Effect of the shot peening process on the fatigue strength of SAE 5160 steel

EFFECT OF THE SHOT PEENING PROCESS ON THE FATIGUE STRENGTH OF SAE 5160 STEEL Héctor E. Jaramillo S.1, Nelly Alba de Sánchez1, Julian A. Avila D.2 1 Departamento de Energética y Mecánica, Universidad Autónoma de Occidente, Cali-Colombia 2 São Paulo State University (UNESP), Campus of São João da Boa Vista, Av., São João da Boa Vista, SP, Brazil Corresponding author [email protected] ABSTRACT The fatigue behaviour of SAE 5160 steel was evaluated before and after applying a shot peening process, using different Almen intensities and surface coverings (uncovered, partial coverage and total coverage). In the high cycle fatigue tests maximum stresses of 0.8 Sut, 0.7 Sut and 0.6 Sut were applied in the three-point bending test on an Instron 8872 servo-hydraulic machine at a frequency of 10Hz and a constant stress ratio of Smin/Smax= 0.2 for all tests. The fatigue tests were performed based on the ASTM E8, the specimens were classified into six groups for each stress evaluated, and each group consisted of three standardized specimens (ASTM E466). Also, yield strength, ultimate strength, hardness and microhardness were obtained. The Wilcoxon's non-parametric test was used to statistically compare all the mechanical properties obtained from the base material with those obtained after the application of the shot peening, for the different surface coverings and Almen intensities. The results showed that the shot peening process significantly increases the fatigue strength of the material, with a 94% increase in fatigue strength of the fully coated specimens. However, no significant increase in fatigue strength was found due to a change in the Almen intensity value. A high correlation factor was found between the increase in ultimate resistance and the increase in Almen intensity; however, for the yield stress the correlation was medium and inverse. For hardness and microhardness, the correlation factor was very low. Finally, the microhardness values revealed a 3% increase in Vickers microhardness of the shot peening specimens compared to the untreated specimens. Key words: SAE 5160 steel, fatigue strength, shot peening, Vickers microhardness. 1. INTRODUCTION The shot peening process has two main applications: as a method to improve the fatigue lifetime of mechanical components subjected to high stresses; and as a method to make complex and precise shapes in thin sheets. The process forms a surface layer of residual compressive stresses that prevents the initiation and growth of cracks in the components when operating under cyclic load operating conditions (1). This technique is also used to inhibit or reduce the concentration of stress, especially in fillets and notches of metal components. In this direction, machine elements are increasingly subjected to high mechanical performance and require, in particular, better fatigue lifetime and corrosion resistance. However, fatigue and corrosion are closely related to the surface finish of the part, hence the great importance of shot peening in surface improvement. The positive effect of shot peening (2) is to induce uniform compressive stress on the component surface and as a result an improvement in: fatigue strength, corrosion fatigue, hydrogen-induced corrosion, and abrasion and erosion corrosion. The shot peening-induced dislocation structure of the surface layer and thereby reducing plastic strain amplitudes, and reduces pit initiation by embedding surface inclusions into a compressive stress field susceptible to modify the structure of steel-inclusion interfaces (3) improving the fatigue strength and corrosion. Shot peening is used to improve the mechanical properties of steel used in springs and on the shafts to reduce the effects of the concentration of stresses produced by holes and section changes. In steels used for springs, SAE 1074 and SAE 6150, increases in the fatigue strength limit of up to 60% have been obtained and, for stainless steels, up to 100% (4). On the other hand, in gear trains, increases in fatigue strength have been reported from 438% to 1150%. In steering knuckles, the increases in fatigue strength have been greater than 121%, while in axles they were 100% to 1000% (5). Haradaa et al. (6), reported the results obtained in a rotary fatigue test with fully inverted load on an S-N curve for different shot peening treatments on a hardened and tempered tool steel (JIS-SKH51) at 550°C. As a result, there was a noticeable increase in the lifetime span to fatigue due to the effect of shot peening. Boswel et al. (7), reported the effect of shot peening on the S-N curve of a cantilever spring of a