A single welding defect can result in a cascade of failures that lead to disaster. That’s why responsible fabrication shops strive for the highest possible weld integrity. It takes time and adds cost, but it’s the only way if you want to be sure a weld won’t let you down.
Welding is a complex process, and many things can create defects in the weld. A small variance from the weld procedure specification (WPS) can result in porosity, inclusions, incomplete fusion, or other defects that drastically reduce joint strength and lead to premature failure.
This blog identifies the most common types of weld defects and describes how we go about preventing them. Read to the end and you’ll get a good sense of what’s involved in making high-quality welds.
There are two problems to consider: flaws in the melted metal that result in weakness, and distortion of the welded fabrication.
If distortion takes key dimensions outside their tolerance range the part probably isn’t usable, which at least avoids the risk of in-service failure. A bigger concern is when distortion causes cracking. That reduces strength and can cause failure.
Flaws in the melted metal take many forms and we’ll spend more time on these. The main types of flaws are
- Porosity: Porosity is gas bubbles in the weld metal. These may be on the surface but can also be underneath and therefore hidden from view.
- Inclusions: These are particles of foreign material that either didn’t melt or solidified at a different temperature to create locations with very different properties to the rest of the weld.
- Incomplete fusion: This occurs when the parent metal isn’t heated enough to melt and contribute to the weld pool. Instead, molten filler metal has taken up the space in the joint, but hasn’t fused with the parent pieces.
- Insufficient depth: If it’s going to be as strong as the parent pieces, the weld must go all the way through. If it goes only part of the depth the weld almost certainly won’t be strong enough.
- Undercuts: A good weld has a domed region over the join where filler metal was added. To create a smooth appearance this may be ground down level with the parent pieces. An undercut is when the edges of the weld are below the surface of the parent pieces. This won’t clean up and can leave regions where corrosion can begin.
- Overlaps: This is when the top of the weld – the domed region – spreads out over the surface of the parent metal. (In the section it looks like a rivet head.) This also creates a place for corrosion to start.
Causes and Prevention of Weld Defects
A welding defect is usually caused by one or more of:
- Poor preparation: Achieving good fusion is largely a matter of getting the correct gap and angles. Cleaning is also very important.
- Poor fit-up: If components aren’t positioned and clamped appropriately the assembly will distort as it cools and welds might crack.
- Poor welding technique: Moving the torch too quickly or tilting it can result in slag inclusions and/or poor fusion. Having the arc current too low is another contributor.
- Contaminated material: Moisture or grease on the surfaces being welded can release hydrogen that causes porosity.
- Wrong filler or electrode: Flux-cored wire should be compatible with the metals being welded. Using the wrong wire can cause inclusions.
- Inadequate gas shielding: Allows air into the weld pool which causes porosity.
Welding defect prevention starts with planning. Determine the appropriate filler or electrode to use, gaps required, and welding parameters. These should all go into a WPS.
Preparation comes next. Pieces being welded must be cleaned and dried thoroughly. Faces must be cut or ground to the optimal angle and fit up should hold pieces with the correct gap between them.
Finally, weld parameters should be set correctly for the weld. It’s often useful to make a test weld if the cost of the job permits. (On very large and high-value projects this isn’t always practical.) Then perform a thorough inspection to verify weld integrity.
The three methods for verifying weld quality are visual inspection, non-destructive evaluation, and destructive testing and analysis.
Visual inspection can pick up undercuts, overlaps, cracks, and surface-breaking porosity. It cannot reveal poor fusion or sub-surface porosity. Liquid penetrant testing is a non-destructive technique that highlights surface defects the human eye might otherwise miss. For defects below the surface eddy current, ultrasound and X-ray imaging are all possible. Their limitations are that they need skilled technicians and tend to be slow and costly.
On critical welds, sectioning is a destructive approach that provides additional assurance the welding process will yield good results. Bending and tensile testing are also effective, although as with sectioning, these only verify process parameters and not individual welds.
The Importance of Training and Certification
As the discussion here has shown, there’s a lot that can go wrong when welding, so it’s essential the welder knows their craft. This is why welders go through extensive training, (a mix of classroom and on-the-job), followed by certification. At Wiley, our welders are certified to American Welding Society (AWS) standards by the Hobart Institute of Welding Technology, so you can be confident of their ability to deliver high-quality work.
We’re also passionate about life-long learning, so we push our welders to keep their skills current. This is especially important as welding technology continues to advance and new features and capabilities appear on the equipment.
Welding Done Right
When you commission a welded fabrication you expect the weld is going to hold up. That’s something we sign up to deliver when we take on the job, and we understand just how important it can be for durability, and most importantly, safety.
As we’ve described here, we follow best practices and go to great lengths to eliminate the possibility of a single welding defect leaving our shop. It’s not the fastest or the cheapest approach, but when safety matters we’re not prepared to cut corners.
If you have a fabrication project that needs dependable, high-quality welds, we’d like to help. Contact us for a discussion or quote.