Study on Inclusions Distribution and Cyclic Fatigue Performance of Gear Steel 18Crnimo7-6 Forging

Article Study on Inclusions Distribution and Cyclic Fatigue Performance of Gear Steel 18CrNiMo7-6 Forging

Min Wang 1,*, Wei Xiao 1, Peng Gan 2, Chao Gu 1 and Yan-Ping Bao 1

1 State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (W.X.); [email protected] (C.G.); [email protected] (Y.-P.B.) 2 CISDI Engineering Co. Ltd., Chongqing 400013, China; [email protected] * Correspondence: [email protected]

Received: 12 December 2019; Accepted: 25 January 2020; Published: 31 January 2020

Abstract: The three-dimensional morphologies of inclusions in gear steel 18CrNiMo7-6 forging were investigated by a non-destructive extraction method, and the cleanliness of radial positions was analyzed, mainly including the variation of total oxygen content and the distribution of size and quantity of inclusions. In addition, fatigue performance was tested using an ultrasonic fatigue machine to investigate the fatigue characteristics of the steel. The results show that the quantity density of inclusions per unit volume in gear steel 18CrNiMo7-6 decreases exponentially with increasing size, oxide inclusions with a size less than 8 µm account for more than 90%, while sulfide inclusions account for more than 85%. The average value of the oxygen content can reflect the level of inclusions that were evenly distributed in the molten steel, and the accumulative total oxygen content increases significantly with increasing inclusion size. The fatigue specimen failed after the stress exceeded the critical value, and fatigue failure hardly occurred when the stress was below the critical value. Meanwhile, large-sized nondeformable inclusions such as Al2O3-CaO in gear steel 18CrNiMo7-6 are closely related to fatigue failure. It is recommended that the area from the center to the 1/2 radius with low cleanliness should be avoided, while the area from the 3/4 radius to the edge with high cleanliness should be selected during the machining of the gear.

Keywords: gear steel; inclusions distribution; cyclic fatigue; three-dimensional morphology

1. Introduction As a necessary transmission part of mechanical equipment, different kinds of high-alloy gears have been widely applied and popularized in industrial production to meet the requirements of aerospace, metallurgical manufacturing, automobile production, mining, lifting transportation and building materials machinery, etc. [1,2]. Among them, the application of 18CrNiMo7-6 according to the standard of DIN (German Institute for Standardization) in Germany has become increasingly widespread with the expansion of high-alloy gear steel production, and the predecessor of this steel is 17CrNiMo6 gear steel [2]. Carburizing gear steel 18CrNiMo7-6 is a surface-hardened steel with high strength, high toughness and high hardenability, which is widely used in mining, transportation, locomotive traction, lifting and internal power, and other industrial fields [3]. JFE steel corporation has developed a new carburizing steel that realizes an intermediate heat treatment-free process (omission of annealing before cold forging and normalizing before carburizing) in the manufacture of carburized gear parts, which makes it possible to reduce the cost and improve the fatigue life of gear parts [4]. The mechanical properties in relation to fracture toughness are often governed by the independent or conjoint influences of the following [5,6]: (a) chemical composition, (b) steel cleanness including inclusions and harmful elements in the matrix, (c) processing history and development of intrinsic microstructural features, (d) service

Metals 2020, 10, 201; doi:10.3390/met10020201 www.mdpi.com/journal/metals Metals 2020, 10, 201 2 of 15 conditions and circumstances of the components including loading-rate, working temperature and the presence of constraints at the crack tip. High surface quality was the essential condition to meet the requirement of harsh working conditions of the gear steel, so it is necessary to control the steel cleanliness and inclusions characterization, including morphologies, size, number and distribution, for guaranteeing the fatigue property of the steel [7–10]. An et al. [11] studied the inclusions evolution of gear steel in the refining process, and discovered the formation mechanism of oxide-sulfide duplex inclusion. Dong et al. [12,13] found that the large inclusions in gear steel are mainly external during the refining process. Nordin et al. [14] summarized that there were three main factors from shot peening that influence the fatigue life of gear steel: residual stresses, microstructure and surface roughness. However, previous studies did not cover the in-depth study of the relationship between inclusions distribution and the fatigue property of final products of gear steel. It is necessary to discover an optimum method to evaluate the fatigue life under various manufacturing conditions because of the long duration of fatigue life of high-quality steel. Recently, the ultra-sonic method has been one of the available methods for the testing of cracks and fatigue life of steel. In this paper, gear steel 18CrNiMo7-6 forging is studied. The three-dimensional morphologies of inclusions in the gear steel were determined by the non-destructive extraction method of inclusions, and the variation of total oxygen content as well as the size, quantity and composition of inclusions in different radial positions were also investigated to clarify the distribution of the cleanliness of the gear steel and the area where the inclusions accumulate. In addition, fatigue tests were also carried out using an ultrasonic fatigue testing machine to investigate the fatigue characteristics of the steel. This provides theoretical guidance for avoiding areas of poor cleanliness in follow-up processing, thereby laying the foundation for further improving the quality of gears [15].

