沃斯回火延性鑄鐵(Austempered Ductile Iron, ADI)是將球墨鑄鐵
經由沃斯回火處理使基材組織變態為針狀肥粒鐵及高碳沃斯田鐵為主之雙
相組織. 本研究探討Bi之添加與否以及不同厚斷面之球墨鑄鐵,在不同
之沃斯回火熱處理條件(於900C/ 1.5小時沃斯田鐵化後,分別在360C及300
C作不同時間之沃斯回火),以得到具高強度,高韌性之ADI為目的,並探討質
量效應與熱處理變數對平面應變破裂韌性值之影響. 實驗結果發
現:(1)經過ADI處理後之破裂韌性值均較鑄態大幅提高50%以上, (2)就質
量效應而言,未添加鉍元素者其厚斷面鑄件的試片之破裂韌性值較高,而有
添加鉍元素者,則為薄斷面鑄件的試片之破裂韌性值較高,此可能原因為未
添加鉍元素者,其裂縫的成長受石墨影響較大,而添加鉍元素者,則受基地
影響較大. 唯其差異僅鑄態較大,經熱處理後(無論是360C或300C沃斯
回火處理)均無太明顯之質量效應顯現, (3)就成分效應而言,有添加鉍者,
在鑄態下及經360C或300C沃斯回火處理之破裂韌值均較未添加鉍元素者
高,此乃因為鉍元素的添加可以增加球墨數,改善球墨分佈情形,在熱處理
後可得到較均勻的組織,且其裂縫成長受基地影響較大,故添加鉍元素的材
料之破裂韌性值較未添加者高.

Microstructure of austempered ductile iron (ADI) composes
of acicular ferrite and high carbon austenite. It is the
purpose of this experimentto evaluate the plane strain fracture
toughness KIC of ductile iron with different section size and
composition when subjected to austempering heattreatment
processing of different temperature and time. The results
showed that (1)the fracture toughness values of materialsafter
ADI heat-treatment raise more than 50% in comparison with as-
cast, (2)for mass effect, ADI without Bi addition has higher KIC
values in thick section size, while ADI with Bi addition has
highervalues in thin section size. This is because the crack
growth in ADI without Bi addition is affected by graphite while
ADI with Bi addition influenced by matrix. The mass effect on
fracture toughness is profound at as-cast, but after ADI heat-
treatment (no matter austempered at 360C or 300C) the change is
not very significant, (3)for composition, the material with Bi
addition before/ after ADI heat-treatment has higher KIC values
then that without Bi addition. This is because addition of
bismuth can increase nodules count and prmote graphite
dispersion at as-cast, and more uniform matrix after ADI heat-
treatment can beobtained.