Have you ever wondered if there is one type of welding discontinuity or defect that is the absolute worst one to have? Have you heard that cracks are by the far the worst? Or do you think others like lack of fusion, undercut or overlap can be worse? The reality is that there isn’t a clear answer to this question, at least not if you are trying to determine the worst type of discontinuity.
However, what we can say is that the most dangerous type of discontinuity or defect in welding is that which goes undetected and out for service. Cracks, undercut, overlap, lack of fusion, excessive convexity, undersized welds, slag inclusion and every single welding discontinuity can be detected during the fabrication process and fixed before the end product is put into service. However, quality control practices vary widely across different organizations.
There are many different levels of quality standards when it comes to welding. Even not having a quality standard is a quality standard. There are shops in which the quality standard is simply “no cracks, no porosity, no undercut.” That is a quality standard. Perhaps not a very robust one, but one nonetheless.
So which are the welding discontinuities that are more susceptible to go undetected?
First, those which are underneath the surface of the weld. Things like lack of fusion, internal porosity and slag inclusions cannot be detected by visual inspection. These discontinuities are avoided by ensuring the use of qualified welding procedures which are used by qualified welders. Only then can we feel comfortable about a weld based solely on exterior appearance. However, even that is not always a guarantee.
Even some surface defects such as cracks can be easily missed due to their small size. Nondestructive examination methods such as dye penetrant (PT) can detect these types of defects that are hard or impossible to see with the naked eye. Of course, if the quality standards don’t include these checks the discontinuities will go undetected.
There is another potentially catastrophic problem with welding that can go undetected, not because of the lack of nondestructive examination, but due to the timing of inspection. Hydrogen induced cracking can manifest itself up to 48 hours after welding. This is why this type of cracking is also called delayed cracking. So naturally, if inspection is carried out immediately after welding this problem goes undetected.
Codes such as AWS D1.1 require that inspection be done no less than 48 hours after welding in applications that are susceptible to hydrogen induced cracking.
Another example of a common mistake that goes undetected happens when welding stainless steels. For example, when welding the most common stainless base metal, 304, many fabricators make a big mistake…assuming it can be welded with any shielding gas in the GMAW process. It is quite troubling that many fabricators believe it is OK to use a 90/10 (argon/carbon dioxide) mix to weld stainless, or even a 75/25. The problem with this is that the weld can look great, but this will result in premature corrosion once in service. At this point it is very costly to repair.
Many times this mistake is made because 300 series stainless flux-cored filler metals can be used with 75/25, but this is because of the alloy additions in the flux. However, solid wires used in the GMAW don’t have this option. You cannot add additional alloying elements to it.
Similar to the above problem,, even when using the correct shielding gas, is sensitization. This is a problem that occurs in austenitic stainless steels where excessive heat input, and thus slow cooling, cause excessive depletion of the chromium available to form chromium oxide, the protective layer that provides corrosion resistance. This results in aggressive corrosion in the heat affected zone. And again, it happens weeks or months later, once the product is out in service, making repairs extremely difficult and costly.
There are many other welding problems that are hard to detect when we don’t have good quality control. So how do we fix this? It starts with setting up the right processes and procedures. Rather than do more inspection, start by developing and qualifying welding procedures for all applications that are considered critical. Don’t make the mistake of assuming that what works on carbon steel will work on stainless steel or vice versa.
Always make sure you understand the base metals you are welding. If you are not familiar with a base metal do research, consult reputable resources and qualify your processes and procedures by testing. If an applicable structural code or other welding standard exists for the welding of that base metal, follow it. Even if you are not required by your customer. This is always good practice.
Are you quality standards addressing these kinds of potential problems?
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