Zhuojin Pipe Fitting Co., LtdAnalysis of Failed Elongation After Fracture in Seamless Steel Pipes
Literature Overview
This paper by Peng Xianming, Zhao Jianming, and Ma Shuai from Hengyang Valin Steel Pipe Co., Ltd., published in the Chinese journal Steel Pipe in 2021, investigates the root causes of failed elongation after fracture (Agu) test results in high-pressure boiler seamless steel pipes produced on an Assel three-roll piercing mill. The study is particularly relevant to engineers working with alloy seamless pipes for critical applications where mechanical properties must meet stringent standards.
Problem Statement and Investigation Methodology
The grade in question is 12Cr1MoVG, a low-alloy heat-resistant steel widely used in high-pressure boiler applications per ASTM A213 and GB/T 5310 standards. The elongation after fracture (Agu) is a critical mechanical property that measures the ductility of the material, and failures in this property can indicate material quality issues, processing defects, or testing methodology problems.
The authors employed a systematic investigation approach:
- Comparison of specimens before and after straightening: Elongation values were compared for specimens taken before and after the straightening process to isolate the effect of straightening-induced residual deformation.
- Effect of internal helical grooves: Specimens with varying depths of internal helical grooves (caused by the piercing process) were tested to quantify the impact of this manufacturing feature on elongation.
- Finite element analysis (FEA): Simulations were conducted on both helical-grooved and smooth 12Cr1MoVG specimens under identical loading conditions to predict elongation behavior.
Root Cause Analysis
The study identified internal helical grooves as a primary contributing factor to low and failed elongation results when using flat (rectangular strip) specimens. The internal helical grooves, which are an inherent feature of the Assel piercing process, create stress concentration sites and non-uniform deformation during tensile testing.
| Condition | Specimen Type | Elongation Result |
|---|---|---|
| No internal helix | Strip specimen | Meets standard |
| Internal helix present | Strip specimen | Below standard (failed) |
| Internal helix present | Round bar specimen | Meets standard |
| Internal helix present | FEA prediction | Confirms strip specimen failure |
The key finding is that when the wall thickness conforms to product standard requirements but the internal helix depth exceeds standard limits, the strip specimen fails the elongation test while the round bar specimen passes. This indicates that the failure is related to the specimen geometry interaction with the internal helix rather than a fundamental material ductility deficiency.
Engineering Practice and Quality Control Implications
This finding has significant implications for quality control procedures at seamless pipe manufacturers:
- Specimen selection: When internal helix depth exceeds standard requirements but wall thickness is within specification, using round bar specimens for tensile testing provides a more representative assessment of the material's actual ductility. This is a practical quality assurance recommendation that can prevent unnecessary rejection of otherwise acceptable product.
- Process control: The Assel three-roll piercing process inherently produces internal helical grooves, and the depth of these grooves is related to piercing parameters such as reduction ratio, piercing speed, and mandrel geometry. Process optimization to reduce helix depth remains important for overall pipe quality.
- Standard interpretation: The study highlights an important nuance in standards compliance: the strip specimen elongation failure may not indicate a material quality problem but rather a testing methodology artifact. Engineers should understand this distinction when making acceptance or rejection decisions.
Study Insights
The systematic approach taken by the authors—combining physical testing, comparison studies, and finite element simulation—provides a robust basis for the conclusions drawn. The FEA confirmation of the physical test results adds credibility to the finding that internal helix geometry, not material quality, is the dominant factor in the elongation failure. For engineers involved in seamless pipe procurement and quality assurance, this paper provides a practical framework for distinguishing between genuine material quality issues and testing artifacts, which is essential for avoiding unnecessary scrap while maintaining quality standards.