2. Experimental Materials and Methods

2.1. Experimental Materials The smelting technological process of gear steel 18CrNiMo7-6 is as follows: EAF LF VD Ingot → → → cast. The ingot was forged into a piece for the gear process. The following steps are required for the forge piece to be processed into a gear: normalizing rough turning quenching and tempering ﬁnish → → → turning cutting teeth carburizing, quenching and tempering making spline gear grinding. → → → → A forge piece with a diameter of 280 mm was prepared before cutting teeth for the experiment. The chemical composition of the steel is listed in Table1.

Table 1. Chemical composition of gear steel 18CrNiMo7-6 (wt %).

C Si Mn P S Cr Ni Mo Al Mg 0.15–0.20 0.40 0.50–0.90 0.02 0.01–0.02 1.50–1.80 1.40–1.70 0.25–0.35 0.03 0.0003 ≤ ≤

Diﬀerent types of samples as described in Figure1 were prepared for total oxygen testing, inclusions extraction, inclusions quantitative analysis and cyclic fatigue performance evaluation. Fourteen rod-like specimens with sizes of φ10 mm 80 mm were machined along the edge of the × forging ingot for the fatigue performance test. Cubic specimens with sizes of 15 mm 15 mm 15 mm × × were machined in the locations of center, 1/2 radius, 3/4 radius, and outer edge of the forge piece for inclusions quantitative analysis by automatic scanning technology. Rod-like specimens for total oxygen testing were cut from the center, 1/4 radius, 1/2 radius, 3/4 radius and outer edge of the forge piece. Metals 2020, 10, 201 3 of 15 Metals 2020, 10, FOR PEER REVIEW 3 of 15

FigureFigure 1. 1. SamplingSampling preparation preparation of of 18CrNiMo7-6 18CrNiMo7-6 forge forge piece. piece. 2.2. Experimental Methods 2.2. Experimental Methods The total oxygen in radial positions of the gear steel was analyzed using the infrared absorption The total oxygen in radial positions of the gear steel was analyzed using the infrared absorption method. Inclusions were extracted from the steel matrix using a non-destructive method with aqueous method. Inclusions were extracted from the steel matrix using a non-destructive method with solution (deionized water + potassium chloride + citric acid) and collected on the conductive adhesive aqueous solution (deionized water + potassium chloride + citric acid) and collected on the conductive after separating with a filter membrane to observe the three-dimensional morphologies of the inclusions adhesive after separating with a filter membrane to observe the three-dimensional morphologies of under a scanning electron microscope [16]. The Oxford INCA System (EVO18, Zeiss, Oberkochen, the inclusions under a scanning electron microscope [16]. The Oxford INCA System (EVO18, Zeiss, Germany) was adopted to analyze the sizes, shapes and chemical compositions of inclusions on a Oberkochen, Germany) was adopted to analyze the sizes, shapes and chemical compositions of scanning area of 53.5 mm2. Cycle fatigue performance was tested using an ultrasonic fatigue system inclusions on a scanning area of 53.5 mm2. Cycle fatigue performance was tested using an ultrasonic (CSPL-20, Chengdu MCT Technology Co. Ltd., Chengdu, China) with 20 KHz frequency under fully fatigue system (CSPL-20, Chengdu MCT Technology Co. Ltd., Chengdu, China) with 20 KHz reversed tension-compression (R = 1) loading condition. The stability diagram of the Fe-Si-Al-Mg-O frequency under fully reversed tension-compression− (R = −1) loading condition. The stability diagram system inclusions was calculated in liquid steel at 1600 ◦C by using FactSage 7.2 (The Integrated of the Fe-Si-Al-Mg-O system inclusions was calculated in liquid steel at 1600 °C by using FactSage Thermodynamic Databank System, GTT-Technologies, Herzogenrath, Germany). 7.2 (The Integrated Thermodynamic Databank System, GTT-Technologies, Herzogenrath, Germany). 3. Results and Discussion 3. Results and Discussion 3.1. Morphological Characteristics of Inclusions in the Gear Steel 3.1. Morphological Characteristics of Inclusions in the Gear Steel As showed in Figure2, four di fferent types of inclusions, namely, MnS, Al2O3, Al2O3-MgO, and AlAs2 Oshowed3-SiO2, in are Figure found 2, on four the forgingdifferen cross-sectiont types of inclusions, after the treatmentnamely, MnS, of non-destructive Al2O3, Al2O3-MgO, extraction. and Al2O3The-SiO morphology2, are found on of the Al2 forgingO3 is mostly cross-section massive, after with the a diametertreatment of of 5–15 non-destructiveµm, and it is extraction. a common deoxidizationThe morphology product of in Al the2O3 steelmaking is mostly massive, process with with a a diameter large quantity. of 5–15 The μm, Al and2O 3it-MgO is a common spinel is deoxidizationsimilar to the Alproduct2O3 inclusions in the steelmaking with small process clusters, with and botha large of themquantity. are harmfulThe Al2O to3-MgO the fatigue spinel life is similarof steel to due the to Al the2O3 highinclusions hardness with and small low clusters, deformability and both [17 of– 19them]. Additionally, are harmful to some the sphericalfatigue life and of steelspheroidal due to silicates, the high which hard possessedness and relativelylow deformability low hardness [17–19] and. goodAdditionally, formability some compared spherical with and the spheroidalAl2O3 and spinel,silicates, were which also possessed found. relatively low hardness and good formability compared with the AlMnS2O3 and inclusions spinel, in were the steelalso found. matrix are mostly elongated or spindle-shaped after the rolling process due toMnS their inclusions good plasticity. in the steel It has matrix been pointed are mostly out thatelongated spindle or MnS spindle-shaped inclusions are after beneficial the rolling to the processmechanical due propertiesto their good and plasticity free-cutting. It has properties been pointed of steel, out butthat have spindle almost MnS no inclusions effect on are the beneficial quality of tosteel the [ 20mechanical]. properties and free-cutting properties of steel, but have almost no effect on the quality of steel [20].