helicopter component made of SAE 1095 steel, which resulted in a 70% increase in the fatigue lifetime of the springs. Ofsthun (8) reported an increase in fatigue lifetime of approximately 100% in tests performed on aluminum alloy joints with shot peening. However, when the material was exposed to high temperatures, the benefit obtained in the residual stresses was significantly lost, cancelling in some cases the improvement in lifetime to the fatigue produced by the shot peening. Yves (9) demonstrated that by hardening the sintering limits of steel alloys by powder metallurgy, their strength can be increased by up to 20% and their fatigue lifetime by a factor of 10. Shot peening also has a favourable effect on the variation of the stress intensity factor and crack growth per duty cycle. In this direction, Dorr and Wagner (10) using an aluminum alloy 2024-T3 demonstrated how shot peening improves crack growth per cycle, so, for the same value of tenacity variation with shot peening, lower crack growth values are obtained for the same work cycle. In this direction, Feng et al. (11) found that for both metals (steel and magnesium), the crack growth rate decreased after the shot peening process was applied, resulting in a higher number of component duty cycles. Shot peening also inhibits stress corrosion because the surface acquires a state of compressive stress, which prevents corrosion from entering the grain boundaries. Rahmatalla et al. (12) demonstrated the benefits of the shot peening effect on 7075-T6 aluminum, where there was a substantial increase in lifetime when the material was subjected to two corrosive working environments. Lamaire et al. (13) worked on the effect of shot peening on the prevention of intergranular corrosion in austenitic steels, proving that the film of plasticised material produced by the shot peening effect protects the material from corrosion. In this direction, this paper shows the evaluation of the fatigue behaviour of SAE 5160 steel before and after applying a shot peening process, using different Almen intensities (15A, 17A and 22A) and different percentages of sample surface coverage (uncovered, partial coverage and total coverage). The purpose of this is to find the process conditions that allow the best behaviour of SAE 5160 steel to fatigue, so that the steel can be used to manufacture leaf spring. 2. METHODS AND MATERIALS The shot peening process was carried out in a centrifugal turbine blasting machine, which is equipped with a Programmable Logic Control (PLC), by means of which the parameters were controlled to provide the appropriate intensity. A CW-35 round cut wire (0.3 mm diameter) shot with a hardness of 55-60 HRC was used in the process. The specimens underwent a heat treatment of hardening at 850°C, tempering at 480°C (2.5 hours) and the Shot Peening process. The specimens were classified into six groups (Table 1): one group without shot peening, groups 1A (with Almen intensity of 22), 2A (with Almen intensity of 17) and 3A (with Almen intensity of 15) with total coverage (Figure 1a), group 2B (with Almen intensity of 17) with partial coverage (Figure 1b) and group 2C (with Almen intensity of 17) with low coverage (Figure 1c). In this direction, groups 1A, 2A and 3A have the same coverage but different Almen intensity and groups 2A, 2B and 2C have the same intensity and different coverage. The gathering of these groups is due to the fact that in the first group of groups (1A, 2A and 3A) we wish to analyse the incidence of the Almen intensity and, in the second group of groups (2A, 2B and 2C) we wish to analyse the effect of the covering on the mechanical properties of the steel. The mechanical properties of SAE 5160 steel, with and without the Shot Peening process, were determined by tension and fatigue tests. The tensile tests were carried out under the ASTM E8 (14) (Figure 2a), a universal UTS 200.3 testing machine of 20 kN capacity was used, with a load application time of 90 seconds, a preload of 700 N and a 50 mm extensometer. The three-point bending fatigue tests were performed using specimens designed in accordance with the ASTM E466 (15) (Figure 2b), using an Instron 8872 servo-hydraulic machine, at a frequency of 10 Hz and maximum stresses of 0.8 Sut, 0.7 Sut and 0.6 Sut with a constant stress ratio (Smin/Smax = 0.2). To perform the fatigue tests (Figure 3), three samples per each level of stress were used, in total nine samples per group (without shot peening, 1A, 2A, 3A, 2B and 2C) were tested. Prior to the stress test, the Rockwell C (HRC) hardness of SAE 5160 steel was evaluated for each group (ASTM E18-17e1) (16), with three indentations on the surface of each