coatings

Article Microstructure Evolution and Failure Behavior of Stellite 6 Coating on Steel after Long-Time Service

Jiankun Xiong 1,2,3, Fuheng Nie 3, Haiyan Zhao 1,*, Liangliang Zheng 3, Jun Luo 3, Lin Yang 2,3 and Zhongbo Wen 3

1 Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China 2 State Key Laboratory of Long-life High Temperature Materials, Dongfang Turbine Co., Ltd., Deyang 618000, China 3 Manufacturing Technology Department, Dongfang Turbine Co., Ltd., Deyang 618000, China * Correspondence: [email protected]; Tel.: +86-838-268-7424

 Received: 30 July 2019; Accepted: 20 August 2019; Published: 22 August 2019 

Abstract: The microstructure evolution, elements diffusion and fracture behavior of the Stellite 6 weld overlay, deposited on 10Cr9Mo1VNbN (F91) steel by the inert gas (TIG) cladding process, were investigated after long-time service. Obvious diffusion of Fe occurred from the steel and fusion zone to the Stellite overlay, resulting in the microstructure evolution and hardness increase in the coating, where hard Co–Fe phases, σ phases (Fe–Cr metallic compounds) and Cr-rich carbides (Cr18.93Fe4.07C6) were formed. Besides, the width of the light zone, combined with the fusion zone and diffusion zone, increased significantly to a maximum value of 2.5 mm. The fracture of the Stellite coating samples mainly occurred in the light zone, which was caused by the formation and growth of circumferential crack and radial crack under high temperature and pressure conditions. Moreover, the micro-hardness values in the light zone increased to the maximum (470–680 HV) due to the formation and growth of brittle Co–Fe phases. The formation of these cracks might be caused by formed brittle phases and changes of micro-hardness during service.

Keywords: Stellite 6 alloy; 10Cr9Mo1VNbN steel; TIG cladding; cracking; microstructure evolution

1. Introduction 10Cr9Mo1VNbN (F91) steel, as a martensitic, heat-resistant steel, has outstanding high-temperature performance and corrosion resistance, and is massively applied in manufacture of steam boiler, valve body, tube and turbine components [1]. The valve discs and seats mainly made of F91 steels are easily subjected to the severe erosion and wear of solid particles under high temperature and pressure in the service process. To enhance the service life of the valve discs and seats, protective coatings with adequate mechanical properties, wear and spalling resistance, are usually required. Stellite alloy (Co-based alloy), especially Stellite 6 alloy, is often used as a coating material in valve disc manufacturing, due to its excellent wear resistance, corrosion resistance and high temperature properties [2–4]. In previous work, different overlaying methods have been performed to investigate the properties of the wearing coating, including laser cladding, arc welding, plasma transferred arc (PTA) welding and gas dynamic cold spray [5–14]. Kusmokoet et al. [15] deposited the Stellite 6 alloy on P22 steel and P91steel plates by laser cladding and investigated the sliding wear characteristics of the coating. The worn surface of the coating on P91 steel was much rougher compared to that on P22 steel plate, and fewer strong carbide-forming elements resulted in the reduction of the amount of loss in the coating on P22 steel. Cincaet et al. [16] investigated the properties of Stellite 6 deposition on low alloy made by cold gas spraying and found that the increase of gas pressure and distance could led to the reduction of deposition efficiencies.

Coatings 2019, 9, 532; doi:10.3390/coatings9090532 www.mdpi.com/journal/coatings Coatings 2019, 9, 532 2 of 11

Ferozhkhanet et al. [17] investigated the microstructure, hardness and wear mechanism of Stellite 6alloy coating deposited on 9Cr–1Mo steel by plasma transferred arc welding process, and 309-16L was used as the interlayer between the coating and 9Cr–1Mo steel. It was found that the amount of alloying elements (Cr, W and Co) in the Stellite 6 coating was higher than that in the nominal composition, and the dilution of Fe in Stellite coating was below 2%. Besides, the wear mechanism of the coating was the combination of delamination and abrasive wear. Tungsten inert gas (TIG) welding has the advantages of convenience operations, excellent arc stability and welding quality. Wear-resistant layer surfacing in the valve body is often performed by TIG cladding in the production of power generation assembly. Mirshekariet et al. [18] and Molleda et al. [19] both characterized the microstructure and phase features of Stellite 6 surfacing layers deposited on steel substrates. However, the Stellite coating deposited on the steel components by TIG cladding could fracture after a long period of service process in power plants, due to the effect of the high temperature and pressure environment. The fracture and spalling of the Stellite layer is one of the key problems for the manufacturing of the power generation assembly, especially a valve body; however, there are no systematic researches to be reported about the fracture mechanism and alloying elements diffusion of Stellite alloy-deposited steel parts after long-time service in power plants. In this paper, the TIG cladding method is used to deposit the surfacing resistance coating (Stellite 6 alloy) onto the F91 steel, and the microstructure evolution, alloying the elements diffusion and failure behavior of Stellite coating samples after long-time service process, were investigated.

