Investigation of the Relation between Rolling Contact Fatigue Property and Microstructure on the Surface Layer of D2 Wheel Steel

Through the rolling contact fatigue experiment under the condition of the lubricating oil, this article investigated the relation between contact fatigue property and microstructure on the surface layer of D2 wheel steel. The results showed that although the roughness of the original specimen induced by mechanical processing would diminish to some extent in the experiment, the 0.5 1.5 μm thick layer of ultrafine microstructure on the original mechanically-processed specimen surface would still become micro-cracks and small spalling pits due to spalling, and would further evolve into fatigue crack source. Additionally, even under the impact of the load that was not adequate to make the material reach fatigue limit, the ferrite in the microstructure underwent plastic deformation, which led the refinement of proeutectoid ferrite grains. During the experiment, the hardening and the refinement caused by plastic deformation consisted with the theory that dislocation gave rise to plastic deformation and grain refinement. The distribution laws of hardness and ferrite grain sizes measured could be explained by the distribution law of the shearing stress in the subsurface.


Introduction
Due to the operational cyclic loading at the wheel-rail interface, contact fatigue snows, water or oil will generate hydrodynamic pressure inside these cracks and reduce the friction on their surface, thus accelerating the propagation of fatigue cracks [5] [6] [7] [8] [9]. Surface roughness can result in high contact strain [10], and the stress around roughness peaks is 8 times higher than Hertzian contact stress under ideally smooth condition.
Fatigue failure formed on the friction surface is related to the microstructure of the material. White etching layers on the wheel-rail friction surface exert immense influences on the initiation and propagation of fatigue cracks and the subsequent spalling at the wheel-rail contact interface [11] [12] [13] [14]. The research results of Chen [14] and others reveal that the white etching layers formed through local laser quenching will accelerate the formation of fatigue cracks. Lian and others [15] conclude that the formation of white etching layers will speed up crack formation and failure of wheel/rail materials. Nevertheless, relative investigations into the impacts of the damaged layer of wheel/rail materials induced by mechanical processing during the process of contact fatigue haven't been that detailed [16] [17] [18].
This present paper is investigating the contact fatigue property of D2 highspeed wheel steel by using rolling fatigue tester. It investigates the relationship between the initiation of fatigue cracks and the deformed microstructure induced by mechanical processing at the outermost layer, and analyzes the evolution of the microstructure in the subsurface during the process of contact fatigue. The aim of the current paper is to provide references for further study on the contact fatigue of actual wheel/rail materials and design and processing of material compositions.

Experimental Materials and Methods
The experimental material was D2 wheel steel. Its main chemical compositions were as follows：0.53% C, 1.0% Si and 1.0% Mn. The experiment was performed on the GPM-30 rolling contact fatigue tester. The actual wheel-rail contact state was simulated by rolling the two-disc specimens, which followed the procedures in YB/T 5345-2014 Metallic Materials-Rolling Contact Fatigue Test [19]. The outer diameters of the main and accompanying specimen were both 60 mm, the width and the height of the protruding part of the main specimen were 5 mm and 3 mm respectively, while the width of the accompanying specimen was 20 mm, seen in Figure 1.  D2 and U71Mn steel, selected as the materials for wheel and rail specimens respectively, were both forged and heat-treated so that they had the similar microstructure and hardness as the actual wheels and rails. The hardness of the D2 steel specimen gained was 255 HV -265 HV, and the hardness of the U71Mn steel specimen was 295 HV -305 HV. The microstructure of both specimens consisted of lamellar pearlite and proeutectoid ferrite; however, less proeutectoid ferrite existed in the U71Mn steel.
According to the rolling angular velocity of high-speed trains at the speed of 250 mph and 0.5% of slip ratio, the rotation speeds of the main and accompanying specimens were set at 1440 r/min and 1446 r/min respectively. Based on Hertz Contact Theory [20], the maximum actual wheel-rail contact stress caused by 17-ton axle load was 1147 MPa. In the experiment, six groups of contact stresses were selected as follows: 1200 MPa, 1250 MPa, 1300 MPa, 1350 MPa, 1400 MPa and 1450 MPa. Then each stress was tested with four sets of specimens. The contact surface of the main and accompanying specimen was constantly lubricated by No. 20 engine oil. When spalling pits that were equal to or larger than 3 mm 2 appeared on the specimen surface, fatigue failure could be determined. Subsequently, the Universal Serial Bus test microscope with 300X magnification was utilized to record the macro-morphology of fatigue damage; then SUPRA 55 field emission scanning electron microscope (SEM) was employed to observe the damage morphology of the specimen surface and the microstructure of the surface layer, and FM-700 micro-hardness tester was used to measure the hardness of the specimen surface, with the load of 25 g and the duration of 15 s. The hardness at different depth of the cross-section was the average value calculated by the hardness measured from five points selected at the same depth. The first depth was 20 μm from the surface, and was increased with an increment of 10 μm. The surface hardness was the average value acquired by 10 points randomly selected along the direction perpendicular to the contact surface after mechanical polishing. Table 1 showed the experimental results of the D2 wheel steel rolling contact fatigue property: when the contact stress was lower than 1300 MPa, the fatigue life

The S-N Curve of D2 Wheel Steel
In Equation (1), S represented the contact stress (MPa), and N signified rotations. When N = 1 × 10 7 was put into the above-mentioned equation, the fatigue limit of the D2 wheel steel was 1290 Mpa.

