Microstructure and Wear Behavior of Tungsten Hot-Work Steel After Boriding and Boroaluminizing

lubricants

Technical Note Microstructure and Wear Behavior of Tungsten Hot-Work Steel after Boriding and Boroaluminizing

Undrakh Mishigdorzhiyn 1,2,*, Yan Chen 3, Nikolay Ulakhanov 1 and Hong Liang 3 1 Department of Mechanical Engineering, East Siberia State University of Technology and Management, Ulan-Ude 670013, Russia; [email protected] 2 Institute of Physical Material Science of the Siberian Branch of the Russian Academy of Science, Ulan-Ude 670047, Russia 3 J. Mike Walker ’66 Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA; [email protected] (Y.C.); [email protected] (H.L.) * Correspondence: [email protected]; Tel.: +7-9148-373-153

Received: 4 June 2019; Accepted: 17 February 2020; Published: 2 March 2020

Abstract: (1) Background: Boron-based diffusion layers possess great application potential in forging and die-casting due to their favorable mechanical and thermophysical properties. This research explores the enhanced wear resistance of tungsten hot-work steel through boriding and boroaluminizing. (2) Methods: Thermal-chemical treatment (TCT) of steel H21 was carried out. Pure boriding was introduced to the substrate through heating a paste of boron carbide and sodium fluoride 1050 ◦C for two hours. As for boroaluminizing, 16% of aluminum powder was added to the boriding paste. (3) Results: It was shown that boriding resulted in the formation of an FeB/Fe2B layer with a tooth-like structure. A completely different microstructure was revealed after boroaluminizing—namely, diffusion layer with heterogeneous structure, where hard components FeB and Mx (B,C) were displaced in the matrix of softer phases—Fe3Al and α-Fe. In addition, the layer thickness increased from 105 µm to 560 µm (compared to pure boriding). The maximum microhardness values reached 2900 HV0.1 after pure boriding, while for boroaluminizing it was about 2000 HV0.1. (4) Conclusions: It was revealed that the mass loss during wear test reduced by two times after boroaluminizing and 13 times after boriding compared to the hardened sample after five-min testing.

Keywords: thermal–chemical treatment; boriding; boroaluminizing; treatment paste; microstructure; microhardness; wear resistance; hot-work steel

1. Introduction Boriding has been studied for more than half a century. Until recently, the process continues to be developed to satisfy extended fields of applications, such as aerospace, petrochemical refining, textile and food processing, biojoints, or implanting materials manufacturing [1–3]. In comparison to nitriding and carburization, boriding is of limited use. The constraining factors are high brittleness of boride layers and overheating of the substrate metal due to long high-temperature exposure. Researchers have made progress in increasing the plasticity of the layer by means of a single Fe2B layer formation. The Fe2B layer can be obtained using low-temperature boriding, ultra-fast boriding in molten electrolyte, direct and alternating current field, concentrated energy flows, etc. [4–17]. Once the brittleness is reduced, boriding acquires potential to be utilized in dynamically loaded machine parts, for instance, to increase surface durability of dies made of hot-work tool steels, which are widely used in forging, die-casting, bending, among others. Due to the wide application of steel H21 in the aforementioned manufacturing processes, the issue of its surface properties enhancement is of great interest. One approach to improve surface

Lubricants 2020, 8, 26; doi:10.3390/lubricants8030026 www.mdpi.com/journal/lubricants Lubricants 2020, 8, 26 2 of 15 properties of hot-work tool steel was provided by Chander and Chawla in [18]. The authors proposed duplex/hybrid treatment (plasma nitriding + PVD) as a surface modifying process to enhance wear resistance, hardness, toughness, and fatigue resistance of H21 steel. The plasma-nitrided and duplex CrTiN/AlCrN coatings significantly contributed to the lifetime enhancement of dies during hot forging industrial tests by 55% and 76%, respectively, compared to conventionally heat-treated H21 steel [18]. Despite that, the proposed techniques demand complex equipment, which increases the treatment costs. Besides, one of the major PVD coatings drawbacks is their poor adhesion to the substrate compared to diffusion layers. The authors in [18] clearly indicate that the single-component thermal-chemical treatment (TCT) methods are surpassed by duplex methods, where at least two elements diffuse into the substrate. The multi-component boron-based diffusion layer has a number of advantages compared to the usual borided layer [19–21]. TCT techniques with boron and a non-metal (boronitriding, borocarburing) or boron and a metal (borochromizing, borotitanizing, boronickelizing, boroaluminizing, etc.) are investigated [22–27]. One of these methods is boroaluminizing (joint saturation with boron and aluminum), which allows increased resistance to wear, high-temperature oxidation, and corrosion [28–30]. Enhancement of the listed properties is provided by the iron borides and aluminides formation, where borides FeB and Fe2B enhance surface microhardness and wear resistance, while aluminides resist surface oxidation. Recent studies indicate that the required surface properties can be provided by boroaluminizing in self-protective pastes with heating in muffle furnaces [31]. Elevated treatment temperatures of 1050 ◦C and 1100 ◦C adduce to the development of heterogeneous structured layers on top of carbon steels, which are superior to the layers, received at inferior temperatures of 950 ◦C and 1000 ◦C in wear resistance. Two-component treatment with boron and aluminum is preferable in comparison to pure boriding due to the potential resistance to oxidation and punching load. As such, further understanding and approaches in boriding are needed. In the current work, we studied the possibility of improvement of wear resistance of hot-work steel H21 by means of boriding and boroaluminizing. The microstructure evolution and wear behavior of treated samples will be discussed, where the advantages of heterogeneous and tooth-like structure of the layers will be considered with respect to mechanical properties. This research is beneficial for the future design of highly wear-resistant coatings in the prospect of their application in forging and die-casting.

2. Materials and Methods Steel 3Kh2V8F (the analog of AISI H21 steel; for convenience, the latter title will be used throughout the paper later on) was used as a tested material. It is a high-quality hot-work tool steel, alloyed with W, Cr, and V. The full chemical composition of steel is given in Table1. It is used for manufacturing heavy-duty pressing tools (small inserts of ﬁnal forming stream, dies and extrusion punches, etc.) during hot deformation of alloyed structural steels and high-temperature alloys, molds for injecting molding copper alloys [32].

Table 1. The chemical composition of steel H21, wt %.

C Si Mn P S Cr W V Fe 0.26–0.36 0.15–0.50 0.15–0.40 up to 0.03 up to 0.03 3.0–3.75 8.5–10.0 0.3–0.6 84.33–87.58

A steel billet was machined in rectangular pieces, with nominal dimensions of 20 12 10 1/2 × × ± mm (length by width by height). Four major faces on each of the samples were ground to a 1000 grit. Finally, samples were rinsed in ethyl alcohol, dried and processed further for TCT under the following conditions. Diﬀusion layers were developed using self-protective pastes which comprise sodium ﬂuoride (NaF) as a chemical activator and boron carbide (B4C) as a boriding agent. Al-powder was used in addition to the aforementioned chemical components for boroaluminizing, where the boron carbide Lubricants 2020, 8, x FOR PEER REVIEW 3 of 15

LubricantsDiffusion2020, 8, 26layers were developed using self-protective pastes which comprise sodium fluoride3 of 15 (NaF) as a chemical activator and boron carbide (B4C) as a boriding agent. Al-powder was used in addition to the aforementioned chemical components for boroaluminizing, where the boron carbide and aluminumaluminum ratioratio was was 5:1, 5:1, respectively. respectively. The The percentage percentage ratio ratio of treatment of treatment paste paste components components is given is ingiven the in following the following equations equations for both for methods: both methods:

