4340鋼과 類似한 組成을 갖는 0.36%C-Ni-Cr-Mo 鋼의 變態組識 즉 tempered martensite, tempered lower bainite+10% martensite 및 tempered upper bainite의 組織에 따른 衝擊靭性을 比較 調査 하였다. 破面遷移溫度 T_(rE) 를 決定하였던 바 tempered martensite가 가장 낮은 溫度를 나타내어 靭性이 가장 좋다는 것을 알았고 tempered upper bainite의 靭性이 가장 不良했다. 各 組織別 靭性을 그 組織內 cementite의 分布狀熊와 關聯시켜 檢討하였다.

Impact toughness of 0.36% C-Ni-Cr-Mo steel, whose composition is similar to the AISI 4340 steel, has been studied in relation to microstructures: tempered martensite, tempered lower bainite +10% martensite and tempered upper bainite. The transition temperature determined by the average energy criterion, has been found to be lowest in the tempered martensite and highest in the tempered. upper bainite. The relation between impact toughness end distribution of cementite precipitates in the microstructures has been discussed.