A Novel Decarburizing-Nitriding Treatment of Carburized/Through-Hardened Bearing Steel Towards Enhanced Nitriding Kinetics and Microstructure Reﬁnement

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Article A Novel Decarburizing-Nitriding Treatment of Carburized/through-Hardened Bearing Steel towards Enhanced Nitriding Kinetics and Microstructure Reﬁnement

Fuyao Yan 1, Jiawei Yao 1, Baofeng Chen 1, Ying Yang 1, Yueming Xu 2, Mufu Yan 1,* and Yanxiang Zhang 1,*

1 School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China; [email protected] (F.Y.); [email protected] (J.Y.); [email protected] (B.C.); [email protected] (Y.Y.) 2 Beijing Research Institute of Mechanical & Electrical Technology, Beijing 100083, China; [email protected] * Correspondence: [email protected] (M.Y.); [email protected] (Y.Z.); Tel.: +86-451-86418617 (M.Y.)

Abstract: Decarburization is generally avoided as it is reckoned to be a process detrimental to material surface properties. Based on the idea of duplex surface engineering, i.e., nitriding the case-hardened or through-hardened bearing steels for enhanced surface performance, this work deliberately applied decarburization prior to plasma nitriding to cancel the softening effect of decarburizing with nitriding and at the same time to significantly promote the nitriding kinetics. To manifest the applicability of this innovative duplex process, low-carbon M50NiL and high-carbon M50 bearing steels were adopted in this work. The influence of decarburization on microstructures and growth kinetics of the nitrided layer over the decarburized layer is investigated. The metallographic analysis of the nitrided layer thickness indicates that high carbon content can hinder the growth of the nitrided Citation: Yan, F.; Yao, J.; Chen, B.; layer, but if a short decarburization is applied prior to nitriding, the thickness of the nitrided layer Yang, Y.; Xu, Y.; Yan, M.; Zhang, Y. A can be significantly promoted. The analysis of nitriding kinetics shows that decarburization reduces Novel Decarburizing-Nitriding the activation energy for nitrogen diffusion and enhances nitrogen diffusivity. Moreover, the effect of Treatment of Carburized/through- decarburization in air can promote surface microstructure refinement via spinodal decomposition Hardened Bearing Steel towards during plasma nitriding. Enhanced Nitriding Kinetics and Microstructure Refinement. Coatings Keywords: duplex surface engineering; decarburization; carburization; plasma nitriding; nitrid- 2021, 11, 112. https://doi.org/ 10.3390/coatings11020112 ing kinetics

Academic Editor: Emerson Coy Received: 1 December 2020 Accepted: 13 January 2021 1. Introduction Published: 20 January 2021 M50 steel and its variant M50NiL steels are used as aero-engines bearings for high- temperature applications [1]. To maintain structural integrity in a more severe service Publisher’s Note: MDPI stays neutral environment, excellent combined surface properties, such as hardness, wear resistance with regard to jurisdictional claims in and rolling contact fatigue resistance, are required for bearings. Traditional carburized published maps and institutional afﬁl- case or through-hardened steel with secondary hardening effect is typically less than iations. 65 HRC (~860 HV), with limited resistance in wear or other surface damages. On the other hand, nitrided layers can exhibit hardness greater than 1000 HV, but the typical case thickness is only 1/10 of the carburized case, and with a sharp decreasing hardness proﬁle, which is not conductive to load-bearing capacities and fatigue resistance. Combination Copyright: © 2021 by the authors. of carburizing/through-hardening and nitriding therefore becomes the typical duplex Licensee MDPI, Basel, Switzerland. hardening techniques. This article is an open access article For M50 steel with ~0.8 wt.% carbon, duplex hardening achieved by applying a distributed under the terms and thin nitride layer over the secondary-hardened matrix [2], has been applied to further conditions of the Creative Commons improve the surface properties. Ooi et al. reported that duplex hardening treatment Attribution (CC BY) license (https:// can increase surface compression stress and therefore improve rolling contact fatigue creativecommons.org/licenses/by/ resistance of the M50 components [3]. Rhoads et al. improved the surface hardness of 4.0/).

