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Pinning Down the Cause of Porosity in SAW
Q: Our company produces large structures fabricated from mild steel plate with a small amount of HSLA forgings ranging from 0.5 in. to 4 in. thick. Our main process is submerged arc welding (SAW) using a mild steel electrode and a neutral-bonded flux. We have been experiencing some issues with random porosity and we're not sure what's causing it. We've tried using the same classification of flux and wire from different manufacturers without success.
A: The information provided is limited; with more detail we could analyze the situation better. Therefore, we will make some general assumptions to address the core issue:
- The wire/flux is the best match for the mechanical requirements, process, and weld conditions.
- The weld parameters are optimized and the mechanicals are verified.
- Your welding equipment is in good working condition, calibrated, and on a PM schedule.
- There have been no changes in the base material and weld joint conditions.
- You properly store and handle consumables.
- You are preheating the base material when appropriate (assuming you are working to a code or weld standard that dictates this).
The previous bulleted items are all significant for successful SAW applications. If any of those items have changed or are not maintained, we suggest you start there to trace the root cause of the porosity. Additionally, you should consider a few more details.
Depending on when the porosity problems arise, the first thing you should look at is the time of year and the weather conditions. On hot days and with higher humid, bonded fluxes can absorb more moisture from the air if they are not stored or handled with extra care. Your flux manufacturer should provide you with instructions to properly store, heat, and rebake if you suspect this may be one of the causes.
Also, during the spring and fall when temperature swings are greater, the dew point can cause moisture to form on the base material and, ultimately, it can end up in the weld joint. Likewise, if material is stored outside during winter months and brought inside for production, be sure to let it sit for 24 hours or more to warm it to a temperature of at least 50 degrees F (welding code may dictate this information).
Flux recovery is another area of concern. Clean flux can be reused in many applications; however, it does tend to break down into smaller granular sizes and can actually prevent the weld from degassing properly, causing porosity.Again, the previous bulleted list is important because parameters such as ESO (electrical stick-out), voltage, current, flux burden (amount of flux deposited onto the weld), contaminants in the weld joint, insufficient preheat, and improperly stored or handled consumables can greatly affect weld quality and performance. Hopefully, something here will help you to uncover and correct the causes of your porosity issues.
This article originally appeared in The WELDER magazine.
It is reprinted here with permission of the Fabricators & Manufacturers Association, Intl.