Porosity in Welding: What it Is, Causes & Remedies
Last Updated on
You’ve got your hood down. You’re welding a groove weld nice and smooth. Everything is looking consistent. You’re in the zone.
Then the weld puddle starts sputtering a little bit. You think, “It’s just some extra mill scale burning off of the edge of the plate. No big deal, right?” You continue welding. Then you lift up your hood to admire your work. You knock off the slag, anticipating the luster of a perfectly placed weld. But your heart sinks. You’ve got porosity.
Now you’re kicking yourself for not paying attention to the irregularities of the arc. What would have been an easy fix had you stopped welding when you noticed the sputtering is now going to take you an hour to fix. It’s easy to lay weld in a joint. But to take a bad weld out, that’s a whole different story.
Undercut, an occasional slag inclusion, or cold lap, these defects are bad enough. But they can be repaired without removing a whole lot of the original weld. But porosity is another story. If you have a lot of it, sometimes it requires moving the entire weld.
In short, porosity is where gases, most often oxygen or atmospheric gases, get trapped inside of the weld during cooling. This can occur for several reasons which we will cover in this article. We will also cover how to identify it and fix it.
Improper Gas Coverage
One of the most common causes of porosity is little to no gas coverage. Of course, this is assuming that you are using a welding process that utilizes shielding gas such as MIG or Dual Shield Flux-cored welding. The primary purpose of a shielding gas is to shield the weld pool from outside contaminants, including atmospheric gases that can get inside the weld. The shielding gas pushes the atmospheric gases away and protects the molten weld pool.
Presence of Moisture
Moisture, as a major cause of porosity, is usually more of a problem when working outside in inclement weather. But a slightly damp piece of steel is not going to ruin your whole weld. The heat from the arc can usually dry that moisture right up without any major problems.\
Not all the steel that you get comes perfectly clean. Often the case is that the supplier had it sitting outside in the yard collecting rainwater, bird droppings, grease, etc. When that piece of steel is cut and fit-up, you might assume that it’s ready to weld. After you weld, you notice a lot of irregular porosity.\
Too Long of an Arc
This often works in combination with poor gas coverage, especially if you are welding MIG or FCAW. If you are welding in a tough-to-reach spot, or you are welding a groove weld between thick plate where your welding gun cannot reach well, sometimes the only way to get the job done is to long-arc it.
Frequently Asked Questions
Can you weld over porosity?
In general, it is not a good idea to weld over porosity. If it runs deep, then it should be ground out and rewelded. However, there are instances where maybe the weld isn’t too critical and has a couple of pinholes. Most structural welding codes have an allowance for a certain number of pinholes for a specific length. It’s always good to check with the CWI (Certified Welding Inspector) to see if you can’t just trigger a couple of pinholes.
What causes wormhole porosity?
Wormhole porosity is a defect where there are indentations on the surface of the weld. This usually happens with FCAW when there is nitrogen trying to escape the weld as it is solidifying. The result is an elongated divot in the weld, often accompanied by pinholes. This is usually caused by surface contamination of the base metal. You may also be repairing a weldment that has already been painted. If you don’t grind off or burn off enough paint before welding, that paint will find its way into your weld and can cause wormhole porosity.
What is a crater hole in welding?
A crater hole can be classified as a type of porosity. This happens if you are welding a bead and taper off too quickly as you are finishing. The result is that the weld pool does not get filled in completely as the weld is being finished. Sometimes there is a small to a medium-sized hole in this crater. The solution to this is simple. As you are finishing the weld, hold the arc at your stopping point, drag it 1/8” to ¼” back over the weld, and return to the very end. This will help round out the ending point of your weld.
By understanding the causes of porosity in welding, you should know how to avoid most of it. If your machine is set right, you have enough gas coverage, and your material is clean, then you’ve already won half the battle. Just remember that is far more efficient (and less frustrating) to stop welding to check if it’s turning out alright than it is to have to grind out 16 inches of bad weld, especially if it’s in a tight spot. Knowing how a good weld feels can give you a baseline. This will help you judge if something “just doesn’t feel right.”
Featured image credit: Praphan Jampala, Shutterstock