B4C 96% + NaF 4% (Boriding) (1) B4C 96% + NaF 4% (Boriding) (1)

B4C 80% + Al 16% + NaF 4% (Boroaluminizing) (2) B4C 80% + Al 16% + NaF 4% (Boroaluminizing) (2) Some sources indicate the necessity of providing a protective atmosphere while carrying out boriding in treatment pastes [[1,31,333].]. On the contrary, conducted experimentsexperiments have shown the ability of B4C–NaF paste to resist against oxygen penetration. Boron carbide reacts with oxygen forming a of B4C–NaF paste to resist against oxygen penetration. Boron carbide reacts with oxygen forming a fusible boronboron oxide oxide ﬁlm film on on the the paste’s paste’s surface, surface, which which protects protects inner inner compounds compounds and the and steel the sample steel fromsample oxidation: from oxidation:

B4C4 + 4O22 → 2B22O3 + CO22 (∆G = 2230 kJ/mol) (3) B C + 4O → 2B O + CO (∆G = –2230− kJ/mol) (3)

2B2B4C4C+ + 7O7O22 → 4B2O33 ++ 2CO2CO ( (∆∆GG == –43874387 kJ/mol) kJ/mol) (4) (4) → − The components were pre-kneaded with organic binder and acetone until achieving a slurry composition. The The acquired slurry (paste) with the steel samples inside it were placed in cylindricalcylindrical molds andand tamped,tamped, as as shown shown in in Figure Figure1. After1. After that, that, the moldsthe molds were were emptied emptied and theand briquettes the briquettes were drainedwere drained for one for hour one at hour 100 ◦atC in100 a drying°C in a oven. drying Then, oven. the Then, briquettes the briquettes with steel sampleswith steel positioned samples insidepositioned were inside exposed were in the exposed following in the time-temperature following time-temperature parameters: 2 hparameters: exposure and 2 h 1050 exposure◦C heating and in1050 a mu °Cﬄ heatinge furnace in withouta muffle afurnace protective without atmosphere. a protective It is importantatmosphere. to placeIt is important the briquettes to place into the preheatedbriquettes atinto 1050 the◦ Cpreheated furnace at at the 1050 beginning °C furnace of the at process. the beginning Cooling of was the conducted process. Cooling in still air was at ambientconducted temperature, in still air at where ambient the temperature, cooling rate was where measured the cooling as 3 ◦rateC/s. was measured as 3 °C/s.

Figure 1. Scheme of treatment paste briquettes formation [34]. Figure 1. Scheme of treatment paste briquettes formation [34]. The metallographic analysis was conducted on scanning electron microscope (SEM) JSM-6510LV (JEOLThe Ltd, metallographic Tokio, Japan) and analysis optical microscopewas conducted Metam on RV-34 scanning (JSC LOMO,electron St. Petersburg,microscope Russia)(SEM) JSM-6510LV (JEOL Ltd, Tokio, Japan) and optical microscope Metam RV-34 (JSC LOMO, St. using Altami Studio software (Altami LLC, St. Petersburg, Russia). In addition, the atomic force Petersburg, Russia) using Altami Studio software (Altami LLC, St. Petersburg, Russia). In addition, microscope (AFM) Nano-R2 (Pacific Nanotech. Inc., Santa Clara, California, United States) was the atomic force microscope (AFM) Nano-R2 (Pacific Nanotech. Inc., Santa Clara, California, United used to observe the layer zones in order to see the fine-scale morphology. The three-dimensional States) was used to observe the layer zones in order to see the fine-scale morphology. The AFM images were measured in close-contact mode on an area of 25 µm 25 µm. EDS analysis three-dimensional AFM images were measured in close-contact mode on an× area of 25 μm × 25 μm. of the boroaluminized layers was conducted using the INCA Energy 350 (Oxford Instruments plc, EDS analysis of the boroaluminized layers was conducted using the INCA Energy 350 (Oxford Abingdon, UK) microanalysis system installed on the SEM JSM-6510LV (JEOL Ltd, Tokio, Japan) with Instruments plc, Abingdon, UK) microanalysis system installed on the SEM JSM-6510LV (JEOL Ltd, an acceleration voltage of 20 keV at the Science Center “Progress” of ESSUTM (East Siberia State Tokio, Japan) with an acceleration voltage of 20 keV at the Science Center “Progress” of ESSUTM University of Technology and Management, Ulan-Ude, Russia). It should be noted that carbon content (East Siberia State University of Technology and Management, Ulan-Ude, Russia). It should be noted is given to reflect the concentration variations depending on the distance from the surface and does that carbon content is given to reflect the concentration variations depending on the distance from not present actual values. The boron content was evaluated rather qualitatively than quantitatively as the surface and does not present actual values. The boron content was evaluated rather qualitatively well. The final decision on the presence of boron content phases was made based on XRD analysis than quantitatively as well. The final decision on the presence of boron content phases was made and microhardness measurement. For the latter, a microhardness tester PMT-3M (JSC LOMO, St. based on XRD analysis and microhardness measurement. For the latter, a microhardness tester Petersburg, Russia) was used. A pyramid indented with an angle of 136◦ between opposite faces at a PMT-3M (JSC LOMO, St. Petersburg, Russia) was used. A pyramid indented with an angle of 136°

Lubricants 2020, 8, 26 4 of 15 Lubricants 2020, 8, x FOR PEER REVIEW 4 of 15 betweenload of 100 opposite gf was appliedfaces at witha load a 10 of s 100 for eachgf was download, applied hold,with anda 10 upload s for each at constant download, strain hold, rate. and The uploadaverage at value constant of 5 printsstrain wasrate. takenThe average to avoid va dismissal.lue of 5 prints was taken to avoid dismissal. The diffractometer diffractometer D8 Advance (Bruker AXS Inc. Inc.,, East Cheryl Parkway Madison, WI, USA) was utilized to explore the phase composition in CuK α radiation at the Laboratory of Oxide Systems of BINM SB RAS (Russian AcademyAcademy ofof Science,Science, SiberianSiberian Branch,Branch, Ulan-Ude,Ulan-Ude, Russia).Russia). The following parameters were applied to obtain didiffractionffraction patterns:patterns: signal signal accumulation accumulation of of 3 s/point, s/point, angular interval 2 θ = 55–80−80°,◦, pitch pitch 0.021°. 0.021◦. The wear test was conducted in accordance with the block-on-ring scheme with dry sliding −1 1 friction on on friction friction machine machine SMTs-2 SMTs-2 (F (Figureigure 2).2). The The sliding sliding speed speed was was 0.8 0.8 m/s m /(sn (=n 300= 300 min min) and− ) andthe contactthe contact pressure pressure (P = (P6.86= 6.86N/mm N/mm2). A2 ).hardened A hardened disk diskof high-speed of high-speed steel steel R6M5 R6M5 (analog (analog to AISI to AISI M2 high-speedM2 high-speed steel) steel) with with a hardness a hardness of 63 of ± 632 HRC2 HRCwas used was usedas a roller as a roller (ring). (ring). The tested The tested samples samples were ± previouslywere previously borided borided and andboroaluminized boroaluminized in accord in accordanceance with with the the aforementioned aforementioned modes. modes. The The bare bare steel and steel samples after hardening were subjecte subjectedd to the test for comparison. The sample’s wear behavior was determined by itsits massmass loss,loss, onon anan analyticalanalytical scalescale withwith anan accuracy accuracy of of ±1010 −44 g. Due to ± − the significant significant difference difference in surface hardness and, consequently, mass loss, it was decided to weight bare and heat-treatedheat-treated steelssteels every every minute, minute, and and borided borided or or boroaluminized boroaluminized ones ones every every 5 or 5 10 or min. 10 min. The Theworn worn surfaces surfaces were were examined examined using using SEM SEM after after the test the finish.test finish.

