It Really Really Hurts to have a failure. Makes you more humble... and smarter. How is this possible?!? I can weld as good as the next guy. I have years of experience. There are a million things I have learned in the trenches. Stringer passes are better than one weave pass. Clean everything between passes. Pre Heat, Inter-Pass, and Post Heat temperatures... yes actually are important. But sometimes you are wrong... maybe even the whole industry is wrong.
Here is what I learned:
Just because they hand you a typical fitting and tell you to weld it to a typical looking pipe, doesn't mean your weld will be a success.
Temperature control is a huge issue!! Rapid cooling, weld speed, filler rod size are all important issues.
When welding fittings on Stainless Steel Chill Pipe
Thread-O-Let joint type - 2:1 depth to face ratio
Rapid cooling during a weld
Lots of weld passes / Small Filler Rod
Fast welding speed
Concave weld shape
Half Coupling joint type - .5:1 depth to face ratio
(make sure your fitting is coped to match the pipe)
Less Passes / Bigger Filler Rod
Convex weld shape
Clean between passes
Adequate Fill on Stop crater
Excellent video on the subject
Carbon Steel Crystal Structure http://youtu.be/Tj9K66mcTPU
This article really nails it.
(this is the same information given to me by a local SCWI about the subject, confirming my research)