metals Article
Effect of Tempering Temperatures on Tensile Properties and Rotary Bending Fatigue Behaviors of 17Cr2Ni2MoVNb Steel Sheng-Guan Qu *
ID
, Ya-Long Zhang, Fu-Qiang Lai and Xiao-Qiang Li
School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China;
[email protected] (Y.-L.Z.);
[email protected] (F.-Q.L.);
[email protected] (X.-Q.L.) * Correspondence:
[email protected]; Tel.: +86-020-8711-1983 Received: 31 May 2018; Accepted: 27 June 2018; Published: 2 July 2018
Abstract: With the rapid development of the automotive industry in China, the common gear steels no longer meet the high speed and heavy load requirements of the automotive industry. 17Cr2Ni2MoVNb steel is a new type of gear steel in the automotive industry, but the mechanical properties of 17Cr2Ni2MoVNb are not well documented. In this study, the tensile properties and rotary bending fatigue behaviors of 17Cr2Ni2MoVNb were investigated, (quenched at 860 ◦ C and tempered at 180, 400, 620 ◦ C); the microstructures and fracture surface were analyzed using an optical microscope, scanning electron microscopy and transmission electron microscopy. The results show that at higher tempering temperatures, the tissue was denser, and the residual austenite transformed into lower bainite or tempered martensite. Dislocation density reduced while tempering temperature increased. Moreover, the samples with a tempering temperature of 180 ◦ C exhibited the highest tensile strength of 1456 MPa, in addition to fatigue limits of 730, 700 and 600 MPa at temperatures of 180, 400, and 620 ◦ C, respectively. Keywords: 17Cr2Ni2MoVNb; tempering temperature; tensile properties; rotary bending fatigue
1. Introduction In the field of gear strength design, it is important to take various material parameters such as tensile strength and fatigue strength limits into consideration. The strengths of common gear steels 20CrMnTiNb and 20CrMn were reported to be 1280 MPa and 1260 MPa, respectively [1], and these materials cannot meet the high speed and heavy load requirements of the automotive industry. Fe-based alloys are always the first-choice material for the automotive industry, because of favorable mechanical properties and low cost [2]. Numerous studies mainly focus on Fe-based alloys to improve mechanical properties in gear steel [3–5]. Pereloma et al. found that Cr can delay Fe3 C precipitation in low carbon steels [6]. In addition, a small amount of Ni was shown to improve the ductility. Adding an excessive amount would reduce ductility [7]. The addition of an appropriate amount of Mo contributes to precipitation strengthening and provides phase balance strengthening [8]. In particular, Wang’s investigations showed that V can effectively prevent abnormal austenite grain growth in 10 h [9]. Xiao et al. demonstrated that Nb can significantly retard recrystallization. It can also effectively refine the recrystallized austenite grain; therefore, fine microstructures can be obtained [10]. Chen and Geng studied the influence of heat treatment on mechanical properties of Cr-Ni-Mo carburized gear steel, and the result indicated that the optimal austenitizing temperature is between 840 and 860 ◦ C [11]. The early work by Wu et al. showed that the mechanical properties of Cr-Ni-Mo steel were linearly related to the tempering parameter in the tempering process, and the tested steel exhibited the tensile strength of 1150 MPa [12].
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17Cr2Ni2MoVNb steel is a type of low alloy steel. According to the service condition of the gear, it presents new challenges to mechanical properties. Because of the broad applications of 17Cr2Ni2MoVNb steel in the automotive industry, it is necessary to study the effect of tempering temperature on the tensile properties and rotary bending fatigue behaviors of 17Cr2Ni2MoVNb. In this paper, these properties, in addition to fracture morphology and fracture mechanism, are discussed. 2. Experimental Procedure The chemical compositions of 17Cr2Ni2MoVNb steel are presented in Table 1 and were prepared by the sequential processes of melting in electric furnace—ladle refining—vacuum degassing, and then continuous rolling into a ϕ100 mm round bar. This was followed by quenching at 860 ◦ C for 1 h and tempering at the different temperatures (shown in Table 2). Table 1. Chemical compositions of 17Cr2Ni2MoVNb steel (wt %). C
Si
Mn
Cr
Ni
Al
Cu
Mo
V
Nb
Mg
S
P
0.188
0.015
0.40
1.83
1.63
0.048
0.01
0.31
0.093
0.059
0.007
0.001
0.009
Table 2. Heat treatment of 17Cr2Ni2MoVNb steel. Samples Label
Quenching Temperature
Quenching Method
Tempering Temperature
Tempering Method
1 2 3
860 ◦ C × 1 h 860 ◦ C × 1 h 860 ◦ C × 1 h
oil oil oil
180 ◦ C × 2 h 400 ◦ C × 2 h 620 ◦ C × 2 h
water water water