Figure 2. TheThe scheme scheme of of the wear resistance test. 3. Results and Discussion 3. Results and Discussion Boriding in a treatment paste at 1050 ◦C leads to the formation of layers with a tooth-like Boriding in a treatment paste at 1050 °C leads to the formation of layers with a tooth-like microstructure (Figure3). XRD analysis has shown FeB, BN, and Cr 2O3 presence (Figure4). The outer microstructure (Figure 3). XRD analysis has shown FeB, BN, and Cr2O3 presence (Figure 4). The iron boride FeB is normally accompanied by Fe2B as the inner phase on the border with the base metal. outer iron boride FeB is normally accompanied by Fe2B as the inner phase on the border with the The latter phase, probably, was not detected due to insuﬃcient X-ray penetration in the boride layer. base metal. The latter phase, probably, was not detected due to insufficient X-ray penetration in the Fe2B presence can be partially conﬁrmed by metallographic and EDS analysis. The presence of BN can boride layer. Fe2B presence can be partially confirmed by metallographic and EDS analysis. The be explained by reaction with atmosphere nitrogen: presence of BN can be explained by reaction with atmosphere nitrogen:

2B + N22→ 2BN (∆G = 316 kJ/mol) (5) 2B + N → 2BN (∆G =− –316 kJ/mol) (5)

Lubricants 2020, 8, 26 5 of 15 LubricantsLubricants 20202020,, 88,, xx FORFOR PEERPEER REVIEWREVIEW 55 ofof 1515

Figure 3. SEM image of borided steel H21 after 2-h exposure at 1050 °C. Figure 3. SEMSEM image image of of borided steel H21 after 2-h exposure at 1050 ◦°C.C.

Figure 4. XRD pattern of steel H21 after boriding at 1050 °C for 2 h. Figure 4. XRDXRD pattern pattern of steel H21 after boriding at 1050 °C◦C for 2 h.

Chromium is prone to oxidation above 600 °C. Consequently, Cr2O3 can form as a result of Chromium isis proneprone to to oxidation oxidation above above 600 600◦C. Consequently,°C. Consequently, Cr2O Cr3 can2O3form can asform a result as a ofresult surface of surfacesurfaceoxidation oxidationoxidation during high-temperatureduringduring high-thigh-temperatureemperature exposure exposureexposure in the non-controlled inin thethe non-controllednon-controlled atmosphere. atmosphere.atmosphere. Another reason AnotherAnother for reasonreasonchromium forfor chromiumchromium oxide appearance oxideoxide appearanceappearance is its presence isis itsits presence inpresence polishing inin polishingpolishing suspension suspensionsuspension as an abrasive asas anan abrasiveabrasive agent. Due agent.agent. to DueDuethe fact toto thethe that factfact the thatthat samples thethe samplessamples were ﬁrstly werewere firstlyfirstly investigated investinvestigatedigated on optical onon opticaloptical microscopy microscopymicroscopy and then andand subjectedthenthen subjectedsubjected to XRD toto XRD analysis, some of the Cr2O3 particles (dµ = 1–2 μm) can penetrate the surface during the XRDanalysis, analysis, some ofsome the Crof2 Othe3 particles Cr2O3 particles (d = 1–2 (dm) = can1–2 penetrateμm) can thepenetrate surface duringthe surface the cross-section during the cross-sectioncross-sectionpreparation procedure. preparationpreparation procedure.procedure. EDSEDS analysisanalysis hashas shownshown thatthat that ironiron iron boridesborides areare are hihi highlyghlyghly alloyedalloyed alloyed withwith with W,W, W, Cr,Cr, and and V V (Table (Table 2 2).2).). ForFor instance, tungsten concentration in the FeB zone is around 8%, while ~10% in the inner-Fe2B zone. instance, tungstentungsten concentrationconcentration in in the the FeB FeB zone zone is aroundis around 8%, 8%, while while ~10% ~10% in the in inner-Fethe inner-Fe2B zone.2B zone. The TheThetungsten-rich tungsten-richtungsten-rich zone zone waszone revealed waswas revealedrevealed below thebelowbelow compound thethe cocompoundmpound layer. It is layer.layer. formed ItIt dueisis formedformed to tungsten duedue displacementtoto tungstentungsten displacementdisplacementfrom the surface fromfrom by boron,thethe surfacesurface while Crbyby and boron,boron, V concentration whilewhile CrCr and isand relatively VV concentrationconcentration stable throughout isis relativelyrelatively the layer stablestable and throughoutthroughoutthe base metal. thethe Microhardnesslayerlayer andand thethe basebase measurements metal.metal. MicrohardnessMicrohardness have shown measurements unusualmeasurements high values, havehave shown whichshown can unusualunusual be referred highhigh values,values, whichwhich cancan bebe referredreferred toto asas ironiron boridesborides alloyingalloying withwith thethe aforementionedaforementioned elements.elements. TheThe summarizedsummarized datadata areare presentedpresented inin TableTable 33 inin comparisoncomparison toto thethe literatureliterature data.data.

Lubricants 2020, 8, 26 6 of 15 to as iron borides alloying with the aforementioned elements. The summarized data are presented in Table3 in comparison to the literature data.

Table 2. The elemental composition of borided steel H21, wt %.

Distance from B 1 C 1 V Cr Fe W Total the Surface 25 µm 12.90 6.35 0.44 2.29 69.57 8.45 100.00 75 µm 4.44 3.49 0.52 2.69 79.00 9.86 100.00 125 µm - 9.91 0.50 2.40 73.44 13.76 100.00 175 µm - 11.61 0.55 2.50 70.80 14.54 100.00 225 µm - 6.67 0.53 2.84 77.8 12.16 100.00 1 Boron and carbon content is given to reﬂect the concentration variation depending on the distance from the surface. The actual values are not possible to deﬁne by EDS analysis.

Table 3. Some parameters of boride layers after thermal-chemical treatment (TCT) in diﬀerent treatment pastes.

