OPTIMIZATION OF THE FATIGUE PROPERTIES OF 56SiCr7 SPRING STEEL

Transcription

1 Proceedings of the 5th International Conference on Integrity-Reliability-Failure, Porto/Portugal July 2016 Editors J.F. Silva Gomes and S.A. Meguid Publ. INEGI/FEUP (2016) PAPER REF: 6291 OPTIMIZATION OF THE FATIGUE PROPERTIES OF 56SiCr7 SPRING STEEL Roselita Fragoudakis 1(*), Georgios Savaidis 2, Nikolaos Michailidis 3 1 Department of Mechanical Engineering, Merrimack College, Massachusetts, USA 2 Aristotle University of Thessaloniki, Laboratory of Machine Elements and Machine Design, Thessaloniki, Greece 3 Aristotle University of Thessaloniki, Physical Metallurgy Laboratory, Thessaloniki, Greece (*) fragoudakisr@merrimack.edu ABSTRACT This study investigates the micro- and macro- hardness of 56SiCr7 leaf spring specimens. The investigation occurs at different stages of the manufacturing process of the leaf springs. Hardness measurements are taken both after thermal treatment and surface shot-peening treatment steps of the manufacturing process, in order to determine the effect of these processes. Residual stress measurements show how heat-treatment and shot-peening affect the mechanical properties of the surface and core of the spring material. The effect of these processes on the fatigue life of the leaf springs is demonstrated by experimental Wöhler curves at stress ratios of R=0 and R=0.4. Keywords: Macro-hardness, micro-structure, fatigue life, residual stresses, high-strength steels, leaf springs. INTRODUCTION Load carrying-applications, such as in axle suspension springs in the automotive industry, will typically use components made of high strength steel. The formation of nucleation sites, often followed by initiation and propagation of defects, is a microstructural change that indicates the degradation of a component s properties when subjected to cyclic loading. Microstructural changes are one of the primary factors to affect a component s fatigue life, and is therefore one of the designated performance parameters for leaf springs (Suresh, 1991). The different steps of leaf spring manufacturing are designed to alter the microstructure of the spring material, and therefore improve the fatigue life and performance of the leaf spring. Both heat treatment and surface shot-peening, being crucial steps in the spring manufacturing process, have a direct effect on the fatigue life and strength of the end product (Fragoudakis et al., 2013). High strength materials still present challenges when used to design components under FKM guidelines (Kleemann, 2015). This study focuses on understanding the mechanical surface properties of 56SiCr7 leaf spring specimens, and how these processes can be optimized by altering and optimizing their manufacturing process. The fatigue life of the leaf springs and the effect of heat treatment and surface shot-peening are of particular interest

2 Topic_C: Fracture and Fatigue SPECIMEN PREPARATION 56SiCr7 flat bars approximately 1200mm long with cross-sectional dimensions of 90x12mm (profile type B acc. to EN (EN (2006))) (Fig. 1) were used for the preparation of specimens for the investigation of the steel microstructure and hardness at different steps of the leaf spring manufacturing process. The microstructure of the raw steel material consists mainly of ferrite and perlite with different grain sizes throughout the thickness of the leaf. The heat treatment process involved heating the bars to complete austenization, and then cooling them rapidly by quenching in an oil bath to achieve a martensitic formation. Following quenching, the leaves were tempered. Half of the heat-treated bars were shot-peened in order to induce compressive residual stresses on the surface of the specimens. The shot type used was S330 at a velocity of 65m/sec, yielding approximately 100% coverage. Specimens of raw material, heat-treated and heattreated-shot-peened specimens were studied. Fig. 1 - Specimen's profile according to EN :2003 (left), four-point test rig (right) The specimens were cut to the desired dimensions of 35x35x12 mm (Figure 2) using initially a SiC disc, followed by precision diamond cutting to avoid thermal straining due to localized thermal stresses. Metallographic protocols were then followed for the preparation of the sample surface to be studied. The surfaces were examined under an Olympus BX60 microscope, equipped with a Leica DFC 290 camera. Fig. 2 - Metallographic specimen (left) and micro-hardness specimen (right) -230-