2. Materials and Methods Stellite6 alloy coating was deposited onto a F91martensitic heat-resistant steel valve disc (240 mm 107 mm) by a multi-pass tungsten inert gas (TIG) cladding process with AWS ERCoCr-A × (Shanghai, China) consumable (4.0 mm). The TIG welding machine is theTSP-300 (Panasonic, Kadoma, Japan) equipped a positioner and an automatic wire feeder. The nominal composition of F91 steel and ERCoCr-A wire were shown in Table1. In addition, the micro-hardness of the Stellite 6 alloy and F91 steel are about 380–450 HV and 200 HV, respectively. Before cladding welding, the F91 steel should undergo a preheating treatment (200 ◦C for 1 h) in the electric furnace. During the cladding process, the welding current (200–290 A), arc voltage (22–30 V), wire feed speed (50–70 mm/min) and travel speed (80–180 mm/min) were kept constant. The shielding gas is argon, with a flow rate of 15–20 L/min. There were three layers in the multi-pass surfacing, and the thickness of the surfacing is about 4 mm. The extension length and diameter of the tungsten electrode was around 8 and 4.0 mm, respectively. After cladding welding, the weldments were subject to post-welding heat treatment, which was performed at 400 ◦C for 1 h. Two components with Stellite 6/F91 TIG cladding structure were obtained. The component#1 is on the pre-service condition, while the component #2 has been used in the main stop valve of the steam turbine for about 26,280 h, operating at 566 ◦C and 9.94 MPa. The weldments before and after long-time service in a power plant were marked as #1 and #2, respectively.

Table 1. The nominal composition of F91 steel and consumable.

Element (wt %) Material C Cr Mo Ni W Nb V F91 0.091 9.01 0.90 0.010 – 0.083 0.17 Stellite 6 0.93 27.3 0.26 1.81 3.36 – – Element (wt %) Material N Si Mn P S Co Fe F91 0.040 0.33 0.42 0.014 0.0011 – Bal. Stellite 6 – 1.08 0.13 0.014 0.0063 Bal. 2.17 Coatings 2019, 9, x 532 FOR PEER REVIEW 33 of of 11

The main microstructures of F91 steel are tempered martensite (Figure 1a), and the Stellite 6 The main microstructures of F91 steel are tempered martensite (Figure1a), and the Stellite 6 alloy alloy consists of Co-rich dendrites matrix and eutectic carbides within grain boundaries (GBs) consists of Co-rich dendrites matrix and eutectic carbides within grain boundaries (GBs) (Figure1b). (Figure 1b). The samples were cut from weldment #1 and #2 for microstructure analysis, and then The samples were cut from weldment #1 and #2 for microstructure analysis, and then they were etched they were etched for 20 s with FeCl3 solution after sectioning, grinding and polishing. The microstructurefor 20 s with FeCl features3 solution were afterstudied sectioning, using an grinding optical microscope and polishing. (OM, The Leica, microstructure DM2700M, Wetzlar, features Germany).were studied In usingorder an to optical observe microscope the microstructu (OM, Leica,re evolution, DM2700M, the Wetzlar, samples Germany). were examined In order by to scanningobserve the electron microstructure microscopy evolution, (SEM, theVEGA3 samples SBH, were TESCAN, examined Brno, by scanning Czech electronRepublic), microscopy and the micro-hardness(SEM, VEGA3 SBH, profile TESCAN, across Brno, the sample Czech Republic), was meas andured. the Moreover, micro-hardness the composition profile across in the different sample regionswas measured. of samples Moreover, was measured the composition with energy in di ffdispersiveerent regions spectrum of samples (EDS, was VEGA3 measured SBH, with TESCAN), energy anddispersive the phase spectrum identification (EDS, VEGA3 of the SBH, coating TESCAN), layer and was the performed phase identification by X-ray ofdiffraction the coating (XRD, layer D/MAX-1200,was performed Rigaku by X-ray Industrial diffraction Co (XRD,rporation, D/MAX-1200, Osaka, RigakuJapan). IndustrialIn addition, Corporation, the microstructure Osaka, Japan). of Stellite-depositedIn addition, the microstructure steel samples ofwa Stellite-depositeds investigated by steeltransmission samples el wasectron investigated microscopy by (TEM, transmission Tecnai G2electron 20 S-TWIN, microscopy FEI, Hillsboro, (TEM, Tecnai OR, G2 USA), 20 S-TWIN, and the FEI, TEM Hillsboro, samples OR, were USA), prepared and the by TEM focused samples ion beam were (FIB,prepared AURIGA, by focused Zeiss, ion Oberkochen, beam (FIB, AURIGA, Germany). Zeiss, Figure Oberkochen, 1c,d illustrates Germany). the schematic Figure1c,d of illustrates the Stellite the 6 alloyschematic weld ofoverlay the Stellite on F91 6 alloy steel weldby the overlay TIG claddi on F91ng steelprocess by theand TIG the cladding samples processfor microstructural and the samples and micro-hardnessfor microstructural analyses. and micro-hardness analyses.