B4C-NaF, B4C-B4O7-Na3AlF6 B4C-SiC-KBF4 Characteristics 1050 ◦C, 120 min, 1000 ◦C, 20 min, 1000 ◦C, 20 min, Tool Steel H21 Low-Carbon Steel [33] Steels [9] Layer thickness, µm 105 50 50–120 Microhardness, HV0.1 2700–2900 1800–2100 1450–2400 Phase composition FeB/Fe2B FeB/Fe2B FeB/Fe2B

A diffusion layer with the heterogeneous microstructure characterized by the thickness up to 560 µm was formed after boroaluminizing, which is much thicker than the one after pure boring (Figure5). The obtained layer can be separated into four structural zones, each on a certain layer depth (Figure5a). Porous outer Zone 1 contains two types of crystals—the light fine ones and coarse sharp crystals, embedded in a softer matrix (Figure5b). Zone 2 is placed underneath and characterized by the presence of gray and light crystals with fine or elongated shape displayed in the matrix as in the previous zone (Figure5c). Zone 3 consists of bainite needles encircled by a light net of the second phase (Figure5d). The upper zone of the base metal can be designated as a transition sublayer and marked as Zone 4, where the martensitic structure is observed. Metallographic analysis along with the EDS data has shown that each zone comprises two main phases with peculiar elemental composition and microstructure (Table4). For instance, aluminum-containing phases such as Fe3Al (17.5 wt % of Al) and aluminum solid solution in α-Fe (10.35 wt % of Al) concentrate in the upper zones and play the role of the soft matrix. The former phase concentrates in Zone 1, while the latter in Zone 2. The XRD analysis confirms the presence of Fe3Al. Besides, other compounds were revealed such as Fe2O3, FeB, Fe7W6, and AlN (Figure6). The discovered iron oxide Fe2O3 concentrates in the upper Zone 1 and results in its high porosity. The presence of hematite means that oxygen penetrates through the boron oxide film. At the same time, there were no oxygen traces detected in the inferior zones. Aluminum nitride formed on the surface of the layer as a product of aluminum reaction with atmospheric nitrogen [30,31]. The hard phases in the upper Zone 1 are represented by iron boride FeB, alloyed with Al, W, Cr, and V. Unlike pure boride layer, the crystal growth of iron boride in the (001) direction was changed after two-component TCT. They orient at different angles relative to the diffusion direction (Figure5b). Lubricants 2020, 8, 26 7 of 15 Lubricants 2020, 8, x FOR PEER REVIEW 7 of 15

FigureFigure 5. Microstructure ofof boroaluminized boroaluminized steel steel H21 H21 treated treated at 1050at 1050◦C for°C 2for h, where2 h, where (a) is the(a) wholeis the wholelayer; (layer;b–d) are(b– magniﬁedd) are magnified pictures pictures of the layer of the zones layer 1, 2,zones and 3;1, (2,e) and is the 3; atomic (e) is forcethe atomic microscope force microscope(AFM) image (AFM) of Zone image 3. of Zone 3.

Table 4. The elemental composition of boroaluminized steel H21, wt %. Table 4. The elemental composition of boroaluminized steel H21, wt %. Probable DistanceDistance from from the Probable Phase Phase B 1 B 1 Al Al CC 1 1 VV CrCr FeFe W W TotalTotal the SurfaceSurface Composition 25–100 µm (Zone 1)Composition FeB/Fe3 Al 11.50/- 0.45/17.50 2.93/2.22 0.64/- 4.73/0.63 73.00/78.52 6.75/1.13 100.00/100.00 25–100175–225 μµm (Zone 2) Mx(B,C)y/α-Fe(C,Al)11.50/ 9.84 /--0.45/1/10.35 2.93/2.28.60/5.71 1.42/- 1.514.73/0./0.34 22.3973.00/78/82.08 56.246.75/1.1/1.52 100.00100.00/1/100.00 375–475 µm (Zone 3)FeB/Fe Mx3AlC/bainite -/- 1.24/5.11 7.22/2.92 0.64/-0.69/- 4.09/- 76.65/90.46 9.73/1.51 100.00/100.00 below(Zone 560 1)µ m (Zone 4) MxC+ martensite- -7.50 -2 7.90 0.48 2.5063 80.04.52 9.083 100.0000.00 175–2251 Boron μm and carbonMx(B,C)y/ contentα-Fe is given9.84/ to reﬂect the-/ concentration 8.60/ variation1.42/ depending 1.51/ on the22.39/ distance from56.24/ the surface 100.00/ (Zoneand 2) phase composition.(C,Al) The actual values- are10.35 not possible 5.71 to deﬁne by- EDS analysis.0.34 82.08 1.52 100.00 375–475 μm -/ 1.24/ 7.22/ 0.69/ 4.09/ 76.65/ 9.73/ 100.00/1 MxC/bainite (Zone 3) - 5.11 2.92 - - 90.46 1.51 00.00 below 560 μm MxC+ - - 7.90 0.48 2.50 80.04 9.08 100.00 (Zone 4) martensite 1 Boron and carbon content is given to reflect the concentration variation depending on the distance from the surface and phase composition. The actual values are not possible to define by EDS analysis.

Lubricants 2020, 8, 26 8 of 15 Lubricants 2020, 8, x FOR PEER REVIEW 8 of 15

FigureFigure 6. 6. XRDXRD pattern pattern of of steel steel H21 H21 after after boroaluminizing boroaluminizing at at 1050 1050 °C◦C for for 2 2 h. h.

ConcerningConcerning revealed revealed Fe Fe77WW66, ,it it is is known known that that Fe Fe77WW66 isis a a high-temperature high-temperature compound compound stable stable at at temperaturetemperature range range 1190–1637 1190–1637 °C◦C[ [35].35]. Another Another point point is is that that ferrotungsten ferrotungsten main main production production methods methods areare the carbothermic and carbothermic–silicothermiccarbothermic–silicothermic reduction from tungsten trioxide. Thus, Thus, the the formationformation of of Fe Fe77WW66 isis unlikely. unlikely. Carbide Carbide presence presence is is more more realistic realistic instead. The The precipitation precipitation of of M xCC andand M M2323CC6 6carbidescarbides occur occur during during H21 H21 tool tool steel steel solidific solidificationation process process and and heat heat treatment, treatment, where where M M is is Fe,Fe, Cr, Cr, and W [27]. [27]. It It is is known known that that boron boron can su substitutebstitute up up to to 80% of carbon in cementite Fe 33CC at at 10001000 °C◦C[ [36].36]. As As a a result, result, carboborides Fe3 (C,B)(C,B) and and Fe Fe2323(C,B)(C,B)66 formform during during the the solidification solidification process process inin the the ternary ternary Fe–C–B Fe–C–B system. system. These These compounds compounds can can be be present present next next to to each each other other and and Fe Fe22BB[ [37].37]. MM33(C,B),(C,B), M M2323(C,B)(C,B)6 6carboborides,carboborides, and and solid solid solutions solutions (α (α-Fe(Al,C))-Fe(Al,C)) represent represent the the phase phase composition composition in in ZoneZone 2, 2, where where M M is is predominantly predominantly Fe and W according to EDS data (Table 4 4).). MetallographicMetallographic analysis analysis revealed revealed bainite bainite in in Zone Zone 3 3 (Figure (Figure 55d).d). Its formation can bebe explainedexplained byby the the presence of undissolved carbides that stim stimulateulate the bainite transformation [38]. [38]. The The second second phasephase in in Zone Zone 3 3 segregates segregates the the form form of of the the light light ne nett around bainite plates. It It is is known known that that H21 H21 steel steel tendstends to to form form a a brittle brittle eutectic eutectic carbide carbide network network [38]. [38 ].In In[39], [39 it], was it was reported reported that that boriding boriding resulted resulted in embrittlementin embrittlement of the of theH11 H11 steel steel compared compared to the to thenon-treated non-treated samples. samples. The The authors authors explained explained such such an effectan eff ectby the by thefact factthat that the layer the layer itself itself and andthe subl the sublayerayer zone zone contributed contributed to the to bulk the bulkembrittlement embrittlement due todue the to transcrystalline the transcrystalline cleavage. cleavage. The topography The topography map mapobtained obtained by AFM by AFM analysis analysis shows shows carbides carbides on theon thehigh high ground, ground, while while bainite bainite is isin inthe the lower lower position position (Figure (Figure 5e).5e). The The element composition ofof carbidescarbides in in Zone Zone 3 3 differs differs significantly significantly comparing to upper Zone 2. For For instance, instance, tungsten tungsten content content deceaseddeceased to to 9.83 9.83 wt wt %, %, which which is is 5.5 5.5 times times less less than than in in Zone Zone 2. 2. In In addition, addition, no no boron boron was was revealed revealed by by EDSEDS analysis analysis (Table (Table 44).). TheThe microhardness microhardness profiles profiles corresponding corresponding to to the the structural zones are given in Figure7 7.. TheThe profilesprofiles didifferffer significantlysignificantly depending depending on on the the TCT TCT method. method. Thus, Thus, boriding boriding has resulted has resulted in iron in borides iron boridesformation, formation, with the with superior the superior microhardness microhardness values ofvalues 2700–2900 of 2700–2900 HV0.1, HV0.1, compared compared to the valuesto the valueson Armco on Armco iron and iron carbon and carbon steels, steels, which which lay at anlay intervalat an interval of 1800–2400 of 1800–2400 HV0.1 HV0.1 [9]. The [9]. base The metalbase metalmicrohardness microhardness is almost is almost constant constant throughout throughout the cross-section the cross-section and its and value its isvalue about is 550about HV0.1. 550 HV0.1.Microhardness Microhardness of the baseof the metal base ismetal much is lowermuch comparedlower compared to the layer.to the Thislayer. high This drop high in drop values in valuesindicates indicates the absence the absence of the derived of the transitionderived transi zonetion with zone the variable with the element variable and element phase composition. and phase composition.That fact could That result fact in could the layer result chipping in the layer under chipping industrial under exploitation, industrial especially exploitation, at shock especially loads. at A shockcompletely loads. di Aff completelyerent profile different has been profile obtained has onbeen the obtained boroaluminized on the boroaluminized samples. Each particularsamples. Each zone particularpossesses zone certain possesses fluctuation certain of microhardness fluctuation of microhardness values. The high values. initial The peak high of aboutinitial 2000peak HV0.1 of about on 2000 HV0.1 on the profile corresponds to the iron boride zone (Zone 1) on top of the layer. Then, the microhardness drops to its minimum value of around 300 HV0.1. Such a great drop corresponds to