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M50 to HV 1000~1250 and prolonged the rolling contact fatigue life by 8~10 times with duplex hardening [4]. Streit et al. reported that duplex-hardened M50 bearings had lower spall propagation rates over conventional counterparts in low or boundary lubrication conditions and contaminated lubrication conditions due to the high surface hardness [5]. For steels with reduced carbon content, duplex hardening can be achieved for surface by nitriding the pre-carburized case [6]. Carburizing is to obtain a thick hardening case to resist the Hertz stresses during loading, while nitriding can further increase the outmost surface hardness and generate more compressive residual stresses. For example, Davies et al. showed that duplex hardening can improve the axial and rolling contact fatigue life of helicopter gears made of Vasco X-2M steel [7]. However, duplex hardening processes are generally time-consuming due to the carbon/nitrogen trapping model [8–11]. Bloyce et al. applied duplex hardening to M50NiL steel and produced a nitrided case thickness of 100 µm with surface hardness over 1000 HV after nitriding at 400 ◦C for 60 h [12]. Egert et al. observed decarburization of steel surface during plasma nitriding and found that the removal of surface carbon can facilitate the diffusion of nitrogen into the bulk [13]. In general, decarburization during heat treatment is well-limited, since it can greatly decrease hardness and wear resistance of the surface [14]. However, this work is intended to deliberately apply decarburization prior to plasma nitriding, with the aim of cancelling the softening effect of decarburization with nitrided case, and at the same time aiming to promote the nitriding kinetics. In this work, low-carbon M50NiL steel and high-carbon M50 steel that represent two grades of steels were adopted as examples. The inﬂuence of decarburization on microstructures and growth kinetics of the nitrided layer is investigated in detail.

2. Materials and Methods 2.1. Materials The materials used in this work were high-carbon M50 steel with the nominal composition Fe-0.85C-4Cr-4Mo-1V-0.15Mn-0.1Si (in wt.%) and low-carbon M50NiL steel with the nominal composition Fe-0.13C-4.1Cr-3.4Ni-4.2Mo-1.2V-0.13Mn-0.18Si (in wt.%). The steel bars were cut into 13 mm × 13 mm × 5 mm pieces for thermochemical treatment and microstructural characterizations. All specimens were mechanically ground by 240- and 800-grit sandpapers and ultrasonically cleaned in ethanol before thermochemical treatment.

2.2. Thermochemical Surface Treatment The thermochemical surface treating process (taken as “duplex process” in the fol- lowing for short) for M50NiL and M50 are schematically shown in Figure1, in which a decarburizing step was added prior to plasma nitriding. Figure1a shows the “duplex process” for M50NiL, which followed the sequence of gas carburizing, decarburizing and plasma nitriding. In Figure1b, the “duplex process” for M50 was carried out ﬁrst with solution heat treatment in air as a manner of decarburization, followed by tempering and plasma nitriding. The details of the duplex processes are described as follows. Coatings 2021, 11, x FOR PEER REVIEW 3 of 10

pered at 520 °C for 4 h. The duplex process is schematically shown in Figure 1b. For comparison, a “Blank” sample was solution treated in vacuum, followed by tempering at the same condition as the decarburized sample. It should be noted that the oxidized layers of air-decarburized samples were grinded off prior to plasma nitriding. (3) Plasma nitriding Plasma nitriding was performed in a 30-kW pulse plasma multi-element furnace (LDMC-30AFZ, 30 kW). The atmospheric pressure was 200 Pa, and the voltage was kept at 650 V. The heating rate was 4 °C/min. M50NiL steel samples were nitrided at 520 °C, 540 °C and 580 °C for 4, 8 and 16 h in a gas mixture of N2 and H2 with a flow ratio 1:1. M50 Coatings 2021, 11, 112 steel samples were nitrided at 500 °C, 520 °C and 540 °C for 4, 8 and 12 h in a gas mixture3 of 10 of N2 and H2 with a flow ratio 1:3. After nitriding, the specimens were furnace cooled to room temperature in N2 flow with a cooling rate of ~2 °C/min.

FigureFigure 1.1. SchematicSchematic diagramdiagram ofof duplexduplex processprocess forfor ((aa)) M50NiLM50NiL steel, steel, ( b(b)) M50 M50 steel. steel.

