Very-High-Cycle Fatigue and Charpy Impact Characteristics of Manganese Steel for Railway Axle at Low Temperatures

applied sciences

Article Very-High-Cycle Fatigue and Charpy Impact Characteristics of Manganese Steel for Railway Axle at Low Temperatures

Byeong-Choon Goo 1,* , Hyung-Suk Mun 2 and In-Sik Cho 3

1 Advanced Railroad Vehicle Division, Korea Railroad Research Institute, Uiwang 16105, Korea 2 New Transportation Innovative Research Center, Korea Railroad Research Institute, Uiwang 16105, Korea; [email protected] 3 Department of Advanced Materials Engineering, Sun Moon University, Asan 31460, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-31-460-5243

Received: 29 June 2020; Accepted: 21 July 2020; Published: 22 July 2020

Featured Application: This work can be applied to the development of new axle materials and to the high-cycle fatigue testing of materials at low temperatures.

Abstract: Railway vehicles are being exposed with increasing frequency to conditions of severe heat and cold because of changes in the climate. Trains departing from Asia travel to Europe through the Eurasian continent and vice versa. Given these circumstances, the mechanical properties and performance of vehicle components must therefore be evaluated at lower and higher temperatures than those in current standards. In this study, specimens were produced from a commercial freight train axle made of manganese steel and subjected to high-cycle fatigue tests at 60, 30, and 20 C. − − ◦ The tests were conducted using an ultrasonic fatigue tester developed to study fatigue at low temperatures. Charpy impact testing was performed over the temperature range of 60 to 60 C − ◦ to measure the impact absorption energy of the axle material. The material showed a fatigue limit above 2 million cycles at each temperature; the lower the test temperature, the greater the fatigue limit cycles. The impact absorption energy at 60 C was 81% less compared to the value at 20 C. − ◦ ◦ The axle material became completely brittle in the temperature range of 30 to 40 C. − − ◦ Keywords: railway axle; very-high-cycle fatigue; low temperature; Charpy impact test; microstructure; ultrasonic fatigue test; manganese steel

1. Introduction With recent changes in climate, railroad vehicles are now operating in a wider range of conditions than in the past, including deep cold and heatwaves. Under such circumstances, the dynamic characteristics of the vehicle may be changed, and unexpected failures may occur [1,2]. For these reasons, interest in the mechanical properties of components at low and high temperatures is increasing. The axle is one of the key components aﬀecting the structural safety of rolling stock. The failure of an axle can lead to the derailment of a vehicle, fatal accidents, property loss, etc. When ﬂying ice cubes by train gust or gravel on the track impact the axle, a defect may occur on the surface of the axle [3]. Cracks may grow from the defect and lead to the axle breaking. So, in addition to fatigue strength, resistance to shock is one of the required standards of axles [4,5]. The fatigue strength and the impact resistance of a material depend on temperature, but the Korean standard [5] does not specify the temperature during tests. In general, railway vehicles are designed to have long lifetimes, from 20 to more than 40 years. Their components need to have long durability. During the lifetime of a railway

