The Study Of Mechanical Properties And Fatigue Crack Growth Behavior On Hypereutectoid Rail

As a new type of pearlite steel rail,the hypereutectoid steel rail has excellent hardness and wear resistance,and is suitable for laying on a small radius curve.At this stage,China's hypereutectoid rails are in the stage of research and development,and the research on the mechanical properties of their tissues is still in the preliminary stage,and there is no research on their fatigue crack growth behavior.This article takes the BG400 hypereutectoid rail produced by a factory as the research object.By adjusting the heat treatment process,the effect of the heat treatment process on its mechanical properties and fatigue crack growth behavior is studied Finally,by changing the stress ratio,the effect of stress ratio on its fatigue crack growth behavior is discussedThe experimental results show that the microstructure of BG400 hypereutectoid rail after heat treatment has a significant change compared to the rolled state.With the decrease of isothermal temperature and isothermal time,the pearlite layer spacing decreases,the cementite content increases,and the eutectoid ferrite decreases;as the cooling rate increases,the pearlite layer decreases first and then increases slightly.The mechanical properties are also improved The mechanical properties of the experimental steel under heat treatment process 2(600?isothermal)are more excellent,of which the mechanical properties of the 8#experimental steel(600?-30s-8?/s)are the most prominentUsing SincoTec fatigue testing machine,the fatigue crack growth a-N curves of the experimental steel under different heat treatment processes were measured,and the effects of different heat treatment processes on the fatigue crack growth rate of the experimental steel were studied.The experimental results showed that:with the decrease of isothermal temperature and cooling rate,fatigue Increase the number of cycles.The fatigue crack growth rate of the experimental steel under heat treatment process 1(630? isothermal)is higher than that of the heat treatment state,and the performance is general;The fatigue crack growth rate of 7#experimental steel with a cooling rate of 5?/s in heat treatment process 2(600? isothermal)It is better than the heat-treated state and is the best.The performance of 8#experimental steel with a cooling rate of 8?/s is second.The fatigue crack growth rate corresponding to?K=10 MPa·(?)and ?K=13.5 MPa·(?)under this process is better than the national recommended standard.Fatigue fracture morphology belongs to quasi-cleavage fracture.In area I of fatigue crack growth near the threshold area,the main factor affecting fatigue crack growth is the thickness of cementite flakes;In area II of fatigue crack growth,the main factors affecting fatigue crack growth are the spacing of pearlite flakes and cementite content.The comprehensive mechanical properties and fatigue properties can be seen:All the properties of 8#experimental steel(600?-30s-8?/s)under heat treatment process 2 are better than the national recommended standards.The fatigue performance of 8#experimental steel is slightly better than the heat treatment state,the hardness and tensile strength are increased by 20.29%and 5.12%respectively compared with the heat treatment state,and the overall performance is optimalBy changing the stress ratio,it can be seen that as the stress ratio increases,the number of fatigue cycles increases;due to the crack closing effect,under the same ?K,as the stress ratio increases,the fatigue crack growth rate increases.After heat treatment,the experimental steel can reduce the influence and sensitivity of the change of stress ratio on the fatigue crack growth to a certain extent,so that it can increase the service life of the rail under abject conditions to a certain extent.