Effects of Vanadium on Microstructure and Wear Resistance of ... - MDPI

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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 M7 C3 and (Cr4.4–4.7 Fe2.1–2.3 V0.2–0.5 )C3 type carbides are formed. When the V is 2.32 wt %, (Cr0.23 V0.77 )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

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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 M7 C3 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 M7 C3 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 Table 1 and shown in Figure 1 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-Al2 O3 ) 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 Table 2. Table 1. Chemical composition of D32 wt %. Materials C D32

0.13

Si

Mn

P

S

Cu

Cr

Ni

Nb

V

Ti

Mo

Al

Fe

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 1# 2# 3# 4#

C

Cr

V

Si

Mn

Fe

3.62 3.59 3.58 3.56

19.48 20.19 19.40 19.57

– 0.83 1.50 2.32

0.86 0.68 0.68 0.64

2.31 2.25 2.06 1.91

Bal. Bal. Bal. Bal.

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Figure Schematic of of the the electroslag Figure 1. 1.Schematic electroslagsurfacing. surfacing.

The hardfacing samples for metallography were polished to mirror finish and etched with

The hardfacing samples were polished to smirror and etched with Figure 1. Schematic of the electroslag surfacing. Villella’s reagent (5 mL HCl,for 1 g metallography picric acid in 100 mL ethanol) for 30 to revealfinish the microstructure. Villella’s reagent (5 mL HCl, 1 g picric acid in 100 mL ethanol) for 30 s to reveal the microstructure. The microstructure of samples was characterized using an optical microscope (OM) and scanning The hardfacing samples for metallography were polished to mirror finish and etched with The microstructure of samples was characterized using an optical microscope (OM) and scanning electron microscope (SEM). The area fraction of M7C3 carbides present in the microstructure was Villella’s reagent (5 mL HCl, 1 g picric acid in 100 mL ethanol) for 30 s to reveal the microstructure. electron microscope (SEM). Theusing areadigitalized fraction of M7 C3 The carbides present the A microstructure measured by Image-Pro-Plus pictures. average carbideinareas at the position was The of samples was characterized using on an optical microscope (OM) weremicrostructure used to determine theusing refining effect of vanadium the carbides, which can beand described by measured by Image-Pro-Plus digitalized pictures. The average carbide areas A atscanning the position electron microscope (SEM). The area fraction of M 72C3 carbides present in the microstructure was (1). Where Sthe is the area ofeffect the carbides, μm ; n ison the number of the carbides one image. by were Equation used to determine refining of vanadium the carbides, which caninbe described measured by Image-Pro-Plus using digitalized pictures. average carbide areas20 A images at the position It can(1). be seen thatSthe smaller the finer the carbides. this arrangement, each 2 ; nThe Equation Where is the areathe ofA, the carbides, µm isWith the number of the carbides inof one image. were used to determine the refining effect of vanadium on the carbides, which can be described specimen at magnification of 500 times are randomly selected and processed for the calculation of by A. It canEquation be seen (1). thatWhere the smaller the A, the finer the carbides. With this arrangement, 20 images of 2 S is the area of the carbides, μm ; n is the number of the carbides in one image. each A =carbides. S/nselected (1) of A. specimen of 500the times arefiner randomly for20 the calculation It canat bemagnification seen that the smaller A, the the Withand thisprocessed arrangement, images of each specimen at magnification of 500 times are randomly selected processed for the calculation A. The phase composition of the samples was studied by and X-ray diffractometer (XRD) withofCu A = S/n (1) rotating anode. The composition of carbide and the matrix was measured by using atom probe A = S/n (1) tomography. The Fe-C equilibrium phase diagram with different V additive was calculated by The The phase composition of ofthe samples was studied byX-ray X-raydiffractometer diffractometer (XRD) composition the was studied by (XRD) withwith Cu Cu softwaresphase Thermo-Calc (Version: P, samples databases: TCFE2, Stockholm, Sweden) and JMatPro (Version: rotating anode. The composition ofofThe carbide and thedifferent matrixwas wasmeasured measured rotating anode. The composition carbide and of the matrix by by using atomatom probe 7.0, Guildford, UK). in this work. hardness hardfacing layers wasusing measured byprobe tomography. The Fe-C equilibrium phase diagram with different V additive was calculated by by tomography. The Fe-C equilibrium phase diagram with different V additive was calculated conventional Rockwell tester in C scale (1470 N load and a diamond cone indenter typical for this softwares Thermo-Calc (Version: P, databases: TCFE2, Stockholm, Sweden) and JMatPro (Version: softwares Thermo-Calc (Version: P, databases: TCFE2, Stockholm, Sweden) and JMatPro (Version: scale). The abrasive wear behavior of HCCI hardfacing layer was investigated by the belt sander of 7.0, Guildford, in this work. The hardness hardness different hardfacing layers waswas measured type P40-X. UK). TheUK). wear tests were performed with a of standard ASTM G65-2004. As shown inmeasured Figureby 2, by 7.0, Guildford, in this work. The of different hardfacing layers conventional Rockwell tester in C scale (1470 N load and a diamond cone indenter typical for belt this rounded Rockwell quartz particles belt were mean diameter between 100 μm and typical 145 μm.for The conventional testeron in the C scale (1470with N load and a diamond cone indenter this scale). scale). The abrasive wear behavior of HCCI hardfacing layer wasand investigated by the belt sander of was changed after each test. The normal loads, rate of revolution wear distance for each sample The abrasive wear behavior of HCCI hardfacing layer was investigated by the belt sander of type P40-X. type P40-X. The wear performed withThe a standard ASTM G65-2004. As shown in Figureon 2, weretests 100 N, 240performed r/mintests and were 200 m, weight loss of specimen was2,measured The wear were with arespectively. standard ASTM G65-2004. Aseach shown in Figure rounded quartz rounded quartz particles on the belt were with mean diameter between 100 μm and 145 μm. The belt an electronic balance with the accuracy of 0.1 mg. The wear surface of each sample was observed by particles on the belt were with mean diameter between 100 µm and 145 µm. The belt was changed was changed after each test. The normal loads, rate of revolution and wear distance for each sample SEM. after were each 100 test.N,The loads, rate of revolution and wear for eachwas sample wereon 100 N, 240 normal r/min and 200 m, respectively. The weight loss distance of each specimen measured 240 r/min and 200 m, respectively. The weight lossThe of wear each surface specimen wassample measured on an electronic an electronic balance with the accuracy of 0.1 mg. of each was observed by SEM. balance 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.

