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 will simply guarantee failure.
Today, welding associations, such as the Canadian Welding Bureau, offer certification courses to help welders learn the ins and outs of proper inspection processes, but before taking the course, you should understand some of the basics in welding inspection.
The following is a guide, gleaned from industry experts and industry-related online sources.
“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 many 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.
Another relatively 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 is passed 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 problem in the weld. While a good method, ESAB advises that welders using this process have a high level of operator training and competence. The method is good for ferrous and non-ferrous materials.
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 though, 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, the magnetic particle testing is a better choice. It can detect cracks, as well as porosity, seams, inclusions and poor fusion. This method works by establishing a magnetic field in the part to be test – a magnet attracts the magnetic particles to its ends and the magnetic lines flow between the poles of the magnet. If the part being test has no cracks or other weld problems, magnetic particles will not be attracted. 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.
As the name implies, destructive welding inspection techniques involves the physical destruction of the welded part to evaluate the part. Applications include welding procedure qualification and welder performance qualification testing, sampling inspection of production welds, research inspection and failure analysis work. Inspection techniques usually involve sectioning or breaking the welded component to evaluate mechanical and physical characteristics, according to ESAB.
Information based on online research from:
