The Effect of the Transformation of Ε-Fe2-3N Into Γ′-Fe4n Phase On

Article The Eﬀect of the Transformation of ε-Fe2-3N into 0 γ -Fe4N Phase on the Fatigue Strength of Gas-Nitrided Pure Iron

Sunkwang Kim 1 , Sungook Yoon 2, Jun-Ho Kim 1,* and Soon Park 3,*

1 Dongnam Regional Division, Korea Institute of Industry Technology (KITECH), Jungang-ro 33-1, Yangsan 50623, Korea; [email protected] 2 Korea Shipbuilding & Oﬀshore Engineering Co., Ltd., Bangeojinsunhwando-ro 1000, Dong-gu, Ulsan 44032, Korea; [email protected] 3 School of Materials Science and Engineering, University of Ulsan, Techno saneop-ro 55 beon-gil 12, Nam-gu, Ulsan 44776, Korea * Correspondence: [email protected] (J.-H.K); [email protected] (S.P.); Tel.: +82-55-367-9403 (J.-H.K.); +82-52-712-8064 (S.P.) Received: 24 May 2020; Accepted: 17 June 2020; Published: 19 June 2020

Abstract: The effect of the iron nitride phases, ε-Fe2-3N and γ0-Fe4N, on the fatigue strength was investigated. Pure iron was used to observe only the effect of nitride, excluding the effects of factors, such as residual stress, depending on the alloy composition and microstructural change according to working on the fatigue strength. In this work, ε and γ0 phases were respectively grown at a time on the surface of the pure iron specimens using the appropriate nitriding potential KN, the mixture rates of ammonia and hydrogen gases, at same temperature of 570 ◦C according to the Fe-N Lehrer diagram. Another γ0 phase was prepared by first growing the ε phase and then transformed from ε phase into γ0 phase by changing the KN at the same temperature of 570 ◦C in the 2-stage gas nitriding. The fatigue strengths of the iron nitride consisted of ε and γ0 phases, γ0 phase, and γ0 phase grown by the 2-stage gas nitriding were evaluated, respectively. As a result, first, it can be seen that the diffusion layer of ε phase was deeper than γ0 phase, but fatigue strength was lower. On the other hand, fatigue strength of both the γ0 phases are higher than that of the ε, and the fatigue strength of γ0 phase nitride grown by 2-stage gas nitriding was almost similar to that of γ0 phase nitride grown at a time, i.e., fatigue strength was not significantly related to diffusion depth and depended on nitride phases in this study. Secondly, we cannot clearly conclude that there was the difference in fatigue strength according to the thickness of nitride layer consisted of γ0 phase. However, it is clear that when ε phase was transformed to γ0 phase, fatigue strength had the same level as γ0 phase formed at one time.

Keywords: 2-stage gas nitriding; phase-controlled gas nitriding; phase transformation; Lehrer diagram; fatigue strength

1. Introduction Gas nitriding is of considerable technological importance, because it can make a pronounced improvement on the fatigue strength, the wear, and the corrosion resistance of metals [1–3]. In gas nitriding, the surface properties of metals are improved by iron nitride layer and diffusion zone formed through the following mechanisms: Nitrogen (N) atoms are transferred into the iron and steel during the nitriding [4–6]. These N atoms were dissolved in the interstitial sites of the ferrite lattice and its mobility on the substrate of the interstitial sites can be large and its interaction with matrix atoms also be strong because of its surrounding strain fields accommodating the misfit of the size between

Metals 2020, 10, 823; doi:10.3390/met10060823 www.mdpi.com/journal/metals Metals 2020, 10, 823 2 of 13 the atom and the interstitial site [7,8]. Through these mechanisms, the iron nitrides layer, consisting of ε-Fe2-3N(ε) and/or γ0-Fe4N(γ0) phases, is formed with thickness of 10–20 µm on the surface of metals and largely depends on nitrogen concentration and presented in the Fe-N phase diagram [9,10]. The diffusion layer is also formed at a depth of about 0.2 to 1.0 mm from surface. Recently, many studied have been conducted to improve the fatigue strength of irons and steels by gas nitriding [11–14]. However, most of these studies have documented that thick diffusion layers have a significant effect on increased fatigue strength [15,16]. On the other hand, some studies report that iron nitride formed on the surface rather than diffusion layer affects the fatigue strength [17,18]. In particular, some studies indicate that nitride consisting of only γ0 phase has higher fatigue strength than nitrides consisting of only ε phase or ε/γ0 mixed phases. Therefore, this study was conducted to confirm the following two points. #1: It was investigated whether the fatigue strength was affected by iron nitrides formed on surface, not improved by diffusion layer. To this end, iron nitrides were formed by gas nitriding on pure iron, unlike the existing literature [11–14]. The reason for using pure iron was to minimize the effect of other factors of steels, which are such as residual stress depending on the alloy composition and microstructural change according to working, on the fatigue strength. In addition, the iron nitrides formed on the surface was subjected to a phase-controlled gas nitriding to accurately form ε or γ0 phases. Phase-controlled gas nitriding is a gas nitriding technology capable of controlling and forming iron nitrides corresponding to a stable iron nitride phase region by gas nitriding with conditions of a specific temperature and nitriding potential, KN, in the Fe-N Lehrer diagram. #2: As mentioned in #1, it was reported that the fatigue strength of irons and steels was improved when iron nitride consisted of only γ0 phase rather than ε phase. In addition, in the literature, it is shown that that fatigue strength was improved when γ0 phase in iron nitride layer occupied a large portion [12]. Therefore, this study is intended to investigate the effect on fatigue strength by forming thick iron nitride consisted of γ0 phase through 2-stage gas nitriding. 2-stage gas nitriding is a technique in which an iron nitride layer consisting of ε phase, which is grown to be relatively thick compared to that consisting of γ0 phase at the same time, is first formed on the surface, and then iron nitride consisting of ε phase is phase-transformed to γ0 phase by controlling KN [12,19,20]; i.e., it was intended to investigate how the fatigue strength of iron nitride consisting of γ0 phase, which was phase-transformed in ε phase through 2-stage gas nitriding, and that of iron nitride consisting of γ0 phase, grown at one time through phase-controlled gas nitriding, are compared.

2. Materials and Methods A commercial pure iron shaped into a round bar was used as a nitriding specimen and its chemical composition is presented in Table1. The pure iron round bar was cut out from a round bar with the dimensions of about 30 mm in diameter and 8 mm in thickness, and annealed at 870 ◦C for 2 h in a pure H2 gas atmosphere. Before nitriding, the surface of specimen was polished with a ﬁnal stage of 1 µm diamond.

