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Double soaking of medium manganese steels
Glover, Alexandra G.
Glover, Alexandra G.
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2020
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2022-04-14
Abstract
In this thesis, a novel heat treatment, termed double soaking, is investigated to create microstructures containing significant volume fractions of metastable austenite and athermal martensite. The double soaking heat treatment utilizes two heat treatments. The first, at a low intercritical annealing temperature produces a microstructure of manganese enriched austenite and ferrite. The second intercritical annealing or austenitizing heat treatment, which can be applied in a continuous or discontinuous manner, partially replaces ferrite with athermal martensite. The substitution of athermal martensite for ferrite in medium manganese steels, combined with significant volume fractions of retained austenite, is expected to result in improvements in strength and work hardening rate as compared to the ferrite austenite microstructure typical of medium manganese steels. In this investigation a 0.14C 7.17Mn steel which had previously undergone a commercial batch intercritical annealing heat treatment was utilized. The second intercritical annealing heat treatment, referred to as secondary soaking, was applied at temperatures between 700 and 800 °C for times between 30 and 120 s. This was shown to produce a microstructure with 19 to 43 vol pct retained austenite and 4 and 80 vol pct athermal martensite. Changes in grain size and solute concentration in both the BCC microstructural constituents and austenite, in response to the double soaking heat treatment, were characterized using a combination of transmission Kikuchi diffraction (TKD), scanning transmission electron microscopy (STEM) energy dispersive X ray spectroscopy (EDS) and DICTRATM simulations. With increasing secondary soaking time, athermal martensite volume fraction increased and the average grain size of both FCC and BCC microstructural constituents was reduced. Additionally, the carbon concentration of retained austenite was reduced, along with a small reduction in retained austenite manganese concentration, due to partitioning out of austenite during the secondary soaking heat treatment. The microstructural changes which occur during the secondary soaking heat treatment resulted in systematic changes in tensile properties. With increasing secondary soaking time and temperature, samples transitioned from discontinuous yielding with moderate strength and ductility to continuous yielding and a combination of high strength and reduced ductility. In the highest strength conditions this resulted in an ultimate tensile strength above 1700 MPa and total elongation at or above 15 pct. A linear relationship was found between athermal martensite volume fraction and tensile properties. Finally, the interaction between retained austenite and athermal martensite was explored during deformation using in situ neutron diffraction. The substitution of athermal martensite for ferrite was found to improve strength by contributing to an increased rate of work hardening within the BCC microstructural constituents. Additionally, the substitution of athermal martensite for ferrite eliminated discontinuous yielding and resulted in greater volume fractions of austenite available for strain induced transformation which may improve the strength ductility combination. Overall, the application of the double soaking heat treatment to medium manganese steels was shown to produce a combination of strength and ductility desired for the third generation of advanced high strength steels.
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