| Fusion welding has widely been used in a manufactured product for a long timeand is essential in many regions. As an engineering structure is developed to beupsizing, such as large oil storage tanks and ships, more and more thick steel plateshave been used and joined with welding. High heat-input welding technology hasbeen used to increase welding efficiency. The microstructures and properties of heataffected zones (HAZs) would worsen when the heat-input increased, making theweld joint unsatisfy its allowable properties. The brittleness is the most harmful one.Recently, the popular ways to reduce the brittleness of welding heat affected zonesfocus on refining grain size and generating beneficial microstructures. Alloying is animportant way to achieve this purpose. Zr/Ti/Nb was found to improve the formation ofbeneficial microstructures. And the tiny carbon-nitrides would pin the austenitegrain boundary and then refining the grain size. The research about the ripening anddissolution of the carbon-nitrides formed by Zr/Ti/Nb addition is not enough. Inaddition, impurity elements, such as Sb, P and Sn, are inevitable to exist in steels.These impurity elements induce brittleness when they segregate to grain boundaries,but nowadays the research on their grain boundary segregation during weldingthermal cycles and the effect of the segregation on brittleness is little.Therefore, in this thesis, grain boundary segregation of Sb, P and Sn duringwelding thermal cyles and segregation-induced embrittlement of HAZs were studedby use of welding simulation and materials analyses for two types of low alloys, i.e.,Cr-Mo and C-Mn steels. Besides, the ripening and dissolution of carbon-nitrides ofZr/Ti/Nb and their effect on austenite grain growth were also studied. The mainresearch contents and results are as follows.A Gleeble-1500D thermomechanical simulator was used to simulate weldingthermal cycles of HAZs for the undoped, Sb-doped and P-doped2.25Cr-1Mo steelsamples with a peak temperature of1320oC under the heat-inputs of36,60and100kJ/cm (corresponding cooling times from800to500oC are27s,59s and149s for a25mm thick plate with two dimensions heat conduction), respectively. Opticalmicroscopy (OM), scanning electron microscopy (SEM), electron backscatterdiffraction (EBSD), field emission gun scanning transmission electron microscopy(FEGSTEM) were employed to analyze the microstructures of the simulated heataffected zones, and FEGSTEM equipped with energy dispersive X-ray spectroscopy(EDS) was used to measure Sb and P grain boundary concentrations of the Sb-dopedand P-doped samples thermally cycled with different heat-inputs. Grain boundary segregation of Sb and P was found to occur during welding thermal cycles. Whenthermally cycled with a heat-input of60kJ/cm, the maximum segregation level ofSb and P was produced. The segregation behavior of Sb and P can be explained bythe non-equilibrium segregation theory. According to the non-equilibriumsegregation theory, a critical heat-input at which the maximum segregation could beobtained during corresponding thermal cycling. The critical heat-input wascalculated for the P-doped samples thermally cycled with different peaktemperatures. The calculations indicate that the critical heat-input changes only alittle when the peak temperature decreases because both the austenite grain size andcooling time decrease. When peak temperature is1320oC, the critical heat-input iscalculated to be67kJ/cm which agrees well with the experiment result. Theductile-to-brittle transition temperatures (DBTTs) of the simulated welding sampleswere measured. The results show that Sb or P grain boundary segregation can raisethe DBTT of HAZs, making the DBTT increase with increasing its segregation.Accordingly, this critical heat-input should be avoided in engineering practice inorder to reduce embrittlement of HAZs caused by impurity grain boundarysegregation during welding thermal cycles.Sn grain boundary segregation during thermal cycles for an Sn-doped C-Mnsteel was studied. The grain boundary was apparently segregated with Sn whenthermally cycled with a heat-input of36kJ/cm and embrittlement was induced bySn grain boundary segregation. When the heat-inputs were60and100kJ/cm, all thegrain boundaries are almost decorated with proeutectoid ferrite, the grain boundarysegregation was suppressed, and embrittlement induced by grain boundarysegregation did not take place. A large amount of acicular ferrite was found in theundoped and Sn-doped samples thermally cycled with a heat-input of100kJ/cm,leading to a lower DBTT.The Zr/Ti/Nb-doped C-Mn steel samples were simulated with the peaktemperatures of1100,1200,1250and1320oC under the heat-inputs of30,60and100kJ/cm. The prior austenite grain sizes of the thermally cycled samples weremeasured with the linear intercept method and the type, size and distribution ofsecond phase particles were measured with the use of FEGSTEM. Most of the finesecond phase (the particle size is smaller than100nm) in the samples was composedof Ti-Nb compounds. There were two types of Ti-Nb compounds, one of which hadhigher Nb content and lower dissolution temperature, and the other had lower Nbcontent and higher dissolution temperature. The ripening and dissolution of secondphase were modeled with the ripening and dissolution theory. The results show thatthe average particle size of second phase increases and its volume fraction decreases with increasing peak temperature. The existence of higher Nb-content Ti-Nbcompounds with a lower dissolution temperature could promote particle coarsening.The ripening and dissolution of second phase could cause a dramatic increase ofaustenite grain size when the welding peak temperature is higher than1250oC.According to the ability of pinning effects, the austenite grain growth duringwelding thermal cycles was divided into two regimes. Finally, the austenite grainsizes in the samples thermally cycled with different peak temperatures underdifferent welding heat-inputs were quantitatively analyzed, and an empirical formulaof grain growth was established. |
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