Welding on work hardened austenitic stainless steel

[Bartholomew69]

Goodday all;

I have a spherical presure vessel, about 8m in diameter, 5 bar design presure, 75 deg C design temperature, manufactured in 1980 to the Swedih Pressure Vessel Code, and the Swedish Cold Stretch Code. This code was basically used prior to the advent of duplex materials, by using thinner austenitic (316L) steel and 'stretching' it by overpressure to over its yield, thereby work hardening it and increaing its strength.

My problem is, we have a defect (stress corrosion crack under a compad) that needs repair. The original code calls for the cold stretch procedure to be repeated after any major repairs (roughly defined as greater than 100 mm diameter insert, or major welding).

We do not want to go this route and propose to cut out the defective area and insert a circular section from 316L (thicker (16mm) section calculated to ASME and tapered 1:3 to 6mm original shell).

What procedures do I need to take when welding onto work hardened austenitic s/s??

 

[metengr]

Two most important questions;

1. What repair code are you required to follow?

2. What Jurisdiction does this vessel belong too?

You reference the ASME Boiler and Pressure Vessel code- this is a construction code NOT a repair code.

 

[Carburize]

Basically in the heat affected zone adjacent to the repair weld you will soften the original material of the vessel. That is the material which was strengthened by the original cold stretch procedure, which is why the code calls for any large repair to be stretched again to restore the properties.
If the heat affected zone is very narrow it might not have a major effect on the overall integrity but I suspect that you will need to do an extensive set of weld procedure trials and mechanical tests to justify your proposal. If the vessel comes under any kind of third party inspection or is covered by a major insurance company you need to get them involved early to agree the procedure.

 

[metengr]

Carburize;
This type of repair can become a serious issue related to safety of the vessel. This can be construed as an illegal alteration to an existing pressure vessel because it violates the original code of construction!

In my view, it is not simply installing a "window patch" as in the original post. In addition to what you stated, I would expect some level of stress analysis because you have introduced a zone of inherent weakness in the pressure boundary of the shell that could effect membrane stresses and ultimately the fatigue life of the vessel.

 

[Carburize]

Yes,I agree completely, those and other issues were what I was trying to highlight by suggesting that the certifying authority/insurance company be involved right from the beginning. Must guess is that they would say no!