Microstructure And Bending Fracture Behavior Of 75kg/m Heavy Rail Steel

During locomotive operation,rail fatigue and even fracture may occur under bending load.Therefore,the study of rail bending fracture is very important.This paper,the characteristics of fatigue crack growth rate of U75V heavy rail steel under bending stress state and tension compression stress state are compared and analyzed.The effects on the normal mechanical properties,fatigue bending and fracture flexibility of rail are studied.The results show that the quality of U75V heat treatment is better than that of rolling.The specific research work is as follows:The fatigue crack growth rate curves of U75V heavy rail steel under different stress conditions were measured,the Paris formula was fitted,and the three stages of fatigue crack growth were observed.It was found that the fatigue crack growth rate of on-line heat-treated samples was lower than that of rolling samples at all stages,and the rolling samples reached the crack instability stage faster than that of on-line heat-treated samples.Through the observation of fatigue fracture,it is concluded that the fatigue striation of on-line heat-treated sample is more dense and fine,which proves that the fatigue performance of heat-treated sample is better than that of rolled sample;the fatigue crack growth rate in bending state was compared with that in tension state of heavy steel at U75V,the results of crack growth rate of two stress states meet the requirements of national standard,which indicates that the fatigue crack growth rate of U75V heavy rail steel in two states is higher than that in rolling state The fatigue test results are consistent.By observing the microstructure,fracture shape and fatigue crack locus of U75V heavy rail steel,it was measured that the average of the heat treatment state test pearlite single layer spacing was 148.4 nm.Rolled and heat-treated bending fatigue fractures are all quasi-dissociated fractures of crystals,with an average fatigue stigma interval of 252.5 nm for rolled-state test pieces and 215.4 nm for heat-treated test pieces.Three-point bending fatigue cracks in the rolled state and heat-treated state diffuse by a mixed method of through fracture and along-line fracture,but the U75Vheavy rail steel in the rolled state often breaks along the line,and the U75V heavy rail steel in the heat-treated state diffuses the cracks.Is more greatly inhibited.When observing the trajectory of fatigue crack elongation under the tensile stress of U75V heavy rail steel in the rolled state,the fatigue crack elongation and the three-point bending fatigue crack elongation are the same,and the fatigue cracks in the two stress states are the same.There was a match in how they grew.A 3-point bending fracture toughness test of U75V heavy rail steel was carried out under static stress conditions.The results showed that the fracture toughness K_IC value of the heat-treated test piece was greater than that of the rolled test piece.Three-point fractures of the bending toughness test piece in the rolled state and the heat-treated state were observed.The toughness nest in the rolled state test piece was a large tough nest,and the toughness nest in the heat-treated test piece was a mixture of large and small tough nests.appeared.Compared with the three-point bending fatigue test specimen under the linear elastic load condition and the three-point bending fracture toughness test specimen under the elastic-plastic load condition,the U75V heavy rail steel is in the bending stress state,the load mode is different,and the fracture shape is different.