How to solve the uneven wall thickness of stainless steel welded pipes?

In the production and application of stainless steel welded pipes, uneven wall thickness is a prevalent and critical issue. It manifests in several forms:

- Spiral wall thickness variation

The wall thickness changes in a spiral pattern along the pipe length, resembling the texture of a screw.

- Linear wall thickness variation

The thickness fluctuates along a straight section of the pipe, leading to inconsistencies.

- Head and tail thickness variation

The pipe’s ends have significantly different thicknesses compared to the middle section, either thicker or thinner.

Impact of Uneven Wall Thickness

Uneven wall thickness negatively affects both quality and functionality:

Structural Stability: It disrupts the uniform performance of the pipe, compromising strength and durability.

Pressure Systems: In applications like water or gas pipelines, thinner sections may fail under normal pressure, leading to deformation, leaks, or rupture.

Construction Support: Pipes with inconsistent thickness have reduced load-bearing capacity, which can compromise structural safety and increase the risk of accidents.

Causes of Uneven Wall Thickness in Stainless Steel Welded Pipes

The issue arises from multiple factors, mainly related to equipment calibration, process control, and material properties.

1. Equipment and Process Factors

Misaligned rolling centerline: If the piercing machine is not aligned properly, the tube experiences uneven force, leading to spiral thickness variation.

Unequal roller inclination angles: Different angles exert varying rolling forces on each side of the pipe, creating an uneven thickness distribution.

Insufficient plug pressure: If the plug’s front pressure is too low, the tube billet does not deform uniformly, contributing to thickness inconsistencies.

Improper centering roller settings: Premature opening or instability of the centering roller can cause unsteady tube movement, resulting in spiral thickness variation.

Mandrel positioning errors: Incorrect mandrel height or misalignment affects metal flow, leading to linear thickness variations or concave defects.

Inconsistent rolling pressure: Incorrect roll gaps or misalignment of the rolling mill affects force distribution, further exacerbating thickness irregularities.

2. Raw Material Factors

Tube billet imperfections: Excessive cutting angles, curvature, or off-center holes make the billet behave unpredictably during perforation, leading to uneven wall thickness at the pipe ends.

High elongation coefficient & roller speed: Excessive elongation or unstable perforation causes head and tail thickness inconsistencies.

Perforation instability: If the perforator operates unevenly, thickness irregularities are more likely at the tail end of the pipe.

Five Key Measures to Improve Wall Thickness Uniformity

1. Precision Equipment Calibration

Align the rolling centerline of the piercing machine to ensure roller inclination consistency (error ≤ ±0.5°).

Optimize the centering roller control to match pipe diameter changes dynamically (accuracy ±0.1mm).

2. Process Parameter Optimization

Control plug front pressure reduction at 30%-40% of wall thickness to ensure proper metal deformation.

Implement a gradual pressure reduction across rolling frames (first frame 35% → last frame 15%), adjusting roll gaps in real time based on online thickness measurements.

3. Raw Material Quality Control

Limit billet front-end cutting inclination to ≤1.5mm/m, curvature to ≤L/1000, and centering hole eccentricity to ≤0.5mm.

Conduct full-size thickness measurement of raw materials upon arrival (accuracy ±0.05mm), rejecting out-of-spec batches.

4. Strict Operational Standards

Regular ejector rod inspections and lubrication every 2 hours to minimize vibration (amplitude < 0.5g).

Maintain welding current/voltage fluctuation within ±5% to ensure even heat input.

5. Comprehensive Inspection and Feedback System

Deploy 8 ultrasonic thickness gauges to inspect 100% of the pipe wall (accuracy ±0.02mm).

Establish a defect database, using SPC (Statistical Process Control) to analyze trends and trace deviations in production.

Conclusion

Controlling wall thickness uniformity in stainless steel welded pipes is essential for ensuring structural integrity, safety, and performance. By optimizing equipment calibration, process parameters, material quality, and inspection protocols, manufacturers can significantly reduce defects and improve overall product reliability.