Figure Schematicofofabrasion abrasiontest testapparatus. apparatus. 1-sand arm, 5-Weight. Figure 2. 2.Schematic 1-sand belt, belt,2-Steel 2-Steeldisc, disc,3-Specimen, 3-Specimen,4-Ever 4-Ever arm, 5-Weight.

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3.Results Resultsand andDiscussion Discussion 3.3. Results and Discussion 3.1.Interface Interfaceofof ofthe theHCCI HCCIElectroslag ElectroslagHardfacing HardfacingLayer Layerand andLow LowAlloy AlloySteel Steel 3.1. 3.1. Interface the HCCI Electroslag Hardfacing Layer and Low Alloy Steel Theinterface interfacebetween betweenHCCI HCCIhardfacing hardfacinglayer layerand andlow lowalloy alloysteel steelisis isfound founddefect-free defect-freeas asshown shown The The interface between HCCI hardfacing layer and low alloy steel found defect-free as shown in Figure 3a. Figure 3b shows that an austenitic band is formed in the interface, adjacent to which in isis inFigure Figure3a. 3a.Figure Figure3b 3bshows showsthat thatan anaustenitic austeniticband bandisisformed formedin inthe theinterface, interface,adjacent adjacentto towhich which is hypoeutectic microstructure of primary austenite and eutectic mixture. However, the fraction hypoeutectic hypoeutectic microstructure microstructure of of primary primary austenite austenite and and eutectic eutectic mixture. mixture. However, However, the the fraction fraction of of of primary austenite decreases with distance away from the interface. Because of the variation primary austenite decreases with distance away from the interface. Because of the variation of primary austenite decreases with distance away from the interface. Because of the variation of of microstructure, a perfect metallurgical bonding between andalloy lowsteel alloyisissteel is obtained. microstructure, aaperfect metallurgical bonding between HCCI and obtained. After microstructure, perfect metallurgical bonding between HCCIHCCI andlow low alloy steel obtained. After After electroslag surfacing, the microstructure of substrate still ferrite and pearlite. In addition, electroslag surfacing, the of isis still and In the electroslag surfacing, the microstructure microstructure of substrate substrate stillisferrite ferrite and pearlite. pearlite. In addition, addition, the the temperature filed of electroslag surfacing was measured by infrared imaging device as shown in temperature filed of electroslag surfacing was measured by infrared imaging device as shown temperature filed of electroslag surfacing was measured by infrared imaging device as shown in in ◦ C/mm. With the Figure 4. The temperature gradients on both X and Z direction are less than 23.1 Figure Figure 4.4. The The temperature temperature gradients gradients on on both both XX and and ZZ direction direction are are less less than than 23.1 23.1 °C/mm. °C/mm. With With the the assistanceof ofsoftware softwareJMatPro, JMatPro,the themaximun maximunthermal thermalstress stressisis iscalculated calculatedas as29.5 29.5MPa, MPa,which whichisis ismuch much assistance assistance of software JMatPro, the maximun thermal stress calculated as 29.5 MPa, which much lessthan thanthe theyields yieldsstress stressof ofthe thesubstrate substrate(R (REhEh >365 365MPa MPa[24]). [24]).So Soelectroslag electroslagsurfacing surfacingisisisan aneffective effective less less than the yields stress of the substrate (R Eh>>365 MPa [24]). So electroslag surfacing an effective method of depositing HCCI hardfacing layer on low alloy steel that produces sound interface. method methodof ofdepositing depositingHCCI HCCIhardfacing hardfacinglayer layeron onlow lowalloy alloysteel steelthat thatproduces producessound soundinterface. interface.