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Figure 2. Three-dimensional morphologies, (a) Al2O3, (c) Al2O3-MgO, (e) Al2O3-SiO2, and (g) MnS; and Figure 2. Three-dimensional morphologies, (a) Al2O3,(c) Al2O3-MgO, (e) Al2O3-SiO2, and (g) MnS; compositions of inclusions, (b) Al2O3, (d) Al2O3-MgO, (f) Al2O3-SiO2, and (h) MnS. and compositions of inclusions, (b) Al2O3,(d) Al2O3-MgO, (f) Al2O3-SiO2, and (h) MnS.

In FigureFigure3 3,, inclusion inclusion stability stability diagrams diagrams of of the the Fe-Si-Mg-O Fe-Si-Mg-O system system (a) (a) and and the the Fe-Si Fe-Si -O system-O system (b) at(b) 1600 at 1600◦C were°C were calculated. calculated. It can It becan seen be seen that Althat2O Al3-MgO2O3-MgO (spinel) (spinel) and Aland2O Al3 can2O3 becan formed be formed and existand stablyexist stably in molten in molten steel aftersteel theafter deoxidization. the deoxidization. The formationThe formation time oftime sulfide of sulfide inclusions inclusions is later is thanlater thatthan ofthat oxide of inclusions,oxide inclusions, most of most which of arewhich formed are duringformed the during solidification the solidification of molten of steel. molten Therefore, steel. theTherefore, oxide inclusions the oxide caninclusions easily providecan easily nucleation provide nucleation conditions conditions for the formation for the offormation sulfide inclusions. of sulfide Itinclusions. has been pointedIt has been out pointed that the out MnS that inclusion the MnS can inclusion almost precipitatecan almost [ 21precipitate] on any kind[21] on of oxideany kind when of woxide(S) is when more thanw(S) 0.01%.is more The than content 0.01%. of The S in content the forging of S of in gear the steelforging 18CrNiMo7-6 of gear steel is more18CrNiMo7-6 than 0.01%, is resultingmore than in 0.01%, complex resulting inclusions in complex with oxide inclusions inclusions with oxide as the inclusions core and MnSas the forming core and wraps MnS forming around thewraps periphery. around Duethe periphery. to the better Due plasticity to the andbetter softer plas textureticity and of MnSsofter inclusions, texture of the MnS harmful inclusions, effects the of Alharmful2O3, Al effects2O3-MgO, of Al silicate2O3, Al and2O3-MgO, other oxidesilicate inclusions and other on oxide the steel inclusions matrix canon the be reducedsteel matrix to a certaincan be extent.reduced Moreover, to a certain there extent. is a Moreover, small amount there of is high-melting a small amount CaS of inclusions high-melting in gear CaS steel inclusions 18CrNiMo7-6. in gear Thesesteel 18CrNiMo7-6. inclusions are alsoThese easily inclusions deformed are during also easily rolling, deformed and the three-dimensional during rolling, and morphology the three- is basicallydimensional in a morphology regular strip is shape. basically The in formation a regular of strip such shape. inclusions The formation is caused byof excessivesuch inclusions calcium is contentcaused by during excessive the refining calcium process conten [t22 during,23]. the refining process [22,23].