Appl. Sci. 2020, 10, 5042; doi:10.3390/app10155042 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 5042 2 of 18 vehicle, which is typically 3 106 km, an axle installed on the vehicle experiences very high cycles × of fatigue loading (1.0~1.2 giga-cycles). However, the giga-cycle fatigue properties of axles have not been studied much because of the restrictions of testing time and costs. In the past, conducting fatigue tests in the giga-cycle range was not easy. Even when a fatigue test is performed at 100 Hz without rest, 115 days are needed to apply 109 loading cycles. Therefore, components under fatigue loading have been designed by using an S–N (stress–number of cycles to failure) curve, with an endurance limit of 2 106 cycles. However, according to the results of very-high-cycle fatigue tests, the fatigue × strength of some materials decreases as the loading cycles increase above 2 106 cycles. In other words, × fatigue limits were not observed for those materials. Under these circumstances, studies on fatigue characteristics at very high cycles are required. High-cycle fatigue testing technologies were widespread in the early 2000s [6–8] with the advent of new technologies such as ultrasonic resonance fatigue testing. Ultrasonic fatigue specimens have a cylindrical or rectangular cross section. Specimen dimensions can be calculated by solving the elastic wave equations [6,7]. In general, a 20 kHz ultrasonic transducer is widely used. Most tests are performed under tension–compression loading. Stanzl-Tschegg et al. [9], Bathias [10], and Nikitin et al. [11] presented test machines for torsional ultrasonic fatigue tests. Recently, fatigue specimens have been designed and studied under simultaneous tensile and shear stress [12,13]. Wang et al. [14] compared the fatigue behavior of two spring wires for automobile use (CrV and CrSi) under a giga-cycle regime. The CrV steel showed constant fatigue strength over 106 cycles, which means that this material has a fatigue limit. However, the CrSi had decreasing fatigue strength with loading cycles. In the case of fatigue life beyond 107 cycles, fatigue cracks occurred from the defects in the interior of the specimens. According to their calculated results, the crack initiation life was more than 99% of the total fatigue life. In their other research [15] on ultra-high-strength steels, they found that high-cycle fatigue cracks initiated from subsurface inclusions of sulfides. Ochi et al. [16] presented similar results with high-strength steels SUJ2 (high-carbon-chromium bearing steel) and SNCM439 (nickel chromium molybdenum steel). The fatigue strengths decreased over 107 cycles. In the short life regime, fatigue cracks started from the surface. In the high-cycle regime, fatigue cracks occurred from internal defects or nonmetallic inclusions such as TiN, Al2O3, MnS, and Al2O3. The effect of nonmetallic inclusions on the fatigue strength of high-strength steels was studied by Murakami and Endo [17] and by Murakami et al. [18]. They expressed fatigue strength as a function of Vickers hardness, stress ratio, and the root of the projected area of inclusion. Most studies have focused on high-strength steels or alloys with hardening heat treatment. However, the railway axle material in this study is normalized and has a medium strength level (tensile strength 590 MPa), and it does not contain Cr, Mo, Ni, Cu, etc. It is composed of five elements (C, Mn, Si, S, P) and Fe. Whether this kind of manganese steel has a fatigue limit or not is interesting. Zettl et al. [19] investigated the giga-cycle fatigue behavior of normalized carbon steel (Ck15, tensile strength 418 MPa) using an ultrasonic fatigue tester. The chemical composition of the material is 0.15 wt % C, 0.45 Mn, 0.20 Si, P, S, Fe balance. All fatigue cracks initiated at surfaces. The fatigue strength decreased until 107 cycles; after that, it was nearly constant. The fatigue strength at 109 cycles was 2.5% lower than at 107 cycles. The highest number of cycles to failure was 1.3 107 cycles. Zhao et al. [20] carried out fatigue tests in a giga-cycle × regime using specimens produced from a real axle with chemical composition C, Si, Mn, Cr, Ni, Cu, Al, and Fe. A duplex S–N curve (inclined–horizontal–inclined–horizontal curve) was obtained. All cracks initiated at surfaces. Damaged spots around subsurface inclusions that occurred during production were observed, but did not propagate beyond a certain size. Qian et al. [21] compared the fatigue strengths of structural steel 40Cr (0.4% C, 1% Cr) in air, fresh water, and 3.5% NaCl aqueous solution. Specimens were tested to 2 109 cycles with a rotary × bending machine. The S–N curves in air and in water showed inclined–horizontal–inclined shapes, but the S–N curve in NaCl presented a continuously decreasing shape. According to the experimental results for 25Cr2Ni2MoV welded specimens by Wu et al. [22], the S–N curve at room temperature presented duplex slopes. However, at 370 ◦C, the fatigue strength decreased continuously. The crack Appl. Sci. 2020, 10, 5042 3 of 18 initiation mechanism was similar at both temperatures. Fatigue studies at high temperatures are numerous, but comparative studies on fatigue properties at room temperature and low temperatures for manganese steels are rarely found in the literature. The purpose of this study is to investigate the fatigue and impact characteristics at low and high temperatures in consideration of the actual Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 18 operating environment of railway vehicles. Ultrasonic fatigue testing equipment capable of fatigue tests tonumerous,160 C wasbut comparative developed, studies and the on fatiguefatigue properties and Charpy at room impact temperature characteristics and low oftemperatures the axle material − ◦ were examinedfor manganese over steels a temperature are rarely found range in the of literature.60 to +The60 purposeC. Crack of this initiation study is to and investigate fracture the surfaces − ◦ were investigated.fatigue and impact characteristics at low and high temperatures in consideration of the actual operating environment of railway vehicles. Ultrasonic fatigue testing equipment capable of fatigue 2. Materialstests to and−160 Methods°C was developed, and the fatigue and Charpy impact characteristics of the axle material were examined over a temperature range of −60 to +60 °C. Crack initiation and fracture surfaces were Theinvestigated. main chemical composition of the axle material is C, Si, Mn, and Fe (Table1). The specimens were obtained from a new railway vehicle commercial axle. The longitudinal direction of the specimen 2. Materials and Methods coincided with that of the axle. The surface was polished by lapping. Tensile tests were performed according toThe ASTM main chemical A370. The composition measured of the yield axle and material tensile is C, strengths Si, Mn, and of Fe three (Table cylindrical 1). The specimens specimens of Φ8.75 werewere withinobtained the from ranges a new of railway 392–440 vehicle and 656–698 commercial MPa, axle. respectively The longitudinal (Figure direction1). Figure of2 thepresents specimen coincided with that of the axle. The surface was polished by lapping. Tensile tests were a microstructural image. The material is composed of ferrite (black part) and pearlite (gray part). performed according to ASTM A370. The measured yield and tensile strengths of three cylindrical The pearlitespecimens has of a varietyΦ8.75 were of shapes. within the The ranges distance of 392–440 between andFe 656–6983C layers MPa, in respectively a pearlite grain(Figure is 1). different for eachFigure grain. 2 presents Figure3 a showsmicrostructura the fatiguel image. test The specimen material andis compos the samplinged of ferrite location (black ofpart) the and specimen from thepearlite axle. (gray The minimumpart). The pearlite and maximum has a variety diameters of shapes. ofThe the distance specimen between were Fe 43C and layers 16 in mm, a pearlite respectively. The specimengrain is different was designed for each grain. according Figure to3a referenceshows the fatigue [7]. The test stress specimen and and strain the sampling in the specimen location were obtainedof the from specimen the elastic from wave the axle. theory The [minimum7]. Natural and vibration maximum frequencies diameters of were the specimen obtained were using 4 and ABAQUS 16 mm, respectively. The specimen was designed according to reference [7]. The stress and strain in software. The convergence of the solution was checked as the number of elements increased. The material the specimen were obtained from the elastic wave theory [7]. Natural vibration frequencies were propertiesobtained listed using in Table ABAQUS2 were software. used for The the convergence simulation. of the The solution Young’s was modulus checked as and the Poisson’s number of ratio of the specimenelements were increased. measured The material according properties to ASTM listed 1876-01. in Table Properties 2 were used of thefor hornthe simulation. were taken The from the literature.Young’s Figure modulus3 shows and the Poisson’s natural ratio modes of the of specim the specimen,en were measured the horn, according and the to assembly. ASTM 1876-01. The lateral displacementProperties of theof the specimen horn were in thetaken middle from ofthe the literature. hourglass Figure section 3 shows was zero,the natural and the modes natural of the frequency was 19,784specimen, Hz. Thisthe horn, frequency and the isassembly. close to The the lateral excitation displacement frequency of the of specimen the input in generator, the middle 20of the kHz. hourglass section was zero, and the natural frequency was 19,784 Hz. This frequency is close to the excitation frequency of the input generator, 20 kHz. Table 1. Composition of the axle material (wt %) [5].

