本研究主要是探討獲得718超合金不同細晶等級的製程，首先是針對一般工業應用的細晶製程進行探討，因後熱處理-靜態再結晶，無法調整細化718超合金的晶粒組織，ASTM No.6~8一般工業應用等級的細晶材料，必須使用高能量和高精密的設備，嚴格管控熱成形製程，使產生完全的動態再結晶組織，但本研究探討另一可行的方法，藉由熱加工後的冷卻方式，控制相的析出量，結合靜態再結晶處理，獲得均勻的晶粒組織；研究製程是將完鍛後的工件進行水冷，以抑制δ相析出，使δ相無法影響再結晶之進行，並保存熱成形過程中的應變能，以低於δ相固溶限 (solvus) 30°C的溫度加熱，促使連續再結晶的發生，能獲得平均ASTM No.7 (31.8μm)的均勻細晶組織，突破以往無法使用靜態再結晶，調整晶粒組織的限制。
而ASTM No.9或更細之718超合金細晶材料，傳統之細晶製程必須在應變成形之前，施以時效處理，使析出大量的針狀δ相，藉由接續熱成形製程中之動態再結晶，或冷間成形應變後的靜態再結晶過程中，利用先前析出大量的δ相，抑制再結晶晶粒的成長，而獲得細晶組織，但是通常會有局部未完全再結晶組織；本研究研發的新細晶製程，反而先在高於δ相固溶限以上的溫度，進行固溶化處理，讓基地中原有的δ相完全固溶，此時Nb的固溶量趨於飽和狀態，再施以冷成形應變，使基地產生大量差排成為δ相析出成核位置，在750~950°C的靜態再結晶處理時，促使球狀微細的δ相快速析出，利用其強大抑制晶界移動的效果阻礙晶粒成長，即使在950°C的高溫熱處理，仍能獲得2.5μm或更細的細晶組織；研究方法藉由加熱溫度的控制，即能獲得200nm~2.5μm不同等級的細晶組織，較國際發表使用傳統細晶製程，所獲得4μm細晶組織更為細緻，且由硬度、常溫拉伸和650°C高溫拉伸和650°C應力破斷測試，各種細晶等級材料各具不同的應用特色。

This study investigated the process for grain refinement of superalloy 718 with various fine grain levels. Post heat treatment fails to refine the grain structure of superalloy 718. The fine grain grade of ASTM No.6~8 for general industry application should be controlled during manufacturing by inducing full dynamic recrystallization through means of a carefully-controlled hot forming process performed in a powerful and precise forging machine. This work presents an alternative method for obtaining a fine and uniform grain structure through means of static recrystallization and proper control of the δ phase formation. In the proposed method, the component is cooled in water immediately after forging to suppress δ phase precipitation and preserve the internal strain energy produced by the hot deformation process. The component is then heated to a temperature 30°C lower than the δ solvus temperature of 1030°C, resulting in continuous recrystallization of the microstructure. Our results demonstrate that the proposed static recrystallization method yields a fine microstructure with an average grain size of ASTM No.7 (31.8 μm).
For acquiring a finer grain structure, Inconel 718 is traditionally refined by aging treatment, and a high volume fraction of acicular δ phase precipitates before forming. During the following static or dynamic recrystallization process, the existing δ phase inhibits recrystallized grain growth and acquires a fine grain structure. In the proposed approach, the Inconel 718 specimens are re-solution heat treated at a temperature higher than the δ solvus temperature to ensure thorough dissolution of the precipitated δ phase into the austenitic matrix and produce a niobium oversaturated matrix. Finally, the specimens are cold compressed to produce a dislocation saturated matrix and are then recrystallized at 750~950°C to induce the rapid precipitation of fine δ phase. The δ phase precipitates exert a strong grain-boundary pinning effect, and thus a fine grain of less than 2.5μm is obtained despite the high recrystallization temperature of 950°C. The average grain size in the refined microstructure is found to be 200nm~2.5 μm, which is finer than the finest grain size of 4μm reported in the literature, achieved by using the conventional process. Hardness testing and tensile testing at 25°C and 650°C revealed that various fine grain structures have their own characteristics.

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