In-Situ High-Energy X-ray Diffraction Study of Austenite Decomposition During Rapid Cooling and Isothermal Holding in Two HSLA Steels |
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Authors: | Lin Sen Borggren Ulrika Stark Andreas Borgenstam Annika Mu Wangzhong Hedström Peter |
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Affiliation: | 1.Department of Materials Science and Engineering, KTH Royal Institute of Technology, 10044, Stockholm, Sweden ;2.SSAB AB, 78184, Borlänge, Sweden ;3.Helmholtz Zentrum Geesthacht, Institute of Materials Research, Max Planck Straße 1, 21502, Geesthacht, Germany ; |
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Abstract: | In-situ high-energy X-ray diffraction experiments with high temporal resolution during rapid cooling (280 °C s?1) and isothermal heat treatments (at 450 °C, 500 °C, and 550 °C for 30 minutes) were performed to study austenite decomposition in two commercial high-strength low-alloy steels. The rapid phase transformations occurring in these types of steels are investigated for the first time in-situ, aiding a detailed analysis of the austenite decomposition kinetics. For the low hardenability steel with main composition Fe-0.08C-1.7Mn-0.403Si-0.303Cr in weight percent, austenite decomposition to polygonal ferrite and bainite occurs already during the initial cooling. However, for the high hardenability steel with main composition Fe-0.08C-1.79Mn-0.182Si-0.757Cr-0.094Mo in weight percent, the austenite decomposition kinetics is retarded, chiefly by the Mo addition, and therefore mainly bainitic transformation occurs during isothermal holding; the bainitic transformation rate at the isothermal holding is clearly enhanced by lowered temperature from 550 °C to 500 °C and 450 °C. During prolonged isothermal holding, carbide formation leads to decreased austenite carbon content and promotes continued bainitic ferrite formation. Moreover, at prolonged isothermal holding at higher temperatures some degenerate pearlite form. |
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