Table 1. Composition of the axle material (wt %) [5]. C Si Mn P S O H Fe 0.35–0.48 0.15–0.40C Si 0.40–0.85Mn <0.035P

Figure 1. Tensile stress–strain curves. Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 18 Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 18 Appl. Sci. 2020, 10, 5042 4 of 18 Figure 1. Tensile stress–strain curves. Figure 1. Tensile stress–strain curves.

Figure 2. Microstructures of the axle material. Figure 2. Microstructures of the axle material.

(a) Fatigue test specimen and sampling location from the axle. (a) Fatigue test specimen and sampling location from the axle.

(b) Natural mode of the specimen, 19,551 Hz. (b) Natural mode of the specimen, 19,551 Hz.

(d) Natural mode of the horn–specimen assembly, 19,784 Hz. (d) Natural mode of the horn–specimen assembly, 19,784 Hz. Figure 3. Modal analysis of the specimen and horn using the finite element method. Figure 3 3.. Modal analysis of the specimen and horn using the finite ﬁnite element method. Figure 4 shows the newly developed piezoelectric UFT (ultrasonic fatigue test) equipment, Figure 4 shows the newly developed piezoelectric UFT (ultrasonic fatigue test) equipment, whichFigure operates4 shows at a frequency the newly of developed20 kHz. The piezoelectric stress ratio (R UFT = minimum (ultrasonic load/maximum fatigue test) equipment,load) was R which operates at a frequency of 20 kHz. The stress ratio (R = minimum load/maximum load) was R =which −1. The operates UFT system at a frequency consisted of 20of kHz.a power The generator, stress ratio a (R converter= minimum (piezo load actuator),/maximum and load) a horn was = −1. The UFT system consisted of a power generator, a converter (piezo actuator), and a horn attachedR = 1. mechanically The UFT system to the consisted converter of by a powera screw. generator, A personal a convertercomputer (piezocontrolled actuator), the generator and ahorn and attached− mechanically to the converter by a screw. A personal computer controlled the generator and converterattached mechanicallyto impose a vibratory to the converter amplitude by at a the screw. end Aof personalthe horn. computerThe power controlled generator theconverts generator a 60 converter to impose a vibratory amplitude at the end of the horn. The power generator converts a 60 Appl. Sci. 2020, 10, 5042 5 of 18 Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 18

Appl.and HzSci. converter 2018voltage, 8, x FORsignal to imposePEER into REVIEW a vibratory20 kHz sinusoidal amplitude ultrason at the endic electrical of the horn. signal. The The power converter generator converts converts5 of 18the 20 a 60kHz Hz ultrasonic voltage signal excitation into a signal 20 kHz into sinusoidal a mechan ultrasonicical signal. electrical The boost signal. and The horn, converter withconverts a natural Hzthe voltagefrequency 20 kHz signal ultrasonic of 20into kHz, a excitation20 amplify kHz sinusoidal signalthe vibration. into ultrason a mechanical Displacementic electrical signal. signal. at Thethe Thehorn boost converter end and was horn, converts measured with a the natural using 20 a kHzfrequencyphotonic ultrasonic ofsensor 20excitation kHz, to calculate amplify signal the into vibration.stress a mechan value Displacement ofical th signal.e specimen. The at the boostThe horn stress and end valuehorn, was measuredatwith the middlea natural using of athe frequencyphotonicspecimen of sensor 20was kHz, tolinearly calculateamplify proportional the stressvibration. valueto theDisplacement of disp thelacement specimen. at theof The hornthe stress specimen end value was endmeasured at the and middle the using dynamic of thea photonicspecimenmodulus sensor was of linearlyelasticityto calculate proportional of thethe specim stress to valueen the [6,7]. displacement of Atth e20 specimen. kHz, of thehigh-cycle specimenThe stress fatigue end value and testing at the the dynamicgenerates middle modulusof excessive the specimenofheat elasticity in was the of linearlyspecimen, the specimen proportional so countermeasures [6,7]. At to 20 the kHz, disp high-cycleagainstlacement heat fatigue ofgeneration the testing specimen are generates necessary. end excessiveand To the ensure dynamic heat inthat the the modulusspecimen,specimen of elasticity so was countermeasures not of heated, the specim the against testen [6,7]. was heat Atperformed generation 20 kHz, high-cycleby are applying necessary. fatigue a load To ensuretesting to the thatgenerates specimen the specimen excessive for 0.3 wass and heatnotstopping in heated, the specimen, it the for test 3 s. wasso As countermeasures a performed result, theby increase applying against of specimenheat a load generation to temperature the specimen are necessary. due for to 0.3 the sTo and deformationensure stopping that the itcaused for specimen3 s.by As the awas result,load not was the heated, increasenegligible. the oftest specimenSeveral was performed specimens temperature by were applying due subjected to the a load deformation to topreliminary the specimen caused tests by for theto 0.3 confirm load s and was the stoppingnegligible.variation it for Severalof 3 specimen s. As specimens a result, temperature the were increase subjected using of an specimen to infrared preliminary temperaturecamera. tests The to low-temperature conﬁrmdue to the the deformation variation test condition of specimencaused was bytemperature theachieved load was by using injectingnegligible. an infrared liquid Several camera. nitrogen specimens The into low-temperature thewere chamber subjected through test to conditionpreliminary a nozzle. was testsThe achieved temperatureto confirm by injecting the of the variationliquidchamber nitrogen of specimen was into controlled thetemperature chamber by opening throughusing an and ainfrared nozzle. closing camera. The the temperature nozzle. The low-temperature The ofresolution the chamber of test temperature was condition controlled was control by achievedopeningwas about by and injecting closing±2 °C. Asliquid the shown nozzle. nitrogen in The Figure resolutioninto 4, the the chamber horn of temperature and through the specimen control a nozzle. waswere The about located temperature2 inC. the As chamber, shown of the in in ± ◦ chamberFigurewhich4 was, thea low controlled horn temperature and theby specimenopening below −and160 were °Cclosing located could the inbe nozzle. therealized. chamber, The The resolution indeveloped which aof lowequipment temperature temperature is expectedcontrol below to was160 beabout usefulC could±2 for°C. below-temperatureAs realized. shown in The Figure developedfatigue 4, the testing. horn equipment and the is specimen expected were to be located useful forin the low-temperature chamber, in − ◦ whichfatigue a low testing. temperature below −160 °C could be realized. The developed equipment is expected to be useful for low-temperature fatigue testing.

