It is supposed to be 316, and we have been told the water(filtered ) is heated to 190f. The whole thing has problems, the heating portion is a jacket about half way up the tank that is 4"~ thick. I think it's a burner under the bottom of the tank like a giant tea kettle. But we have no specs or drawings of any kind. They call it a water heater, it seems to be more of a very thin boiler to me.

That's why I hedged my comments. We don't know all the facts. At best we can only apply the SWAG method to derive a solution to the problem this welder encountered.

Al

I agree Niekie that the switch to L grade or stabilized grades of austenitic stainless would have little influence on the body of the vessel, but in the HAZ it would make a difference. As you noted, sensitization in the HAZ can be the culprit, but that is a localized condition. The problems with the dished head and shell are both have residual stresses of the same magnitude as the yield strength of the base metal, i.e., both were most likely cold formed without the benefit of a subsequent stress relief heat treatment.

The best approach, if practical, is to stress relieve the vessel after all the fabrication is completed. One of my clients was experiencing a similar problem as described in this post. The solution was a full stress relief after all fabrication was completed. It was not inexpensive, but it worked. They have not had a failure since PWHT has been instituted. They also switched to a different insulation material to rid themselves of the chloride problem.

Then again, we still do not know the entire story or all the facts. I believe we have provided some conjecture, some "food for thought," and made it known that there is the need to gather more information before a workable solution can be derived.  It is essential that the base metal alloy be determined, method of manufacture be known (PWHT?), the operating conditions as well as past history, cleaning methods, flushing after cleaning, chemistry of the water, and whether the vessel is insulated with any material that contains chlorides that could leach out. It is a real "who done it" for someone that specializes in that type of forensics and metallurgy. The bottom line is the brewery is going to be forced to buy a new vessel. I suspect similar cracks will be found through out the bottom dished head and possibly the lower portions of the shell as well if there is an external source of chlorides. If the insulation (if that is the source of chlorides) is the "bad guy," the cracking will be more pronounced on the tank's exterior it is continually wet.  Now, this is what I call fun!

Best regards - Al