Abstract: During manufacturing the Φ711 mm×11 mm X70 elbow, the bended section is made with the quenching+ whole pipe body tempering process, which leads to the problem that after tempering, the impact toughness of the straight section of the elbow fails to meet applicable technical requirement. Accordingly the causes for deterioration of the impact resistance of the X70 elbow weld are investigated. It is identified via the analysis that there exist in the as-welded weld of the said elbow chemical element Cr that possesses strong hardenability, and comes from the base metal, and is helpful for the precipitation of coarse strip proeutectoid ferrite with poor toughness, which leads to the low impact toughness of the elbow weld;as for the weld of the straight section of the elbow, after being tempered, the proeutectoid ferrite grows bigger obviously, and furthermore the C and N as precipitated from the proeutectoid ferrite turn into carbonitrides which in turn cause the further reduction of the impact toughness of the weld.