(b) Test equipment (a) Schematic diagram (a) Schematic diagram (b) Test equipment Figure 4. Ultrasonic resonant fatigue test equipment. FigureFigure 4. 4.UltrasonicUltrasonic resonant resonant fatigue fatigue test test equipment. equipment. Figure 5 shows a drawing and photograph of the Charpy impact test specimen. The specimens wereFigureFigure produced 5 5shows shows accordinga adrawing drawing to and andASTM photograph photograph A370 from of of th a thee commercial Charpy Charpy impact impact railway test test specimen.vehicle specimen. axle. The The The specimens specimens surface was werewerepolished produced produced by accordinglapping. according The to to ASTMlongitudinal ASTM A370 A370 fromdirection from a commercial a commercialof the specimen railway railway coincided vehicle vehicle axle. with axle. The that The surface of surface the axle. was was The polishedpolishedspecimen by bylapping. size lapping. was The 10 The longitudinal× 10 longitudinal × 55 and direction a V-shaped direction of thenotc of specimen theh was specimen machined coincided coincided in itswith center. that with ofThe that the Charpy ofaxle. the The impact axle. specimenThe specimen size was size 10 was× 10 10× 55 and10 a55 V-shaped and a V-shaped notch was notch machined was machined in its center. in its The center. Charpy The impact Charpy test is performed by hitting× the× opposite side of the impact specimen fixed at both ends with a free- testimpact isfalling performed test hammer. is performed by hittingThe test by the hittingevaluates opposite the oppositetheside impact of the side impactre ofsistance the specimen impact of a specimenspecimen fixed at ﬁxedbothby measuring ends at both with ends thea free- with energy a fallingfree-fallingconsumed hammer. hammer. when The thetest The impactevaluates test evaluates specimen the impact the breaks. impact resistance Under resistance actualof a ofspecimen conditions, a specimen by gravel bymeasuring measuring or ice the rubble the energy energy on the consumedconsumedtracks maywhen when be the thelifted impact impact by the specimen wind generated breaks.breaks. UnderUnder by the actual actual vehicle conditions, conditions, and hit the gravel gravel axle, or impacting iceor ice rubble rubbleon the theon surface tracksthe of tracksmaythe may be axle lifted be and lifted by creating the by wind the surface wind generated generateddefects. by theImpact by vehicle the resistan vehicle and hitce and is the proportionalhit axle, the impactingaxle, impacting to the the ability surface the surfaceto of resist the axleofcrack theand axlepropagation. creating and creating surface Therefore, surface defects. impact defects. Impact resistance resistanceImpact resistan is is one proportional ceof isthe proportional key to mate the abilityrial to specifications the to resistability crack to thatresist propagation. ensure crack axle propagation.Therefore,durability. impactTherefore, resistance impact is resistance one of the is key one material of the speciﬁcationskey material specifications that ensure axle that durability. ensure axle durability.

(a) Geometry of Charpy specimen (b) Charpy impact specimen (a) Geometry of Charpy specimen (b) Charpy impact specimen FigureFigure 5. 5.Geometry Geometry and and photograph photograph of of the the Charpy Charpy impact impact specimen. specimen. Figure 5. Geometry and photograph of the Charpy impact specimen. 3. Results and Discussion 3. Results and Discussion