3 Proceedings of the 5th International Conference on Integrity-Reliability-Failure Similar samples were then used for micro- and macro-hardness measurements. A Wilson Rockwell hardness tester was used for macro-hardness measurements in the Rockwell C scale, while a Shimadzu hardness tester was used for the Vickers micro-hardness measurements. The area and location of the micro-hardness measurements are shown in Fig.2. X-Ray Diffraction was used to measure residual stresses on the surface and down to a depth of 500 µm of the shot-peened specimens. In order to determine the effect of shot-peening on the induction of residual stresses, similar measurements were carried out on non-peened specimens. Fatigue testing was performed on larger leaf spring samples (1200x90x12mm) at two stress ratios (R=0 and R=0.4). The choice of the stress ratios is based on the fact that these are among the most commonly used stress ratios in the leaf suspension industry. The fatigue tests resulted in the construction of Wöhler curves and the determination of Schütz mean stress sensitivity factors (Schütz, 1967). RESULTS AND DISCUSSION Surface and Core Microstructure The microstructure of the core and the two surfaces of a leaf spring was observed under a stereoscope and compared for the cases of raw material and heat-treated material. Figure 3 shows the microstructures of the surfaces and core for the case of raw material and heattreated materials following two different heat-treatment protocols (Table 1). Fig. 3 - Microstrure of surfaces and core -231-

4 Topic_C: Fracture and Fatigue It is clear from the figure that the raw material exhibits a ferritic-pearlitic microstructure. The light colored regions are regions of proeutectoid ferrite, while darker regions are pearlitic (exhibiting the characteristic lamella structures of ferrite and cementite (Fe 3 C)). The grain boundaries are visible and allow determination of the amount of pearlite and proeutectoid ferrite at the core and surfaces of the material. At the surfaces the ferritic grains appear larger in size, while they shrink towards the core of the material. As a result, the surfaces present a ferritic layer, approximately 100 µm deep, which is a strong indication of decarburization, possibly due to prior treatment of the spring steel. Table 1 - Heat Treatment Protocols Austenization Quenching Tempering Protocol o C for min In oil 40 o C 500 o C for min Protocol o C for 30 min In oil 40 o C 450 o C for min The protocols for the heat-treated samples are shown in Table 1. Both heat-treated samples show the martensitic formation, being the result of the austenization and quenching of the spring steel, which is one of the major differences between the raw and heat-treated steels. A carburization layer, µm deep, can also be observed at the two surfaces. Comparison of the results of the two protocols yields to the conclusion that protocol 1 is not an acceptable process to fully transform the ferritic-pearlitic microstructure of the raw material to martensite. Ferritic grains are still present in the microstructure, while the ferritic layer on the surface, still present, has a depth of 45 µm. Protocol 2 on the other hand has a smaller carburization layer (20-80 µm deep depending on the surface), the core presents a satisfactory tempered martensitic formation, and the ferritic grains are present only on one of the two surfaces. Based on the above observations, it can be concluded that protocol 2, is the optimum heattreatment to achieve a satisfactory martensitic formation. Micro/Macro-hardness measurements and Residual Stresses The results of the hardness macro-measurements on specimens prepared by the two protocols are summarized in Table 2. Table 2 - Micro-hardness measurements Rockwell C Specimen Core Surface 1 Surface 2 Raw Material 28.5±0.5 HRC 23±1 HRC 23±1 HRC Heat Treated (Protocol 1) 49.5±1 HRC 39.5±1.5 HRC 46±1 HRC Shot-peened (Protocol 1) 49±1 HRC 40±1 HRC 45±1.5 HRC Heat Treated (Protocol 2) 49±0.5 HRC 38±1 HRC 45.5±2 HRC Shot-peened (Protocol 2) 50±0.5 HRC 40.5±1.5 HRC 45±3.5 HRC -232-

5 Proceedings of the 5th International Conference on Integrity-Reliability-Failure Macro-hardness testing in the Rockwell C scale shows that heat-treatment followed by shotpeening results in an increase of the surface hardness of approximately 40% compared to the hardness of the raw material. It is worth mentioning that the macro-hardness results give valuable information about the heat-treatment process. As it can be observed from the comparison of the results of the macro-hardness of the two surfaces, although heat-treating of the two surfaces is assumed to be uniform, it does not appear to be so based on these results. The results also show that the core in all cases has a higher macro-hardness. This is due to the fact of the ferritic layer on the surfaces. Protocol 1 Protocol 2 Fig. 4 - Vickers micro-hardness distribution on surfaces and core of raw, heated and heat-treated-shot-peened material specimens according to Protocols 1 and 2 Vickers micro-hardness measurements (Figure 4) also show the beneficial effect in the increase of core and surface hardness, after heat-treatment of the steel. These measurements however, do not show any differences based on the choice of the heat-treatment protocol. The effect of shot-peening on the hardness of the material is insignificant for both heattreatment protocols followed. Fig. 5 - Residual Stress Distribution of Protocol 2 specimens -233-