Figure 3. Interface of the HCCI hardfacing layer (a) The macro-profile, (b) The micro-structure. Figure Figure3.3.Interface Interfaceof ofthe theHCCI HCCIhardfacing hardfacinglayer layer(a) (a)The Themacro-profile, macro-profile, (b) (b) The The micro-structure. micro-structure.

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

Figure Figure4.4.Temperature Temperaturefiled filedof ofthe theback backsurface surfaceof ofD32 D32during duringthe theelectroslag electroslagsurfacing surfacingprocessing. processing.

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3.2. Microstructure of 3.2. Microstructure of V V Alloyed Alloyed HCCI HCCI Hardfacing Hardfacing Layers Layers Detailed analysis of of HCCI layers with Detailed microstructural microstructural analysis HCCI hardfacing hardfacing layers with different different V V content content has has been been investigated. OM micrographs are showing in Figure 5, whereas phase diagram shown in Figure 6, 6, investigated. OM micrographs are showing in Figure 5, whereas phase diagram shown in Figure is is calculated calculated to to understand understand the the effect effect of of V V on on microstructure. microstructure. Figure 5a shows that without the V additive, the the microstructure is consisting of eutectic and bulky Figure 5a shows that without the V additive, microstructure is consisting of eutectic and primary M C carbides. Figure 6a illustrates that the solidification starts with the formation of primary 7 3 M7C3 carbides. Figure 6a illustrates that the solidification starts with the formation bulky primary of ◦ C. When the temperature is decreased to 1278 ◦ C, the austenite (γ) and M C carbide 1300 7 C3 at M 7 3 primaryM carbide 7C3 at 1300 °C. When the temperature is decreased to 1278 °C, the austenite (γ) and carbides simultaneously develop and form eutectic. M7C3 carbides simultaneously develop andthe form the eutectic. The microstructure of HCCI hardfacing layer The microstructure of HCCI hardfacing layer with with 0.83 0.83 wt wt % %V V is is still still consisting consisting of of eutectic eutectic and and primary M C as shown in Figure 5b; however, the primary carbides are refined as compared 7 3 primary M C3 as shown in Figure 5b; however, the primary carbides are refined as compared to to Figure shifts to to the the right right side side in in the the phase phase diagram. diagram. Therefore, Therefore, Figure 5a. 5a. Figure Figure 6b 6b shows shows the the eutectic eutectic point point shifts the C33”” phase phase zone zone is is smaller smaller than than the the temperature temperature range range of of “L “L ++ M M77C the temperature temperature range range shown shown in in Figure 6a, so the grain growth time of primary carbides is shorter accordingly. Figure 6a, so the grain growth time of primary carbides is shorter accordingly. Figure when thethe V concentration is raised to 1.50towt %, the is eutectic. Figure 5c 5cshows showsthat that when V concentration is raised 1.50 wt microstructure %, the microstructure is As shownAs inshown Figure in 6c,Figure the C content eutecticofpoint is 3.6point wt %, to is thesame C content eutectic. 6c, the Cofcontent eutectic is which 3.6 wt is %,same which to the of C HCCI hardfacing layers. So the formed microstructure is eutectic only. content of HCCI hardfacing layers. So the formed microstructure is eutectic only. The The microstructure microstructure of of HCCI HCCI hardfacing hardfacing layer layer with with 2.32 2.32 wt wt % %V V is is consisting consisting of of primary primary austenite austenite and eutectic as shown in Figure 5d. Figure 6d shows that when the V content is 2.32 wt %, the position and eutectic as shown in Figure 5d. Figure 6d shows that when the V content is 2.32 wt %, the position of eutectic point moves to the right side a little more. The solidification of HCCI passes through of eutectic point moves to the right side a little more. The solidification of HCCI passes through the the “L+γ” phaseregions regionssequentially, sequentially, so so primary primary austenite austenite and and eutectic eutectic microstructure microstructure “L+γ” and and “L+γ+M “L+γ+M77CC33””phase is is obtained obtained after after cooling. cooling.

Figure 5. Microstructure of 3.6C-20Cr-Fe HCCI hardfacing layers with different V, (a) 0 V, (b) 0.83 V, Figure 5. Microstructure of 3.6C-20Cr-Fe HCCI hardfacing layers with different V, (a) 0 V, (b) 0.83 V, (c) 1.50 V, V,(d) (d)2.32 2.32V. V. (c) 1.50

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Figure 6. layers with (a)(a) 3.6C-20Cr-Fe, (b) (b) 3.6C-20Cr-Fe-0.83V, (c) Figure 6. Phase Phasediagrams diagramsofofthe thehardfacing hardfacing layers with 3.6C-20Cr-Fe, 3.6C-20Cr-Fe-0.83V, 3.6C-20Cr-Fe-1.50V, (d)(d) 3.6C-20Cr-Fe-2.32V. (c) 3.6C-20Cr-Fe-1.50V, 3.6C-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 Figure 7. Line (a) shows that the 3.6C-20Cr-Fe HCCI hardfacing layer contains M7 C3 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 Figure 8a, 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. Figure 8b, 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 Figure XRD of of the the hardfacing hardfacing layer layer with with different different V V contents, contents, (a) (a)00V, V, (b) (b)0.83 0.83V, V, (c) (c) 1.50 1.50 V, V, (d) (d) 2.32 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-20CrFigure 8. Microstructure of eutectic zone of HCCI hardfacing layer, (a) 3.6C-20Cr-Fe, (b) 3.6C-20CrFigure 8. Microstructure of eutectic zone of HCCI hardfacing layer, (a) 3.6C-20Cr-Fe, (b) 3.6C-20Cr-Fe-1.50V. Fe-1.50V. Fe-1.50V.

