Variations of VHCF Characteristics by Microstructure of a Spring Steel to UNSM Treatment
Article information
Abstract
The bainitic structure resulting from the austempering of spring steel exhibits high strength and ductility. On the other hand, there appears to be no study on the effects of very high cycle fatigue (VHCF) and ultrasonic nanocrystal surface modification (UNSM) of this bainite structure. Therefore, this study compared the fatigue properties of VHCF using spring steel with bainitic and martensitic structures and bearing steel data. This study analyzed the characteristics of microstructure transformation associated with the heat treatment cycles and studied and evaluated the fatigue strength characteristics because of the UNSM in terms of fracture mechanics method and fracture surface analysis through electron backscatter diffraction, scanning electron microscopy fracture analysis, and energy-dispersive spectroscopy analysis. The fatigue limit of UNSM-treated spring steel was improved significantly by approximately 33% to 50% compared to the fatigue test results of the untreated material in the VHCF. In the long life range of bainized spring steel, fish-eye cracks appear in the form of the fine granular area, and fatigue cracks occur in the form of fish-eye cracks that occur in the bainite facet and matrix, resulting in a significant increase in fatigue strength and fatigue life.
1. Introduction
The microstructure and mechanical properties of spring steel quenched and tempered (QT) and austempering treated (AT) specimens were examined. The results showed that the multiphase structure with bainite (B), martensite (M), and retained austenite structures exhibited high strength and good ductility (Kouters et al., 2014; Nie et al., 2009; Sourmail et al., 2013; Suzuki and Furuhara, 2009; Wei et al., 2004). The final microstructure of this spring steel has a martensitic structure tempered by QT heat treatment, but this structure suffers from hydrogen embrittlement during strengthening (Kouters et al., 2014). Therefore, replacing the final microstructure of spring steel with a bainite or mixed structure will help develop high-strength and long-life products.
In the case of spring steel, a B + M mixed structure was reported to have better fatigue strength than the martensitic structure (Nie et al., 2009). The present study first examined whether these bainite and martensite microstructure changes could be applied industrially through long-life fatigue tests. Research on very high cycle fatigue (VHCF) of bainite steel is necessary for the durability design of next-generation springs (Chai, 2006; Nakasone and Hara, 2004; Sakai et al., 2000) because springs are major components of all ships, marine structures, and machinery. Nevertheless, there has been little research on surface treatments such as ultrasonic nanocrystal surface modification (UNSM) (Suh et al., 2007; Pyun et al., 2012; Suh et al., 2010; Suh and Pyun, 2011). Accordingly, efforts focused on securing data applicable to bainitic microstructure spring steel.
A previous study on VHCF of bearing steel found that fish-eye cracks were more likely to occur than surface cracks and that a duplex S-N curve in which the S-N curve descended secondarily was formed (Chai, 2006; Nakasone and Hara, 2004; Sakai, et al., 2000). In this study, the target VHCF area was 106 to 109, but it was usually expressed as S-N curve data from 103 to 109 to compare the data required for the current era and the next generation.
Therefore, in this study, QT heat-treated martensitic specimens (referred to as QT material) and austempered bainitic specimens (referred to as AT material) were manufactured using SAE9254 (similar to SUP12) material. These specimens exhibited variations in mechanical properties caused by microstructure change, the influence of UNSM surface treatment, and VHCF characteristics, which were studied and analyzed using fracture mechanics techniques and fracture analysis. In addition, this study compared and reviewed these results with the VHCF research results of bearing steel.
2. Test Material and Experimental Methods
2.1 Test Material and Heat Treatment Methods
The spring steel used in the experiment was SAE9254, which mainly contains 0.55% C, 1.5% Si, 0.7% Mn, and 0.7% Cr and is referred to as an AR (as received) material. The QT heat-treated specimen used in this study was cut to 13 mm in diameter and 110 mm in length, austenitized in air at 980 °C for 15 minutes, cooled in water, and quenched at 60 °C for five minutes. The specimen was then salt-bathed at 430 °C for 90 minutes and air-cooled. In addition, the austempered specimen was austenitized in air at 980 °C for 15 minutes, quenched, and cooled in air by a salt bath at 300 °C for 30 minutes.
The heat-treated specimens were machined into fatigue and tensile test specimens (ASTM A370 standard) and used in experiments. In addition, some of the fractured fatigue specimens were cut in places without deformation and used to observe microstructure and measure hardness. For microstructure, an etching solution of 5 ml HCl + 1 g picric acid + 100 ml ethanol (95%) was used.
2.2 The Tensile Test
The tensile test was performed on tensile specimens processed according to ASTM A370 standards using an Instron universal testing machine. The test results were received as an Excel file and expressed as a stress and strain diagram, as shown in Fig. 1. The results are summarized in Table 1. The AR material obtained a tensile strength of 991.5 MPa, a 0.2% yield strength of 526.5 MPa, and an elongation rate of 19.5% from this diagram. In addition, the tensile strength of QT material was 1723.1 MPa, and AT material was 1824.4 MPa, which increased by 73.8% for QT material and 84% for AT material compared to the AR material. In addition, the tensile strength of AT material increased by 5.9% compared to QT material.
Stress-strain diagrams of QT and AT materials
Mechanical properties
The tensile strength of AT and QT materials increased significantly compared to AR materials because of the microstructural and shape transformations caused by the formation of bainite and martensite structures through heat treatment, as shown in Fig. 1.
In general, the elongation and reduction of area tend to decrease as the tensile strength of steel increases. On the other hand, AT material with a bainitic structure showed increased elongation and decreased area compared to QT material with a martensitic structure, which is believed to be a characteristic of the bainitic microstructure.
2.3 UNSM Technology and Processing Conditions
UNSM technology applies ultrasonic vibration energy to apply more than 20,000 blows per second (∼1,000–10,000 times/mm2) to the metal surface with a ball subject to very large static and dynamic loads. The surface texture changes into a nanoscale microstructure. This is a patented technology that modifies the microstructure of the surface layer into a nanocrystalline structure and adds a very large and deep compressive residual stress (Suh et al., 2007; Pyun et al., 2012; Suh et al., 2010; Suh and Pyun, 2011).
Fig. 2(a) presents a UNSM processing system that includes a generator that generates ultrasonic frequencies, an air compressor that applies a static load, a transducer that generates mechanical ultrasonic waves, a booster that amplifies the generated ultrasonic waves, and a horn that transmits the amplified ultrasonic waves without loss. The system has a ball tip that transmits ultrasonic vibration energy to the workpiece.
UNSM equipment (a) micro forging traces by SEM (b) by 3D of Atomic Force Microscopy (AFM) (c) after UNSM treatment.
Fig. 2(b) gives an example of micro forging traces that appeared on the surface after UNSM treatment observed at 3000× by scanning electron microscopy (FE-SEM, S-4300, Hitachi, Japan). Fig. 2(c) shows traces of 3D forging by atomic force microscopy (AFM, Scanning Probe Microscope, NX20, USA).
When UNSM technology is applied to the surface in this way, it has characteristics such as the formation of micro-dimples on the surface, improvement of surface roughness, increase in surface hardness, nanoization of crystals, and the formation of deep and large compressive residual stress. Therefore, applying this technology significantly increases the durability and fatigue strength in areas such as high cycle fatigue (HCF), very long life fatigue (VHCF), and stress corrosion cracking (SCC). In addition, the friction coefficient was reduced, showing greatly improved wear resistance (Suh et al., 2007; Pyun et al., 2012; Suh et al., 2010; Suh and Pyun, 2011).
Fig. 3 shows that when the static force of UNSM processing on SCM435 was controlled to three values of 40 N, 70 N, and 100 N, there was a change in Vickers hardness before and after UNSM, as shown in Fig. 3(a). In addition, the changes in the depth profile of compressive residual stress were observed, as shown in Fig. 3(b) (Suh et al., 2013; Suh et al., 2011b).
Variation of Vickers hardness (a) and formation of compressive residual stress and change with depth before and after UNSM treatment for SCM435 (b).
In other words, a high increase in hardness and a deep and large compressive residual stress were formed on the surface of the UNSM-treated material. Owing to this beneficial UNSM characteristic, the rotational bending fatigue test results tended to increase by approximately 30% or more compared to the untreated material in the VHCF test (Suh and Pyun, 2011).
Table 2 lists the UNSM treatment conditions used in this study, and the fatigue specimens were subjected to an elaborate surface treatment. The fatigue test results were obtained from this treated fatigue specimen according to the UNSM treatment. The fatigue strength and VHCF characteristics were analyzed quantitatively by comparing them with the untreated specimen. In addition, the characteristics of the unique fracture surface were compared and evaluated through a fracture mechanics inspection and component analysis.
UNSM conditions
2.4 Vickers Hardness Test
The hardness was measured using a Vickers hardness tester (JP/HM-112, Mitutoyo). The measurements were taken 12 times, twice with a 5 kN load based on the center of the test specimen, as shown in Fig. 4(a). Fig. 4(b) shows the change in the measured Vickers hardness (Hv). Table 3 summarizes the mean values. The standard deviation (SD) was the largest for the QT material at 14.9 and the lowest for the AT material at 10.1.
Vickers hardness measurement method (a) and change in hardness according to depth (b)
Results of the Vickers hardness test
In addition, compared to the AR material, the hardness of QT material increased by 84.5%, and AT material increased by 102.6%. In addition, the AT material increased by 9.8% compared to the QT material. The hardnesses of the AT and QT materials were significantly higher than the AR material because of the presence of bainite and martensite phase structures and a microscopic structural transformation depending on the heat treatment conditions.
2.5 Fatigue Test
Fig. 5(a) shows the shape and dimensions of the fatigue specimen. The diameter and radius of curvature of the fatigue specimen were 3.0 mm and 7.0 mm, and the stress concentration coefficient Kt = 1.02. The surface of the test piece was mirror-finished using polishing paper from No. 100 to No. 2,000. The fatigue life of spring steel at room temperature was tested at a frequency of 53 Hz and a stress ratio of R = −1 using a cantilever rotary bending fatigue tester (YRB200, Yamamoto, Japan) that simultaneously uses four test specimens, as shown in Fig. 5(b). Fatigue tests were conducted in air.
