B31.1 DMW weld question 1

[metalhoosier]

Per 2007 B31.1 Section 5, 127.2.1, E)"When austentic steels are joined to ferritic steels, the weld metal shall have an austenitic structure."

Does anyone have any thoughts on this statement for the example below?

For example, if one had a threaded/seal welded thermowell attachment where the high energy pipe material was SA-335-P22 and the thermowell material is 316SS, wouldn't it make more sense to weld this with filler material similar to the base metal (in this case 9018-B3)? This would meet the intent of B) "If base metals of different tensile strengths are to be joined,the nominal tensile strength of the weld metal shall equal or exceed the minimum specified tensile strength of the weaker of the two." and move the DMW interface against the face of the thermowell. I would think this might produce less headaches than following point E) above.

Your thoughts would be much appreciated.
MetalHoo

 

[metengr]

Does anyone have any thoughts on this statement for the example below?

Yes.

For example, if one had a threaded/seal welded thermowell attachment where the high energy pipe material was SA-335-P22 and the thermowell material is 316SS, wouldn't it make more sense to weld this with filler material similar to the base metal (in this case 9018-B3)? This would meet the intent of B) "If base metals of different tensile strengths are to be joined,the nominal tensile strength of the weld metal shall equal or exceed the minimum specified tensile strength of the weaker of the two." and move the DMW interface against the face of the thermowell. I would think this might produce less headaches than following point E) above.

No, it would not make sense to use a B3 filler metal for joining an austenitic stainless steel to a ferritic base material (lke Grade P22) because of dilution concerns that would render the weld metal adjacent to the Grade P22 base metal susceptible to cracking after welding or in-service. So, this is why for DMW's filler metal composition is critical and the avoidance of harmful phase transformations after the weld metal cools.

 

[GRoberts]

In this case, the code requirement is also sage advice. Using B3 composition filler could be the real headache. To confirm that you are using appropriate filler metal that does indeed produce an austenitic microstructure, you should use a microstructure diagram such as the 1992 WRC diagram or DeLong diagram. You will be miles ahead if you can find someone who would show you how to use one. Otherwise, test the weld metal from the PQR for ferrite or chemical composition. You are going to have a qualified welding procedure, right?

 

[metalhoosier]

meteng and GRoberts,

Thank you for the insight. It spurred me to reference my Welding Metallurgy, 2nd ed. by Sindo Kou which contains the 1992 WRC diagram. I think I was able to follow the diagram; so basically I'll have a solidification structure of martensite thus increasing the possibility of cracking.

Our typical repair has been to follow the 127.2.1 E) with a nickel alloy applied with an approved weld procedure. Back when the unit was built, it was common practice to use a stainless steel filler metal (309)to seal weld the thermowell. Over time it creates cracking due to the thermal expansion difference at the toe of the weld at the base metal interface which eventally leads to leaks. This is problematic. Is the best answer to grind out the 309 filler and replace with inconel if the 316 thermowell is still used?

 

[metengr]

Over time it creates cracking due to the thermal expansion difference at the toe of the weld at the base metal interface which eventally leads to leaks. This is problematic. Is the best answer to grind out the 309 filler and replace with inconel if the 316 thermowell is still used?

The failure of the weld in elevated temperature service is caused by carbon diffusion where the higher carbon content of the Grade 22 base metal along the fusion zone of the weld combines with chromium in the 309 weld deposit. Over time, the loss of carbon along the edge of the fusion zone results in lower creep strength and what you have is a local creep failure. The failure looks like a massive lack of fusion weld failure.

Yes, remove all of the 309 filler metal, NDT and re-weld using Inconel.