Table 1. Chemical composition of pure iron in this study.

Composition C Si Mn P S Fe Pure iron 0.0036 0.026 0.257 0.0111 0.0053 Bal.

Phase-controlled and 2-stage gas nitriding were carried out in a horizontal quartz tube furnace with a diameter of 50 mm. The temperature was controlled within 3 C in about 5 cm of the uniformity ± ◦ temperature zone in furnace, where the specimen was placed by a ceramic boat. The ﬂow of NH3 and H2 was controlled by a separate mass-ﬂow controller. Phase-controlled gas nitriding to form iron nitrides consisted of ε phase or γ0 phase on the surface of pure iron were controlled accurately with 1/2 1/2 operating parameters at 570 ◦C, KN = 1.4 atm− for 240 min and at 570 ◦C, KN = 0.38 atm− for Metals 2020, 10, 823 3 of 13

240 min. 2-stage gas nitriding was performed to transform the ε phase into γ0 phase with operating 1/2 parameters as follows: at ﬁrst stage, 570 ◦C and KN = 1.4 atm− for 240 min, at second stage, 570 ◦C 1/2 and KN = 0.38 atm− for 30, 60, and 120 min. All of gas nitriding conditions are summarized in Table2.

Table 2. Experimental conditions of gas-nitrided pure iron with various nitriding potential.

Gas Nitriding Phase-Controlled Phase-Controlled Phase-Transformed γ0-Fe4N Conditions ε-Fe2-3N γ0-Fe4N 1st Stage 2nd Stage

Temperature (◦C) 570 570 570 570 Time (min) 240 240 240 120 1/2 Nitriding potential (KN, atm− ) 1.41 0.38 1.41 0.38 Pressure (atm) 1 1 1 1

The furnace was first preheated and purged at temperature of 250 ◦C for about 100 min, and then raised the nitriding temperature of 570 ◦C. KN was periodically measured to achieve the accurate values which depends on ammonia dissociation percentage during gas nitriding. In this work, the value of in-process KN was little changed during gas nitriding because of very low ammonia dissociation in a horizontal tube furnace that was less than 3 vol.% of the input ammonia gas. As soon as the gas nitriding is completed, the specimen was cooled by blowing cold air continuously to prevent peeling off of the nitride layer when it was dropped into water for quenching. The cooling time at which the specimens reached room temperature was less than 5 min as sample size was small. After gas nitriding, the microstructure of iron nitride was observed by the optical microscopy and FESEM. Additionally, the gas-nitrided specimens were characterized by high-resolution electron back-scatter diffraction (EBSD). Data were recorded and analyzed with the TSL OIM analysis software. To index the ε and γ0 phases, these phases are defined using the lattice constants and phase–space groups reported in the literature [21]. The hexagonal closed-packed ε phase belongs to the P63/mmc space group and has the nominal lattice parameters of a = 2.529 Å and c = 4.107 Å [8,21]. The face-centered cubic γ0 phases belong to the Pm3m space group and has lattice parameter of a = 3.798 Å [9,22]. X-ray diffraction (XRD) analysis was performed using a Rigaku Ultima 4 X-ray diffractometer to determine iron nitride phases. Measurements were made using Cu Kα (λ=0.15406 nm) radiation at 40 kV and a dual position graphite monochromator for Cu in the diffracted beam. The range of the diffraction angle (2θ) was from 20◦ and 80◦ with a step size of 0.02◦. XRD patterns were recorded from the surface of the all the nitrided specimen. To identify the nitride phase from the position of the diffraction peaks, data from ICSD database were used. The Knoop micro hardness test was conducted using a Mitutoyo HM-109 hardness tester with a load of 50 gf to determine the hardness of iron nitride and diffusion layer. For each nitride specimen, the hardness profile was plotted by the mean value of three measurements of hardness. For the rotary bending fatigue test, the hourglass shaped test specimens were prepared according to the specification as shown in Figure1, and then gas nitriding was performed under the planned conditions, and a sample was separately prepared to confirm the phase of the nitrided specimens. The fatigue test specimens were fixed with a jig during gas nitriding to minimize distortion. As a result, dimensional changes did not occur in all specimens after gas nitriding. MetalsMetals2020 2020,, 10 10,, 823x FOR PEER REVIEW 44 ofof 1313

Figure 1. Geometry dimensions and photo of rotating bending fatigue test specimen which used in thisFigure study. 1. Geometry dimensions and photo of rotating bending fatigue test specimen which used in this study. The main purpose of most engineering fatigue tests is to determine the relationship between the applied load and the number of load applications to cause failure, and to obtain some estimate of the 32 × (1) probability of failure under speciﬁed loading conditions. Fatigue2 strength was determined using a = rotating bending fatigue machine. Rotating bending fatigue × tests were conducted under the conditions of loadwhere frequency x (= 12 of mm) 3000 is rpm the indiameter a laboratory of the air effectiveness atmosphere. region Hence, in thethe tensilespecimen, and W compressive(kgf) is the stressesapplied onload the on specimen the specimen surface and were L is calculated the distance by Equationbetween (1).the points of load application. In each test, the number of cycles to fatigue failure was noted; the tests were terminated if no failure had 32 WL occurred after 107 cycles. ASTM E466 standardσ = was× employ2 ed to determine fatigue strength [23]. (1) π x3 × where3. Resultsx (= and12 mm) Discussion is the diameter of the eﬀectiveness region in the specimen, W(kgf) is the applied load on the specimen and L is the distance between the points of load application. In each test, the number3.1. Calculated of cycles Thermodynamics to fatigue failure was noted; the tests were terminated if no failure had occurred after 107 cycles. ASTM E466 standard was employed to determine fatigue strength [23]. The Fe-N Lehrer diagram, which is well known in other literature [10,24–26], was absolutely 3.necessary Results to and accurately Discussion control iron nitrides, ε and γ’ phases, with a suitable KN in this study. In this study, the Fe-N Lehrer diagram was calculated using Equation (2), (3) and nitrogen activity obtained 3.1.from Calculated the Fe-N Thermodynamicsphase diagram database of Commercialized Thermo-Calc. Program (CHALPAD) and shown in Figure 2. The Fe-N Lehrer diagram showed almost similar patterns to that of H. Du. because The Fe-N Lehrer diagram, which is well known in other literature [10,24–26], was absolutely of Gibbs energy of equations (4) given in the research of H. Du [24]. necessary to accurately control iron nitrides, ε and γ0 phases, with a suitable KN in this study. In this study, the Fe-N Lehrer diagram was calculated using Equations (2)∆ and (3) and nitrogen activity obtained(2) = from the Fe-N phase diagram database of Commercialized Thermo-Calc. Program (CHALPAD) and shown in Figure2. The Fe-N Lehrer diagram showed almost similar patterns to that of H. Du. because of Gibbs energy of equations (4) given in the researchN of H. Du [24]. is usually regarded as nitriding potential (K ) or N potential in experiments. 0 ! 1/2 PNH3 ∆G P = exp 3 (2)(3) N2 ∆ 3/=2 − + P RT H2 2

