1. A welding method for welding base materials of
modified 9Cr-lMo steel together with a welding
material by submerged arc welding,
wherein the welding material is a chemical composition defined by ASME SFA-5.23 F9PZ-EB9-B9,
the base materials are welded with a welding heat input of 27.0 kJ/cm or higher and 33.0 kJ/cm or lower, and
post-heat treatment is performed with an L.M.P. of 21.04 103 or more and 23.30 103 or less as calculated by equation (1) below:
L.M.P. = T(20 + logiot) ... (1)
where T is a holding temperature [K] and t is a holding time [hour].
2. The welding method as claimed in claim 1,
wherein the welding heat input is 28.8 kJ/cm or
higher and 33.0 kJ/cm or lower.

3. The welding method as claimed in claim 1 or 2, wherein the base material is a member for constructing process equipment for a plant.