Thermoplastic Forming Behavior and Microstructure Evolution of 422 Super-critical Blade Steel

The blades is an important part of super-critical steam turbine operating long-term in bad environment with high temperature, high pressure, so good forming quality and organizational performance is required. AISI 422(C422) steel is a new important material for super-critical steam turbine blades, has good high temperature performance, which can adapt to the complex environment. However, establishing and optimizing for forging process just rely on the experience at present, not only high cost, but also poor stability. so it is necessary to research on the thermodynamic properties and microstructure evolution of 422 steel. In this Paper, high temperature plastic deformation behavior and dynamic recrystallization of 422 martensitic stainless steel are studied based on the thermal simulation compression experiment and metallographic test, the main research work is as follows:

Analysis the thermoplastic forming behavior of 422 stainless steel, and establish the flow stress model. The isothermal compression experiments were conducted by Gleeble machine to get and explore the flow stress under different deformation conditions, and analysis the effect of temperature, strain rate, strain on flow behavior. The Arrhenius constitutive equation Z parameter correction and BP neural network prediction model was establish, contrast the prediction values and experiment values found the precision of the models is high enough to describe the rheological behavior of 422 steel.

Research on the formability of 422 steel. Establish the hot processing map under different strain based on dynamic material model and flow instability criterion, analysis the maps to explore the thermodynamic properties and inference the stable and unstable region, combine with the phase diagram to confirm the optimum deformation zone of steel.

Build the dynamic recrystallization model for 422 steel.

Introduce the work hardening rate to judge the critical strain of recrystallization, set up the relationship of critical strain and peak strain, and build the equation of peak strain.

Establish the equation of dynamic recrystallization volume percentage and grain size model own by Deform.

Test the applicability of the material model established for 422 steel. Import the constitutive equation and dynamic recrystallization model into Deform to simulation the isothermal compression process, analysis the result from the forming performance and microstructure transformation two aspect, contrast the simulation results with experiments to test the applicability of the model established.

The can provide practices guidance for 422 super-critical steam turbine blade steel forging process development and optimization, mold design, equipment selection, which has great engineering application value.