Effect of Intercritical Thermomechanical Processing on Austenite Retention and Mechanical Properties in a Multiphase TRI
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THE excellent mechanical properties of transformation-induced plasticity (TRIP)-assisted steels are mainly a result of a ductile ferritic matrix, strengthening by bainite or martensite, and the occurrence of the TRIP effect in the retained austenite which enhances strain hardening and delays necking.[1,2] From this perspective, the volume fraction and distribution of each phase and the stability of retained austenite have a great impact on the final mechanical properties of the steels.[3]
ALIREZA MOHAMADIZADEH, Postgraduate Student, ABBAS ZAREI-HANZAKI, Professor and Director, and MOHAMMAD MOALLEMI, Postgraduate Researcher, are with The Complex Laboratory of Hot Deformation & Thermomechanical Processing of High Performance Engineering Materials, School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O.Box 11155-4563, Tehran, Iran. Contact e-mail: [email protected] SAARA MEHTONEN, Postgraduate Researcher, is with the Center for Advanced Steels Research, Materials Engineering Research Group, University of Oulu, P.O.Box 4200, 90014 Oulu, Finland. DAVID PORTER, Professor and the Head of the Materials Engineering Research Group and the Director of Center for Advanced Steels Research, Materials Engineering Research Group, is with the University of Oulu. Manuscript submitted April 11, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
Conventionally, TRIP-assisted steels are being isothermally treated at bainite formation temperatures in order to achieve a considerable amount of retained austenite located between the bainite laths.[4] Alternatively, quench and partitioning would be employed to produce considerable amounts of retained austenite through decarburization of martensite.[5,6] In addition to the amount of austenite, the stability of austenite also depends on the partitioning of alloying elements such as carbon during intercritical annealing.[7–9] It is reported that the austenite with low levels of carbon (1.7 wt pct C) is stable enough which does not transform at all.[11] Zaefferer et al.[12] have shown that long holding time at relatively high bainite formation temperatures leads to suitable stabilization of austenite and optimum mechanical properties due to homogeneous carbon content. The morphology of retained austenite and the interaction of adjacent micro-constituents play important roles in determination of the stability of the retained austenite in multiphase TRIP-assisted steels. According to Jeong et al.,[3] retained austenite can be found at various locations, e.g., in contact or out of contact with bainite, within the microstructure of a TRIP-assisted steel which leads to different manners of TRIPing. For
instance, the equiaxed morphology of retained austenite is reported to be less stable under straining compared to lamellar austenite formed between bainite laths.[13] Additionally, an enhanced ductility has been achieved in TRIP-assisted steels with a network or isolated fine and acicular structure of second phase, i.e., bainite or ferrite, due to synergic effect o
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