Figure 1. 1. (a()a )Microstructure Microstructure of of F91 F91 steel; steel; (b ()b Microstructure) Microstructure of ofStellite Stellite 6 alloy; 6 alloy; (c) (Thec) The schematic schematic of theof theoverlay overlay weldment weldment of ofStellite Stellite 6 6alloy alloy on on F91 F91 steel; steel; (d (d) )the the sample sample for for microstructural microstructural and micro-hardness analysis. 3. Results 3. Results 3.1. Microstructure 3.1. Microstructure The typical microstructures at cross sections of sample #1 are shown in Figure2. The boundary The typical microstructures at cross sections of sample #1 are shown in Figure 2. The boundary between the base material and coating was obvious, and a wide fusion zone was observed in Figure2a. between the base material and coating was obvious, and a wide fusion zone was observed in Figure Moreover, the microstructure of the Stellite weld overlay zone (WOZ) was a typical hypoeutectic 2a. Moreover, the microstructure of the Stellite weld overlay zone (WOZ) was a typical dendritic in Figure2b, which consisted of Co solid solution and a network of small carbides particles. hypoeutectic dendritic in Figure 2b, which consisted of Co solid solution and a network of small The Co solid solution consisted of fcc γ and hcp ε phases, and the carbides particles were mainly carbides particles. The Co solid solution consisted of fcc γ and hcp ε phases, and the carbides M23C6, Cr7C3 and Co3W intermetallic phases, where M was (Cr, Co, W, Ni, Fe) [2,20].During the TIG particles were mainly M23C6, Cr7C3 and Co3W intermetallic phases, where M was (Cr, Co, W, Ni, Fe) cladding process, thermal cycle could result in the formation of microstructures with different features [2,20].During the TIG cladding process, thermal cycle could result in the formation of in the heat-affected zone (HAZ), which was divided into a coarse grain heat-affected zone (CG-HAZ), microstructures with different features in the heat-affected zone (HAZ), which was divided into a fine grain heat-affected zone (FG-HAZ) and partial normalized zone (PNZ), as shown in Figure2c–e. coarse grain heat-affected zone (CG-HAZ), fine grain heat-affected zone (FG-HAZ) and partial Obviously, grain coarsening occurred in the CG-HAZ, where the austenitizing induced by the high normalized zone (PNZ), as shown in Figure 2c–e. Obviously, grain coarsening occurred in the peak temperature happened during TIG cladding, following a significant growth of austenite grains. CG-HAZ, where the austenitizing induced by the high peak temperature happened during TIG Finally, coarse original austenite grains remained after cooling. The fine austenite grains formed in cladding, following a significant growth of austenite grains. Finally, coarse original austenite grains FG-HAZ due to relatively low heat input compared with CG-HAZ, in which the pinning effect of remained after cooling. The fine austenite grains formed in FG-HAZ due to relatively low heat undissolved precipitated phases would restrain the grain boundary migration. Furthermore, a part of input compared with CG-HAZ, in which the pinning effect of undissolved precipitated phases would restrain the grain boundary migration. Furthermore, a part of the microstructure in PNZ

Coatings 2019, 9, 532 4 of 11 Coatings 2019, 9, x FOR PEER REVIEW 4 of 11 theunderwent microstructure an austenitizing in PNZ underwent transformation, an austenitizing while th transformation,e other part remained while the of other martensite part remained structure, of martensitewhich eventually structure, generated which eventually a mixture generated structure a mixturewhich consisted structure whichof un-tempered consisted ofmartensite un-tempered and martensiteover-tempered and over-tempered martensite. martensite.

FigureFigure 2. 2.Microstructure Microstructure ofof sample sample #1: #1: (a(a)) Stellite-steel Stellite-steel interface; interface; ( b(b)) Stellite Stellite coating; coating; ( c()coarsec)coarse grain grain heat-aheat-affectedffected zone zone (CG-HAZ); (CG-HAZ); (d )( fined) fine grain grain heat-a heat-affectedffected zone zone (FG-HAZ); (FG-HAZ); (e) partial (e) partial normalized normalized zone. zone. The fusion zone had broadened in the Stellite 6 coating layer near F91 steel due to the diffusion of elementsThe fusion from zone the steelhad broadened to the Stellite in the 6 coating Stellite after 6 coating the service layer near process. F91 steel The due elements to the di diffusionffusion eventuallyof elements made from a the wider steel light to th zonee Stellite form, 6 consistingcoating after of the the servic originale process. fusion zoneThe elements and new diffusion mutual dieventuallyffusion zone, made as shown a wider in Figurelight zone3. The form, light consisting zone still showedof the original a dendritic fusion microstructure zone and new as same mutual as thatdiffusion in our zone, Stellite as weld shown layer in Figure of sample 3. The #2. light The widthzone still of the showed fusion a zonedendritic was measuredmicrostructure as 31, as 28 same and 49asµ thatm at in di ourfferent Stellite locations weld layer of sample of sample #1 in #2. Figure The3 widtha, indicating of the fusion the width zone of was the measured fusion zone as 31, was 28 non-uniform.and 49 μm at Afterdifferent long-time locations service, of sample the width #1 in of Figure the fusion 3a, indicating zone increased the width significantly of the fusion to form zone a largewas dinon-uniform.ffusion zone After as marked long-time in Figure service,3b (sample the width #2), of which the fusion could evenzone reachincreased to around significantly 2.5 mm, to indicatingform a large that diffusion a high temperature zone as marked environment in Figure could 3b (sample promote #2), the which extent could of the even light reach zone to through around influencing2.5 mm, indicating the diffusion that a of high alloying temperature elements environment and the formation could promote and growth the extent of metallic of the compound light zone phasesthrough and influencing carbides. the Figure diffusion3c,d shows of alloying the XRD elem resultsents ofand phases the formation in the light and zone growth of samples of metallic #1 andcompound #2. It can phases be seen and from carbides. the XRD Figure results 3c,d inshows Figure the3c XRD that theresults phases of phases in the lightin the zone light mainly zone of consistsamples of #1 Co–Fe and substitution#2. It can be solidseen from solutions the XRD and σresultphasess in (Fe–CrFigure metallic3c that the compounds), phases in the and light the twozone elements,mainly consist Co and of Fe,Co–Fe have substitution good solid solubility,solid solutions thus and they σ could phases dissolve (Fe–Cr with metallic each compounds), other by almost and anythe proportion.two elements, The Co carbides and Fe, werehave notgood found solid fromsolubility, the XRD thus results they could due to dissolve the smaller with quantityeach other and by smallalmost size. any The proportion. type of phases The incarbides the light were zone ofnot sample found #2 from was similarthe XRD to thatresults of sampledue to #1; the however, smaller therequantity existed and a small more size. evident The di typeffraction of phases peak ofin carbidesthe light inzone Figure of sample3d, which #2 was were similar Cr 18.93 toFe that4.07C of6 carbides.sample #1; It meanthowever, that there the formationexisted a more and growth evident of di theffraction precipitated peak of phases carbides happened, in Figure and 3d, the which Co matrixwere Cr had18.93 evolvedFe4.07C6 tocarbides. form Co–Fe It meant phases that during the formation the service, and which growth could of result the inprecipitated the increasing phases of thehappened, micro-hardness and the and Co brittlenessmatrix had of evolved this light to zone. form Co–Fe phases during the service, which could result in the increasing of the micro-hardness and brittleness of this light zone. 3.2. Distribution of Alloying Elementsand Phases The microstructure of the boundary between the Stellite alloy and F91 steel of sample #1 and #2 were shown in Figure4, and the major composition of di fferent positions in Figure4 was measured by EDS, as shown in Table2. It can be seen that the microstructure evolution was obvious on the F91 steel side, where a mass of carbides within grains dissolved, precipitated and then grew along GBs, forming coarse prior austenite grain boundaries (PAGBs). Comparing the composition in position A with that in position C, it was found that the content of Fe elements increased obviously, while the content of Co, Cr and W decreased. It indicated that the diffusion process of Fe from the fusion zone to the Stellite layer predominated. The decrease of Cr content might be attributed to the diffusion of Fe, the formation of a Co–Fe matrix and a composition fluctuation of alloying elements. Thus, the content of