P NH3 3/2 is usually regarded as nitriding potential (KN) or N potential in experiments. P 3 (4) H ∆ =− + + 2 2 = +45,403 − 128.94 × − 10.785 0 0 3 0 ∆G = GNH + GH (3) ×() − 3 2 2

∆G0 is the standard Gibbs∆ energyG0 = Gof0 the+ NH3 dissociation3 G0 reaction.+ G0 − NH3 2 H2 N From this calculation, in order to stably= +control45, 403 nitride128.94 phasesT 10.785based on the above calculation(4) results, in the Fe-N Lehrer diagram of Figure 2, KN to− form Jiron× −nitride consisted of ε phase was TlnT mol selected as 1.4 atm−1/2 and KN to form iron consisted of× γ’ phase was selected 0.38 atm−1/2 at 570 °C. 0 ∆G is the standard Gibbs energy of the NH3 dissociation reaction.

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From this calculation, in order to stably control nitride phases based on the above calculation Metalsresults, 2020, in 10, the x FOR Fe-N PEER Lehrer REVIEW diagram of Figure2,K N to form iron nitride consisted of ε phase5 of was 13 Metals 2020, 10, x FOR PEER1/2 REVIEW 1/2 5 of 13 selected as 1.4 atm− and KN to form iron consisted of γ0 phase was selected 0.38 atm− at 570 ◦C.

Figure 2. Lehrer diagram which are calculated with Thermo-Calc. software in this study. FigureFigure 2.2. LehrerLehrer diagramdiagram whichwhich areare calculatedcalculated withwith Thermo-Calc.Thermo-Calc. softwaresoftware inin thisthis study.study. 3.2.3.2. Microstructural Microstructural and and Phase Phase Analysis Analysis 3.2. Microstructural and Phase Analysis 3.2.1.3.2.1. OM, OM, FESEM, FESEM, and and GDS GDS Analysis Analysis 3.2.1. OM, FESEM, and GDS Analysis TheThe cross-sectional cross-sectional optical optical images images of of the the microstructures microstructures of of pure pure irons irons each each subjected subjected to to gas gas The cross-sectional optical images of the microstructures of pure irons each subjected to gas nitridingnitriding with with various various K KNN atat temperature temperature of of 570 570 °C◦C mentioned mentioned in in the the previous previous experimental experimental method method nitriding with various KN at temperature of 570 °C mentioned in the previous experimental method areare shown shown in in Figure Figure 33.. The nitride layers were observedobserved with various thicknesses near the surface, are shown in Figure 3. The nitride layers were observed with various thicknesses near the surface, butbut diffusion diﬀusion zone zone usually usually in in gas-nitrided gas-nitrided iron iron an andd steel steel was was not not clearly clearly visible visible in in these these optical optical but diffusion zone usually in gas-nitrided iron and steel was not clearly visible in these optical micrographs.micrographs. The The thickness thickness of of the the nitride nitride layer layer on on the the surface surface decreased decreased with decreasing in K N.. The The micrographs. The thickness of1 /2the nitride layer on the surface decreased with decreasing in KN. The purepure iron iron nitrided nitrided at at 1.4 1.4 atm −−1/2 ofof nitriding nitriding potential potential has has aa thickness thickness of of 20.6 20.6 ± 55 μµmm and and there there are are −1/2 ± pure iron nitrided at 1.4 atm of nitriding potential has a thickness of 20.6 ± 5 μm and there are1/2 manymany pores in thethe layerlayer as as shown shown in in Figure Figure3a. 3(a). On the On other the other hand, hand, the pure the iron pure nitrided iron nitrided at 0.3 atm at −0.3 many pores in the layer as shown in Figure 3(a). On the other hand, the pure iron nitrided at 0.3 atmof nitriding−1/2 of nitriding potential potential has a thickness has a thickness of 7 2 ofµ m7 ± and 2 μ therem and were there a few were pores a few in thepores layer in the as shownlayer as in −1/2 ± atm of nitriding potential has a thickness± of 7 2 μm and there were a few pores1/2 in the layer1 /as2 shownFigure 3inb. Figure The pure 3(b). iron The nitrided pure iron with nitrided phase transformationwith phase transformation with changing with 1.4 changing atm − to 1.4 0.38 atm atm−1/2− to shown in Figure 3(b). The pure iron nitrided with phase transformation with changing 1.4 atm−1/2 to 0.38of the atm nitriding−1/2 of the potential nitriding haspotential a thickness has a thickness of 13.0 5ofµ 13.0m and ± 5 thereμm and were there many were pores many in pores the layer in the as 0.38 atm−1/2 of the nitriding potential has a thickness± of 13.0 ± 5 μm and there were many pores in the layershown as inshown Figure in3 c.Figure 3(c). layer as shown in Figure 3(c).