Lubricants 2020, 8, 26 9 of 15

Lubricantsthe proﬁle 2020 corresponds, 8, x FOR PEER to REVIEW the iron boride zone (Zone 1) on top of the layer. Then, the microhardness9 of 15 drops to its minimum value of around 300 HV0.1. Such a great drop corresponds to Zone 2 and Zonerelates 2 toand its ferriticrelates structure.to its ferritic Despite structure. the presence Desp ofite hard the carbidespresence (some of hard coarse carbides crystals (some have coarse shown crystalsmicrohardness have shown over microhardness 3000 HV0.1) in over Zone 3000 2, theHV0.1) average in Zone value 2, the is 500–700 average HV0.1.value is It500–700 is known HV0.1. that Ittungsten is known carbides that tungsten possess carbides high hardness possess up high to 3100 hardness HV0.1 up [40 to]. 3100 Another HV0.1 source [40]. indicatesAnother ternarysource indicatescarbides (Feternary3W3C carbides and Fe6 W(Fe6C)3W microhardness3C and Fe6W6C) of microhardness 1560 HV0.1 [41 of]. 1560 The lowHV0.1 average [41]. The microhardness low average in microhardnessZone 2 can be explainedin Zone 2 by can the be shape explained and size by of carbides.the shape Shallow and size and of/or carbides. elongated Shallow crystals and/or (up to elongated20 µm) undergo crystals indentation (up to 20 μ andm) undergo displacement indentation in the plasticand displacement matrix under in the a load plastic of 100 matrix gf. Thus, under the a loadmeasurement of 100 gf. reﬂectsThus, the soft measurem matrix microhardnessent reflects soft in matrix this particular microhar zone.dness in this particular zone.

FigureFigure 7. 7. MicrohardnessMicrohardness distribution distribution of of boride boride an andd boroaluminized boroaluminized layers layers on on steels steels H21. H21.

TheThe average average microhardness microhardness in in Zone Zone 3 3 is is about about 8 85050 HV0.1, HV0.1, where where the the hardness hardness of of bainite bainite is is 600 600 HV0.1,HV0.1, and and of of the the carbide carbide network network is is about about 1400 1400 HV0.1. TheThe second peakpeak ofof aboutabout 1450 1450 HV0.1 HV0.1 on on microhardness microhardness proﬁle profile was was measured measured in transition in transition Zone Zone4. This 4. growthThis growth can be can attributed be attributed to the to carbide-rich the carbide-rich zone, whichzone, which was formed was formed as a result as a ofresult carbon of carbondisplacement displacement from the from surface. the Carbonsurface. distributionCarbon distribution is a systematic is a systematic phenomenon phenomenon for tool steels for with tool a steelshigher with alloying a higher level. alloying It is driven level. byIt is the driven insoluble by the nature insoluble of carbides nature inof boridescarbides [39 in]. borides It is known [39]. It that is knownthe enhancement that the enhancement of tungsten of concentration tungsten concentration in M23C6-carbide in M23 canC6-carbide increase can its increase hardness its from hardness initial from1080 HV0.1initial 1080 to 1450 HV0.1 HV0.1 to 1450 [37]. HV0.1 [37]. BeneathBeneath the the carbide-rich carbide-rich zone, zone, the the values values grad graduallyually decrease decrease from from around around 1000 1000 HV0.1 HV0.1 to to 550 550 HV0.1.HV0.1. The minimumminimum value value coincides coincides with with the the literature literature data, data, indicating indicating 520 HV0.1 520 afterHV0.1 the after vacuum the vacuumheat treatment heat treatment of H21 steelof H21 [18 ].steel The [18]. base The metal base microhardness metal microhardness after boroaluminizing after boroaluminizing is slightly is slightly superior to the one after pure boriding until achieving a distance of 1.9 mm from the surface. Generally, the microhardness values of the multicomponent layer range significantly and are characterized by sharp rises and drops, which indicate the phase and elemental composition diversity.

Lubricants 2020, 8, 26 10 of 15 superior to the one after pure boriding until achieving a distance of 1.9 mm from the surface. Generally, the microhardness values of the multicomponent layer range significantly and are characterized by Lubricants 2020, 8, x FOR PEER REVIEW 10 of 15 sharp rises and drops, which indicate the phase and elemental composition diversity. It is known that wear and plastic deformation are the main failure processes of hot forging It is known that wear and plastic deformation are the main failure processes of hot forging dies dies [39]. The wear behavior of steel H21 shows that the samples after TCT are superior in wear [39]. The wear behavior of steel H21 shows that the samples after TCT are superior in wear resistance in comparison with the steels without diffusion layers on the surface (Figure8). Both resistance in comparison with the steels without diffusion layers on the surface (Figure 8). Both heat-treated (quenching followed by tempering) and bare steel have shown critical mass loss since the heat-treated (quenching followed by tempering) and bare steel have shown critical mass loss since very beginning of the test and characterized by a sharp wear curve ascent. However, the former is the very beginning of the test and characterized by a sharp wear curve ascent. However, the former more wear-resistant compared to the non-treated samples and the mass loss was measured as 0.06 g is more wear-resistant compared to the non-treated samples and the mass loss was measured as 0.06 against 0.17 g after the five-min test, respectively. The continuing of the test was decided redundant g against 0.17 g after the five-min test, respectively. The continuing of the test was decided for these samples due to intense mass loss and high wear rate. redundant for these samples due to intense mass loss and high wear rate.