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Cr decreased in the matrix and formed Cr-rich second phases in some positions. Moreover, the content of Co in positions B and D were similar, demonstrating that the diffusion rate of Co elements from the Stellite layer to the steel was relatively low under the high temperature and pressure condition.

Figure 3. The interface between Stellite 6 and F91 steel of samples (a) #1 and (b) #2; The X-ray diffraction (XRD) results of phases in (c) fusion zone of sample #1 and (d) light zone of sample #2.

3.2. Distribution of Alloying Elementsand Phases The microstructure of the boundary between the Stellite alloy and F91 steel of sample #1 and #2 were shown in Figure 4, and the major composition of different positions in Figure 4 was measured by EDS, as shown in Table 2. It can be seen that the microstructure evolution was obvious on the F91 steel side, where a mass of carbides within grains dissolved, precipitated and then grew along GBs, forming coarse prior austenite grain boundaries (PAGBs). Comparing the composition in position A with that in position C, it was found that the content of Fe elements increased obviously, while the content of Co, Cr and W decreased. It indicated that the diffusion process of Fe from the fusion zone to the Stellite layer predominated. The decrease of Cr content might be attributed to the diffusion of Fe, the formation of a Co–Fe matrix and a composition fluctuation of alloying elements. Thus, the content of Cr decreased in the matrix and formed Cr-rich second phases in some positions. Moreover,Figure 3. 3.theThe The content interface interface of between betweenCo in Stellitepositions Stellite 6 and 6 B F91and and steel F91 D of steelwere samples of similar, samples (a) #1 anddemonstrating (a) (b #1) #2; and The (b X-ray) #2that; diThe fftheraction X -diffusionray rate(XRD) diffractionof Co resultselements (XRD) of phases fromresults inthe of (c phases)Stellite fusion in zonelayer (c) fusion of to sample the zone steel #1 of andwas sample ( drelatively) light #1 and zone ( lowd) of light sampleunder zone #2.the of samplehigh temperature #2. and pressure condition.

FigureFigure 4. 4.Photos Photos of of the the microstructure microstructure around around the the boundary boundary between between Stellite Stellite alloy alloy and and F91 F91 steel steel of of samplesample ( a()a) #1 #1 and and ( b(b)) #2. #2.

Table 2. The major composition in different positions around the boundary between Stellite alloy and Table 2. The major composition in different positions around the boundary between Stellite alloy F91 steel. and F91 steel.

ElementElement (mol(mol %) %) PositionPosition FeFe CoCo Cr Cr W W AA 46.86 46.86 28.4828.48 20.17 20.17 4.49 4.49 BB 89.1389.13 1.321.32 9.55 9.55 – – CC 76.6976.69 13.2113.21 8.08 8.08 2.02 2.02 D 88.47 – 11.53 – D 88.47 – 11.53 –

TheThe microstructure microstructure around around the the boundary boundary between between the the Stellite Stellite and and the the steel steel of samplesof samples #1and #1 and #2 are#2 shownare shown in Figure in Figure5a,e, and 5a,e, the and distributions the distribution of Co,s Cr of and Co, Fe Cr in and the rectangleFe in the area rectangle are displayed area are indisplayed Figure5b–d,f–h. in Figure It 5b–d,f–h. can be seen It can that be Co seen mainly that Co formed mainly a Coformed matrix a Co in matrix the Stellite in the layer, Stellite and layer, Fe mainlyand Fe distributedmainly distributed in the steel in the region steel toregion form to Fe-base form Fe-base phases phases and Fe-rich and Fe-rich carbides carbides in Figure in Figure5b,d. Besides, the distribution of Cr was uniform from the Stellite layer to the steel in Figure5c. It was found

