1. Introduction
The surface layers (SLs) of engineering components are directly exposed to various impacts (mechanical, thermal, chemical, physical) as a result of their interactions with other elements and the environment. These components’ metals and alloys possess a non-uniform crystal structure characterized by many micro-defects. Some defects are introduced in the blank-making process (metallurgical defects), and others appear during the machining process. As a result, the degradation of structural elements due to fatigue, wear, and corrosion occurs primarily in the SL and depends primarily on the combination of topographic, mechanical, physical, chemical, and metallurgical properties of the SL, known as surface integrity (SI). Thus, modifying only the SL is the basis of the modern multi-disciplinary direction of surface engineering (SE), whose purpose is to develop efficient and cost-effective finishing processes to improve SI in correlation with the operational behavior of the respective components.
Classifications are available according to different characteristics of the various processes within the scope of SE [
1,
2]. According to the chemical composition change, the three main techniques for SL modification are as follows: 1) the diffusion of new chemical elements in the SL; 2) the addition of new material in the form of coatings or thin films; 3) SL modification without changing the chemical composition of the material. The latter is the object of the present study.
Cost-effective SE processes without changing the chemical composition of the material are based on surface cold working (SCW) methods [
3,
4,
5]. The essence of SCW lies in the surface plastic deformation of an SL at a temperature lower than the material’s recrystallization temperature. In addition, the SCW methods are eco-friendly, as the mass of the components is conserved. The two types of SCW methods are dynamic and static [
6].
Dynamic methods (e.g., shot peening, laser shock peening, water cavitation) can be applied to the complex surfaces. With static SCW methods (i.e., burnishing methods), the surface plastic deformation is continuous over time, and the equivalent plastic strain can be controlled by the corresponding technological parameters (governing factors). A significant advantage of burnishing methods is their implementation on conventional and CNC machines, and thus, they are suitable for finishing rotary and flat surfaces. Regardless of the burnishing method classification [
6], the type of contact between the deforming element and the treated surface (i.e., rolling or sliding friction) has a determining role for the SI. In some cases [
6], slide burnishing (SB) provides less roughness, significantly greater microhardness, larger-in-absolute-value compressive residual stresses, and, as a result, greater mega-cycle fatigue strength compared to roller burnishing (RB) and deep rolling because of the greater equivalent plastic strain of the SLs and near-subsurface layers.
The deforming element in SB typically has a spherical tip and, less often, a cylindrical tip [
6]. The most common SB method is diamond deforming, which employs diamond deforming elements. Synthetic polycrystalline composites are the most commonly used diamond [
7,
8,
9,
10]. Other hard materials, such as single-crystal diamond [
11,
12,
13], hard alloy [
14,
15,
16,
17,
18], and hardened steel [
19,
20], are used occasionally. Recent research on SB on the correlation between SI and the operating behavior of slide-burnished metallic components was reviewed in [
21].
Maximov et al. presented a new static SCW method for finishing cylindrical surfaces, called slide roller burnishing (SRB) [
22]. Similar to the conventional RB method, SRB uses a deforming roller with a toroidal working surface, but its axis crosses that of the workpiece at an angle (i.e., crossing angle). As a result, a relative sliding velocity occurs in the contact zone between the roller and the treated surface. The magnitude and direction of this sliding velocity can be controlled depending by adjusting the crossing angle.
Of the three main burnishing effects (smoothing, cold work, and beneficial compressive residual stresses) on SL, the significantly lower roughness (resulting in mirror-like surfaces) is the indisputable advantage of SRB over RB. For example, in [
22], AISI 316 steel exhibited a roughness improvement of 42%, while surface microhardness improvement was only 7%. In addition, SRB achieves an oriented (anisotropic) surface texture. As a result, SRB is particularly suitable for finishing cylindrical structural elements made of stainless steel, whose typical applications require excellent corrosion resistance and very low roughness. These requirements are crucial for structural elements in the automotive and transport, aviation, medical equipment, and food industries. In these applications, the structural elements are subjected to dynamic or cyclic loads, and thus, the material fatigue is critical.
