Effects of Vanadium on Microstructure and Wear Resistance of High Chromium Cast Iron Hardfacing Layer by Electroslag Surfacing

Article Effects of Vanadium on Microstructure and Wear Resistance of High Chromium Cast Iron Hardfacing Layer by Electroslag Surfacing

Hao Wang, Sheng Fu Yu *, Adnan Raza Khan and An Guo Huang State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China; [email protected] (H.W.); [email protected] (A.R.K.); [email protected] (A.G.H.) * Correspondence: [email protected]; Tel.: +86-027-87559960

Received: 18 May 2018; Accepted: 11 June 2018; Published: 15 June 2018

Abstract: The 3.6C-20Cr-Fe-(0–2.32)V high chromium cast iron (HCCI) hardfacing layers were deposited on low alloy steel by electroslag surfacing. The microstructure of hardfacing layers were observed and the carbide types, size and area fraction were measured. In addition, the hardness and wear resistance were tested. Results show that the interface between hardfacing layer and low alloy steel is defect free. 3.6C-20Cr-Fe hardfacing layer contains primary carbides and eutectic. Increasing V wt % in the hardfacing layer, primary carbides are decreasing by increasing eutectic along with martensite formation. For 1.50 wt % of V, the microstructure contains a lot of eutectic and a little of martensite. For 2.32 wt % of V, primary austenite formed, the microstructure is primary austenite, eutectic and a little of martensite. In the V alloyed hardfacing layers, V has strong afﬁnity with carbon than chromium, hence V can replace a part of Cr in M7C3 and (Cr4.4–4.7Fe2.1–2.3V0.2–0.5)C3 type carbides are formed. When the V is 2.32 wt %, (Cr0.23V0.77)C carbides are formed in the hardfacing layer. The hardness and wear resistance are improved by increasing V from 0 to 1.50 wt %. However, when the V is 2.32 wt %, the primary austenite has reduced the hardness and wear resistance of hardfacing layer.

Keywords: electroslag surfacing; high chromium cast iron; vanadium alloying; microstructure; wear resistance; carbide reﬁnement

1. Introduction Being an important class of wear resistant material, high chromium cast iron (HCCI) can be used for surfacing low alloy steel to form bimetallic material. This bimetallic material overcomes the poor toughness of HCCI and has been used more and more in metallurgy, machinery, mining and other ﬁelds [1–3]. Whereas the traditional method of surfacing is arc welding that holds techniques of open arc welding and submerged arc welding, etc. Because of the high carbon equivalent of HCCI, it is very easy to form cracks during surfacing process. Although the cracks can release the stress and avoid the spalling of hardfacing layer during the wear process, whereas in many applications, HCCI hardfacing layer is required to have good structural integrity and certain corrosion resistance. Thereby, cracks cannot be allowed in the hardfacing layer [4]. There is a need for a new surfacing method that can reduce the thermal stress during the surfacing process and avoid cracks in the HCCI hardfacing layer. The latest research [5] reveals that in electroslag welding due to high heat input, temperature gets evenly distributed in the workpiece, which reduces the cooling rate, residual stresses and cracking tendency in the hardfacing layer. The content and size of carbides in HCCI hardfacing layer has signiﬁcant impact on its wear resistance [6–8]. Normally, HCCI with high carbon content contains a large amount of carbides,

Metals 2018, 8, 458; doi:10.3390/met8060458 www.mdpi.com/journal/metals Metals 2018, 8, 458 2 of 16 but a considerable part of them are large-size primary carbides. These coarse primary carbides are easy to be fractured and spalled during the wear process with heavy impact [9,10]. So the refinement of primary carbides and increasing of eutectic carbides content are of significant importance to improve the wear resistance of HCCI hardfacing layer. The volume fraction and size of M7C3 carbides in HCCI can be affected by alloying elements such as Nb, Ti and V [11–13]. Additions of Nb and Ti can refine the microstructure of the HCCI alloy but cannot increase the content of eutectic M7C3 carbides [14,15]. Studies [16,17] reveal that V can promote the precipitation of secondary carbides during the solid-state phase transformation. Therefore, it is possible that V-alloyed electroslag HCCI hardfacing layer can obtain good wear resistance without post surfacing heat treatment. Thermodynamic analyses [18] reveal that vanadium partitioning in between matrix and the carbide happens, which increase the abrasive wear resistance effectively. Thereby, vanadium’s twofold effect on both the microstructure and the stereological characteristics of carbides makes it a sensible choice of HCCI hardfacing alloying. Reasonable works [17,19–23] have been done relating the effect of vanadium alloy on HCCI microstructure and property. However, these results are focusing on HCCI in casting process. In the current study, the 3.6C-20Cr-Fe HCCI hardfacing layers with varying V additive (0–2.32 wt %) were deposited on low alloy steel D32 by electroslag surfacing. Furthermore, the effects of V additive on the microstructure, carbides, the corresponding hardness and wear resistance of electroslag HCCI hardfacing layer were discussed in detail. This work is intended to provide data that will underpin future development of the HCCI electroslag surfacing and will also be useful for the production operations.

2. Materials and Methods In this study, HCCI hardfacing layers were deposited on low alloy steel plate D32 (Thickness: 30 mm) by electroslag surfacing. The chemical composition of D32 and schematic of electroslag surfacing are listed in Table1 and shown in Figure1 respectively. The microstructure of D32 steel is ferrite and pearlite. During the surfacing process, the consumable guide tube (ISO-C101, Outer dia. 10 mm, inner dia. 4 mm) was inserted successively and vertically in the electroslag surfacing plate cavity. The cavity contains a 30 × 30 mm2 water-cooled copper block and a run-on tab. The run-on tab is connected to the power supply. The other pole of the electrical power is connected to the guide tube. For the first step of the process, some slags (Sintered base CaF2-CaO-Al2O3) were inserted in the run-on tab and then melted by turning on the power (U-40 V, I-320 A). The liquid slag filled the plate cavity. Afterwards, more slags were continuously added to the slag bath with the aim of increasing the electrical resistance. The slags were used for heating the solid material and producing a controlled molten metal bath from the consumable guide tube and flux cored wire. To investigate the effect of V additive on HCCI hardfacing layers, the mass fraction of ferrovanadium added into the flux cored wire was 0 wt %, 2 wt %, 4 wt % and 6 wt % respectively. The chemical contents of each group are listed in Table2.

Table 1. Chemical composition of D32 wt %.

Materials C Si Mn P S Cu Cr Ni Nb V Ti Mo Al Fe D32 0.13 0.22 1.30 0.01 0.01 0.32 0.20 0.39 0.04 0.08 0.02 0.05 0.02 Bal.

Table 2. Chemical compositions of high chromium cast iron hardfacing layer wt %.

Hardfacing C Cr V Si Mn Fe 1# 3.62 19.48 – 0.86 2.31 Bal. 2# 3.59 20.19 0.83 0.68 2.25 Bal. 3# 3.58 19.40 1.50 0.68 2.06 Bal. 4# 3.56 19.57 2.32 0.64 1.91 Bal. Metals 2018, 8, 458 3 of 16 Metals 2018, 8, x FOR PEER REVIEW 3 of 16

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FigureFigure 1. 1.Schematic Schematic ofof the electroslag surfacing. surfacing.

The hardfacing samples for metallography were polished to mirror finish and etched with TheVillella’s hardfacing reagent (5 samples mL HCl, forFigure 1 g metallographypicric 1. Schematic acid in 100of the weremL electroslag ethanol) polished surfacing. for 30 to s mirror to reveal finish the microstructure. and etched with Villella’sThe microstructure reagent (5 mL of HCl, samples 1 g picricwas characterized acid in 100 mLusing ethanol) an optical for microscope 30 s to reveal (OM) the and microstructure. scanning The hardfacing samples for metallography were polished to mirror finish and etched with The microstructureelectron microscope of samples (SEM). T washe area characterized fraction of M using7C3 carbides an optical present microscope in the microstructure (OM) and scanningwas Villella’s reagent (5 mL HCl, 1 g picric acid in 100 mL ethanol) for 30 s to reveal the microstructure. electronmeasured microscope by Image (SEM).-Pro-Plus The using area d fractionigitalized of pictures M7C3. Tcarbideshe average present carbide in area thes A microstructure at the position was measuredThewere microstructure used by Image-Pro-Plus to determine of samples the using refining was digitalized characterizedeffect of vanadium pictures. using onan The theoptical average carbides, microscope carbide which can(OM) areas be and Adescribed at scanning the position by electron microscope (SEM). The area fraction of M72C3 carbides present in the microstructure was wereEquation used to determine(1). Where S the is the refining area ofeffect the carbides, of vanadium μm ; n is on the the number carbides, of the which carbides can in be one described image. by measured by Image-Pro-Plus using digitalized pictures. The average carbide areas A at the position EquationIt can (1). be seen Where thatS the is thesmaller area the of A the, the carbides, finer the µcarbides.m2; n is With the numberthis arrangement, of the carbides 20 images in of one each image. werespecimen used at to magnification determine the of refining 500 times effect are randomlyof vanadium selected on the and carbides, processed which for canthe calculationbe described of byA. It canEquation be seen ( that1). Where the smaller S is the the area A, of the the finercarbides, the carbides.μm2; n is the With number this arrangement,of the carbides 20in imagesone image. of each specimenIt can atbe magnification seen that the smaller of 500 the times A, the are finer randomly theA =carbides. S/n selected With and thisprocessed arrangement, for 20 the images calculation of each(1) of A. specimenThe phaseat magnification composition of 500of the times samples are randomly was studied selected by andX-ray processed diffractometer for the calculation(XRD) with of Cu A. A = S/n (1) rotating anode. The composition of carbide andA = S/nthe matrix was measured by using atom probe(1) tomography. The Fe-C equilibrium phase diagram with different V additive was calculated by The phase composition of the samples was studied by X-ray diffractometer (XRD) with Cu softwareThe sphase Thermo composition-Calc (Version: of the P, samplesdatabases: was TCFE2, studied Stockholm, by X-ray Sweden) diffractometer and JMatPro (XRD) (Version:with Cu rotating7.0rotating, anode.Guildford, anode. The UKThe composition). compositionin this work. of of The carbidecarbide hardness and ofthe the different matrix matrix washardfacing was measured measured layers by byusingwas using measured atom atom probe by probe tomography.conventionaltomography The. RockwellThe Fe-C Fe-C equilibrium testerequilibrium in C scale phasephase (1470 diagramdi agramN load withand with adifferent diamond different V cone Vadditive additive indenter was wastypical calculated calculated for this by by softwaresscale).software Thermo-Calc Thes Thermo abrasive-Calc wear (Version: (Version: behavior P, P, databases:of databases: HCCI hardfacing TCFE2, Stockholm,layer Stockholm, was investigated Sweden) Sweden) and by and JMatProthe JMatProbelt sander(Version: (Version: of 7.0, Guildford,type7.0, Guildford, P40-X. UK).The UK wear in). thisintests this work.were work. performed The The hardnesshardness with a of ofstanda different differentrd ASTM hardfacing hardfacing G65-2004. layers layersAs shownwas was measured in measuredFigure by 2, by conventionalconventionalrounded Rockwellquartz Rockwell particles tester tester on in the C in scale beltC scale were (1470 (1470 with N load Nmean load and diameter and a diamond a diamond between cone cone 100 indenter μm indenter and typical14 typical5 μm. for The for this beltthis scale). scale). The abrasive wear behavior of HCCI hardfacing layer was investigated by the belt sander of The abrasivewas changed wear after behavior each test. of HCCI The normal hardfacing loads, layer rate of was revolution investigated and wear by the distance belt sander for each of sample type P40-X. type P40-X. The wear tests were performed with a standard ASTM G65-2004. As shown in Figure 2, The wearwere tests100 N, were 240 performedr/min and 200 with m, a respectively. standard ASTM The weight G65-2004. loss of As each shown specimen in Figure was2 ,measured rounded on quartz anrounded electronic quartz balance particles with on the the accuracy belt were of with 0.1 mg. mean The diameter wear surface between of each 100 μmsample and was145 μm.observed The belt by particles on the belt were with mean diameter between 100 µm and 145 µm. The belt was changed SEM.was changed after each test. The normal loads, rate of revolution and wear distance for each sample afterwere each 100 test. N, The 240 normalr/min and loads, 200 m, rate respectively. of revolution The weight and wear loss distanceof each specimen for each was sample measured were on 100 N, 240 r/minan electronic and 200 balance m, respectively. with the accuracy The weight of 0.1 mg. loss The of wear each surface specimen of each was sample measured was observed on an electronic by balanceSEM. with the accuracy of 0.1 mg. The wear surface of each sample was observed by SEM.

Figure 2. Schematic of abrasion test apparatus. 1-sand belt, 2-Steel disc, 3-Specimen, 4-Ever arm, 5-Weight.

FigureFigure 2. 2Schematic. Schematic of of abrasion abrasion testtest apparatus.apparatus. 1 1-sand-sand belt, belt, 2 2-Steel-Steel disc, disc, 3 3-Specimen,-Specimen, 4- 4-EverEver arm, arm, 5-Weight 5-Weight..

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3.3.3. Results Results Results and and and Discussion Discussion Discussion

3.13.13.1.. .Interface Interface Interface of of of the the the HCCI HCCI HCCI Electroslag Electroslag Electroslag Hardfacing Hardfacing Hardfacing Layer Layer Layer and and and Low Low Low Alloy Alloy Alloy Steel Steel TheTheThe interfaceinterface interface betweenbetween between HCCIHCCI HCCI hardfacinghardfacing hardfacing layerlayer layer andand and lowlow low alloyalloy alloy steelsteel steel isis is foundfound found defectdefect defect-free--freefree asas as shownshown shown ininin FigureFigure Figure 33aa.a. . FigureFigure Figure 333bbb shows shows that that anan austeniausteni austenitictictic bandband band isis is formedformed formed inin in thethe the interface,interface, interface, adjacentadjacent adjacent toto to whichwhich which iiss hypoeutectichypoeutecticis hypoeutectic microstructurmicrostructur microstructureee ofof primary ofprimary primary austeniteaustenite austenite andand and eutecticeutectic eutectic mixture.mixture. mixture. HowevHowev However,er,er, thethe thefractionfraction fraction ofof primaryprimaryof primary austeniteaustenite austenite decreasedecrease decreasesss withwith with distancedistance distance awayaway away fromfrom from thethe the interface.interface. interface. BecauseBecause Because ofof thethe of thevariationvariation variation ofof microstructure,microstructure,of microstructure, a a perfect perfect a perfect metallurgical metallurgical metallurgical bonding bonding bonding between between between HCCI HCCI HCCI and and low low and alloy alloy low steel steel alloy is issteel obtained. obtained. is obtained. A Afterfter electroslagelectroslagAfter electroslag surfacing,surfacing, surfacing, thethe microstructuremicrostructure the microstructure ofof substratesubstrate of substrate isis stillstill is ferriteferrite still ferrite andand pearlite. andpearlite. pearlite. InIn addition,addition, In addition, thethe temperaturetemperaturethe temperature filedfiled ﬁled ofof elecelec of electroslagtroslagtroslag surfacingsurfacing surfacing waswas was measuredmeasured measured byby by infraredinfrared infrared imagingimaging imaging devicedevice device asas as shownshown shown inin in ◦ FigureFigureFigure 444.. . The TheThe temperature temperaturetemperature gradients gradientsgradients on onon both bothboth XX and and ZZ directiondirection areare lesslessless thanthanthan 23.1 223.13.1 °C°CC/mm.//mm.mm. WithWith With thethe the assistanceassistanceassistance of of of software software software JMatPro, JMatPro, JMatPro, the the the maximun maximun maximun thermal thermal thermal stress stress stress is is is calculated calculated calculated as as as 29.5 29.5 29.5 MPa, MPa, MPa, which which which is is is much much much lesslessless than than than the the the yields yields yields stress stress stress of of of the the substrate substrate ( ( (RRREhEh > >> 365 365 365 MPa MPa MPa [ [24 [2424]]).]).). So So So electroslag electroslag electroslag surfacing surfacing surfacing is is is an an an effective effective effective methodmethodmethod of of of depositing depositing depositing HCCI HCCI HCCI hardfacing hardfacing hardfacing layer layer on on low low alloy alloy steel steel that that produces produces sound sound interface. interface.