The platers (8 mm × 8 mm × 8 mm) were cut from the heat-treated samples at different temperatures, which were then polished and corroded with 4% alcohol nitric acid solution for 15 s. Afterwards, their microstructures were observed using an optical microscope (OM, LEICA M165C, Barnack, Germany) and a transmission electron microscope (TEM, FEI Tecnai G20, Oregon, USA) operating at 200 kV. An X-ray diffractometer (XRD, Cu Kα radiation, Amsterdam, Holland) was used to identify the metallographic phase of the materials. The hardness of the samples was measured with durometer (SCTMC, HV-50, Shanghai, China), using an applied load and dwelling time of 200 g and 15 s respectively. After being quenched and tempered treatment, the samples were machined into standard specimens for room temperature tensile (Chinese standard, GB/T228-2002) and fatigue tests (Chinese standard, GB/T4337-2015). The tensile tests using a universal testing machine (CMT5105, Shenzhen, China), and the dimension of tensile specimens are shown in Figure 1a. The fatigue tests were assessed using a rotating bending fatigue test machine (QBWP-10000X, Jinan, China), and the dimension of fatigue specimens is shown in Figure 1b. Fatigue strength of 1 × 107 cycles is usually defined as the fatigue limit. All specimens were tested at room temperature at a frequency of 6000 r/min and under load control at stress ratios R = −1, and the regarded load was taken as the control variable [13–15]. The dimensions of fatigue specimens are shown in Figure 1b. Fracture surfaces were examined with a scanning electron microscope (NOVA NANOSEM 430, Eindhoven, Holland). Before being examined, the surfaces were cleaned using alcohol for 5 min.
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(a)(a)
(b) (b)
Figure 1. Dimensionsofofthe thespecimens specimensfor fortensile tensile tests and rotary bending specimen Figure Dimensions androtary rotarybending bendingfatigue fatiguetests: tests:(a) (a) specimen Figure 1.1.Dimensions of the specimens for tensile tests and fatigue tests: (a) specimen of tensile tests; (b) specimenofofrotary rotarybending bendingfatigue fatigue tests. of tensile tests; (b) specimen of tensile tests; (b) specimen of rotary bending fatigue tests.
3. Results and Discussion Results and Discussion 3.3.Results and Discussion MicrostructuralCharacterization Characterization 3.1.3.1. Microstructural 3.1. Microstructural Characterization Figure 2a–c shows the microstructural structures of 17Cr2Ni2MoVNb at the Figure 2a–c shows thethe microstructural structures of 17Cr2Ni2MoVNb temperedtempered at the temperature Figure 2a–c shows microstructural structures of 17Cr2Ni2MoVNb tempered at the temperature of 180, 400 and 620 °C, respectively. The tempering temperature had a great influence of 180, 400 and 620 ◦400 C, respectively. The tempering temperature had a great influence on martensitic temperature of 180, and 620 °C, respectively. The tempering temperature had a great influence on martensitic shape; there were a large number of lath martensites, which were a product of shape; there were a large number ofalath martensites, which were a productwhich of standardized quenching on martensitic shape; there large number of lath2c martensites, were a product standardized quenching aswere shown in Figure 2a. Figure shows that after quenching and highof as shown in Figure 2a. Figure 2c shows that after quenching and high tempering, the standardized shown in Figure 2a. Figure 2c shows that temperature after quenching andmore high temperaturequenching tempering,asthe structure maintained the microstructures, though they were structure maintained the microstructures, though they were more detailed and fuzzy. Figure 2d–f temperature tempering, the 2d–f structure the microstructures, though were more detailed and fuzzy. Figure displaymaintained the transmission electron microscopy (TEM) they microstructures display the transmission electron microscopy (TEM) microstructures of long and slender ferrite and detailed and fuzzy. Figure 2d–f display the transmission electron microscopy (TEM) microstructures of long and slender ferrite and retained austenite at each tempering temperature. The results reveal austenite at each tempering The results reveal that there were muchreveal more that and there were much more dislocations, and the areas marked with “1” illustrate the decomposition ofretained long slender ferrite and retainedtemperature. austenite at each tempering temperature. The results dislocations, and the areas marked with “1” illustrate the decomposition process of residual austenite. of residual austenite. The residual austenite gradually transformed intothe lower bainite or thatprocess there were much more dislocations, and the areas marked with “1” illustrate decomposition The residual austenite gradually transformed into lower bainite or tempered martensite each tempered martensite at each heat treatment condition, and finer bainite were achieved atat the process of residual austenite. The residual austenite gradually transformed into lower bainite or ◦ heat treatment condition, finertreatment bainite were achieved at the of 180 C; a at similar temperature of 180 at °C;each aand similar transformation of microstructures was previously reported by tempered martensite heat condition, and finertemperature bainite were achieved the transformation previously reported by Shendy was [16]. previously reported by Shendy [16]. temperature of of 180microstructures °C; a similar was transformation of microstructures
Shendy [16].
