Effect of Adiabatic Heating on Strain Induced Phase Transformations in Stainless Steels
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During plastic deformation, metastable austenitic stainless steels can go through a phase transformation from unstable austenite to martensite. This leads to an increase in the work hardening rate. Since steels are widely used metals, a lot of research efforts have been directed towards better understanding of their behavior. The phase transformation is one of the questions that have attracted a lot of attention from the researchers. This thesis focuses on studying the role of strain rate and adiabatic heating on the phase transformation from austenite to martensite during plastic deformation of metastable austenitic steels. The materials used in this study were EN 1.4318 stainless steel and titanium Ti-6Al-4V alloy. Mechanical testing was carried out with a servohydraulic materials testing machine at strain rates ranging from 0.0003 s-1 to 1 s-1 and at heating rates ranging from 1.4K/min to 10K/min. The results indicate that the strain induced phase transformation is affected by adiabatic heating but also by the strain rate itself. The results were obtained by measuring the temperature change during high strain rates and then applying the same heating rate to a lower strain rate test where test conditions without the heating would have been practically isothermal. The flow stress after 15% strain for the stainless steel at the strain rate of 0.03 s-1 was lower than the flow stress for the strain rate of 0.0003 s-1 with the continuous heating even though the temperature conditions for both strain rates were artificially kept similar throughout the test. Based on this observation it was concluded that the change in the strain rate had a noticeable effect on the flow stress. To confirm that this behavior was directly related to the phase transformation, similar tests were conducted for the titanium alloy, which does not go through any phase transformation during plastic deformation. The same heating rate that was measured during the deformation at the strain rate of 0.025 s-1 (none-isothermal deformation) was applied to the test that was performed at the strain rate of 0.0003 s-1 (isothermal deformation). The results showed that the flow stress for the strain rate of 0.0003 s-1 with continuous heating was lower than the flow stress for strain rate of 0.025 s-1 which was to be expected.