(1) GasFor clarification, carburizing and detailed decarburizing processing for conditions M50NiL For and M50NiL corresponding steel, a “boost-diffusion” sample denota- tionsgas for carburizingM50NiL and and M50 a samples short decarburizing are listed in Tables step were 1 and performed 2. at continuous gas carburizing furnace (FAW Group) in Changchun, China, with the process schematically shownTable in 1. Figure Duplex1a. process For comparison, for M50NiL. a group of as-carburized samples (denoted as ‘C’) were kept at the same temperature and time with decarburizing while the Sample Carburizing atmosphere maintained withDecarburizing the carbon potential of Cp = 0.85%. Plasma Nitriding Denotation Boost (2) DecarburizingDiffusion and tempering for M50 To decarburize M50, specimens were solution C 850 °C, 30 min, Cp =◦ 0.85% treated in a box furnace in air at 1100 C for 20 min. Then the specimens- were CD 930 °C, encapsulated910 °C, in a vacuum 850 quartz °C, 30 tube min, and air tempered at 520 ◦C for 4 h. The duplex C-N 430 min, process280 min, is schematically850 shown°C, 30 min, in Figure Cp = 0.85%1b. For comparison, 520 °C a, “Blank”540 °C, 580 sample °C Cp = 1.15% wasCp = solution 0.85% treated in vacuum, followed by tempering at the same4 h, condition8 h, 12 h as the CD-N 850 °C, 30 min, air decarburized sample. It should be noted that the oxidized layers ofN2 air-decarburized:H2 = 1:1 samples were grinded off prior to plasma nitriding. (3) Plasma nitridingTable 2. PlasmaDuplex process nitriding for wasM50. performed in a 30-kW pulse plasma multi- element furnace (LDMC-30AFZ, 30 kW). The atmospheric pressure was 200 Pa, and Sample Denotation Solutioning/Decarburizingthe voltage was kept at 650 V. The Tempering heating rate was 4 ◦C/min. Plasma M50NiL Nitriding steel samples ◦ ◦ ◦ Blank 1100 were°C, 20 nitrided min, vacuum at 520 C, 540 C and 580 C for 4, 8 and 16 h in a gas mixture of N2 - ◦ ◦ D 1100and °C, H 220with min, a air flow ratio 1:1. M50520 steel °C, samples were nitrided at 500 C, 520 C and ◦ N 1100 540°C, 20C min, for 4,vacuum 8 and 12 h in a gas mixture4 h, of N2 and H2 500with °C, a 520 flow °C, ratio 540 1:3.°C After nitriding, the specimens were furnacevacuum cooled to room temperature4 h, 8 h, in 12 N 2h flow with a D-N 1100cooling °C, 20 rate min, of air ~2 ◦C/min. N2:H2 = 1:3 For clarification, detailed processing conditions and corresponding sample denota- tions for M50NiL and M50 samples are listed in Tables1 and2.

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Table 1. Duplex process for M50NiL.

Carburizing Sample Decarburizing Plasma Nitriding Denotation Boost Diffusion 850 ◦C, 30 min, C Cp = 0.85% - 850 ◦C, 30 min, CD ◦ ◦ 930 C, 910 C, air 430 min, 280 min, 850 ◦C, 30 min, C-N Cp = 1.15% Cp = 0.85% ◦ ◦ ◦ Cp = 0.85% 520 C, 540 C, 580 C 4 h, 8 h, 12 h 850 ◦C, 30 min, CD-N N :H = 1:1 air 2 2

Table 2. Duplex process for M50.

Sample Denotation Solutioning/Decarburizing Tempering Plasma Nitriding Blank 1100 ◦C, 20 min, vacuum ◦ - D 1100 ◦C, 20 min, air 520 C, 4 h, vacuum ◦ ◦ ◦ N 1100 ◦C, 20 min, vacuum 500 C, 520 C, 540 C 4 h, 8 h, 12 h D-N 1100 ◦C, 20 min, air N2:H2 = 1:3