(a)
(d)
(d)
(a) Martensite
Dislocations
Martensite
Dislocations
Ferrit
RA
Ferrit
FB
FB
20 μm
20 μm
(e)
RA 100 nm
100 nm
(e) Dislocations RA
FB
RA
FB
Dislocations
20 μm
100 nm
Figure 2. Cont.
20 μm
100 nm
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(f) (f) FB FB
RA RA 20 μm 20 μm
Dislocations Dislocations
100 nm 100 nm
Figure 2. Optical different Figure 2. Optical microscopy microscopy (OM) (OM) micrographs micrographs of of 17Cr2Ni2MoVNb 17Cr2Ni2MoVNb steel steel tempered tempered at at different Figure 2. Optical microscopy (OM) micrographs of and 17Cr2Ni2MoVNb steel tempered at different ◦ ◦ ◦ temperatures of (a) 180 °C, (b) 400 °C, (c) 620 °C transmission electron microscopy temperatures of (a) 180 C, (b) 400 C, (c) 620 C and transmission electron microscopy (TEM) (TEM) temperatures of(d) (a)180 180◦°C, °C, (b) 400 (c) ◦620 °C and transmission electron microscopy (TEM) ◦ C, °C, micrographs of (f) micrographs of (d) 180 C, (e) (e) 400 400 °C, (f) 620 620 °C. C. micrographs of (d) 180 °C, (e) 400 °C, (f) 620 °C.
Figure 3 shows the X-ray diffraction (XRD) patterns. The results were similar and indicate that Figure 33 shows shows the the X-ray X-ray diffraction diffraction (XRD) (XRD) patterns. patterns. The The results results were were similar and and indicate indicate that that Figure the test conditions were stable and the samples preparation was qualified.similar The carbides formed at the test test conditions conditions were were stable stable and and the the samples preparation preparation was was qualified. qualified. The The carbides carbides formed at at the different tempering temperatures were samples related to the diffusion rate of alloying elements informed iron [17]. different tempering temperatures were related to the diffusion rate of alloying elements in iron [17]. different temperatures to the diffusion rate of such alloying elements in iron [17]. Differenttempering carbides were formed atwere eachrelated heat treatment temperature, as Fe 3C, MoC and Cr7C3. Different carbides carbides were were formed formed at at each each heat heat treatment treatment temperature, temperature, such such as as Fe Fe33C, C, MoC MoC and and Cr Cr77CC33. . Different Fe3C was precipitated at a low tempering temperature, the diffusion of Cr to Fe3C exceeded a certain Fe33CCwas wasprecipitated precipitated ataalow low tempering temperature, temperature, the the diffusion diffusion of of Cr to to Fe Fe33CCexceeded exceeded aa certain certain Fe limit, and Cr7Fe3 was at formed tempering at the tempering temperature of 400 ◦°C.CrThe tempering temperature limit, and Cr Fe was formed at the tempering temperature of 400 C. The tempering temperature limit, and Cr Fe33was at the temperingtotemperature continued to 77rise, thusformed Cr7C3 and C combined form (FeCr)3of C. 400 °C. The tempering temperature continued to to rise, rise, thus thus Cr Cr77CC33and andCCcombined combinedto toform form(FeCr) (FeCr)3C. 3 C. continued
Figure 3. X-ray diffraction (XRD) patterns of 17Cr2Ni2MoVNb steel tempered at different Figure 3.3.X-ray diffraction (XRD) patterns of 17Cr2Ni2MoVNb steel tempered different temperatures. Figure X-ray diffraction (XRD) patterns of 17Cr2Ni2MoVNb steel attempered at different temperatures. temperatures.
3.2. Hardness 3.2. Hardness The results of the hardness experiment are shown in Figure 4. The process of tempering involved The results of the hardness experiment are [18,19]. shown inItFigure The process tempering involved dissolving the supersaturated solid solution can be4. seen that theofhardest parts of the dissolving the supersaturated solid solution [18,19]. It can be seen that the hardest parts of the
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The results of the hardness experiment are shown in Figure 4. The process of tempering involved dissolving the supersaturated [18,19].value It canofbethe seen that the hardest parts of samples were the outer edges,solid andsolution the hardness samples with tempered at the 400samples °C and ◦ ◦ C were were the outer edges, and the hardness value of the samples with tempered at 400 C and 620 620 °C were not significantly different. For all samples, the hardness at a tempering temperature of ◦ C was not different. For all samples, a tempering temperatureand of 180 180 significantly °C was significantly higher than that ofthe thehardness other twoattempering temperatures, the highest significantly higher than that of the tempering(HV temperatures, the highest value hardness value was observed at other that two temperature 389). It isand probable that hardness the increased was observed at that temperature (HV 389). It is probable that the tempering temperature tempering temperature reduced the solid solution strengthening ofincreased carbon and the decomposition of reduced the solid solution strengthening of carbon and the decomposition of alloy elements. alloy elements.
