By -
Date 07-29-2001 20:26
I could not link to the site suggested, so I do not know what advice is given there. For smaller articles, the purging time is not such an issue, because by the time you are ready to start welding, the article is generally already sufficiently purged. For larger articles, the way that I would suggest is as follows:
Calculate the volume of the article. Based on your purge gas flow rate, you can calculate how quickly the article will be filled. You have to keep in mind that Ar is heavier than air, and will tend to fill your article from the bottom. You "air leak holes" therefore needs to be placed on the highest point of the article. (As far as possible.) Just a cautionary note: Do not have too high a flow rate of Ar, because then you get a mixing of the Ar with the air, resulting in a much longer time required before a propper purge is achieved.
When purging with N, your approach needs to be different, because it is the same density as air. It will therefore not "sink to the bottom". You are then essentially working on a dilution principle where you are continuously diluting the air within the article with N, untill the O2 concentration is low enough. To calculate the time, decide what level of O2 is allowed, and then do your calculation based on the fact that you are starting with an O2/N mixture of 21/79%. Here a higher flow rate is not a problem.
Regarding the type of purge, I must say that I have never used anything other than Ar. When using N, you always run a chance of ending up with Nitrides in your weld. They tend to embrittle your weld. I know that some people put a certain percentage of N in purge and shielding gas when welding duplex stainless steels. I personally have never done it, and have always obtained good welds. At the end of the day, you can use anything that your PQR tests have shown will work. On 347 S/Steel I would stay away from N.
Unless you are using "thick" walled piping, (Above approx. 8mm) I would maintain the purge for all the passes, if for no other reason than to allow the weld to look a bit better.
Hope this helps.
Regards
Niekie
Hello Chaim:
I just want add one thing to Nieke's answer. As a general rule, you can determine the prepurge time allowing about five volume changes in the pipe system or air dam volume (time=5V/Q); you must to know the flow rate Q. A tipical gas flow rate during prepurge is near to 20 L/min (42 cfm) and during the welding near to 15 L/min (32 cfm). If you have the way, look at the Welding Journal of August 2000, where there is an interesting article related with the purging. One of the main reasons for use weld root purging is avoid the formation of oxides after welding, wich are caused for the high temperatures of the steel in presence of air, then if you replace the air (with Argon, for example) you have resolve the trouble; in my opinion, on very thin materials you can't remove the purging gas because you certainly will have very high temperatures in the root pass (by reheating of the cover passes) and is still probable the oxides formation.
Jorge Giraldo
Medellín, Colombia