1

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5b,d. Besides, the distribution of Cr was uniform from the Stellite layer to the steel in Figure 5c. It thatwas changingfound that of thechanging Co and of Cr the content Co and around Cr cont theent Stellite-steel around the interface Stellite-steel was not interface significant was after not thesignificant service processafter the in service Figure 5processf,h. However, in Figure as shown5f,h. However, in Figure as5h, shown an apparent in Figure increase 5h, an of apparent the Fe contentincrease was of happeningthe Fe content in the was Stellite happening layer near in the interface,Stellite layer indicating near the that interface, a mass of indicating Fe had diff thatused a frommass the of steelFe had into diffused the Stellite from because the steel of theinto large the diStelliteffusion because rate of of Fe. the Obvious large diffusion mutual di rateffusion of Fe. of majorObvious elements mutual happened diffusion near of thema boundaryjor elements during happened the service near process the boundary due to the during high temperature the service condition,process due thereby to the leading high totemperature the formation condition, of a mutual thereby diff usionleading zone to inthe the formation Stellite layer of a near mutual the fusiondiffusion zone. zone in the Stellite layer near the fusion zone.

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5b,d. Besides, the distribution of Cr was uniform from the Stellite layer to the steel in Figure 5c. It was found that changing of the Co and Cr content around the Stellite-steel interface was not significant after the service process in Figure 5f,h. However, as shown in Figure 5h, an apparent increase of the Fe content was happening in the Stellite layer near the interface, indicating that a mass of Fe had diffused from the steel into the Stellite because of the large diffusion rate of Fe. Obvious mutual diffusion of major elements happened near the boundary during the service process due to the high temperature condition, thereby leading to the formation of a mutual diffusion zone in the Stellite layer near the fusion zone.

FigureFigure 5. 5.The The alloying alloying elements elements distribution distribution around around the the Stellite-steel Stellite-steel boundary boundary of samples of samples (a–d )(a #1–d and) #1 (eand–h) ( #2.e–h) #2.

TheThe rapidrapid didiffusionffusion ofof FeFe couldcould changechange thethe structurestructure andand induceinduce thethe formationformation ofof newnew phases,phases, resultingresulting inin the property property variation variation of of the the Stellite Stellite coating coating components components after after the theservice service process. process. The Thedistribution distribution curve curveFigure of Fe, 5. of The Co Fe, alloying and Co elements andCr content Cr distribution content perpendi around perpendicular the cularStellite-steel to boundarythe to fusion the of fusionsamples line (a – lineofd) #1sample of sample #1 and #1 and#2 is and (e–h) #2. #2presented is presented using using EDS, EDS, as shown as shown in Figure in Figure 6. It6 can. It canbe seen be seen that thatthe Co the content Co content in the in steel the steel was wasvery verylow. low.The Theamount amount Theof rapidFein of Fein diffusion thethe Stellite of Stellite Fe could coating coatingchange nearthe near stru thecture the fusion and fusion induce lineline the wasformation was around around of new 30phases, 30 mol mol %–60 mol %,%, indicating the diresultingffusion in processthe property had variation been of occurred.the Stellite coating The components amount ofafter Fe the reached service process. 30 mol The %–35 mol % at indicating the distributiondiffusion curve process of Fe, Cohad and been Cr content occurr perpendied. cularThe to amount the fusion lineof Feof sample reached #1 and 30 #2 molis %–35 mol % theat the distance distance of presented 2.2of 2.2 mm mm using from from EDS, the as the shown Stellite-steel Stellite-steel in Figure 6. interface It caninterface be seen in that in the the Stellite Co Stellite content coating. incoating. the steel Besides,was Besides, very the the variation variation of the amount oflow. Cr The was amount not ofsignificant. Fein the Stellite Therefore,coating near the the fusion di fflineusion was around of Fe 30 was mol the%–60main mol %, alloy elements of the amount indicatingof Cr was the diffusion not signif processicant. had beenTherefore, occurred. Thethe amount diffusion of Fe reached of Fe 30 was mol %–35the mainmol % alloy elements migrationmigration in in the theat weldthe weld distance overlay, overlay, of 2.2 mm resulting resultingfrom the Stellite-steel in microstructurein microstructure interface in the evolution Stellite evolution coating. in theBesides, in lightthe the light variation zone. zone. Comparing Comparing the distributionthe distribution of Fe,of ofthe Co Fe,amount and Co of Crand Cr ofwas Crsample not of signif sampleicant. #2 with Therefore, #2 with that the ofthat diffusion sample of sample of Fe #1, was the #1,theFe mainthe increased alloyFe increased elements obviously obviously in the in migration in the weld overlay, resulting in microstructure evolution in the light zone. Comparing lightthe light zone zone near the thenear distribution fusion the fusion lineof Fe, dueCo line and to Crdue di offf sampleusionto diffusion #2 of with Fe that during of of sampleFe during the #1, service. the Fethe increased service. The obviously diff erenceThe in difference of Fe content of Fe betweencontent samplebetweenthe #1 lightsample and zone #2 near decreased#1 theand fusion #2 graduallylinedecreased due to diffusion when grad of awayually Fe during fromwhen the theservice. away fusion The from difference line the in of thefusion Fe Stellite line coating. in the In addition, thecontent variation between tendency sample #1 ofand Cr #2 anddecreased Co wasgradually not when very away significant from the fusion after line the in service. the Stellite coating.Stellite In addition, coating. In addition, the variation the variation tendency tendency of of Cr Crand andCo was Co not was very significantnot very after significant the after the service. service.

Figure 6. Element distribution perpendicular to fusion line of samples #1 and #2.

Coatings 2019, 9, x FOR PEER REVIEW 7 of 11 Coatings 2019, 9, 532 7 of 11 Figure 6. Element distribution perpendicular to fusion line of samples #1 and #2.

