Effects Of Trace Tin And Cerium On Embrittlement Of High Heat-input Welding Heat-affected Zones In SA508CL-3 Steel

SA508CL-3 steel is a typical low alloy pressure vessel steel which possesses moderate strength, good high temperature properties, good resistance to neutron irradiation, excellent malleability and weldability. It is widely used in pressure vessels for thermal and nuclear power plants.Because of the large engineering structures, thick steel plate s are required for pressure vessels. On the one hand, for penetration and reduction of welding passes, the high heat input welding methods are applied to them. On the other hand, the consequences of high welding heat input is creation of coarse-grained heat-affected zones, making the performance deterioration of the heat-affected zones, especially in toughness. Meanwhile, since recycling of steel scraps, some trace elements such as Sn, Sb and As could be introduced steel melting. There would be some impact of these trace elements on the microstructures and properties of weldments.In this study, coarse-grained heat-affected zones(CGHAZs) of the above SA508CL-3 steel samples undoped, doped with Sn and doped with Ce are simulated by virtue of a Gleeble-1500 D thermalmechanical simulation system with different welding heat input. Advanced materials analysis techniques are employed to evaluate microstructure, behavior of Sn and Ce, and their influence on the properties of CGHAZs..In the experiments, the peak temperature creating CGHAZs is 1320 oC and heat inputs are 30 k J/cm, 50 k J/cm, 70 k J/cm, 100 k J/cm, respectively, which correspond to different welding thermal cycles. Ductile-to-brittle transition temperatures(DBTTs) are measured to characterize the toughness of CGHAZs. In general, the lower the DBTT, the higher is the toughness. The results indicate that the DBTT increases from-23 oC to-1 oC with increasing heat input from 30 k J/cm to 100 k J/cm for the undoped sample, from-13 oC to-1 oC for the Sn-doped sample, and from-80 oCto-15 oC for the Ce-doped sample. Apparently, the DBTTs are lowered by Ce addition while they are augmented by Sn addition. This implies that the toughness of CGHAZs is improved by Ce addition and worsen by Sn addition. Optical microscopy, electron backscatter diffraction and transmission electron microscopy demonstrate that the microstructure changes from mainly granular bainite in the high heat input sample to mainly lath bainite in the low heat input sample. In addition, the microstrcture in the Ce-doped sample is obviously refined under all the heat input conditions, while the microstrcture in the Sn-doped sample is almost unchanged. All the above factors enable the Ce-doped sample with the heat input of 30 k J/cm to possess the best toughness. Field emission gun scanning transmission electron microscopy-energy dispersive spectroscopy(FEGSTEM-EDS) indicates that grain boundary segregation of Ce occurs during welding thermal cycles. The quantity of segregation increases with decreasing heat input, showing a non-equilibrium characteristic. Ce may enhance the grain boundary cohesion, thereby improving the toughness of CGHAZs.