From the above above discussion, the the microstructure of of 3.6C-20Cr-Fe hardfacing hardfacing layer layer is is primary From From the the above discussion, discussion, the microstructure microstructure of 3.6C-20Cr-Fe 3.6C-20Cr-Fe hardfacing layer is primary primary carbides and eutectic. With the V content increasing, the eutectic point moves to the right side,side, the carbides and eutectic. With the V content increasing, the eutectic point moves to the right carbides and eutectic. With the V content increasing, the eutectic point moves to the right side, the eutectic area is increasing until the microstructure of hardfacing layer is only eutectic. When the V the eutectic is increasing until microstructure hardfacinglayer layerisisonly onlyeutectic. eutectic. When When the the V V eutectic areaarea is increasing until thethe microstructure ofofhardfacing content exceedsthethe limited value, primary austenite and eutectic colonies formed in the content limited value,value, primary austenite and eutectic formed in theformed microstructure. content exceeds exceeds the limited primary austenite andcolonies eutectic colonies in the microstructure. In addition, V alloying also promotes the formation of martensite in the HCCI In addition, V alloying also promotes the formation of martensite in the HCCI hardfacing microstructure. In addition, V alloying also promotes the formation of martensite 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. Effect of V on Carbides in HCCI Hardfacing Layers 3.3.1. Carbide Types 3.3.1. Carbide Types 6 shows 3.3.1.Figure Carbide Types that only M7 C3 type carbides are formed in the 3.6C-20Cr-Fe hardfacing layer. WhenFigure 0.83 wt % andthat 1.50only wt % added, carbide in the hardfacing layers is M the 6 shows MV 7Care 3 type carbides are formed in the 3.6C-20Cr-Fe hardfacing layer. 7 C3 . When Figureis6 2.32 shows only M7Cin 3 type carbides are formed in the 3.6C-20Cr-Fe hardfacing layer. V content wtthat %, hardfacing M7 C3 and MC. dispersive When 0.83 wt % and 1.50carbides wt % V arethe added, carbidelayers in the are hardfacing layers is Energy M7C3. When the V When 0.83 wt(EDS) % and 1.50 wtof%the V matrix, are added, in are theperformed hardfacingaslayers is MFigures 7C3. When the V spectroscopy M7 Ccarbide shown 9 and 10, content is 2.32 wt analysis %, carbides in the hardfacing layers are M7C3 and MC. in Energy dispersive 3 and MC content is 2.32 wt %, carbides in the hardfacing layers are M 7C3 and MC. Energy dispersive and the results are shown inofTable 3. By comparing Figure it can be that little of9 V is spectroscopy (EDS) analysis the matrix, M7C3 and MC are 9e,f, performed as seen shown in aFigures and 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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dissolved in austenite, while V is mostly existed in M7 C3 carbides. As shown in Table 3, the chemical dissolved in austenite, while V is mostly existed in M7C3 carbides. As shown in Table 3, the chemical composition of M7 C3 in 3.6C-20Cr-Fe HCCI hardfacing layer is (Cr4.7 Fe2.3 )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 % 1.50 wtwt%, hassubstituted substituted a part ofand Cr Fe and Fe atoms M7 C3 , content to 0.83 wtand % and 1.50 %,VVatoms atoms has a part of Cr atoms into M7into C3, the the composition of which is (Cr Fe V )C and (Cr Fe V )C , respectively. With increasing 4.62.2V2.2 0.4 3 composition of which is (Cr4.6Fe 0.2)C30.2 and 3(Cr4.5Fe2.1V4.5 0.4)C3,2.1 respectively. With increasing V content V content further %,Vmore atoms has replaced atoms the M7and C3 carbide and its further to 2.32towt2.32 %, wt more atomsVhas replaced Cr atoms Cr in the M7Cin 3 carbide its chemical composition is (Cris 4.4Fe 2.1V )C2.1 3. It is well thatknown the atomic of vanadium chromium and chemical composition (Cr V0.5 )C3 . known It is well thatnumber the atomic numberand of vanadium 4.40.5Fe are 23are and respectively. Their atomic radii are similar, but the but electrons in the d in layer the of chromium 2324, and 24, respectively. Their atomic radii are similar, the electrons the of d layer vanadium atom are less than chromium atom. So the vanadium have stronger affinity with carbon the vanadium atom are less than chromium atom. So the vanadium have stronger affinity with carbon than chromium, thereby V can replace a part CrininM M7C C3 type carbides. than chromium, thereby V can replace a part ofofCr 7 3 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 testtest point of of MM (b) 0.83 0.83V,V,(c) (c)1.50 1.50 2.32 V, (e) of matrix, 7C 3 3(a) 7C (a) 00 V, V, (b) V,V, (d)(d) 2.32 V, (e) EDSEDS of matrix, test point of matrix, 2-EDS point (f) EDS M7of C3M . 7C3. (f)of EDS


