Abstract
Bearing ring blanks of the high-speed rail bearings, machine tool spindle bearings and wind power bearings have been manufactured through the ring rolling technology. Yet, the microstructure evolution of bearing ring in cold ring rolling remains unclear. In this work, the plastic strain distribution of the rolled ring is first simulated. Then, the microstructure of bearing ring is studied by the electron backscatter diffraction technique. Furthermore, the microstructure changes in cold ring rolling are investigated through band contrast images, and the texture evolutions after various deformation ratios are analysed by the technique of 3D-Euler space distribution. This work provides valuable guidelines for enhanced understanding the role of the cold ring rolling technology on the microstructure evolution of bearing rings.
Introduction
Cold ring rolling is widely used to manufacture precise seamless rings, such as bearing rings, ring gears and connecting flanges due to its significant advantages including uniform quality, smooth surface, close tolerance and high-geometric precision. However, the influence of cold ring rolling on the plastic deformation and microstructure of bearing rings made of GCr15 steel has received little attention. So, it is of great importance to investigate the plastic deformation and microstructure of bearing ring blanks in cold ring rolling.
Previously, many studies have been carried out on the microstructural transformations of GCr15 bearing steel (AISI 52100 steel, 100Cr6 steel) under various conditions. Chakraborty and Manna1,2 studied the size, morphology and distribution of the phase through the thermo-mechanical routine included 5% hot deformation before, during or after austenitising at 1223 K for 15 min, and investigated the nanohardness, wear resistance and elastic modulus of GCr15 bearing steel during the thermo-mechanical processing. Rogal and Dutkiewicz3,4 investigated the effect of annealing on microstructure, phase composition and mechanical properties of 100Cr6 steel, and revealed the decomposition of austenite into ferrite and the transformation of martensite into ferrite under the heat treatment conditions. Yin et al.5,6 studied the effect of initial size and plastic strain on the microstructure evolution of the GCr15 steel by setting different heating temperature, holding time and deformation degree, based on which the constitutive equations for flow stress, austenite grain growth and dynamic recrystallisation of GCr15 steel were formulated by linear regression method and genetic algorithm. The microstructure of the obtained thixo-cast of 100Cr6 steel consisting of primary globular grains (average size 340 μm) surrounded by a secondary fine grains (average size 20 μm) formed from liquid phase during cooling was presented by Łukasz et al. 7 Li et al. 8 studied the effect of prior cold deformation on the microstructure evolution during austenitising through interrupted quenching, and investigated the dissolution of cementite, mean diameter of cementite particles in the cold deformation. Guo et al. 9 analysed the grain refinement limit during hot radial ring rolling of as-cast GCr15steel, in which the optimising selections of rolling parameters based on grain limit was discussed.
On the other hand, in the aspects of plastic deformation and microstructure of bearing ring blanks in ring rolling, some studies mainly focused on the geometric deformation of the bearing ring blank in ring rolling. The effects of process parameters, such as rolling force, 10 rolling speeds, 11 ring blank design 12 and forming process design 13 on the geometric deformation of ring blanks, were emphasised. Other researches mainly devote to the study of microstructure of GCr15 steel in ring rolling. Sun et al. 14 studied the pearlitic transformation mechanisms in ultra-fine grained GCr15 steel during the process of the warm rolling, in which the result that grain size had a great influence on pearlitic transformation by increasing the diffusion rate of carbon atoms in the ultra-fine grained steel was confirmed. Ryttberg et al.15,16 investigated the microstructure and texture of 100Cr6 steel in a certain rolled deformation and revealed the texture components in the central zone, outer and inner surfaces of bearing rings.