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Figure 3. CalculatedCalculated inclusion stability diagrams of the Fe-Si-Mg-O system (a) and the Fe-Si-O system (b) atat 16001600 ◦°C.C. 3.2. Distribution of Inclusions in Radial Directions 3.2. Distribution of Inclusions in Radial Directions The quantitative distribution of the main inclusions in the center, 1/2 radius, 3/4 radius and The quantitative distribution of the main inclusions in the center, 1/2 radius, 3/4 radius and edge edge of the forging were analyzed using the Oxford INCA System. Four diﬀerent types of oxide of the forging were analyzed using the Oxford INCA System. Four different types of oxide inclusions, inclusions, mainly including Al2O3-CaO, Al2O3, Al2O3-MgO and silicates, existed in the steel matrix. mainly including Al2O3-CaO, Al2O3, Al2O3-MgO and silicates, existed in the steel matrix. Figure 4a Figure4a shows the distribution of Al 2O3-CaO inclusions. The total amount of such inclusions is small, shows the distribution of Al2O3-CaO inclusions. The total amount of such inclusions is small, but a but a large proportion of such inclusions are distributed on the edge of the forge piece, which is very large proportion of such inclusions are distributed on the edge of the forge piece, which is very damaging for the working condition of the gear because this area is very close to the service face of damaging for the working condition of the gear because this area is very close to the service face of the gear. Figure4b shows the size distribution of Al 2O3 inclusions. The Al2O3 inclusions with sizes the gear. Figure 4b shows the size distribution of Al2O3 inclusions. The Al2O3 inclusions with sizes of of 2–4 µm occupy the largest proportion of 50.7%. With increasing size, the proportion of inclusions 2–4 μm occupy the largest proportion of 50.7%. With increasing size, the proportion of inclusions decreases gradually. The largest proportions of inclusions with sizes below 4 µm from the center to decreases gradually. The largest proportions of inclusions with sizes below 4 μm from the center to the edge reach 78.3%, 60.0%, 72.7% and 68.8%, respectively. Large-sized inclusions above 10 µm only the edge reach 78.3%, 60.0%, 72.7% and 68.8%, respectively. Large-sized inclusions above 10 μm only appear at the location of the 1/2 radius and account for 10% of the total. The sizes of Al2O3-MgO appear at the location of the 1/2 radius and account for 10% of the total. The sizes of Al2O3-MgO inclusions are all below 10 µm, and the number of 2–4 µm reaches 59.6%. The largest proportions of inclusions are all below 10 μm, and the number of 2–4 μm reaches 59.6%. The largest proportions of inclusions with sizes below 4 µm from the center to the edge reach 100.0%, 90.9%, 73.3%, and 80.0%, inclusions with sizes below 4 μm from the center to the edge reach 100.0%, 90.9%, 73.3%, and 80.0%, respectively. Figure4d shows the size distribution of silicate inclusions, and large silicate inclusions respectively. Figure 4d shows the size distribution of silicate inclusions, and large silicate inclusions with sizes above 10 µm appear at the location of the 1/2 radius, accounting for 44.4% of the total with sizes above 10 μm appear at the location of the 1/2 radius, accounting for 44.4% of the total silicate inclusions. silicate inclusions.

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Figure 4. SizeSize distribution distribution of of (a (a) )Al Al2O2O3-CaO,3-CaO, (b ()b Al) Al2O23,O (c3),( Alc)2O Al3-MgO,2O3-MgO, (d) Al (d2)O Al3- 2SiOO32-, SiOand2 (,e and) all (oxidee) all oxideinclusions inclusions at different at diﬀ locations.erent locations.

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TheThe relationship relationship between between the the accumulative accumulative oxide oxide density density and theand size the in size per unitin per volume unit volume is shown is inshown Figure in5, Figure indicating 5, indicating that the quantitythat the quanti of inclusionsty of inclusions with a size with less a size than less 8 µthanm accounts 8 μm accounts for more for thanmore 90%. than With 90%. increasing With increasing inclusion inclusion size, the size, quantity the quantity density tendsdensity to decrease.tends to decrease. The quantity The quantity density ofdensity Al2O3 ofinclusions Al2O3 inclusions along the along radius the shows radius little shows change. little change. The quantity The quantity densities densities of Al2O of3 -MgOAl2O3-MgO and 2 2 silicateand silicate inclusions inclusions reach reach the highest the highest at 0.82 at/mm 0.82/mmand 0.362 and/mm 0.36/mm, respectively.2, respectively.

Figure 5. Figure 5.Accumulative Accumulative quantity quantity density density of of di differentfferent sizes sizes of of oxide oxide inclusions. inclusions. Figure6 shows the size distribution of sulfide inclusions. The largest proportion of inclusions Figure 6 shows the size distribution of sulfide inclusions. The largest proportion of inclusions with sizes below 2 µm reaches 50.5%. From the center to the edge, the number of sulfide inclusions with sizes below 2 μm reaches 50.5%. From the center to the edge, the number of sulfide inclusions decreases gradually, and the average sizes increase slightly, which is due to the segregation of sulfur decreases gradually, and the average sizes increase slightly, which is due to the segregation of sulfur during the solidification. during the solidification.

