Analysis of Internal Folding Defects in Microalloyed Non-Quenched Seamless Steel Pipes

Literature Overview and Defect Description

The paper by Fang Jian and Yuan Zexi (2007, Physical Testing and Chemical Analysis - Physical Methods, Vol. 43, No. 7, pp. 325-327), authored by researchers from the Hubei Key Laboratory of High Temperature Ceramics and Refractories at Wuhan University of Science and Technology, presents a metallurgical analysis of internal folding defects observed in N80 grade microalloyed non-quenched seamless steel pipes during production. The defect was investigated using metallographic examination, chemical composition analysis of inclusions, and scanning electron microscopy (SEM) observation. The root cause was traced to the presence of a large number of inclusions in the centre of the continuous casting billet, which led to internal folding during the piercing and rolling process.

Defect Characterisation and Metallurgical Analysis

Internal folding (also referred to as internal seams or laps) in seamless steel pipes is a serious surface-breaking or near-surface defect that can significantly reduce the fatigue strength and pressure-bearing capacity of the pipe. The defect manifests as a crease or fold in the pipe wall, often with oxide scale trapped between the folded layers. The following table summarises the key findings from the metallurgical investigation.

The N80 grade steel is a microalloyed non-quenched steel designed for oil and gas well casing applications, requiring high yield strength (minimum 552 MPa) and good toughness without post-weld heat treatment. The microalloying elements (typically Ti, Nb, V) are added to achieve precipitation hardening during the controlled rolling and cooling process. However, the same microalloying elements can promote inclusion formation, particularly Ti-containing inclusions if titanium is used for sulphur control.

Root Cause Analysis Using FMEA Approach

Applying a Failure Mode and Effects Analysis (FMEA) framework, the root cause chain for the internal folding defect can be traced as follows:

1. Root cause: Inadequate inclusion control during the steelmaking and continuous casting process, resulting in a high density of inclusions concentrated in the centre of the billet.
2. Contributing factor: The microalloying additions (Ti, Nb, V) promote the formation of oxide and nitride inclusions, particularly if the deoxidation practice is not optimised.
3. Defect initiation: During the piercing process, the billet centre is subjected to severe plastic deformation. The inclusion-rich centreline zone has reduced ductility and cohesive strength, leading to micro-cracking along the inclusion clusters.
4. Defect propagation: During subsequent hot rolling, the micro-cracks are stretched and opened, forming the internal fold. Oxide scale from the billet surface is drawn into the fold during rolling, creating a characteristic oxide-containing seam.

Preventive Measures and Process Improvement

Based on the analysis, the following corrective and preventive measures are recommended:

1. Steel composition optimisation: Control the total sulphur content below 0.015% and use calcium treatment to modify inclusions into spherical, fluidised CAS inclusions that are less likely to align during solidification.
2. Continuous casting optimisation: Adjust the casting parameters (pulling speed, superheat, secondary cooling intensity) to minimise centreline segregation and inclusion accumulation. The use of electromagnetic stirring (EMS) or soft reduction in the secondary cooling zone can help break up the centreline segregation zone.
3. Billet inspection: Implement ultrasonic testing (UT) or electromagnetic acoustic testing (EMAT) of the billets before piercing to detect centreline defects and inclusion clusters.
4. Piercing process control: Optimise the piercing plug geometry, rolling reduction, and temperature to minimise the severity of centreline deformation. A lower piercing reduction per pass can reduce the risk of centreline cracking.
5. Rolling process monitoring: Use inline eddy current or magnetic flux leakage detection to identify surface-breaking folds and reject affected pipe lengths.

Engineering Practice and Quality Control Implications

For seamless steel pipe manufacturers producing microalloyed non-quenched grades for oil and gas applications, the prevention of internal folding defects is critical for both product quality and safety. The API 5CT standard specifies strict requirements for the mechanical properties and NDT acceptance criteria of casing and tubing, and internal folding defects would result in rejection of the affected pipe.

The quality control plan for microalloyed seamless pipe production should include:

• Incoming material control: Verification of billet chemistry (particularly S, P, and microalloying element content) and UT inspection of billets.
• Process parameter control: Documentation and monitoring of piercing plug geometry, rolling temperature, reduction schedule, and cooling rate.
• In-line NDT: Magnetic flux leakage or eddy current inspection of the finished pipe, supplemented by hydrostatic testing for pressure integrity.
• Final inspection: Mechanical property testing (tensile, impact, hardness) and chemical analysis of the finished pipe, with sampling from both the head and tail ends of each heat.

Summary and Conclusions

This case study provides a clear and instructive example of how metallurgical defects in the continuous casting billet can propagate through the seamless pipe manufacturing process to produce internal folding defects in the finished product. The root cause analysis demonstrates the importance of inclusion control in microalloyed steels, where the microalloying elements that provide beneficial precipitation hardening also increase the risk of inclusion formation. The recommended corrective measures focus on upstream process optimisation in steelmaking and continuous casting, which is the most effective strategy for preventing this type of defect. For quality assurance engineers, this case underscores the importance of traceability from billet to finished pipe and the value of comprehensive NDT at each stage of the manufacturing process.