## 1. Introduction

Machinery parts will be affected by some degrees of impact, friction, wear or corrosion in their working process. Especially under the action of long-term alternating load, fatigue failure of parts will occur, which will not only reduces mechanical efficiency but also increase power consumption and shorten service life of mechanical equipment [

1,

2,

3]. Generally, fatigue cracks initiate and expand mainly in the material of part’s surface. Therefore, surface strengthening—i.e., improving surface strength, pre-stressing surface material, or changing the state of surface residual stress—can effectively restrain the initiation and propagation of cracks and prolong parts’ service life. Surface hardening, carburizing, shot peening, and rolling are of surface strengthening technology used in industry. Rolling strengthening is the most commonly used kind of process technology that causes plastic deformation on the surface of parts through mechanical processing.

The principle of rolling for surface strengthening is shown in

Figure 1. A rolling head, such as hard and smooth ball or roller, exerts press force on the part’s surface during rolling process. Therefore, contact stress must exist between the surfaces of rolling head and part rolled by the head. Contact stress will increase with the increasing of press force exerted on part’s surface. The surface layer material will show elastic–plastic deformation as contact stress becomes larger than its yield strength. The plastic deformation of the part’s surface layer can effectively improve the surface roughness and surface hardness of rolled part. Meanwhile, the rolling process can make compressive stress in a part’s surface layer—i.e., residual compressive stress—which can directly increase the fatigue strength and fatigue life of rolled part [

4,

5,

6].

Surface rolling strengthening technology was initially mainly used for surface strengthening of low carbon steel, alloy steel, pure aluminum, brass, and other metal materials in the 20th century. With the development and application of this technology, rolling process has gradually been applied to surface strengthening for hardened steel [

7] and composite materials [

8]. Juijerm et al. [

9] investigated cyclic deformation behavior and s/n curves of deep rolled Al–Mg–Si–Cu alloy by stress-controlled fatigue tests at elevated temperatures up to 250 °C and compared to the polished condition as a reference. Their research shows that effects on the fatigue lifetime and residual stresses from deep rolling are significant. Avilés et al. treated AISI 1045 normalized steel by means of low-plasticity ball rolling process. Compared with the non-treated specimens, the fatigue strength of the ball-rolled specimens is increased up to 21.25%. Their work also provides experimental data and analyses of the surface roughness, fractography, in-depth residual stresses, and cyclic relaxation effects [

10]. After deep cold rolling, the part can produce a larger hardening layer and residual compressive stress, which has a good finishing effect [

11,

12]. Bouzid et al. [

13] studied the change of surface roughness of carbon steel after surface rolling strengthening. Based on the classical Hertz contact theory, a prediction model of surface roughness of rolling strengthening was established. In this model, the original surface roughness of workpiece surface was simplified and regarded as a smooth plane. The surface of the specimen was rolled by formula described in [

13]. The reduction of surface roughness is regarded as the applied force smooths out the irregularities of the surface, which can be used to predict the change of surface roughness of specimens after surface rolling strengthening. Hiegemann et al. [

14] considered the surface roughness of specimens before rolling and simplified the peak of surface roughness to discrete hemisphere or pyramid. The prediction model of surface roughness about rolling pressure in surface rolling strengthening was established, which has high accuracy. Korzynski et al. [

15] considered the influence of surface roughness of specimens, and proposed a prediction model for the mechanism of surface rolling hardening and finishing. Based on contact mechanics, the model included the elastic–plastic properties of materials, simplified the rolling process as the action process of normal rolling pressure on strip roughness peaks, and obtained rolling pressure and roughness. The relationship between peak drop height and rolling test was carried out to verify the model, and the accuracy of the prediction model was confirmed. Zhang et al. [

16] measured the residual stress on the surface of the specimens after surface rolling strengthening and carried out fatigue test. The results show that the residual compressive stress on the surface of the specimens after surface rolling strengthening has significantly increased, which can effectively inhibit the formation and development of surface cracks and increase the fatigue life of the specimens by 50%. It proves that surface rolling pressure strengthening is an effective method to improve the fatigue life of parts.

In the rolling process as mentioned above, elastic deformation and plastic deformation in the surface layer of a workpiece would result in dislocation configuration change and phase change. At the same time, subgrains of the rolling layer are refined to form gradient nanocrystalline layer [

17,

18,

19]. After rolling, the roughness of the workpiece surface can be reduced [

20], the work hardening phenomenon can be produced [

21,

22], and residual compressive stress can be introduced on the workpiece surface [

23], which can prevent the occurrence of fatigue failure and improve the fatigue strength and fatigue life of the workpiece [

24,

25].

In this paper, the rolling test of 210Cr12 axle is carried out, the rules and mechanism of rolling strengthening of this kind of material are revealed, and the rolling strengthening process parameters of 210Cr12 axle parts are put forward, which provides a theoretical basis and technical support for the practical application of surface rolling strengthening technology.

## 4. Conclusions

If the press force exerted on specimen surface is great enough during the rolling process, surface layer material will deform plastically. Meanwhile, grain refinement, dislocation configuration change, or phase change in surface layer material may occur, so that surface material mechanical properties can be changed by means of rolling process. Our rolling test results for 210Cr12 shaft (initial residual stress on the specimen surface was tensile stress of 52 MPa, and initial R_{a} of unrolled specimen is of 3.079 μm) shows that: (1) Surface residual stress changes significantly from the residual tensile stress of 52 MPa to the residual compressive stress −216 MPa after rolling process for 210Cr12 shaft. (2) The absolute value of residual compressive stress increases obviously with the increase of contact stress as the rolling contact stress is lower than 717 MPa. The absolute value of residual compressive stress increases gradually slowly, and finally close to a certain value with the increase of contact stress when rolling contact stress is over 717 MPa. (3) The absolute value of residual pressure stress increases obviously as the number of rolling pass cycles increases from 1 to 3. With the increase of the number of rolling pass cycles when it is greater than 3, the absolute value of residual pressure stress increases slowly and gradually approaches a certain value. (4) The absolute value of the residual compressive stress increases first and then decreases with the increase of the depth. The minimum residual compressive stress exists at the position beneath specimen surface about 0.025 mm. The depth of residual stress layer is about 0.2 mm. (5) Rolling process can effectively improve the surface microhardness of the specimen. Microhardness of rolled specimen increases within small range monotonously with the increasing of contact stress. Microhardness of rolled specimens shows a trend of first increasing and then becoming stable with the increase of rolling passes. (6) R_{a} decreases with the increase of rolling parameters (both contact stress and the number of rolling pass cycles). The minimum surface roughness R_{a} reaches 1.019μm, and the roughness decreases up to 67%. (7) Considering the effect of rolling strengthening such as residual stress, surface roughness, and microhardness, the reasonable rolling process parameters should be contact stress of 700−800MPa, the number of rolling pass cycles not larger than 3, when rolling process parameters including roller’s dimension, spindle speed, and feeding rate are as same as what was mentioned in our investigation. (8) Future work is to investigate the surface microstructures of the rolled surface at different contact stresses and number of rolling passes.