3.1. Fatigue Test Appl. Sci. 2020, 10, 5042 6 of 18

3. Results and Discussion

3.1. Fatigue Test Figure6a presents an S–N diagram showing the fatigue test data at a room temperature of about 20 ◦C. The logarithm of the number of cycles to failure is on the X-axis, and stress amplitude is on the Y-axis. When 200 MPa stress amplitude was applied to the center of the specimen, fatigue failure did not occur until 109 cycles, which means that the axle material had typical carbon steel properties with a fatigue limit. However, the fatigue limit was not 2 106 cycles. Some specimens fractured × in the 2 106 to 1 107 cycle range. The fracture surfaces in Figure6b,c show the fracture surfaces × × at 213,000 and 1.02 106 cycles, respectively. A crack occurred on the surface, gradually grew × elliptically, and then reached an unstable state at a certain crack size. The specimen was finally broken. Some inclusions were found on the fracture surface. According to [1], inclusions found in axles are typically aluminates, sulfides, silicates, oxides, etc. The specification [4] specifies the acceptable size and quantity of inclusions. Figure7a presents an S–N diagram at 30 C. Fatigue failure in four specimens occurred in the − ◦ 2 106 to 1 107 cycle range, beyond the 2 million cycles traditionally regarded as the infinite life of × × steel. Figure7b–e shows the fracture surfaces; cracks formed on the surface as in the room temperature case and gradually progressed. Once they reached a certain size, they entered an unstable growth mode. Some black voids or inclusions were observed on the fracture surface. Figure8a presents an S–N diagram at 60 C. Similar to at 30 C, fatigue failure in five specimens occurred in the 2 106 to − ◦ − ◦ × 1 107 cycle range, beyond the 2 million cycles. Figure8b,c shows the fracture surfaces; cracks formed × on the surface as in the room temperature case and gradually progressed. Once they reached a certain size, they entered an unstable growth mode. Some black voids or inclusions were observed on the fracture surface. Further, it was observed that long, white ridges were decreased in the stable crack propagation region. Figure9 compares fatigue test data for 20, 30, and 60 C. It can be seen that the − − ◦ fatigue strength increases with decreasing temperature. In general, for metals, the lower the temperature, the higher the hardness, the tensile, and yield strengths. Fatigue strength is proportional to hardness and tensile strength. When the amplitude of the acting stress is Sa and the number of cycles to failure under a given stress amplitude ∆S is N, the straight section of the Sa–N diagrams at each temperature can be expressed as Equations (1)–(3). R is the coefficient of determination. The fatigue strengths at 2 million cycles for +20, 30, and 60 C − − ◦ were 210.1, 225.2, and 251.9 MPa, respectively. Fatigue strength at 30 and 60 C increased by 7.1 and − − ◦ 19.9%, respectively, compared to the fatigue strength at room temperature. Statistical analysis of the S–N curves was performed using the Excel program. The results are summarized in Table3.

2 +20 ◦C: Sa = 25.786 log(N) + 372.57, R = 0.895 (1) − 2 30 ◦C: Sa = 28.946 log(N) + 407.61, R = 0.931 (2) − − 2 60 ◦C: Sa = 34.207 log(N) + 467.43, R = 0.888 (3) − −

Table 3. Statistical analysis of the S–N curves.

Coeﬃcient of Determination Standard Deviation Standard Error of the Mean

+20 ◦C 0.895 27.95 6.58 30 C 0.931 20.03 5.78 − ◦ 60 C 0.888 36.19 10.44 − ◦ Appl. Sci. 2020 10 Appl. Sci. 2018, , 8, 5042x FOR PEER REVIEW 77 of of 18 18

(a) S–N diagram at room temperature.

(b) Fracture surface at room temperature, 224 MPa, 213,000 cycles.

Figure 6. Cont. Appl. Sci. 2020, 10, 5042 8 of 18 Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 18

Figure 6.6. S–N diagram and fracture surface at room temperature.temperature.

(a) S–N diagram at –30 °C.

Figure 7. Cont. Appl. Sci. 2020, 10, 5042 9 of 18 Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 18

(b) Fracture surface at –30 °C, 240 MPa, 854,000 cycles.

Figure 7. Cont. Appl. Sci. 2020, 10, 5042 10 of 18

Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 18

(d) Fracture surface at –30 °C, 210 MPa, 7.12 × 106 cycles.

(e) Fracture surface at –30 °C, 200 MPa, 107 cycles.

FigureFigure 7.7. S–NS–N diagramdiagram andand fracturefracture surfacesurface atat –3030 ◦°C.C. − Appl. Sci. 2020, 10, 5042 11 of 18 Appl. Sci. 2018, 8, x FOR PEER REVIEW 11 of 18

(a) S–N diagram at –60 °C.

(b) Fracture surface at –60 °C, 280 MPa, 840,000 cycles.

Figure 8. Cont. Appl.Appl. Sci. Sci.2020 2018, 10, 8,, 5042x FOR PEER REVIEW 1212 of of 18 18 Appl. Sci. 2018, 8, x FOR PEER REVIEW 12 of 18

6 Figure 8. S–N diagram(c) Fracture and fracture surface surface at –60 at°C, –60 240 °C, MPa, 240 MPa,5.5 × 10Nf6 cycles.= 5.5 × 10 cycles.

FigureFigure 8.8. S–N diagram andand fracturefracture surfacesurface atat –6060 °C,C, 240 MPa, Nf = 5.55.5 × 10106 6cycles.cycles. − ◦ ×

Figure 9. S–N diagram at room temperature (RT, 20◦C), 30 ◦C, and 60 ◦C. Figure 9. S–N diagram at room temperature (RT, 20°C),− −30 °C, and −–60 °C.

