Zhuojin Pipe Fitting Co., LtdConstitutive Relationship and Fracture Criterion of X90 Ultra-High Strength Gas Transmission Steel Pipe
Literature Overview
This paper by Yang Fengping, Luo Jinheng, Li He, Guo Yazhou, and Feng Jian was published in Acta Petrolei Sinica, Volume 38, Issue 1, pages 112–118, in 2017. The research was supported by the National Natural Science Foundation of China (Grant No. 51404294), the Shaanxi Provincial Natural Science Foundation (Grant No. 2014JQ2-1004), and the China National Petroleum Corporation Major Science and Technology Program (Grant No. 2012E-2801). The study focuses on the material constitutive relationship and fracture criterion of X90 ultra-high strength gas transmission steel pipe, addressing a critical need for advanced material models in pipeline integrity assessment.
Material Properties and Test Program
The X90 steel pipe, with a minimum yield strength of 620 MPa (90 ksi), represents the upper end of the strength spectrum for gas transmission pipelines. The test program was extensive and included:
| Test Type | Configuration | Purpose |
|---|---|---|
| Quasi-static tensile test | 5 different round bar notch geometries | Stress triaxiality and fracture strain characterization |
| Conventional tensile test | Standard dogbone specimens | Baseline mechanical properties |
| Split Hopkinson Tensile Bar (SHTB) test | High strain rate loading | Strain rate effect on fracture behavior |
| Stress triaxiality calculation | FEA-based post-processing | Quantification of stress state |
The yield strength of the X90 steel pipe was determined to be approximately 620–650 MPa, with an ultimate tensile strength of 700–750 MPa and an elongation of 20–25%. The material exhibits a fine-grained microstructure with a combination of ferrite and acicular ferrite phases, which provides the necessary combination of strength and toughness.
Core Technical Findings
The research produced several significant findings that are critical for pipeline integrity assessment:
- The presence of notches in the specimens increased the stress triaxiality by a factor of 2.43, while reducing the fracture strain by 29% and decreasing the damage strain energy by 71%. This demonstrates the profound influence of stress state on the fracture behavior of X90 steel.
- The strain rate effect on fracture strain was found to be relatively modest, with the maximum difference between quasi-static and high strain rate conditions being approximately 10%. This finding has important implications for fracture mechanics-based pipeline assessments.
- A Johnson-Cook constitutive model incorporating strain rate effects was established for the X90 pipeline steel, providing a reliable description of the material's stress-strain behavior under dynamic loading conditions.
- A failure model considering both strain rate and stress triaxiality was developed, offering a more comprehensive prediction of fracture initiation and propagation.
- Based on damage mechanics theory, a fracture criterion for X90 pipeline steel was derived using the plastic uniform elongation and damage strain energy approaches.
- The fracture characteristic length was related to stress triaxiality and specimen diameter through a relationship based on the assumption of constant critical material damage strain energy density.
Technical Parameters and Models
| Parameter | Symbol | Value/Expression | Significance |
|---|---|---|---|
| Yield strength | σ_y | 620–650 MPa | Minimum design strength |
| Ultimate tensile strength | σ_u | 700–750 MPa | Maximum stress capacity |
| Stress triaxiality increase factor | — | 2.43× | Notch effect on stress state |
| Fracture strain reduction | — | 29% | Notch effect on ductility |
| Damage strain energy reduction | — | 71% | Notch effect on energy absorption |
| Strain rate effect on fracture strain | — | ~10% maximum | Quasi-static vs. dynamic |
| Johnson-Cook strain hardening | B | Material-specific | Plastic deformation resistance |
| Johnson-Cook strain rate sensitivity | m | Material-specific | Dynamic loading response |
| Fracture characteristic length | L_f | f(σ*, D) | Size-dependent fracture prediction |
Standards and Engineering Practice
The X90 steel pipe must comply with API 5L, ISO 3183, or EN 10216 standards, which specify requirements for chemical composition, mechanical properties, and testing. The chemical composition typically includes carbon content of 0.05–0.10%, manganese of 1.2–1.6%, and controlled levels of niobium, vanadium, and titanium as microalloying elements. The welding of X90 pipeline steel presents unique challenges due to the high strength and the susceptibility of the heat-affected zone to hydrogen-induced cracking.
Key welding considerations for X90 pipeline steel include:
- Preheating temperatures of 100–150°C are typically required to control the cooling rate and prevent HAZ cracking.
- Interpass temperature control of 150–250°C is necessary to maintain the desired microstructure.
- Hydrogen control through low-hydrogen electrodes or fluxes is essential to prevent delayed hydrogen cracking.
- Post-weld heat treatment (PWHT) is generally not required for X90 steel when proper welding procedures are followed, but the HAZ microstructure must be verified by metallographic examination.
- The Charpy V-notch (CVN) impact energy at the HAZ should meet the minimum requirements specified in API 5L, typically 40 J at the minimum design temperature.
Study Insights and Implications
This research makes a significant contribution to the field of pipeline integrity assessment by providing validated material models for X90 ultra-high strength steel. The finding that strain rate effects on fracture strain are relatively modest (approximately 10%) simplifies the implementation of dynamic fracture assessments, as quasi-static test data can be used with reasonable accuracy for dynamic scenarios. The stress triaxiality-dependent fracture criterion is particularly valuable for assessing pipeline defects of varying geometry, as the stress state at the defect tip is directly related to the defect depth-to-length ratio and the applied stress level.
For steel pipe manufacturers, the research underscores the importance of maintaining tight control over the chemical composition and microstructure of X90 steel to ensure consistent fracture behavior. The welding quality of girth welds in X90 pipelines is critical, as any defects in the weld or HAZ can serve as initiation sites for fracture under operating pressure. The fracture characteristic length concept provides a practical tool for scaling laboratory test results to full-scale pipeline conditions, enabling more accurate risk assessments for in-service pipelines.