Figure temperatures. Figure 4. 4. Hardness Hardness of of 17Cr2Ni2MoVNb 17Cr2Ni2MoVNb steel steel tempered tempered at at different different temperatures.
3.3. Tensile Properties 3.3. Tensile Properties 3.3.1. Tensile Strength 3.3.1. Tensile Strength It is is well well known known that that the the effective effective methods methods to to improve improve the the strength strength in in steel steel mainly mainly include include fine fine It grain strengthening, strengthening, and precipitation strengthening the most most grain and precipitation strengthening [20]. [20]. The The fine fine grain grain strengthening strengthening is is the effective method method of of improving improving the the strength. strength. The is that that the the smaller effective The reason reason is smaller the the grain grain size size is, is, the the larger larger the number number of of boundaries. boundaries. The grain size size is is investigated investigated by by the the Scherrer Scherrer equation, equation, as as follows: follows: the The grain
Kλ D = Kλ , , D= ββ cos cosθθ
(1) (1)
where is the size of crystal, K is theK dimensionless shape factor,shape λ is the X-ray λ wavelength, β is is mean the mean size of crystal, is the dimensionless factor, is the X-ray where DD the breadth atβhalfisthe and θ is the Bragg angle (inθdegrees). themaximum breadth atpeak halfintensity, the maximum peak intensity, and is the Bragg angle (in wavelength, The relationships between the mean size of crystal and the tempering temperature are shown degrees). in Figure 5. It should be noted that the higher the tempering temperature, the larger the size: The relationships between the mean size of crystal and the tempering temperature aregrain shown in ◦ at tempering temperatures of 180, 400, and 620 C, the mean grain sizes are approximately 24.53, Figure 5. It should be noted that the higher the tempering temperature, the larger the grain size: at 25.55 and 28.10 nm, respectively. Moreover, carbides strengthening, tempering temperatures of 180, 400, and 620 alloying °C, the mean grainimprove sizes areprecipitation approximately 24.53, 25.55 while increased tempering temperature strengthening effect. The samples, tempered at and 28.10 nm, respectively. Moreover, reduces alloyingthe carbides improve precipitation strengthening, ◦ have the most alloying carbides. Therefore, samples at 180 ◦ C have the best precipitation 180 whileCincreased tempering temperature reduces the strengthening effect. The samples, tempered strengthening effect. at 180 °C have the most alloying carbides. Therefore, samples at 180 °C have the best
precipitation strengthening effect.
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Figure 5. The relationships between the mean size of crystal and tempering temperature. Figure 5. The relationships between the mean size of crystal and tempering temperature. Figure 5. The relationships between the mean size of crystal and tempering temperature.
Figure 6 shows the stress-strain curves at room temperature for samples tempered at different Figure 66 shows at tempered at tempering The samplescurves tempered at 620temperature °C exhibitedfor thesamples lowest tensile strength, which Figuretemperatures. shows the the stress-strain stress-strain curves at room room temperature for samples tempered at different different ◦ tempering temperatures. The samples tempered at 620 C exhibited the lowest tensile strength, which was 1019 MPa, due partlyThe to Cr inhibiting the precipitation of Fe3C and the retardation of martensite tempering temperatures. samples tempered at 620 °C exhibited the lowest tensile strength, which was 1019 MPa, due partly to Cr inhibiting the precipitation of Fe C and the retardation of martensite decomposition, andpartly also to in Cr part due to the theprecipitation alloy carbide the saturation of C was 1019 MPa, due inhibiting of precipitates Fe33C and theand retardation of martensite decomposition, and also in part due to the alloy carbide precipitates and the saturation of C decreasing decreasing at a high tempering temperature. Data presented of tensile properties, including 2% yield decomposition, and also in part due to the alloy carbide precipitates and the saturation of C at a high (2% tempering temperature. Data presented ofand tensile properties, including 2% yield 2% strength strength tensile strength (UTS)presented elongation are theincluding average of three decreasing at aYS), highultimate tempering temperature. Data of tensile(EL), properties, yield (2% YS), ultimate tensile strength (UTS) and elongation (EL), are the average of three experimental experimental the tensile properties of (UTS) 17Cr2Ni2MoVNb steel are listed 3. of three strength (2% results; YS), ultimate tensile strength and elongation (EL), are in theTable average results; the tensile properties of 17Cr2Ni2MoVNb steel are listedsteel in Table 3. experimental results; the tensile properties of 17Cr2Ni2MoVNb are listed in Table 3.