Authors of [
21] established that, to increase the fatigue life and limit, the corresponding finishing must provide an appropriate combination of SI characteristics: low roughness, increased microhardness, compressive axial and hoop residual stresses, and modified microstructure. This correlation has been confirmed experimentally for different austenitic stainless steel grades [
10,
23,
24].
Researches in [
22] determined optimal values of the governing factors (radius of the roller toroidal surface r, burnishing force
, feed rate f, and crossing angle α) for three SRB processes—smoothing, hardening, and mixed—for treating of AISI 316 stainless steel. They established “finishing – SI characteristics” correlations for these optimized SRB processes with the following SI characteristics: 2D height roughness parameters Ra, Rq, Rp and Rv, skewness Rsk and kurtosis Rku shape parameters; surface microhardness; and axial and hoop residual stress distributions.
To fully evaluate the effect of relevant SRB processes on SI, we need a database comprising the microhardness profile and the microstructure near the SL. Besides the markedly low 2D height roughness parameters, all three SRB processes provide negative skewness and kurtosis with values equal to or greater than three. In the case of low integral roughness Ra (or Rq)
this combination of shape roughness parameters improves lubrication and reduces friction and sliding wear [
25,
26]. However, according to Zabala et al. [
27], these shape parameters worsen the fatigue behavior.
Thus, we are interested in evaluating the influence of the skewness Rsk and kurtosis Rku shape parameters on fatigue behavior. Of note, the specified geometric and physical–mechanical characteristics of SI are interdependent, collectively affecting the operational behavior (including fatigue) of the respective components. For instance, the surface plastic deformation causes smoothing and strain hardening effects, which are evaluated based on the surface texture parameters, microhardness profile, residual stress distribution, and microstructure evolution. On the one hand, cyclic loading causes an evolution of the microstructure, leading to the relaxation of the introduced residual compressive stresses. On the other hand, the desired fatigue strength can be obtained for combinations of SI characteristics corresponding to different finishing processes. Therefore, for a comprehensive evaluation of the impact of the obtained SI characteristics and the effectiveness of optimized SRB processes on fatigue behavior, we need to determine the SI–fatigue behavior correlation.
This study aims to evaluate the effectiveness of the SRB method for increasing the fatigue life and fatigue limit of the most-used grade austenitic stainless steel, AISI 304, based on a comparison with the conventional RB method. The present study extends our previous work [
22], which described SRB kinematics in detail.
2. Materials and Methods
This study employed AISI 304 austenitic stainless steel in the form of hot-rolled bars with a 20 mm diameter in the as-received state. We established the chemical composition via optical emission spectrometry. Tensile tests at room temperature (20°C) were conducted using a Zwick/Roell Vibrophore 100 testing machine. The main mechanical characteristics were defined as the arithmetic mean values of three specimens [
28] (
Figure 1a). The hardness was measured via a VEB-WPM tester with a spherical-ended indenter having a diameter of 2.5 mm, loading of 63 kg, and holding time of 10 s.
RB and SRB (
Figure 2) were implemented on an Index Traub CNC lathe using Vasco 6000 lubricant. The governing factors were the radius
of the roller toroidal surface, burnishing force
, feed rate
, and velocity
of a point along the burnished surface. Specific combinations of governing factors defined different burnishing processes. According to Ecoroll [
29], two processes are critical in practice: smoothing and hardening.
The main goal of the smoothing process is to achieve a mirror-finish surface and high surface bearing ratio, while the hardening process aims to increase fatigue life and fatigue strength by inducing significant residual compressive stresses and work hardening in the SL. This study employed the optimal governing factors for smoothing and hardening RB processes obtained in [
22] for AISI 316 steel. In addition, the optimal values of angle α (the crossing angle between the workpiece and deforming roller axes) for smoothing and hardening SRB processes are and
, respectively [
22] and used in this work. The tested steel in [
22] was AISI 316, which has a significantly lower yield limit, tensile strength and hardness, and greater elongation than the AISI 304 steel used in the present study.