TEMTEM investigations investigations were were performed performed to analyzeto analyze the phasesthe phases in the in di fftheusion diffusion zone. Figurezone. 7Figure shows 7 theshows TEM the images TEM and images diffraction and diffraction patterns of patterns the microstructure of the microstructure in the light zonein the between light zone the between Stellite and the theStellite steel and of sample the steel #2. of The sample TEM #2. image The of TEM the matriximage of in the matrix diffusion in zonethe diffusion was displayed zone was in Figure displayed7a, whichin Figure was 7a, cubic which structure was cubic Co–Fe structure phase, accordingCo–Fe phase, to the according diffraction to the pattern diffraction in Figure pattern7b, as in well Figure as XRD7b, as results well andas XRD EDS analysis.results and It meant EDS analysis. that Fe gradually It meant di thatffused Fe fromgradually the fusion diffused zone from and the the steel fusion to thezone Stellite and layerthe steel in the to the high Stellite temperature layer in environment, the high temperature and then theenvironment, Fe and Co and were then mutually the Fe soluble and Co withwere each mutually other tosoluble form thewith Co–Fe each matrixother to in fo therm coatingthe Co–Fe layer. matrix in the coating layer. Furthermore,Furthermore, there there existed existed somesome Cr-richCr-rich phasesphases inin thethe didiffusionffusion zone,zone, asas shownshown inin FigureFigure7 c,7c, whichwhich were were black black knife-like knife-like particles particles with with a sharp a sharp edge. edge. It can It be can determined be determined that these that Cr-rich these phasesCr-rich werephases (Cr, were Fe)23 C(Cr,6 carbides Fe)23C6 (Crcarbides18.93Fe 4.07(CrC18.936)Fe through4.07C6) through indexing indexing the corresponding the corresponding diffraction diffraction pattern. Thepattern. black The particles black in particles the light in zone the werelight σzonephases were (Fe–Cr σ phases phases) (Fe–Cr with phases) a size of with about a hundredssize of about of nanometers.hundreds of According nanometers. to the According Fe–Cr phase to diagram, the Fe–Cr these phaseσ phases diagram, formed these intemperatures σ phasesranging formed fromintemperatures 450–830 ◦C, ranging and were from also 450–830 detected °C, and in alloyswere also with detected 23.4 wt in %–33.7 alloys with wt % 23.4 Cr wt during %–33.7 plastic wt % deformationCr during plastic followed deformation by aging due followed to a composition by aging fluctuationdue to a composition of alloying elementsfluctuation [21 ,of22 ].alloying Thus, underelements the condition[21,22]. Thus, of in-service under temperaturethe condition (566 of ◦inC)-service and a large temperature stress load, (566 the °C) formation and a oflarge multiple stress brittleload, σthephases formation could be of predicted, multiple which brittle could σ phases lead to deteriorationcould be predicted, of both corrosion which resistancecould lead and to ductilitydeterioration of the of coatings. both corrosion resistance and ductility of the coatings. FigureFigure7 g,h7g,h shows shows the the TEM TEM images images of of the the microstructure microstructure in in the the light light zone zone between between the the Stellite Stellite andand the the steel. steel. ItIt cancan bebe seenseen thatthat therethere werewere manymany small,small, blackblack precipitateprecipitate particlesparticles inin thethe matrix.matrix. TheseThese nano-scaled nano-scaled carbides carbides were wereM6C carbidesM6C carbides according according to their smallto their size andsmall dispersive size and distribution. dispersive Mdistribution.6C carbides couldM6C carbides significantly could inhibit significantly the dislocation inhibit motion the dislocation within the matrix,motion therebywithin inducingthe matrix, a strongerthereby strengtheninginducing a stronger effectthan strengthening that of M23 effectC6 carbides. than that Therefore, of M23C6 thecarbides. strength Therefore, and hardness the strength of the matrixand hardness was improved. of the matrix was improved.

FigureFigure 7. 7.TEM TEM imagesimages (a(a,c,,ce,e)) and and di diffractionffraction patterns patterns ( b(b,d,d,f,)f) of of the the microstructure microstructure in in light light zone zone of of samplesample #2; #2; TEM TEM bright bright field field images images in in (g ()g the) the fusion fusion zone zone and and (h ()h di) diffusionffusion zone zone of of sample sample #2. #2.

3.3.3.3. Failure Failure Analysis Analysis FigureFigure8a shows8a shows the micro-hardnessthe micro-hard profilesness profiles perpendicular perpendicular to the fusion to linethe alongfusion the line cross-section along the ofcross-section the samples. of It the can samples. be seen that It can the be micro-hardness seen that the micro-hardness in the weld overlay in the was weld higher overlay than thatwas inhigher the steel.than Meanwhile,that in the steel. the micro-hardness Meanwhile, the in themicro-hardness HAZ was higher in the compared HAZ was with higher F91 steelcompared for the with sample F91 #1.steel After fora the long sample period #1. of service,After a the long micro-hardness period of service, values the of samplemicro-hardness #2 increased values to the of maximumsample #2 (470–680increased HV) to the in the maximum weld overlay (470–680 near HV) fusion in the zone weld due overlay to the formationnear fusion and zone growth due to of the hard formation matrix phases.and growth Moreover, of hard as for matrix sample phases. #2, the Moreover, micro-hardness as for valuessample in #2, the the HAZ micro-hardness dropped to be values the same in asthe HAZ dropped to be the same as that in the base metal, and the micro-hardness values in the Stellite

