Wear And Rolling Contact Fatigue Properties Of Welded Joints Of Bainitic Rails For Heavy Haul Railways

With the rapid development of railway industry towards high-speed and heavy-haul,the service conditions of rails become more and more harsh.However,it is difficult for traditional pearlitic rails to further improve their strength and wear resistance by means of heat treatment and alloying to meet the service requirements of rails.Bainitic rail is expected to become a new generation of rail steel due to its excellent combination of strength and toughness,and has received a lot of research and attention.However,given that the rail welding is the principal method to realize seamless railway,and the current research mainly focuses on the performance of the base material of bainitic rails,the service performance of welded joints as the weak area of the rails has been studied less.Therefore,systematically experimental investigations on the service performance of the welding joints of bainitic rails have great significance for the practical engineering application and theories.This paper took the bainitic rail newly developed by Panzhihua Iron and Steel Group as the research object,conducted on the flash welding technology and carried out three different post-weld treatment processes,Air Cooling(AC),Wind Cooling(WC)and Normalization +Wind Cooling(NWC),to obtain the microstructure of welded joints under different processes,and the friction and wear experiments and rolling contact fatigue tests under different working conditions were systematically carried out.Combined with advanced microscopic characterization methods,the microstructures of different regions of the heat affected zone(HAZ)of each welded joint under different processes and base material,the wear and rolling contact fatigue damage behaviors under different working conditions were studied,and the influence mechanism of damage behavior on service performance was discussed.The main conclusions are as follows:(1)Based on the microstructure characterization,it can be found that the base material consisted of carbide-free bainite with obvious prior austenite grain boundaries,and the welding center of the three welded joints were mainly composed of granular bainite,while the content of M/A(Martensite / Austenite)islands in the overheated zone significantly increased along with a large number of white micro area and banded structures.The grains in the finegrained HAZ were refined,and the size of M/A islands was smaller than that in the overheated zone.The microstructure in the inter-critical HAZ was mainly coarsened granular bainite,and the grain boundaries of parent phase austenite disappeared.In contrast,the microstructure of the subcritical HAZ was mainly consisted of interlaced lath bainite.(2)The worn scar morphologies indicate that the wear scars mainly present ploughing and delamination,showing the abrasive and adhesive wear mechanism.The wear results show that the wear resistance of microstructure in the inter-critical HAZ for the AC and NWC welded joints was the worst,but the best for the microstructure in the overheated zone.While for the WC joint,the microstructure in the fine-grained HAZ has the worst wear resistance,but the best for the microstructures in the overheated zone under the low load condition and in the subcritical HAZ under the high load condition.In general,the welded joint obtained by the air cooling(AC)has the best wear resistance among the three welded joints.(3)The results of rolling contact fatigue test show that the wear rate of welded joints and base material generally increased first and then decreased with the increase of axle load,regardless of low speed(80 km/h)or high speed(120 km/h),which may be due to different crack propagation behaviors:(Ⅰ)crack propagation towards the depth of the matrix,(Ⅱ)crack propagation parallel the surface in the sub-surface layer,which of the latter was susceptible to be formed under the medium-load condition in this paper and then led to the severe delamination of material,hence resulting in serious material loss.Meanwhile it can also be found that the cracks were prone to propagate in the bainitic ferrite between the M/A islands,which to some extent hinder the propagation of the cracks.The characterization analysis on wear scars demonstrates that there were a large number of carters in the welding center without an evident peeling,while the overheated zone showed a slight loss with marginal peeling,and no visible peeling could be found in the fine-grained HAZ generally.The degree of peeling on the surface of the base material was generally higher than that of the welded joints.(4)Based on the experimental results above,it indicates that the welding joint subjected to air cooling(AC)has a promised wear resistance and rolling contact fatigue properties compared with the normalizing + wind cooling(NWC)welded joint.