0.18

0.16

0.14 annealing quenching + tempering 0.12 boriding 0.1 boroaluminizing 0.08 ΔM, G ΔM,

0.06

0.04

0.02

0 0 102030405060 Time, min

Figure 8. WearWear resistance of steel H21.

The borided samplesample hashas demonstrated demonstrated the the highest highest resistance resistance to to dry dry friction. friction. In theIn the meantime, meantime, the thesample sample after after boroaluminizing boroaluminizing possesses possesses the corresponding the corresponding slope ofslope the of wear the curve wear aftercurve the after six-min the six-mintest. Apparently, test. Apparently, the elevated the elevated wear rate wear at the rate beginning at the beginning of the test of resulted the test from resulted the presence from the of presenceporous Zone of porous 1. It is Zone known 1. It that is known high porosity that high accompanied porosity accompanied by the brittle by the phase brittle presence phase resultspresence in resultssignificant in significant wear rate wear enhancement rate enhancement during abrasive during abrasive wear without wear without lubrication lubrication [42]. As [42]. soon As as soon the asporous the porous zone is zone removed, is removed, the wear the curve wear slopecurve changed slope changed the replicating the replicating borided borided sample’s sample’s wear profile. wear profile.It was revealed It was revealed that the that mass the loss mass had loss reduced had reduced by two by times two aftertimes boroaluminizing after boroaluminizing and by and 13 timesby 13 timesafterboriding after boriding compared compared to the to hardened the hardened sample sample after the after five-min the five-min weartest. wear test. The wearwear mechanismmechanism on on steel steel H21 H21 diff ersdiffers significantly significantly depending depending on the on treatment. the treatment. The abrasive The abrasivegrooves orientedgrooves tooriented the sliding to the direction sliding aredirection visible are on barevisible and on heat-treated bare and heat-treated steel (Figure steel9a,b). (Figure Such 9a,b).behavior Such is behavior inherent inis theinherent abrasion in the wear abrasion mechanism wear mechanism [43]. At the [43]. same At time, the localsame traces time, oflocal adhesive traces ofwear adhesive can be wear observed can be on observed bare steel, on where bare abrasivesteel, wh groovesere abrasive are crumpled grooves are or filledcrumpled with aor small filled number with a smallof particles. number This of indicatesparticles. the This first indicates stage of the material first transfer,stage of wherematerial the transfer, shear and where rupture the of shear adhesive and rupturejunctions of in adhesive asperities junctions of tested in material asperities results of tested surface material damage. results surface damage.

Lubricants 2020, 8, 26 11 of 15 Lubricants 2020, 8, x FOR PEER REVIEW 11 of 15

Figure 9. SEMSEM images images of of worn worn surfaces surfaces on on steel steel H21 H21 after after wear wear tests: tests: ( (aa)) bare bare steel; steel; ( (bb)) heat heat treatment; treatment; (c) boriding; ( d) boroaluminizing.

Another type type of of worn surfaces was was obtained obtained after after TCT. TCT. As can be seen in Figure 99c,d,c,d, bothboth borided and boroaluminized samples possess large de delaminatinglaminating and flattened ﬂattened (crumpled) areas. In addition toto mentionedmentioned damaged damaged surfaces, surfaces, few few separate sepa abrasiverate abrasive grooves grooves parallel parallel to the sliding to the direction sliding directionare visible are on visible the surface on the of surface the borided of the sample.borided sample. In [44], theIn [44], authors the authors indicated indicated that borided that borided carbon carbonsteels are steels extremely are extremely resistant resistan to adhesiont to adhesion and have and low have welding low welding tendencies tendencies due to highdue ironto high borides iron boridesmicrohardness. microhardness. This statement This statement can be partially can be attributed partially toattributed borided steelto borided H21, wherein steel H21, a combination wherein a combinationof abrasion andof adhesionabrasion wearand adhesion takes place. wear Apparently, takes place. the latterApparently, process the is alatter predominant process wearis a predominantmechanism in wear this particularmechanism case. in this particular case. As forfor thethe two-component two-component di ﬀdiffusionusion layer, layer, no abrasiveno abrasive grooves grooves are visible. are visible. The damaged The damaged surface surfaceis characterized is characterized by delaminated by delaminated craters and craters smeared and adhesion smeared lumps.adhesion Such lumps. a surface Such was a surface formed was as a formedresult of as intense a result adhesive of intense material adhesive transfer andmaterial its following transfer plastic and its deformation following resulting plastic indeformation the galling resultingbuild-up. in It the is known galling that build-up. stainless It steelis known and aluminum that stainless possess steel the and most aluminum tendencies possess to gall the during most tendenciescontact with to the gall tool during surface contact [45,46 ].with According the tool to surface the EDS [45,46]. analysis, According the aluminum to the was EDS content-rich analysis, the by aluminum17.5% in the was upper content-rich Zone 1 of by the 17.5% boroaluminized in the uppe layer,r Zone which 1 of contributes the boroaluminized to its adhesion layer, behavior which contributes(Table4). to its adhesion behavior (Table 4). EDS analysis of worn surface after wear test ha hass shown a high high amount of oxygen on the the borided borided sample (Figure(Figure 1010a).a). TheThe lowest lowest oxygen oxygen content content of aboutof about 3.9% 3.9% was was measured measured in the in delaminated the delaminated crater crater(Spectrum (Spectrum 1). The 1). rest The three rest spectrums three spectrums have shown have oxygen shown concentration oxygen concentration from 41.4% from to 49.8%. 41.4% This to 49.8%.means This that means oxidation that wearoxidation takes wear place takes in addition place in toaddition mentioned to mentioned wear mechanisms. wear mechanisms. High contact High contactpressure pressure results in results friction in heating, friction whichheating, accelerates which accelerates the formation the offormation oxides on of wear oxides surfaces. on wear The surfaces.thickness The of the thickness oxide layer of the is unevenoxide layer and itsis lowestuneven value and its corresponds lowest value to elevated corresponds tungsten to elevated content tungstenaccompanied content by vanadiumaccompanied presence by vanadium in the vicinity presence of Spectrum in the vicinity 1. According of Spectrum to [43 1.], theAccording high wear to resistance of heat-treated H21 steel is attributed to undissolved carbides Fe W C. Thus, both types of [43], the high wear resistance of heat-treated H21 steel is attributed to undissolved3 3 carbides Fe3W3C. hard compounds-iron borides and carbides M C (where M is Fe, W, Cr, V) contribute to wear resistance Thus, both types of hard compounds-iron boridesx and carbides MxC (where M is Fe, W, Cr, V) contributefor borided to steel. wear resistance for borided steel. The wear surface of of the boroaluminized sample sample was was severely severely oxidized oxidized as as well well (Figure (Figure 10b).10b). Oxygen content ranges ranges from from 38.9% 38.9% to to 53.4% in points (Spectrums 1–4) 1–4) and and over over 55% 55% in an area (Spectrum 5)5) according according to to EDS EDS measurements. measurements. Aluminum Aluminum with thewith content the content of 2.6%–3.6% of 2.6%–3.6% was revealed was revealed in addition to alloying metals. Generally, the elemental composition is relatively uniform

Lubricants 2020, 8, 26 12 of 15 Lubricants 2020, 8, x FOR PEER REVIEW 12 of 15 throughoutin addition tothe alloying wear surface. metals. The Generally, oxide layer the elemental mainly consists composition of iron is relativelyand aluminum uniform oxides, throughout which arethe considered wear surface. as Thetribo-oxides, oxide layer limiting mainly material consists transfer of iron andduring aluminum wear [43,44]. oxides, which are considered as tribo-oxides, limiting material transfer during wear [43,44].

