Abstract
Effect of nitrogen addition on notch toughness of 2 1/4Cr-1Mo submerged arc weld metals was investigated. Nitrogen makes the grain size of weld metal smaller, and lowers the transition temperature of weld metal after stress-relieving and also after step-cooling. In stress-relieved weld metal, abundant and homogeneously distributed CrN precipitates are found, in addition to Mo2C, M7C3 and M23C6, by means of TEM observation of extracted carbon replica. Larger reduction of hardness after PWHT results in lower vTr40. From the results, the mechanism on notch toughness improvement in nitrogen added 2 1/4Cr-lMo weld metal is considered as follows. Nitrogen in weld metal makes the transformation temperature low and the grain size fine. The weld metal in as-welded condition is hard and brittle because it contains much nitrogen as solute atom. However the weld metal is softened by PWHT, and the softening is promoted by precipitation of CrN, in addition to Mo2C, M7C3 and M23C6. Consequently, the notch toughness improvement is achieved by both the fine grain size and relatively soft microstructure.