Four processes were defined for this study using the governing factor magnitudes in
Table 1—RB-A, SRB-A, RB-B, and SRB-B—corresponding to the smoothing and hardening processes established in [
22] for AISI 316 steel. The deforming rollers, having a diameter of 26 mm, were made of 1.3343 tool steel. After heat treatment, the measured hardness was 63 HRC.
The steel’s 2D roughness parameters were measured using a Mitutoyo Surftest SJ-210 surface roughness tester and obtained as average arithmetic values from the measurements on six equally spaced sample generatrixes.
A ZHVµ Zwick/Roell microhardness tester was used to establish the microhardness of the surface (0.05 kgf loading) and microhardness profile along the depth (0.01 kgf loading) with a holding time of 10 s. The final surface microhardness value was considered the median of the clustering of ten measurements.
To determine the content of strain-induced martensite in the surface layer (relative to austenite), we used а Bruker D8 Advance diffractometer and a DIFFRAC.Dquant V1.5 specialized software developed by Bruker (Billerica, MA, USA) [
30]. The characteristics of the residual stress X-ray measurement are shown in
Table 2.
The microstructures and surface fractures were observed via scanning electron microscopy (SEM, Zeiss Evo 10).
Rotating bending fatigue tests were conducted on a UBM testing machine. The loading frequency was 50 Hz in air. Each specimen was tested to complete destruction. Exceptions were the samples that reached
-cycle fatigue strength, after which the test was terminated. The specimen geometry is depicted in
Figure 1b [
31]. Five groups of specimens were manufactured. The first group was treated by turning and polishing and served as a reference condition (RC). The remaining four groups were treated by RB-A, SRB-A, RB-B, and SRB-B processes.
Author Contributions
Conceptualization, J.M. and G.D.; methodology, J.M. and G.D.; software, J.M., G.D., A.A., V.D.; validation, J.M. and G.D.; formal analysis, J.M., G.D. and Y.A.; investigation, A.A., V.D., Y.A., K.A., G.D., and J.M.; resources, J.M., G.D. and K.A.; data curation, J.M., G.D. and Y.A.; writing—original draft preparation, J.M. and G.D.; writing—review and editing, J.M. and G.D.; visualization, J.M., G.D., A.A., V.D. and Y.A.; supervision, J.M. and G.D.; project administration, J.M. and G.D.; funding acquisition, J.M. and G.D. All authors have read and agreed to the published version of the manuscript.
Figure 1.
Specimen geometry: (a) tensile test; (b) rotating bending fatigue test.
Figure 1.
Specimen geometry: (a) tensile test; (b) rotating bending fatigue test.
Figure 2.
Kinematic scheme of RB (a) and SRB (b), and burnishing device photo (c).
Figure 2.
Kinematic scheme of RB (a) and SRB (b), and burnishing device photo (c).
Figure 3.
Roughness profiles due to: (a) RB-A; (b) SRB-A; (c) RB-B; and (d) SRB-B.
Figure 3.
Roughness profiles due to: (a) RB-A; (b) SRB-A; (c) RB-B; and (d) SRB-B.
Figure 4.
Comparison of different processes under criterion: (а) roughness parameter Ra; (b) microhardness HV0.01.
Figure 4.
Comparison of different processes under criterion: (а) roughness parameter Ra; (b) microhardness HV0.01.
Figure 5.
Microhardness distribution along the depth from the surface.
Figure 5.
Microhardness distribution along the depth from the surface.
Figure 6.
Phase analysis results.
Figure 6.
Phase analysis results.
Figure 7.
Residual stress distribution: (a) axial; (b) hoop.
Figure 7.