However, it should be noted that a majority of the studies above were primarily concentrated on the microstructural transformations of GCr15 bearing steel under the various conditions, while little attention was paid to the effect of cold ring rolling on plastic deformation and microstructure of bearing ring blank. Despite Ryttberg et al.15,16 employed the EBSD technique to analyse the microstructure and texture in the central zone, outer and inner surfaces of cold rolled bearing ring in a certain rolled deformation. They mainly focused on the comparison of microstructure and texture between the central zone, outer and inner surfaces for a rolled bearing ring. To further reveal the evolution mechanism of microstructure and texture of bearing ring blanks in cold ring rolling, it is necessary to study the microstructure of bearing ring blank based on the plastic deformation in cold ring rolling.
In this paper, the distribution of the plastic deformation of the bearing ring blank in cold ring rolling was analysed using the simulation method, and the relevant experiment was made using the D56G90 CNC ring rolling mill. Then, the rolled rings with the different outer diameters are cut to observe their microstructures using the EBSD technique, in which the texture evolution is revealed in cold ring rolling.
Materials and methods
Chemical compositions of GCr15 steel producing bearing ring blank (wt-%)
The cold ring rolling tests are conducted using the D56G90 CNC ring rolling mill, and the general principle of the cold ring rolling process in this machine is applied,
17
as shown in Table 2. The rotational speed of the forming roll is 2.43 rev s−1, and the feed per revolution of mandrel roll is 0.8 mm s−1. The bearing ring blanks with the rectangular cross-section are rolled to obtain the rolled rings with different outer diameters by stopping the rolling process, as shown in Fig. 1. Ring 1 is the original blank with outer diameter of 58 mm, inner diameter of 37 mm, height of 16.5 mm and thickness of 10.5 mm. The outer diameter of rolled ring 2 is expanded up to 13.8%, and for rolled ring 3 it is enlarged up to 41.3% (closing to the rolling forming limit).
Rolled rings with different outer diameters: ring 1, 0%; rolled ring 2, 13.8%; rolled ring 3, 41.3% Parameters for DS6G90 CNC cold ring rolling mill
To study the microstructure of these rolled rings, the samples with the circumferential sections (shown in Fig. 2) are prepared. Surfaces of the samples are first polished using 100, 200, 400, 600, 800 and 1000 grit sandpaper, sequentially. Then, these samples are prepared by mechanical grinding and polishing with diamond suspension down to 1 μm, followed by polishing using colloidal silica suspension. The microstructure evolution of these rolled rings is observed by means of EBSD. The analysis is done using CHANNEL 5 software
18
where band contrast (BC), band slope and 3D-Euler space distribution results achieved through automatic indexing of Kikuchi patterns. The deformed areas at a depth of ∼750 µm below the outer surface of rolled rings were selected to carry out the EBSD measurements. Some areas of EBSD measurement about 70 µm × 70 µm were investigated at each location, resulting in approximately 100–250 refined grains per location being analysed, except the original blank. The step size of the EBSD measurement was 0.1 µm, and the resulting percentage of indexing was ranged from 72 to 93%, with the somewhat lower values for the most severely deformed samples.
Observed sections of rolled rings and definition of coordinate system used throughout this work (RD, rolling direction; ND, normal direction; AD, axial direction)
Results and discussion
To study the plastic deformation of the bearing ring blank in cold ring rolling, the simulated model of cold ring rolling is established using the simulation method in Hua et al.,
19
as shown in Fig. 3. Rolls are set as rigid bodies. Forming roll is free to rotate around axial direction. Mandrel is free to rotate around axial direction and move along feed direction. Guide roll is free to move along feed direction and circumferential direction. For the contact, friction of the contact surfaces between rolls and ring is adapted Coulomb Friction model, and the friction coefficient of contact surfaces between forming roll and ring, mandrel and ring is taken as 0.15, while the friction coefficient of contact surface between guide roll and ring is assumed as 0 because that the force of guide roll acting to the ring is very small and can be neglected. Dynamic explicit method is adopted for the model calculation to avoid long time and convergence problem in calculation which may easily occur in implicit method. Eight-noded first-order reduction integration continuum element is adopted. Arbitrary Lagrange-Euler adaptive meshing is used to control deformation of elements. Mass scaling is adopted in model calculation. During the calculation, appropriate mass scaling factors are selected, computational efficiency of model is promoted, and computational accuracy is also satisfied. In the simulation, the density, Young's modulus and Poisson ratio of GCr15 steel are 7850 kg m−3, 210 GPa and 0.3, respectively. The constitutive equation is as follows
18
:
Finite element model of cold ring rolling in Hua et al.
