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Comparison of oil and intensive quenching via coupled thermal, transformation, and mechanical modeling
Baker, Daniel S.
Baker, Daniel S.
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2016
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A series of simulations were performed on a 25.4 mm (1 in) diameter 254 mm (10 in) long cylindrical bar. These simulations included three carburization levels: non-carburized, carburized to 0.8 wt pct C and 1.0 wt pct C at the surface utilizing a plain carbon steel (1020) and three alloy steels (4120, 4320, and 8620) representing a range of hardenabilities. Both industrially standard oil quenching as well as high intensity quenching which has a heat transfer rate of 20 kW/(m2 °C) were simulated. After quenching, the non-carburized and oil quenched bars were predicted to have tensile residual hoop stresses at the surface while the carburized bars were predicted to have compressive residual hoop stresses. All carburization levels of 1020 were predicted to have compressive residual hoop stresses after quenching. After high intensity quenching, all four alloys at all three carburization levels were predicted to have compressive residual stresses at the surface. It was shown that the high intensity quenching compressive residual hoop stresses at the surface were a result of the high heat transfer rate decreasing the temperature within fractions of a second resulting in a martensitic shell forming around a high temperature austenitic core. As the core cooled and thermally contracted, the shell was pulled inward to maintain coherency between the shell and the core. When the core austenite transforms, the volume expansion was insufficient to overcome the thermal contraction resulting in large compressive stresses at the surface and core, and a large tensile stress at the mid-radius. This profile was not found in literature. 1020 was found to transform to a mixture of ferrite and pearlite. As the core contracted and transformed, the volume expansion initially resulted in tensile hoop stresses near the surface. These tensile hoop stresses were decreased and became compressive due to the thermal contraction after transformation. A critical heat transfer rate for each of the alloys was determined where the tensile residual hoop stresses were reversed to compression. This critical heat transfer rate was 3.2 kW/(m2 °C), 9.0 kW/(m2 °C), 8.9 kW/(m2 °C), and 9.0 kW/(m2 °C) for 1020, 4120, 4320, and 8620 respectively. This was generalized to a Biot number with a minimum of 2.5 needed to create compressive hoop stresses at the surface.
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