Creep-ratcheting effect and lifetime prediction of advanced 9-12% Cr ferritic steel at 600 ℃.

A thorough understanding of the physical mechanisms that govern material deformation under cyclic thermal and mechanical stresses is crucial for ensuring the safety and efficiency of critical applications such as steam turbines, boilers, and heat exchangers. These components often experience severe conditions, including creep-fatigue loading during start-up and shut-down operations. 9-12% Cr ferritic steel, with its excellent mechanical properties and corrosion resistance, has emerged as a promising material for high-temperature applications. Extensive research has been conducted to investigate the high-temperature behavior of this steel under cyclic stress- and strain-controlled conditions. Previous studies have focused on examining microscopic failure mechanisms, the evolution of internal stress components, and the development of prediction models for creep-ratcheting. However, there is still a need for an improved understanding of the creep-ratcheting behavior of this material, especially when subjected to varying peak holding periods. Bridging this knowledge gap is essential for accurate lifetime prediction and structural optimization, which are critical for safety assessment.

In a recently published study in the peer-reviewed International Journal of Fatigue, PhD candidate Pei-Shan Ding and Professor Xiao-Tao Zheng from the Wuhan Institute of Technology in China investigated the creep-ratcheting behavior of advanced 9-12% Cr ferritic steel at 600°C under various peak holding times. The authors developed a modified plasticity-creep superposition model that takes into account anelastic creep recovery and plastic creep ratcheting over the load history. This model was used to predict both the accumulated creep-ratcheting deformation and the rupture lifespan of the material. To conduct their investigation, they utilized a Gleeble-3500 thermomechanical simulator and employed cylinder-shaped samples measuring 6 mm in diameter and 12 mm in gauge length. Various tests, including monotonic tensile tests, creep tests, and creep-fatigue tests, were performed on these samples. Axial strain was measured using extensometers, and stress-strain curves were recorded. Additionally, selected specimens underwent metallographic investigation using scanning and transmission electron microscopy to examine microstructural changes.

The stress-strain responses of the material revealed three distinct phases of deformation: an initial rapid decrease, a steady state, and a subsequent significant increase leading to rupture. Understanding the material’s behavior under different conditions was crucial for accurately predicting its deformation and potential failure. The authors defined the anelastic recovery effect as the partial or complete recovery of creep strain in the material when it was discharged from stress. The duration of the peak holding time during creep loading influenced the degree of recovery during unloading, thereby impacting the stress-strain curve upon reloading. The primary creep regeneration effect described the steady behavior observed during primary creep, where the evolution of internal stresses influenced the material’s creep strain rate. The effect of creep regeneration remained constant throughout each loading and unloading cycle. Shorter peak holding times were found to significantly reduce the rupture time. The evolution of strain was affected by factors such as anelastic recovery, primary creep regeneration, and applied stress levels.

To capture the creep-ratcheting behavior, the authors developed a constitutive model that incorporated the principle of superposition, wherein the total strain rate equaled the sum of the plastic strain rate and the creep strain rate. A damage parameter was introduced to modify the creep strain rate in response to cyclic loading and material degradation. An additional term was added to the model to account for ratcheting deformation, which occurred when a material experienced cyclic loading with an increasing mean stress level. The proposed model was validated using experimental data obtained from 9-12% Cr ferritic steel at 600°C subjected to varying peak holding times. The model accurately predicted accumulated creep-ratcheting deformation and rupture lifetime.

This new study significantly enhances our understanding of the creep-ratcheting behavior of advanced 9-12% Cr ferritic steel at high temperatures and provides a valuable tool for forecasting material behavior under such circumstances. The proposed model can be instrumental in component design and optimization for high-temperature applications where creep and ratcheting deformation are critical factors to consider. By improving our ability to predict and mitigate potential failures, this research contributes to enhancing the safety and efficiency of crucial industrial components such as steam turbines, boilers, and heat exchangers.