Fatigue Spalling Damage
Macroanalysis was initially conducted on the fatigue spalling of the specimen surface under contact fatigue. The results indicated that great amounts of V-shaped spalling appeared on the specimen surface and small amounts of pox-like or irregular-shaped spalling also existed, shown in Figure 3. V-shaped spalling was also known as arrow-shaped spalling, whose apex angle oriented towards the rolling direction of the specimen. Through statistics, the degree of the apex angle was between 70˚ and 140˚. According to the morphology of the fracture surface, the fatigue cracks radiated from the V-shaped tip and propagated towards the opposite to the rolling direction. Then the specimen was cut from the center of the V-shaped spalling along the rolling direction of the specimen, and what was found was that the depth of the cracks reached about 1.    In view of the feature that V-shaped fatigue spalling generally originates from rolling contact fatigue spalling [17], the surface morphology of the contact fati-Materials Sciences and Applications In the analysis of the path along which the cracks propagated, it was discovered that fatigue cracks preferentially grew towards the proeutectoid ferrite, and then propagated along the interface between the proeutectoid ferrite and pearlitic colony ( Figure 6(a)). In certain areas where cracks penetrated pearlite, what could be observed was that pearlite on both sides of the cracks was twisted due to plastic deformation.

The Analysis of the Surface Microstructure
In order to investigate the formation of contact fatigue cracks on the surface and its relationship with the surface microstructure, slight polishing and etching was firstly conducted on the surface of the original mechanically-processed specimen and the surface of the failure specimen, and then SEM was employed to observe both of them.   varied obviously with the increasing depths: as depths increased, grains were refined step by step; when the depth reached about 200 μm, the grains were the smallest, and then gradually recovered to the size of the original microstructure again, as was seen in Figure 8. Figure 9 showed the microstructure at the depth 200 μm from the surface of the specimen on which contact fatigue failure did not appear after 1 × 10 7 r under the load of 1200 MPa. It could be seen that compared with the original central microstructure, proeutectoid ferrite was refined obviously, while the contrast between ferrite subgrains inside lamellar pearlite was enlarged-signifying that the misorientation between each subgrain was increased and the refinement of ferrite subgrains was not that obvious.

The Distribution of Shear Stress on the Surface
To discuss the reasons why the surface microstructure and its hardness varied, the distribution of shear stress had been calculated. Based on Hertz Contact Theory [20], the maximum contact stress P 0 and contact half width α in the contact region of the main and accompanying specimens were as follows: In the formulas, P 0 represented the maximum contact stress (MPa); π equaled 3.1416; L was the contact length of the specimen, and in this experiment, L 5 mm; F was the load (N) exerted on the specimen; V 1 and V 2 were Poisson's ratios of the upper and lower specimens, V 1 = V 2 = 0.3; ρ 1 , ρ 2 were the radii of curvature of the upper and lower specimens, ρ 1 , ρ 2 = 30 mm; E 1 and E 2 were the modulus of elasticity of the upper and lower specimens, E 1 = E 2 = 206 GPa; α was contact half width (μm). Then the data and the maximum contact stress of the design (P 0 ) were put into formulas (2) and (3), and the load of the specimen (F) and the contact half width (α) could be calculated and gained, shown in Table 2.
The tangential force q on the surface caused by slip ratio could be acquired through Amonton Law: In the formula, μ was the friction parameter, and in the experiment different values of contact stress all approximated 0.1.
Under the combined impacts of positive force and tangential force, the shear stress [21] beneath the contact surface was: ( ) In the formula, σ x , σ z , τ xz were stress components respectively under the combined impacts of positive force and tangential force: The stress components generated by positive force p and tangential force q were as follows: Parameters m and n were put into formula (7) and the following formulas were gained:  Put parameters m and n in formula (8) and the friction parameter μ = 0.1 into formula (7), and then stress components generated by the positive force p and the tangential force q were acquired. Subsequently, put the stress components into formula (6) in order to get σ x , σ z , τ xz ; finally, put σ x , σ z , τ xz into formula (5), and gained the distribution function of the shear stress τ 1 on the X-Z plane. Figure 11 displayed the contour line of the shear stress on the X-Z plane, signifying that the maximum of the shear stress was 0.321 P 0 at the depth of 0.6a.
In order to better observe the variations of shear stress with depths under different contact stress, shear stresses at different depths were projected onto the Y-Z plane, and then the variations of shear stress with the depths were gained, shown in Figure 12. It was discovered that the variation tendency of shear stress from the surface to the central part of the specimens was the same as the variation tendency of hardness measured: under different contact stresses, the depth of the subsurface maximum hardness from the surface corresponded quite well to that of the maximum shear stress calculated.  standard must be larger than the observed one (0.5 -1.5 μm) in this article.