(a)

(b)

Figure 10. EDSEDS analysis analysis results results of of worn worn surfaces after wear test on steel H21: ( (aa)) boriding; boriding; ( (bb)) boroaluminizing.1 BoronBoron and and carbon carbon content content is is given given to to refl reﬂectect the the concentration concentration variation depending on the distance from the surface. The actual values are not possi possibleble to define deﬁne by EDS analysis.

4. Conclusions

Lubricants 2020, 8, 26 13 of 15

4. Conclusions

This research generated the following findings: boriding has resulted in FeB/Fe2B layer formation with the tooth-like structure, wherein obtained iron borides are highly alloyed with tungsten and chromium. The two-component thermal-chemical treatment (TCT) results in the surface layer formation with a heterogeneous structure. There are four sublayers (zones) that can be distinguished, each with specific microstructure and properties. The layer thickness has been increased by five times compared to pure boriding and reached 560 µm. The nature of the formation of the heterogeneous microstructure is not established yet. However, it is apparently affected by aluminum presence in the treatment paste. Microhardness of steel H21 after pure boriding reached up to 2900 HV0.1. Such elevated microhardness is due to iron borides alloying with W, Cr, and V. The microstructure complexity and phase diversity of boroaluminized steel result in microhardness profile variations, where maximum values correspond to iron boride and carbides zones, and minimum to Fe3Al in Zone 2. Both boriding and boroaluminizing lead to a significant wear resistance improvement of steel H21. However, the wear mechanism of boride and boroaluminized layers differs and largely refers to their microstructure and composition. It has been established that galling is the principal wear mechanism for the two-component layer. On the contrary, the boride layer has a low welding tendency. This property contributes to a higher wear resistance of the borided sample compared to the boroaluminized one. Besides, the tribo-oxide layer forms during the wear test.

Author Contributions: Conceptualization, H.L. and U.M.; methodology, N.U. and U.M.; investigation, Y.C., N.U., and U.M.; data curation, U.M.; writing—original draft preparation, U.M.; writing—review and editing, H.L. All authors have read and agreed to the published version of the manuscript. Funding: The study was carried out with a grant from the Russian Science Foundation (Project No. 19-79-10163). Acknowledgments: This work was supported by the Fulbright Visiting Scholar Program. Undrakh Mishigdorzhiyn’s internship research results at the Department of Mechanical Engineering, Texas A&M University, in 2018 are used in the paper. The scholar is grateful to Alex Fang, Eugene Chen and Lian Ma of Department of Mechanical Engineering, Texas A&M University, for providing research facilities, consults, and assistance in experiments. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

References

1. Kulka, M. Trends in thermochemical techniques of boriding. In Current Trends in Boriding, Engineering Materials; Springer: Cham, Switzerland, 2019; pp. 17–98. 2. Campos-Silva, I.; Palomar-Pardavé, M.; Pérez Pastén-Borja, R.; Kahvecioglu Feridun, O.; Bravo-Bárcenas, D.;

López-García, C.; Reyes-Helguera, R. Tribocorrosion and cytotoxicity of FeB-Fe2B layers on AISI 316 L steel. Surf. Coat. Technol. 2018, 349, 986–997. [CrossRef] 3. Ribeiro, R.; Ingole, S.; Usta, M.; Bindal, C.; Ucisik, A.H.; Liang, H. Tribological characteristics of boronized niobium for biojoint applications. Vacuum 2006, 80, 1341–1345. [CrossRef] 4. Keddam, M.; Kulka, M. A kinetic model for the boriding kinetics of AISI D2 steel during the diﬀusion annealing process. Prot. Met. Phys. Chem. Surf. 2018, 54, 282–290. [CrossRef] 5. Timur, S.; Kartal, G.; Eryilmaz, O.L.; Erdemir, A. US Patent No. 8,951,402, 10 February 2015.

6. Xie, F.; Sun, L.; Cheng, J. Alternating current ﬁeld assisted pack boriding to Fe2B coating. Surf. Eng. 2013, 29, 240–243. [CrossRef] 7. Kulka, M.; Pertek, A. Characterization of complex (B+C+N) diﬀusion layers formed on chromium and nickel-based low carbon steels. Appl. Surf. Sci. 2003, 218, 114–123. [CrossRef] 8. Novakova, A.A.; Sizov, I.G.; Golubok, D.S.; Kiseleva, T.Y.; Revokatov, P.O. Electron-beam bonding of low-carbon steel. J. Alloys Compd. 2004, 383, 108–112. [CrossRef] 9. Krukovich, M.G.; Prusakov, B.A.; Sizov, I.G. Plasticity of Boronized Layers; Springer International Publishing: Cham, Switzerland, 2016; pp. 111–227. 10. He, X.; Xiao, H.; Ozaydin, M.F.; Balzuweit, K.; Liang, H. Low-temperature boriding of high-carbon steel. Surf. Coat. Technol. 2015, 263, 21–26. [CrossRef] Lubricants 2020, 8, 26 14 of 15

