Effect of Strain Rate and Temperature on the Tensile Flow Behavior and Microstructure Evolution in Fe-0.3 Pct C-CrMoV Grade Steel

Abstract

The effect of temperature and strain rate on the tensile flow behavior of Fe-0.3 pct C-CrMoV grade steel was studied over a wide range of strain rates (10⁻⁴ to 10⁻¹ s⁻¹) and temperatures (700 °C to 950 °C). The flow curves of the steel showed typical dynamic recovery (DRV)-type characteristics at low temperature, high strain rate, and dynamic recrystallization (DRX) type at high temperature > 775 °C. Stress regimes with stress exponent (n) of 3.6 to 5.5 for low–high stresses were observed. The ‘n’ values at temperatures of 850 °C and 900 °C were found to be > 4, which correspond to dislocation climb as the rate controlling mechanism. At 950 °C, ‘n’ value was found to be < 4, where viscous glide is the rate controlling mechanism. The apparent activation energy (Q) was found to be 320 ± 12 kJ mol⁻¹. Hence, the dominant high-temperature deformation mechanism was identified as high-temperature climb of edge dislocations. The strain rate sensitivity index (m) of the steel was evaluated using jump strain rate tests and cyclic temperature and strain rate jump tests over temperatures of 700 °C to 950 °C and strain rates of 10⁻⁴ to 10⁻³ s⁻¹ . Although, ‘m’ value as high as 0.5 was observed, cavitation resulted in premature failure during deformation resulting in low elongation. The volume fraction of cavities was inversely proportional to the strain rate at all temperatures. The fine-grained microstructure aids grain boundary sliding, and diffusion thereby favors the cavity growth at low strain rates. Microstructures evolved during the high-temperature tensile tests were analyzed and the optimum conditions for hot deformation i.e., hot rolling/hot forming schedules were determined as the temperature range of 850 °C to 950 °C and strain rate range of 10⁻³ to 10⁻⁴ s⁻¹. The flow stress data for the steel were found to follow the universal Dorn sine hyperbolic equation.