Leakage Failure Analysis of Cold-Extruded Tees in Pipeline Engineering

Literature Overview

This failure analysis study, published in Metal Heat Treatment (2007, Vol. 32, Issue Z1, pp. 106–110) by Liu Yinglai from the Petroleum Pipe Research Institute of China National Petroleum Corporation, investigates the leakage failure of a tee fitting used in pipeline engineering. The analysis employed a comprehensive suite of examination techniques including macroscopic inspection, fracture surface microscopy, metallographic examination, chemical composition analysis, and mechanical property testing. The study ultimately identified cold extrusion-induced cracking as the root cause of the failure.

Failure Investigation Methodology

The investigation followed a systematic approach consistent with established failure analysis protocols:

Examination Method	Purpose
Macroscopic inspection	Identify visible crack origins and propagation paths
Fracture surface microscopy (SEM)	Characterize fracture mode (ductile vs. brittle, fatigue vs. overload)
Metallographic examination	Assess microstructure, grain flow, and internal defects
Chemical composition analysis	Verify material grade compliance
Mechanical property testing	Evaluate hardness, tensile strength, and toughness

The root cause was determined to be cracks generated during the cold extrusion forming process that underwent unstable propagation during hydrostatic pressure testing. This is a classic case of a manufacturing defect being masked during production inspection and only manifesting under service-relevant loading conditions.

Cold Extrusion Forming of Tees: Process and Defect Mechanisms

Cold extrusion is a widely used manufacturing method for producing seamless tee fittings from forged or rolled stock. The process involves forcing material through a die at room temperature to achieve the desired tee geometry. While cold extrusion offers advantages in terms of grain flow continuity, surface finish, and dimensional accuracy, it is also susceptible to several defect modes:

Cracking at the branch junction: The severe plastic deformation at the tee branch intersection can exceed the material's formability limits, particularly if the material is not adequately annealed prior to forming or if the forming parameters (die angle, lubrication, reduction ratio) are not optimized.
Residual stress concentration: Cold working introduces high residual tensile stresses, particularly at geometric discontinuities. These stresses can serve as driving forces for crack initiation and propagation under subsequent loading.
Grain flow disruption: If the forming process is not properly controlled, the natural grain flow lines established during forging can be disrupted, creating planes of weakness.

Quality Control Implications and Countermeasures

The study reports that a comprehensive quality assessment was performed on all 21 units from the same production batch, and the resulting countermeasures were accepted by the project management department. Based on the failure mechanism identified, the following quality control measures are critical for cold-extruded tee production:

Pre-forming heat treatment: Ensure adequate solution treatment or annealing to restore ductility and relieve prior processing stresses before cold extrusion.
Forming parameter control: Maintain strict control over die geometry, lubrication conditions, and forming speed to minimize the risk of cracking. Process windows should be established through trial production and validated through destructive testing.
Post-forming inspection: Implement non-destructive testing (NDT) protocols that are sensitive to surface and near-surface cracks. Magnetic particle testing (MT) is particularly effective for detecting surface-breaking cracks in ferromagnetic materials. Ultrasonic testing (UT) should be applied to detect subsurface defects.
Post-forming stress relief: Perform a controlled stress relief anneal after cold extrusion to reduce residual tensile stresses without significantly compromising the mechanical properties.
Hydrostatic testing protocol: While hydrostatic testing is a standard pressure containment verification method, it should be recognized as a potential crack propagation accelerator rather than a purely benign test. Test pressures should comply with applicable standards (e.g., ASME B16.9, ASTM A403) and should not be excessively above the design pressure.

FMEA Perspective on Cold-Extruded Tee Manufacturing

Applying a Failure Mode and Effects Analysis (FMEA) framework to the cold extrusion tee manufacturing process yields the following critical failure modes:

Failure Mode	Potential Cause	Effect	Suggested Control
Surface cracking at branch junction	Excessive deformation, inadequate lubrication	Leakage under pressure	MT inspection, forming parameter control
Subsurface cracking	Inclusion stringers, poor annealing	Delayed failure under cyclic loading	UT inspection, heat treatment verification
Residual stress cracking	Insufficient stress relief	Stress corrosion cracking in service	Post-forming stress relief, hardness verification
Dimensional deviation	Die wear, tool misalignment	Fit-up problems, stress concentration	In-process dimensional checks, die maintenance

Study Insights and Engineering Practice

This failure analysis case underscores a fundamental principle in pipe fitting quality assurance: manufacturing defects that survive production inspection can propagate under service loading conditions, leading to catastrophic failures. The cold extrusion process, while producing high-quality fittings when properly controlled, demands rigorous process discipline and comprehensive inspection. The fact that 21 units were evaluated from the same batch highlights the importance of batch-level quality assessment rather than isolated component acceptance. For engineering organizations specifying or procuring cold-extruded tee fittings, the lessons from this case study should inform both specification requirements and incoming inspection protocols. The integration of metallurgical knowledge with non-destructive testing capabilities is essential for preventing recurrence of such failures.