Dynamic recrystallization and microstructural evolution in 40CrNi2MoV steel during forging.

Area of Science:

• Materials Science
• Metallurgical Engineering
• Mechanical Engineering

Background:

• Reactor pressure vessel (RPV) studs demand uniform microstructures for operational reliability.
• The specific steel alloy 40CrNi2MoV is critical for RPV stud integrity.

Purpose of the Study:

• To establish predictive models for the microstructural evolution of 40CrNi2MoV steel during processing.
• To identify optimal processing parameters for achieving uniform and refined microstructures in RPV studs.

Main Methods:

• Development of a strain-compensated Arrhenius constitutive model and a dynamic recrystallization (DRX) model.
• Isothermal compression experiments across a range of temperatures (900°C-1180°C) and strain rates (0.001-0.5 s⁻¹).
• Finite element analysis (FEA) for microstructural evolution prediction, validated by processing maps and electron backscatter diffraction (EBSD).

Main Results:

• Discontinuous DRX dominated at 900°C/0.5 s⁻¹, forming necklace structures.
• Continuous DRX was favored at higher temperatures or lower strain rates.
• Optimal conditions (1060°C/0.05 s⁻¹) achieved 99.28% DRX volume fraction with 25.2 μm grains.
• Simulations predicted uniform grain refinement (144 μm to 17 μm) under optimized parameters (1180°C, 15 mm/s die speed).

Conclusions:

• The developed models accurately predict microstructural evolution in 40CrNi2MoV steel.
• A framework for predictive control of RPV stud microstructures was established.
• Optimized processing significantly refines and homogenizes RPV stud microstructures, enhancing reliability.