welding issue for different material

Dear Member,

I have high stress at the reducer (6"x8"), so one of the options considered is to upgrade the reducer material to Duplex (A790 S31803)from A312 TP316. In consequence, 8" reducer side(duplex) has to be welded directly into SS 316 pipe. Is it acceptable practice to connect two different material by welds without flange? Thermal coefficient for both materials differes, which may cause a local stress.

Your advise please

Regards
STC

You should be optimized piping routing ways to solve the high stress problem.

The reply is disappointing. I want to know two material can be connected by welds.

The reply is certainly not disappointing, you are looking for an easy answer. Using CII is always going to be an approximation for what happens in the field, you should always consider the option of re-routing if it means you can substantially reduce stresses.

Yes, two different types of material can be welded, that is a fabrication issue solved by weld procedures, however you need to be consulting your welding engineer for that query.

As you are located in Australia, most companies there have very good experience and you should definitely be asking these questions to your supervisors or colleagues, as you will get prompt and appropriate responses.

Thanks Mr the Dude,

This is existing piping, and unfortunately the layout change is not option. The high stress is due to seismic induced X-mas tree rotation. All other stresses (sustained,expansion, seismic and slug) are well within the code allowable. The material upgrade is one of the options I would like to present to clinet with a number of other possible solution. All I want to know is the weldibility between two specific materials. Yes we have material specialist in our company to consult about this issue.

Sounds like a metallurgy issue here, hence the short replies so far.
Yes changing the material could be an option, but can't you just use a thicker reducer (same material) and avoid any welding issues? It would seem like the simpler solution.

I agree with Perseus. You need more section modulus, it is easier to use a thicker item to take the loads/stresses.

Thanks for your input. Sorry I did not explain the situation fully from beginning. My query focus on the materail issue.

The increase ruducer to Sch 160 was considered as another option, but my concern was with the chamfer to mate with Sch 120. We need to grind off the inside of reducer to match with sch 120, so I think we would not gain much benefit from this option. Stress problem occurs at 6" sch 120 part.

I'm not a fabricator but could you not reduce the length of the reducer to match the I.D.s rather than reduce the thickness of the reducer? - Fillet weld on O.D. not I.D.