RE Alloying Of GH4169 And Additive Forging Of Large-scale Disc Used In Heavy-duty Gas Turbine

Large-scale disc is one of the core components of heavy-duty gas turbine,and the main technical difficulties in preparation include the casting of homogenized large-size alloy ingot and the die forging of large-scale disc.By the using of 800MN die forging hydraulic press,the die forging forming problem of large-scale disc has been solved,and the making of homogenized large alloy ingots has now become the most urgent problem.Different from the life requirement of 1000-3000h of aeroengine,the heavy-duty gas turbine requires stable service for more than 20000h.The improving of thermal stability of microstructure and mechanical performance of turbine disc materials is also one of the difficulties that must be overcome in the making of heavy-duty gas turbine.Aiming at increasing service life and preparing homogenized large-scale turbine disc,on the one hand,exploring rare earth(RE)micro-alloying GH4169,the effects of rare earth elements on the hot working process and long-term thermal stability were studied.On the other hand,the interface oxidation and oxides evolution of GH4169 were studied,and the methods to reduce or even completely eliminate the interface oxides were explored.The main contents and conclusions in this thesis are as follows:(1)GH4169 alloy with different contents of Ce was prepared as raw material to study the effect of Ce on hot working process and alloy microstructure.The results show that Ce element can refine the as-cast dendrite and aggravate the segregation of Nb element.Interdendritic LE phase transforms to Laves phase with Ce addition,and Ce rich phase precipitates around MX phase.During homogenization treatment,some of the insoluble Ce tends to segregate at grain boundaries.Another part of insoluble Ce forms low melting phase,which will lead to forging cracking due to its uncoordinated deformation with matrix.Only when the content of Ce is less than 110ppm and completely solid solute in matrix,can the alloy be carried out with normal hot working,while Ce will slightly increase the deformation resistance.(2)The effect of Ce element on the precipitation of γ" and δ phases was studied.The results show that the nucleation of γ",phase has not been obviously effect,but Ce can inhibit the coarsening of y" phase,and they tend to grow up uniformly.Ce element inhibits the nucleation of δ phase in matrix,including nucleation on coarsening γ" phase and direct nucleation.Ce has no effect on the direct nucleation of δ phase at austenite grain boundary,but also inhibits the coarsening behavior of δphase and it tends to grow up by extending into austenite grain with needle like shape.(3)The effects of Ce on long-term microstructure,mechanical performance and thermal stability of GH4169 alloy were studied.Ce inhibits the coarsening of γ" andδ phases.Compared with GH4169 alloy,the mechanical property degradation of GH4169 alloy with Ce slows down obviously in the first 96h of exposing at 750℃,and the biggest difference occurs at 24h exposing.α-Cr phases are observed after 400h exposing at 750℃ in GH4169 alloy,and the amount of precipitation increases with the exposing time increased,while none significant α-Cr phase precipitation is observed until the aging time of 1000h in the alloy with Ce addition.The elongation and cross-sectional shrinkage of the alloy with Ce addition are significantly higher than those of GH4169 alloy after the exposing time more than 400h.Trace addition of Ce can improve the long-term thermal stability of microstructure and properties of GH4169 alloy.(4)Aiming at simulating the behavior of bonding interface at high temperature in additive forging,the oxidation and oxides evolution of GH4169 alloy under enclosed vacuum condition were studied.Under the condition of enclosed vacuum,the initial oxide film is formed by the inner layer of δ*-Al2O3 particles and outer layer of anatase-type Ti1-xAlxO2 particles with rutile-type Ti1-xNbxO2 particles at their oxide/vacuum interface.After long time oxidation,the number of rutile-type Ti1-xNbxO2 particles becomes very small,and all anatase-type Ti1-xAlxO2 particles are transformed into surface polygonal rutile-type TiO2 and internal δ*-Al2O3 particles.Rod-like α-Al2O3 appears deepest in matrix with the orientation extending to matrix.The stable order of oxides in this system is rutile-type Ti1-xNbxO2<anatase-type Ti1-xAlxO2<rutile-type TiO2<δ*-Al2O3<α-Al2O3.(5)The characteristics of interface oxides under different vacuum degrees and the effects of different surface treatments on interface oxides and mechanical properties were studied.The results show that,under low vacuum.continuous oxide film rich in Ni,Cr,Al,Fe and Ti forms at interface.Under high vacuum,the oxide is mainly small and separately distributed.Under both high and low vacuum,with the increase of high temperature holding time,the oxide can eventually change into Al2O3 particles.Different surface treatment methods significantly affect the quantity and distribution of interface oxides.After mechanical polishing,a large number of interfacial oxides can be observed,while after surface electrolysis and pickling treatment,the number of interfacial oxides is the smallest,and the tensile and stress rupture properties of the interface are equivalent to those of the control group(with the same hot working processes as test group)without interface.Surface electrolysis and pickling treatment can restore the mechanical properties of the interface after additive forging to the same level of matrix,and the evaluation results of the 2t engineering turbine disc verify the correctness of the above results.