Figure 6. Tensile properties of 17Cr2Ni2MoVNb steel tempered at different temperatures. Figure properties of of 17Cr2Ni2MoVNb 17Cr2Ni2MoVNb steel steel tempered tempered at at different different temperatures. temperatures. Figure 6. 6. Tensile Tensile properties Table 3. Mechanical properties of 17Cr2Ni2MoVNb steel.
Sample Label 1 2 3
Table 3. 3. Mechanical Mechanical properties properties 2% of 17Cr2Ni2MoVNb 17Cr2Ni2MoVNb steel. Sample YS UTS Table of steel. Temperature (°C) EL (%) Label (MPa) (MPa) Sample 2% YS UTS Temperature (°C) EL (%) 1 180 1362 1456UTS (MPa) 13.76 ◦ Label Temperature ( C) 2% YS(MPa) (MPa) (MPa) 21 400 1288 1341 12.48 180 1362 1456 13.76 180 1362 1456 32 620 964 1019 14.8 400 1288 1341 12.48 3
400 620
620
1288 964 964
1019
1341 14.8 1019
EL (%) 13.76 12.48 14.8
3.3.2. Fracture Analysis 3.3.2. Fracture Analysis 7a–c shows the SEM micrographs of cracks. There were a large number of tearing ridges 3.3.2.Figure Fracture Analysis and dimples on the fracture surface, indicatingofthat the tensile fracture mode was a of ductile fracture. Figure 7a–c shows the SEM micrographs cracks. There were a large number tearing ridges Figure 7a–c shows the SEM micrographs of cracks. There were aof large number of tearing ridges In addition, the hardness of (FeCr) 3 C and Cr 7 C 3 was higher than that Fe 3 C, which was more likely and dimples on the fracture surface, indicating that the tensile fracture mode was a ductile fracture. and dimples on the fracture surface, indicating that the tensile fracture mode was a ductile fracture. to uneven slip or microcracks theCr surrounding. Due to that the different slip coefficients the In bring addition, the hardness of (FeCr)3Cin and 7C3 was higher than of Fe3C, which was more of likely In addition, thein hardness of (FeCr) Crslip C3 was higher than Feover was more and likelythe 3 C and 7surrounding. 3 C, which crystal different directions, could notDue be extended one to bringgrains uneven slip or microcracks inthe the tothat theofdifferent slip direction coefficients of to bring uneven slip or microcracks in was the surrounding. Due to front the different slip coefficients ofresulting the crystal displacement crystal directions, grains accumulated atnot the of the grain in crystal grainsofinthe different the slip could be extended over boundary, one direction and the grainsstress in different directions, the slip could not be extended over oneofdirection and the high concentration and subsequently leaving aat tortuous area on displacement the specimen displacement of the crystal grains was accumulated the front of thedeformation grain boundary, resulting in high stress concentration and subsequently leaving a tortuous area of deformation on the specimen
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of the crystal grains was accumulated at the front of the grain boundary, resulting in high stress Metals 2018, 8, x FOR PEER REVIEW 7 of 11 concentration and subsequently leaving a tortuous area of deformation on the specimen surface [21,22]. Itsurface should[21,22]. be noted that the size the of crack of size samples wasofmeasured about 3.43, 3.64, 8.333.43, µm It should belargest noted that largest of crack samples was measured about ◦ C, respectively. achieved with the tempering temperatures of 180, 400, and 620 3.64, 8.33 μm achieved with the tempering temperatures of 180, 400, and 620 °C, respectively. Figure thethe energy dispersive X-rayX-ray analysis (EDS) (EDS) patterns of oxides, the chemical Figure7d–f 7d–fshows shows energy dispersive analysis patterns of and oxides, and the ◦ compositions are listed in 4. in TheTable samples, tempering at 620 C, contained chemical compositions areTable listed 4. The samples, tempering at 620 approximately °C, contained 33.45 wt % O. This indicates theindicates higher the of O the elements and more oxides sample approximately 33.45 wt % O.that This thatcontent the higher content of the O elements and athe more has, the more cracks there will be. This is in good agreement with Wang [23], so it can be concluded oxides a sample has, the more cracks there will be. This is in good agreement with Wang [23], so it that mismatch between oxides andbetween the baseoxides metal and of the modulus may be one mechanism can the be concluded that the mismatch theelastic base metal of the elastic modulus may that generated the microcracks. be one mechanism that generated the microcracks.
(d)
(e)
(f)
Figure 7. Typical SEM micrographs of cracks at different temperatures of (a) 180 °C, (b) 400 °C, (c) Figure 7. Typical SEM micrographs of cracks at different temperatures of (a) 180 ◦ C, (b) 400 ◦ C, 620 °C and EDS graphs of oxides at different temperatures of (d) 180 °C, (e) 400 °C, (f) 620 °C. (c) 620 ◦ C and EDS graphs of oxides at different temperatures of (d) 180 ◦ C, (e) 400 ◦ C, (f) 620 ◦ C.