specimen. Also, microhardness tests were performed using an Indentec ZHV Micro Hardness Tester and a load of 0.5 kg (ASTM E92-17)(17). Three measurements were made from the application side of the shot peening to the inside of the specimen. Finally, the metallographic analysis was performed using an Olympus PM-3 microscope and the Image Pro-plus image analyzer, a bakelite assembly was performed with cross-section cuts of the fractured specimens in the fatigue test. Then, they were polished with 1000 reference sandpaper and given a mirror finish using cloths, and finally they were attacked with 2% Nital chemical reagent to reveal their microstructure (ASTM E3-11) (18). The data were statistically evaluated using Minitab (Inc., State College, Pennsylvania, USA) and Excel software (Microsoft, USA). Due to the low number of specimens, the nonparametric Wilcoxon-signed rank test (19) was used to detect differences between the different specimen groups. As a criterion it was defined that if the p-value is less than 0.1 (p<0.1) there are significant differences between the analysed groups. The correlation coefficient (R) was calculated to obtain the correlation between the different mechanical properties obtained versus the Almen intensity and the surface coverage. 3. RESULTS From the stress tests, the ultimate strength and yield stress were 1484.3 MPa (SD=25.4 MPa) and 1262.5 MPa (SD=41.3 MPa) respectively (Table 2, Figure 4). For the specimens with total coverage and with different Almen intensity, groups 1A (22A), 2A (17A) and 3A (15A), an increase of 3%, 4% and 4% respectively of the ultimate strength of the samples with shot peening compared to the ultimate strength of the specimens without the process was found. In this direction, a strong correlation (R=0.998) was found between the increase in the Almen intensity and the increase in the ultimate strength. In the case of yield stress, a medium and inverse correlation (R=-0.690) was found with the increase of the Almen intensity. In the fatigue tests, for specimens with total coverage and different Almen intensity, groups 1A (22A), 2A (17A) and 3A (15A), an increase in cycles of 1426%, 1425% and 1578% respectively was found with respect to specimens without the shot peening (Figure 5). A high correlation (R=0.887) was also found between the increase in Almen intensity and fatigue lifetime. In the case of groups 2A, 2B and 2C a low correlation was found between coverage and fatigue lifetime (R=0.328). However, the fatigue lifetime increased (Figure 6) by the effect of partial coverage by 95.6% and 80.8% for total coverage compared to uncovered specimens. No increase in fatigue strength was found for low coverage. The Wilcoxon test found statistically significant differences (pvalue<0.1) between all 1A, 2A and 3A Without Shot Peening groups, as well as between all 2A, 2B and 2C Without Shot Peening groups. The characterization of the fracture zones in the specimens before and after the fatigue test was performed (Figure 7). The upper side of the neutral axis of the sample is under to compression stresses and the underside to tension stresses (Figure 7b), which favored the crack nucleation on the tension side as show on the fracture surfaces. Samples with Shot Peening show fewer crack initiation points than non-penned samples on the underside, thus, some nucleation occurred on the right side, which agrees with a beneficial aspect of the compressive residual stresses induced by shot peening. Crack propagates from the initiation points through the samples until the final crack; therefore, some beach marks are well-defined on the penned samples (Figure 7e and Figure 7f.). Some river patterns enclosed in a semi-ellipse and radial marks pointing to the origin of the crack (Figure 7c), then the crack propagation continuous in a smooth way. In the case of the non-penned sample tested with 0.7Sut (Figure 7f), the crack propagation has a smoother surface and small striation steps indicating that the crack tip was undergo low stresses than the 0.8Sut tested sample (Figure 7e). About the last stage (final fracture), was characterized by a fibrous aspect that can be correlated with a ductile tearing fracture. The metallographic analysis of the steel revealed a microstructure with tempered martensite, which is a characteristic phase of this steel, after having undergone a heat treatment of hardening and tempering. Rockwell C hardness was between 41 and 45 HRC. Particularly, for the Without Shot Peening group an average of 42.6 HRC (SD=0.67 HRC) was obtained, for the 1A group 42.7 HRC (SD=1.66 HRC), for the 2A group 43.7 HRC (SD=1.41 HRC) and for the 3A group 43.3 HRC (SD=0.87) (Figure 8). Statistically, a low correlation (R=0.390) was found between Rockwell C hardness and Almen intensity. Finally, Vickers microhardness tests revealed an increase in hardness at a depth of 2 mm from the surface (Figure 9), with an average of 447 HV (SD = 6.6 HV). An average inverse correlation of R=-0.795 was found between the increase in Almen intensity (groups 1A, 2A and 3A) and Vickers microhardness. Similarly, when comparing the different coatings applied (groups 2A, 2B and 2C) with respect to the Vickers microhardness, a correlation of R=-0.652 was found. Negative values of the correlation coefficient (R) indicate a decrease in hardness with increasing Almen intensity and percentage of coverage. 