2.3. Microstructure Characterizations The cross-sections of the modified layers were metallographically polished and etched by 4% Nital solution, and then observed by optical microscopy (OM, CMM-33E, Jinan Fengzhi Test Instrument Co., Ltd., Jinan, China). The nitrided layer thickness was identified by the etched boundary evident in OM observations. The phase structures in the surface layer were analyzed by the X-ray diffractometer (type D/max-rB) with Cu-Kα radiation (λ = 0.15406 nm) and by transmission electron microscopy (TEM, JEM-2100, JEOL, Tokyo, Japan) in the bright field and selected area diffraction (SAD) modes. TEM specimens were prepared by single-sided ion thinned from the base side without damage to the nitrided surface. The microhardness on the surface and along the cross-section was measured by Vickers hardness tester (type HV-1000, Shanghai Jvjing Precision Instrument Manufacturing Co., Ltd., Shanghai, China) under a load of 100 g and a dwelling time of 15 s. Three indentations at each depth were taken and averaged. All tests were conducted in air.

3. Results and Discussion 3.1. Microstructures of Modified Surface Layers of M50NiL Steel Figure2a is an optical micrograph of the cross-section of Sample C showing that excessive carbon forms carbide (bright) primarily at prior austenite grain boundaries during carburizing. The region enclosed by the carbides has a characteristic scale of around 50 µm, corresponding to the prior austenite grain size. X-ray diffraction of Sample C outmost surface in Figure2b proves the carbides to be β-Mo2C and Cr7C3, which is consistent with the predicted stable phases of M50NiL at the carburizing temperature (910 ◦C~920 ◦C) when the carbon content is greater than 0.85 wt.% (using TCFE9 thermodynamic database). For Sample CD (decarburized in air) in Figure2c, carbides along the prior austenite grain boundaries coarsen, but not forming into significant networks. The hardness profiles of M50NiL Sample C and Sample CD along the diffusion direction in Figure2d reflects the bulk diffusion of carbon atoms during carburizing and decarburizing. After carburizing, the carburized case thickness reaches about 1.2 mm. The surface hardness is ~720 HV0.1 and the peak hardness (~800 HV0.1) offsets to about 350 µm from the outmost surface due to the presence of retained austenite as indicated by X-ray diffraction in Figure2b. For the decarburized sample, the surface hardness is as low as the matrix and the hardness peak is Coatings 2021, 11, 112 5 of 10

Coatings 2021, 11, x FOR PEER REVIEW 5 of 10 closer to the surface, manifesting the thickness of decarburized layer is about 100~200 µm where the hardness is lower than the carburized layer.

