Study On Spalling Behavior And Microstructure Evolution Of Wheel/Rail Steels In Wear

The advancement of Chinese railways in terms of high speed and heavy load has attracted increasing attention to wheel–rail contact surface damage.This damage reduces the service life of the wheel and rail and increases the operation and maintenance costs.Therefore,understanding the damage mechanism is crucial for developing new wheel–rail materials and ensuring the running order and safety of high-speed trains.In this study,the rolling wear experiment of the wheel–rail steel was performed using a two-disk wear test machine.The fatigue spalling behavior of spot-like and strip white etched areas(WEAs)simulated via U71 MnG laser quenching and the changes of the strip WEA and its adjacent substrate during the wear process are studied.The influence of the strip WEA on the damage of ER8 C wheel steel is discussed,and the influence of the WEA on the formation of rolling wear polygonization is compared and analyzed.The main results are summarized as follows.(1)When the WEA is spot-like,only the surface pearlite has obvious plastic deformation under normal stress and creep force at the initial wear stage.The incoordination of plastic deformation between the substrate and the WEA leads to crack formation at the interface between the WEA and substrate,which then propagates to the inside of the WEA.As the cycle increases,the crack develops from the initial to the end position of the white corrosion zone,and the mutual delivery of cracks during the propagation process leads to spalling.(2)When the WEA is a strip,the incoordination of plastic deformation between the substrate and strip WEA leads to crack initiation easily at their interface.As the strip WEA is thin at the initial position,the fracture and spalling initially occur under the plastic deformation of the adjacent substrate.As the cycle increases,the middle position of the strip WEA gradually bulges,resulting in greater contact stress and creep force on the contact surface.Cracks propagate to the surface and in the depth of the WEA after their initiation at the interface of the WEA and substrate,respectively.Their mutual delivery promotes the spalling of the WEA.(3)The pearlite plastic deformation under shear stress during wear can be divided into three categories based on severity: slight plastic deformation,obvious plastic deformation,and severe plastic deformation.In the slight plastic deformation area,the density of geometrically necessary dislocation increases,and a low angle misorientation of 2°–10° is formed between the ferrite sheets.In the obvious plastic deformation area,ferrite subgrain with a misorientation of 2°–10° begins to form inside the eutectoid ferrite lamellar and transforms to a high-angle boundary;consequently,these grains start becoming refined,and the density of geometrically necessary dislocation gradually decreases.Meanwhile,in the severe plastic deformation area,the pearlite plastic deformation increases,cementite fractures,the proportion of high-angle boundary in ferrite increases,and grain refinement intensifies.(4)There is a contact area with a substrate(S-CA)and a contact area with a strip WEA(WEA-CA)on the surface of ER8 C wheel steel.As the cycle increases,the surface wear mechanism of S-CA and WEA-CA gradually changes from oxidation and adhesive wear to oxidation and fatigue wear.During the wear process,obvious plastic deformation occurs on the surface of both areas;however,the surface fatigue wear,microstructure refinement,and hardness are higher in WEA-CA than in S-CA.(5)Polygonization wear under the test conditions used in this study can be attributed to the coupling of vibration and wear.In other words,vibration increases wear,and severe wear increases amplitude to promote vibration.Additionally,the joint action of the two promotes the formation and development of polygonization wear.The polygonization order(K)has a quantitative relationship with the dominant frequency(f is a constant)and rotational speed(ω)of the device vibration,i.e.,K = f/ω.When the polygonization wear appears,there is a slight crest surface wear,primarily oxidation wear.The trough surface mainly undergoes fatigue wear,and the surface microstructure shows greater plastic deformation and microstructure refinement and hardening.The existence of the WEA will promote the occurrence of polygonization wear because the initial position of the WEA is easy to peel off and gradually evolve into a trough during the wear process.