Research On Shcct Curves And Repair Welding Processes Of A Bainite Steel Frog

Frog was one of the key track components for train to switch from one line to another, and was the maximum damage location of railway structure. After the wear failure or fatigue damage failure of rail frog, surfacing welding could be used to repair it. To guarantee the safety and reliability in the train operation, there should be very high demands of the surfacing metal’s microstructures and properties. It was very important for obtaining high quality surfacing joints. Bainite steel was another frog material coming out after high manganese steel. It had perfect weldability and excellent comprehensive mechanical properties. Now there were some researches on bainite steel rail and frog at home and abroad, but mainly focused on material components design, the weldability with rail and CCT curves measurement. SHCCT diagram of bainite steel frog and repair welding prcesses had important practical significance, but there was rare report about it at home and abroad.This paper introduced a kind of bainite steel used for frog, and the evolution of microstructure in Heat Affected Zone under thermal cycle was simulated by Gleeble-3500. The microstructures and properties of HAZ were investigated by optical microscopy (OM), scanning electron microscopy (SEM), hardness test and so on. The SHCCT diagram of bainite steel was constructed, and it showed that there was only bainite and martensite transformation region. The results showed that with the increase of the cooling rate, the microstructures changed from mainly acicular lower bainite+little martensite (when the cooling rate was within0.2℃/s～1℃/s), to lath bainite+martensite(when the cooling rate was within1.5℃/s～5℃/s), and finally to mainly lath martensite+little bainite(when the cooling rate was within7.5℃/s-40℃/s). SHCCT diagram could be used to predict microstructures and properties of HAZ, and had very important significance in guiding repair welding processes of bainite steel forg.In this paper, four different welding processes were designed. Two different welding current and two different preheat temperature were used for surfacing test of bainite steel frog. Optical microscopy, vicker hardness tester, wear tester, impact tester, SEM etc. were used to test microstructures, hardness, wear weight loss, impact absorbing energy and wear morphology. The conclusion was that when the current was120A and the preheat temperature was250℃the hardness of surfacing metal was the highest, the wear weight loss was least and the impact absorbing energy was the highest. In practical repairing works, this process could be considered.Finally, multiple repair welding test was conducted, in this paper repair welding was conducted for two times, and the best process(120A, preheating250℃) was used. Then optical microscopy, vicker hardness tester, wear tester, impact tester, SEM etc. were used to test microstructures, hardness, wear weight loss, impact absorbing energy and wear morphology. Comparing the results of two times repair welding with one time, we could concluded that the impact absorbing energy of two times surfacing was higher than that of one time surfacing, the hardness was lower and the wear weight loss was more. In practical repair welding processes, the less the repair welding time was the better the process was.