Research On High-temperature Oxidation And Decarburization Of U71Mn Steel

At present, the R&D and production capacities of the rail are far from meeting needs onrapid development of the railway transport industry in China. The quality, types, specifications,production, technology of rail are still substantially lower than those advanced countries.U71Mn steel is one of the most widely used heavy rail steels. Therefore, people put forwardhigher request to its yield, surface quality, internal quality, mechanical properties andappearance sizes.Steel in the processes of heat treatment, oxidation and decarburization were inevitable onthe billet surface. All kinds of experiment data showed that oxidation film and decarburizationlayer reduced the yield of steel, and steel mechanical properties such as strength, hardness andfatigue performance were affected, this caused great energy lost and waste of resources.Moreover, many experts and scholars are focus on how to reduce steel oxidation anddecarburization in actual production. Oxidation and decarburization simultaneous occurred onU71Mn steel during heating process. In this article, we studied the oxidation anddecarburization mechanisms of U71Mn steel during heated, then analyzed the effect of gas,temperature, time on oxidation and decarburization of U71Mn steel.We made a systematical study on U71Mn steel oxidation and decarburization under airatmosphere and oxygen atmosphere with an experimental method. The isothermal oxidationbehavior of U71Mn steel at temperature range of950-1250℃in the environment of air andoxygen were studied, oxidation kinetics curves were drawn by weighting the samplecontinuously, oxidation constant rate Kp and activation energy Q in corresponding conditionwere calculated, and the influences of different factors were analyzed. Samples were cut intoappropriate size to make metallographic specimen, then all metallographic specimen werepolished and etched with3%nital for microstrutural observations, measured the thickness of thedecarburization layer with significant microstructure method, curves of decarburization layer-time were plotted, the diffusion rate of carbon atoms in decarburization process and diffusion activation energy were computed, influences of heating temperature, time and atmosphere infurnace and oxidation, ect on its decarbonizing characteristic.The studies showed that at temperature range of950-1250℃in the environment of air, theoxidation weight gain and decarburization layer of U71Mn steel gradually added as the heatingtemperature and holding time rised, oxidation kinetics followed the parabolic law, oxidation rateand carbon diffusion rate increased with temperature, the activation energy of oxidation was211.25KJ/mol, the diffusion activation energy of carbon was133.08KJ/mol. In oxygen, thesame temperature conditions, the oxidation weight and decarburization layer of U71Mn steelgradually added as the heating temperature and holding time, oxidation kinetics followed astraight line, the activation energy of oxidation was106.93KJ/mol, the decarbonization layerdepth increased gradually with the temperature heated at950-1150℃, the diffusion activationenergy of carbon was87.41KJ/mol, there was a largest depth of decarbonization layer at1150℃. At1150-1250℃, the oxidation scalewas broken badly, the decarbonization layer wasoxidized, created new compact oxidation film on the surface of steel. At the same temperatureand holding time conditions, the oxidation and decarburization in oxygen were more powerfulthan in air.Research U71Mn heavy rail steel oxidation and decarburization phenomenon in heattreatment processes, provided the experimental basis and theoretical supports to reduce theoxidation burning lost and the depth of decarburization layer. Optimizing heating process,improved the quality of steel in the process of actual production.