Residual stress distribution: (a) axial; (b) hoop.
Figure 8.
Microstructure of a specimen processed by RB-A process.
Figure 8.
Microstructure of a specimen processed by RB-A process.
Figure 9.
Microstructure of a specimen processed by SRB-A process.
Figure 9.
Microstructure of a specimen processed by SRB-A process.
Figure 10.
Microstructure of a specimen processed by RB-B process.
Figure 10.
Microstructure of a specimen processed by RB-B process.
Figure 11.
Microstructure of a specimen processed by SRB-B process.
Figure 11.
Microstructure of a specimen processed by SRB-B process.
Figure 13.
Fracture surface of the fatigue specimen treated via turning and polishing (stress amplitude of 580 MPa; number of cycles to failure: 5700).
Figure 13.
Fracture surface of the fatigue specimen treated via turning and polishing (stress amplitude of 580 MPa; number of cycles to failure: 5700).
Figure 14.
Fracture surface of the fatigue specimen treated via RB-A process (stress amplitude of 580 MPa; number of cycles to failure: 6600).
Figure 14.
Fracture surface of the fatigue specimen treated via RB-A process (stress amplitude of 580 MPa; number of cycles to failure: 6600).
Figure 15.
Fracture surface of the fatigue specimen treated via SRB-A process (stress amplitude of 580 MPa; number of cycles to failure: 7500).
Figure 15.
Fracture surface of the fatigue specimen treated via SRB-A process (stress amplitude of 580 MPa; number of cycles to failure: 7500).
Figure 16.
Fracture surface of the fatigue specimen treated via RB-B process (stress amplitude of 580 MPa; number of cycles to failure: 13,300).
Figure 16.
Fracture surface of the fatigue specimen treated via RB-B process (stress amplitude of 580 MPa; number of cycles to failure: 13,300).
Figure 17.
Fracture surface of the fatigue specimen treated via SRB-B process (stress amplitude of 580 MPa; number of cycles to failure: 13,500).
Figure 17.
Fracture surface of the fatigue specimen treated via SRB-B process (stress amplitude of 580 MPa; number of cycles to failure: 13,500).
Table 1.
Magnitudes of the governing factors used in this study.
Table 1.
Magnitudes of the governing factors used in this study.
Burnishing processes |
r, mm |
|
|
|
RB-A, SRB-A |
3 |
250 |
0.05 |
60 |
RB-B, SRB-B |
4 |
550 |
0.11 |
60 |
Table 2.
Characteristics of the residual stress X-ray measurement.
Table 2.
Characteristics of the residual stress X-ray measurement.
Measuring device |
Bruker D8 Advance diffractometer |
X-ray tube |
Long focus Cr – Kα |
Crystallographic plane |
Fe(γ) - (220) |
Diffraction angle (2θ) |
128.78° (124° - 133°) |
Measuring method |
Offset coupled TwoTheta/Theta (sin2ψ method) |
Scan mode |
Continuous PSD fast |
X-ray detector |
SSD160-2 (1D scanning) |
Collimator spot size |
Standard Φ1.0 mm |
Measurement time for single scan |
Approx. 35 s |
Elastic constant s1
|
|
Elastic constant 1/2s2
|
|
Voltage |
30 kV |
Current |
40 mA |
Step size |
0.5° |
Time for step |
1 s |
Table 3.
Chemical composition (in wt%) of the used AISI 304 steel.
Table 3.
Chemical composition (in wt%) of the used AISI 304 steel.
Fe |
C |
Si |
Mn |
P |
S |
Cr |
Ni |
Nb |
Ti |
Mo |
Cu |
Co |
W |
V |
68.7 |
0.0383 |
0.108 |
1.72 |
0.0299 |
0.0315 |
20.7 |
7.63 |
0.0356 |
0.0049 |
0.318 |
0.38 |
0.096 |
0.09 |
0.09 |
Table 4.
Main mechanical characteristics of the tested AISI 304 steel.