The Microstructure on the Surface Layer and the Hardening Process
Therefore, the actual microhardness of this layer of ultrafine microstructure should far exceed the one measured in the experiment, which is related to the actual microhardness of the surface layer, the combining force between the surface layer and the matrix and the hardness of the matrix [23] [24]. Based on the fact that the matrix hardness of the material in the experiment was relatively low, it could be speculated that the actual hardness of this layer induced by mechanical processing would be extremely high, exerting negative impacts on fatigue properties [25]. During this investigation, it was found that this thin layer was inclined to crack and spall (seen in Figure 6), indicating that it possessed relatively high hardness.
From the results in Figure 8 and Figure 9, it could be seen that even fatigue failure specimens did not appear in the operation under laboratory conditions, the microstructure in the surface layer underwent refinement and the hardness of the surface layer was also increased, shown in Figure 10. Additionally, this type of refinement and hardening process accorded with the variation law of the shear stress calculated (displayed in Figure 11, Figure 12 and Figure 13). This  pearlite is about 600 MPa [28], under the impact of contact stress, proeutectoid ferrite preferentially underwent plastic deformation and dislocation accumulation constantly drove the increment of dislocation density, which gradually refined grains. Nevertheless, in that the ferrite inside pearlite was limited by the enhancement of lamellar cementite, the ferrite was not inclined to deform and did not reach the limit of grain refinement-but only increased the misorientation between ferrite subgrains inside pearlitic colonies.

The Formation and Propagation of Fatigue Cracks
Multitudinous research [1] [8] [10] [29] indicated that main forms of contact fatigue failure on the wheel/rail surface during actual operation all resulted from the micro-roughness, falling into the category of contact fatigue caused by a crack source on the surface. In the investigation of this article, the same result was acquired under the lubricating condition (shown in Figure 3 and Figure 5). Additionally, it was found that the layer of ultrafine microstructure with high hardness and low toughness on the mechanically-processed surface layer was nonuniform. Although protruding points like peaks and valleys on the surface were worn smooth during the rolling contact fatigue experiment and hence the roughness was diminished (seen in Figure 5), regional spalling would take place on this thin layer and finally became the crack source under the combined impact of contact stress and additional hydrodynamic stress from liquid medium.  [32] along the direction at a certain angle with the contact surface (about 30˚ -45˚) under the above-mentioned stresses (displayed in Figure 4).
During the process of crack propagation, the stress field at the tip of cracks would increase under the impact of incompressible liquid, leading the material at the tip of cracks to undergo plastic deformation. Apparently, due to the low strength of ferrite, plastic deformation tended to take place, making cracks propagate more easily; however, since lamellar pearlite possesses high strength and strong resistance to plastic deformation, it cannot be easily penetrated by cracks [28]. In certain regions, cracks penetrated lamellar pearlite and the stress field at the tip of cracks caused pearlitic lamellae to twist obviously (seen in Figure 7), displaying apparent characteristic of fatigue crack propagation and signifying lamellar pearlite held relatively strong resistance to crack propagation.
Besides, the contact fatigue experiment conducted under the condition below fatigue limit indicated that even though fatigue cracks did not appear after 1 × 10 7 rotations, the microstructure and the hardness at certain depths inside the surface layer did experience changes (seen in Figure 8 and Figure 10). The process of these changes could also be understood through the assistance of material regional properties proposed by Johnson and others [33]. Under the load of 1200 MPa and 1250 MPa, though the surface and the surface layer both experienced plastic deformation, they were roughly in the shakedown limit and did not reach the extent of fatigue failure; nonetheless, when the load was higher than 1300 MPa, the surface of the material had already entered the stage of ratcheting effect, causing plastic deformation to accumulate continuously and further resulting in the formation of fatigue cracks.

Conclusions
In this article, through the rolling contact fatigue experiment, the contact fatigue property of the wheel steel used in high-speed railway under the condition of oil lubrication was investigated; in particular, great emphasis was laid on the effects of the microstructure at the outermost layer on fatigue property. The conclusions were as follows: 1) The layer of ultrafine microstructure on the original specimen surface in- 3) Even under the impact of contact stress below fatigue limit, the microstructure in the surface layer tended to undergo plastic deformation, dislocations began to be released inside ferrite, the density of dislocations was increased, and then evolved into the stage where proeutectoid ferrite grains refined themselves, the misorientation between ferrite subgrains inside lamellar pearlite was enlarged and the hardness was increased; inside the subsurface appeared grain refinement and the hardness maximum, whose variation laws were accorded with that of the shear stress calculated. This is of great significance in delving into the effects of the microstructure and inclusions on crack formation and propagation and predicting ultra-long fatigue life during the process of contact fatigue.