4. DISCUSSION AND CONCLUSIONS The decrease in yield stress and the increase in ultimate strength due to the effect of shot peening can be explained by the surface hardening effect produced by the local plastic deformation caused by the application of the process (20,21). Surface hardening is evidenced by the increased surface hardness and microhardness when shot peening is applied to the specimens (Figures 8 and 9). The appreciable increase in the fatigue strength of SAE 5160 steel was proved by finding that the specimens of the Without Shot Peening group did not exceed one hundred thousand cycles, at a maximum stress of 856 MPa, while the groups with shot peening (1A, 2A and 3A) significantly increased the number of cycles, including some samples reaching the infinite lifetime zone in fatigue (N> 106 cycles). In relation to coverage there was no increase in fatigue strength when increasing the partial to total coverage, which shows that a total coverage is not necessary to obtain the benefits of shot peening. This coincides with what was reported by Prevey and Cammett (22) obtained for AISI 4340 steel. Regarding the increase in fatigue lifetime due to the effect of the increase in Almen intensity the results obtained were in accordance with what was reported in the literature for other steels (23–25). For example, Torres and Voorwald (26) carried out shot peening tests on AISI 4340 steel, where they found an increase in fatigue strength of up to 12% due to slight increases in Almen intensity. The reason for the increased fatigue strength produced by the shot peening process can be attributed to the formation of compressive residual stresses on the surface of the material. Compressive stresses decrease the stress generated by the external working forces of the piece, as well as delay the generation of cracks, thus increasing fatigue lifetime. Although shot peening produces a beneficial hardening and compressive residual stress on the surface, it also damages the surface producing craters and crevices that cause cracks (27). On the other hand, the Rockwell C hardness values (42.6 HRC, SD=0.67 HRC) were very close to those reported in the research by Mattson and Roberts (28) for the same steel (48.5 HRC). The small difference between the two hardness values may be due to the difference in the parameters of the hardening and tempering process used in each case. Likewise, the Vickers microhardness values (447 HV, SD= 6.6 HV) were very close to the lower range of hardness values reported by Ruy et al. (29) (491 HV, SD=34 HV). Some limitations were inherent in this study. First, a limited number of specimens were available, which did not allow for a better statistical characterization of fatigue strength. Second, it was impossible to measure the values of the residual stresses and even more so to make a profile of these, due to the impossibility of accessing technology such as Xray diffraction (30), which would make it possible to measure the residual stresses. In conclusion, the following can be said: 1. The influence of a shot peening treatment on the flexural strength of SAE 5160 steel for springs was investigated. Fatigue lifetime increases of more than 80% were obtained at different values of coverage and Almen intensities. 2. It was found that total coverage is not necessary to obtain the benefits of shot peening, partial coverage is sufficient. 3. It was found that the increase in Almen intensity produces an improvement in the fatigue strength of the material. 4. It was shown that it is very complex to determine the best shot peening condition to increase fatigue resistance, as it depends on many variables. The intensity of shot peening that produces the best results in the lifetime of fatigue is influenced by several factors: the relaxation of the compressive stresses induced during the fatigue process; surface conditions created by shot peening; and the possibility of finding cracks below the surface. 5. 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