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Table 4. Content of oxides. Table 4. Content of oxides.
Element Element
C C O OMg Mg Al Al Cr Cr Fe Fe
Weight % Weight A B % C A 7.81 4.99B 2.41 C 19.23 32.54 7.81 4.99 33.452.41 19.23 32.54 1.2133.45 0.63 1.15 0.63 1.15 1.21 21.2 24.66 21.2 24.66 25.34 25.34 1.47 1.621.62 1 1.47 50.13 35.19 50.13 35.19 35.9735.97
3.4. Fatigue Fatigue Characteristics Characteristics 3.4. 3.4.1. S-N S-N Curve Curve 3.4.1. The S-N S-Ncurves curvesofofthe the samples with different tempering temperatures are shown in Figure The samples with different tempering temperatures are shown in Figure 8. The8. The arrows indicate the unbroken specimens in the fatigue tests. The most interesting finding of S-N arrows indicate the unbroken specimens in the fatigue tests. The most interesting finding of S-N curves was was that thatthe thefatigue fatiguelife lifeincreased increasedwhen whencyclic cyclicstress stressdeclined, declined,with withthe theS-N S-N curves showing curves curves showing a 7 cycles of the three specimens a general trend of this material. The fatigue strength limits at 10 general trend of this material. The fatigue strength limits at 107 cycles of the three specimens were about were700 about 700 and 600 MPa, obtained the standard limit for thethe samples withtemperatures the tempering 730, and 730, 600 MPa, obtained at the standardatlimit for the samples with tempering of ◦ temperatures 180, 400 and 620 C, respectively. should be noted the dislocation was an 180, 400 and 620of°C, respectively. It should be noted thatItthe dislocation was that an important factor effecting important effecting the fatigue strength limits and high-density dislocations contributed to the fatigue factor strength limits and high-density dislocations contributed to the improvement in fatigue the improvement strength [24]. With the tempering rising, the stacking of strength [24]. With in thefatigue tempering temperature rising, the stackingtemperature of dislocations coalesces, and this dislocations coalesces, and this resulted in a decreasing dislocations density. Therefore, the samples resulted in a decreasing dislocations density. Therefore, the samples tempered at 180 °C were logical to ◦ C were logical to get the highest fatigue strength limits. tempered at 180 get the highest fatigue strength limits.
Figure Figure 8. 8. S-N S-Ncurve curveof of17Cr2Ni2MoVNb 17Cr2Ni2MoVNb steel steel tempered tempered at at different different temperatures. temperatures.
3.4.2. 3.4.2. Fractography Fractography Figure the wear wear zone. zone. The Figure 99 reveals reveals the the SEM SEM micrographs micrographs of of the The local local plastic plastic strain strain accumulated accumulated during the loading loadingtimes, times,and andthe the micro strain gradient increased, producing the redistribution of during the micro strain gradient increased, producing the redistribution of micro micro strain in the samples. The repeated opening and closing of the crack propagation region caused strain in the samples. The repeated opening and closing of the crack propagation region caused friction friction in the test fatigue test process, illustrated in the formation of river patterns on thesurface. fractureAfter surface. in the fatigue process, illustrated in the formation of river patterns on the fracture the After the initiation of cracks, wear occurred between the two parts on the fracture surface, and there initiation of cracks, wear occurred between the two parts on the fracture surface, and there were scars were scarsdebris and in wear in this zone. The the arrows indicate the direction of fatigue cracks and wear this debris zone. The arrows indicate direction of fatigue cracks propagation, which propagation, which are evidence of fatigue rupture characteristics. These debris between contact are evidence of fatigue rupture characteristics. These debris between the contact surfacesthe remained surfaces remained and agglomerated as plowing moving particles, plowing the surfaces to generate abrasive and agglomerated as moving particles, the surfaces to generate abrasive pits. Hardness of pits. Hardness of 17Cr2Ni2MoVNb steel markedly reduced when the tempering temperature 17Cr2Ni2MoVNb steel markedly reduced when the tempering temperature increased; the debris was increased; debris was gradually worn out disappearing due was to friction. Thus, debris there was no gradually the worn out before disappearing duebefore to friction. Thus, there no obvious on the obvious debris on the surface of the samples with tempering temperature of 620 °C. ◦ surface of the samples with tempering temperature of 620 C.