Figure 3. Interface of the HCCI hardfacing layer (a) The macro-profile, (b) The micro-structure. FigureFigure 3 3.. InterfaceInterface of of the the HCCI HCCI hardfacing hardfacing layer layer ( (aa)) The The macro macro-proﬁle,-profile, ( (bb)) The The micro micro-structure.-structure.

Figure 4. Temperature ﬁled of the back surface of D32 during the electroslag surfacing processing. FigureFigure 4 4. .Temperature Temperature filed filed of of the the back back surface surface of of D32 D32 during during the the electroslag electroslag surfacing surfacing processing processing. .

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3.2.3.2. Microstructure of V Alloyed HCCI Hardfacing Layers Detailed microstructural analysis of HCCI hardfacing layer layerss with different V content has been investigated. OM OM micrographs micrographs are are showing showing in in Figure Figure 55,, whereaswhereas phasephase diagramdiagram shownshown inin FigureFigure6 6, , is calculatedcalculated toto understandunderstand thethe effecteffect ofof VV onon microstructure.microstructure. Figure5 a5a shows shows that that without without the the Vadditive, V additive the, microstructurethe microstructure is consisting is consist ofing eutectic of eutectic and bulky and primarybulky primary M7C3 carbides. M7C3 carbides Figure. 6Figurea illustrates 6a illustrate that thes that solidification the solidification starts with starts the with formation the formation of primary of ◦ ◦ γ carbideprimary M carbide7C3 at M 13007C3 atC. 1300 When °C. the When temperature the temperature is decreased is decreas to 1278ed toC, 1278 the °C austenite, the austenite ( ) and (γ) M and7C3 carbidesM7C3 carbides simultaneously simultaneously develop develop and form and theform eutectic. the eutectic. TheThe microstructure of HCCI hardfacinghardfacing layer with 0.83 wt % V is still consisting of eutectic and primary M7C33 asas shown shown in in Figure 55b;b; however,however, thethe primaryprimary carbidescarbides areare refinedrefined asas comparedcompared toto Figure5 5a.a. FigureFigure6 b6b shows shows the the eutectic eutectic point point shifts shifts to to the the right right side side in in the the phase phase diagram. diagram. Therefore, Therefore, the temperaturetemperature rangerange ofof “L“L ++ MM77C3”” phase phase zone is smaller than the temperature range shown in Figure6 6a,a,so sothe thegrain grain growth growth time time of of primary primary carbides carbides is is shorter shorter accordingly. accordingly. Figure5 c5c shows shows that that when when the the V concentration V concentration is raised is raised to 1.50 to wt 1.50 %, thewt microstructure%, the microstructure is eutectic. is Aseutectic. shown As in shown Figure 6inc, Figure the C content6c, the C of content eutectic of point eutectic is 3.6 point wt %, is which 3.6 wt is %, same which to theis same C content to the of C HCCIcontent hardfacing of HCCI hardfacing layers. So thelayers formed. So the microstructure formed microstructure is eutectic only.is eutectic only. TheThe microstructure ofof HCCIHCCI hardfacinghardfacing layerlayer withwith 2.322.32 wtwt % V is consistingconsisting of primary austenite and eutectic as shown in Figure5 5d.d. FigureFigure6 d6d shows shows that that when when the the V V content content is is 2.32 2.32 wt wt %, %, the the position position of eutectic point movesmoves to the right side a littlelittle more.more. TheThe solidificationsolidification of HCCI passes through the “L+“L+γ”γ” and “L+“L+γ+M77CC33”” phase phase regions regions sequentially sequentially,, so so primary primary austenite austenite and eutectic microstructure is obtained after cooling.

Figure 5.5. Microstructure of 3.6C 3.6C-20Cr-Fe-20Cr-Fe HCCI HCCI hardfacing hardfacing layers layers with with different different V, V, ( (aa)) 0 0 V V,, ( (bb)) 0.83 0.83 V, V, ((c)) 1.501.50 V, ((dd)) 2.322.32 V.V.

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Figure 6 6.. PhasePhase diagrams diagrams of of the the hardfacing hardfacing layers layers with with (a) (3.6Ca) 3.6C-20Cr-Fe,-20Cr-Fe, (b) (3.6Cb) 3.6C-20Cr-Fe-0.83V,-20Cr-Fe-0.83V, (c) (3.6Cc) 3.6C-20Cr-Fe-1.50V,-20Cr-Fe-1.50V, (d) ( d3.6C) 3.6C-20Cr-Fe-2.32V.-20Cr-Fe-2.32V.

The X-ray diffraction (XRD) spectrum of hardfacing layers with varied V content is shown in The X-ray diffraction (XRD) spectrum of hardfacing layers with varied V content is shown Figure 7. Line (a) shows that the 3.6C-20Cr-Fe HCCI hardfacing layer contains M7C3 and austenite in Figure7. Line (a) shows that the 3.6C-20Cr-Fe HCCI hardfacing layer contains M 7C3 and phase. Line (b), (c) and (d) show that by increasing V, martensite is formed in the hardfacing layers. austenite phase. Line (b), (c) and (d) show that by increasing V, martensite is formed in the As shown in Figure 8a, the eutectic colony in 3.6C-20Cr-Fe hardfacing layer comprises of eutectic hardfacing layers. As shown in Figure8a, the eutectic colony in 3.6C-20Cr-Fe hardfacing layer austenite and carbides. While in 3.6C-20Cr-Fe-1.50V hardfacing layer, as shown in Figure 8b, the comprises of eutectic austenite and carbides. While in 3.6C-20Cr-Fe-1.50V hardfacing layer, as shown in eutectic colony comprises of eutectic austenite and plate martensite surrounding eutectic carbides. Figure8b, the eutectic colony comprises of eutectic austenite and plate martensite surrounding eutectic This is because after adding V, more eutectic carbides are formed in the hardfacing layer, so the C carbides. This is because after adding V, more eutectic carbides are formed in the hardfacing layer, and Cr contents in the austenite are decreasing, which has raised the Ms temperature of austenite so the C and Cr contents in the austenite are decreasing, which has raised the Ms temperature of and promoted the transformation of martensite [4]. austenite and promoted the transformation of martensite [4].

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Figure 7.7. XRD of the hardfacing layer layer with with different different V V contents, contents, ( (a)a) 0 0 V V,, (b) (b) 0.83 0.83 V, V, (c) (c) 1.50 1.50 V, V, (d) (d) 2.32 V V.. Figure 7. XRD of the hardfacing layer with different V contents, (a) 0 V, (b) 0.83 V, (c) 1.50 V, (d) 2.32 V.

