Welding characteristics and stress corrosion cracking behavior of nickel-based alloys 690 by laser beam and arc welding processes

• Main Authors: Jia-LinWu, 吳佳霖
• Other Authors: Hwa-Teng Lee
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/30412290082092113370

博士 === 國立成功大學 === 機械工程學系碩博士班 === 98 === This study initially investigates the influences of three mean Nd:YAG laser output powers (1250, 1500, and 1750W) combined with rectangular pulse wave (PW) and continuous wave (CW) on weld bead profiles, melting ratios (MR), microstructures, and microhardnesses of alloy 690 for bead on plate (BOP) welding specimens, in order to achieve the critical welding speed for the just fully penetrating the butt weld. Moreover, precise measurements of the welding thermal cycles (i.e. the peak temperature, the heating rate and the cooling rate) and temperature distributions (i.e. temperature gradient) continuously at various points of the alloy 690 butt weldment during laser beam welding (LBW) and gas tungsten arc welding (GTAW) processes were taken. The resulting thermal cycle and temperature distribution profiles were then correlated with the distribution of residual stress, grain boundary character distribution (GBCD), degree of sensitization (DOS), and carbide type with an aim to study and establish the combined influences of peak temperature, heating rate, cooling rate, and temperature distribution on the intergranular corrosion (IGC) and stress corrosion cracking (SCC) resistance of alloy 690 weldments. The results showed that PW specimens have significantly higher MR values than those of CW specimens under constant mean laser output power. The PW specimen is characterized by its full penetration capability at higher critical welding speed, which results in a narrower weld bead. As a result, a denser dendritic structure and higher microhardness can be obtained in the fusion zone (FZ). For the butt welding specimens, the LBW process, with a power density as high as 104?105 W/ mm2, has significantly lower heat input (112 J/mm) on the weldment than the GTAW process, which provides a very steep temperature distribution, a very great heating rate (16080?C/s in the FZ), and a very high cooling rate. As a result, the sub-grain structure in the FZ of the LBW weldment appears to be denser than that in the FZ of the GTAW weldment. Moreover, the very steep temperature distribution in the LBW weldment effectively reduced the size of the longitudinal tensile residual stress zone relative to that in the GTAW weldment. A comparison of the double-loop electrochemical potentiokinetic reactivation (DL-EPR) curves on the FZs and weld decay zones (WDZs) of the LBW and GTAW weldments revealed that the Ir/Ia values in the FZ and WDZ of the LBW specimen were about 0.15% and 0%, respectively, while those of the corresponding regions of the GTAW specimen were about 0.41% and 0.6%, respectively. Therefore, the DOS value of the LBW specimen was much lower than those of the GTAW specimens. Furthermore, the GBCDs by orientation image maps showed that the fraction of low energy Σ coincidence site lattice boundaries (1≦Σ≦9) in the FZ of the LBW weldment is significantly higher than that in the FZ of the GTAW weldment. In particular, the modified Huey test results revealed that in the LBW weldment, compared with the GTAW weldment, interdendritic corrosion (IDC) and IGC were significantly arrested in the FZ and the WDZ, respectively. This occurred because the very rapid cooling rates in the FZ and WDZ during welding leads to an insufficient exposure time of around 1.2–1.6s through the Cr23C6 carbide precipitation temperature range of 620?1020?C, suppressing Cr-carbide precipitation and Cr-depletion along grain boundaries in the FZ and WDZ, respectively. Consequently, both the U-bend constant deformation and slow strain rate testing results showed that the LBW technique makes a significant improvement in the SCC resistance of alloy 690 weldments.