Abstract: The toughness weak points of bainite weld metal in multi pass weld of 30Cr2Ni4MoV refractory steel low-pressure steam turbine welded rotor are found and studied. Impact toughness experiment is used to evaluate the toughness of the weld. The quasi-static fracture test is used to observe the crack growth behavior. The energy spectrum analysis is used to analysis the composition. The impact energy of the weld is 118 J, the experimental results of impact toughness show that the weld owns good toughness. In quasi-static fracture tests the cracks propagate in unstable manner in the incomplete phase change zone of layers between two adjacent beads, so the incomplete phase change zone of welded layers are relatively weak in toughness. Through the optical microscopy analysis of specimens etching by picric acid aqueous solution, many black lines are found in the incomplete phase change zone of welded layers. The cracks propagate in unstable manner along the black lines. There is much more carbon in the black lines than in the parent metal. It could be inferred that the carbon-rich areas contain the microstructure which is disadvantageous to the toughness.

Key words: Bainite, Carbon-rich area, Multi pass weld, Toughness, Weld metal, integrated design optimization, macro fiber composites, MFC- Layout, structural shape control