Figure 8. Microstructure of eutectic zone of HCCI hardfacing layer, (a) 3.6C-20Cr-Fe, (b) 3.6C-20Cr- Figure 8. Microstructure of eutectic zone of HCCI hardfacing layer, (a) 3.6C-20Cr-Fe, (b) 3.6C-20Cr- FigureFe-1.50V 8.. Microstructure of eutectic zone of HCCI hardfacing layer, (a) 3.6C-20Cr-Fe, (b) 3.6C-20Cr-Fe-1.50V. Fe-1.50V. From the above discussion, the microstructure of 3.6C-20Cr-Fe hardfacing layer is primary From the above discussion, the microstructure of 3.6C-20Cr-Fe3.6C-20Cr-Fe hardfacing layer is primary carbides and eutectic. With the V content increasing, the eutectic point moves to the right side, the carbides and and eutectic. eutectic. With With the the V Vcontent content increasing, increasing, the the eutecti eutecticc point point moves moves to the to theright right side, side, the eutectic area is increasing until the microstructure of hardfacing layer is only eutectic. When the V theeutectic eutectic area area is increasing is increasing until until the the microstructure microstructure of of hardfacing hardfacing layer layer is is only only eutectic. eutectic. When When the the V content exceeds the limited value, primary austenite and eutectic colonies formed in the content exceedsexceeds the the limited limited value, value, primary primary austenite austenite and eutectic and colonies eutectic formed colonies in the formed microstructure. in the microstructure. In addition, V alloying also promotes the formation of martensite in the HCCI Inmicrostructure. addition, V alloying In addition, also promotes V alloying the formationalso promotes of martensite the formation in the HCCIof martensite hardfacing in layers.the HCCI hardfacing layers. hardfacing layers. 3.3. Effect of V on Carbides in HCCI Hardfacing Layers 3.3. Effect of V on Carbides in HCCI Hardfacing Layers 3.3.1.3.3. Effect Carbide of V on Types Carbides in HCCI Hardfacing Layers 3.3.1. Carbide Types 3.3.1.Figure Carbide6 shows Types that only M 7C3 type carbides are formed in the 3.6C-20Cr-Fe hardfacing layer. WhenFigure 0.83 wt6 shows % and that 1.50 only wt %M V7C are3 type added, carbides carbide are formed in the hardfacing in the 3.6C layers-20Cr-Fe is Mhard7C3facing. When layer. the Figure 6 shows that only M7C3 type carbides are formed in the 3.6C-20Cr-Fe hardfacing layer. VWhen content 0.83 iswt 2.32 % and wt %,1.50 carbides wt % V inare the added, hardfacing carbide layersin the arehardfacing M7C3 and layers MC. is EnergyM7C3. When dispersive the V When 0.83 wt % and 1.50 wt % V are added, carbide in the hardfacing layers is M7C3. When the V spectroscopycontent is 2.32 (EDS) wt analysis%, carbides of the in matrix, the hardfacing M7C3 and MClayers are performedare M7C3 and as shown MC. inEnergy Figures dispersive9 and 10, content is 2.32 wt %, carbides in the hardfacing layers are M7C3 and MC. Energy dispersive andspectroscopy the results (EDS) are shownanalysis in of Table the 3mat. Byrix, comparing M7C3 and FigureMC are9e,f, performed it can be as seen shown that in a Figure little ofs V9 and is spectroscopy (EDS) analysis of the matrix, M7C3 and MC are performed as shown in Figures 9 and 10, and the results are shown in Table 3. By comparing Figure 9e,f, it can be seen that a little of V is 10, and the results are shown in Table 3. By comparing Figure 9e,f, it can be seen that a little of V is

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Metals 2018, 8, x FOR PEER REVIEW 8 of 16 dissolved in austenite, while V is mostly existed in M7C3 carbides. As shown in Table3, the chemical dissolved in austenite, while V is mostly existed in M7C3 carbides. As shown in Table 3, the chemical composition of M7C3 in 3.6C-20Cr-Fe HCCI hardfacing layer is (Cr4.7Fe2.3)C3. With increasing V composition of M7C3 in 3.6C-20Cr-Fe HCCI hardfacing layer is (Cr4.7Fe2.3)C3. With increasing V content to 0.83 wt % and 1.50 wt %, V atoms has substituted a part of Cr and Fe atoms into M C , content to 0.83 wt % and 1.50 wt %, V atoms has substituted a part of Cr and Fe atoms into M7C3, the 7 3 the compositioncomposition ofof which is is (Cr (Cr4.6Fe4.62.2FeV2.20.2)CV30.2 and)C 3(Crand4.5Fe (Cr2.1V4.50.4)CFe3,2.1 respectively.V0.4)C3, respectively. With increasing With V content increasing V contentfurther further to 2.32 to wt 2.32 %, wtmore %, V more atoms V has atoms replaced has replacedCr atoms Crin the atoms M7C in3 carbide the M 7andC3 carbideits chemical and its chemicalcomposition composition is (Cr is4.4Fe (Cr2.1V4.40.5Fe)C2.13. ItV 0.5is well)C3. known It is well that known the atomic that number the atomic of vanadium number and of vanadium chromium and chromiumare 23 are and 23 24, and respectively. 24, respectively. Their atomic Their atomicradii are radii similar, are similar,but the butelectrons the electrons in the d inlayer the of d layerthe of the vanadiumvanadium atom atom are are less less thanthan chromiumchromium atom. atom. So So the the vanadium vanadium have have stronger stronger affinity afﬁnity with with carbon carbon than chromium, thereby V can replace a part of Cr in M7C3 type carbides. than chromium, thereby V can replace a part of Cr in M7C3 type carbides.

Figure 9. EDS analysis of different phases in HCCI hardfacing layers with different V contents, 1-EDS Figure 9. EDS analysis of different phases in HCCI hardfacing layers with different V contents, 1-EDS test point of matrix, 2-EDS test point of M C (a) 0 V, (b) 0.83 V, (c) 1.50 V, (d) 2.32 V, (e) EDS of matrix, test point of matrix, 2-EDS test point of M7 7C3 3 (a) 0 V, (b) 0.83 V, (c) 1.50 V, (d) 2.32 V, (e) EDS of matrix, (f) EDS(f) of EDS M7 ofC 3M. 7C3.

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FigureFigure 10 10.. EDSEDS of of MC MC carbide carbide in in 3.6C 3.6C-20Cr-Fe-2.32V-20Cr-Fe-2.32V hardfacing layer layer..

TableTable 3 3.. ChemicalChemicalFigure 10 component component. EDS of MC of carbide carbides in 3.6C in HCCI -20Cr- Fehardfacing -2.32V hardfacing layers with layer .different V.V.