FigureFigure 3. 3. Cross-sectionalCross-sectional OM OM images images from from the the gas-nitrided gas-nitrided pure pure iron iron with with various various nitriding nitriding potential: potential: Figure 3. Cross-sectional OM images from the gas-nitrided pure iron with various nitriding potential: ((aa)) 1.4, 1.4, ( (bb)) 0.38 0.38 and and ( (cc)) 1st 1st stage: stage: 1.4 1.4 and and 2nd 2nd stage: stage: 0.38 0.38 of of nitriding nitriding potential potential at at 570 570 °CC for for ( (aa),), ( (bb)) 4 4 h h (a) 1.4, (b) 0.38 and (c) 1st stage: 1.4 and 2nd stage: 0.38 of nitriding potential at 570◦ °C for (a), (b) 4 h andand ( (cc)) 1st 1st stage: stage: 4 4 h h and and 2nd 2nd stage: stage: 2 2 h. h. and (c) 1st stage: 4 h and 2nd stage: 2 h. TheThe FESEM FESEM cross-sectional cross-sectional images images of of the the nitrided nitrided pure pure irons irons are are shown shown in in Figure Figure 44,, and the GDS The FESEM cross-sectional images of the nitrided pure irons are shown in Figure 4, and the GDS analysisanalysis results results are are also also superimposed superimposed on on the the images images and and plotted plotted in in the the direction direction of of nitrogen nitrogen diffusion diﬀusion analysis results are also superimposed on the images and plotted in the direction of nitrogen diffusion fromfrom the the surface surface into into inside. inside. The The thicknesses thicknesses of of all all ni nitridestrides layers layers are are almost almost similar similar to to those those observed observed from the surface into inside. The thicknesses of all nitrides layers1 /are2 almost similar to those observed withwith optical optical microscope microscope and the values are 1919 ± 2 μµm for 1.4 atm −1/2ofof the the nitriding nitriding potential, potential, 8 8 ± 11 μµmm with optical microscope and the values are 19± ± 2 μm for 1.4 atm−1/2of the nitriding potential, 8± ± 1 μm for 0.38 atm−1/2 of the nitriding potential, and 13 ± 2 μm for phase transformation with changing 1.4 for 0.38 atm−1/2 of the nitriding potential, and 13 ± 2 μm for phase transformation with changing 1.4 atm−1/2 to 0.38 atm−1/2 of the nitriding potential. The maximum nitrogen concentration in nitride layer atm−1/2 to 0.38 atm−1/2 of the nitriding potential. The maximum nitrogen concentration in nitride layer of the pure irons nitrided by 1.4 and 0.3 atm−1/2 of the nitriding potential and phase-transformed with of the pure irons nitrided by 1.4 and 0.3 atm−1/2 of the nitriding potential and phase-transformed with changing 1.4 atm−1/2 to 0.38 atm−1/2 of the nitriding potential were about 9.4 wt.%, 5.8 wt.% and 5.9 changing 1.4 atm−1/2 to 0.38 atm−1/2 of the nitriding potential were about 9.4 wt.%, 5.8 wt.% and 5.9 wt.%, respectively. The nitride phases on the surface can be deduced through qualitative relation wt.%, respectively. The nitride phases on the surface can be deduced through qualitative relation

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1/2 for 0.38 atm− of the nitriding potential, and 13 2 µm for phase transformation with changing 1/2 1/2 ± 1.4 atm− to 0.38 atm− of the nitriding potential. The maximum nitrogen concentration in nitride 1/2 layer of the pure irons nitrided by 1.4 and 0.3 atm− of the nitriding potential and phase-transformed 1/2 1/2 Metalswith changing2020, 10, x FOR 1.4 atmPEER− REVIEWto 0.38 atm− of the nitriding potential were about 9.4 wt.%, 5.8 wt.%6 of and 13 5.9 wt.%, respectively. The nitride phases on the surface can be deduced through qualitative relation betweenbetween the Fe-N phasephase diagramdiagram and and the the nitrogen nitrogen concentration concentration depth depth proﬁle profile results. results. As As a result, a result, the thenitride nitride layer layer of Figure of Figure4a may 4(a) consist may consist of ε/γ0 phasesof ε/γ’ phases and the and nitride the layersnitride of layers (b) and of (c) (b) may and consist (c) may of consistγ0 phase, of γ i.e.,’ phase, it can i.e., be seenit can that be seenε and thatγ0 correspondingε and γ’ corresponding to Figure to4a,b Figure are controlled 4 (a) and (b) with are an controlled accurate withnitride an phasesaccurate by nitride the nitriding phases potentials by the nitriding selected potentials in the Lehrer selected diagram in the of Lehrer Figure diagram2b. In addition, of Figure the 2 (b).γ0 nitride In addition, phase the corresponding γ’ nitride phase to Figure corresponding4c can also to be Figure inferred 4 (c) that can the also phase be inferred transformation that the from phaseε transformationto γ0 phase, which from is ε theto γ objective’ phase, which in this is study,the objective has occurred in this study, almost has completely occurred almost by the completely 2-stage gas bynitriding. the 2-stage In addition, gas nitriding. a small In numberaddition, of a pores small were number observed of pores in nitridewere observed consisting in ofnitrideγ0 phase, consisting which ofis mentionedγ’ phase, which in the is optical mentioned microscopic in the optical image. microscopic image.

(a) (b) (c)

FigureFigure 4. 4. Cross-sectionalCross-sectional SEM SEM micrograph micrograph obtained obtained perpen perpendiculardicular to to the the nitrogen nitrogen diffusion diﬀusion direction direction fromfrom the the gas-nitrided gas-nitrided pure pure iron iron wi withth various various nitriding nitriding potential potential and and th thee change change in in the the nitrogen nitrogen weight weight percentpercent from GDS analysis: Gas- Gas-nitridednitrided pure pure iron iron with ( aa)) 1.4, 1.4, ( (b)) 0.38 0.38 and ( c) 1st stage: 1.4 1.4 and and 2nd 2nd stage:stage: 0.38 0.38 of of nitriding nitriding potential potential at at 570 570 °C◦C for for ( (aa),), ( (bb)) 4 4 h h and and ( (cc)) 1st 1st stage: stage: 4 4 h h and and 2nd 2nd stage: stage: 2 2 h. h.

3.2.2.3.2.2. XRD XRD and and EBSD EBSD Analysis Analysis AlthoughAlthough the the nitride nitride phases phases were were inferred inferred from from the the results results of ofcross-sectional cross-sectional observation observation and and N concentrationN concentration analysis, analysis, XRD XRD analysis analysis was was performed performed to observe to observe the more the more accurate accurate nitride nitride phases. phases. The 1/2 XRDThe XRDresults results of pure of pure iron ironnitrided nitrided with with 1.4 and 1.4 and0.3 atm 0.3 atm−1/2 of− nitridingof nitriding potential potential and andthe 2-stage the 2-stage gas nitridinggas nitriding were were presented presented in Figure in Figure 5. It 5can. It be can seen be that seen the that peaks the peaks (33.401, (33.401, 41.22, 41.22,47.97, 47.97,70.180, 70.180, 84.76, γ 89.51)84.76, [27] 89.51) of [the27] ofγ’ thephase0 phasewere detected were detected in all nitrided in all nitrided pure irons. pure irons.On the Onother the hand, other the hand, peaks the ε (38.338,peaks (38.338, 43.768, 43.768, 57.625, 57.625, 77.037) 77.037) [28] of [ 28the] ofε thephasephase were weredetected detected only onlyin pure in pure iron ironnitrided nitrided with with 1.4 1/2 atm1.4 atm−1/2 of− nitridingof nitriding potential potential and and 2-stage 2-stage gas gas nitriding. nitriding. From From the the results, results, it itcan can be be seen seen that that nitride nitride 1/2 γ formedformed by by gas gas nitriding nitriding with 0.3 atm −1/2 ofof nitriding nitriding potential potential was was completely completely controlled controlled in in γ’ 0phase.phase. ε ε However,However, in 2-stage gas nitriding, ε phase was detected, and and it it was was considered considered that that ε phase was was not not γ completelycompletely phase-transformed intointo γ0 ’phase phase and and remained. remained. The The reason reason for for this this was was estimated estimated that that the ε ε theinitial initialphase ε phase nitride nitride layer layer was was thick thick and and there there was was not enoughnot enough time time to completely to completely transform transform all the all thenitride ε nitride phase. phase. InIn addition, addition, magnetite magnetite peaks peaks were were observed, observed, whic whichh are thought to be be formed formed by by oxygen oxygen that that was was notnot removed removed when nitrogen was cooled after gas nitriding.

