Zhuojin Pipe Fitting Co., LtdAnalysis of Reasons for Unqualified Elongation After Fracture in Seamless Steel Pipes
Literature Overview
The paper by Peng Xianming, Zhao Jianming, and Ma Shuai from Hengyang Valin Steel Pipe Co., Ltd. investigates the root cause of unqualified elongation after fracture (Ae) values in high-pressure boiler seamless steel pipes produced on an Assel three-roll piercing mill. Published in the journal "Steel Pipe" in 2021 (Vol. 50, No. 3, pp. 75-78), this study addresses a critical quality issue that directly impacts the mechanical performance and service safety of boiler tubes used in power generation.
Technical Background
High-pressure boiler seamless steel pipes, typically made from 12Cr1MoVG or similar alloy grades, must meet stringent mechanical property requirements including minimum elongation after fracture values specified in standards such as GB/T 5310, ASTM A213, or EN 10216-2. The elongation after fracture is a critical indicator of ductility and the ability of the pipe to withstand plastic deformation under operating conditions.
The Assel three-roll piercing mill is a modern continuous piercing technology that produces seamless pipe blanks with improved productivity and dimensional accuracy compared to conventional two-roll piercing mills. However, the unique deformation pattern of the Assel process introduces specific metallurgical characteristics that may affect mechanical properties.
Investigation Methodology
The authors conducted a systematic investigation using the following approach:
- Comparison of elongation values for specimens taken before and after straightening
- Evaluation of elongation values for specimens with different internal spiral (helical) depths
- Finite element analysis of 12Cr1MoVG specimens with and without internal spirals under identical loading conditions
Key Findings
Effect of Internal Spiral on Elongation
The internal spiral (helical groove) is a characteristic feature of seamless pipes produced by the Assel piercing process. During the piercing operation, the deformation of the billet creates a helical pattern on the internal surface of the pipe blank. This internal spiral has significant implications for mechanical testing:
| Condition | Specimen Type | Elongation Result | Compliance |
|---|---|---|---|
| No internal spiral | Strip specimen | Meets standard | Pass |
| Internal spiral present | Strip specimen | Below standard | Fail |
| Internal spiral present | Round bar specimen | Meets standard | Pass |
| After straightening | Strip specimen | Improved | Conditional pass |
Finite Element Analysis Results
The finite element simulation of 12Cr1MoVG specimens under tensile loading confirmed that the presence of an internal spiral significantly reduces the measured elongation after fracture for strip-shaped specimens. The helical geometry creates stress concentration effects at the root of the spiral groove, leading to premature localized deformation and fracture that does not represent the true bulk ductility of the material.
Root Cause Analysis
The fundamental issue is that the internal spiral acts as a geometric discontinuity that:
- Creates stress concentration at the groove root
- Reduces the effective cross-sectional area of the strip specimen
- Introduces anisotropic deformation behavior in the circumferential direction
- Causes premature necking and fracture at locations not representative of bulk material ductility
This is particularly problematic when the internal spiral depth exceeds the tolerance limits specified in product standards. The standard typically allows a certain degree of internal spiral as a manufacturing feature, but when this spiral is excessive, it compromises the reliability of mechanical testing results.
Standard Interpretation and Recommendations
The paper provides a practical recommendation for quality control: when specimens exhibit internal spiral depths exceeding standard requirements but wall thickness meets product specifications, round bar specimens should be used for tensile testing instead of strip specimens. This approach eliminates the geometric influence of the internal spiral and provides a more accurate assessment of the bulk material ductility.
| Testing Approach | Specimen Type | Internal Spiral Influence | Reliability |
|---|---|---|---|
| Strip specimen (standard method) | Rectangular strip | High | Low when spiral is excessive |
| Round bar specimen (alternative) | Cylindrical bar | Minimal | High |
Engineering Practice Implications
For manufacturing engineers and quality assurance personnel, this study highlights several important considerations:
- Specimen selection: The choice of specimen geometry must account for manufacturing-induced geometric features. When internal spirals are present, round bar specimens provide more reliable ductility assessment.
- Process control: The Assel piercing process parameters should be optimized to minimize internal spiral depth, particularly for high-pressure boiler tubes where mechanical property consistency is critical.
- Quality dispute resolution: When elongation failures are observed, the presence and severity of internal spirals should be evaluated before attributing the failure to material quality issues.
Study Insights
From a metallurgical perspective, this study underscores the importance of understanding the interaction between manufacturing processes and mechanical testing methodologies. The Assel piercing process, while offering significant productivity advantages, introduces geometric features that can confound quality assessment if not properly accounted for. This is a reminder that mechanical testing standards must be applied with engineering judgment, particularly when manufacturing processes introduce non-standard specimen geometries.
For engineers involved in the procurement and qualification of boiler tubes, this research provides valuable guidance for interpreting test results and resolving quality disputes related to elongation failures. The recommendation to use round bar specimens when internal spirals exceed standard limits is a practical and technically sound approach that should be incorporated into internal quality procedures.