3.2. Charpy Impact Test

Charpy impactFigure testing 9. S–N diagram was carried at room out temperature in the temperature (RT, 20°C), − range30 °C, ofand –6060 to°C.+ 60 C, and the − ◦ average3.2. Charpy value Impact for the Test three specimens was taken at each temperature. Table4 shows the brittle fracture

3.2. Charpy Impact Test Appl. Sci. 2020, 10, 5042 13 of 18 appearance, lateral expansion, and impact absorption energy. The brittle fracture appearance is the ratio of the ductile area to the full fracture surface, which is inversely proportional to the impact absorptionAppl. Sci. energy.2018, 8, x FOR Figure PEER REVIEW10 shows the change in the shock absorption energy with temperature.13 of 18 At temperatures below 30 ◦C, full brittle fracture occurred. The three specimens exhibited nearly Charpy impact testing− was carried out in the temperature range of −60 to +60 °C, and the average constant shock absorption energy. However, at temperatures above 20 ◦C, along the edge of the value for the three specimens was taken at each temperature. Table 4− shows the brittle fracture fracture surface, ductile failure was observed. The shock absorption energies of the three specimens appearance, lateral expansion, and impact absorption energy. The brittle fracture appearance is the had slightlyratio of ditheff erentductile values. area to Lateral the full expansion fracture surf refersace, towhich the increaseis inversely in the proportional width of theto the specimen impact cross sectionabsorption measured energy. in the Figure groove 10 shows direction the change after the in the test, shock which absorption tends to energy be proportional with temperature. to the At impact absorptiontemperatures energy. below According –30 °C, tofull the brittle test results,fracture theoccurred. axle material The three became specimens completely exhibited brittle nearly in the rangeconstant of 30 toshock40 absorptionC, and the energy. shock-absorbing However, at energytemperatures became above very –20 small, °C, along converging the edge to of a constantthe − − ◦ value.fracture The minimumsurface, ductile absorbed failure energywas observed. required The byshock the absorption KS standard energies [5] isof 31the J. three The specimens European EN had slightly different values. Lateral expansion refers to the increase in the width of the specimen standard [4] requires 30 J at 20 ◦C. The test results satisfied the two standards. Figure 11 shows fracture cross section measured in the groove direction after the test, which tends to be proportional to the surfaces after the impact test. At cross sections below 40 ◦C, there were no ductile deformations impact absorption energy. According to the test results, −the axle material became completely brittle (the slightly black areas near the edges). As the temperatures rose from -30 C, the areas of ductile in the range of −30 to −40 °C, and the shock-absorbing energy became very small,◦ converging to a deformation near the edges of the specimens increased. Further, the deformation of the opposite edge constant value. The minimum absorbed energy required by the KS standard [5] is 31 J. The European of theEN groove standard increased. [4] requires It was 30 J inferredat 20 °C. The from test the results low-temperature satisfied the two impact standards. test data Figure that 11 theshows current axle material is vulnerable to external impacts at temperatures below 30 C, which means that a fracture surfaces after the impact test. At cross sections below −40 °C,− there◦ were no ductile defectdeformations can be easily (the caused slightly byblack an areas external near impact.the edges). Notably, As the temperatures the carbon rose steel from axle -30 used °C, the in thisareas study did notof ductile contain deformation alloying elements, near the edges such asof the Cr, Cu,specimens Mo, Ni, increased. and V, includedFurther, the in thedeformation axle component of the of the ENopposite standard edge [ 4of]. the It wouldgroove beincreased. effective It was to add inferr alloyinged from elements the low-temperature to improve impact resistance test data toimpact that at low temperature.the current axle Figure material 12 is shows vulnerable close-up to external images impacts of the at marked temperatures sites below A and −30 B in°C, Figure which 11meansa. Slides, cracks,that and a defect voids can were be easily observed, caused as by shown an external in Figure impact. 12 Notably,a. When the the carbon temperature steel axle was used increased in this to study did not contain alloying elements, such as Cr, Cu, Mo, Ni, and V, included in the axle 60 C, ridges and dimples were observed, as shown in Figure 12i. ◦ component of the EN standard [4]. It would be effective to add alloying elements to improve resistance to impact at low temperature. Figure 12 shows close-up images of the marked sites A and Table 4. Charpy impact test results. B in Figure 11a. Slides, cracks, and voids were observed, as shown in Figure 12a. When the Temperaturetemperature (wasC) increased60 to 60 °C,50 ridges and40 dimples30 were observed,20 as 0 shown 20 in Figure 40 12i. 60 ◦ − − − − − Brittle fracture 100Table 100 4. Charpy 100 impact 90 test results. 90 80 70 40 50 appearance (%) Lateral expansionTemperature (mm) 0.05(°C) 0.03−60 − 0.0650 −40 0.17−30 0.22−20 0 0.3420 0.6740 0.8760 1.24 Brittle fracture appearance (%) 100 100 100 90 90 80 70 40 50 Impact absorption 6.42 6.58 7.00 11.32 12.98 23.40 34.59 46.87 68.19 energyLateral (J) expansion (mm) 0.05 0.03 0.06 0.17 0.22 0.34 0.67 0.87 1.24 Impact absorption energy (J) 6.42 6.58 7.00 11.32 12.98 23.40 34.59 46.87 68.19

FigureFigure 10. 10.Charpy Charpy impact absorption absorption energy. energy. Appl. Sci. 2020, 10, 5042 14 of 18 Appl. Sci. 2018, 8, x FOR PEER REVIEW 14 of 18 Appl. Sci. 2018, 8, x FOR PEER REVIEW 14 of 18

(a) −60 °C (b) −50 °C (c) −40 °C (d) −30 °C (a) −60 °C (b) −50 °C (c) −40 °C (d) −30 °C

(g) 40 °C (h) 60 °C (e) −(e20) −°C20 °C (f) (20f) 20°C °C (g) 40 °C (h) 60 °C

FigureFigureFigure 11.11. 11.Images Images of of theof the the fracture fracture fracture surfaces surfaces surfaces taken taken taken with withwith an an optical optical optical microscope. microscope. microscope.