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Figure 6. Size distribution of (a) Al O -MnS, (b) Al O -MgO-MnS, (c) Al O -CaO-MnS, (d) MnS, Figure 6. Size distribution of (a) Al2O2 3-MnS,3 (b) Al2O23-MgO-MnS,3 (c) Al2O32-CaO-MnS,3 (d) MnS, and and(e) all (e )sulfide all sulfide inclusions inclusions at different at different locations. locations. Figure7 shows the distribution of sulfide inclusions per unit volume. It is found that the quantity Figure 7 shows the distribution of sulfide inclusions per unit volume. It is found that the quantity of inclusions decreases exponentially with increasing size. As can be seen from Figure6, the quantity of inclusions decreases exponentially with increasing size. As can be seen from Figure 6, the quantity of sulfide inclusions with a size less than 8 µm accounts for more than 85%. of sulfide inclusions with a size less than 8 μm accounts for more than 85%.

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FigureFigure 7. 7.Accumulative Accumulative densitydensity ofof didifferentﬀerent sizes sizes of of sulﬁde sulfide inclusions. inclusions.

The machinability of gear steel is caused by a large amount of sulfide inclusions in the steel. The machinability of gear steel is caused by a large amount of sulfide inclusions in the steel. Compared with oxide inclusions, the size distribution of sulfide inclusions is similar to the former, Compared with oxide inclusions, the size distribution of sulfide inclusions is similar to the former, and their range is wider. As for the distribution of inclusions, the amount of sulfide inclusions in each and their range is wider. As for the distribution of inclusions, the amount of sulfide inclusions in each position is much larger than that of oxide inclusions. Owing to the fact that the sulfide inclusions position is much larger than that of oxide inclusions. Owing to the fact that the sulfide inclusions contained some composite inclusions of oxides and sulfides, it is well explained that the cumulative contained some composite inclusions of oxides and sulfides, it is well explained that the cumulative total oxygen content of the oxide inclusions is significantly lower than the total oxygen content total oxygen content of the oxide inclusions is significantly lower than the total oxygen content measured by the infrared absorption method. measured by the infrared absorption method. Based on the above analysis, inclusions with sizes above 10 µm mainly exist in a band area from Based on the above analysis, inclusions with sizes above 10 μm mainly exist in a band area from the center to the 1/2 radius. Actually, the band with large sizes of inclusions is far from the working the center to the 1/2 radius. Actually, the band with large sizes of inclusions is far from the working face of the gear, and that is also good for the fatigue life of the gear. So, it is critical to control both the face of the gear, and that is also good for the fatigue life of the gear. So, it is critical to control both the sizes and distribution of inclusions on the section of the forging [24]. sizes and distribution of inclusions on the section of the forging [24]. In summary, considering the distribution of total oxygen content, inclusion quantity distribution In summary, considering the distribution of total oxygen content, inclusion quantity distribution andand size size distribution distribution of of gear gear steel steel 18CrNiMo7-6 18CrNiMo7-6 forging, forging, the the area area from from the the 3 3/4/4 radius radius to to the the edge edge with with highhigh cleanliness cleanliness is is more more reasonable reasonable to to be be selected selected for for the the machining machining of of the the gear gear face. face. 3.3. Relationship between Total Oxygen and Inclusions 3.3. Relationship between Total Oxygen and Inclusions The accumulative oxygen content in oxide inclusions with different sizes can be calculated by The accumulative oxygen content in oxide inclusions with different sizes can be calculated by Equations (1)–(4) [25–27] to estimate the contributions of oxides inclusions to the total oxygen in the Equations (1)–(4) [25–27] to estimate the contributions of oxides inclusions to the total oxygen in the steel. In these equations, DeHoff’s formulas are expressed by Equations (1) and (2), the volume fraction steel. In these equations, DeHoff’s formulas are expressed by Equations (1) and (2), the volume of oxides can be obtained by Equation (3), and the oxygen content in oxide inclusions is described by fraction of oxides can be obtained by Equation (3), and the oxygen content in oxide inclusions is Equation (4). described by Equation (4). 2 N N = a , (1) v 2 N N π= · d· a, (1) V π d 1 1 X 1 = , (2) 1n 1 d1 d = ∑ i , (2) d n d π 3 i V = d Nv, (3) 6 · π 3 V = !d ·N , (3) ρox6 V [O]ox= V (O)ox, (4) ρFe · · ρox (4) [O]ox= ·V·(O)ox, ρFe

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ρox (4) Metals 2020, 10, FOR PEER REVIEW [O]ox= ( ) ∙V∙(O)ox, 10 of 15 Metals 2020, 10, 201 ρFe 10 of 15