(a) Configuration of the fatigue specimen (unit; mm) and (b) fatigue testing machine
The macroscopic characteristics of the fatigue fractured surface were sketched individually and recorded. The morphology and chemical composition were analyzed by FE-SEM (S-4300, Hitachi, Horiba, Japan).
3. Research Results and Discussion
3.1 Microstructure Observation According to Heat Treatment Cycle
Fig. 6 presents an optical microscopy (OM) image of the specimen. Fig. 6(a) shows a 500× image of the AR material, showing the mixed structure of ferrite, pearlite, and cementite, with an average particle size about 11 μm. Fig. 6(b) shows an OM image of the QT material at 500×, showing that the microstructure has been refined. Some white parts were observed in the QT material because some ferrite appeared due to composition heterogeneity; the remaining part transformed into martensite. The traces of tempering could not be distinguished from current microstructure photographs. Fig. 6(c) presents the microstructure at 500x after austempering. The AT specimen showed microstructural transformation. The lath (Fig. 10) was well-developed and exhibited a needle-like structure.
Typical OM images of three test specimens ((a) AR, (b) QT, and (c) AT))
Phase map of EBSD analysis of the AT specimen
Fig. 7 shows an SEM image of the microstructure at 20,000×. Figs. 7(a) and 7(b) present the QT and AT materials, respectively. In Fig. 7(a), martensite and tempered martensite structures were observed. Many large and small bainite structures and retained austenite structures (γ-Fe, indicated by arrows) were observed in Fig. 7(b). Other studies also reported that a structure with a dual phase structure of bainite and martensite was obtained through austempering (Nie et al., 2009; Sourmail et al., 2013; Suzuki and Furuhara, 2009; Wei et al., 2004).
SEM images at 20,000× of QT(a) and AT(b) specimens
3.2 Quantitative Microstructure Analysis by EBSD
3.2.1 EBSD Microstructure Analysis of AR Material
Fig. 8 presents the electron backscattered diffraction (EBSD, DigiView EBSD Camera, EDAX, USA) data of an AR specimen. The average particle size of the microstructure was ∼11 μm. The measurement conditions were the acceleration voltage, magnification, and measurement unit of 18 kV, 2.0 kx, and 1 μm, respectively. The samples used for EBSD of spring steel were electropolished in a mixed solution of 10% perchloric acid and 90% acetic acid. No additional washing process was used, and the crystal size was 0.3 μm or more. Fig. 8 presents the phase map of AR material, which is the result of mapping two phases with 76.9% ferrite and 23% Fe3C. The microstructure of the spring steel used in this study was composed of grain sizes with a uniform orientation.
Cross-sectional EBSD analysis of an AR specimen. (a) inverse pole figure (IPF) map and (b) phase map
3.2.2 EBSD Microstructure Analysis of the QT Material
Fig. 9 presents an enlarged phase map observed with EBSD of a QT material. In the phase map, the red color is the α-Fe portion with a body-centered cubic (BCC) crystal structure, and the light green color is the γ-Fe portion with a face-centered cubic (FCC) crystal structure. In addition, the part shown in black was not expressed during filtering because it did not meet the conditions. The data showed that it was a lath-type martensite (BCT phase) with an average width of less than 0.81 μm (maximum 2.76 μm, minimum 0.55 μm). For reference, the EBSD equipment used in this study recognizes all body-centered tetragonal (BCT) phases as BCC. In addition, the phase fraction of FCC of γ-Fe was almost non-existent (< 1%); most of the phases are α-Fe. This suggests that the QT heat treatment cycle conducted in this study transformed the existing austenite into martensite through a quenching and tempering process.
Phase map of EBSD analysis of the QT specimen
3.2.3 EBSD Microstructure Analysis of AT Material
Fig. 10 presents a phase map of AT material observed with EBSD. Some dark localized areas appeared among many laths, but they were removed by filtering. In this data, a lath, a long plate-shaped structure, had a maximum and minimum size of 29.4 μm and 7.1 μm, respectively, and an average length of 15.5 μm because of the heterogeneous chemical composition. On the other hand, most structures were lath-type bainite with a width of less than 1.76 μm and an average length of 15.5 μm.
In addition, the γ-Fe phase fraction of the AT material, which is FCC, was approximately 6.5%, which was much larger than that of the QT material (< 1%), as shown in Fig. 9. On the other hand, the α-Fe phase fraction fitted to the BCC phase comprised 93.5%. Hence, unlike the existing austenite, which had sufficiently undergone phase transformation into martensite, a significant amount of retained austenite was generated, as shown in Fig. 7(b).
