Porosity can happen to even the best welders. That moment you see little pits in the surface of your weld or on an x-ray can be frustrating, to say the least. These small little defects can cause big problems compromising the integrity of your weld.
There are two times to fix porosity. After it has happened, the only reliable way to fix it is to remove the affected weld and start over. Fixing porosity before it happens is more complicated and can involve:
- gas flow
- workplace conditions
In this article, we’ll look at the best ways to fix porosity after it has happened and the best practices to keep porosity from happening again after we learn a little bit about what causes porosity.
How to Fix Porosity After It Has Happened
Once you have identified porosity in your weld, there is nothing for it but to remove it and start over. The question is, how?
The most basic solution is to grind off the areas of your weld that have porosity and reweld them. In a short and concise answer letter to a reader’s question, Professor Hammer of Hot Rod suggests using an abrasive cut-off disc to make a tiny slice in the weld.
However, it’s generally not a good idea to weld a gap that is larger than the material’s thickness. If the porosity covers a larger area, a broader approach may need to be taken. Consider cutting out a circle of the affected area with a step drill and welding a sheet metal plug in its place.
But if the diameter is larger than ¾ inch, consider removing the area in the shape of a rectangle using an abrasive die grinder. Then you can weld a patch into place. Just bear in mind that it’s hard to weld a strip of metal that is ½ inch or less because it melts easily.
Another option is to use a stainless steel rod to fill a small pinhole of porosity. However, don’t do this if you have to file down your weld. Stainless steel is tough. It can be cut, but it won’t file down.
One thing you should never do is try to cover up porosity by welding over a bad weld. This only disguises the problem. It doesn’t fix it. And if your weld is structurally integral, you could be covering up a weakness that should be properly remedied.
The best way to approach porosity in welding is to try to prevent it, “fix it” before it happens. The things that we will discuss now are not just helpful as prevention, but as ways to troubleshoot the problem if you have had to cut out a bad weld and start over.
But first, you’ll need to know a little bit about what porosity is and what causes it before we take a look at addressing some of the porosity’s key contributors.
What Is Porosity?
Simply put, porosity occurs when nitrogen, oxygen, and hydrogen get absorbed into the weld pool and become trapped when the weld solidifies. These trapped gasses create cavities or pits that weaken the weld.
Nitrogen and oxygen often seep into the weld pool due to poor gas shielding that can result from a faulty gas line, too high of a flow rate, or an outside draught. Hydrogen usually comes from moisture or grease and oil at a weld site that hasn’t been properly cleaned.
Any one of these things can cause one of four different kinds of porosity:
- Distributed – This kind of porosity exists inside the weld and can only be revealed through an x-ray machine.
- Surface breaking pores – Like the name, surface-breaking pores can be detected as pits or cavities on the surface of a bead.
- Wormholes – These are long holes the create a herringbone look on an x-ray.
- Crater pipe – This kind of porosity forms during the final solidification of the weld.
What Causes Porosity?
The causes of porosity are many, but they can generally be broken down into two categories: gas flow and dirty welding surface.
Whatever is on the surface of your weld will go on to affect the weld itself. These items can include general dirt or grease, moisture, and thick paint or primer coatings.
Gas flow can cause porosity if the setting is either too high or too low. If it is too high, the shielding gas can get trapped in the pool. If it is too low, gas doesn’t cover the pool well enough and allows atmospheric gasses to intrude. Unfortunately, there is no “just right” setting that is appropriate for all situations and materials.
TWI points out that some materials are more susceptible to porosity with certain kinds of gas. This how they break it down along with recommended cleaning methods:
|C-Mn Steel||Hydrogen, Nitrogen, and Oxygen||Grind to remove scale coatings|
|Stainless Steel||Hydrogen||Degrease + wire brush + degrease|
|Aluminum and alloys||Hydrogen||Chemical clean + wire brush + degrease + scrape|
|Cooper and alloys||Hydrogen, Nitrogen||Degrease + wire brush + degrease|
|Nickel and alloys||Nitrogen||Degrease + wire brush + degrease|
How to Fix Porosity Before it Starts
Now that we know a little bit more about porosity and what causes it let’s take a look at some basic ways to prevent porosity. Just as the causes generally fall into two categories, so do the solutions.
It is important that you make sure your metals, tools, and working areas are clean before you begin welding.
Check the metals you are working with. Be sure they are clean of moisture, grease, dirt, and rust. You also want to pay close attention to primer and paint that may be on your metal.
Thick layers of paint can allow impurities into the weld pool, so you want to remove any coatings that expose the bright metal. Check metals that have a manufacturer’s primer on them to be sure that the primer is not too thick. The primer on a plate should not be thicker than 20µm.
Also, be mindful that some non-primed metal strips can come with a coating to prevent oxidation during storage. You will want to scrub this off as well.
Cleaning includes not just the metal you are working with but the area you are working in. After you grind, scrape, and scrub the metals you are welding together, take a moment to thoroughly clean your workstation.
You may have gotten primers and impurities off your metal, but that will do you no good if it gets stirred up from your work area and caught in the weld pool.
Clean your workstation thoroughly with a solvent that doesn’t leave a greasy film behind.
Finally, make sure your tools are clean and that your electrodes are kept in a dry, clean storage place. Damp electrodes can cause porosity.
Having the right gas flow is critical when preventing porosity. There is no magic solution for this. Welding Supplies from IOC suggests that gas flow depends on the position and size of the weld, your equipment, and your own preference, as well as other factors.
In most cases, your machine will have a list of suggested flow rates and settings from which you can make adjustments as necessary.
Porosity is both fixable and preventable. If the old adage that an ounce of prevention is worth a pound of cure is true, then learning how to prevent porosity will also help you fix it when you have to start over.