Welded Pipe Weld Seam Inspection

I. The Importance of Weld Seam Inspection
The weld seam is the area where the edges of the pipe billet are joined by fusion welding. During the welding process, thermal stress, metallurgical reactions, and operational factors can easily lead to various defects, such as porosity, slag inclusions, incomplete penetration, lack of fusion, and cracks. These defects act like hidden mines within the pipe wall. Under pressure, corrosion, or alternating loads, they can become the source of crack initiation and propagation, ultimately causing pipe leaks or even bursts, resulting in significant economic losses, environmental damage, and personal injury. Therefore, comprehensive quality inspection of weld seams on welded pipes has the following core objectives:

Ensuring structural integrity: Ensuring that the weld seam can withstand the internal pressures and external loads required by design.

Preventing premature failure: Promptly identifying and removing products with serious defects to prevent accidents during service.

Assessing process stability: Using inspection results as feedback, we optimize welding process parameters and improve production quality consistency.

Meeting regulatory standards: Complying with national, industry, and customer-specific quality regulations and certification requirements.

II. Main Inspection Methods and Their Applications
There are numerous methods for inspecting weld seams in welded pipes. These methods can be categorized into two main types: nondestructive testing and destructive testing, depending on whether they damage the workpiece.

(I) Nondestructive Testing

Nondestructive testing (NDT) is a method for detecting internal or surface defects without damaging or affecting the performance of the object being inspected. It is the mainstream method for inspecting weld seams in welded pipes.

Appearance Inspection

Overview: This is the most basic and intuitive inspection method, usually the first step in inspection.

Method: Rely on the naked eye or with the aid of tools such as a low-power magnifying glass or weld inspection ruler.

Inspection Content: Checks for uniform and full weld formation; the presence of external defects such as surface cracks, undercuts, weld bumps, arc pits, and spatter; and measures weld height reinforcement, width, and other dimensions to ensure they meet standards.

Features: Simple, fast, and low-cost, but only detects surface defects.

Radiographic Testing

Overview: X-rays or gamma rays are used to penetrate the workpiece. Due to the different absorption capacities of defective and intact areas, images are formed on film or digital imaging plates, which are then used to identify internal defects.

Methods: Primarily include film radiography and digital radiography.

Inspection Content: Highly sensitive to volumetric defects (such as pores and slag inclusions), it can also effectively detect area defects such as incomplete penetration and lack of fusion.

Features: Intuitive results and traceable imaging records; however, the detection rate for directional defects such as cracks is affected by the angle, and there are radiation safety issues and high costs.

Ultrasonic Testing

Overview: High-frequency sound waves propagate through the workpiece. When they encounter defects, they are reflected. The reflected waves are analyzed to locate and evaluate the defects.

Methods: The pulse reflection method is often used, with the probe moving along the weld for scanning.

Inspection Content: Highly sensitive to area defects (such as cracks and lack of fusion), it has a large detection depth and can accurately determine the depth and height of the defect.

Features: Requires high workpiece surface requirements and requires a coupling agent; results are not intuitive and highly dependent on the operator's skill level and experience; fast inspection, radiation-free, and suitable for thick-walled pipes and automated in-line inspection.

Magnetic Particle Testing

Overview: Magnetizes the surface or near-surface of ferromagnetic welded pipes. Defects generate a leakage magnetic field that attracts the applied magnetic particles, resulting in visible magnetic traces.

Methods: Includes continuous and residual magnetization methods.

Testing Scope: Specifically designed for detecting surface and near-surface linear defects, such as cracks.

Features: High sensitivity and simple operation; only suitable for ferromagnetic materials and has certain requirements for workpiece shape.

Penetrant Testing

Overview: A penetrant containing a dye is applied to the workpiece surface and allowed to penetrate into open surface defects. After removing any excess penetrant, a developer is applied to draw out the penetrant from the defect, creating a magnified trace.

Methods: Available as dye penetrant and fluorescent penetrant.

Testing Scope: Used to detect open surface defects in non-porous materials.

Features: Simple equipment, easy operation, and suitable for a variety of metal materials; however, it cannot detect non-opening defects or near-surface defects.

(II) Destructive Testing

Destructive testing involves cutting specimens from a production batch of welded pipe or test plates for mechanical or metallographic analysis to verify their properties.

Mechanical Property Testing

Tensile Testing: Determines the strength and ductility of the weld.

Bend Testing: Includes face, back, and side bends to test the ductility and surface quality of the weld.

Impact Testing: Determines the toughness of the weld under impact loads, which is particularly important in low-temperature environments.

Metallographic Testing

Overview: A cross-section of the weld is cut, ground, polished, and etched, and its microstructure is observed under a microscope.

Testing Content: Analyzes the microstructure of the weld, heat-affected zone, and base metal; inspects for internal defects such as lack of fusion, cracks, and slag inclusions; and measures penetration and weld dimensions.

Features: Accurate results make it an important tool for analyzing welding quality and process rationality, but it is destructive.

Hardness Test

Measures the hardness values of different areas on a weld cross-section to assess the hardening or softening tendency of the weld joint and determine its resistance to stress corrosion and fatigue.