3.2.4 Quantitative Analysis of Lath of QT and AT Specimens
Table 4 lists the length and width of the lath formed in the map in Fig. 10 for the AT material, which has a bainitic structure, and Fig. 9 for the QT material, which has a martensitic structure. The lath of the AT material was approximately five times longer than that of the QT material, and its width was approximately double. Furthermore, the maximum lath length of AT material tended to be three times longer than that of QT, and the width tended to be twice as large. The fact that the length and width of the lath of this AT material were larger than that of the QT material is a good parameter that can coincide with the fact that the AT material with a bainitic structure is superior to the QT material with a martensitic structure.
Comparison of the length and width of the lath in the QT and AT specimens
3.3 S-N Curve and VHCF Fatigue Characteristics
Fig. 11 compares the S-N curves of the rotational bending fatigue test of the three spring steels used in this study. Fig. 11 shows the S-N data obtained by the fatigue test of the AR material at room temperature in the form of ‘△.’ A knee point appeared at about 450 MPa, and the fatigue limit appeared. The number 2 in the figure indicates that the two data overlap, and the arrow indicates no fracture at the current fatigue life.
Comparison of S–N curves of spring steel before (AR) and after the heat treatments (QT and AT)
In addition, the QT material, which has a martensitic structure, is indicated in the ‘●’ shape, and its fatigue limit was 600 MPa, which was approximately 33.3% higher than the 450 MPa of the AR material. In addition, the AT material, which has a bainitic structure, was expressed as a ‘■’ shape. A knee point appeared at 700 MPa, and the fatigue limit increased by 55.6% and 16.6% compared to the AR and QT materials, respectively.
In this way, the fatigue strength of the AT material, which has a bainitic structure, increased compared to QT material, which has a martensitic structure, and the fatigue strength of AT material increased significantly in the long life range because the lath of the bainitic structure is long and wide, as explained in Figs. 8–10. This was attributed to changes in the shape of the microstructure.
In other words, the structural transformation with the phase structure of bainite and martensite was analyzed to explain why the strength, ductility, and fatigue limit of the steel were superior to those of untreated steel. This phenomenon is in good agreement with the results of this study, which reviewed organizational transformation and quantitative analysis of the lath. Other studies have shown that the strength and toughness of steels with a dual phase structure of bainite and martensite are superior to those of single martensite structure (Kouters et al., 2014; Nie et al., 2009; Sourmail et al., 2013; Suzuki and Furuhara, 2009; Wei et al., 2004).
Wei et al. (2004), in a study on the fatigue behavior of 1500 MPa bainitic-martensitic duplex-phase high-strength steel, reported that the fatigue strength and fatigue crack threshold were higher than that of all martensitic steels and the crack propagation rate was lower.
3.4 S-N Curves and UNSM Effects According to Microstructure
3.4.1 S-N Curve According to UNSM Treatment of AR Material
Fig. 12 compares the ‘▲’ type fatigue test results of the UNSM treated UAR material with the ‘△’ type S-N data obtained by the fatigue test of the AR material. Here, since the knee point of the UAR material was formed at 600 MPa, the fatigue strength increased by about 33.3 % compared to 450 MPa of the AR material.
Comparison of the S-N curves before and after UNSM treatment of AR specimen
The test results of VHCF characteristics and S-N curves according to the UNSM treatment effect were obtained using the fatigue test specimen (Fig. 5(a)) and the fatigue tester (Fig. 5(b)), which were surface treated under the UNSM treatment conditions, as shown in Table 2. Fig. 12 compares the ‘▲’ type fatigue test results of the UNSM-treated UAR material with the ‘△’ type S-N data obtained by the fatigue test of the AR material. The fatigue strength increased by approximately 33.3% compared to 450 MPa of the AR material because the knee point of the UAR material formed at 600 MPa.
The macroscopic fracture surface characteristics were sketched and compared with the S-N data of the AR material at three stress levels (Fig. 12). The AR3 material at 600 MPa fractured at a lifespan of 7.3 × 104 due to the generation, growth, and coalescence of three surface cracks. The AR1 material at 650 MPa fractured at 1.5 × 104 because of the formation, growth, and coalescence of seven surface cracks. In addition, the AR5 material at 870 MPa fractured at 7 × 103 because of the formation, growth, and coalescence of six surface cracks.
The characteristics of the three stress levels above were those at low stress levels. The number of surface cracks forming decreased, and the number of surface cracks increased at high-stress levels, showing a tendency to grow, coalesce, and fracture.
The macroscopic fracture surface characteristics were sketched at three stress levels in the S-N data of the UAR material (Fig. 12). The UAR7 material at 650 MPa fractured at 5.3 × 105 because of the formation, growth, and coalescence of three surface cracks. The UAR4 material at 750 MPa fractured at 7.25 × 104 because two surface cracks formed, grew, and merged to form a ridge (Marked in black on the picture). In addition, the 850 MPa, UAR6 material fractured at 1.07 × 104 because four surface cracks formed, grew, and coalesced, forming three prominent ridges.
The characteristics of the three stress levels above are that the number of surface cracks that form at low-stress levels decreases, and the number of surface cracks increases at high-stress levels. Hence, the tendency to coalesce and break quickly is similar to AR materials. On the other hand, the number of cracks forming on the surface was lower than that on the AR materials because of the UNSM effect, and the fatigue life and the fatigue strength increased. These UNSM treatment effects are similar to research results on other materials (Suh et al., 2007; Pyun et al., 2012; Suh et al., 2010).
