구상흑연주철의 소성 및 파단거동 (破斷擧動) 에 관한 연구 = A Study on the Plastic and fracture Behavior of Nodular Graphite Cast Iron

국문 초록 (Abstract)

基地가 pearlite, pearlite와 ferrite의 混合 및 ferrite로 되어있는 3種의 球狀黑鉛鑄鐵에 對한 塑性 및 破斷擧動을 調査한 結果 다음과 같은 結論을 얻었다. (1) 基地가 거의 大部分 ferrite로 分解된 것은 軟鋼과 비슷한 應力-變形曲線을 나타낸다. (2) 3種의 球狀黑鉛鑄鐵은 모두 變形速度 1.5×10^(-5)∼1.5×10^(-3) sec^(-1)인 범위內에서는 降伏强度 및 引張强度의 變形速度依存性은 거의없다. (3) 3種의 球狀黑鉛鑄鐵의 應力-變形曲線은 모두 σt=Kε^n_t 式을 잘 滿足시킨다. (4) 이들 球狀黑鉛鑄鐵의 引張强度, 伸張 및 衝擊値를 綜合한 結果 ferrite 基地를 갖는것이 强度와 延性의 가장 좋은 組合을 보여준다. (5) 衝擊試驗에 있어서 pearlite 基地들 갖는것은 典型的인 脆性破斷擧動을 나타내지만 다른 두組織을 갖는것들은 근소한 延性 破斷擧動을 보여준다. 이들 3種의 試料에 對한 衝擊 에너지는 모두 crack 發生 에너지에 相當 한다고 볼 수 있다.

다국어 초록 (Multilingual Abstract)

Nodular graphite cast irons which are consisting of three kinds of matrix, i.e. pearlite, mixture of pearlite and ferrite, and ferrite, have been prepared, then the plastic deformations and fracture behaviors have been studied by the methods of observing the stress-strain curves in tension test and load-time curves in impact test respectively.
The results obtained are summarized as follows:
(1) The stress-strain curves for the nodular graphite cast iron of ferrite matrix have shown discontinuous yielding behaviors as observed in those of mild steel.
(2) The tensile and yield strength for the three kinds of nodular graphite cast iron are almost independent of the strain rate between 1.5×10^(-5) and 1.5×10^(-3)sec^(-1).
(3) The equation σ_t=Kε^n_t seems to be correct in the plastic regions of the stress-strain curves for all kinds of nodular graphite cast irons we prepared.
(4) The nodular graphite cast iron of ferrite matrix shows better performance is combination of strength and ductility compared with other two cast irons.
(5) The nodular graphite cast irons of two types of matrix, ferrite, and mixture of pearlite and ferrite, show the quite similar fracture behaviors in impact tests.
(6) It is concluded that the impact energies of the nodular graphite cast irons of three types of matrix are not more than the crack initiation energies.