FigureFigure 2. ((aa)) Optical Optical micrograph micrograph of of cross-sectional microstructuremicrostructure of the carburized layerlayer ofof SampleSam- ple C; (b) XRD pattern of Sample C surface; (c) Optical micrograph of cross-sectional microstruc- C; (b) XRD pattern of Sample C surface; (c) Optical micrograph of cross-sectional microstructure of ture of the carburized layer of Sample CD; (d) Microhardness profiles along the modified layers of the carburized layer of Sample CD; (d) Microhardness profiles along the modified layers of Sample C Sample C and Sample CD. and Sample CD. Figure 3 represents the cross-sectional micrographs and microhardness profiles of Figure3 represents the cross-sectional micrographs and microhardness profiles of the the nitrided layers of M50NiL samples nitrided at 520 °C for 4 h. The nitrided layer is nitrided layers of M50NiL samples nitrided at 520 ◦C for 4 h. The nitrided layer is etched etcheddark close dark to close the surface.to the surface. For Sample For Sample C-N that C-N is thethat directly is the directly nitrided nitrided over the over carburized the car- burizedcase, the case, nitrided the nitrided layer thickness layer thickness is only 75 isµ onlym in 75 Figure μm 3ina, Figure half of the3a, half thickness of the of thickness Sample ofCD-N Sample (see CD-N Figure (see3c) thatFigure is decarburized3c) that is decarburized prior to plasma prior to nitriding. plasma nitriding. This indicates This indi- that catesthe pre-existing that the pre-existing C atoms may C atoms remarkably may remark hinderably the hinder diffusion the ofdiffusion N atoms. of N The atoms. hardness The hardnessprofile of profile the nitrided of the casenitrided in Figure case 3inb Figure shows 3b that shows the surface that the hardness surface hardness of Sample of CD-N Sam- plecan CD-N reach can ~1000 reach HV, ~1000 whereas HV, thewhereas counterpart the counterpart of Sample of C-NSample is only C-N ~750 is only HV, ~750 slightly HV, slightlyhigher thanhigher the than hardness the hardness of the carburized of the carburized case as case in Figure as in Figure2d. According 2d. According to Figure to Fig-3d urethat 3d shows that shows the diffraction the diffraction patterns patterns of the of nitrided the nitrided samples, samples, Sample Sample C-N contains C-N contains large largeamount amount of FCC-Fe of FCC-Fe phase, phase, which which refers refers to retained to retained austenite austenite in the in carburized the carburized case. Priorcase. Priorwork work [15] on [15] gas on nitriding gas nitriding of maraging of maraging steel withsteel with different different amount amount of retained of retained austenite aus- tenitedemonstrates demonstrates that the that hardness the hardness of the nitrided of the layernitrided drops layer with drops theincreased with the amountincreased of amountretained of austenite. retained Byaustenite. analyzing By theanalyzing intensity the of intensity the peaks of inthe Figure peaks3d, in the Figure percentage 3d, the percentageof austenite of in austenite the surface in the of Sample surface CD-Nof Sample nitrided CD-N at nitrided 520 ◦C for at 4520 h is°C about for 4 10%h is about while 10%the percentagewhile the percentage is 15% in the is surface15% in the of Sample surface C-Nof Sample nitrided C-N at 520nitrided◦C for at 4 520 h. This °C for agrees 4 h. Thiswith agrees the findings with the in findings [15], and in it [15], can and further it can support further that support the nitrided that the nitrided case hardness case hard- and nessnitrided and layernitrided thickness layer thickness can be enhanced can be enhanced when C atoms when andC atoms therefore and therefore retained austeniteretained austeniteare partially are removedpartially fromremov theed surfacefrom the layer. surface layer.

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FigureFigure 3. 3. ((aa)) Optical Optical micrograph micrograph of of cross-sectional cross-sectional micros microstructuretructure of of the the nitrided nitrided layer layer of of M50NiL M50NiL SampleSample C-N C-N nitrided nitrided at at 520 520 °C◦ Cfor for 4 h; 4 ( h;b) (Microhardnessb) Microhardness profiles proﬁles along along the modified the modiﬁed layers layers of of Sample C-N and Sample CD-N nitride at 520 °C for 4 h; (c) Optical micrograph of cross-sectional Sample C-N and Sample CD-N nitride at 520 ◦C for 4 h; (c) Optical micrograph of cross-sectional microstructure of the nitrided layer of M50NiL Sample CD-N nitrided at 520 °C for 4 h; (d) XRD microstructure of the nitrided layer of M50NiL Sample CD-N nitrided at 520 ◦C for 4 h; (d) XRD patterns of Sample C-N and CD-N nitride at 520 °C for 4 h. patterns of Sample C-N and CD-N nitride at 520 ◦C for 4 h.