In particular, the UAR material had a significantly increasing fatigue life in VHCF compared to the AR material test specimen. In addition, despite some data variability in the S-N curve and VHCF area of the UAR material, the tendencies for the fatigue strength and fatigue life to increase significantly were attributed to the effects of UNSM treatment on the test specimen.
3.4.2 S-N Curve According to the UNSM Treatment of QT Materials
Fig. 13 compares the ‘○’ type S-N data obtained by the fatigue test of QT material, which is a martensitic structure, with the ‘●’ type fatigue test result of the UQT material. The knee point of the UQT material was formed at 900 MPa, and the fatigue strength increased by approximately 50% compared to 600 MPa of the QT material.
Comparison of the S-N curves before and after UNSM treatment of the QT specimen
In addition, the fracture surface characteristics were compared by sketching the macroscopic fracture surface characteristics at three stress levels in the S-N data of the QT material in Fig. 13. The QT10 material at 600 MPa fractured at 1.94 × 105 with four surface cracks forming, growing, and coalescing, forming four prominent ridges. The QT9 material at 650 MPa fractured at 5.4 × 105 after two surface cracks formed, grew, and coalesced. In addition, the QT3 material at 700 MPa developed, grew, and coalesced four surface cracks and fractured at a fatigue life of 1.1 × 105.
The macroscopic fracture surface characteristics were sketched at three stress levels in the S-N data of UQT material (Fig. 13). The UQT1 at 900 MPa developed and grew as a single surface crack and fractured at 9.98 × 105. The UQT4 material at 1000 MPa fractured at 1.09 × 105 with three surface cracks forming, growing, and coalescing. Furthermore, the UQT7 at 1100 MPa, similar to the UQT1 material, developed and grew as a single surface crack and fractured at 2.3 × 107.
The characteristic of these three stress levels is the UNSM effects, where the number of cracks forming on the surface of UQT material was lower than the QT material, and the fatigue lifespan and fatigue strength were higher. In particular, the low volatility and stability of data in the S-N curve and VHCF area of the UQT material were evaluated according to the effects of UNSM processing. When the UNSM treatment was performed on the QT material, the fatigue strength and lifespan increased significantly, which is closely related to the effects of the UNSM treatment. Therefore, its application is expected to expand in engineering and industrial terms.
3.4.3 UNSM Treatment Effects of AT Material
Fig. 14 compares the ‘□’ type S-N data obtained from the fatigue test of the AT material, which has a bainitic structure, with the ‘■’ type fatigue test results of UAT material conducted under the AT material conditions (Table 2). The fatigue strength of the UAT material was approximately 50% higher than that of the 700 MPa AT material because the knee point of the UAT material is formed at 1050 MPa.
Comparison of the S-N curves before and after the UNSM treatment of the AT specimen
The macroscopic fracture surface characteristics were sketched at three stress levels in the S-N data of the AT material in Fig. 14. The AT2 material at 800 MPa fractured at 1.9 × 107 as a fish-eye crack and grew inside the specimen. Like the AT2 material, the 1000 MPa AT4 material also generated a fish-eye crack and fractured at 3 × 105. Furthermore, the AT7 at 1000 MPa fractured at 2.82 × 105 due to the generation, growth, and coalescence of one fish-eye crack and one surface crack.
The macroscopic fracture surface characteristics were sketched on the S-N data of the UAT material, as shown in Fig. 14. The UAT10 material at 1100 MPa developed and grew as a single fish-eye crack and fractured at 1.93 × 108. The UAT5 at 1200 MPa had the same fracture type as the UAT10 material, and one fish-eye crack formed, grew, and fractured at 6.4 × 105. Because the stress level of the 1400 MPa UAT1 material was very high, unlike other UAT materials, multiple surface cracks formed, grew, and coalesced, resulting in fracture at 1.2 × 104, which has a very short fatigue life. At these three stress levels, the fish-eye cracks that formed at a higher stress level for UAT materials than for AT materials determined the fatigue life and fatigue strength because of the UNSM effects.
In particular, the VHCF area of the UAT material showed little data volatility and a stable S-N curve. On the other hand, the fatigue tests of UQT and UAR materials were attributed to the influence of microstructure, with greater data variability in the VHCF area than that of UAT materials. In particular, the low volatility and stability of the data in the S-N curve and VHCF area of the UAT material were attributed to the effects of the UNSM treatment and the effect of the bainite structure with a large length and width of lath.
The fatigue test results of the UNSM-treated UAT material, UQT material, and UAR material had fatigue strengths of approximately 33–50% higher than the results of the fatigue test of untreated AT material, QT material, and AR material in the VHCF area, respectively (Figs. 12–14). These research results are similar to those of other studies (Suh et al., 2007; Pyun et al., 2012; Suh et al., 2010; Suh and Pyun, 2011).