Table 4.
Main mechanical characteristics of the tested AISI 304 steel.
Yield limit, MPa |
Tensile strength, MPa |
Elongation, % |
Hardness, HB |
|
|
|
|
Table 5.
2D roughness parameters obtained via turning, RB and SRB.
Table 5.
2D roughness parameters obtained via turning, RB and SRB.
Processes |
2D roughness parameters |
|
|
|
|
|
|
|
|
|
Turning |
0.482 |
0.576 |
1.224 |
1.402 |
-0.046 |
2.195 |
1.746 |
0.276 |
0.408 |
RB-A |
0.186 |
0.237 |
0.470 |
0.885 |
-0.791 |
3.703 |
0.545 |
0.142 |
0.378 |
SRB-A |
0.073 |
0.098 |
0.229 |
0.526 |
-1.072 |
7.117 |
0.219 |
0.093 |
0.166 |
RB-B |
0.105 |
0.131 |
0.305 |
0.456 |
-0.317 |
3.067 |
0.329 |
0.099 |
0.160 |
SRB-B |
0.055 |
0.076 |
0.215 |
0.388 |
-0.675 |
6.584 |
0.167 |
0.074 |
0.133 |
Table 6.
Errors in residual stress measurement.
Table 6.
Errors in residual stress measurement.
Turning |
RB-A |
SRB-A |
RB-B |
SRB-B |
Depth mm |
Error, MPa |
Depth mm |
Error, MPa |
Depth mm |
Error, MPa |
Depth mm |
Error, MPa |
Depth mm |
Error, MPa |
Axial |
Hoop |
Axial |
Hoop |
Axial |
Hoop |
Axial |
Hoop |
Axial |
Hoop |
0 |
50 |
59 |
0 |
126 |
38 |
0 |
84.7 |
62.1 |
0 |
47.7 |
41.5 |
0 |
50.3 |
25.7 |
0.03 |
28.8 |
34.3 |
0.02 |
48 |
55.6 |
0.02 |
18.9 |
35.5 |
0.01 |
52.8 |
20.0 |
0.01 |
58.8 |
31.7 |
0.08 |
34.7 |
33.5 |
0.06 |
57.1 |
33.8 |
0.05 |
41.9 |
33.8 |
0.04 |
51.4 |
75.7 |
0.05 |
54.8 |
46.5 |
0.17 |
32.3 |
35.6 |
0.11 |
32.9 |
28.8 |
0.09 |
23.8 |
39.5 |
0.10 |
63.6 |
42.1 |
0.11 |
25.8 |
66.7 |
0.24 |
66.4 |
35.9 |
0.15 |
31.9 |
31.5 |
0.15 |
23.3 |
29.7 |
0.16 |
27.4 |
47.9 |
0.21 |
31.1 |
49.1 |
0.3 |
38.4 |
31.9 |
0.24 |
41 |
33.2 |
0.19 |
13.7 |
45.3 |
0.24 |
37.0 |
30.5 |
0.26 |
72.7 |
21.2 |
- |
- |
- |
0.32 |
41.9 |
32.3 |
0.26 |
52.8 |
31.4 |
0.32 |
27.2 |
23.9 |
0.31 |
70.4 |
23.6 |
- |
- |
- |
0.4 |
33.3 |
22.9 |
0.39 |
31.8 |
39.4 |
0.37 |
32.3 |
37.0 |
0.35 |
46.1 |
49.6 |
- |
- |
- |
- |
- |
- |
0.45 |
67.7 |
18.7 |
0.44 |
42.8 |
55.1 |
0.40 |
65.1 |
56.8 |
- |
- |
- |
- |
- |
- |
- |
- |
- |
0.51 |
18.4 |
37.4 |
0.45 |
62.5 |
49.2 |
- |
- |
- |
- |
- |
- |
- |
- |
- |
- |
- |
- |
0.5 |
59.3 |
51.0 |