Stainless steel pipe welding is gravy work. Always has been. Always will be.
If you can do it and do it well, you are the man and you will never go hungry. But it takes some attention to detail to do it right.
The main thing that separates the men from the boys or should I say, the real welders from the rod burners is the willingness to follow procedures for welding stainless steel. Procedures like waiting for the pipe to cool in between passes, keeping the hot tip of the rod shielded with argon, snipping the tip of the tig welding rod when it gets crapped up, keeping your tungsten sharp.
One of the main procedures I am talking about is PURGE. Stainless steel absolutely has to be purged with argon gas in order to prevent sugaring. Sugaring is also called granulation but it is severe oxidation no matter what you call it.
You cant even get away with tack welding without an argon purge that shield the back side or inside of the pipe. So how do you do it? How do you protect the back side with argon?
Usually it is done with aluminum duct tape. The ends of a stainless steel pipe are taped shut and the whole inside is purged with argon gas. Even the opening in the pipe joint itself is taped so that argon can fill the entire pipe and push out any oxygen. The tape is peeled back just enough to get a tack once the pipe is purge enough.
What is enough? The only way to tell for sure is with an oxygen analyzer. A bic lighter will not do the trick. Even 2% oxygen content will blow out a bic lighter. And 2% oxygen will result in a black root pass.
There are some other down and dirty ways to check the purge but an oxygen analyzer is the sure way.
Here are a few tips that will help you get a good purge on the stainless steel pipe you weld.
Make sure there is no water inside the pipe...not even a drop. One single drop of water will throw a monkey wrench in your purging until it evaporates. Remember what is in water? Hydrogen and oxygen.
Poke a vent hole in the uppermost place you can find. Think of argon as if it was water. Because it is heavier than air, it will fill up any space like water, displacing the air as it fills every nook and cranny.
More purge gas flow is not always better. A high and turbulent flow of argon will not necessarily purge a pipe quicker than a lower flow rate. And a diffuser on the end of the end of your argon hose usually allows you to use a higher flow rate. A homemade diffuser can be made with some stainless steel wool , some perforated stainless sheet, and a bit of sheet metal.
Once you verify a good purge, you are ready to tack. Peel the tape back just enough to tack and let cool a bit and re tape. Then tack again 180 degrees apart. Continue until all tacks are in place.
More tips for welding stainless pipe...
Laying an 1/8 inch wire in the root works better than dipping a 3/32 inch rod.
Use a feather edge bevel. No land needed
You can fix areas that do not penetrate by re welding
Use only stainless steel wire brushes and only use files that have only been used on stainless steel.
Use just enough amperage. Using too much amperage will overheat the stainless steel and make things go south quick