American Welding Society Forum
How do I know we need to use backing gas for welding when we create a new WPS? and what is the purpose of it?
The bottom line is that with some metals it is not possible to achieve a sound weld without a backing gas to exclude the oxygen from the hot weld metal in the root. Typically anything that contains more than about 5% Cr and most corrosion resistant alloys fall in this category. Some client specifications may be tighter and require anything with more than 2.5% Cr to be purged.
You are very helpfull, can you tell me where can I find these requirement in the code book and if I qualify WPS/ QPR by plate, what is best methode to apply the backing gas?
The codes I use do not generally specify the use of the backing gas. They merely state that if you need to use backing gas to get an acceptable result in your PQR test, then you need to also use the same for production welding. The need for backing gas may be specified in client specifications, but generally it is a matter of basic welding engineering. Given certain materials, with certain processes, you just need to protect the root from oxygen (or nitrogen) to get an acceptable result. Typically GTAW welding of stainless steels.
The easiest way to purge a plate coupon (there are a number of ways) would be to tack weld an angle iron onto the back of the plate, and seal the sides with tape. (e.g. masking tape) Apply the gas from one end of the angle iron, and make a small drain hole in the other end to prevent excessive pressure build up when closing the weld. (Or else it will cause root concavity.) Place a piece of masking tape along the top of the joint, and progressively remove it as the weld progresses. For most stainless steels, ensure you keep the root purged till you have about 6mm of weld deposited.
In possibly the most BASIC of considerations, for me, if a metal contains more than 2.5% chromium I use a purge. This isn't a code requirement but something you'll have to study. There are lot's and lot's of factors besides chromium content, the above is just an example.
I just wonder about it, some client want to do it, so I just want to know where it is came from.
I believe the typical codes of construction (ASME Sec I, Sec VIII, B31.1, B31.3) do not address backing gas or even have "root oxidation" as an unacceptable condition. Individual project specifications would dictate when it is required and in addition, the WPS that is used would indicate what is required.
Getting a code acceptable root surface would be very difficult on many base materials whether purging is "required by code" or not.
Have a good day
Looks like I wrote the same thing everyone else did. Should have read a little closer. Anyway, What they said !!
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