Although welding results in premature failure by type IV fracture under high temperature creep conditions, the alloy design of light elements such as boron addition and nitrogen reduction enhances the creep lifetime of 9Cr heat resistant steel. In particular, the simulated heat affected zone (SHAZ) sample of new 9Cr steel (called TA steel) shows about 10 times longer creep lifetime than that of the standard Gr. 91 steel. The welded TA steel is thus expected to exhibit good creep properties because its SHAZ sample has coarser grains and suppresses type IV fracture. The preservation of base metal’s microstructure after welding results from the precipitate morphology, such as high grain boundary coverage by precipitates and low amount of MX being nucleation sites of ferrite grains during the α-γ phase transformation. In addition, the increase of stability of M23 C6 affects high pinning pressure toward grain boundary migration upon rapid heating during welding. First-principles calculations confirm the increased stability when boron is absorbed by M23 C6. Moreover, the calculations reveals that boron decreases the coherency between matrix and M23 C6, suppressing grain coarsening during creep tests in TA steel. It is concluded that the increased microstructural stability during welding and long high temperature exposure generates the elongated creep lifetime in welded TA steel including about 0.01 wt% boron and less than 0.01 wt% nitrogen.