3.2.3.2. Microstructures Microstructures of of Modifi Modifieded Surface Surface Layers Layers of of M50 M50 Steel Steel SimilarSimilar improvement improvement in in nitrided nitrided layer layer thic thicknesskness can can be be observed observed in in pre-decarburized pre-decarburized M50M50 samples. AccordingAccording toto Figure Figure3a,b, 3a,b, the the nitride nitride layer layer thickness thickness of pre-decarburized of pre-decarburized M50 M50Sample Sample D-N D-N nitrided nitrided at 500 at◦ C500 for °C 4 hfor is 4 107 h isµ 107m, 40% μm, greater 40% greater than that than of that Sample of Sample N, which N, whichwas directly was directly nitrided nitrided at the same at the plasma same plasma nitriding nitriding conditions conditions without without decarburization. decarburiza- This tion.indicates This thatindicates for high-carbon that for high-carbon steels, partly steels, removing partly the removing solutioned the C solutioned atoms can C promote atoms canthe promote nitriding the kinetics. nitriding Meanwhile, kinetics. Meanwhile the surface, hardnessthe surface of hardness the nitrided of the layer nitrided for Sample layer ◦ forD-N Sample nitride D-N at 500 nitrideC for at 4500 h reaches°C for 4 1200h reaches HV0.1 1200which HV is0.1 more which significantly is more significantly improved improvedcompared compared with the non-decarburized with the non-decarburized sample. sample. AccordingAccording to XRD patterns inin FigureFigure4 c,4c, the the peaks peaks of of M50 M50 matrix matrix significantly significantly shift shift to tothe the right right after after decarburization. decarburization. This This indicates indicates the the shrinkageshrinkage ofof FeFe latticelattice by the removal ofof solutioned solutioned carbon carbon atoms. atoms. By Bycomparing comparing the the(110) (110) peak peak of M50 of samples M50 samples with and with with- and outwithout decarburization decarburization after aftertempering tempering as in asthe in inset the insetof Figure of Figure 4c, it4 c,can it be can observed be observed that besidesthat besides shifting, shifting, the peak the peakis greatly is greatly narrowed narrowed after after decarburization, decarburization, which which reflects reflects the there- laxationrelaxation of ofthe the residual residual stresses stresses in in the the surfac surface.e. The The residual residual compressive compressive stresses stresses induced induced byby martensitic martensitic transformation isis parallelparallel to to the the surface, surface, which which can can hinder hinder nitrogen nitrogen diffusion diffu- sionperpendicular perpendicular to the to the surface surface upon upon nitriding nitriding [16 [16].]. I. I. Calliari Calliari [ 17[17]] foundfound the white-color microstructuremicrostructure appearing appearing in the the nitrided nitrided laye layerr is is caused caused by by decarburization. decarburization. As As suggested suggested ◦ byby Figure Figure 44d,d, after nitridingnitriding forfor 44 hh atat 500500 °C,C, a low-nitrogen compound FeNFeN0.0760.076 isis detected detected inin the the surface surface of Sample D-ND-N inin contrastcontrast to to the the formation formation of of traditional traditionalγ ’-Feγ’-Fe4N4N in in Sample Sample N Nthat that is directlyis directly nitrided nitrided without without decarburization. decarburization. The The formation formation of FeN of0.076 FeNin0.076 the in nitrided the ni- tridedlayer duringlayer during plasma plasma nitriding nitriding is proved is prov to exhibited to exhibit high hardness high hardness [18], which [18], thereforewhich there- can foreinterpret can interpret the high the hardness high hardness of D-N samples. of D-N samples.

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FigureFigure 4.4.( (aa)) Optical Optical micrograph micrograph of of cross-sectional cross-sectional microstructure microstructure of M50of M50 Sample Sample N nitridedN nitrided at 500 at 500◦C for°C 4for h; 4 (h;b) ( Opticalb) Optical micrograph micrograph of of cross-sectional cross-sectional microstructure microstructure of of M50 M50 Sample Sample D-N D-N nitrided nitrided atat 500500◦ °CC forfor 44h; h; ( c(c)) XRD XRD patterns patterns of of the the surface surface of of M50 M50 Sample Sample ‘Blank’ ‘Blank’ and and Sample Sample D withoutD without nitriding; nitriding; (d) XRD patterns of M50 Sample N and Sample D-N nitrided at 500 °C for 4 h. (ing;d) XRD (d) XRD patterns patterns of M50 of SampleM50 Sample N and N Sample and Samp D-Nle nitridedD-N nitrided at 500 at◦ C500 for °C 4 for h. 4 h.

