T91铁素体耐热钢过冷奥氏体转变过程中临界冷却速度的研究

利用DIL805A/D高精度差分膨胀仪,通过线膨胀行为测量与微观组织分析,获得T91铁素体耐热钢连续冷却转变过程中相关动力学信息,结合冷却后T91钢组织特征,确定了T91钢过冷奥氏体转变过程中的临界冷却速度.研究表明:T91钢过冷奥氏体连续冷却过程中只存在铁素体和马氏体转变区,而不出现贝氏体和珠光体转变.在冷却速度为10K/min时该钢获得完全板条马氏体组织,9K/min时组织中开始出现铁素体,即10K/min可以定为T91钢奥氏体向马氏体转变的上临界冷却速度;当冷却速度介于3～9K/min时为马氏体和铁素体的混合组织,冷却速度为2K/min时T91钢中不存在马氏体转变,室温组织为铁素体,即2K/min可以定为T91钢奥氏体向马氏体转变的下临界冷却速度.

Determination of Critical Cooling Rates in Undercooled Austenite Transformation Process of T91 Ferritic Heat-resistant Steel

The critical cooling rates in undercooled austenite transformation process of T91 ferritic heat-resistant steel were studied by DIL805A/D high-resolution differential dilatometer. According to the dilatometric measurements, phase transformation kinetic information was obtained with the aid of the analysis of the lever rule from the recorded dilatometric curves. The microstructures of the T91 steel were adopted to determine the corresponding critical cooling rates. The results show that 10K/ min is the upper limit for the formation of lath martensite, instead of ferrite appearing at 9K/min. Hence 10K/min could be regarded as the upper critical cooling rate of Ms transformation of T91 steel. In the range of 3-gK/min, the microstructure of the transformed steel is a mixture of the martensite and ferrite. Martensite disappears at 2K/min, and the corresponding structure is full ferrite, which suggests that 2K/min could be regarded as the lower critical cooling rate of Ms transformation of T91 steel.