| With the development of society, road congestion and environmental pressure have become increasingly prominent, the railway with high efficiency of transportation, low cost and less environmental pollution has become the first choice for the development of transport industry around the world, especially the development of high-speed railway for passenger transport and the high-haul railway for freight. Determined by Chinese national characteristics, such as vast territory, large population, scarce arable land and the relatively fragile innate ecological environment and so on, a highly efficient transport system based on railway must be established. After entering the new century, the high-speed and heavy-haul railway has been developed rapidly, which raise a higher demand for the heavy rail as an important part of railway. At present, several heavy rail manufacturers in China have been equipped with universal rolling mill with an international advanced level, and the dimension and accuracy of heavy rail can basically meet the requirements of high-speed railway. However, with the growth of rolling speed and cooling time, it will lead to the obvious growth of grain, the decrease of wear resistance and impact toughness, thickening of oxide scale on the surface, the serious curvature of rolled stock after cooling. The most feasible and useful way to improve the product quality is full-length heat treatment for heavy rail. But under the current conditions, the innovative technology, which need little investment and enhance the heavy rail quality, is to develop on-line heat treatment process of heavy rail. Consequently, it is imperative for heavy rail plants in our country to develop on-line heat treatment production line of heavy rail independently.In this paper, according to intensions about development of on-line heat treatment equipment and of rocesses proposed by a heavy rail plant, we choose U75V60kg/m heavy rail used most widely as the research object, under the laboratory’s condition, the on-line heat treatment of U75V60kg/m heavy rail was simulated at1:1scale, and the equipment and process parameters was confirmed. The main work of this paper is summarized as follows:(1) The compressed air was chosen to be coolant for on-line heat treatment of heavy rail, and experiment principle was determined. First of all, the heavy rail should be heated in the furnace until its temperature is the same as that of universal rolling products, stimulating the state of heavy rail under hot rolling; subsequently, heavy rail should be cooled for a period in open air, simulating the temperature drop when it is besides the track of cooling bed; and then, the heavy rail was dragged into air spraying channel to make it cooled on the top and two sides of rail head; finally, heavy rail was dragged out of air spraying channel and laid on the rollers to cooling in open air, simulating cooling on the cooling bed.(2) The design of heavy rail on-line heat treatment experimental equipment:the design of heavy rail heating device, which consists of furnace and protective gas pipes, and it is responsible for heating heavy rail to the austenitic state. The design of compressed air system, which is responsible for providing compressed air for heavy rail during heat treatment and transporting it back to the air spraying channel after cooling to certain temperature, can be achieved as follows:Firstly, the air consumption was determined according to the length of air spraying channel, which is also called air compressor displacement; Then, according to the air compressor displacement, the corresponding water cooler was chosen; Subsequently, the sub-cylinder was designed according to the intake of air spraying channel; Finally, according to the safety of compressed air system and the need of stable air transmission, the corresponding pipes and valves was designed. Air spraying channel as the core of experimental equipment, which is responsible for the cooling of heavy rail head, includes the design of the air boxes, nozzles and air box bracket. The design of rollers, including:its length and size of roller.(3) The installation and debugging of experimental equipment:The experimental equipment should be debugged after installing, including:temperature uniformity of the heating system, the patency of protective gas pipes; gas-supply capacity of air compressor, and cooling capacity of water cooler; the adjustment of position of air spraying channel and air boxes; the security and tightness of compressed air system.(4) Through observing the microstructure of U75V heavy rail steel obtained by thermal simulation experiment under different cooling rates, which provides cooling rate inference for heavy rail heat treatment for getting fine lamellar pearlite, the CCT curve of U75V heavy rail steel was drawn by using the expansive method, and the temperature range of pearlite transformation was determined, which provides a basis for the setting of the start cooling temperature and the end cooling temperature of heavy rail heat treatment; Through observing the microstructure of heavy rail samples under different holding time, the holding time of heavy rail was determined, and austenite grain size of heavy rail was tested after heating.(5) Through the on-line heat treatment of U75V60kg/m heavy rail at1:1scale, the effects of equipment parameters on the thickness and hardness of the hardened layer was determined:By adjusting the distance between nozzle and surface of rail head, the surface temperature field of heavy rail before and after heat treatment was recorded by thermal imager, and the effects of air spraying distance on the thickness and hardness of the heat treatment hardened layer was studied; By changing the temperature of compressed air, the surface temperature field of heavy rail before and after heat treatment was recorded by thermal imager, and the effects of air spraying temperature on the thickness and hardness of the heat treatment hardened layer was studied. Finally, the air spraying distance and air spraying temperature of equipment parameters were determined.(6) By changing the staying time of heavy rail in the air spaying channel, the surface temperature field of heavy rail before and after heat treatment was recorded by thermal imager, and the effects of air spraying time on the hardened layer thickness and hardness was studied. Finally, the suitable spraying time of U75V60kg/m heavy rail heat treatment was confirmed.(7) Through adjusting the compressed air pressure in the air boxes, the surface temperature field of heavy rail before and after heat treatment was recorded by thermal imager, and the effects of air pressure on the hardened layer thickness and hardness was studied. Finally, the suitable air pressure of U75V60kg/m heavy rail heat treatment was confirmed.(8) According to the temperature field before and after heat treatment recorded by thermal imager in different processes, the average cooling rate at different positions of rail head was calculated, it can be known that the main parameter affecting the hardened layer thickness is air spraying pressure. Therefore, the microstructure of hardened layer after heat treatment was observed using transmission electron microscope (TEM), and the pearlite lamellar spacing at different positions of the rail head was measured precisely using electron probe microanalysis (EMPA). The hardness, tensile strength and impact toughness of heavy rail under different heat treatment processes were measured also, finally, the relationship among heat treatment process, organization and performance of U75V60kg/m heavy rail was determined.(9) According to the heat treatment process parameters determined by experiment, heat treatment experiment was done using4m heavy rail, and the bending of heavy rail was measured during the process of air cooling after heat treatment, residual stress was also analyzed. In addition, the effect of alloying element on U75V60kg/m heavy rail was studied, especially the distribution of V and its strengthening effect on heavy rail.The U75V60kg/m heavy rail was set as the research object in this paper, its on-line heat treatment equipment and process parameters were determined. The performance of heavy rail after heat treatment was better than the requirements of railway industry standard "Technical specification for the heat treatment rails", which further tested the feasibility of heat treatment experimental equipment and process. So the experimental results provide an important reference for the construction of heavy rail heat treatment production line and the setting of on-line heat treatment process. |
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