Abstract: For improving the toughness of 2.25Cr-1Mo-0.25V heat resisting steel welding joint, the impact test and the three point bending test are done in order to locate the weakest part of the joint, and much more micro analysis is necessary to find out the reason of that. The toughness of weld metal is lower than the other parts of the joint according to the result of the impact test, so the crack growth process in weld metal is further researched with three point bending test. A zigzag crack is found in weld metal after three point bending test. Dimple and quasi cleavage are both exist on the surface of the fracture through scanning electron microscope(SEM) technic. Zigzag crack appears in the columnar grain zone of weld bead with metallographic observation, and its direction is parallel or vertical to the direction of columnar grains. After dealing with picric acid, it appears that the parts of the zigzag crack which are parallel to the columnar grains are the original austenite grain boundaries, the result of electron backscattered diffraction(EBSD) test also proves that conclusion, and the parts which are vertical to them are layer lines. Because of that, the original austenite grain boundaries and layer lines of submerged arc welding (SAW) weld bead should be paid especially attention. The toughness of the joint can be obviously improved by adjusting the composition of welding wire or changing welding parameters, the number of layer lines is decreased due to it.

Key words: Stratified segregation, Submerged arc welding, Toughness