6 Topic_C: Fracture and Fatigue The effects of shot-peening on the induction of residual stresses in the heat-treated specimens are shown in Fig. 5. As expected the non-peened samples show not worth mentioning residual stresses, while the shot-peened specimens show that maximum compressive residual stresses are induced down to a depth of 150 µm. The peening process significantly affects the surface area, while the core, at approximately 500 µm, past a threshold of 200 µm, shows negligible induction of residual stresses. Wöhler curves and Schütz mean stress sensitivity factors To determine the fatigue life of 56SiCr7, flat bars (1200mm long with cross-sectional dimensions of 90x12mm (profile type B acc. to EN (EN (2006))) heattreated following both protocols and shot-peened, were tested under four point bending on the servohydraulic fatigue rig shown in Fig. 1. Tests were carried out at two stress ratios, R=0 and R=0.4. The results of the tests were plotted in the form of Wöhler curves (Fig. 6). Stress amplitude σ a [MPa] k peened,r=0 =4.39 k thermal,r=0 =3.5 Experimental resuts - R=0 Experimental results - R= Number of cycles to failure N Material: 56SiCr7 Protocol 2 k peened,r=0.4 =6.35 k hermal,r=0.4 =6.17 Stress amplitude σ a [MPa] Protocol 1 Protocol 2 Production batch 1 Production batch Number of cycles to failure N Material: 56SiCr7 R=0 Protocols 1 and 2 Fig. 6 - Wöhler Curves of Protocol 2 specimens at R=0 and R=0.4 (left), for both Protocol 1 and 2 Specimens at R=0 (right) -234-

7 Proceedings of the 5th International Conference on Integrity-Reliability-Failure Though the number of available fatigue life results is relatively narrow, statistically calculated approximations for their slopes (k-values) are given on the left diagram of Fig. 6. In order to better determine the strains developed during the deformation of the beams under loading, strain gages were positioned along the surface of the beams. Strains have been transformed to stresses by means of the Theory of Elasticity using the standardized Young s modulus for steel E=210 GPa. Comparison of the Wöhler curves for beams prepared following the two protocols shows that protocol 2 significantly increases the fatigue life of the beams by almost an order of magnitude, at high stress amplitudes (600 MPa). Comparing Wöhler curves of solely heat treated specimens with ones that have been additionally shot peened reveals the significance of the shot peening process applied to fatigue life. The mean stress sensitivity factor, M, according to Schütz for heat-treated and shot-peened bars amounts to 0.4, while those not subjected to shot-peening after heat-treatment have 0.3. CONCLUSION This study has shown the effect of heat and surface shot-peening treatment on the fatigue properties of 56SiCr7 spring steel. An optimized heat-treatment was suggested, yielding high surface hardness and better fatigue life of spring steel bars. It was shown that although surface treatment by shot-peening does not significantly affect the macro- and micro-hardness of the steel, it can significantly improve the residual stress state on the surface area and, therewith, its fatigue life by at least an order of magnitude at high amplitude stresses. It is important to mention that the optimized heat-treatment yields the desired hardness properties due to the fact that it effectively transforms the perlitic/ferritic microstructure of the raw material to the desired tempered martensitic formation. ACKNOWLEDGMENTS The authors gratefully acknowledge the General Secretariat for Research and Technology of Greece and the European Union for financially supporting this investigation within the framework of ESPA , Support of New, Small and Medium Enterprises. REFERENCES [1]-Suresh S., Fatigue of Metals, Cambridge University Press, 1991, Cambridge. [2]-Fragoudakis R. et al., Fatigue assessment and failure analysis of shot-peened leaf springs, Fatigue Fract Engng Mater Struct, 36 (2013) [3]-Tekeli S., Enhancement of fatigue strength of SAE 9245 steel by shot peening, Mater Lett Vol. 57 (2002), p

8 Topic_C: Fracture and Fatigue [4]-Kleemann A, Bergmann J, Thumser R., Kleemann S. Mean stress influence on endurance of very high strength steels. Proc. of 4 th Int. Conference on Engineering Against Failure (ICEAF IV), 2015, p , Skiathos, Greece [5]-EN Steels for quenching and tempering. Technical delivery conditions for alloy steels (2006). [6]-Schütz, W. Über eine Beziehung zwischen der Lebensdauer bei konstanter und veränderlicher Beanspruchungsamplitude und ihre Anwendbarkeit auf die Bemessung von Flugzeugbauteilen. Zeitschrift für Flugwissenschaften, Vol. 15, p (1967)