Crack Cause Analysis of Equal-Diameter Tee in Oil Pipeline

Literature Overview

The paper by Jiang Zhiguo and Diao Zhuo (2014), published in "Electric Welder" (Vol. 44, No. 11, pp. 40-42), investigates the root cause of cracking in an equal-diameter tee fitting used in an oil pipeline system. The investigation employed a comprehensive suite of examination techniques including macroscopic inspection, microstructural analysis, fracture surface examination, and mechanical property testing. The key finding is that cracks existed on the inner surface of the tee at the crack location prior to service, and these cracks originated during the cold forming process. This case study is highly instructive for engineers dealing with formed pipe fittings and highlights the importance of process control in the manufacturing of seamless and formed tees.

Investigation Methodology

The investigation followed a systematic approach typical of failure analysis, which can be structured using the PDCA (Plan-Do-Check-Act) framework:

Plan: Define the scope of investigation, identify the failure mode, and establish the examination plan.
Do: Conduct macroscopic examination, microstructural analysis, fracture surface analysis, and mechanical property testing.
Check: Analyze the collected data to identify the root cause and validate the hypothesis.
Act: Recommend corrective and preventive actions.

Examination Techniques Employed

Technique	Purpose	Key Findings
Macroscopic inspection	Identify crack location, orientation, and surface characteristics	Cracks found on inner surface at the branch intersection
Metallographic examination	Analyze microstructure, grain flow, and deformation patterns	Evidence of cold working and strain localization
Fracture surface analysis (SEM)	Determine fracture mode and initiation sites	Ductile fracture with initiation from pre-existing surface cracks
Mechanical property testing	Assess material properties against specification	Properties generally met specification requirements

Root Cause Analysis

Cold Forming Process and Crack Initiation

The tee fitting was manufactured using a cold forming process, which involves deforming a pipe or tube to create the branch opening without heating. Cold forming is widely used for tee production because it preserves the surface integrity and material properties better than hot forming. However, it also introduces significant plastic deformation that can lead to cracking if not properly controlled.

During cold forming, the material at the branch intersection undergoes severe plastic strain. The strain state is complex, involving both stretching and bending. The outer surface of the branch experiences tensile strain, while the inner surface experiences compressive strain. However, due to the geometry of the forming process, strain localization can occur at specific locations, leading to excessive strain concentrations that exceed the material's ductility limit.

The following factors contribute to crack initiation during cold forming:

Excessive strain concentration: If the forming process is not optimized, localized strain can exceed the uniform elongation of the material.
Low temperature forming: Cold forming at low temperatures reduces material ductility and increases cracking susceptibility.
Material anisotropy: The forming process can induce directional properties that reduce crack resistance in certain orientations.
Surface defects: Pre-existing surface defects such as scratches, inclusions, or laps can act as crack initiation sites during forming.

Crack Propagation During Service

Once cracks are initiated during the forming process, they may remain dormant until the fitting is placed into service. In an oil pipeline, the tee is subjected to:

Internal pressure (hydrostatic stress)
Thermal cycling (thermal stresses)
Mechanical loading (pipe weight, support reactions)
Possible corrosion (if the oil contains corrosive species)

The combination of these stresses can cause the pre-existing cracks to propagate. The fracture surface examination revealed a ductile fracture mode, indicating that the material had adequate toughness to arrest crack growth under certain conditions. However, the pre-existing cracks reduced the effective cross-sectional area and created stress concentrations that accelerated the failure process.

Process Control and Prevention

Critical Process Parameters for Cold Forming

Parameter	Recommended Range	Control Method
Forming temperature	Ambient to 50°C (for carbon steel)	Temperature monitoring and control
Strain rate	Low to moderate	Forming speed control
Die geometry	Optimized for uniform strain distribution	CAD/CAM design and validation
Material ductility	Elongation ≥ 30% (minimum)	Pre-forming material testing
Surface preparation	Clean, defect-free surface	Visual inspection and surface treatment

Preventive Measures

Based on the findings of this investigation, the following preventive measures should be implemented:

Pre-forming inspection: All raw materials should be inspected for surface defects and material properties before forming.
Process optimization: The forming process should be optimized through trial forming and strain analysis to ensure that strain concentrations remain within acceptable limits.
Post-forming inspection: All formed tees should undergo non-destructive testing (NDT) such as magnetic particle testing (MT) or dye penetrant testing (PT) to detect surface cracks.
Heat treatment: For materials with limited ductility, a post-forming stress relief or annealing treatment should be considered.
Material selection: Materials with higher ductility and formability should be selected for cold forming applications.

Engineering Practice Implications

This case study underscores several important lessons for engineering practice:

Manufacturing quality is critical: Even if the material meets specification requirements, improper manufacturing processes can introduce defects that lead to premature failure.
NDT is essential: Non-destructive testing of formed fittings should be mandatory, not optional. Surface cracks that are invisible to the naked eye can be detected by MT or PT.
Root cause analysis is valuable: A thorough investigation of failures provides valuable data for process improvement and can prevent similar failures in the future.
Traceability is important: Maintaining traceability of material, manufacturing process, and inspection records is essential for effective failure analysis.

Key Reflections

The most striking aspect of this case study is that the failure was not caused by a material defect or a design error, but by a manufacturing process issue. This highlights the importance of process control in the production of pipe fittings. The cold forming process, while economical and widely used, requires careful attention to detail to avoid introducing cracks. Engineers involved in the design and specification of pipe fittings must ensure that the manufacturing processes are suitable for the material and the application.

The investigation also demonstrates the value of a systematic approach to failure analysis. By combining multiple examination techniques, the investigators were able to establish a clear timeline of events: crack initiation during forming, followed by crack propagation during service. This kind of detailed analysis is essential for developing effective corrective actions.

Conclusion

The crack in the oil pipeline tee was caused by pre-existing surface cracks that originated during the cold forming process. The investigation demonstrated that a systematic approach combining macroscopic, microstructural, fracture surface, and mechanical property analysis can effectively identify the root cause of fitting failures. The key preventive measures include pre-forming material inspection, process optimization, post-forming NDT, and appropriate heat treatment. This case study serves as a reminder that manufacturing quality is as important as material quality and design adequacy in ensuring the reliability of pipe fittings.