I can give you some general advice without knowing the specific service application. Typically, it is better to design around dissimilar metal combinations especially those dealing with welding against creep strength enhanced steels. These steels are specially formulated to provide enhanced creep strength by using very precise thermal treatments. If the welding is not performed correctly or lack of knowledge in using this material, the material will become unpredictable.
Given the cautionary warnings above and assuming you need to use this material combination in an elevated temperature environment, here is what I would suggest
1. My recommendation is to weld the 316H fitting to the P91 base material with Inco A filler metal. The Grade 91 will require a 500 deg F local preheat, prior to welding.
2. Perform local PWHT of the attachment weld at 1385-1420 deg F for 1 hour per inch of weld deposit thickness at temperature.
3. After all welding is completed, perform a final liquid penetrant examination of the fitting attachment weld.
Precaution – Since Grade 91 is a creep strength enhanced ferritic steel, it must be welded with extreme caution. One must follow strict preheat and PWHT requirements, and make sure the PWHT temperature range is not exceeded. Enforce strict temperature controls and proper hold points – preheat temp, fit up, PWHT.
Qualify this welding procedure and welder(s) to ASME Boiler and Pressure Vessel Code, Section IX.
If necessary, hire a 3rd party weld or NDT inspector to enforce the above.
metengr, I read somewhere that local PWHT of P-91 is not permitted, because there will always be a zone where temperature is undesirable. Please clarify your practice on this matter, because I may need to know this in future.
brimstoner;
There is nothing in the US boiler and pressure vessel or piping Codes that I am aware of that prohibits use of local PWHT for Grade 91, or other creep strength enhanced ferritic steels. There are ASME B&PV Code Case rules that specify what to do if the component is subjected to temperatures in excess of current PWHT temperature requirements - cut the material out and replace it or re-heat treat.
Typically, one should try to avoid or minimize welding to Grade 91 after final heat treatment because within the HAZ in the Grade 91 base material there is a fine grained region that is more susceptible to Type IV creep damage. In this case, one needs to understand this limitation and design for lower stresses. However, it is not always practical to locate attachment welds in low stress regions, and as such, welding and local PWHT are sometimes necessary.
you are correct in that this is for steam applications, more specifically the connection/transistion from P-91 to 316 S.S. for super critical boiler instrumentation tubing
the next question is, is there any issues if the point where the transistion weld is made then exceeds 800 degF during normal operation?
metengr, am i correct in assuming that your initial comment 1 kind of suggests a BW instead of SW?
It isn't uncommon to weld an alloy 600 or 800 stub to the 91, then PWHT. After that you can weld and austenitic fitting to the austenitic stub without PWHT requirements.
There are special fillers used for this work. Check some of the EPRI welding reports.
