The most common practice is to have the bosses where pipe attachments will take place overlayed with 309L at the foundry, then given a PWHT. Subsequent welding of a pipe to the overlay will not require PWHT. However, it is a little to late for that now. Besides the risk of cracking or service failure depending on your environment, it may be prohibited by code, so check that out also. If you have to weld without PWHT, which is not recommended, but is sometimes done under certain circumstances, you would want to use an austenitic filler metal like 309L, or nickel based like Inco 82 or Incoweld A (NiCrFe-2) so that even though you will have a hard HAZ, less hydrogen will diffuse into the HAZ, thus reducing the likelihood of cracking. It would also be beneficial to weld with a temper bead technique. Deposit a layer of 309L/Inco over the area to be attached to, then grind half of it off, and weld another layer of 309/Inco over the first layer so that the HAZ of the first weld will be tempered by the second one. (This technique is described further in ASME Section III) Then attach the pipe to the 309L/Inco overlay. The lower the carbon in the pump case, the better off you are also.