本研究主要探討S50C鋼材經不同沃斯回火熱處理條件處理後的機械性質變化。本研究藉由系統化實驗的執行，探討不同沃斯田鐵化或雙相 退火溫度（900、745oC）、時間（5、15、30、60分鐘）、不同回火溫度（300、325、350、375、400oC）、時間（15、30、60、120、180 分鐘）下S50C鋼材拉伸強度、硬度、延伸率等機械性質的變化，以期在最簡易的熱處理方法下得到所需的機械性能，符合大量生產及低成 本的工業界要求。

實驗結果顯示，S50C經900oC沃斯田鐵化再恆溫回火之後，可得下變韌鐵組織或麻田散鐵與下變韌鐵的混合組織，其拉伸強度隨回火溫度的上 升而降低。S50C經745oC雙相退火再恆溫回火之後，可得初析肥粒鐵、下變韌鐵與球狀波來鐵的混合組織，其拉伸強度與硬度會隨回火溫度的 上升而增加，延伸率則隨之降低，此趨勢則與傳統沃斯回火熱處理相反，其主要原因是S50C經745oC雙相退火再經恆溫回火之後，其機械性質 主要是由殘留波來鐵的再結晶數量所主導，而且其主導強硬度變化的殘留波來鐵含量會隨著雙相退火時間的增加而減少，因此拉伸強度隨回 火溫度上升而增加的程度，亦會隨雙相退火時間的增加而降低。在回火時間對微結構的影響方面，不論是在沃斯回火恆溫處理或雙相退火恆 溫處理，在固定的回火溫度下S50C經過30分鐘的恆溫持時後即可完成主要的相變態，更長的回火時間不會對微結構產生明顯的改變，對機械 性質的影響也不大。因此在成本考量之下，實際應用只需要進行30分鐘的恆溫回火持時就已足夠。

S50C藉由不同沃斯回火熱處理條件的組合，可以得到相當廣泛的機械性質，抗拉強度範圍為596 ~ 1930 MPa，硬度範圍為HR30N 36 ~ 72，延 伸率範圍為4.6 ~ 28.7%，將可適用於不同用途之機械零組件。

The purpose of this research is to study the mechanical properties of S50C steel under various austempering treatments. Through systematic experiments, the variation of mechanical properties (tensile strength, hardness, and elongation) with the temperature (900 and 745oC) and time (5, 15, 30, and 60 min) in austenitizing or intercritical annealing and temperature (300, 325, 350, 375, and 400oC) and time (15, 30, 60, 120, and 180 min) in isothermal tempering was characterized. In this way, it is hoped that a simple low-cost heat-treatment technique suitable for mass production could be developed to meet the various needs of mechanical properties of S50C steel in industry.

Results showed that the S50C could have a microstructure of bainite or mixed martensite and lower bainite after going through the given austempering treatments and that the tensile strength was reduced when the tempering temperature was increased. On the other hand, S50C can have a microstructure of mixed proeutectoid ferrite, bainite and globoular pearlite after going through the given intercritical annealing plus isothermal tempering treatments and that the tensile strength and hardness were increased with tempering temperature. This trend is opposite to that in conventional austempering treatment and the main reason is that the S50C mechanical properties are controlled by the extent of recrystallization of the residual pearlite after going through the intercritical annealing and isothermal tempering. The content of residual pearlite was reduced when the time of intercritical annealing is increased. Accordingly, the degree of increase in tensile strength with tempering temperature would be reduced with an increase in intercritical annealing time. The effects of tempering time on the microstructure became saturated after 30 min in both austempering treatment and intercritical annealing plus isothermal tempering treatment. This means that for a given tempering temperature the transformation of microstructure was almost completed within 30 min. In this regard, increasing tempering time longer than 30 min will not change the microstructure and mechanical properties. Under consideration of cost, it is therefore suggested to let the isothermal tempering process last just for 30 min in practical use.

By the combination of different austempering conditions, S50C steel can have a variety of mechanical properties with the tensile strength ranging from 596 to 1930 MPa, the hardness ranging from HR30N 36 to 72, and the elongation ranging from 4.6 to 28.7%. This wide range of mechanical properties could make S50C steel suitable for use in various mechanical components.