Coatings 2019, 9, 532 8 of 11 Coatings 2019, 9, x FOR PEER REVIEW 8 of 11 thatoverlay in the had base increased metal, and (450–500 the micro-hardness HV). In the HAZ values of in F91 the steel, Stellite the overlay micro-hardness had increased decreased (450–500 because HV). Inof the the HAZ coarsening of F91 steel,of carbides. the micro-hardness decreased because of the coarsening of carbides. ThereThere were were two two kinds kinds of of cracks cracks existing existing in in the the weld weld overlay overlay of of sample sample #2 #2 as as shown shown in in Figure Figure8b. 8b. OneOne kind kind of of them them is theis the circumferential circumferential crack crack paralleling paralleling to the to fusion the fusion line, andline, the and other the oneother is aone radial is a crackradial perpendicular crack perpendicular to the fusion to the line. fusion When line. the Wh twoen the kinds two of kinds cracks of merged cracks gradually,merged gradually, the coating the couldcoating fracture could andfracture fall o andff. Figure fall off.8c showsFigure the8c show failures the locations failure of locations sample #2,of sample and it can #2, beand seen it can that be theseen fracture that the mainly fracture occurred mainly in occurred the light in zone. the light zone.

FigureFigure 8. 8.(a )( Transversea) Transverse micro-hardness micro-hardness profiles prof ofiles samples of samples #1 and #2;#1 ( band) Image #2; of(b cracks) Image distribution of cracks ofdistribution sample #2; (ofc) sample Image of#2; fracture (c) Image paths of fracture in partial paths region in partial of sample region #2. of sample #2.

4.4. Discussion Discussion AfterAfter the the service service process, process, hard hard Co–Fe Co–Fe matrix matrix and andσ phases σ phases formed, formed, caused caused by theby the diff usiondiffusion of Fe of fromFe from steel steel into theinto Stellite the Stellite layer, layer, resulting resulting in the in formation the formation of a wide of a light wide zone, light which zone, consistedwhich consisted of the originalof the original fusion zone fusion and zone new and mutual new di mutualffusion diffusion zone. The zone. growth The of growth the Co–Fe of the matrix Co–Fe phases matrix with phases high hardnesswith high in hardness the light zonein the led light to formationzone led to of formation an interface of betweenan interface the lightbetween zone the and light Stellite zone layer. and TheStellite micro-hardness layer. The inmicro-hardness the light zone in was the higher light thanzone that was in thehigher Stellite than layer that significantly, in the Stellite and layer the maximumsignificantly, diff erenceand the in maximum micro-hardness difference between in micro-hardness the two zones between was even the up two to 230zones HV. was Moreover, even up fromto 230 the HV. Stellite Moreover, layer to from the light the Stellite zone near layer the to interface, the light the zone micro-hardness near the interface, suddenly the micro-hardness changed from 450suddenly to 680 HVchanged as displayed from 450 in Figureto 6808 HVa. It as indicated displayed that in the Figure changes 8a. ofIt micro-hardnessindicated that the between changes the of lightmicro-hardness zone and Stellite between layer happenedthe light zone after and a long Stellite period layer of service happened process. after Large a long stress period concentrations of service couldprocess. easily Large occur stress on this concentrations interface because could of changeseasily occur in micro-hardness, on this interface resulting because in theof formationchanges in ofmicro-hardness, a vast nucleation resulting site of in micro-cracks the formation in theof a interface vast nucleation as shown site in of Figure micro-cracks9a. In addition, in the interface the σ phases,as shown which in Figure are hard 9a. andIn addition, brittle, harmfully the σ phases, affect which the mechanical are hard and properties brittle, harmfully of the coatings affect bythe creatingmechanical local properties embrittlement of the and coatings by forming by creating micro-cracks local embrittlement at the γ/σ phase and interfaces by forming during micro-cracks loading. Afterat the nucleating γ/σ phase at these interfaces interfaces, during the cracks loading. aggregated After andnucleating grew gradually at these in interfaces, Figure9b. Finallythe cracks the cracksaggregated propagated and grew into thegradually light zone in Figure and became 9b. Finally large the cracks. cracks The propagated large cracks into gradually the light increased, zone and andbecame thus large led to cracks. the fracture The large and fallcracks off ofgradually the coating. increased, and thus led to the fracture and fall off of the coating.

Coatings 2019, 9, 532 9 of 11 Coatings 2019, 9, x FOR PEER REVIEW 9 of 11

FigureFigure 9. 9.Schematic Schematic diagrams diagrams of of initiation initiationand andpropagation propagationof ofcracks: cracks: (a(a)) InitiationInitiation of of micro-cracks; micro-cracks; (b(b) Aggregation) Aggregation and and growth growth of of micro-cracks; micro-cracks; (c ()c Propagation) Propagation of of cracks. cracks.

AsAs shown shown in in Figure Figure8 b,c,8b,c, the the formation formation of of circumferential circumferential cracks cracks on on the the interface interface was was related related to to thethe hardness hardness changes changes between between the the light light zone zone and and Stellite Stellite layer, layer, which which could could be be separated separated from from the the StelliteStellite weld weld layer layer and and the the light light zone, zone, and and fracture fracture under under the combinedthe combined effect effect of high of high temperature temperature and impactand impact load underload under service service conditions. conditions. Besides,Besides, inter-dendriticinter-dendritic regionsregions werewere weakweak zones,zones, wherewhere impurities,impurities, coarsecoarse carbidescarbides andand micro-cracksmicro-cracks caused caused by by thermal thermal stress stress and and residual residual stress stress existed, existed, which which could could induce induce the the formation formation ofof radial radial cracks. cracks. The The high high hardness hardness made made it it easier easier for for the the micro micro cracks cracks to to form form and and propagate propagate in in the the lightlight zone. zone. The The cracks cracks which which initiated initiated on theon interfacethe interface between between the light the zonelight andzone Stellite and Stellite layer could layer propagatecould propagate into the into light the zone light gradually zone gradually and form and a large form macro-crack. a large macro-crack. Thus, the Thus, light zonethe light becomes zone thebecomes weakest the region weakest and region fracture and failure fracture could failure happen could in happen this zone. in this zone.

