Buttering for welding SA-335 Gr P91 to SS 316

[Hollan Petersen]

By the design and material selection of a boiler under the scope of ASME VIII, it happens that I need to weld pipes SA-335 Gr P91 to weld neck flanges SS 316. Since I have been consulted that such a welding requires buttering, could any one explain me what buttering is and why it is considered as necessary for such a welding. Moreover, may I have some instructions for the qualification of the welding procedure.

[wgadmin]

In accordance with ASME IX, buttering is the addition of material, by welding, on one or both faces of a joint, prior to the preparation of the joint for final welding, for the purpose of providing a suitable transition weld deposit for the subsequent completion of the joint.

As far as this specific welding is concerned, the reason that you need to do buttering is to avoid PWHT after the completion of the welding. According to ASME VIII, table UCS-56 the welding of SA-335 Gr P91 (P-no 5B, Group-no 2) needs PWHT in a temperature higher than 700 deg C. But if you do PWHT in such temperatures there is the possibility the austenitic SS to be made vulnerable to cracking
http://www.weldinguide.com/viewtopic.php?f=5&t=29
or to lose its corrosion resistance
http://en.wikipedia.org/wiki/Intergranular_corrosion

For that reason you shall overlay (buttering) the SA-335 Gr P91 with a nickel based consumable like AWS 5.14, ErNiCr-3 (or similar), then you shall PWHT the buttered pipe and finally you shall complete the welding by joining the SS flange to the buttered pipe using the same welding consumable as the one that used for the buttering. Since after completion of the welding the HAZ shall not be extended to the SA-335 Gr P91, the actual thickness of the buttering (after completion of bevel preparation) must be sufficient to permit HAZ to be included in the buttered area.

As for the PQR requirements, you could have a look to the ASME IX, QW-283 that gives the scope and the requirements for the buttering.