Weld Quality Testing
To ensure the satisfactory performance of a welded structure, the quality of the welds must be determined by adequate testing procedures. Therefore, they are proof-tested under conditions that are the same or more severe than those encountered by the welded structures in the field.
This page contains visual inspection tips. The following pages contain inspection methods for GMAW and physical weld testing.
These tests reveal weak or defective sections that can be corrected before the material is released for use in the field. The tests also determine the proper welding design for ordnance equipment and forestall injury and inconvenience to personnel.
NDT refers to nondestructive testing. It is an approach to testing that involves evaluating the weld without causing damage. It saves time and money including the use of remote visual inspection (RVI), x-rays, ultrasonic testing, and liquid penetration testing.
In most welds, quality is tested based on the function for which it is intended. If you are fixing a part on a machine, if the machine functions properly, then the weld is often considered correct. There are a few ways to tell if a weld is correct:
- Distribution: Weld material is distributed equally between the two materials that were joined.
- Waste: The weld is free of waste materials such as slag. The slag after cooling should peel away from the project. It should be removed easily. In MIG welding, any residue from the shielding gas should also be removed with little problem. TIG, being the cleanest process, should also be waste-free. In Tig, if you see waste, it usually means that the material being welded was not cleaned thoroughly.
- Porosity: The weld surface should not have any irregularities or any porous holes (called porosity). Holes contribute to weakness. If you see holes, it usually indicates that the base metal was dirty or had an oxide coating. If you are using MIG or Tig, porosity indicates that more shielding gas is needed when welding. Porosity in aluminum welds is a key indicator of not using enough gas.
- Tightness: If the joint is not tight, this indicates a weld problem. In oxyacetylene welding, if using autogenous welding, where there is no filler material, the weld must be tight. Same for Tig autogenous welding. The gap is not as critical in other types of welds since any gap is filled in by the filler material. That said, gaps, in general, indicate a potential quality problem.
- Leak-Proof: If you are repairing an item that contains liquid, a leak is a sure-fire way (and obvious way) to see that there is a problem. Same for something that will contain a gas. One testing method is to use soap bubbles to check for problems (can be easily applied with a squirt bottle.
- Strength: Most welds need to demonstrate the required strength. One way to ensure proper strength is to start with a filler metal and electrode rating that is higher than your strength requirement.
Other checks using visual methods include checks before (root face, gap, bevel angle, joint fit), during (electrode consumption rate, metal flow, arc sound, and light), and after welding (undercut, root fusion issue, pinholes, excessive spatter, weld dimensions).
Visual Inspection (VT)
Visual inspection is a non-destructive testing (NDT) weld quality testing process where a weld is examined with the eye to determine surface discontinuities. It is the most common method of weld quality testing.
Advantages of nondestructive weld quality testing:
- Inexpensive (usually only labor expense)
- Low-cost equipment
- No power requirement
- Quick identification of defects and downstream repair costs due to issues that weren’t caught early
Disadvantages:
- Inspector training necessary
- Good eyesight required or eyesight corrected to 20/40
- Can miss internal defects
- Report must be recorded by inspector
- Open to human error
Visual Weld Quality Testing Steps
Practice and develop procedures for consistent application of the approach
- Inspect materials before welding
- Weld quality testing when welding
- Inspection when weld is complete
- Mark problems and repair the weld
Visual Inspection During Welding
- Check electrodes for size, type and storage (low hydrogen electrodes are kept in a stabilizing oven)
- Watch root pass for susceptibility to cracking
- Inspect each weld pass. Look for undercut and required contour. Ensure the weld is cleaned properly between each pass.
- Check for craters that need to be filled
- Check weld sequence and size. Gauges are used to check size.
Inspection After Welding
- Check weld against code and standards
- Check size with gauges and prints
- Check finish and contour
- Check for cracks against standards
- Look for overlap
- Check undercut
- Determine if spatter is at acceptable levels
How to know if it’s a Weld Discontinuity or a Welding Defect
A weld discontinuity is easy to recognize because it looks like an interruption of normal flow. It is also known as a weld imperfection, and it can be found in the weld metal or the parent metal. A discontinuity happens in the weld metal due to the wrong welding pattern or welding technique. It can differ from the weld bead shape and thickness and ultimately, the quality.
In general, a discontinuity should be avoided and dealt with, but they are marginally less serious than a welding defect. Nonetheless, if you do find a weld discontinuity, you need to fix it.
That said, a group of weld discontinuities can become a weld defect, if they exceed the limits stated in your project. This goes in accordance with your country, the material, and the type of environment you are in.
Ultimately, the most effective way to inspect a welding job, especially in tight spaces, is using a welding camera. Here are some video examples of how that works.
Identifying Welding Defects Before It’s Too Late
It is impossible to overstate the importance of catching welding defects in time. Even the slightest defect can be catastrophic. From taking care of the welding technique, down to the welding current and supervision, welding is a complex craft that requires the utmost attention.
Investing in the right tools will make it easy and safe to get the job done. It is crucial for companies in the oil, gas, and mining industries to understand how to identify the most common welding defects, so they can act.