利用DIL805A/D高精度差分膨胀仪，通过线膨胀行为测量获得相关动力学信息，结合冷却后的组织特征，研究了T91钢不同冷却速度（2～6000℃/min）下过冷奥氏体的相变过程和产物，确定了该钢组织转变的临界冷却速度，研究了冷却速度对马氏体转变点及组织的影响，绘制了连续冷却转变曲线，研究表明：T91钢的连续冷却过程中只存在铁素体和马氏体转变区，10℃/min和2℃/min分别为马氏体转变的上、下临界冷却速度。不同淬火速度对T91钢马氏体开始转变温度有较大的影响，其不同于随冷速增加而相变点升高的经典理论。淬火速度通过碳原子气团、内应力的形成来影响过冷奥氏体状态，从而影响相变点；随淬火速度的增加，过冷奥氏体转变后的组织呈细化趋势，但当速度大于1000℃/min时，冷速的变化对组织形貌几乎没有影响。

The influences of cooling rate in a wide range of 2～6000℃/min on phase transformation process and microscopic structure of T91 ferritic heat-resistant steels were studied by DIL805A/D high-resolution differential dilatometer. The rule of phase transformation of T91 steel during continuous cooling is clarified . The results show that the martensite critical cooling speed（10℃/min） is determined, and the continuous cooling transformation (CCT) curve is obtained. The quenching rate has a significant effect on the onset temperature of Martensite transformation Ms. This observation is different from the classical theory that claimed the transformation point falls when cooling rate reduces. The quenching rate determines the transform point by affecting the carbon atomic group and internal stress. With the quenching rate increasing, the structure has a tendency of fining, whereas the cooling rate beyond 1000℃/min, it has nothing on the morphology of the structure.