Y. I. Komizo, H. Terasaki, Osaka University, Ibaraki, Osaka, Japan
For the weld metal of 1000 MPa grade high strength steels, the martensite structure is candidate due to its high strength. However, when it is formed as weld condition, the weld defects: hot cracking and cold cracking, are occurred. In order to prevent the defects, phase design during welding is effective. When delta-ferrite exists during solidification, the high solubility of delta-ferrite for phosphorus and sulfur is effective for reducing the risk of hot cracking. When austenite is retained to the room temperature, the high absorption of hydrogen is effective for avoiding the cold cracking. On the other hand, the delta-ferrite and austenite are detrimental for toughness and strength, respectively. It means that the adequate phase combination should be designed during welding by optimizing the chemical composition of weld metal.
In the present work, the phase evolution during welding was in-situ observed by using time-resolved x-ray diffraction system, in order to assess the design of weld metal with martensite microstructure. The experiments were performed in the undulator beam line of synchrotron radiation source. The time-resolved diffraction patterns in the solidification process of 13Cr-9Ni-0.5Mo mass% steel were measured while the welding torch moves vertically to the x-ray indcident beam. In the solidification process, delta-ferrite was detected as a primary phase with the liquid phase. The following is the peritectic reaction between liquid, delta-ferrite and austenite phase. After that, it could be confirmed that the delta-ferrite extinguished after peritectic reaction. It meant that delta-ferrite played the best role during solidification to emphasis the weldability. The austenite transformed to martensite about 373 K but the diffractions peaks corresponding to austenite phase were retained. Those phase evolutions in reciprocal lattice space were presented with measured temperature and the related with TEM micrograph of weld metal and the results of mechanical tests.
Summary: Phase evolutions during welding were in-situ accessed in order to develop the high-strength weld metal with martensite structure. Those phase evolutions in reciprocal lattice space were presented with measured temperature and the related with TEM micrograph of weld metal and the results of mechanical tests.
