Consumables Corner: Diagnosing the obvious and not-so-obvious causes of porosity
Q: We are a carbon steel pipe and vessel shop located in the South. We are having problems with weld porosity on some of our pipe fabrications verified by X-ray inspection. We do an open-root weld with gas metal arc welding (GMAW) and then fill and cap it with flux-cored arc welding (FCAW). We use a 75% argon/25% CO2 shielding gas blend at 55 CFH for both processes. Typically, the porosity occurs more during the warmer months of the year. Any ideas how to eliminate it?
A: You mention a couple things that may point directly to your problem. Let’s assume that your weld joints are free of heavy rust, mill scale, paint, grease, or any other contaminants that can cause porosity.
When it comes to dealing with porosity, the first thing to look at is if it is caused by moisture. Has the welding electrode been stored properly? Flux-cored and stick electrodes that are not stored properly are notorious for absorbing moisture and causing porosity or worm tracking. Another source of moisture can be condensation on the steel in the joint. It is not uncommon with warm weather to have high dew point values that can lead to problems. Usually preheating the weld joint to at least 225 degrees F can determine if this is a contributing factor. Last, make sure you are following the electrode manufacturer's recommended welding parameters. Excessive voltage or electrical stick-out can cause moisture from the air to be dissolved into the weld pool.
Let's address the not-so-obvious clue, which is your location down south and the fact that you see more porosity during the warmer months. You probably keep the shop doors open during the summer if your shop isn’t climate-controlled, which most shops aren’t. The problem is that substantial drafts or air currents move through the shop and can rapidly displace your shielding gas. Remember, shielding gases such as the blend that you use protect the weld because they are heavier than air and will displace atmospheric air locally around the molten weld pool, thereby preventing contamination.
The more obvious clue is your shielding gas flow rate of 55 CFH. The general recommendation for shielding gas flow rate is 35 to 45 CFH for GMAW and FCAW. However, every application needs to be considered on a case-by-case basis. If you're welding aluminum with a high-helium-mix shielding gas, then you need a high flow rate because helium has lower density compared to other shielding gases.
If you are using GMAW and a small-diameter wire, such as 0.023 in., on very thin-gauge material, keep your flow rate down to around 25 CFH. If you are welding on steel that has a substantial amount of mill scale with a large-diameter flux-cored wire, your flow rate should be on the higher end of the range.
It is likely that your flow rate is producing a turbulent shielding gas stream and aspirating atmospheric air into the mix, contaminating your shielding gas plume. Additionally, in many open-root weld applications such as yours, welders prefer to use a tapered nozzle for better access to the root and a better view of the weld puddle. The combination of a tapered nozzle and high shield gas flow rate increases the likelihood of atmospheric air contamination that is leading to porosity.
For a set flow rate, the gas exits the nozzle at a certain velocity. If you were using a 5/8-in.-diameter nozzle and changed to a 3/8-in.-diameter at the same flow rate, the smaller nozzle would force the gas to exit at a higher velocity to maintain volumetric flow. Higher flow rate plus higher velocity creates a very turbulent gas stream.
To help prevent this from happening, use weld curtains to protect the weld area from strong air currents. Make sure your welders don’t have a fan blowing on them directly while they’re welding. Keep your shield gas flow rates between 35 and 45 CFH. If your welders are using a small, tapered nozzle for the GMAW root pass, reduce that flow rate to 25 to 30 CFH.