TEMTEM imageimage andand selectedselected areaarea electronelectron diffractiondiffraction (SAED)(SAED) patternpattern ofof thethe surfacesurface ofof M50M50 Sample D-N nitrided nitrided at at 500 500 °C◦ Cfor for 4 h 4 is h shown is shown in Figure in Figure 5. Consistent5. Consistent with withthe phase the phasestructure structure of Sample of Sample D-N identified D-N identified by XRD by in XRD Figure in Figure4b, FeN4b,0.076 FeN and0.076 BCC-Feand BCC-Feare indexed are indexedas the primary as the primaryphases in phases the nitrided in the surface. nitrided Moreover, surface. Moreover,SAED pattern SAED is an pattern indicator is an of indicatorevident microstructure of evident microstructure refinement refinementthrough concen throughtric concentricrings, collection rings, of collection large number of large of numbercrystals ofwith crystals different with differentorientations. orientations. The morphology The morphology of refined of refinedmicrostructure microstructure is mani- is manifestedfested in Figure in Figure 5a, which5a, which exhibits exhibits the thelight light and and dark dark parallel parallel stripes stripes typical typical spinodal spinodal de- decompositioncomposition character character [19]. [19 In]. our In our previous previous study, study, a thermodynamic a thermodynamic model model has hasbeen been cre- createdated to touse use the the limit limit of of thermal thermal stability stability of of solid solid solution solution to explain thethe mechanismmechanism ofof microstructuremicrostructure refinement refinement during during nitriding nitriding as spinodalas spinodal decomposition decomposition [20]. [20]. With With spinodal spi- decomposition,nodal decomposition, conventional conventionalγ’-Fe4N γ’-Fe nitride4N nitride is compressed is compressed by the by formation the formation of nitrogen- of ni- leantrogen-lean FeN0.076 FeNand0.076 nitrogen-rich and nitrogen-rich martensite. martensite. In this way, In this refined way, microstructure refined microstructure can enhance can theenhance diffusion the diffusion of nitrogen of atomsnitrogen along atoms highly along dense highly grain dense boundaries grain boundaries [21]. [21].

FigureFigure 5.5. ((a)) TEM image and and ( (bb)) the the corresponding corresponding SAED SAED of of the the surface surface of of the the M50 M50 Sample Sample D-N D-N nitridednitrided atat 500500◦ °CC forfor 44 h.h.

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3.3. The Kinetics of Nitrogen Diffusion during Plasma Nitriding 3.3. TheSince Kinetics surface of Nitrogen can be easily Diffusion saturated during with Plasma the Nitriding nitrogen species during plasma bom- bardingSince the surface surface, can and be no easily white saturated nitride layer with wasthe observednitrogen species in M50 during or M50NiL plasma nitrided bom- samples,barding the nitrogen surface, diffusion and no into white the nitride matrix la isyer the was primary observed rate-limiting in M50 or step M50NiL [16,22 ].nitrided There- fore,samples, the kinetics nitrogen of diffusion nitriding followsinto the thematrix parabolic is the primary rate law, rate-limiting i.e., the thickness step of[16,22]. the nitrided There- layerfore, (the∆) cankinetics be expressed of nitriding by Equationfollows the (1) para [23],bolic rate law, i.e., the thickness of the nitrided layer (Δ) can be expressed by Equation (1) [23], ∆2 = k D t (1) 1 N ∆ 𝑘𝐷𝑡 (1) where D (in m2/s) is the diffusivity of N in steel, t (in s) is the nitriding time, and k is a where DNN (in m2/s) is the diffusivity of N in steel, t (in s) is the nitriding time, and k11 is a concentration-dependent constant. The dependence of D on temperature is described by concentration-dependent constant. The dependence of DNN on temperature is described by thethe ArrheniusArrhenius relationshiprelationship [[24,25],24,25], i.e.,i.e., Q −( ) D = D e RT (2) N𝐷N 𝐷0 𝑒 (2)