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Figure 10. 10. EDS EDS of of MC MC carbide carbide in in 3.6C-20Cr-Fe-2.32V 3.6C-20Cr-Fe-2.32V hardfacing hardfacing layer. layer. Figure Figure 10. EDS of MCof carbide in 3.6C-20Cr-Fe-2.32V hardfacing layer.different Table 3. 3. Chemical Chemical component of carbides in HCCI HCCI hardfacing hardfacing layers with with different V. Table component carbides in layers V.

Hardfacing 7C3 hardfacing layers MCwith MC Table 3.Hardfacing Chemical component of carbides inM HCCI different V. LayerLayer M C 7 3

3.6C-20Cr-Fe (Cr(Cr 4.7Fe2.3)C3 -Hardfacing Layer M74.7 C3Fe2.3 )C3 MC 3.6C-20Cr-Fe – 3.6C-20Cr-Fe-0.83V (Cr 4.6Fe2.2 V)C 0.2V )C3 3.6C-20Cr-Fe-0.83V (Cr – 3.6C-20Cr-Fe (Cr 4.7 FeFe 2.32.2 3 0.2 )C3 -- -4.6 3.6C-20Cr-Fe-1.50V Fe V – 3.6C-20Cr-Fe-1.50V 4.5 Fe 2.1 V2.1 0.4 )C 4.52.2 0.4 3.6C-20Cr-Fe-0.83V (Cr (Cr(Cr 4.6Fe V 0.2 )C 3 3 )C3 -- -3.6C-20Cr-Fe-2.32V (Cr Fe V )C (Cr V0.77 )C 4.4 2.1 0.5 3 0.23 3.6C-20Cr-Fe-2.32V (Cr 4.4 Fe 2.1 V 0.5 )C 3 (Cr 0.23 V 0.77 )C 3.6C-20Cr-Fe-1.50V (Cr4.5Fe2.1V0.4)C3 -3.6C-20Cr-Fe-2.32V

(Cr4.4Fe2.1 V0.5)C3

(Cr0.23V0.77)C

When V content content isis2.32 2.32wt wt%, %,the theamount amountofofV V solid solution austenite reaches limit, When V solid solution in in austenite reaches thethe limit, andand the When Vincontent is 2.32 wtalso %, the amount ofsaturation V solid solution in austenite reaches the limit, and 10, the content of V M 7 C 3 carbide reaches the limit. At this time, as shown in Figure content of V in M7 C3 carbide also reaches the saturation limit. At this time, as shown in Figure 10, the content of V in Min 7C3 carbide also reaches the saturation limit. At this time, as shown in Figure 10, the is present present MC carbides carbides which which are are precipitated precipitated from from austenite. austenite. Figure the excess excess V V is in MC Figure 11 11 shows shows the 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 equilibrium phase mass friction of the 3.6C-20Cr-Fe-2.32V It illustrates illustratesthat that the equilibrium phase mass friction of the 3.6C-20Cr-Fe-2.32VHCCI HCCIhardfacing hardfacing layer. layer. It ◦ C, which the precipitation temperature ofcarbides MC carbides is 990 almost 990 °C, whichthan is lower than temperature the solidus precipitation temperature of MC is almost is lower the solidus the precipitation temperature of MC carbides is almost 990 °C, which is lower than the solidus ◦ C. So the temperature 1240MC °C. carbides So So thethe MC are fromaustenite austenite secondary carbides. EDS 1240 arecarbides precipitated from austenite as secondary carbides. EDSEDS reveals temperature 1240 °C. MC carbides areprecipitated precipitated from as as secondary carbides. reveals that these MC secondary carbides are enriched in V and Cr elements with the chemical that these enriched V and in CrVelements with thewith chemical formula of revealsMC thatsecondary these MC carbides secondaryare carbides are in enriched and Cr elements the chemical formula of (Cr 0.23 V 0.77 )C. The same type of carbide was found by de Mello [25] and Mirjana formula of (Cr 0.23 V 0.77)C. The same type of carbide was found by de Mello [25] and Mirjana (Cr V )C. The same type of carbide was found by de Mello [25] and Mirjana M. Filipovi´ cM. [15]M. in 0.23 0.77 Filipović [15] in iron-chromium-carbon-vanadium white cast irons. Filipović [15] in iron-chromium-carbon-vanadium white cast irons. iron-chromium-carbon-vanadium white cast irons.