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Figure 9. 9. Micrographs Micrographsofofwear wearzone: zone: tempered at 180 °C maximum 900 and MPa95,257 and 95,257 ◦ C maximum Figure (a)(a) tempered at 180 stressstress 900 MPa cycles Figure 9. Micrographs of wear zone: (a) tempered at 180 °C maximum stress 900 MPa and 95,257 cycles to failure; (b) tempered at 400 °C, maximum stress 750 MPa and 352,388 cycles to failure; (c) ◦ to failure; (b) tempered at 400 C, maximum stress 750 MPa and 352,388 cycles to failure; (c) tempered cycles ◦to failure; (b) maximum tempered stress at 400 750 °C, MPa maximum stresscycles 750 MPa and 352,388 cycles to failure; (c) tempered at 620 °C, and 46,005 to failure. at 620 C, maximum stress 750 MPa and 46,005 cycles to failure. tempered at 620 °C, maximum stress 750 MPa and 46,005 cycles to failure.
Fatigue consists of the cracks initiation zone, crack propagation zone, and the final fatigue zone. Fatigue consists of cracks initiation initiation zone, crack crack propagation propagation zone, zone, and and the the final final fatigue fatigue zone. zone. Fatigue consists of the the to cracks All the cracks are prone initiate at or zone, near the specimen surface because the cyclic plastic All areare prone to initiate at or near specimen surface because thebecause cyclic plastic deformation All the thecracks cracks prone initiate at orthe near the specimen thethat cyclic plastic deformation at or near the to surface acquires the maximum cyclic surface stress compared to of internal at or near the surface acquires the maximum cyclic stress compared to that of internal regions [25]. deformation at or near the surface the maximum cyclic stress compared to that regions [25]. Figure 10 displays theacquires typical SEM micrographs of the ridges produced by of theinternal strong Figure 10 displays the typical SEM micrographs of the ridges produced by the strong deformation regions [25]. at Figure 10 displays the typical of distributed the ridges produced by the strong deformation the samples surfaces. CracksSEM andmicrographs slip lines were on the ridges surfaces: at the samples surfaces. Cracks and slip lines were distributed on the ridges surfaces: the furthest deformation at the samples Cracks and were slip lines were distributed the ridges surfaces: the furthest distances of slip surfaces. lines within samples measured about 3.91, on 2.63, 1.61 μm achieved distances of distances slip lines of within samples were measured about 3.91,about 2.63, 3.91, 1.61 2.63, µm achieved with the the furthest slip lines within were measured 1.61 μmtempering achieved with the tempering temperatures of 180,samples 400, and 620 °C, respectively. Samples at the ◦ C, respectively. Samples at the tempering temperature of tempering temperatures of 180, 400, and 620 with the tempering temperatures of 180, 400, and 620 °C,onrespectively. Samplesitathad thethe tempering temperature of 180 °C had the fewest number of cracks the ridges because highest ◦ C had the fewest number of cracks on the ridges because it had the highest strength among the 180 temperature of 180 °C had the fewest number of cracks on the[26], ridges because it had themodel highest strength among the samples. This was consistent with Neumann who proposed a slip of samples. This was with Neumann [26], with who Neumann proposed a[26], slip who model of fatigue cracked, and strengthcracked, among theconsistent samples. This was consistent proposed a slip fatigue and confirmed that cracks growths were caused by the expansion of the slipmodel lines.of confirmed that cracks growths were by the expansion of the fatigue cracked, and confirmed that caused cracks growths were caused byslip the lines. expansion of the slip lines.
Figure 10. Cont.
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◦ C maximum Figure 10. 10. Micrographs Micrographsofofthe the ridges: tempered at 180 °C maximum 900 and MPa95,257 and 95,257 Figure ridges: (a)(a) tempered at 180 stressstress 900 MPa cycles ◦ cycles to failure; (b) tempered at 400 °C, maximum stress 750 MPa and 352,388 cycles to (c) to failure; (b) tempered at 400 C, maximum stress 750 MPa and 352,388 cycles to failure; (c)failure; tempered ◦ tempered at 620 °C, maximum stressand 75046,005 MPa and 46,005 cycles to failure. at 620 C, maximum stress 750 MPa cycles to failure.
4. Conclusions 4. Conclusions The increased thethe detail of The increased tempering tempering temperature temperaturereduced reducedthe thedislocations dislocationsdensity, density,improved improved detail the microstructures, and the residual austenite gradually transformed into lower bainite or tempered of the microstructures, and the residual austenite gradually transformed into lower bainite or martensite. tempered martensite. The tensile strengths were were 1456, 1456, 1341 1341 and and 1019 1019 MPa MPa with with the temperatures of of 180, 180, 400 400 The tensile strengths the tempering tempering temperatures and 620 °C respectively. The size of cracks increased when the tempering temperature decreased, ◦ and 620 C respectively. The size of cracks increased when the tempering temperature decreased, with the the largest largest crack crack size size of of 8.33 8.33 µm μm appearing appearing at at the thetempering temperingtemperature temperatureof of620 620◦°C. with C. The fatigue strength limits decreased with an increase of the tempering temperature. Fatigue The fatigue strength limits decreased with an increase of the tempering temperature. Fatigue strength limits were measured about 730, 700 and 600 MPa, obtained with tempering temperatures strength limits were measured about 730, 700 and 600 MPa, obtained with tempering temperatures of of 180, and◦ C, 620respectively. °C, respectively. The distance of slip lines when decreased when thetemperature tempering 180, 400,400, and 620 The distance of slip lines decreased the tempering temperature rose; the furthest distance of slip lines at the tempering temperature of 180 °C wasµm. about ◦ rose; the furthest distance of slip lines at the tempering temperature of 180 C was about 3.91 3.91 μm. Author Contributions: Both authors contributed equally to the work. Author Contributions: Both authors contributed equally to the work. Funding: This research received no external funding.
