R350HT钢轨钢的连续冷却转变曲线

在Gleeble-3500热模拟试验机上对欧标R350HT钢轨钢进行不同冷却速度的热模拟试验,观察显微组织并测量硬度,绘制试验钢的连续冷却转变(CCT)曲线。结果表明,在冷却速率为0.5~2.5 ℃/s时,组织以珠光体为主,有少量先共析铁素体。当冷却速度为3 ℃/s时,组织中出现马氏体。由于珠光体轨钢中不允许有马氏体组织,因此冷却速度应小于3 ℃/s。同时,随着冷却速率的增大,直至10 ℃/s,珠光体开始转变温度降低,这是因为随着冷却速率的增大,在高温区停留时间缩短,珠光体转变来不及发生,并且发生珠光体相变需要较大的过冷度。随着冷却速率增加至20 ℃/s,组织基本上为马氏体。当冷速大于20 ℃/s后,组织为单一马氏体。因此,马氏体临界转变冷速为20 ℃/s。

Continuous cooling transformation curve of R350HT rail steel

Thermal simulation test of European standard R350HT rail steel with different cooling rates was carried out on the Gleeble-3500 thermal simulation testing machine. The microstructure was observed, the hardness was measured, and the continuous cooling transformation (CCT) curves of the tested steel was drawn. The results show that when the cooling rate is between 0.5-2.5 ℃/s, the microstructure consists of pearlite and a small amount of proeutectoid ferrite. When the cooling rate is 3 ℃/s, martensite appears in the structure. Since the pearlite rail steel should not contain martensite, the cooling rate of the tested steel should be less than 3 ℃/s. At the same time, with the increase of cooling rate until 10 ℃/s, the pearlite transformation start temperature decreases, which is because the residence time in the high temperature region is shortened with the increase of the cooling rate, and the pearlite transformation is too late to occur, and the pearlite transformation requires a large degree of undercooling. In addition, with the cooling rate increasing to 20 ℃/s, the microstructure is basically composed of martensite. When the cooling rate is greater than 20 ℃/s, the microstructure only contains martensite. Hence, the critical cooling rate of martensite transformation is 20 ℃/s.