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Figure 5. The XRD patterns for Gas-nitrided pure iron with 1.4, 0.38 and 1st stage: 1.4 1.4 and 2nd 2nd stage: 0.38 of nitriding potential at 570 ◦°C.C.

Although the nitride phases were analyzed by XRD, the nitride phases were observed by the Although the nitride phases were analyzed by XRD, the nitride phases were observed by the EBSD analysis to further increase accuracy. In particular, it was intended to measure the distribution EBSD analysis to further increase accuracy. In particular, it was intended to measure the distribution and amount of ε phase nitride remaining in the nitride layer after the 2-stage gas nitriding. The EBSD and amount of ε phase nitride remaining in the nitride layer after the 2-stage gas nitriding. The EBSD results are shown in Figure6. The FESEM images were taken with back-scattered electron (BE) contrast results are shown in Figure 6. The FESEM images were taken with back-scattered electron (BE) from the nitrided layer shown in left side of Figure6. The phase map from the EBSD analysis of the contrast from the nitrided layer shown in left side of Figure 6. The phase map from the EBSD analysis same area in right side of Figure6 corresponds perfectly with the BE image of the FESEM but with a of the same area in right side of Figure 6 corresponds perfectly with the BE image of the FESEM but better contrast showing the grain boundaries and the change in the microstructure from the surface with a better contrast showing the grain boundaries and the change in the microstructure from the into the bulk material. surface into the bulk material. In EBSD analysis results, the nitride layer consists of bilayer of ε and γ phases in Figure6a: the In EBSD analysis results, the nitride layer consists of bilayer of ε and 0γ’ phases in Figure 6 (a): thick outer layer consisted of the ε phase nitrides (yellow color) and the thin inner layer consisted the thick outer layer consisted of the ε phase nitrides (yellow color) and the thin inner layer consisted of γ phase nitrides (red color), and there was mono layer consisting γ phase nitrides in Figure6b. of γ’0 phase nitrides (red color), and there was mono layer consisting γ’ phase0 nitrides in Figure 6 (b). In Figure6c, it was observed that ε phase nitrides detected in XRD analysis were very slightly In Figure 6 (c), it was observed that ε phase nitrides detected in XRD analysis were very slightly distributed only in the grain boundary, and most of the phase was transformed into γ phase. It was distributed only in the grain boundary, and most of the phase was transformed into γ0’ phase. It was presumed that the remaining ε phase nitride was present on in the grain boundary because the presumed that the remaining ε phase nitride was present on in the grain boundary because the relatively high N concentration was maintained by the movement and diﬀusion of N atoms despite the relatively high N concentration was maintained by the movement and diffusion of N atoms despite lowering of nitriding potential in the second stage of 2-stage gas nitriding. the lowering of nitriding potential in the second stage of 2-stage gas nitriding. The thicknesses of the nitride layers observed in the EBSD analysis were almost similar to those The thicknesses of the nitride layers observed in the EBSD analysis were almost similar to those of nitride layers measured by OM and FESEM. The thicknesses of nitride layer shown in Figure6a of nitride layers measured by OM and FESEM. The thicknesses of nitride layer shown in Figure 6 (a) consisting of ε and γ phases were 17.5 2 µm and 4.1 1 µm, respectively. The thicknesses of the consisting of ε and γ0’ phases were 17.5 ± 2 μm and 4.1 ± 1 μm, respectively. The thicknesses of the nitride layers shown in Figure6b,c consisting of γ phase were 6 2 µm and 12 3 µm, respectively. nitride layers shown in Figure 6 (b) and (c) consisting0 of γ’ phase± were 6 ± 2 ±μm and 12 ± 3 μm, Compared to the growth thickness of nitrides, it can be seen that ε phase nitride was grown faster layer respectively. Compared to the growth thickness of nitrides, it can be seen that ε phase nitride was than γ phase nitride at the same time, i.e., the result that can be inferred through the previous analysis grown0 faster layer than γ’ phase nitride at the same time, i.e., the result that can be inferred through will be possible to form in a short time the thicker single γ phase nitride layer by phase transformation the previous analysis will be possible to form in a short time0 the thicker single γ’ phase nitride layer of 2-stage gas nitriding as suggested in the literature [12]. by phase transformation of 2-stage gas nitriding as suggested in the literature [12].

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(a) (b) (c)

Figure 6.6. (Right(Right side) side) phase phase map map from from the cross-sectionalthe cross-sectio gas-nitridednal gas-nitrided pure iron,pure with iron, the with phase the relevant phase

colorrelevant coded color map, codedα-Fe map, or α α-Fe[N]-Fe or (red),α-Fe[N]γ0 phase(red), γ (green),’ phase ε(green),phase(yellow), ε phase (yellow), (Left side) (Left Cross-sectional side) Cross- SEMsectional images SEM from images EBSD from area: EBSD Gas-nitrided area: Gas-nitrided pure iron with pure ( airon) 1.4, with (b) 0.38(a) 1.4, and ( (bc)) 0.38 1st stage: and (c 1.4) 1st and stage: 2nd