Table 3. ChemicalHardfacing component Layer of carbides inM HCCI7C3 hardfacing layersMC with different V. Hardfacing Layer M7C3 MC 3.6C-20Cr-Fe (Cr4.7Fe2.3)C3 -- 3.6C-20Cr-FeHardfacing Layer (CrM74.7C3Fe 2.3)C3 MC – 3.6C-20Cr-Fe-0.83V (Cr4.6Fe2.2V0.2)C3 -- 3.6C-20Cr-Fe-0.83V3.6C-20Cr-Fe (Cr (Cr4.74.6FeFe2.32.2)CV3 0.2)C3 -- – 3.6C-20Cr-Fe-1.50V3.6C3.6C-20Cr-20Cr-Fe-Fe-1.50V-0.83V (Cr(Cr (Cr4.54.6Fe4.52.22.1FeVV2.10.20.4)CV)C0.43 3 )C3 -- -- – 3.6C-20Cr-Fe-2.32V (Cr Fe V )C (Cr V )C 3.6C3.6C-20Cr-20Cr-Fe-Fe-2.32V-1.50V (Cr(Cr4.44.5FeFe4.42.1V V2.10.40.5)C)C0.53 3 3(Cr0.23-- V0.770.23)C 0.77 3.6C-20Cr-Fe-2.32V (Cr4.4Fe2.1 V0.5)C3 (Cr0.23V0.77)C When V contentcontent isis 2.322.32 wt wt %, %, the the amount amount of of V V solid solid solution solution in in austenite austenite reaches reaches the the limit, limit, and and the When V content is 2.32 wt %, the amount of V solid solution in austenite reaches the limit, and thecontent content ofV of in V Min7 MC37Ccarbide3 carbide also also reaches reaches the the saturation saturation limit. limit. At At thisthis time,time, asas shown in Figure 1010,, the content of V in M7C3 carbide also reaches the saturation limit. At this time, as shown in Figure 10, the excess V is present in MC carbidescarbides which are precipitatedprecipitated fromfrom austenite.austenite. Figure 11 shows the the excess V is present in MC carbides which are precipitated from austenite. Figure 11 shows the equilibrium phase mass friction of the 3.6C-20Cr-Fe-2.32V HCCI hardfacing layer. It illustrates that equilibriumequilibrium phase phase mass mass friction friction of theof the 3.6C-20Cr-Fe-2.32V 3.6C-20Cr-Fe-2.32V HCCI HCCIhardfacing hardfacing layer.layer. It It illustrates illustrates that that the the precipitation temperature of MC carbides is almost◦ 990 °C, which is lower than the solidus precipitationthe precipitation temperature temperature of MCcarbides of MC carbides is almost is 990almostC, which990 °C, is which lower is than lower the than solidus the temperaturesolidus ◦ 1240temperaturetemperatureC. So the1240 MC1240 °C. carbides°CSo. Sothe the MC MC are carbides carbides precipitated are are precipitated precipitated from austenite from from austenite asaustenite secondary as assecondary secondary carbides. carbides. carbides. EDS EDS reveals EDS revealthat thesereveals that MCs thatthese secondary these MC MC secondary carbidessecondary ca are carbidesrbides enriched areare enriched inenriched V and in Crin V V elements and and Cr Cr elements with elements the with chemical with the thechemical formula chemical of (Crformula0.23formulaV0.77 of )C.(Cr of The 0.(Cr23V0. same0.2377V)C0.77.)C typeThe. The ofsame carbidesame type type was ofof carbide foundcarbide by waswas de found Mellofound by [ 25by de] andde Mello Mello Mirjana [25 ][ 25and M.] and Filipovi´c[Mirjana Mirjana M.15 ]M. in Filipovićiron-chromium-carbon-vanadiumFilipović [15] [in15 ]iron in iron-chromium-chromium-carbon-carbon white-vanadium- castvanadium irons. white cast cast irons. irons.

FigureFigure 11. Equilibrium11. Equilibrium phase phase mass mass friction friction of the 3.6C 3.6C-20Cr-Fe-2.32V-20Cr-Fe-2.32V HCCI HCCI hardfacing hardfacing layer. layer.

Figure 11. Equilibrium phase mass friction of the 3.6C-20Cr-Fe-2.32V HCCI hardfacing layer.

Metals 2018, 8, 458 10 of 16

In the HCCI hardfacing alloy, the formation of carbides is related to the system energy. Thermodynamics can be used to analyze the precipitation possibility of VC carbides. The reactions equation and free energy of VC can be described in Equation (2):

0 [V] + [C] = VC(S) ∆GV = −104038 + 97.45T (2)

0 where ∆GV is the Gibbs free energy of V, T is the Kelvin temperature. The energy change ∆G in the formation of VC carbide can be calculated using the Gibbs free energy given by Equation (3):

∆G = ∆G0 − RT ln(f VωVf CωC) = ∆G0 − RT (lnf V + lnωV + lnf C + lnωC) (3) where, f V and f C are the activity coefficients of V and C, respectively. ωV and ωC are the mass fractions of V and C in the hardfacing alloy. When the equilibrium reaches, then the activity coefficient can be expressed as: 1873 ln 10 ln f = lg f 1873 (4) x T x 1873 x j lg fx = exωx + ∑ exωj (5) j where, ex is the interaction coefficient of the j element with the x element, whereas the activity coefficient of V and C can be expressed as:

1873 ln 10 ln f = eVω + eCω + eCrω + eSiω + ··· (6) V T V V V C V Cr V Si 1873 ln 10 ln f = eCω + eCrω + eSiω + eMnω + ··· (7) C T C C C Cr C Si C Mn The interaction coefﬁcient of alloying elements in molten iron at 1600 ◦C is shown in Table4[ 26].

Table 4. The interaction coefﬁcient of alloying elements in molten iron at 1600 ◦C.

Alloying Elements C Cr Si Mn V j 0.14 −0.024 0.08 −0.012 −0.077 eC j −0.34 0.042 0.015 eV

By substituting the composition of the HCCI hardfacing layer and the relevant data from Table4 into Equations (6) and (7), Equations (8) and (9) can be obtained as follows:

lnf V = 0.03 ωV + 2.75 (8)

lnf C = −0.18 ωV + 0.13 (9) If the VC carbides form, the energy change

∆G ≤ 0 (10)

By combining Equations (3), (8)–(10), we can get Equation (11):

0.15 ωV + lnωV ≥ 6.38 (11)

It is well known that, VC can be formed only when the mass fraction of V reaches the requirement of Equation (11). However, in this paper, the mass fraction of V is not enough to meet Equation (11). Therefore, VC cannot be formed as primary carbide. Metals 2018, 8, 458 11 of 16

Meanwhile, by assuming that VC precipitates from the molten iron as primary carbide, then the relation between the VC precipitation temperature and the mass fraction of V and C elements in electroslag molten iron can be expressed by Equation (12) [20].