V −3 a wherewhere N NV is is the the number number of of oxide oxide inclusions inclusions per per unit unit volume volume in in the the specimen, specimen, in in m m-3; ;N Na is is the the number number −2 density of oxide inclusions per unit area in the specimen, in m-2 ; di is the apparent 3particle size of the densitywhere N ofV oxideis the inclusions number of per oxide unit inclusions area in the per specimen, unit volume in m in the; di specimen,is the apparent in m− particle; Na is thesize number of the ̅ 2 i-idensity-thth oxide oxide of inclusions inclusions oxide inclusions among among pern n oxides, oxides, unit area in in m; m; in 푑 the̅ isis specimen, the the harmonic harmonic in m mean− mean; di isof of theoxide oxide apparent inclusions inclusions’ particle’ particle particle size ofsize, size, the ox inini-th m; m; oxide V V is is the inclusionsthe volume volume among fraction fraction n oxides,of of oxide oxide in inclusions; m;inclusions;d is the harmonic[ O[O]]ox is is the the mean oxygen oxygen of oxide content content inclusions’ in in oxide oxide particle inclusions inclusions size, in in in ox −3 Fe −3 ox thethem; steel; Vsteel;is the ρ ρox volumeis is the the density density fraction of of ofoxide oxide oxide inclusions, inclusions, inclusions; inin [ kg∙mOkg·m]ox -is3; ; theρ ρFe is oxygenis the the density density content of of inthe the oxide steel, steel, inclusions inin kg∙mkg·m-3; ; in( (OO the))ox 3 3 isissteel; the the oxygen ρoxygenox is the content content density of of ofoxide oxide oxide inclusions. inclusions. inclusions, in kg m− ; ρ is the density of the steel, in kg m− ;(O)ox is · Fe · the oxygenFigureFigure 8 content8 shows shows ofthe the oxide relations relationship inclusions.hip between between oxide oxide sizes sizes and and the the accumulative accumulative oxygen oxygen in in oxides oxides per per unitunit volume. volume.Figure8 It showsIt was was reported thereported relationship by by [21] [21] betweenthat that the the proportion oxideproportion sizes of andof bound bound the accumulative oxygen oxygen in in oxides oxides oxygen with with in oxidesdiameters diameters per smallersmallerunit volume. than than 5 5 Itμm μ wasm exceeded exceeded reported 80% 80% by of [of21 the the] that total total the oxygen oxygen proportion in in Al Al-k- ofkilledilled bound steel. steel. oxygen Under Under in the the oxides present present with res results, diametersults, the the oxygenoxygensmaller in in than oxide oxide 5 µ inclusions minclusions exceeded with with 80% sizes sizes of the below below total 8 oxygen8 μm μm occup occupies in Al-killedies less less than steel.than 13% 13% Under of of the the total total present oxygen oxygen results, in in the the steel.steel.oxygen in oxide inclusions with sizes below 8 µm occupies less than 13% of the total oxygen in the steel. The accumulative total oxygen content increases with increasing inclusion size when the size is less than 8 μm. The quantity of inclusions with a size less than 8 μm accounts for more than 90%, making a vital contribution to total oxygen. Except for the sharp increase in the oxygen content due to the large-sized inclusions appearing at the 1/2 radius, the oxygen content increases steadily as the size of inclusions increases at other positions, and the cumulative oxygen content is stabilized at a fixed value.

Figure 8. RelationshipRelationship between between oxide oxide sizes and accumu accumulativelative oxygen in oxides per unit volume.

The accumulative total oxygen content increases with increasing inclusion size when the size is The accumulative total oxygen content increases with increasing inclusion size when the size is less than 8 µm. The quantity of inclusions with a size less than 8 µm accounts for more than 90%, less than 8 μm. The quantity of inclusions with a size less than 8 μm accounts for more than 90%, making a vital contribution to total oxygen. Except for the sharp increase in the oxygen content due making a vital contribution to total oxygen. Except for the sharp increase in the oxygen content due to the large-sized inclusions appearing at the 1/2 radius, the oxygen content increases steadily as the to the large-sized inclusions appearing at the 1/2 radius, the oxygen content increases steadily as the size of inclusions increases at other positions, and the cumulative oxygen content is stabilized at a size of inclusions increases at other positions, and the cumulative oxygen content is stabilized at a ﬁxed value.Figure 8. Relationship between oxide sizes and accumulative oxygen in oxides per unit volume. fixed value. In Al-deoxidization liquid steel, the dissolved oxygen content depends on the Al content. In Al-deoxidization liquid steel, the dissolved oxygen content depends on the Al content. The TheIn reaction Al-deoxidization between [Al] liquid and [O]steel, can the be dissolved expressed oxygen by content depends on the Al content. The reactionreaction between between [Al] [Al] and and [O] [O] can can be be expressed expressed by by

2 3 2[Al]2[Al] + + 3[O] 3[O]3[O] = = Al Al22OO3(s)3(s)(s) (5)(5) (5)

