High-Temperature Toughness Damage of Stainless Steel Pipes Based on Critical Void Growth Ratio Theory

Literature Overview and Research Motivation

This 2023 study by Liu Feng, Chen Chengcheng, Chen Fanhai, Ji Pengliang, Han Yi, and Xu Dong, published in the Journal of Plasticity Engineering, addresses a critical practical problem in the hot forming of stainless steel pipes: the prediction and prevention of cracking during the oblique rolling piercing process. The authors selected TP321 austenitic stainless steel, a widely used alloy in high-temperature and corrosion-resistant applications, and investigated its high-temperature tensile behavior and ductility damage across a temperature range of 850 to 1180 degrees Celsius and strain rates from 0.01 to 10 per second. The work is significant because it bridges the gap between fundamental fracture mechanics research and the practical optimization of rolling mill parameters.

The choice of the critical void growth ratio (V_GC) as the toughness damage criterion is particularly noteworthy. Unlike conventional ductility parameters such as reduction of area or elongation, which are dependent on the specific test configuration and loading path, the critical void growth ratio is derived from stress triaxiality and is therefore more suitable for application to complex forming operations where the stress state varies significantly across the workpiece.

Experimental Methodology and Key Results

The experimental program was conducted on a Gleeble-3800 thermomechanical simulator, which allows precise control of temperature, strain rate, and deformation conditions. Smooth cylindrical specimens of TP321 were subjected to high-temperature tensile testing, and the true stress-true strain curves were recorded at each combination of temperature and strain rate. The equivalent plastic strain at the moment of fracture was also captured, providing a direct measure of the material's ductility under each condition.

The finite element simulation component of this study is equally important. The authors performed numerical simulations of the tensile tests to extract the stress triaxiality distribution within the specimens. This allowed them to calculate the critical void growth ratio parameter V_GC at the moment of fracture for each test condition. The stress triaxiality, defined as the ratio of hydrostatic stress to von Mises equivalent stress, is a critical parameter in ductile fracture modeling because it governs the mode of void growth and coalescence within the material.

A key finding from the experimental data is that within a specific temperature range, the V_GC value at fracture shows relatively weak dependence on strain rate but exhibits a linear relationship with temperature. The authors established a mathematical expression relating V_GC to temperature for TP321, which provides a practical tool for predicting material behavior under varying thermal conditions. This linear temperature dependence suggests that within the austenitic single-phase region, the void growth mechanism is governed primarily by thermal activation of dislocation processes rather than by rate-dependent dynamic effects.

Application to Oblique Rolling Piercing and Defect Prediction

The practical application of the V_GC criterion was demonstrated by applying it to finite element simulations of different roll configurations used in the oblique rolling piercing of TP321 pipe blanks. The authors compared the damage predictions obtained from the V_GC criterion with the actual defect patterns observed in production, and found that the criterion accurately predicted the locations and severity of cracking. This validation is essential because it confirms that the criterion is not merely a theoretical construct but a reliable predictor of real-world forming defects.

The comparison of different roll shapes revealed that roll geometry has a significant influence on the stress triaxiality distribution within the pipe blank. Roll configurations that produce higher stress triaxiality values in the deformation zone are more likely to cause void initiation and growth, leading to surface cracks and internal defects. By adjusting the roll profile to reduce the peak stress triaxiality, the forming process can be optimized to produce defect-free pipes. This finding has direct implications for roll design and rolling schedule optimization in stainless steel pipe mills.

The FMEA approach can be applied to interpret these results systematically. The failure modes in oblique rolling piercing include surface cracking, internal voids, and dimensional deviation. The critical cause identified by this study is excessive stress triaxiality exceeding the V_GC threshold at the local temperature. The detection method is visual inspection of the pierced blank combined with ultrasonic examination. The preventive action is to optimize roll geometry and rolling temperature to keep the local stress triaxiality below the critical value. This structured analysis framework helps production engineers prioritize corrective actions based on the severity and likelihood of each defect mode.

Study Insights and Engineering Implications

This research exemplifies the power of integrating experimental characterization with numerical simulation to solve practical manufacturing problems. The V_GC criterion, while conceptually simple, requires careful calibration through both experimental testing and simulation to be applicable to a specific material and forming operation. The work demonstrates that this calibration is feasible and that the resulting criterion can be used to guide process optimization with confidence.

One area for further development is the extension of this approach to multi-step forming operations where the pipe blank undergoes multiple deformation passes, each with different temperature and strain rate conditions. The accumulation of damage across multiple passes is a complex problem that requires integrating the V_GC criterion with a damage accumulation model. Additionally, the effect of grain size and texture evolution during hot forming on the V_GC values should be investigated, as these microstructural factors are known to influence void growth behavior in austenitic stainless steels.

For stainless steel pipe manufacturers, the key takeaway is that high-temperature forming defects can be predicted and prevented through the systematic application of ductility damage criteria combined with finite element simulation. The linear relationship between V_GC and temperature provides a simple yet powerful tool for setting temperature windows in rolling schedules. Engineers should invest in the development of material-specific V_GC databases that cover the full range of operating conditions encountered in production, as this knowledge base will serve as a foundation for continuous process improvement and quality enhancement.