stage:1.4 and 0.38 2nd of stage: nitriding 0.38 potentialof nitriding at 570 potential◦C for (ata), 570 (b) °C 4 h for and (a (),c) ( 1stb) 4 stage: h and 4 ( hc) and 1st 2ndstage: stage: 4 h and 2 h. 2nd stage: 2 h. 3.3. Phase-Transformation Gas Nitriding 3.3. Phase-TransformationIn this study, ε phase Gas nitride, Nitriding which had not been transformed and left a small amount, was not a significantIn this study, problem ε phase to confirm nitride, whether which had the notγ0 phasebeen transformed nitride of thick and layer,left a small which amount, was formed was not by 2-stagea significant gas nitriding, problem to was confirm able to whether improve the fatigue γ’ phase strength nitride compared of thick layer, to γ0 whichphase nitridewas formed formed by at2- once.stage gas According nitriding, to was the literatureable to improve [12,19, fatigue20], however, strength it wascompared recorded to γ that’ phase the nitrideε phase formed formed atin once. the firstAccording stage wasto the mostly literature transformed [12,19,20], into however,γ0 phase it inwas the recorded second stage that the for aε shortphase time. formed Nevertheless, in the first thisstage study was mostly showed transformed different results into γ than’ phase expected in the thatsecondε phase stage formedfor a short in thetime. first Nevertheless, stage would this be completelystudy showed transformed different intoresults the γthan0 phase expected in the secondthat ε stagephase during formed 2-stage in the gas first nitriding. stage Therefore,would be additionalcompletely 2-stage transformed gas nitriding into the experiments γ’ phase in werethe second conducted stage to during confirm 2-stage whether gasε nitriding.phase would Therefore, not be transformedadditional 2-stage into γ gas0 phase nitriding due to experiments insufficient were time mentionedconducted above.to confirm whether ε phase would not be transformedIn this part, into the γ degree’ phase of dueγ0 phaseto insufficient transformation time mentioned according above. to the change of the second stage processIn this times part, of 30 the min, degree 60 min, of γ and’ phase 120 mintransformation during 2-stage according gas nitriding to the waschange observed of the bysecond analyzing stage withprocess EBSD times measurements of 30 min, 60 min, and and the results120 min are during shown 2-stage in Figure gas nitriding7. In all was the observed 2-stage gas by nitriding,analyzing thewith nitride EBSD layers, measurements which consisted and the of resultsε and γ are0 phases, shown were in Figure almost 7. transformed In all the 2-stage to monolayers gas nitriding, consisting the ofnitrideγ0 phase layers, nitride which and consisted a small amount of ε and of γε’ phasephases, nitride were wasalmost observed transformed at 30 min to andmonolayers 120 min. consisting However, itof wasγ’ phase confirmed nitride that and the a εsmallphase amount was not of observed ε phase nitride at 60 min, was and observed it was completelyat 30 min and transformed 120 min. intoHowever, the γ0 itphase. was confirmed In addition, that the the result ε phase of observation was not wasobserved that thickness at 60 min, of and nitride it was decreased completely from 21.6transformedµm to 10 into1 theµm γ in’ gasphase. nitriding In addition, for a relatively the result short of observation time of 30 minwas tothat transform thickness the ofε nitridephase ± formeddecreased at from one time 21.6 intoμm to a γ100 phase.± 1 μm Thein gas thickness nitriding of for the a relatively nitride, which short wastime phase-transformed of 30 min to transform for 60the min ε phase by increasing formed theat one time time of second into a stage, γ’ phase. was 10 The2 µthicknessm, which of was the similar nitride, to thewhich result was of 30phase- min. ± However,transformed it canfor 60 be min seen by that increasing the thickness the time of nitrideof second the stage, phase-transformed was 10 ± 2 μm, nitride which forwas 120 similar min into the secondresult of stage 30 min. was aboutHowever, 12 it3 µcanm, be which seen was that about the thickness 2 µm thicker of nitride than that the ofphase-transformed the previous two ± nitrides,nitride for but 120 the min amount in the of secondε phase stage mentioned was about several 12 ± 3 times μm, earlierwhich was also about increased. 2 μm thicker than that of theAccording previous two to these nitrides, results, but itthe cannot amount be of concluded ε phase mentioned whether there several was timesε phase earlier that also has increased. not been transformed due to lack of time, but was not observed in the result of 60 min or the change in the concentrationAccording of to nitride these occurring results, it duecannot to excessive be concluded diffusion whether and/or there movement was ε phase of already that has accumulated not been Ntransformed atoms at the due boundary to lackof of 60 time, min. but No was further not analysis observed and in discussion the result will of 60 be min made or in the this change study. Thisin the is becauseconcentration [28] explained of nitride the mechanismoccurring ofdue phase to transformationexcessive diffusion to γ0 phase and/or through movement annealing of afteralready gas nitriding,accumulated but N this atoms cannot at the be clearlyboundary indicated of 60 min. because No further the process analysis conditions and discussion such as will transformation be made in this study. This is because [28] explained the mechanism of phase transformation to γ’ phase through annealing after gas nitriding, but this cannot be clearly indicated because the process conditions such

Metals 2020, 10, 823 9 of 13 Metals 2020, 10, x FOR PEER REVIEW 9 of 13 processas transformation time and temperature process time were and ditemperatureﬀerent to explain were different the mechanism to explain of phase-transformationthe mechanism of phase- gas nitridingtransformation conducted gas nitriding in this study conducted with reference. in this study with reference.

(a) (b) (c) (d)

Figure 7. (Right(Right side) side) phase phase map map from from the the cross cross section section ga gas-nitrideds-nitrided pure iron, with the phase relevant

color coded map, α-Fe or α-Fe[N] (red), γ0’ phase (green), ε phase (yellow); (Left side) Cross-sectional −11/2/2 SEM images fromfrom EBSDEBSD area:area: Gas-nitridedGas-nitrided purepure ironiron withwith1st 1st stage: stage:1.4 1.4atm atm− of nitriding potential −11/2/2 for ((a)a) onlyonly 1st1st stage:stage: 240240 min,min, andand 2nd2nd stage:stage: 0.380.38 atmatm− ofof nitriding potential at 570 ◦°CC for ((b)b) 1st stage: 240 min and 2nd stage: 30 30 min, (c) (c) 1st stage: 240 240 min min and and 2nd stage: 60 60 min min and and (d) (d) 1st stage: 240 min and 2nd stage: 120 min.