Ѳ ∆GѲ = −1218799 + 394.13T (6) ∆G = –12187991218799 ++ 394.13394.13TT (6)(6) − Al O Considering Alf = Of = 1, and when T = 1873 K, there are ConsiderConsideringing ff Al = =f f=O 1,= and1, and when when T =T 1873= 1873 K, there K, there are are 1.5 −7 [%Al][ %O]1.5 = 2.00 × 10−7 (7) %Al %O1.5 = 2.00 × 10 7 (7) [%Al][%O] = 2.00 10− (7) InIn this this steel, steel, the the Al Al content content is is 0.03%, 0.03%, as as showed showed in in Table Table× 1. 1. Thus, Thus, the the dissolved dissolved oxygen oxygen content content is is 3.53.5 ppm ppm,In, this as as calculated steel,calculated the Al by by content Eq Equationuation is 0.03%,( (7).7). as showed in Table1. Thus, the dissolved oxygen content is 3.5 ppm,The total as calculated oxygen content by Equation of gear (7). steel 18CrNiMo7-6 forging is 11 to 12 ppm, with an average of 11.4 ppm.

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MetalsThe 2020 total, 10, FOR oxygen PEER contentREVIEW of gear steel 18CrNiMo7-6 forging is 11 to 12 ppm, with an average11 of of 15 11.4 ppm. FigureFigure9 describes9 describes the the relationship relationship among among dissolved dissolved oxygen, oxygen, bound bound oxygen oxygen in oxides, in andoxides, bound and oxygenbound inoxygen composite in composite inclusions inclusions at different at different locations. loca It cantions. be It seen can thatbe seen a large that proportion a large proportion of oxygen of existsoxygen in theexists composite in the composite inclusions, inclusions, which means which that means a large that proportion a large proportion of oxides is wrappedof oxides byis wrapped sulfides. Higherby sulfides. bound Higher oxygen bound in oxides oxygen than in thatoxides in compositethan that in inclusions composite at inclusions the location at the of thelocation 1/2 radius of the indicated1/2 radius that indicated large oxides that large are di oxidesfficult are to bedifficult wrapped to be by wrapped sulfide. by sulfide.

FigureFigure 9. 9. RelationshipRelationship amongamong dissolveddissolved oxygen,oxygen, boundbound oxygenoxygen inin oxides,oxides, andand boundbound oxygenoxygen inin compositecomposite inclusions inclusions at at di differentﬀerent locations. locations.

SinceSince the the quantity quantity andand thethe size size of of single single oxide oxide inclusions inclusionsin ingear gearsteel steel18CrNiMo7-6 18CrNiMo7-6 areare small,small, thethe accumulative accumulativetotal total oxygen oxygen content content has ahas certain a cert diﬀainerence difference from the from total oxygenthe total content oxygen measured content bymeasured the infrared by the absorption infrared method,absorption but method, the change but inthe the change oxygen in contentthe oxygen at the content four positions at the four is relativelypositions stable.is relatively Therefore, stable. the Therefore, average value the aver of theage oxygen value of content the oxygen can reﬂect content the can level reflect of inclusions the level thatof inclusions are evenly that distributed are evenly in thedistributed molten steel.in the molten steel.

3.4.3.4. CyclicCyclic FatigueFatigue PerformancePerformance TheThe S-N S-N curve curve of of fatigue fatigue tests tests for for the the material material is is shown shown in in Figure Figure 10 10.. The The S-N S-N curve curve of of fatigue fatigue 9 cyclescycles at at the the directions directions of of the the transverse transverse and and longitudinal longitudinal specimens specimens reaches reaches 10 109when when the the loading loading stressstress is is below below 530 530 and and 410 410 MPa, MPa, respectively, respectively, and and the the two two types types of of fatigue fatigue specimens specimens will will fail fail when when thethe loading loading stress stress is is higher higher than than the the above above values, values, respectively. respectively.

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FigureFigure 10. 10.S-N S-N curve curve of of the the internal internal crack crack initiation. initiation.

CombinedCombined with with the the fatigue fatigue test test results, results, when when the the loading loading stress stress of of the the specimens specimens is is higher higher than than thethe critical critical value, value, the the specimens specimens tend tend to to fail, fail, while while if if it it is is lower lower than than the the critical critical value, value, the the fatigue fatigue cycle cycle 9 cancan reach reach more more than than 10 10,9, which which means means the the specimens specimens possess possess good good anti-fatigue anti-fatigue characteristics. characteristics. FigureFigure 11 11 showsshows the fracture fracture morphologies morphologies of of some some specimens. specimens. As Ascan can be seen be seen in the in figure, the ﬁgure, some somespecimens specimens were weretested tested under under a load a higher load higher than the than critical the critical value; value; as a result, as a result, fatigue fatigue failure failure occurs occursrapidly rapidly during during the experiment, the experiment, and andthe thefatigue fatigue source source has has not not been been fully fully expanded expanded due toto thethe interruptioninterruption of of the the fatigue fatigue test. test. AmongAmong them, them, AlAl22OO33-CaO composite inclusions inclusions are are found found in in some some fatigue fatigue sources sources as as showed showed in inFigure Figure 11d. 11d. It Ithas has been been pointed pointed out out [28] [28 ]that that fatigue fatigue fracture fracture for for Al Al2O2O3-CaO3-CaO inclusions inclusions occurred occurred at atlower lower stress stress amplitudes amplitudes than than that that for forTiN TiN or CaO-type or CaO-type inclusions, inclusions, and no and failure no failure occurred occurred for Al for2O3- 7 AlCaO2O3 -CaOinclusions inclusions when when fatigue fatigue life lifewas was far farbeyond beyond 10 107 cycles,cycles, while while TiN TiN and and CaO-type inclusions,inclusions, 9 fracturesfractures were were still still caused caused at at nearly nearly 10 10.9. ThisThis indicates indicates that that the the gear gear steel steel 18CrNiMo7-6 18CrNiMo7-6 contained contained inclusionsinclusions such such as as Al Al2O2O33-CaO,-CaO, whichwhich isis proneprone toto fatiguefatigue failurefailure at at a a critical critical value value due due to to the the e effectﬀect of of AlAl2O2O33-CaO-CaO compositecomposite inclusionsinclusions on on the the matrix. matrix.