19
The equivalent plastic strain (PEEQ) for rolled rings 1, 2 and 3 are simulated, as shown in Fig. 4. There is not plastic deformation in the outer layer, inner layer and core zone of ring 1, as shown in Fig. 4a. For rolled ring 2, a large plastic deformation in the out and inner layers takes place and few plastic deformation appears in the core zone, which is illustrated in Fig. 4b. When the outer diameter is expanded up to 41.3% (rolled ring 3), the plastic deformation is remarkably aggravated and the plastic zone is significantly enlarged in the out and inner layers, as shown in Fig. 4c. Based on the description of the plastic deformation, some analyses can be conducted that the strain hardening is unevenly induced in the axial section of the rolled ring, that is, the strain hardening occurred in the inner layer is largest, it appeared in the outer layer is less, and it at core zone is least. On the other hand, the strain hardening is gradually enhanced with increasing the rolled deformation, confirming that the amount of cold rolled deformation exerts a crucial effect on the strain hardening. Given this, it can be concluded that microstructure in the out and inner layers is notably changed in cold ring rolling. To study the microstructure evolution of the bearing ring blank in cold ring rolling, the plastic deformation zone at a depth of ∼0.75 mm below the outer surface of rolled rings is considered to research the microstructure transformation in cold ring rolling.
PEEQ in axial section of rolled ring under various outer diameters: a ring blank, b rolled ring 2 and c rolled ring 3
Figure 5 shows the grains and subgrains within the matrix under various outer diameter increments. The black line in the matrix outlines the grain boundary (misorientation angle >15°), and the subgrain boundaries are represented with yellow lines (misorientation angle 5–15°) and red lines (misorientation angle 2–5°). It can be seen that fine equiaxed grains exist in the matrix and few low-angle grain boundary appears for the ring blank, which is shown in Fig. 5a. For rolled ring 2, many subgrain boundaries, displayed in Fig. 5b, appear in the circumferential section. When the outer diameter is expanded up to 41.3% (rolled ring 3), dense subgrain boundaries are distributed in the circumferential section, and a multitude of subgrains are formed through the closed connection of subgrain boundaries, indicating the occurrence of grain refinement. On the premise of the propagation of subgrain boundaries, it is worth noting that the degree of cold rolled deformation is an influential factor with a significant effect on the formation of subgrains. That is, the larger the cold rolled deformation of bearing ring blanks, the denser the formation of subgrains is. On other hand, the total area of the subgrain boundaries is markedly increased, which can inhibit the occurrence of the dislocation slips within the matrix.
20
Therefore, it can be concluded that with intensifying the cold rolled deformation of bearing ring blanks, abundant subgrains generate within the matrix of rolled rings meanwhile the large area of the subgrain boundaries is induced in cold ring rolling.