Hence, the formation of large and deep compressive residual stresses on the surface can be attributed to the nano-sized surface texture, improved surface roughness, and improved surface hardness through the UNSM treatment. Nevertheless, the secondary decrease in fatigue limit that occurs in the long life range of bearing steel did not occur in spring steel (Nahm et al., 2018a; Nahm et al., 2018b; Suh et al., 2021).
3.5 Observation of Fatigue Fractured Section
3.5.1 Observation of Fatigue Fractured Section of QT Material
Fig. 15(a) presents an SEM image of the fatigue fracture surface of the QT9 specimen (650 MPa, Nf = 5 × 105) observed at 30×. The three arrows indicate the location of fatigue cracks, and the semicircles represent the formation, growth, and advancement of surface cracks as lines.
(a) Fractography of a small surface crack on the QT9 specimen (650 MPa, Nf = 5 × 105). The arrows in the figure indicate the location of a surface fatigue crack. (b) The result of EDS analysis of the area of marked with a square on the fracture surface in (a).
The adjacent surface cracks S1 and S2 grow into each other and merge on the plane when the crack tips are on the same plane, and the distance between them is small, as shown in Fig. 16(b), then there is less curvature on the fracture surface (Suh and Kitagawa, 1987). On the other hand, as surface cracks S2 and S3 grow together, if the gap between the crack tips is large, as shown in Fig. 16(c), they merge to form a ridge in a shear shape, as shown in number 5 in Fig. 16(a), resulting in protrusions on the fracture surface. This phenomenon is reported elsewhere (Kitagawa et al.,1979; Nahm et al., 2018b).
(a) Surface crack initiation, growth, coalescence, and ridge formation model. (b) No ridge is formed when the crack tips merge in the same plane. (c) If there is a gap between the crack tips, a ridge is formed by combining in a shear type, as shown in number 5 in (a).
In Fig. 15(b), the area marked with a square on the fracture surface in Fig. 15(a) was analyzed by EDS to compare and review the components and data of the main components of this sample. As a result, each element was uniformly distributed, with C 18.52%, Si 0.92%, Cr 0.64%, and Mn 0.59%. Nevertheless, the content of each element was slightly different from the composition (wt.%) of the specimen. In particular, in this analysis, Cr and Si were found in small amounts and overlapped with other components, making detection difficult. Therefore, the central part of the specimen tended to have a high percentage of carbon.
3.5.2 Observation of Fatigue Fractured Section of UNSM-QT Material
Fig. 17 presents a SEM image of the fatigue fracture surface of the UQT7 test specimen (1100 MPa, Nf = 2.3 × 107) for 30× and 50×. The arrow indicates the location of the fatigue crack, and the semicircle in the picture indicates the appearance, growth, and progression of the surface crack as a line. Hence, this specimen was fractured when a small single surface crack formed on the UNSM-treated surface and grew slowly and smoothly to the long-life range like other surface cracks (Suh et al., 2011c). In particular, multiple surface cracks did not occur like in QT9 because it was UNSM treated, and the fatigue life was greatly increased, highlighting the effects of the UNSM treatment.
SEM photographs of fatigue fractured UQT7 specimen (1100 MPa, Nf = 2.3 × 107 when observed at (a) 30× and (b) 50×
In general, fish-eye cracks are the preferred failure mode in the VHCF region. By contrast, fish-eye cracks did not occur in the long-life region of the QT and AR materials, which have a tempered martensitic structure and a pearlitic structure, respectively.
3.5.3 Observation of Fatigue Fractured Section of AT Material
Fig. 18 shows a SEM image of specimen AT4(1000 MPa, Nf = 3 × 105). The fish-eye crack, which is the crack formation area, is indicated by a circle when observed at 100× (a) and 1000× (b). The arrow indicates the crack formation area, and the fine granular area (FGA), which is the internal originating fracture type that formed inside the specimen, is indicated by a circle in Fig. 18(a).
Fractography of a fish-eye crack on AT4 specimen (1000 MPa, Nf = 3×105): (a) Arrow in the image indicates the initiation site of a fish-eye crack; (b) Area indicated by the dotted rectangle is the area where a surface crack formed. (c) EDS analysis results of the surface crack initiation area indicated by the dotted rectangle shown in Fig. 18(b).
The area marked with a dotted rectangle in Fig. 18(b) contains the area where surface cracks form. Therefore, EDS analysis of this place revealed a slight difference from the composition of the test material, as shown in Fig.18(c), but the analysis shows C, 11.51%, Si, 1.05%, Cr, 0.67%, Mn, 0.61%, and Al, 0.57%. No traces of inclusions mainly composed of TiN (polyhedral inclusion, see Fig. 23(a)) and Al2O3 (ellipsoidal inclusion), which are generally observed in the center of fish-eye cracks, were found in the AT material with a bainitic structure (Nahm et al., 2018a; Nahm et al., 2018b; Suh et al., 2021).