(a) (Fracturea) Fracture surfaces surfaces of ofCharpy Charpy impact impact testing testing at at − 60−60 °C. °C.

(b) Fracture surfaces of Charpy impact testing at −50 °C. (b) Fracture surfaces of Charpy impact testing at −50 °C. Figure 12. Cont. Appl. Sci. 2020, 10, 5042 15 of 18

Appl. Sci. 2018, 8, x FOR PEER REVIEW 15 of 18

(d) Fracture surfaces of Charpy impact testing at −30 °C.

(e) Fracture surfaces of Charpy impact testing at −20 °C.

Figure 12. Cont. Appl. Sci. 2020, 10, 5042 16 of 18

Appl. Sci. 2018, 8, x FOR PEER REVIEW 16 of 18

(f) Fracture surfaces of Charpy impact testing at 0 °C.

(g) Fracture surfaces of Charpy impact testing at 20 °C.

(h) Fracture surfaces of Charpy impact testing at 40 °C.

Figure 12. Cont. Appl. Sci. 2020, 10, 5042 17 of 18 Appl. Sci. 2018, 8, x FOR PEER REVIEW 17 of 18

(i) Fracture surfaces of Charpy impact testing at 60 °C.

FigureFigure 12. 12.Images Images ofof fracture surfaces surfaces taken taken wi withth a scanning a scanning electronic electronic microscope. microscope.

4.4. Conclusions Conclusions InIn this this study, study, specimens specimens were were produced produced from from a a commercial commercial traintrain axleaxle mademade ofof manganesemanganese steel and subjectedand subjected to high-cycle to high-cycle fatigue fatigue tests and tests Charpy and Char impactpy impact tests overtests theover temperature the temperature range range of 60of –60 to 60 C − ◦ toto obtain 60 °C fatigue to obtain strength fatigue and strength impact and absorption impact absorption energy data energy about data the about axle the material. axle material. Some importantSome resultsimportant were results as follows. were as follows. AA 20 20 kHz kHz ultrasonic ultrasonic actuatoractuator fatigue tester tester was was su successfullyccessfully developed. developed. It is It capable is capable of fatigue of fatigue tests down to −160 °C by spraying liquid nitrogen into the chamber. tests down to 160 ◦C by spraying liquid nitrogen into the chamber. The fatigue− strength values of the axle material at 2 million cycles at +20, −30, and, −60 °C were The fatigue strength values of the axle material at 2 million cycles at +20, 30, and, 60 C were 210.1, 225.2, and 251.9 MPa, respectively. − − ◦ 210.1, 225.2, and 251.9 MPa, respectively. The fatigue strength of the axle material at −30 and −60 °C increased by 7.1 and 19.9%, The fatigue strength of the axle material at 30 and 60 C increased by 7.1 and 19.9%, respectively, respectively, compared to the fatigue strength at− room temperature.− ◦ comparedFatigue to the failure fatigue occurred strength in atthe room 2 × temperature.106 to 1 × 107 cycle range, above the 2 million cycles Fatigue failure occurred in the 2 106 to 1 107 cycle range, above the 2 million cycles traditionally traditionally considered to be the carbon× steel ×fatigue limit. consideredThe toshock-absorbing be the carbon steelenergy fatigue of the limit. axle material decreased gradually with decreasing temperature,The shock-absorbing and the axle energy material of the became axle material fully brittle decreased in the graduallyrange of − with30 to decreasing −40 °C. The temperature, shock andabsorption the axle materialenergy at became −60 °C was fully 81% brittle lower in than the rangethat at of 20 °C.30 We to inferred40 C. The from shock the impact absorption test results energy at − − ◦ 60thatC the was current 81% lower axle material than that is atvulnerable 20 C. We to inferred external from shocks the at impact temperatures test results below that −30 the °C. current These axle − ◦ ◦ materialresults issuggest vulnerable that it to will external be necessary shocks at to temperatures improve the belowlow-temperature30 C. These properties results of suggest the axle that it − ◦ willmaterial be necessary used in to this improve study for the use low-temperature in cold climates. properties of the axle material used in this study for useAuthor in cold Contributions: climates. Conceptualization and Methodology; Funding Acquisition, H.S.M.; Analysis, Investigation, Data Curation, Writing—Original Draft and Editing, B.C.G.; Ultrasonic Fatigue Testing, I.S.C. Author Contributions: Conceptualization and Methodology; Funding Acquisition, H.-S.M.; Analysis, Investigation,Funding: This Data study Curation, was supported Writing—Original by an R&D program Draft andof Korea Editing, Railroad B.-C.G.; Research Ultrasonic Institute. Fatigue Testing, I.-S.C. All authors have read and agreed to the published version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest. Funding: This study was supported by an R&D program of Korea Railroad Research Institute. ConﬂictsReferences of Interest: The authors declare no conﬂict of interest. 1. Luo, R.; Teng, W.; Wu, X.; Shi, H.; Zeng, J. Dynamics simulation of a high-speed railway car operating in Referenceslow-temperature environments with stochastic parameters. Veh. Syst. Dyn. 2019, doi:10.1080/00423114.2019.1662922. 1. Luo, R.; Teng, W.; Wu, X.; Shi, H.; Zeng, J. Dynamics simulation of a high-speed railway car operating in 2. Luo, R.; Shi, H.; Guo, J.; Huang, L.; Wang, J. A nonlinear rubber spring model for the dynamics simulation low-temperature environments with stochastic parameters. Veh. Syst. Dyn. 2019.[CrossRef] of a high-speed train. Veh. Syst. Dyn. 2019, doi:10.1080/00423114.2019.1624788. 2. Luo, R.; Shi, H.; Guo, J.; Huang, L.; Wang, J. A nonlinear rubber spring model for the dynamics simulation of 3. Zerbst, U.; Beretta, S.; Köhler, G.; Lawton, A.; Vormwald, M.; Beier, H.T.; Klinger, C.; Černy, I.; Rudlin, J.; a high-speed train. Veh. Syst. Dyn. 2019.[CrossRef] Heckel, T.; et al. Safe life and damage tolerance aspects of railway axles—A review. Eng. Fract. Mech. 2013, ˇ 3. Zerbst,98, 214–271. U.; Beretta, S.; Köhler, G.; Lawton, A.; Vormwald, M.; Beier, H.T.; Klinger, C.; Cerny, I.; Rudlin, J.; 4. Heckel,BS EN T.;13261:2009+A1:2010, et al. Safe life and Railway damage applications tolerance aspects-Wheelsets of railway and bogies axles—A -Axles -Pro review.duct Eng.requirements, Fract. Mech. BSI, 2013, 98,London, 214–271. UK. [CrossRef ] 4. BS EN 13261:2009+A1:2010. Railway Applications-Wheelsets and Bogies-Axles-Product Requirements. BSI: London, UK. Appl. Sci. 2020, 10, 5042 18 of 18