5.5. Conclusions Conclusions OverlayingOverlaying of of Stellite Stellite 6 alloy 6 alloy on F91 on steelF91 substratessteel substrates was performed was performed with multi-pass with multi-pass TIG cladding TIG method.cladding The method. effect ofThe in-service effect of environment in-service enviro on thenment element on di theffusion, element microstructure diffusion, evolutionmicrostructure and fractureevolution behavior and fracture of the resultant behavior weld of the overlay resultant were weld investigated. overlay Thewere conclusions investigated. are listedThe conclusions as follows: are Thelisted microstructure as follows: of the Stellite weld overlay near the fusion zone had changed to form a light • • zone,The consistingmicrostructure of Co–Fe of the substitution Stellite weld solid overla solutions,y nearσ phasesthe fusion (Fe–Cr zone metallic had changed compounds) to form and a Crlight18.93 Fezone,4.07C 6consistingcarbides. Theof highCo–Fe temperature substitution environment solid solutions, could promote σ phases the extent (Fe–Cr of themetallic light zone.compounds) After service, and Cr the18.93 widthFe4.07C of6 carbides. the light The zone, high combined temperature with fusionenvironment zone and could diff usionpromote zone, the increasedextent of significantlythe light zone. to formAfter a service, large di fftheusion widt zone,h of whichthe light could zone, even combined reach to with around fusion 2.5 mm.zone Theand obvious diffusion di ffzone,usion increased of Fe occurred significantly from theto form steel aand large fusion diffusion zone zone, to the which Stellite could overlay, even • resultingreach to inaround the microstructure 2.5 mm. evolution and hardness increase in the weld overlay. The content • ofThe Fe increasedobvious diffusion intensively, of butFe occurred the content from of Cothe decreased, steel and whichfusion couldzone eventuallyto the Stellite lead overlay, to the formationresulting ofin hard the andmicrostructure brittle Co–Fe evolution phases. and hardness increase in the weld overlay. The Thecontent micro-hardness of Fe increased in the intensively, Stellite weld but overlay the co wasntent higher of Co than decreased, that in the which steel. could After eventually cladding, • thelead micro-hardness to the formation in the of hard HAZ and increased. brittle Co–Fe After the phases. service process, the micro-hardness values in • theThe Stellite micro-hardness overlay slightly in the increased Stellite to weld 450–500 overla HV,y whilewas thosehigher in than the HAZ that droppedin the steel. where After the precipitatescladding, hadthe coarsened.micro-hardness Moreover, in the the micro-hardnessHAZ increased. values After in thethe lightservice zone increasedprocess, the to themicro-hardness maximum (470–680 values HV),in the resulting Stellite overlay in changes slightly of micro-hardness increased to 450–500 between HV, the basewhile material those in andthe theHAZ Stellite dropped weld where overlay. the precipitates had coarsened. Moreover, the micro-hardness values Thein the fracture light zone of the increased Stellite to coating the maximum samples (470–680 mainly occurredHV), resulting in the in lightchanges zone of • (fusionmicro-hardness zone + di ffbetweenusion zone) the baseafter material the service and process.the Stellite The weld formation overlay. of these cracks might • beThe caused fracture by formed of the brittleStellite phases coating and samples changes mainly of micro-hardness occurred in the during light service. zone (fusion zone + diffusion zone) after the service process. The formation of these cracks might be caused by formed brittle phases and changes of micro-hardness during service. Author Contributions: Conceptualization, J.X. and F.N.; Methodology, L.Y.; Validation, J.X.; Formal Analysis, J.X., F.N.Author and J.L.;Contributions: Investigation, Conceptualization, J.X., F.N., H.Z., J.L. J.X. and and Z.W.; F.N.; Resources, Methodology, H.Z. and L.Y.; Z.W.; Validation, Data Curation, J.X.; Formal H.Z. andAnalysis, L.Z.; Writing—Original Draft Preparation, J.X. and F.N.; Writing—Review and Editing, J.X. and H.Z.; Visualization, L.Y.; J.X., F.N. and J.L.; Investigation, J.X., F.N., H.Z., J.L. and Z.W.; Resources, H.Z. and Z.W.; Data Curation, H.Z. Supervision, L.Z.; Project Administration, J.X.; Funding Acquisition, J.X. and L.Z. and L.Z.; Writing—Original Draft Preparation, J.X. and F.N.; Writing—Review and Editing, J.X. and H.Z.; Funding:Visualization,This researchL.Y.; Supervision, received no L.Z.; external Project funding. Administration, J.X.; Funding Acquisition, J.X. and L.Z.

Funding: This research received no external funding.

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Acknowledgments: This work was supported by State Key Laboratory of Long-life High Temperature Materials of Dongfang Turbine Co., Ltd. in Deyang. This work was technically supported by College of Materials Science and Engineering of Chongqing University in Chongqing. Conflicts of Interest: The authors declare no conflict of interest.

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