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

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

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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):

[V] + [C] = VC(S)

0 ∆GV = −104038 + 97.45T

(2)

0 is the Gibbs free energy of V, T is the Kelvin temperature. where ∆GV 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 V ωV f C ωC ) = ∆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 lg f x1873 (4) ln f x = T lg f x1873 = exx ωx + ∑ ex ωj j

(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 V Cr Si ω + e ω + e ω + · · · eV ωV + eC Cr V C V V Si T

(6)

1873 ln 10 C Si Mn eC ωC + eCr C ωCr + eC ωSi + eC ωMn + · · · T

(7)

ln f V = ln f C =

The interaction coefficient of alloying elements in molten iron at 1600 ◦ C is shown in Table 4 [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 −0.34

−0.024

0.08 0.042

−0.012

−0.077 0.015

eC j eV

By substituting the composition of the HCCI hardfacing layer and the relevant data from Table 4 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.

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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 Metals 2018, 8, x FOR PEER REVIEW 11 of 16 the precipitated temperature of primary phases and eutectic carbides. Therefore, VC carbides are the secondary carbides thattemperature. precipitates austenite. where T is Kelvin [V]from and [C] are mean mass fraction in molten iron of V and C elements, Therefore, when V isEquation added to the hardfacing layer, as has stronger affinity toalloy C than Cr, respectively. From (12), theHCCI precipitated temperatures of V VC carbides in Fe-Cr-C-V 0.83, 1.50 and 2.32 wt % V addition are 1238 K, 1314 K and 1375 K, respectively, which are lower most of with the V is present in M C carbides to replace Cr, while a little of V is dissolved into austenite. 7 3 than the precipitated temperature of primary phases and eutectic carbides. Therefore, VC carbides When V in both austenite and M7 C3 carbides reached the saturated limit, (Cr0.23 V0.77 )C carbides are the secondary carbides that precipitates from austenite. precipitated in the 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 M7 C3 Carbides austenite. When V in both austenite and M7C3 carbides reached the saturated limit, (Cr0.23V0.77)C precipitated in the austenite. Thecarbides area fractions of carbides against various V concentrated samples are show in Figure 12. It can

be seen that, the area fraction of Primary M7 C3 carbides and eutectic M7 C3 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 fractions of carbides against various V concentrated samples are show in Figure 12. It the area fraction of primary carbides is decreased to 10.1% while the area fraction of eutectic carbides is can be seen that, the area fraction of Primary M7C3 carbides and eutectic M7C3 carbides in 3.6C-20Crincreased 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 hardfacing hascarbides the maximum carbides of 46.9%. By increasing the area fraction of primary is decreased to 10.1% area whilefraction the area fraction of eutectic carbides the V concentration to 2.32%, withInthe formation of primary austenite,layers the area fraction of eutectic carbides is increased to 34.5%. 3.6C-20Cr-Fe-1.50V HCCI hardfacing no further primary carbide growth observed. The hardfacing has the maximum carbides area fraction of 46.9%. By increasing is decreased tois44.7%. the V13 concentration 2.32%, with the of primary austenite, the area fraction eutectic Figure shows the to size variation of formation M7 C3 carbides in HCCI hardfacing layers of with different V carbides is decreased to 44.7%. additive. The average area of primary M7 C3 carbides and eutectic carbides in 3.6C-20Cr-Fe hardfacing Figure213 shows the size2 variation of M7C3 carbides in HCCI hardfacing layers with different V layer is additive. 422.3 µm and 43.5 µm respectively, determined by Equation (1). When the V content is The average area of primary M7C3 carbides and eutectic carbides in 3.6C-20Cr-Fe hardfacing 0.83 wt %, the areas of2primary carbides and eutectic carbides are refined to the 172.9 µm2 and 37.1 µm2 , 2 respectively, layer is 422.3 μm and 43.5 μm determined by Equation (1). When V content is 0.83 2 and 2, respectively. theofVprimary concentration is raised 1.50 wtare %,refined the growth the primary carbide is wt %, When the areas carbides and eutectictocarbides to 172.9ofμm 37.1 μm respectively. When the V concentration is raised to 1.50 wt %, the growth of the primary carbide is completely suppressed and the eutectic carbides are further refined. The microstructure is composed suppressed and an the average eutectic carbides further The microstructure composed 2 . However, of smallcompletely eutectic carbides with area ofare28.8 µmrefined. when the Vis concentration 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 1.50 wt1.50 % Vwtadditive. TheThe eutectic carbides hardfacing have grown with % V additive. eutectic carbidesin in 3.6C-20Cr-Fe-2.32V 3.6C-20Cr-Fe-2.32V hardfacing layer layer have grown 2. 2 bigger with an average area of 40.3 µm bigger with an average area of 40.3 μm .

12. Area fraction of 7M carbides in HCCI hardfacing layers.layers. FigureFigure 12. Area fraction of M C7C indifferent different HCCI hardfacing 3 3carbides

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Figure 13. area of M C Figure 13. 13. Average Average area of of M M777C C333 carbides carbides in different HCCI hardfacing layers. Figure Average area carbides in in different different HCCI HCCI hardfacing hardfacing layers. layers.

