Quality Control Essentials: A Guide to Testing ERW Pipe Welds

Introduction: The Critical Importance of the Weld Seam

Electric resistance welded pipe (ERW) is a foundation product in many pipeline and structural applications due to its economic nature and precision. ERW pipe does have a critical characteristic that will ultimately determine its performance as a whole: the weld seam. The ERW pipe's longitudinal weld seam is formed with the aid of high-frequency induction heating. The high-frequency induction heating and pressure is the principal aspect of the weld seam integrity. Therefore, the reliability of the pipe as a whole depends on the integrity of the weld seam.

Given this, ERW pipe manufacturers conduct a very robust multi-layered suite of quality control tests so every length of ERW pipe can be guaranteed to safely contain their operational designed pressures. This guide is intended as an engineer's guide to the various non-destructive and destructive tests that enhance the level of confidence of the integrity of welds on ERW pipe.

I. Non-Destructive Testing (NDT): Inspecting Every Inch

NDT processes are the forefront of quality control, allowing for 100% inspection of the weld seam, without the pipe being damaged.

• Ultrasonic Testing (UT): This is the main NDT tool that is used for ERW welds. High frequency sound waves are directed at the weld zone, and analyzing the returning echoes specialized sensors can find internal flaws, such as lack of fusion, inclusions, or voids that cannot be detected visually. This method provides a cross section of the weld’s internal quality.
• Eddy Current Testing (ECT): An alternating magnetic field is induced in the weld area of the pipe. Any surface or near-surface discontinuities such as crack or gouges will disrupt the flow of the resultant eddy currents. These disruptions will be detected by the testing equipment, hence indicating defects. ECT is a useful method of making detections of surface level issues.
• Hydrostatic (water) Testing: Each length of pipe shall be filled with water and pressurized to a level quantified considerably higher than the pipes intended service pressure, as defined by applicable standards such as API 5L. The pipe is maintained at that pressure for a specified period of time (Eg: 5-10 seconds). The test will serve as a practical proof-of-strength, indicating that the pipe, including any individual weld seam, can sustain the demands of its operational stresses; without leaking and rupturing.
• Hydrostatic Test Pressure Calculation: The pressure obtained by the hydrostatic test is not a random figure, but calculated based on the specifications of the pipe. A simplified version of such a calculation, based on Barlow’s formula, and sometimes specified within standards such as API 5L, would be;

• P= Minimum Hydrostatic Test Pressure (psi)
• S= Stress, which is a specified percentage of the pipe's SMYS (e.g., 60% to 85% of SMYS)
• t= Nominal Wall Thickness (inches)
• D= Outside Diameter (inches)

This calculation ensures the test is both meaningful and safe, verifying the weld's integrity under a quantifiable stress level.

II. Destructive Testing: Validating the Process

Where NDT inspects the final product, destructive testing is performed on samples removed from the production run to establish the welding process parameters and mechanical properties of the material.

• Flattening Test: A ring-type sample of the pipe is placed between two platens that are moved toward each other until the sample reaches a specified height. The weld is placed in the maximum stress location. The sample is inspected for cracks or breaks. A successfully passed test verifies that the weld has good ductility and can easily deform without breaking.
• Tensile Test: A specimen is machined from the weld area, and the specimen is pulled apart in a tensile testing machine. The ultimate tensile strength is derived from this test for the weld. As per standards like API 5L, the weld needs to have the same or greater ultimate tensile strength as the minimum specified tensile strength of the pipe body to ensure that the weld does not fail before the pipe body.
• Bend test: The bend test is performed on a sample that bends the weld 180 degrees around a mandrel. The weld area is then rigorously inspected for cracking to verify the weld can take bending stresses during installation and service.

III. The Role of Standards and Certification

These tests are not random, and instead are rigidly prescribed by industry standards.  Standards like API 5L or ASTM A53 specify:

• Which tests are required.
• How to perform each test.
• Acceptance criteria for passing or failing.

Each of these tests are officially recorded on a Mill Test Certificate (MTC), which provides evidence for the end-user that the ERW pipe satisfies quality and safety standards in the planned application.

Conclusion: A System of Assurances

The dependability of modern ERW pipe is not a matter of luck, but rather a result of an integrated and systematic quality control procedure. Through a combination of 100% non-destructive inspection, quantifiable pressure testing and meticulous destructive testing, the weld seam is validated in a high confidence multi-layered methodology. This framework is justified in used for ERW pipes in everything from low pressure waterlines to heavy-duty structural and energy pipelines.