I would suggest for welding carbon steel to duplex 2304, an
ER 309L to assure adequate mechanical properties of the weld joint. You could use a nickel base filler but it must not contain niobium (columbium). For joining to duplex 2205 or the 25% Cr duplex to carbon steel use ER 309MoL.
For duplex to duplex - use ER 2209
No preheat for temperature at or above 50 deg F and keep the interpass temperature below 300 deg F. Watch your carbon steel thickness, you may need to butter the carbon steel with ER 309 if the code or standard requires a PWHT. Do NOT PWHT the completed weld joint!
This information was obtained from the following source;
"Practical Guidelines for Fabrication of Duplex Stainless Steels" by IMOA.
The reason to use the the high nickel filler for the CS to Duplex weld is to try and keep the weld from being ferritic, or even worse martensitic. You need the weld at least duples so that you have some weld ductility.
If there were a 309/310 filler with low carbon and a stablizer (Ti) this would be my choice. You don't need Mo in the filler since weld corrosion resistance isn't an issue. The CS will have less corrosion resistance .
Actually you might look at using 825 as a filler. It meets the criteria well.
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Corrosion never sleeps, but it can be managed.
Merzi;
Could you please inform us of your specific pressure vessel application in relation to joining the duplex SS to carbon steel. Various parameters go into proper selection of filler metal.
You don't want to use 625 because of the high Mo and Nb stabalization. You would have trouble setting much ductility in the welds.
309 is a good bet.
What is the HX application? Are there any special Code restrictions?
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Corrosion never sleeps, but it can be managed.