11. Kartal, G.; Timur, S.; Sista, V.; Eryilmaz, O.L.; Erdemir, A. The growth of single Fe2B phase on low carbon steel via phase homogenization in electrochemical boriding (PHEB). Surf. Coat. Technol. 2011, 206, 2005–2011. [CrossRef] 12. Xie, F.; Wang, X.-J.; Pan, J.-W. Accelerate pack boriding with reused boriding media by simultaneously employing Al and alternating current field. Vacuum 2017, 141, 166–169. [CrossRef] 13. Kulka, M.; Makuch, N.; Pertek, A.; Piasecki, A. Microstructure and properties of borocarburized and laser-modified 17CrNi6-6 steel. Opt. Laser Technol. 2011, 44, 872–881. [CrossRef] 14. Sizov, I.G.; Smirnyagina, N.N.; Semenov, A.P. Special features of electron-beam boronizing of steels. Met. Sci. Heat Treat. 1999, 41, 516–519. [CrossRef] 15. Mikołajczak, D.; Kulka, M.; Makuch, N.; Dziarski, P. Laser borided heterogeneous layer produced on austenitic 316L steel. Arch. Mech. Technol. Mater. 2016, 36, 35–39. [CrossRef] 16. Mishigdorzhiyn, U.L.; Sizov, I.G.; Polaynsky, I.P. Formation of coatings based on boron and aluminum on the surface of carbon steels by electron beam alloying. Obrab. Met. Met. Work. Mater. Sci. 2018, 20, 87–99. [CrossRef] 17. Christiansen, T.; Bottoli, F.; Dahl, K.; Gammeltoft-Hansen, N.; Laursen, M.; Somers, M. Hard surface layers by pack boriding and gaseous thermo-reactive deposition and diffusion treatments. Mater. Perform. Charact. 2017, 6, 475–491. [CrossRef] 18. Chander, S.; Chawla, V. Characterization and Industrial Performance Evaluation of Duplex-Treated AISI H21 Die Steel during Hot Forging Process. Mater. Perform. Charact. 2019, 8, 197–210. [CrossRef] 19. Rohr, V.; Schutze, M.; Fortuna, E.; Tsipas, D.N.; Milewska, A.; Perez, F.J. Development of novel diffusion coatings for 9–12% Cr ferritic-martensitic steels (SUNASPO). In Novel Approaches to Improving High Temperature Corrosion Resistance. European Federation of Corrosion Publication; Schütze, M., Quadakkers, W.J., Eds.; Woodhead Publishing: Cambridge, UK, 2008; Volume 47, pp. 176–192. 20. Voroshnin, L.G.; Mendeleeva, O.L.; Smetkin, V.A. Teoriya i Tekhnologiya Khimiko-Termicheskoy Obrabotki [Theory and Technology of Thermal-Chemical Processing]; Novoje Znanije: Minsk, Belarus, 2010; p. 304. 21. Czerwinski, F. Thermochemical treatment of metals. In Heat Treatment: Conventional and Novel Applications; Czerwinski, F., Ed.; InTech Open Access: Rijeka, Croatia, 2012; pp. 73–112. 22. Pertek, A.; Kulka, M. Characterization of complex (B+C) diffusion layers formed on chromium and nickel-based low carbon steel. Appl. Surf. Sci. 2002, 202, 252–260. [CrossRef] 23. Bartkowska, A.; Bartkowski, D.; Piasecki, A. Effect of diffusion borochromizing on microstructure, microhardness and corrosion resistance of tool steel with different carbon content. J. Achiev. Mater. Manuf. Eng. 2017, 80, 49–55. [CrossRef] 24. Bartkowska, A.; Pertek, A.; Popławski, M.; Bartkowski, D.; Przestacki, D.; Miklaszewski, A. Effect of laser modification of B-Ni complex layer on wear resistance and microhardness. Opt. Laser Technol. 2015, 72, 116–124. [CrossRef] 25. Sizov, I.G.; Mishigdorzhiyn, U.L.; Leyens, C.; Vetter, B.; Furmann, T. Influence of thermocycle boroaluminizing on strength of steel C30. Surf. Eng. 2014, 30, 129–133. [CrossRef] 26. Zemskov, G.V.; Kogan, R.L. Mnogokomponentnoye Diffuzionnoye Nasyshcheniye Metallov i Splavov [Multi-Component Diffusion Saturation of Metals and Alloys]; Metallurgiya: Moscow, Russia, 1981; p. 208. 27. Balandin, Y.A. Surface Hardening of Die Steels by Diffusion boronizing, borocopperizing and borochromizing in fluidized bed. Met. Sci. Heat Treat. 2005, 47, 3–4. [CrossRef] 28. Zakhariev, Z.; Stambolova, I.; Marinov, M.; Perchemliev, C. Borozar-PII boronaluminising paste for steels treatment. Oxid. Commun. 2012, 35, 491–496. 29. Maragoudakis, N.E.; Stergioudis, G.; Omar, H.; Paulidou, H.; Tsipas, D.N. Boron–aluminide coatings applied by pack cementation method on low-alloy steels. Mater. Lett. 2002, 53, 406–410. [CrossRef] 30. Mishigdorzhiyn, U.; Sizov, I. The influence of boroaluminizing temperature on microstructure and wear resistance in low-carbon steels. Mater. Perform. Charact. 2018, 7, 252–265. [CrossRef] 31. Mishigdorzhiyn, U.; Polyansky, I.; Sizov, I.; Vetter, B.; Schlieter, A.; Heinze, S.; Leyens, C. Thermocyclic boroaluminizing of low carbon steels in pastes. Mater. Perform. Charact. 2017, 6, 531–545. [CrossRef] 32. Roberts, G.A.; Kraus, G.; Kennedy, R.L. Tool Steels, Materials Park; ASM International: Novelty, OH, USA, 1998. 33. Zimmerman, C. Boriding (boronizing) of Metals. In Steel Heat Treating Fundamentals and Processes; Dossett, J.L., Totten, G.E., Eds.; ASM International: Almere, The Netherlands, 2013; pp. 709–724. Lubricants 2020, 8, 26 15 of 15

34. Ulakhanov, N.S.; Mishigdorzhiyn, U.L.; Greshilov, A.D.; Tikhonov, A.G. Formation features of structure, perfor-mance properties and surface layer quality of 3KH2V8F die steel tooling. Vestnik Irkutskogo gosudarstvennogo tehnicheskogo universiteta (Proc. Irkutsk State Tech. Univ.) 2019, 23, 1104–1115. 35. Predel, B. Fe-W (Iron-Tungsten). In Dy-Er—Fr-Mo. Landolt-Börnstein—Group IV Physical Chemistry (Numerical Data and Functional Relationships in Science and Technology); Madelung, O., Ed.; Springer: Berlin/Heidelberg, Germany, 2013; Volume 5, pp. 1–4. 36. Goldschmidt, H.J. Mixed Interstitial Compounds. In Interstitial Alloys; Goldschmidt, H.J., Ed.; Butterworth-Heinemann: Oxford, UK, 1967; pp. 532–601.

37. Lentz, L.; Röttger, A.; Theisen, W. Hardness and modulus of Fe2B, Fe3(C,B), and Fe23(C,B)6 borides and carboborides in the Fe-C-B system. Mater. Charact. 2018, 135, 192–202. [CrossRef] 38. Nurbanasari, M.; Tsakiropoulos, P.; Palmiere, E.J. Inﬂuence of high temperature deformation and double tempering on the microstructure of a H21 tool steel. Mater. Sci. Eng. 2013, 570, 92–101. [CrossRef] 39. Jurˇci,P.; Hudáková, M. Diﬀusion Boronizing of H11 Hot Work Tool Steel. J. Mater. Eng. Perform. 2011, 20, 1180–1187. [CrossRef] 40. Samsonov, G.V.; Vitryanyuk, V.K.; Chaplygin, F.I. Karbidy Vol’frama [Tungsten Carbides]; Naukova Dumka: Kiev, Ukraine, 1974; p. 176. 41. Mercado, W.; Cuevas, J.; Castro, I.; Fajardo, M.; Alcázar, G.; Sthepa, H. Synthesis and characterization of

Fe6W6C by mechanical alloying. Hyperfine Int. 2007, 175, 49–54. [CrossRef] 42. Zhang, L.; Qu, X.-H.; Duan, B.-H.; He, X.-B.; Qin, M.-L. Effect of porosity on wear resistance of SiCp/Cu composites prepared by pressureless infiltration. Trans. Nonferrous Met. Soc. China 2008, 18, 1076–1082. [CrossRef] 43. Zhang, Q.; Wang, S.; Wang, L. Comparative study on wear of hot-working die steels. Appl. Mech. Mater. 2013, 331, 559–562. [CrossRef] 44. Gunes, I.; Kayali, Y.; Ulu, S. Investigation of surface properties and wear resistance of borided steels with different B4C mixtures. Indian J. Eng. Mater. 2012, 19, 397–402. 45. Alotaibi, J.; Yousif, B.; Yusaf, T. Wear behaviour and mechanism of different metals sliding against stainless steel counterface. Proc. Inst. Mech. Eng. Part J. Eng. Tribol. 2014, 228, 692–704. [CrossRef] 46. Pujante, J.; Pelcastre, L.; Vilaseca, M.; Casellas, D.; Prakash, B. Investigations into wear and galling mechanism of aluminium alloy-tool steel tribopair at different temperatures. Wear 2013, 308, 193–198. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).