Abstract
Present work deals with the study of structural changes of medium carbon micro alloyed steel during two heat treatment cycles as isothermal transformation and thermomechanical processing followed by isothermal processing. In isothermal transformations, the steel immediately after austenitization at 1250°C for 20 minutes was transferred a to salt bath at a preselected temperature of 365, 400, 435, 470 or 500°C and after holding for 35 minutes followed by their quenching in water. In the other heat treatment cycle, after austenitization at 1250°C for 20 minutes the steel was subjected to thermomechanical treatment by hot rolling at a finish temperature of 750°C followed by isothermal treatment under the similar conditions as mentioned above.The third heat treatment, Two Step Cooling (TSC), involved ϕ30 mm forged bar of 38 MnSiVS5 soaked at 1200 °C (30 min) → hot rolled at finish rolling temperature of 800 °C in 6 to 8 passes to ϕ 17 mm → air cooling up to 700 °C followed by water quenching (Two Step Cooling) → annealed at 400-550 °C (in steps of 50 °C) for 90 minutes. The effect of microstructure on the fracture toughness behavior of heat treatment routes has been studied and related to their microstructures. The cost effectiveness of this steel for critical aeronautical field is discussed.
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