lg [V][C] = (−105,800 + 94.5 T)/19.1 T (12) where T is Kelvin temperature. [V] and [C] are mean mass fraction in molten iron of V and C elements, respectively. From Equation (12), the precipitated temperatures of VC carbides in Fe-Cr-C-V alloy with 0.83, 1.50 and 2.32 wt % V addition are 1238 K, 1314 K and 1375 K, respectively, which are lower than the precipitatedMetals 2018, temperature8, x FOR PEER REVIEW of primary phases and eutectic carbides. Therefore, VC carbides11 of 16 are the secondarywhere carbides T is Kelvin that temperature. precipitates [V] from and [C] austenite. are mean mass fraction in molten iron of V and C elements, Therefore,respectively. when From V isEquation added to(12 the), the HCCI precipitated hardfacing temperatures layer,as of VVC has carbides stronger in Fe affinity-Cr-C-V toalloy C than Cr, most ofwith the V0.83, is present1.50 and 2.3 in2 M wt7C %3 Vcarbides addition are to replace1238 K, 1314 Cr, K while and 1375 a little K, re ofspe Vctively, is dissolved which are into lower austenite. than the precipitated temperature of primary phases and eutectic carbides. Therefore, VC carbides When V in both austenite and M7C3 carbides reached the saturated limit, (Cr0.23V0.77)C carbides precipitatedare the in secondary the austenite. carbides that precipitates from austenite. Therefore, when V is added to the HCCI hardfacing layer, as V has stronger affinity to C than Cr, most of the V is present in M7C3 carbides to replace Cr, while a little of V is dissolved into 3.3.2. Area Fraction and Size of M7C3 Carbides austenite. When V in both austenite and M7C3 carbides reached the saturated limit, (Cr0.23V0.77)C Thecarbides area fractions precipitated of carbides in the austenite. against various V concentrated samples are show in Figure 12. It can be seen that, the area fraction of Primary M7C3 carbides and eutectic M7C3 carbides in 3.6C-20Cr-Fe 3.3.2. Area Fraction and Size of M7C3 Carbides HCCI hardfacing layer are 15.7% and 25.4%, respectively. When the V concentration is 0.83 wt %, the area fractionThe area of primary fractions carbidesof carbides is against decreased various to 10.1%V concentrated while the samples area fraction are show of in eutectic Figure 12 carbides. It is can be seen that, the area fraction of Primary M7C3 carbides and eutectic M7C3 carbides in 3.6C-20Cr- increased to 34.5%. In 3.6C-20Cr-Fe-1.50V HCCI hardfacing layers no further primary carbide growth Fe HCCI hardfacing layer are 15.7% and 25.4%, respectively. When the V concentration is 0.83 wt %, is observed.the area The fraction hardfacing of primary has carbides the maximum is decreased carbides to 10.1% areawhile fraction the area fraction of 46.9%. of eutectic By increasing carbides the V concentrationis increased to 2.32%, to 34.5%. with In the 3.6C formation-20Cr-Fe-1.50V of primary HCCI hardfacing austenite, layers the area no further fraction primary of eutectic carbide carbides is decreasedgrowth to is 44.7%. observed. The hardfacing has the maximum carbides area fraction of 46.9%. By increasing the V concentration to 2.32%, with the formation of primary austenite, the area fraction of eutectic Figure 13 shows the size variation of M7C3 carbides in HCCI hardfacing layers with different V carbides is decreased to 44.7%. additive. The average area of primary M7C3 carbides and eutectic carbides in 3.6C-20Cr-Fe hardfacing Figure 13 shows the size variation of M7C3 carbides in HCCI hardfacing layers with different V layer is 422.3 µm2 and 43.5 µm2 respectively, determined by Equation (1). When the V content is additive. The average area of primary M7C3 carbides and eutectic carbides in 3.6C-20Cr-Fe hardfacing 2 2 0.83 wt %,layer the is 422.3 areas μm of2 primaryand 43.5 μm carbides2 respectively, and eutectic determined carbides by Equation are refined (1). When to the 172.9 V contentµm and is 0.83 37.1 µm , respectively.wt %, Whenthe area thes of Vprimary concentration carbides and is raised eutectic to carbides 1.50 wt are %, refined the growth to 172.9 of μ them2 and primary 37.1 μm carbide2, is completelyrespectively. suppressed When and the theV concentration eutectic carbides is raised are to 1.5 further0 wt % refined., the growth The of microstructure the primary carbide is composed is of smallcompletely eutectic carbidessuppressed with and anthe averageeutectic carbides area of are 28.8 furtherµm refined2. However,. The microstructure when the V is concentrationcomposed is of small eutectic carbides with an average area of 28.8 μm2. However, when the V concentration is raised to 2.32 wt %, the solid-liquid coexistence temperature range becomes wider comparing to that raised to 2.32 wt %, the solid-liquid coexistence temperature range becomes wider comparing to that with 1.50with wt 1.50 % V wt additive. % V additive. The Th eutectice eutectic carbides carbides inin 3.6C-20Cr-Fe-2.32V3.6C-20Cr-Fe-2.32V hardfacing hardfacing layer layerhave grown have grown 2 bigger withbigger an with average an average area area of 40.3 of 40.3µm μm. 2.

FigureFigure 12. Area 12. Area fraction fraction of Mof 7MC7C3 3carbides carbides in in different different HCCI HCCI hardfacing hardfacing layers. layers.

Metals 2018, 8, 458 12 of 16 Metals 2018, 8, x FOR PEER REVIEW 12 of 16 Metals 2018, 8, x FOR PEER REVIEW 12 of 16

Figure 13. Average area of M7C3 carbides in different HCCI hardfacing layers. Figure 13.13. Average area ofof MM77C3 carbidescarbides in in different different HCCI hardfacing layers. 3.4. Hardness and Wear Resistance of V Alloyed HCCI Electroslag Hardfacing Layers 3.4.3.4. Hardness and Wear Resistance of V Alloyed HCCI Electroslag Hardfacing LayersLayers

3.4.1.3.4.1. Hardness Figure 1414 showsshows thethe hardnesshardness of of HCCI HCCI hardfacing hardfacing layer layer with with different different V V content. content The. The hardness hardne ofss of HCCI hardfacing layer without V is 58.5 HRC. When the V is 0.83 wt %, the primary carbide M7C3 HCCIof HCCI hardfacing hardfacing layer layer without without V isV 58.5is 58.5 HRC. HRC. When When the the V isV 0.83is 0.83 wt wt %, % the, the primary primary carbide carbide M7 MC37Cis3 reﬁnedis refined and and the the area area fraction fraction of of eutectic eutectic carbides carbides is is increased. increased. Carbides Carbides distributiondistribution isis more uniformuniform in the hardfacing layer, which has raised the hardness up to 59.2 HRC. As V is furtherfurther increased to 1.50 wt %, the content of carbides is increased and all the M7C3 carbides are fine eutectic carbides in 1.50 wtwt %%,, thethe contentcontent ofof carbidescarbides isis increasedincreased andand allall thethe MM77C33 carbides a arere ﬁnefine eutectic carbides in the microstructure. InIn thisthis case,case, the the highest highest hardness hardness of of 61.2 61.2 HRC HRC is i achieved.s achieved. When When the the V isV 2.32is 2.32 wt wt %, the%, the formation formation of softof soft austenite austenite phase phase and and the the grain grain coarsening coarsening have have decreased decreased the the bulk bulk hardness hardness of hardfacingof hardfacing to 60.1to 60.1 HRC. HRC.

Figure 14 14.. ChangesChanges in the hardness and wear volume loss of HCCI hardfacing layer.layer.

