本研究是利用退火時間及回火溫度等熱處理條件的改變，探討低合金鎳鉻鉬鑄鋼之低溫衝擊韌性與金相顯微組織的關係。熱處理分為兩種：一種是在920 ℃下退火，分別保持1、3、7及12 hr後，再進行淬、回火熱處理；一種是先進行退火、淬火熱處理，最後再施以400、500、590、620及650 ℃的回火熱處理。熱處理後試片在-40 ℃作低溫衝擊，並以SEM觀察金相與破斷面。
實驗結果顯示：(1)退火及回火的調質熱處理可以明顯提升低合金鎳鉻鉬鑄鋼在-40 ℃的低溫衝擊韌性，但對硬度的影響不大；(2)以-40 ℃的衝擊值為評估依據，最佳的調質熱處理條件為，退火是920 ℃持溫7小時後爐冷，淬冷是940 ℃持溫1小時後以60 ℃油淬冷，回火是在620 ℃持溫1小時後空冷，其衝擊值超過37 J；(3)低溫衝擊韌性的提高，可能與回火熱處理後所產生的下變韌鐵與回火麻田鐵組織有密確關係。

In this study, the heat treating factors of annealing time and tempering temperature were considered to investigate the relationship between low temperature impact toughness and microstructures of low alloy Ni-Cr-Mo steel. Experiments were conducted in two categories: one of them was annealed at 920 ℃, for 1, 3, 7, and 12 hr respectively, followed with a quenching and tempering; the other set of experiments was to conduct tempering in various temperatures as 400, 500, 590, 620 and 650 ℃. Specimens after various heat treatments were impact tested at -40 ℃. The microstructures of heat treated specimens and fracture surface of impact tested specimens were examined with SEM.

The results showed: (1) the annealing together with the quenched and tempered process can significantly improve the -40 ℃temperature impact toughness of low alloy Ni-Cr-Mo steel, but has little effect on hardness; (2) based on the value of impact toughness at -40 ℃, the best condition for quenched and tempered is annealed at 920 ℃ for 7 hr after then furnace cooled, austenitized at 940 ℃ for 1 hr and oil quenched with oil temperature at 60 ℃, and tempered at 620 ℃ for 1 hour followed by air cooling. Followed with this process, the impact value -40 ℃can over 37 J; (3) the increasing of impact toughness at low temperature may be contributed from the presence of low bainite and tempered martensite which are resulted from the designed heat treatment.

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