Mn-Cr-Mo系贝氏体轨钢连续冷却转变的原位观察

In situ observation of continuous cooling transformation of Mn-Cr-Mo bainite rail steel

采用原位观察法研究不同奥氏体化温度1250、950℃及不同相变冷速0.8、1.5℃/s对Mn-Cr-Mo系贝氏体轨钢连续冷却转变的影响。结果表明,贝氏体板条形核长大在晶界处以有序同时或不同时生长、晶粒内部无序不同时生长的特点;晶界、晶粒内形核的贝氏体板条都以近似相等的恒定速率生长,不同位向板条遇晶界或预先形成的贝氏体板条停止生长而形成“交叉”板条组织。板条生长速率主要受相变驱动力控制,而奥氏体晶粒大小及强度是影响最终贝氏体板条长度的主要因素。降低奥氏体化温度、加快相变冷速,可加快板条生长并缩短相变时间,减少贝氏体转变量,并获得细小均匀稳定的贝氏体板条组织。Mn-Cr-Mo系贝氏体轨钢中贝氏体板条平均生长速率为4.053μm/s,支持贝氏体扩散控制相变机制。

Effects of austenitizing temperature of 1250 ℃, 950 ℃ and cooling rate of 0.8 ℃/s, 1.5 ℃/s on continuous cooling transformation of Mn-Cr-Mo bainite rail steel were studied by in-situ observation. The results show that the bainite plates nucleate and grow up orderly on grain boundaries simultaneously or non-simultaneously, but disorderly non-simultaneously in grain. The nucleated bainite plates on grain boundaries and in the grains grow at approximately the same constant rate, and the plates with different orientation stop growing when colliding grain boundaries or the preformed bainite plates and form a "cross-like" bainite structure. The growth rate of the plates is mainly controlled by the driving force of phase transformation, and the size and strength of austenite are the main factors affecting the length of the final bainite plates. It is concluded that lowering the austenitizing temperature and accelerating the cooling rate of bainite transformation can speed up the growth of the plates, shorten the transformation time and reduce the amount of transformed bainite. As a result, a fine, uniform and stable bainite plate structure can be obtained. Further, the average growth rate of bainite plates in Mn-Cr-Mo bainite rail steel is calculated to be 4.053 μm/s, which supports the diffusion-controlled phase transformation mechanism.