雙相區沃斯回火延性鑄鐵之顯微結構研究

Microstructure of Intercritical Austempered Ductile Iron

10.6841/NTUT.2011.00008

陳碧亭

沃斯回火延性鑄鐵 ； 臨界溫度區間 ； 殘留沃斯田鐵 ； 相變態誘發塑性 ； austempered ductile irons ； intercritical austenitizing temperatures ； retained austenite ； TRIP effect

臺北科技大學材料科學與工程研究所學位論文

2011年

碩士

陳貞光

繁體中文

本研究針對未添加合金之商用延性鑄鐵，於雙相區溫度區中進行沃斯田鐵化，觀察其對沃斯回火延性鑄鐵顯微組織及機械性質之影響。實驗採用FCD700鑄鐵母材以775~900°C的溫度進行雙相區沃斯田鐵化後，接著以400°C進行高溫沃斯回火熱處理，其機械性質(強度、延性、韌性)隨著沃斯田鐵化溫度增加，均呈明顯的提升。其中，以接近上臨界沃斯田鐵化溫度的830°C，配合400°C的沃斯回火溫度，可獲得強度974 MPa、16.4 ％伸長率、以及166 J衝擊能之最佳製程，此一數值明顯高於其強度790 MPa、8％伸長率、以及42 J衝擊能之未經熱處理延性鑄鐵。此一較佳特性歸因於控制適當初析肥粒鐵量約8％及殘留沃斯田鐵量約27％於基地中，得以使該沃斯田鐵在加工過程中產生加工硬化，大幅提升延韌性，且配合沃斯肥粒鐵均勻分佈，可達到細晶強化效果而使整體強度獲得提昇。由穿透式電子顯微鏡觀察顯示，次微米沃斯肥粒鐵結構之均勻分佈，在拉伸過程中，以奈米級的尺寸生成麻田散鐵組織，導致加工硬化，對於強度與延性均有相當大的貢獻，此種均勻遍佈於殘留沃斯田鐵當中的奈米結構成長使得在雙相區內進行沃斯回火的鑄鐵材料整體韌性獲得大幅提升。

The current study investigates the influences of intercritical austenitizing temperatures on microstructures and mechanical properties of austempered ductile irons. A series of intercritical austenitizing temperatures ranging from 775 to 900°C are used and austempering is performed at 400°C on a conventional FCD700 ductile iron. Experimental results show that mechanical properties, including strength, ductility, and the toughness, all increase with intercritical austenitizing temperatures till an optimum austenitizing temperature of 830°C. At this optimum processing condition, strength of 974 MPa, impact energy of 166 J, and 16.4% ductility is achieved. These properties are much higher than non-treated ductile iron with 790 MPa strength, 42 J impact energy, and 8.2% ductility. The 830°C austenitized and 400°C austempered microstructure consists of 8% proeutectoid ferrite and 27% retained austenite. The uniform distribution of refined ausferrite grains is responsible for the increase of overall strength. Furthermore, nano-size martensitic transformation induced by plastic strain can be observed via TEM inside the blocky type retained austenite. The increased carbon content, adequate size and distribution of retained austenite combine to increase toughness through martensitic transformation giving rise to transformation induced plasticity (TRIP) effect.

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