3.4. Micro Hardness Profiles Proﬁles

The cross-sectionalcross-sectional hardnesshardness of ofε /εγ/γ0,’,γ γ0 ’and andγ γ0 with’ with a a small small amount amountε phaseε phase of of nitrides nitrides defined defined by theby the previously previously analyzed analyzed results, results, was was measured measured five fiv timese times in in total total to to present present the the averageaverage valuevalue and the hardness depth profilesprofiles areare shownshown inin FigureFigure8 8.. TheThe maximummaximum hardnesshardness valuesvalues andand locationslocations ofof ε/γ , γ and γ with a small amount of ε phase nitrides were about 526 ±25 HK , 400 ±5 HK and ε/γ0’, γ0’ and γ0’ with a small amount of ε phase nitrides were about 526± 25 HK0.050.05, 400± 5 HK0.050.05 and 415 ± 15 HK at 10 µm, 5 µm and 3 µm, respectively. The maximum hardness of γ nitride with a 415 ± 15 HK0.050.05 at 10 μm, 5 μm and 3 μm, respectively. The maximum hardness of γ0’ nitride with a small amount ε phase formed by 2-stage gas nitriding was relatively lower than that of ε nitride and higher than that of γ0’ nitride, which means that most of the ε phasephase havehave beenbeen transformedtransformed into γ0’ phase. In addition, the maximum hardness of γ’0 nitride with a small amount ε phase was slightly higher thanthan thatthat of ofγ 0γnitride’ nitride formed formed at aat time a time because, because, as can as be can seen be from seen the from previous the previous EBSD analysis, EBSD thereanalysis, was there a small was amount a small of amountε phase of that ε phase was not that transformed, was not transformed, i.e., the fact i.e., that the can fact be that confirmed can be throughconfirmed the through hardness the results hardness was results that even was whenthat evenε phase when was ε phase transformed was transformed to γ0 phase, to itγ’ canphase, have it almostcan have the almost same the hardness same hardness as γ0 phase as γ’ formedphase formed at a time. at a time. This This means means that that the the hardness hardness values values of nitridesof nitrides depended depended only only on theon nitrogenthe nitrogen concentration. concentration. The coreThe hardnesscore hardness value value of pure of ironpure was iron about was 160 2 HK± . about± 160 0.05 2 HK0.05.

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700 Phase controlled ε-Fe N 3 1+x Phase controlled γ'-Fe N 600 4 1-y Phase transformed γ'-Fe N 4 1-y

) 500 0.05

400

300 Micro Hardness (Hk

200

100 0 50 100 150 200 250 300 Depth (μm)

Figure 8.8. MicrohardnessMicrohardness proﬁlesprofiles obtained obtained on on gas-nitrided gas-nitrided pure pure iron iron with with 1.4, 1.4, 0.38 0.38 and and 1st step:1st step: 1.4 and 1.4

2ndand step:2nd step: 0.38 0.38 of nitriding of nitriding potential potential at 570 at ◦570C. °C.

3.5. Fatigue Test Results

The fatigue strengths ofof nitridesnitrides consistingconsisting ofof εε andand γγ0’ phase, γ0’ phase, and transformed γ0’ phase were measuredmeasured by rotatingrotating bendingbending fatigue tests, and the resultsresults areare shownshown inin FigureFigure9 9.. TheThe fatiguefatigue 7 limit stresses (failure(failure strengthstrength ofof 10107 cycles) of all nitrides are shown inin TableTable3 3.. It is generallygenerally known that the absolute value of fatigue strength is related to didiffusionffusion layer of nitrided irons and steels [[15,16].15,16]. According to the hardness results, it can be seen that the didiffusionffusion layers of γ0’ phase and transformed γ’0 phase nitrides are almost the same, and only that of ε and γγ0’ phase nitride was relatively thick. In the fatigue test result, however, it can be seen that the diffusion diffusion 7 layer of nitridenitride consistingconsisting of ε phasephase waswas thethe thickestthickest whilewhile fatiguefatigue limitlimit stressstress atat 10107 cycles was the lowest exceptexcept forfor thethe specimenspecimen withoutwithout gasgas nitriding.nitriding. This study conducted a rotary bending fatigue testtest of nitridesnitrides formed by phase-controlledphase-controlled and phase-transformation gasgas nitridingnitriding toto confirmconfirm thatthat nitridenitride consistingconsisting ofof γγ0’ phase was superior to that consisting of ε phase, and the results are shownshown inin FigureFigure9 9.. TheThe fatiguefatigue testtest waswas conductedconducted inin bothboth low-cycle and high-cycle fatigue, and the fatigue limit stress did not cause fracture inin thethe high-cyclehigh-cycle 7 of 107.. There There is is research research that that suggests suggests the the thicker thicker the the diffusion diffusion layer formed by gas nitriding,nitriding, the better the fatigue strength [[12].12]. However, in this study, it can be seen that the ε phase had a thicker didiffusionffusion layer than the γγ0’ phase, but the fatigue strength was much lower. According to the results of fatigue tests, the fatigue strength didifferedffered depending on the phase of the nitride compared to the didiffusionffusion layer.layer. In In the the literature literature [21 [21],], fracture fracture of materialsof materials occurs occurs due todue surface to surface crack nucleationcrack nucleation which iswhich originated is originated from surface from defects surface such defects as grooves, such scratches,as grooves, pores, scratches, holes, orpores, metallic holes, inclusions, or metallic even forinclusions, low stresses. even for low stresses. The second purpose of this study is to verify thethe fact that fatigue strength is improvedimproved whenwhen γγ0’ phase isis thick.thick. However, asas expected,expected, inin thethe fatiguefatigue strength strength result, result, the the nitride nitride transformed transformed from from the theε phaseε phase to to the theγ 0γphase’ phase does does not not show show a a significant significantdi differencefferencein in fatiguefatigue strengthstrength althoughalthough itit is thicker than the γ0’ phase nitride formed at one time. The fatigue strength of nitrided pure iron was higherhigher than that of untreated pure iron from the fatigue test results, and the fatiguefatigue strength was improved when γ0’ phase layer was formed on the surface. According According to to the the literature [29], [29], the fatigue limit increases with increasing hardness up to hardness of about 400 HV, but fatigue limit is lowered when nonmetallic inclusions (carbide, nitride, etc.) exist in pure iron or steels wi withth over 400 HV. In addition, when ε phase with high hardness and brittleness existed on the surface, fatigue crack occurred and propagated due to plastic deformation due to repeated compression and tensile. The nitride consisted of thick ε phase cannot withstand the tensile stress in fatigue test because of directly passing the crack propagation through ε phase layer.