5. Korean Industrial Standards. Wheels for Railway Rolling Stock, KS R 9221; Korean Agency for Technology and Standards: Eumseonggun, Korea, 2008. 6. Ultrasonic Fatigue Testing. Mechanical Testing and Evaluation, ASM Handbook; ASM International: Novelty, OH, USA, 2000; Volume 8, pp. 717–728. 7. Bathias, C.; Paris, P.C. Gigacycle Fatigue in Mechanical Practice; Marcel Dekker: New York, NY, USA, 2005; pp. 9–29. 8. Stanzl-Tschegg, S. Very high cycle fatigue measuring techniques. Int. J. Fatigue 2014, 60, 2–17. [CrossRef] 9. Stanzl-Tschegg, S.E.; Mayer, H.R.; Tschegg, E.K. High frequency method for torsion fatigue testing. Ultrasonics 1993, 31, 275–280. [CrossRef] 10. Bathias, C. Piezoelectric fatigue testing machines and devices. Int. J. Fatigue 2006, 28, 1438–1445. [CrossRef] 11. Nikitin, A.; Bathias, C.; Palin-Luc, T. A new piezoelectric fatigue testing machine in pure torsion for ultrasonic gigacycle fatigue tests: Application to forged and extruded titanium alloys, Fatigue and Fracture of Engineering Materials & Structures. Fatigue Fract. Eng. Mater. Struct. 2015, 38, 1294–1304. 12. Vieira, M.; Reis, L.; Freitas, M.; Ribeiro, A. Strain measurements on specimens subjected to biaxial ultrasonic fatigue testing. Theor. Appl. Fract. Mech. 2016, 85, 2–8. [CrossRef] 13. Costa, P.; Vieira, M.; Reis, L.; Ribeiro, A.; Freitas, M. New specimen and horn design for combined tension and torsion ultrasonic fatigue testing in the very high cycle fatigue regime. Int. J. Fatigue 2017, 103, 248–257. [CrossRef] 14. Wang, Q.Y.; Berard, J.Y.; Rathery, S.; Bathias, C. High-cycle fatigue crack initiation and propagation behaviour of high-strength spring steel wires. Fatigue Fract. Eng. Mater. Struct. 1999, 22, 673–677. [CrossRef] 15. Wang, Q.Y.; Bathias, C.; Kawagoishi, N.; Chen, Q. Effect of inclusion on subsurface crack initiation and gigacycle fatigue strength. Int. J. Fatigue 2002, 24, 1269–1274. [CrossRef] 16. Ochi, Y.; Matsumura, T.; Masaki, K.; Yoshida, S. High-cycle rotating bending fatigue property in very long-life regime of high-strength steels. Fatigue Fract. Eng. Mater. Struct. 2002, 25, 823–830. [CrossRef] 17. Murakami, Y.; Endo, M. Effects of defects, inclusions and inhomogeneities on fatigue strength. Int. J. Fatigue 1994, 16, 163–182. [CrossRef] 18. Murakami, Y.; Takada, M.; Toriyama, T. Super-long life tension-compression fatigue properties of quenched and tempered 0.46% carbon steel. Int. J. Fatigue 1998, 16, 661–667. [CrossRef] 19. Zettl, B.; Mayer, H.; Ede, C.; Stanzl-Tschegg, S. Very high cycle fatigue of normalized carbon steels. Int. J. Fatigue 2006, 28, 1583–1589. [CrossRef] 20. Zhao, Y.X.; Yang, B.; Feng, M.F.; Wang, H. Probabilistic fatigue S–N curves including the super-long life regime of a railway axle steel. Int. J. Fatigue 2009, 31, 1550–1558. [CrossRef] 21. Qian, G.; Zhou, C.; Hong, Y. Crack propagation mechanism and life prediction for very-high cycle fatigue of a structural steel in different environmental medias. Frat. Integrità Strutt. 2013, 25, 7–14. [CrossRef] 22. Wu, W.; Zhu, M.L.; Liu, X.; Xuan, F.G. Effect of temperature on high-cycle fatigue and very high cycle fatigue behaviour of a low-strength Cr–Ni–Mo–V steel welded joint. Fatigue Fract. Eng. Mater. Struct. 2017, 40, 45–54. [CrossRef]

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