Fractography of a fish-eye crack on a common bearing by TiN (a) and a clean bearing by Cr matrix (b). (a): Elemental analysis results of the core of a fish-eye crack composed of TiN (Ti 71.38%, Cr 6.92%, V 2.2%) and (b): Elemental analysis results of the core of a fish-eye crack composed of a Cr matrix (Cr 1.51%, Fe 98%)
SEM magnified at 1000× (Fig. 18(b)) showed that the initial fish-eye crack formed in a separated form in a plane of approximately 35 μm. This fish-eye crack formed as this face separation type was classified as a fish-eye crack that forms in the “Facet” (Fig. 18(b)). In other words, fish-eye cracks formed in places with no inclusions (Nahm et al., 2018a; Suh et al., 2021). Furthermore, Nie et al. (2009) reported a fish-eye crack in the facet, similar to the results of this study in the UFT fatigue test of spring steel.
Fig. 19 presents a SEM image of the fatigue-failed AT7 specimen (1000 MPa, Nf = 2.82 × 105) observed at (a) 100× and (b) 300×, respectively. The FGA of the fish-eye crack, which is the part where the internal crack formed (Fig. 19(a)), is indicated by a circle, and the arrow within the circle indicates the location of the internal crack. The inclusions in general bearing steels are made of TiN and Al2O3. In this AT7 specimen, however, these inclusions resulted in a completely different bainite matrix (Fig. 19(b)) than that observed with the initiation, growth, and coalescence process (Nahm et al., 2018a; Suh et al., 2021).
SEM images of a fatigue-fractured AT7 specimen (1000 MPa, Nf = 2.82 × 105) observed at (a) 100× and (b) 300×
The following characteristics were obtained from fracture observation and fracture analysis of the above two AT specimens. The fish-eye crack of the spring steel in this study was not a fracture type of a fish-eye crack in which fatigue cracks usually originate and grow from internal inclusions, but a fish-eye crack that occurs in the “facet.” This crack type is a fish-eye crack type, as shown in Fig. 18(b), even in materials such as Ti-6Al-4V alloy (Suh et al., 2011a; Suh et al., 2011b), as shown in Fig. 20. In other words, FGA of fish-eye cracks appeared in the long life range of bainitic spring steel. In addition, fatigue cracks formed in the form of fish-eye cracks in the bainite matrix (Fig. 19(b)) and facet (Fig. 18(b)), which were completely different from the initiation, growth, and coalescence processes in the inclusions. Therefore, the form of the fatigue cracks forming in the ‘matrix and facet’ significantly increased the fatigue strength and life.
Closeup view of a fractured UNSM-Ti specimen (700 MPa, Nf = 1.13 × 105). The arrows within the circle indicate the locations (a: 50×; b: 500×) where the facet-type fish-eye cracks formed.
3.5.4 Observation of Fatigue Fractured Section of the UNSM-AT Material
Fig. 21 presents a SEM image of UNSM-treated fatigue specimen UAT5 (1200 MPa, Nf = 6.4 × 105). In addition, the fish-eye crack, which is the crack formation area, is represented by a red circle, the arrow indicates the crack formation area, and the center was observed by enlarging it at 25× (a), 100× (b), and 500× (c), respectively. In addition, Fig. 21 (d) presents a 2000× magnification image of the area indicated by the dotted square in photo (c), and the lath of the bainite structure can be observed.
Fractography of a fish-eye crack on a UAT5 specimen (1200 MPa, Nf = 6.4 × 105). The fish-eye crack is represented by a circle enlarged by 25× (a), 100× (b), and 500× (c). In addition, (d) presents a 2000× enlargement of the area indicated by the dotted square in (c); the lath of the bainite structure can be observed.
In this case, the fish-eye crack was not one in which fatigue cracks form and grow in internal inclusions. Hence, it had the mechanism of fish-eye cracks forming as facets in the bainite matrix and forming in places with no inclusions.
Fig. 22 shows a SEM image of UNSM-treated fatigue specimen UAT10 (1100 MPa, Nf = 1.93 × 108). The fish-eye crack, which is the crack formation area, is indicated by a circle and an arrow, and the center is enlarged at 25× (a), 100× (b), 500× (c), and 2000× (d), respectively.
Fractography of a fish-eye crack on a UAT10 specimen (1100 MPa, Nf = 1.93 × 108). The fish-eye crack is represented by a circle enlarged by 25× (a), 100× (b), 500× (c), and 2000× (d).
Even in this case, the fish-eye crack is not a type of fish-eye crack in which fatigue cracks occur and grow in internal inclusions, as shown in Fig. 23(a). Therefore, it had a fish-eye crack generation mechanism that formed as a “facet” in the bainite matrix. The fish-eye crack formed in a place where there were no inclusions. The mechanism of formation of these fish-eye cracks was analyzed in the same classification as the case of fish-eye cracks that formed on clean bearing materials, such as Fig. 23(b), in which inclusions were removed with TiN and Al2O3 as the main components (Suh et al., 2021).
The center and surrounding areas of the fish-eye crack in Fig. 23(b) were all made of a Cr matrix, and the components and ratios were similar. No discernible inclusions, such as TiN (Fig. 23(a)), were observed, and fish-eye cracks formed and grew on the Cr matrix (Fig. 23(b)), which determines the fatigue life of the test specimen (Suh, et al., 2021).