Metals 2018, 8, 458 13 of 16 Metals 2018, 8, x FOR PEER REVIEW 13 of 16

3.4.2.3.4.2. Wear ResistanceResistance FigureFigure 14 showsshows that that the the variation variation in in the the wear wear resistance resistance is consistent is consistent with with the hardness. the hardness. The 3 The3.6C- 3.6C-20Cr-Fe20Cr-Fe hardfacing hardfacing layer has layer the hashighest the wear highest volume wear loss volume of 211 mm loss3. As of the 211 primary mm . carbides As the primarycontent is carbides decreasing content and the is eutectic decreasing carbides and content the eutectic is increasing carbides in the content hardfacing is increasing layer, the in wear the 3 hardfacingvolume loss layer, of 3.6C the-20Cr wear- volumeFe-0.83V loss hardfacing of 3.6C-20Cr-Fe-0.83V layer is decreasing hardfacing to 160layer mm3. is The decreasing 3.6C-20Cr to- 160Fe-1.50V mm . Thehardfacing 3.6C-20Cr-Fe-1.50V layer with eutectic hardfacing microstructure layer with has eutectic the least microstructure wear volumehas of 86 the mm least3, which wear i volumes only 40% of 3 86of mmthe 3.6C, which-20Cr is-Fe only hardfacing 40% of the sample. 3.6C-20Cr-Fe Whereas, hardfacing the formation sample. of soft Whereas, austenitic the dendrites formation phase of soft in austeniticthe microstructure dendrites of phase 3.6C in-20Cr the- microstructureFe-2.32V HCCI of hardfacing 3.6C-20Cr-Fe-2.32V layer has also HCCI increased hardfacing the layerwear hasvolume also 3 increasedsignificantly the to wear 181 volumemm3. significantly to 181 mm . UnderUnder normalnormal conditions,conditions, thethe carbidecarbide withwith smallsmall sizesize andand uniformuniform distributiondistribution inin matrixmatrix showshow goodgood wearwear resistance resistance [27 [27,28,28]. It]. wasIt was observed observed that that 3.6C-20Cr-Fe 3.6C-20Cr HCCI-Fe HCCI hardfacing hardfacing layer containslayer contains coarse primarycoarse primary carbide carbide which which can be can seen be more seen clearlymore clearly in Figure in Figure 15a. It15 hasa. Italso has beenalso been observed observed that that the austenitethe austenite region region B with B with lower lower carbon carbon and and chromium chromium contents contents is is surrounding surrounding the the primary primary carbidecarbide regionregion C.C. DuringDuring thethe wearwear process,process, thethe coarsecoarse carbidecarbide fracturingfracturing andand spallingspalling fromfrom thethe matrixmatrix underunder tangentialtangential stressstress [[2929].]. ThisThis carbidecarbide fracturefracture cancan bebe clearlyclearly observedobserved inin FigureFigure 1515bb wherewhere thethe crackedcracked primary carbide leaves a large polygonal cave in its original position. Meanwhile the cracked M C primary carbide leaves a large polygonal cave in its original position. Meanwhile the cracked M77C33 carbidecarbide particles,particles, rollingrolling onon thethe wearwear surface,surface, exacerbateexacerbate thethe wear.wear. WhereasWhereas thethe red red box box marked marked in in Figure Figure 16 a1 shows6a shows that that in the in 3.6C-20Cr-Fethe 3.6C-20Cr HCCI-Fe HCCI hardfacing hardfacing layer, thelayer, fractured the fractured primary primary carbides carbides further further act as secondary act as secondary abrasive abrasive particles particles to cause to more cause primary more carbidesprimary breakage.carbides breakage. When the When V content the V iscontent 0.83 wt is %,0.83 carbides wt %, carbides are refined, are refined, the content the content and size and of primarysize of primary carbides carbides is decreased is decreased and more and uniform moreeutectic uniform carbides eutectic are carbides formed. are As formed. shown inAs Figure shown 16 inb, theFigure plow 16 groovesb, the plow on wear grooves surface on arewear shallower surface andare finershallower than thatand showingfiner than in Figurethat showing 15a. When in Figure the V content15a. When is 1.50 the wt V %, content fine eutectic is 1.50 carbides wt %, are fine distributed eutectic carbides uniformly are in thedistribute hardfacingd uniformly layer, as shownin the inhardfacing Figure 16 layer,c, the plowas shown grooves in Figure on wear 16c surface, the plow are groove more shallowers on wear and surface finer are than more that shallower in Figure 16andb, andfiner no than noticeable that in Figure caves 16 haveb, and been no noticeable observed. caves When have the been V content observed reaches. When to the 2.32 V content wt %, primary reaches austeniteto 2.32 wt is% appeared, primary austenite in the microstructure is appeared in and the a microstructure decline in the hardnessand a decline of the in hardfacingthe hardness layer of the is found,hardfacing whereas layer the is found depth, ofwhereas the plow the groovesdepth of is the deeper, plow whichgrooves indicates is deeper, that which the wear indicates resistance that the is decreasedwear resistance further. is However,decreased becausefurther. ofHowever, the absence because of coarse of the primary absence carbides, of coarse caves primary have notcarbides, been observedcaves have on not the been wear observed surface as on shown the wear in Figure surface 16 asd. shown in Figure 16d.

FigureFigure 15.15.Morphology Morphology of of the the primary primary carbide carbide in 3.6C-20Cr-Fein 3.6C-20Cr- HCCIFe HCCI hardfacing hardfacing layer, layer, (a) before (a) before wear, (wear,b) after (b wear.) after wear.

Metals 2018, 8, 458 14 of 16 Metals 2018, 8, x FOR PEER REVIEW 14 of 16

FigureFigure 16.16. MorphologyMorphology ofof thethe wearwear surfacesurface of of HCCI HCCI hardfacing hardfacing layer layer with with different different V V contents, contents, (a )(a 0)V, 0 (Vb,) ( 0.83b) 0.83 V,( V,c) 1.50(c) 1.50 V, ( dV,) 2.32(d) 2.32 V. V.

4.4. ConclusionsConclusions (1)(1) HighHigh chromiumchromium castcast ironiron cancan be used for surfacing low alloy steel to form form bimetallic bimetallic material material.. TheThe temperaturetemperature getsgets evenlyevenly distributeddistributed in thethe workpieceworkpiece during the electroslag surfacing process, thereforetherefore thethe interfaceinterface betweenbetween hardfacinghardfacing layerlayer andand lowlow alloyalloy steelsteel D32D32 isis defectdefect free.free. (2)(2) T Thehe microstructure microstructure of of 3.6C 3.6C-20Cr-Fe-20Cr-Fe hardfacing hardfacing layer layer is primary is primary carbides carbides and eutectic. and eutectic. With With0.83 0.83wt % wt V, % the V, theprimary primary carbides carbides are are refined refined and and small small amount amount of of martensite martensite formed formed in thethe microstructure. With 1.50 wt % V, the microstructure is a lot of eutectic and a little of martensite. The microstructure. With 1.50 wt % V, the microstructure is a lot of eutectic and a little of martensite. microstructure of 3.6C-20Cr-Fe-2.32V is primary austenite, martensite and eutectic. The microstructure of 3.6C-20Cr-Fe-2.32V is primary austenite, martensite and eutectic. (3) In V alloyed HCCI hardfacing layers, V can replace a part of Cr in M7C3 and (Cr4.4–4.7Fe2.1– (3) In V alloyed HCCI hardfacing layers, V can replace a part of Cr in M7C3 and 2.3V0.2–0.5)C3 type carbides formed, while a little of V is dissolved into austenite. When V atoms in both (Cr4.4–4.7Fe2.1–2.3V0.2–0.5)C3 type carbides formed, while a little of V is dissolved into austenite. When V austenite and M7C3 carbides reached the saturated limit, (Cr0.23V0.77)C carbides precipitate from the atoms in both austenite and M7C3 carbides reached the saturated limit, (Cr0.23V0.77)C carbides austenite. precipitate from the austenite. (4) With the increase of V additive, the M7C3 carbides in the hardfacing layers are refined (4) With the increase of V additive, the M7C3 carbides in the hardfacing layers are refined significantly. The eutectic carbides are refined with an average area of 28.8 μm2 when V content is significantly. The eutectic carbides are refined with an average area of 28.8 µm2 when V content is 1.50 wt %. While when the V concentration is raised to 2.32 wt %, the eutectic carbides have grown 1.50 wt %. While when the V concentration is raised to 2.32 wt %, the eutectic carbides have grown bigger with an average area of 40.3 μm2. bigger with an average area of 40.3 µm2. (5) The HCCI hardfacing layer with 1.50 wt % V exhibited the highest hardness and lowest wear (5) The HCCI hardfacing layer with 1.50 wt % V exhibited the highest hardness and lowest wear loss. It appears that the microstructural refinement by limiting the carbon content in the range of loss. It appears that the microstructural refinement by limiting the carbon content in the range of eutectic composition plays a predominant role in improving the wear resistance of the electroslag eutectic composition plays a predominant role in improving the wear resistance of the electroslag HCCI hardfacing layer. HCCI hardfacing layer.

AuthorAuthor Contributions:Contributions:Conceptualization, Conceptualization, H.W. H.W and. and S.F.Y.; S.F Investigation,.Y.; Investigation, H.W. H and.W A.R.K.;. and A Methodology,.R.K.; Methodology, A.G.H.; ProjectA.G.H.; administration, Project administration, S.F.Y.; Supervision, S.F.Y.; Supervision, S.F.Y.; Writing—original S.F.Y.; Writing— draft,original H.W.; draft, Writing—review H.W.; Writing— andreview editing, and H.W.,editing, A.R.K. H.W. and, A.R A.G.H..K. and A.G.H.