0 −−11 wherewhereD D0 isis thethe frequencyfrequency factor factor (pre-exponential (pre-exponential constant), constant),Q Q(in (in J· molJ·mol )) isis thethe activationactivation −11 −1−1 0 energy,energy,R Ris is thethe gasgas constantconstant (8.314(8.314 JJ·mol·mol ·K·K), and), and T (inT (in K) K)is temperature. is temperature. D Dand0 and Q canQ canbe determined be determined by the by thelinear linear fitting fitting of ln of D lnN DandN and 1/T. 1/T. ByBy analyzinganalyzing thethe nitridenitride layerlayer thicknessthickness ofof allall M50M50 andand M50NiLM50NiL samplessamples thatthat werewere nitridednitrided atat differentdifferent nitriding nitriding temperatures temperatures and and time time as as indicated indicated in in Tables Tables1 and 1 and2, a linear2, a lin- relationear relation between between the the calculated calculated ln DlnN DandN and the the reciprocal reciprocal of of nitriding nitriding temperatures temperatureswas was obtainedobtained forfor M50 M50 and and M50NiL M50NiL samples, samples, respectively, respectively, as shownas shown inFigure in Figure6. The 6. slopeThe slope of the of fittedthe fitted lines lines as labeled as labeled in Figure in Figure6 reflects 6 reflects the activation the activation energy energy required required for nitrogen for nitrogen atoms toatoms diffuse to diffuse upon plasma upon plasma nitriding, nitriding, and it canand implyit can carbon’simply carbon’s hindering hindering effect oneffect nitrogen on ni- atoms.trogen Itatoms. is reasonable It is reasonable to find thatto find M50NiL that M50NiL Samples Samples and M50 and N samples M50 N samples that are notthat pre- are decarburizednot pre-decarburized exhibit larger exhibit activation larger activati energyon compared energy compared with pre-decarburized with pre-decarburized samples duesamples to the due carbon to the barrier carbon effect. barrier Moreover, effect. Mo comparedreover, withcompared the activation with the energy activation of nitrogen energy atomof nitrogen to diffuse atom in to bcc-lattice diffuse in ( −bcc-lattice74.791 kJ/mol) (−74.791 [26 kJ/mol)], the values [26], the obtained values obtained in this work in this is muchwork is smaller. much smaller. This implies This thatimplies the that reduced the reduced activation activation energy isenergy resulted is resulted fromincreased fromin- numbercreased number density density of grain of boundary grain boundary and defects and defects via microstructure via microstructure refinement refinement during dur- plasmaing plasma nitriding. nitriding.

FigureFigure 6.6. The activation energy for plasma plasma nitrided nitrided samples samples (a (a) )M50NiL M50NiL C-N C-N samples samples and and CD-N CD-N samples,samples, ((bb)) M50M50 NN samplessamples andand D-ND-N samples.samples.

4.4. ConclusionsConclusions InIn summary,summary, decarburization,decarburization, whichwhich isis generallygenerally consideredconsidered detrimentaldetrimental toto surfacesurface properties,properties, is is innovatively innovatively applied applied before before plasma plasma nitriding nitriding to to promote promote the the nitriding nitriding kinetics kinet- forics case-hardenedfor case-hardened M50NiL M50NiL and and through-hardened through-hardened M50 M50 bearing bearing steel steel in in this this work. work. The The inﬂuenceinfluence ofof decarburizationdecarburization onon propertiesproperties andand growthgrowth kineticskinetics ofof thethe nitridednitrided layerlayer overover thethe decarburizeddecarburized casecase isis investigated.investigated.

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(1) Compared with the conventional “duplex surface engine” method, the duplex process with decarburizing step performed in this work can significantly increase the nitrided layer thickness even by more than ~100% and the surface hardness reach ~1200 HV0.1. (2) The analysis of nitriding kinetics shows that the pre-existing carbon atoms solutioned in Fe lattice can hinder the growth of nitride layers. Decarburization performed in air can produce a decarburized case with less carbon and residual compressive stresses, which can significantly increase the thickness of the subsequent nitride layer. (3) Low-nitrogen compound FeN0.076 with high hardness was produced on the modified surface of the decarburizing sample, which indicates that decarburizing can promote the occurrence of surface microstructure refinement via spinodal decomposition during plasma nitriding. Under the same nitriding conditions, the nanostructure can enhance nitrogen diffusion into the matrix along grain boundaries.

Author Contributions: Conceptualization, F.Y.; methodology, F.Y. and J.Y.; validation, F.Y. and J.Y.; formal analysis, F.Y. and J.Y.; resources, M.Y. and Y.X.; data curation, F.Y., J.Y., and B.C.; writing— original draft preparation, F.Y., J.Y., and Y.Y.; writing—review and editing, M.Y., Y.Z., and Y.Y.; visualization, F.Y.; supervision, M.Y.; project administration, M.Y.; funding acquisition, M.Y. and Y.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Key Research and Development Program of China (2018YFB2001901) and National Key Research and Development Program of China (2017YFB0304601). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data presented is original. Acknowledgments: This work was supported by National Key Research and Development Program of China through award numbers 2018YFB2001901 and 2017YFB0304601. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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