本研究旨在了解熱處理對構造用合金鋼JIS-SNCM 439之影響。Charpy V型槽衝擊試片實施淬火處理經由衝擊試驗、硬度量測以尋找SNCM 439 之最佳淬火條件。淬火試片並於200°C~600°C實施2小時二次之回火處理，以了解淬火及回火對機械性質、顯微組織以及破斷面型態的影響。
實驗結果發現 JIS-SNCM 439 最佳淬火溫度為875°C、加熱時間30 分鐘，而最佳強韌性組合的回火溫度約250°C。隨回火溫度的增加，硬度及抗拉強度皆呈現下降趨勢，衝擊值在300°C附近呈現下降且達到最小值。於200°C~300°C回火之衝擊斷口皆為劈裂 (或準劈裂) 型態，但在300°C回火之衝擊斷口的台階大、韌帶少；於500°C 以上回火之衝擊斷口皆由韌窩及韌帶組成之韌性破
斷。回火溫度在300°C以下，拉伸之巨觀破斷面呈現銑刀狀破斷型態，回火溫度在400°C以上，則呈現杯錐狀破斷型態。

The purpose of this article is to investigate the effect of heat treatment
on Ni-Cr-Mo structural alloy steel JIS-SNCM 439. Charpy V-notch impact
specimens quenching treatment had carried out to find optimum quenching
conditions by conducting impact tests and hardness measurements. The
impact specimens and tensile specimens were quenched using above the
optimum quenching conditions, then tempered for 2 hours double tempering
at 200°C~600°C, and impact tests, tensile tests, hardness measurements,
microstructures and SEM fracture surface morphology observations were
conducted to reveal the effect of quenching and tempering conditions on
mechanical properties, microstructures and fracture surface morphology.
The results show that the optimum quenching temperature is 875°C and austenitizing time is 30 minutes for JIS-SNCM 439, and the tempering
temperature for the optimum combination of strength and toughness is about 250°C. The hardness and tensile strength will decrease with increasing tempering temperature, the toughness increases initially with
tempering temperature but declines sharply reaching a minimum in
toughness at 300°C. The fracture of impact specimens has a cleavage (or
quasi-cleavage) morphology while tempering temperature is at 200°C~
300°C, the fracture has a ductile morphology which shows many dimples
while tempering temperature is above 500°C. The fracture surface of
tensile specimens shows a milling cutter fracture when tempering temperature is under 300°C, and a cup-and-cone fracture when tempering
temperature is above 400°C.