Band contrast image under various outer diameters (grain boundary: >15° black, 5–15° yellow, 2–5° red): a section of ring blank, b circumferential section of rolled ring 2 and c circumferential section of rolled ring 3
To study the relationship of the area of the subgrain boundaries with dislocations within the matrix, the image quality of crystallographic orientation map under various outer diameters is presented in Fig. 6. The image quality is related to the number of lattice defect such as dislocation, it reflects the state of deformation and internal stresses. So it is reasonable that the image quality is used as a measure of dislocation accumulation inducing the stored energy in the deformation microstructure. The stored energy Si, for each lattice site, can be expressed to be proportional to the image quality distribution Hi, their relation is shown as follows
21
:
Spatial distribution of image quality in which the grey is proportional to the local stored energy (green indicating carbides): a ring blank, b circumferential section of rolled ring 2 and c circumferential section of rolled ring 3
From Fig. 6a, it can be seen that the image quality exhibits homogeneous distribution within each grain, which means little lattice distortion occurs within the grains due to no dislocation accumulation is induced by cold ring rolling. When the outer diameter is expanded up to 13.8%, the colour gradation within the grains appears in the circumferential section of rolled ring 3. This is because the plastic deformation causes lattice defects such as dislocations within the grains so that the strained region has low value of image quality. 22 When the outer diameter is expanded up to 41.3%, the degree of colour gradation within grains is increased in the circumferential section of rolled ring 3, which indicates dense dislocations are generated by plastic deformation. These suggest that in cold ring rolling, dislocation density within the grains is increased as the thickness of the rolled ring is gradually decreased.
Combining Fig. 6 with Fig. 5 to analyse the relationship of subgrain boundaries with dislocation density, it can be found that the degree of colour gradation within grains is higher in the circumferential section of rolled ring 3 than those of rolled ring 2. This is because low-angle boundaries are denser within grains of rolled ring 3 relative to that of rolled ring 2. In general, low-angle boundaries have a low mobility, which is attributed to the different structure of low- and high-angle boundaries. For high-angle boundaries, migration of dislocations can occur easily by single atom jumps across the rather open grain boundary structure. For low-angle boundaries, dislocation arrays migrate by a coordinated movement of atoms. An single dislocation can be easily glided, while the motion of dislocation arrays is more difficult, because the motion of arrays causes a change in shape of two (sub)grains, which is resisted by surrounding (sub)grains. During the process of limiting the dislocation array slip, dislocations are accumulated within (sub)grain boundaries, and the deformation energy in this process is stored within (sub)grain boundaries. 22 Therefore, the cold rolled deformation is responsible for the dislocation accumulation along the subgrain boundaries. At the large rolled deformation, dense dislocations accumulated within the subgrain boundaries contribute the remarkable deformation energy stored within the matrix.
To investigate the formation of the textures in cold ring rolling, the amounts of refined grains in the investigated areas of rolled rings 2 and 3 are checked through the CHANNEL 5 EBSD software and meted the requirement of analysing the texture evolution. For the bearing ring blank, the scanned area is expanded enough to fit the texture research. The textures in the 3D-Euler space in the circumferential sections of rolled rings at various outer diameter increments are represented in Fig. 7. The mesh-surface in the 3D-Euler space indicates the ideal distribution of textures. From Fig. 7a, it can be seen that the bearing ring blank contains a Goss { Texture in the 3D-Euler space in circumferential section of rolled ring at various outer diameter increments: a ring blank, b rolled ring 2 and c rolled ring 3
Conclusions
This work has shown that the amount of cold rolled deformation exerts a crucial effect on the strain hardening, in which the microstructure of out and inner layers is notably changed for the rolled ring. Employing the EBSD technique, it can be confirmed that abundant subgrains generate within the matrix of rolled rings meanwhile the large area of the subgrain boundaries is induced in cold ring rolling, indicating the occurrence of grain refinement. The degree of colour gradation within grains suggests that dense dislocations are generated as the thickness of the rolled ring is gradually decreased. Moreover, at the large rolled deformation, dislocations are accumulated within (sub)grain boundaries, and the deformation energy in this process is stored within (sub)grain boundaries. These results will give a guidance to decide the deformation amount in terms of the grain refinement of the bearing ring blank. Besides, the dispersive textures are gathered towards a pronounced texture with enhancing the deformation degree of the rolled bearing ring, and the rolled ring possesses the specific texture constituents in the different stage of cold ring rolling. To get to the needed texture constituent of bearing ring blanks, the amount of rolled deformation should be considered seriously in cold ring rolling.