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Metals 2020, 10, x FOR PEER REVIEW 11 of 13 due to repeated compression and tensile. The nitride consisted of thick ε phase cannot withstand ε theOn tensilethe other stress hand, in fatigueγ’ phase test slowed because down of directly the initiation passing and the propagation crack propagation of cracks through to the insidephase γ layer.because On compressive the other hand, stress0 phaseof γ’ phase slowed layer down presented the initiation much and larger propagation than that of of cracks the ε tophase the inside layer γ ε because[30]. When compressive the compressive stress of residual0 phase stress layer on presented the surface much of largerthe material than that was of higher, the phase the generation layer [30]. Whenof the initial the compressive crack was delayed residual because stress on the the tensile surface stress of the generated material in was the higher, fatigue the test generation was relaxed. of the In initialaddition, crack it was was shown delayed that because nitride the improves tensile stress fatigue generated life because in the it fatigue plays a test role was of preventing relaxed. In progress addition, itand was accumulation shown that of nitride dislocation. improves It seems fatigue to be life due because to ε phase it plays remaining a role of completely preventing untransformed progress and ε accumulationin the second stage of dislocation. nitriding. It seems to be due to phase remaining completely untransformed in the second stage nitriding. Table 3. Fatigue limit stress of the nitride phases formed on surface of pure iron after gas nitriding. Table 3. Fatigue limit stress of the nitride phases formed on surface of pure iron after gas nitriding. Specimen Failure strength of 107 cycles Specimen Failure Strength of 107 Cycles Phase-controlled ε-Fe2-3N 535 Phase-controlled ε-Fe2-3N 535 Phase-controlled γ՜-Fe4N 570 Phase-controlled γ0-Fe4N 570 Phase-transformedPhase-transformed γ՜-Feγ40N-Fe 4N 580580

Figure 9. S-N curve diagram of pure iron after gas nitridingnitriding with various nitriding conditions. 4. Conclusions 4. Conclusions In this study, gas nitriding was conducted to examine the following two points. First, this study In this study, gas nitriding was conducted to examine the following two points. First, this study was conducted to observe if nitride consisting of γ0 phase formed by gas nitriding can improve the was conducted to observe if nitride consisting of γ’ phase formed by gas nitriding can improve the fatigue strength compared to one consisted of ε phase. Secondly, it was shown that the thicker γ0 phase fatigue strength compared to one consisted of ε phase. Secondly, it was shown that the thicker γ’ nitride layer, the better the fatigue strength. Therefore, 2-stage gas nitriding, which was known to be phase nitride layer, the better the fatigue strength. Therefore, 2-stage gas nitriding, which was known able to formed thicker γ0 phase relatively than one formed at one time during same time, was conducted to be able to formed thicker γ’ phase relatively than one formed at one time during same time, was and fatigue strength of nitride consisting of γ0 phase formed by 2-stage gas nitriding was compared to conducted and fatigue strength of nitride consisting of γ’ phase formed by 2-stage gas nitriding was one consisting of γ0 phase formed at one time through fatigue testing. In addition, we observed how compared to one consisting of γ’ phase formed at one time through fatigue testing. In addition, we the fatigue strength of nitride consisted of γ0 phase-transformed from ε phase changed. observed how the fatigue strength of nitride consisted of γ’ phase-transformed from ε phase changed. The experimental results of this study were as follows: The experimental results of this study were as follows: (1) As planned in this study, nitrides consisting of ε phase and γ0 phase were accurately formed by (1) As planned in this study, nitrides consisting of ε phase1/2 and γ’ phase1/2 were accurately formed phase-controlled gas nitriding using appropriate 1.4 atm− and 0.38 atm− of nitriding potential in by phase-controlled gas nitriding using appropriate 1.4 atm−1/2 and 0.38 atm−1/2 of nitriding potential Fe-N Lehrer diagram at 570 ◦C, and a thick γ0 phase could be formed through phase transformation in in Fe-N Lehrer diagram at 570 °C, and a thick γ’ phase could be formed through phase transformation 2-stage gas nitriding. The γ0 phase formed through phase-transformation nitriding was not completely in 2-stage gas nitriding. The γ’ phase formed through phase-transformation nitriding was not phase-transformed in this study, and a small amount of ε phase remained at the grain boundaries. completely phase-transformed in this study, and a small amount of ε phase remained at the grain (2) In the second stage of 2-stage nitriding, the phase-transformation process time was changed to boundaries. 30 min, 60 min, and 120 min, and the process of phase transformation from ε phase to γ0 phase was (2) In the second stage of 2-stage nitriding, the phase-transformation process time was changed observed. A small amount of ε phase was observed at the results of 30 and 120 min in the second stage. to 30 min, 60 min, and 120 min, and the process of phase transformation from ε phase to γ’ phase was observed. A small amount of ε phase was observed at the results of 30 and 120 min in the second

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In addition, at 30 and 60 min, the thickness was slightly reduced compared to the thickness of nitride before phase transformation, but at 120 min, it tended to increase again. (3) As previously known, nitride consisting of ε phase had a maximum hardness value of 526 HK0.05 and γ0 phase and phase γ0 transformed by 2-stage gas nitriding had relatively low hardness values of 400 HK0.05 and 415 HK0.05, respectively. In addition, the hardness profile tended to be very similar to the nitrogen concentration profile, and it can be seen that the maximum hardness values and location of nitrides also depended on the nitrogen concentration. (4) To examine the fact that the target both nitrides consisting of γ0 phases in this study had higher fatigue strength than one consisted of ε phase, the fatigue limit stresses of nitrides were compared with each other at 107 cycles. (5) Second, it was predicted that thick nitride consisting of γ0 phase-transformed through 2-stage gas nitriding would have better fatigue strength than one consisting of γ0 phase formed at one time, but results of the fatigue strength were almost similar. In addition, fatigue strength of nitride consisting of γ0 phase-transformed were significantly improved as compared to that of one consisted of ε phase. However, despite the relatively thick nitride consisting of γ0 phase being transformed, as shown in a literature, the tendency for the thicker nitride consisting of γ0 phase being higher with the fatigue strength did not appear in this study.

Author Contributions: S.K., J.-H.K., and S.P. conceived and designed the experiments; S.K. and S.Y. performed the experiments; S.K., S.Y, J.-H.K., and S.P. analyzed and discussed the data; S.K. and S.Y. wrote the paper; J.-H.K., and S.P. Review & Editing. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (Grant No. 20203030040060, Development of ultra-high pressure hydrogen piping valve (105MPa), dispenser, heat exchanger, and monitoring system with hydrogen embrittlement suppression effect and leak detection function). Conflicts of Interest: The authors declare no conflict of interest.

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