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Figure 11. a c Figure 11. FractureFracture surfacessurfaces ofof thethe failedfailed specimens,specimens, ((a––c);); and and Al Al22OO33-CaO-CaO inclusionsinclusions inin thethe fatiguefatigue sourcesource ((dd).).

In summary, different from the general fatigue test results, there is a critical value in the relationship In summary, different from the general fatigue test results, there is a critical value in the between the fatigue life and the loading stress of 18CrNiMo7-6 gear steel. The specimen will fail after relationship between the fatigue life and the loading stress of 18CrNiMo7-6 gear steel. The specimen the stress exceeds the critical value, and fatigue failure hardly occurs when the stress is below the will fail after the stress exceeds the critical value, and fatigue failure hardly occurs when the stress is critical value. Meanwhile, the gear steel 18CrNiMo7-6 contained inclusions such as Al2O3-CaO, which below the critical value. Meanwhile, the gear steel 18CrNiMo7-6 contained inclusions such as Al2O3- is prone to fatigue failure after reaching the critical value. CaO, which is prone to fatigue failure after reaching the critical value. 4. Conclusions 4. Conclusions (1) The main inclusions in the steel contain oxides and sulfides, where oxides contain Al2O3-CaO, (1) The main inclusions in the steel contain oxides and sulfides, where oxides contain Al2O3-CaO, Al2O3, Al2O3-MgO and silicates, and sulfides contain MnS and composite inclusions of oxides Al2O3, Al2O3-MgO and silicates, and sulfides contain MnS and composite inclusions of oxides and sulfides. and sulfides. (2) The quantity density of inclusions per unit volume in gear steel 18CrNiMo7-6 decreases (2) The quantity density of inclusions per unit volume in gear steel 18CrNiMo7-6 decreases exponentially with increasing size, and the inclusions are mainly less than 8 µm. Oxide exponentially with increasing size, and the inclusions are mainly less than 8 μm. Oxide inclusions with a size less than 8 µm account for more than 90%, while sulfide inclusions with a inclusions with a size less than 8 μm account for more than 90%, while sulfide inclusions with a size less than 8 µm account for more than 85%. size less than 8 μm account for more than 85%. (3) As the size of inclusions increases, the accumulative total oxygen content increases significantly. (3) As the size of inclusions increases, the accumulative total oxygen content increases significantly. The accumulative total oxygen content has a certain difference from the total oxygen content The accumulative total oxygen content has a certain difference from the total oxygen content measured by the infrared absorption method, but the change in the oxygen content at the four measured by the infrared absorption method, but the change in the oxygen content at the four positions is relatively stable, and there are a great deal of sulfide inclusions, including some positions is relatively stable, and there are a great deal of sulfide inclusions, including some composite inclusions of oxides and sulfides. As a result, the average value of the oxygen content can reflect the level of inclusions that were evenly distributed in the molten steel.

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composite inclusions of oxides and sulfides. As a result, the average value of the oxygen content can reflect the level of inclusions that were evenly distributed in the molten steel. (4) Different from the general fatigue test results, there is a critical value in the relationship between the fatigue life and the loading stress of 18CrNiMo7-6 gear steel. The specimen will fail after the stress exceeds the critical value. However, fatigue failure hardly occurs when the stress is below the critical value. Meanwhile, the gear steel 18CrNiMo7-6 contained inclusions such as Al2O3-CaO, which is prone to fatigue failure.

Author Contributions: Data curation, M.W., P.G. and C.G.; writing—original draft preparation, M.W. and W.X.; writing—review and editing, W.X., M.W. and Y.-P.B.; supervision, Y.-P.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (No. 51774031). Acknowledgments: The authors wish to express their gratitude to the foundation for providing financial support. Conflicts of Interest: The authors declare no conflict of interest.

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