Funding: This research received no external funding.

Metals 2018, 8, 458 15 of 16

Funding: This research received no external funding. Acknowledgments: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Here, the authors want to thank the technical support from the Analytical and Testing Center in Huazhong University of Science and Technology. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Sharma, T.; Maria, S.; Dwivedi, D.K. Abrasive wear behavior of Fe-30Cr-3.6C overlays deposited on mild steel. ISIJ Int. 2005, 45, 1322–1325. [CrossRef] 2. Correa, E.O.; Alcântara, N.G.; Tecco, D.G.; Kumar, R.V. The relationship between the microstructure and abrasive resistance of a hardfacing layer in the Fe-Cr-C-Nb-V system. Metall. Mater. Trans. 2007, 38, 1671–1680. [CrossRef] 3. Zahiri, R.; Sundaramoorthy, R.; Patrick, L.; Subramanian, C. Hardfacing using ferro-alloy powder mixtures by submerged arc welding. Surf. Coat. Technol. 2014, 260, 220–229. [CrossRef] 4. Tang, X.H.; Chung, R.; Pang, C.J.; Li, D.Y.; Hinckley, B.; Dolman, K. Microstructure of high (45 wt %) chromium cast irons and their resistances to wear and corrosion. Wear 2011, 271, 1426–1431. [CrossRef] 5. Wang, H.; Yu, S.F. Inﬂuence of heat treatment on microstructure and sliding wear resistance of high chromium cast iron electroslag hardfacing layer. Surf. Coat. Technol. 2017, 319, 182–190. [CrossRef] 6. Zhou, Q.; Su, J. Chromium Family of Wear Resistant Cast Iron; Xi’an Jiaotong University Press: Xi’an, China, 1986; pp. 111–112. 7. Fulcher, J.K.; Kosel, T.H.; Fiore, N.F. The effect of carbide volume fraction on the low stress abrasion resistance of high Cr-Mo white cast irons. Wear 1983, 84, 313–325. [CrossRef] 8. Sapate, S.; Rama Rao, A. Erosive wear behavior of weld hardfacing high chromium cast irons: Effect of erodent particles. Tribol. Int. 2006, 39, 206–212. [CrossRef] 9. Zhi, X.H.; Liu, J.Z.; Xing, J.D.; Ma, S. Effect of cerium modiﬁcation on microstructure and properties of hypereutectic high chromium cast iron. Mater. Sci. Eng. A 2014, 603, 98–103. [CrossRef] 10. Wang, Y.; Gou, J.F.; Chu, R.Q.; Zhen, D.; Liu, S. The effect of nano-additives containing rare earth oxides on sliding wear behavior of high chromium cast iron hardfacing layers. Tribol. Int. 2016, 103, 102–112. [CrossRef] 11. Zhi, X.H.; Xing, J.D.; Fu, H.G.; Xiao, B. Effect of niobium on the as-cast microstructure of hypereutectic high chromium cast iron. Mater. Lett. 2008, 62, 857–860. [CrossRef]

12. Wu, X.J.; Xing, J.D.; Fu, H.G.; Zhi, X. Effect of titanium on the morphology of primary M7C3 carbides in hypereutectic high chromium white iron. Mater. Sci. Eng. A 2007, 457, 180–185. [CrossRef] 13. Filipovic, M.; Kamberovic, Z.; Korac, M.; Gavrilovski, M. Microstructure and mechanical properties of Fe-Cr-C-Nb white cast irons. Mater. Des. 2013, 47, 41–48. [CrossRef] 14. Filipovic, M.; Romhanji, E.; Kamberovic, Z.; Korac, M. Matrix microstructure and its micro-analysis of constituent phases in as-cast Fe-Cr-C-V alloys. Mater. Trans. 2009, 50, 2488–2492. [CrossRef] 15. Mirjana, M. Filipovi´c,Iron-chromium-carbon-vanadium white cast irons: the microstructure and properties. Hem. Ind. 2014, 68, 413–427. [CrossRef] 16. Wu, L.L.; Xiao, F.R.; Wang, Y.C.; Liang, C.B.; Sun, G.P.; Liao, B. Effect of vanadium on microstructure and wear resistance of Ni-Cr alloyed cast iron. Int. J. Cast Met. Res. 2013, 26, 176–183. [CrossRef] 17. Arnoldo, B.J. Microstructure of vanadium-, niobium-and titanium-alloyed high-chromium white cast irons. Int. J. Cast Met. Res. 2001, 13, 343–361. [CrossRef] 18. Waleed, E.; Rashad, R.; Sayed, E.; Saied, E. Inﬂuence of Vanadium and Boron Additions on the Microstructure, Fracture Toughness, and Abrasion Resistance of Martensite-Carbide Composite Cast Steel. Adv. Mater. Sci. Eng. 2016, 2016, 1203756. [CrossRef] 19. Bedolla-Jacuinde, A.; Guerra, F.V.; Mejía, I.; Zuno-Silva, J.; Rainforth, M. Abrasive wear of V-Nb-Ti alloyed high-chromium white irons. Wear 2015, 332–333, 1006–1011. [CrossRef] 20. Qi, X.; Jia, Z.; Yang, Q.; Yang, Y. Effects of vanadium additive on structure property and tribological performance of high chromium cast iron hardfacing metal. Sur. Coat. Technol. 2011, 205, 5510–5514. [CrossRef] Metals 2018, 8, 458 16 of 16

21. Cemil, C. An investigation of the wear behaviors of white cast irons under different compositions. Mater. Des. 2006, 27, 437–445. [CrossRef] 22. Radulovic, M.; Fiset, M.; Peev, K. The influence of vanadium on fracture toughness and abrasion resistance in high chromium white cast irons. J. Mater. Sci. 1994, 29, 5085–5094. [CrossRef] 23. Ogi, K.; Matsubara, Y.; Matsuda, K. Eutectic solidification of high chromium cast iron-eutectic mechanism of eutectic growth. AFS Trans. 1982, 89, 197–204. 24. Liu, X.Y.; Piao, Z.M.; Wang, X.H.; Zhao, J.; Cao, Z.Q.; Li, H.; Lai, C.B.; Wu, B.; Xu, H.Q.; Huang, Z.Y.; et al. GB 712-2011, Ship and Ocean Engineering Structural Steel; Standards Press of China: Beijing, China, 2011. 25. De Mello, J.D.B.; Durand-Charre, M.; Mathia, T. Abrasion mechanisms of white cast iron II: Influence of the metallurgical structure of V-Cr white cast irons. Mater. Sci. Eng. 1986, 78, 127–134. [CrossRef] 26. Liang, Y.J.; Che, Y.C. Inorganic Thermodynamic Data Manual; North East University Press: Shen Yang, China, 1993; pp. 508–510, ISBN 7-81006-532-7. 27. Radzikowska, J.M. Metallography and microstructures of cast iron. In ASM Handbooks: Metallography and Microstructures; ASM International: Novelty, OH, USA, 2004; pp. 565–587. 28. Wu, H.Q.; Sasaguri, N.; Matsubara, Y.; Hashimoto, M. Solidification of Multi-Alloyed White Cast Iron: Type and Morphology of Carbides. Trans. Am. Fish. Soc. 1996, 140, 103–108. 29. Hao, F.; Li, D.; Dan, T.; Ren, X.; Liao, B.; Yang, Q. Effect of rare earth oxides on the morphology of carbides in hardfacing metal of high chromium cast iron. J. Rare Earth 2011, 29, 168–172. [CrossRef]

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