Funding: This research no external Acknowledgments: Thereceived authors would like tofunding thank the National Engineering Research Center of Near-Net-Shape Forming for Metallic Materials, State Key Laboratory of Smart Manufacturing for Special Vehicle as well as Acknowledgments: authors like toofthank theCombustion National Engineering Research Near-NetTransmission System The and State Keywould Laboratory Internal Engine Reliability for Center materialofpreparation Shape Forming for Metallictesting. Materials, State Key Laboratory of Smart Manufacturing for Special Vehicle as well and mechanical properties as Transmission System and State Key Laboratory of Internal Combustion Engine Reliability for material Conflicts of Interest: The authors declare no conflict of interest. preparation and mechanical properties testing. Conflicts of Interest: The authors declare no conflict of interest. References Yang, Y.H.; Wang, M.Q.; Chen, J.C.; Dong, H. Microstructure and mechanical properties of gear steels after 1. References high temperature carburization. J. Iron Steel Res. Int. 2013, 20, 140–145. [CrossRef] 1. Yang, Y.H.; Wang, M.Q.; Chen, J.C.;G.S. Dong, H. Microstructure and mechanical properties of gear after Gurudas, M.; Kumar, T.N.; Kumar, Enhancement of mechanical properties in bainitic steel steels processed 2. high temperature carburization. J. Iron Steel Res. 20, 140–145. from different austenitization temperatures. SteelInt. Res.2013, Int. 2017, 2, 87–92. 2. Gurudas,J.M.; M.; Kumar, T.N.;D.Kumar, G.S. Enhancement of mechanical properties in bainitic steel processed 3. Morave, Mantovani, Biodegradable metals for cardiovascular stent application: Interests and new from different austenitization temperatures. Steel Res. Int. 2017, 2, 87–92. opportunities. Int. J. Mol. Sci. 2011, 12, 4250–4270. [CrossRef] [PubMed] 3. Morave, J.M.; Mantovani, D. Biodegradable metals for application: Interests andalloys new 4. Schinhammer, M.; Hänzi, A.; Löffle, J.; Uggowitzer, P.J.cardiovascular Design strategystent for biodegradable Fe-based opportunities. Int. J. Mol. Acta Sci. 2011, 12, 4250–4270. for medical applications. Biomater. 2010, 6, 1705–1713. [CrossRef] [PubMed] 4. Schinhammer, M.; Hänzi, A.; Löffle,S.;J.; BoccacciniIron, Uggowitzer, P.J. A.R. Design strategy for biodegradable alloys 5. Francis, A.; Yang, Y.; Virtanen, Iron and iron-based alloys Fe-based for temporary for medical applications. Acta Biomater. 2010, 6, 1705–1713. cardiovascular applications. J. Mater. Sci. Mater. Med. 2015, 26, 138–154. [CrossRef] [PubMed] 5. Francis, A.; Yang, S.; Effect BoccacciniIron, A.R.ageing Iron behaviour and iron-based alloys steel. for temporary 6. Pereloma, E.V.; Scott,Y.; R.I.;Virtanen, Smith, R.M. of Cr on strain of low carbon Mater. Sci. cardiovascular applications. J. Mater. Sci. Mater. Med. 2015, 26, 138–154. 2007, 24, 539–543. 6. Pereloma, E.V.; Scott, Cr onY.D. strain ageing behaviour carbon steel. Mater. 7. Yang, Z.; Cong, D.Y.; R.I.; Sun,Smith, X.M.;R.M. Nie, Effect Z.H.; of Wang, Enhanced cyclabilityofoflow elastocaloric effect in Sci. 2007, 24, 539–543. boron-microalloyed NiMn-In magnetic shape memory alloys. Acta Mater. 2017, 127, 33–42. [CrossRef] 7. Yang, Z.; Cong, D.Y.; Sun, X.M.; Nie, Z.H.; Wang, Y.D. Enhanced cyclability of elastocaloric effect in boronmicroalloyed NiMn-In magnetic shape memory alloys. Acta Mater. 2017, 127, 33–42.
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