steel undergoes a tensile deformation (macro-strain), both dislocation density and microstrain increase considerably, 34,35 which further promotes the crack propagation along the direction imposed by the formed dislocations. The eventual micro-crack which can be created under the machining of the notch can also influence the orientation of the crack path.…”