The effect of molybdenum on notch toughness of austenitic stainless steel weld metal was investigated in detail. The weld metals where molybdenum content varied from 0.1 to 2.9% and δ ferrite content 0 to 10.8%, were prepared by submarged arc welding. The absorbed energy of fully austenitic weld metal was constant independent of molybdenum content. However, in the case of eutectic δ ferrite weld metal (EWM), the absorbed energy decreased monotonously with the molybdenum content and in particular, this tendency was remarkable in the weld metal of which the concentration of molybdenum within δ ferrite grain was more than 5%. In the weld metal of primary δ ferrite (PWM), the absorbed energy was constant up to 2.3% Mo and began to drop in the specimen above 2.9% Mo where the concentration of molybdenum within δ ferrite grain became more than 5%. These results revealed that if the concentration of molybdenum within δ ferrite grain exceeded 5%, the notch toughness Called drastically. This was considered to be attributed to following causes: the increase of molybdenum resulted in degradation of the coherency of δ/γ interface and moreovre, in the eutectic δ ferrite of EWM where the concentration of molybdenum was especially high, the formation of carbide of M23C6 was observed which affected harmfully the low temperature toughness, while in the primary δ ferrite of PWM the carbide was not observed at all. The multiple regression analysis revealed that each element of C, Cr and Mo had inherently a reducing action in toughness and particularlly, this action of molybdenum was stronger in EWM than in PWM.
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