Zhuojin Pipe Fitting Co., LtdHigh-Temperature Toughness Damage of Stainless Steel Pipe Based on Critical Void Expansion Ratio Theory
Literature Overview
This paper by Liu Feng, Chen Chengcheng, Chen Fanghai, Ji Pengliang, Han Yi, and Xu Dong, published in the Journal of Plasticity Engineering, Vol. 30, No. 4, 2023, addresses a critical manufacturing challenge in stainless steel pipe production: the prediction and control of cracking defects during hot plastic deformation, specifically during the oblique rolling piercing process. The authors employed Gleeble-3800 thermal simulation testing combined with finite element analysis to establish a quantitative toughness damage criterion based on the critical void expansion ratio parameter V_GC for TP321 austenitic stainless steel.
Core Technical Methodology
The research methodology follows a rigorous experimental-numerical approach. The authors conducted high-temperature tensile tests on TP321 austenitic stainless steel smooth bar specimens across a temperature range of 850–1180°C and strain rate range of 0.01–10 s⁻¹. These tests yielded true stress-true strain curves and the equivalent plastic strain at fracture. The experimental results were then validated through finite element simulation, which provided the stress triaxiality values needed to calculate the critical void expansion ratio V_GC.
The critical void expansion ratio V_GC is a material damage parameter that represents the ratio of void volume at fracture to the original void volume at nucleation. It serves as a material-specific toughness criterion that, when exceeded, indicates the onset of ductile fracture. By determining V_GC as a function of temperature and strain rate, the authors established a quantitative framework for predicting cracking susceptibility during hot forming operations.
| Test Parameter | Range | Purpose |
|---|---|---|
| Temperature | 850–1180°C | Covers typical hot piercing temperature window |
| Strain rate | 0.01–10 s⁻¹ | Spans slow deformation to high-speed rolling conditions |
| Specimen | TP321 smooth bar | Austenitic stainless steel with Ti stabilization |
| Equipment | Gleeble-3800 | Thermal simulation with precise temperature and strain rate control |
Critical Void Expansion Ratio Results
A key finding of this research is that within a specific temperature range, V_GC at fracture shows weak correlation with strain rate but exhibits a linear relationship with temperature. The authors established a mathematical expression relating V_GC to temperature for TP321 austenitic stainless steel. This linear relationship is practically significant because it allows engineers to predict the fracture susceptibility of the material at any temperature within the hot forming range using a simple equation.
The weak dependence of V_GC on strain rate within the studied temperature range suggests that the dominant damage mechanism is thermally activated void growth rather than rate-dependent plastic instability. This finding has important implications for process design: temperature control is more critical than strain rate control for avoiding cracking defects in TP321 hot piercing operations.
Application to Oblique Rolling Piercing Process
The V_GC criterion was applied to finite element simulations of oblique rolling piercing processes with different roll contour geometries. The simulation results demonstrated that the V_GC criterion can accurately predict the locations and severity of cracking defects during the piercing operation. Different roll contours produce different stress states within the workpiece, leading to varying levels of accumulated damage as quantified by the V_GC criterion.
The comparison of different roll contour designs revealed that the roll groove geometry directly influences the stress triaxiality distribution within the workpiece. Roll contours that produce lower stress triaxiality in the critical deformation zones result in lower V_GC values and reduced cracking susceptibility. This finding provides a quantitative basis for optimizing roll contour design to minimize defect formation.
Engineering Practice Integration
For stainless steel pipe manufacturers, this research provides a powerful tool for process optimization and quality control. The V_GC criterion can be incorporated into finite element simulation workflows to evaluate different process parameters before production trials, reducing the cost and time associated with trial-and-error optimization. Key process parameters that can be optimized using this criterion include piercing temperature, roll contour geometry, piercing angle, and the sequence of deformation operations.
The practical implementation of this approach involves establishing a material database of V_GC values for the specific grade and heat treatment condition of the steel being processed, developing validated finite element models of the piercing process, and using the V_GC criterion as a quality gate in the simulation workflow. Any process parameter combination that produces V_GC values exceeding the material's critical threshold in critical deformation zones should be modified before production.
Study Insights and Implications
The most significant contribution of this research is the validation of the V_GC criterion for high-temperature applications in austenitic stainless steel. Previous applications of void growth-based damage criteria have been limited primarily to room temperature and warm forming conditions. The demonstration that V_GC maintains predictive capability at hot forming temperatures (850–1180°C) expands the applicability of this damage criterion to a much wider range of industrial processes.
The linear relationship between V_GC and temperature suggests that the void growth mechanism in TP321 at hot forming temperatures is governed by thermally activated processes, likely involving diffusion-assisted void coalescence and grain boundary sliding. Understanding this mechanism at a fundamental level could lead to further improvements in the predictive model. Additionally, the extension of this approach to other austenitic stainless steel grades (such as TP304, TP316, and 310) would be valuable for broadening the applicability of the methodology.
In conclusion, this paper establishes a rigorous quantitative framework for predicting and controlling cracking defects in TP321 stainless steel hot piercing operations. The combination of thermal simulation testing, finite element analysis, and the V_GC damage criterion provides engineers with a practical tool for process optimization that can significantly reduce defect rates and improve manufacturing yield. The methodology is transferable to other hot forming processes and material grades, making it a valuable addition to the technical toolkit of stainless steel pipe manufacturers.