Numerical Simulation Of Flow Field During Rail-end Air-cooling Quenching Process

As economy of our country is developing at a high speed, the railway transport is required to develope deeply. The use of heavy rail can meet the demand of large amount of load, therefore heat-treatment of rail end is becoming more and more emphasized. Quench cooling type is one of the most important treatments in heavy rail heat-treatments, and it is also a major factor influencing quenching quality. The type of atomize-cool and air-cool are widely used in the world now. Because atomize-cool quench treatment is strictly required in controlling and sensitive to rail surface state, there is sometimes something wrong with the quality. While the velocity of air-cool treatment waves little, what's more, air-cool treatment is not sensitive to rail surface state, influenced little by artificial factor that ensure the quality of quench.Air-cool treatment is recently used in the world, while most quench production lines are changed to air-cool quenching line. As a result, it is necessary to research the distribution regularity of the air-jet flow field and temperature field under air-cooling to support the production. As 60kg/m heavy rail and air-jet were researched, air-cooling process of heavy rail was simulated through finite element software FLUENT. The simulation result is consistent with the theoretic result.Firstly, air-jet flow field of air-cooling process was simulated. While the pressure of air inlet was 0.4MPa, inner and outer flow fields of air-jet were simulated, and air flow velocity of rail surface was analysed. Then, the air flow velocities were analysed when the distances between air-jet and rail tread were 10mm, 15mm, and 20mm. the results showed that the choice of 10mm among the three conditions was the best choice.Secondly, through setting heat transfer coefficient of rail surface in air-cooling process, the temperature field of air-cooling quench of heavy rail was numerical simulated. In calculating process, various physical parameters that changed by temperatures were fully considered. The temperature instant distribution was calculated in simulation. Relative tests showed that the numerical simulation was reasonable.Finally, the phase changing temperature of steel U71Mn was got based on its CCT curves. Through cooling curves of several key points, the cooling rate at phase transition point was calculated. By comparing with every microstructure's critical cooling rate, the final cooling microstructure was predicted. Relative tests showed that the prediction was reasonable.This paper simulated the inner and outer flow field of air-jet, flow field of heavy rail surfaces, temperature field of heavy rail under cooling process, and calculated final cooling microstructure of heavy rail based on the empirical formulation and the CCT. The results of numerical simulation were correct. In actual production, the method can be applied to research the parameters of air-jet, and predict the quenching microstructure, in order to guide the improve of technology and actual production.