高合金化GH4065鎳基變形高溫合金點狀偏析研究

高合金化GH4065鎳基變形高溫合金點狀偏析研究STUDY ON FRECKLE OF A HIGH-ALLOYED GH4065 NICKEL-BASE WROUGHT SUPERALLOY

GH4065合金系新型高合金化鎳基變形高溫合金，大錠型中易形成點狀偏析缺陷。本研究以真空感應熔煉+電渣重熔+真空自耗重熔三聯冶煉GH4065合金直徑280mm棒材為對象，系統研究了該合金點狀偏析的低倍組織、元素分布、第二相及晶粒形貌，分析了典型溶質元素對點狀偏析行為的影響，探討點狀偏析的形成規律與機制及控制思路。結果表明：GH4065合金點狀偏析主要源于枝晶間富Ti、Nb等元素的熔體密度較小沖破枝晶臂流動形成的通道偏析；鍛造后生成較多的板條狀的η相、塊狀的M3B2型硼化物與MC型碳化物。熱力學相計算亦證實了點狀偏析區較正常組織區域更容易生成η相、M3B2和MC。熱處理后點狀偏析中仍存在板條狀η相，較正常組織區域一次γ′相的尺寸和數量明顯增加，二次γ′相尺寸和形貌基本相同但數量減少。分析發現，由于點狀偏析區的成分變化使得γ′相回溶溫度升高，導致鍛造中粗大γ′相阻礙結晶長大，點狀偏析區晶粒尺寸小于正常組織區域。采取前道次冶煉精細化控制、釋放電極殘余應力、適度降低真空自耗重熔熔化速率、加強真空自耗重熔冷卻等措施，可以有效地降低點狀偏析的形成傾向。

GH4065 alloy is a new type of high-alloyed wrought superalloy, in which freckle defect is extremely prone to form in large ingot. In the present research, the freckle of GH4065 alloy Φ280mm bar produced by VIM + ESR +VAR triple smelting is studied. The macrostructure, secondary phases and grain structure of the freckle are investigated, the influences of solute elements on the freckle are analyzed, both the mechanism and control methods are also discussed. The results show that the freckle in GH4065 alloy is caused by channel segregation with the low-density Ti and Nb-rich melt flows. Additionally, lots of lath-like η-phases, block M3B2 borides are formed in the forged condition. It is confirmed with thermodynamic calculations that the η-phases, M3B2 borides, and MC carbides are much easier forming in the freckle than that in matrix. After heat treatment, compared with matrix, the lathy η-phases are still existed in the freckle; the size and quantity of primary γ′ phases increase significantly while the size and morphology of the secondary γ′ phase are basically identical, only with less quantity. It has been found that due to the high content of γ′ phase, the γ′ dissolution temperature in the freckle is higher than that in the matrix. This induces an impededrecrystallization process caused by the coarsened γ′ phases during forging process and the grain size of the freckle region is significantly smaller than that of matrix. Based on this study, the formation of freckle can be effectively controlled by meticulous controlling of the previous smelting process, releasing of electrode residual stress, suitably reducing VAR melting rate, and accelerating VAR cooling.

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