Ensuring weld quality
- August 26, 2012
Welding inspectors use many tools and processes to ensure weld quality
As anyone in the fabricating and welding business knows, process and part inspection is a critical component of a successful business.
Build and weld a poor part simply guarantees failure.
To ensure the quality and integrity of welds, countries have established standards and inspection. In Canada, the Canadian Standards Association (CSA) publishes the “W” series of standards to address welding. To ensure standards are being met requires inspection by qualified and certified welding inspectors and institutions. In Canada, the Canadian Welding Bureau offers welding inspection certification courses.
The following is a guide on some basic tools and processes for weld inspections.
“Welding inspection can often require a wide variety of knowledge on the part of the welding inspector,” notes ESAB’s Tony Anderson in an online article on the company’s web site. “The understanding of welding drawings, welding symbols, weld joint design, welding procedures, code and standard requirements and inspection and testing techniques, to name a few. For this reason many welding codes and standards require that the welding inspector be formally qualified or have the necessary knowledge and experience to conduct the inspection services.”
To begin with, recognize that there are different techniques for welding inspection. They include visual, comparison, radiographic and ultrasonic, surface crack detection (also know as dye penetrant inspection and magnetic particle inspection), pressurized water and gas, and destructive methods.
The most common technique—and least expensive, if done correctly—is visual. Welders look for cracks, poor fusion, overlap, melted edges and other inconsistencies that are visual indicators of a poor weld.
To ensure inspection is conducted by competent individuals, there are standards for welding inspectors and welding inspection companies. Standards for welding inspectors determine requirements for experience, training, and examination to ensure that individuals can demonstrate the required knowledge and skills. In Canada, the CSA W178 series of standards address these requirements: W178.1 for welding inspection companies and W178.2 for individual welding inspectors.
Another inexpensive technique is a simple comparison of the welding inspection job in question to other similar welding jobs. The welding inspector can ask the welders how they created the structure to get a sense of whether they are following the proper procedures for welding, according to ESAB Welding & Cutting. The inspector can also compare the welding job to the blueprint supplied to the welder.
Radiographic and ultrasonic techniques are two of the most common methods of non-destructive testing. The obvious advantage of non-destructive testing is that welders can detect weld problems without having to destroy the welded part. According to ESAB, radiographic testing—penetrating radiation passes through a solid object onto a photographic film—provides a “permanent record of weld quality that is relatively easy to interpret by trained personnel.” It is a slow and expensive method, but is a good technique for detecting porosity, inclusions, cracks and voids in weld interiors.
Ultrasonic testing—mechanical vibrations via a beam of ultrasonic energy are directed into the object—gives welders the ability to determine the exact position of a weld problem.
Liquid dye penetrant testing is a common non-destruction method of detecting cracks in welds. The process works by applying a penetrating liquid to the weld surface and allowing it to soak for a set period of time and then removing excess liquid to check for cracks on the surface. A note of warning: this method is useful only for detecting cracks on the surface; it won’t detect any cracks or weld problems (porosity or fusion defects) sealed in the body of the weld.
To detect a broader range of weld inconsistencies, magnetic particle testing is a better choice. It can detect cracks, as well as porosity, seams, inclusions and poor fusion. Only ferromagnetic materials can be testing with this method.
Pressurized water or gas inspection techniques are used for welded structures designed to transport liquid, such as piping. In this method, pressurized water or nitrogen is sent through the pipe to detect the development of any leaks.
Destructive welding inspection techniques involve the physical destruction of the welded part to evaluate the part. Applications include welding procedure qualification and welder performance qualification testing, and failure analysis work. Inspection techniques usually involve sectioning or breaking the welded component to evaluate mechanical and physical characteristics, according to ESAB. SMT