3.4. and Wear Resistance of V HCCI Electroslag Hardfacing Layers 3.4. Hardness Hardness Resistance of of V V Alloyed Alloyed Hardfacing Layers Layers 3.4. Hardness and and Wear Wear Resistance Alloyed HCCI HCCI Electroslag Electroslag Hardfacing 3.4.1. 3.4.1. Hardness 3.4.1. Hardness Hardness Figure 14 shows the hardness HCCI hardfacing layer with different content. The hardness Figure different VV content. The hardness of Figure 14 14 shows shows the thehardness hardnessofof ofHCCI HCCIhardfacing hardfacinglayer layerwith with different V content. The hardness of HCCI hardfacing layer without V is 58.5 HRC. When the V is 0.83 wt %, the primary carbide M 7C3 HCCI hardfacing layer without V isV58.5 HRC. When thethe V isV0.83 wt wt %, %, thethe primary carbide M7M C37Cis3 of HCCI hardfacing layer without is 58.5 HRC. When is 0.83 primary carbide is and the area fraction eutectic carbides increased. Carbides distribution is more uniform refined and the area fraction is refined refined and the area fractionofof ofeutectic eutecticcarbides carbidesisis isincreased. increased.Carbides Carbidesdistribution distribution is is more more uniform uniform in the hardfacing layer, which has raised the hardness up to 59.2 HRC. As V is further increased to in in the the hardfacing hardfacing layer, layer, which which has has raised raised the the hardness hardness up up to to 59.2 59.2 HRC. HRC. As As V V is is further further increased increased to to 1.50 wt %, the content of carbides is increased and all the M 7C3 carbides are fine eutectic carbides in 1.50 carbides are are fine 1.50 wt wt %, %, the the content content of of carbides carbides is is increased increased and and all all the the M M77C C33 carbides fine eutectic eutectic carbides carbides in in the microstructure. In this case, the highest hardness 61.2 HRC achieved. When V is the HRC is is achieved. When thethe V is wt wt %, the microstructure. microstructure. In Inthis thiscase, case,the thehighest highesthardness hardnessofof of61.2 61.2 HRC is achieved. When the V 2.32 is 2.32 2.32 wt %, the formation of soft austenite phase and the grain coarsening have decreased the bulk hardness the formation of soft austenite phase andand thethe grain coarsening have decreased thethe bulk hardness of %, the formation of soft austenite phase grain coarsening have decreased bulk hardness of to HRC. of hardfacing hardfacing to 60.1 60.1 HRC. hardfacing to 60.1 HRC.

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

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3.4.2. 3.4.2. Wear Wear Resistance Resistance Figure shows that thatthe thevariation variationininthethe wear resistance is consistent the hardness. Figure 14 14 shows wear resistance is consistent withwith the hardness. The 3. The 3.6C-20Cr-Fe hardfacing layer has the highest wear volume loss of 211 mm As the 3.6C-20Cr-Fe hardfacing layer has the highest wear volume loss of 211 mm3. As the primary carbides primary content is eutectic decreasing and content the eutectic carbides content is increasing the content iscarbides decreasing and the carbides is increasing in the hardfacing layer, theinwear 3 hardfacing layer, the wear volume loss of 3.6C-20Cr-Fe-0.83V hardfacing layer is decreasing to 160 mm 3 volume loss of 3.6C-20Cr-Fe-0.83V hardfacing layer is decreasing to 160 mm . The 3.6C-20Cr-Fe-1.50V. The 3.6C-20Cr-Fe-1.50V hardfacing layer withhas eutectic microstructure the least wear isvolume of 3, which hardfacing layer with eutectic microstructure the least wear volumehas of 86 mm only 40% 3 86 , which is only 40% of the 3.6C-20Cr-Fe sample. Whereas, thedendrites formationphase of soft of mm the 3.6C-20Cr-Fe hardfacing sample. Whereas,hardfacing the formation of soft austenitic in austenitic dendrites phase in the microstructure of 3.6C-20Cr-Fe-2.32V HCCI hardfacing layer has also the microstructure of 3.6C-20Cr-Fe-2.32V HCCI hardfacing layer has also increased the wear volume 3 increased the to wear significantly 181volume mm3. significantly to 181 mm . Under Under normal normal conditions, conditions, the the carbide carbide with with small small size size and and uniform uniform distribution distribution in in matrix matrix show show good wear resistance [27,28]. It was observed that 3.6C-20Cr-Fe HCCI hardfacing layer contains coarse good wear resistance [27,28]. It was observed that 3.6C-20Cr-Fe HCCI hardfacing layer contains primary carbidecarbide which which can be can seenbemore in Figure 15a. It15a. hasItalso observed that that the coarse primary seen clearly more clearly in Figure has been also been observed austenite region B with lower carbon and chromium contents is surrounding the primary carbide the austenite region B with lower carbon and chromium contents is surrounding the primary carbide region region C. C. During During the the wear wear process, process, the the coarse coarse carbide carbide fracturing fracturing and and spalling spalling from from the the matrix matrix under under tangential stress [29]. This carbide fracture can be clearly observed in Figure 15b where the cracked tangential stress [29]. This carbide fracture can be clearly observed in Figure 15b where the cracked primary primary carbide carbide leaves leaves aalarge largepolygonal polygonalcave cavein inits itsoriginal originalposition. position.Meanwhile Meanwhilethe thecracked crackedM M7 7CC33 carbide particles, rolling on the wear surface, exacerbate the wear. carbide particles, rolling on the wear surface, exacerbate the wear. Whereas in in Figure 16a16a shows thatthat in the HCCIHCCI hardfacing layer, Whereasthe thered redbox boxmarked marked Figure shows in 3.6C-20Cr-Fe the 3.6C-20Cr-Fe hardfacing the fractured primary carbides further act as secondary abrasive particles to cause more primary layer, the fractured primary carbides further act as secondary abrasive particles to cause more carbides When the V content 0.83 wt are refined, the content and sizeand of primary breakage. carbides breakage. When the V is content is %, 0.83carbides wt %, carbides are refined, the content primary carbides is decreased and more uniform eutectic carbides are formed. As shown in Figure 16b, size of primary carbides is decreased and more uniform eutectic carbides are formed. As shown in the plow16b, grooves on wear surface shallower than that in Figure 15a. When the V Figure the plow grooves on are wear surface and are finer shallower andshowing finer than that showing in Figure content is 1.50 fine eutectic carbides distributed uniformlyare in the hardfacing layer, as shown 15a. When thewtV%,content is 1.50 wt %,are fine eutectic carbides distributed uniformly in the in Figure 16c, the plow grooves on wear surface are more shallower and finer than that in Figure 16b, hardfacing layer, as shown in Figure 16c, the plow grooves on wear surface are more shallower and and noticeable caves16b, have been observed.caves When thebeen V content reaches tothe 2.32 wt %, primary finerno than that in Figure and no noticeable have observed. When V content reaches austenite in the microstructure and decline in the hardness of the layer is to 2.32 wtis%,appeared primary austenite is appeared in theamicrostructure and a decline in hardfacing the hardness of the found, whereas the depth of the plow grooves is deeper, which indicates that the wear resistance is hardfacing layer is found, whereas the depth of the plow grooves is deeper, which indicates that the decreased further. the absence of coarse primary carbides, caves have not been wear resistance is However, decreasedbecause further. of However, because of the absence of coarse primary carbides, observed on the wear surface as shown in Figure 16d. caves have not been observed on the wear surface as shown in Figure 16d.

