[en] The status of welding and weldability studies on modified 9 Cr-1 Mo steel is presented, and microhardness, tensile, creep, and Charpy impact properties of welds made by gas tungsten arc, shielded metal arc, and submerged arc processes are analyzed. Microhardness traverses of modified 9 Cr-1 Mo welds were examined after nominal and extended postweld heat treatments. Microhardness data on modified 9 Cr-1 Mo were also compared with similar results on standard 9 Cr-1 Mo, 2 1/4 Cr-1 Mo, and HT9. Tensile and creep data were primarily on weldment specimens in which the gage length contained the base metal, weld metal, and heat-affected zone. Charpy impact data were primarily on the weld metal, with notch parallel to the welding direction. On the basis of the data presented, it is concluded that standard 9 Cr-1 Mo wire and electrodes can be used to weld modified 9 Cr-1 Mo base metal if welds are given a nominal postweld heat treatment. If welds are to be normalized and tempered, the use of modified wire and electrodes having the base metal composition is recommended