The following characteristics were obtained from fracture observation and fracture analysis of the two UNSM-treated UAT specimens. The fish-eye crack of the austempered bainitic structure of the spring steel in this study was not the fractured form of a fish-eye crack in which fatigue cracks form and grow from internal inclusions. Hence, this is a case where a fish-eye crack forms and grows in the bainite matrix, which determines the fatigue life of the specimen. This is the same mechanism in the Cr matrix for fish-eye cracks that form on clean-bearing materials. Furthermore, the facet shape of bainite without inclusions was similar to the mechanism of fish-eye crack formation in Ti alloy. As these fish-eye cracks formed, the fatigue life of AT materials increased significantly compared to QT and AR materials, and the fatigue life was increased further by the UNSM treatment.
In particular, there were no traces of inclusions composed mainly of TiN and Al2O3, which are commonly observed in the center of fish-eye cracks in bearing steel. Moreover, no inclusions were found in UAT materials as cracks formed in the facet. In addition, fish-eye cracks were not found in the fracture surface observations of the fatigue test of UQT and UAR materials, which were UNSM-treated martensitic structures, even in a very long life fatigue range. In UAT materials, most formed in the very long life fatigue range.
3.6 Fracture Mechanical Analysis of Inclusions
Fig. 24(a) summarizes the relationship between the bending stress and the diameter (μm) of the matrix where fish-eye cracks formed, using the results of bearing materials (Nahm et al., 2018b). The bearing material is distributed widely from a minimum of 65 μm to a maximum of 350 μm. On the other hand, because AT and UAT materials have a diameter change of 106 to 180 μm, they tend to converge to a constant value, as shown in the square-marked data, and similar results were obtained with various bearing data.
Comparison of the bearing data of the diameter (a),
Fig. 24(b) presents the relationship between bending stress and
Fig. 24(c) presents the △K (stress intensity factor range) of the inclusions in the fish-eye crack matrix and compares them with the data of three types of bearing steel. At this time, △K used equation (1).
The matrix △K values of the AT and UAT materials of 3.14–3.41
3.7 Crack Initiation Mechanism
Fig. 25(a) shows the change in compressive residual stress in the depth direction according to UNSM treatment, as shown in Fig. 3(b), and shows a tendency to gradually decrease from the surface to the inside from about −650 to 0 MPa. These test results were similar to those for SCM435, SKD61, and other materials (Suh et al., 2011c). In this way, the large compressive residual stress formed by the UNSM treatment can be organized and interpreted schematically as changing from compressive to tensile stress as it decreases gradually in the depth direction depending on the stress conditions acting on the test specimen, as shown in Fig. 25.
Crack initiation mechanism under the rotating bending fatigue test
Fig. 25(b) shows a schematic diagram of the stress state when subjected to bending stress. The maximum stress at the surface decreased linearly (Fig. 25(a)) because it is a bending load, and there was no stress at the neutral axis. The compressive residual stress decreased with depth, as shown in Fig. 25(a). Therefore, a stress state, as shown in Fig. 25(b), is formed by subtracting the tensile and compressive residual stresses. Hence, a mechanism of surface-originating fatigue crack is formed in all bending states except for special cases.
In particular, fish-eye fatigue cracks form in the facet and matrix, unlike in the case of SKD61 and SCM435, because it is difficult to observe inclusions in bainitic AT and UAT materials (Suh and Pyun, 2011). On the other hand, during bending fatigue tests on pearlitic AR and UAR materials and martensitic OT and UQT materials, only fatigue cracks form on the surface according to the principle in Fig. 25(b) (Chai, 2006).
4. Conclusions
In this study, the QT-treated martensite specimen (QT material) and austempered bainite specimen (AT material) were manufactured using SAE9254 material (AR material) to examine the effects of a UNSM surface treatment and VHCF characteristics according to microstructural changes. The following conclusions were obtained.
(1) The fatigue test results of UNSM-treated UAT material, UQT material, and UAR material of spring steel showed a significant improvement in fatigue limit by approximately 33% to 50% compared to the fatigue test results of AT material, QT material, and AR material in the VHCF area. This results from the effects of UNSM processing, confirming that UNSM processing is effective in the VHCF range. Fish-eye cracks were not found in the fracture surface observation results of the fatigue test of the UNSM-treated UQT and UAR materials. Furthermore, the secondary decrease in fatigue limit that occurs in the long life range of bearing steel did not occur in spring steel.
(2) Fish-eye cracks only form in AT and UAT materials, which are the bainite structure of spring steel and are not a type of fatigue crack that forms and grows in internal inclusions as in general bearing materials. Therefore, in the long-life range of bainitic spring steel, fish-eye cracks appear in the form of FGA (fine granular area), and fatigue cracks occur in the form of fish-eye cracks generated in the bainite facet and matrix, resulting in a significant increase in fatigue strength and fatigue life.
(3) In the data of bending stress and
Notes
The authors declare that they have no conflict of interests.
This study was conducted with support from the 2024 Korea Research Institute of Standards and Science’s hydrogen station reliability evaluation technology development funds. (KRISS-2024-GP2024-0010).