Figure 15. 15.Morphology Morphologyofofthe the primary carbide in 3.6C-20Cr-Fe HCCI hardfacing (a) before Figure primary carbide in 3.6C-20Cr-Fe HCCI hardfacing layer,layer, (a) before wear, wear, (b) after wear. (b) after wear.

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Figure 16. 16. Morphology Morphology of ofthe thewear wearsurface surfaceofofHCCI HCCIhardfacing hardfacinglayer layerwith with different contents, Figure different VV contents, (a)(a) 0 V,0 V, (b) 0.83 V, (c) 1.50 V, (d) 2.32 V. (b) 0.83 V, (c) 1.50 V, (d) 2.32 V.

4. Conclusions 4. Conclusions (1) High chromium cast iron can be used for surfacing low alloy steel to form bimetallic material. (1) High chromium cast iron can be used for surfacing low alloy steel to form bimetallic material. The temperature gets evenly distributed in the workpiece during the electroslag surfacing process, The temperature gets evenly distributed in the workpiece during the electroslag surfacing process, therefore the interface between hardfacing layer and low alloy steel D32 is defect free. therefore the interface between hardfacing layer and low alloy steel D32 is defect free. (2) The microstructure of 3.6C-20Cr-Fe hardfacing layer is primary carbides and eutectic. With (2) The microstructure of 3.6C-20Cr-Fe hardfacing layer is primary carbides and eutectic. 0.83 wt % V, the primary carbides are refined and small amount of martensite formed in the With 0.83 wt % V, the primary carbides are refined and small amount of martensite formed in the 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 M7 C3 and 2.3V0.2–0.5)C3 type carbides formed, while a little of V is dissolved into austenite. When V atoms in both (Cr4.4–4.7 Fe2.1–2.3 V0.2–0.5 )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 M7 C3 carbides reached the saturated limit, (Cr0.23 V0.77 )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 M7 C3 carbides in the hardfacing layers are refined significantly. The eutectic carbides are refined with an average area of 28.8 μm22 when V content is significantly. The eutectic carbides are refined with an average area of 28.8 µm 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 μm22. bigger with an average area of 40.3 µm . (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. Author Contributions: Contributions:Conceptualization, Conceptualization, H.W. S.F.Y.; Investigation, and A.R.K.; Methodology, Author H.W. andand S.F.Y.; Investigation, H.W. H.W. and A.R.K.; Methodology, A.G.H.; A.G.H.;administration, Project administration, S.F.Y.; Supervision, S.F.Y.; Writing—original draft, H.W.; Writing—review and Project S.F.Y.; Supervision, S.F.Y.; Writing—original draft, H.W.; Writing—review and editing, H.W., A.R.K. A.G.H. editing, H.W.,and A.R.K. and A.G.H. Funding: This research received no external funding.

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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.

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