A comparison of TIG welding and laser welding of INCOLOY 800 is presented under two different aspects: weld pool sensitivity to minor elements and modelling of stresses and temperatures distribution. For TIG welding, welding parameters have to be adjusted when changing of INCOLOY 800 master melt. It appeared that laser welding did not depend on the material chemistry and gave a good reproducibility of the results, whatever is the mater melt, with the same parameters. Thermal modelling was developed for a tube-tubesheet configuration. TIG modelling showed a stress localization for several chemistries, which indicated a risk of cracking at the root of the weld. The modelling showed how it is related both to the temperature of the base material close to the molten pool, and the duration of the process. For the INCOLOY 800, the longer the temperature is in the 800-1000 degree(s)C range, the higher the risk of cracking exists. For laser welding, the risk is much lower, because the process is incomparably faster.