Comparison of Wear Performance of Austempered and Quench-Tempered Gray Cast Irons Enhanced by Laser Hardening Treatment

applied sciences

Article Comparison of Wear Performance of Austempered and Quench-Tempered Gray Cast Irons Enhanced by Laser Hardening Treatment

Bingxu Wang 1,2,*, Gary C. Barber 2, Rui Wang 2,3 and Yuming Pan 2

1 Faculty of Mechanical Engineering and Automation, Zhejiang Sci-Tech University, #928 No.2 Street, High Education Zone, Hangzhou 310018, China 2 Department of Mechanical Engineering, School of Engineering and Computer Science, Oakland University, Rochester, MI 48309, USA; [email protected] (G.C.B.); [email protected] (R.W.); [email protected] (Y.P.) 3 College of Modern Science and Technology, China Jiliang University, Hangzhou 310018, China * Correspondence: [email protected]

Received: 30 March 2020; Accepted: 26 April 2020; Published: 27 April 2020

Abstract: The current research studied the effects of laser surface hardening treatment on the phase transformation and wear properties of gray cast irons heat treated by austempering or quench-tempering, respectively. Three austempering temperatures of 232 ◦C, 288 ◦C, and 343 ◦C with a constant holding duration of 120 min and three tempering temperatures of 316 ◦C, 399 ◦C, and 482 ◦C with a constant holding duration of 60 min were utilized to prepare austempered and quench-tempered gray cast iron specimens with equivalent macro-hardness values. A ball-on-flat reciprocating wear test configuration was used to investigate the wear resistance of austempered and quench-tempered gray cast iron specimens before and after applying laser surface-hardening treatment. The phase transformation, hardness, mass loss, and worn surfaces were evaluated. There were four zones in the matrix of the laser-hardened austempered gray cast iron. Zone 1 contained ledeburite without the presence of graphite flakes. Zone 2 contained martensite and had a high hardness, which was greater than 67 HRC. Zone 4 was the substrate containing the acicular ferrite and carbon-saturated austenite with a hardness of 41–27 HRC. In Zone 3, the substrate was tempered by the low thermal radiation. For the laser-hardened quench-tempered gray cast iron specimens, three zones were observed beneath the laser-hardened surface. Zone 1 also contained ledeburite, and Zone 2 was full martensite. Zone 3 was the substrate containing the tempered martensite. The tempered martensite became coarse with increasing tempering temperature due to the decomposition of the as-quenched martensite and precipitation of cementite particles. In the wear tests, the gray cast iron specimens without heat treatment had the highest wear loss. The wear performance was improved by applying quench-tempering heat treatment and further enhanced by applying austempering heat treatment. Austempered gray cast iron specimens had lower mass loss than the quench-tempered gray cast iron specimens, which was attributed to the high fracture toughness of acicular ferrite and stable austenite. After utilizing the laser surface hardening treatment, both austempered and quench-tempered gray cast iron specimens had decreased wear loss due to the high surface protection provided by the ledeburitic and martensitic structures with high hardness. In the worn surfaces, it was found that cracks were the dominant wear mechanism. The results of this work have significant value in the future applications of gray cast iron engineering components and provide valuable references for future studies on laser-hardened gray cast iron.

Keywords: gray cast iron; austempering; quench-tempering; laser surface hardening; wear loss

Appl. Sci. 2020, 10, 3049; doi:10.3390/app10093049 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 3049 2 of 12

1. Introduction Gray cast iron (GI) is a common ferrous material with the presence of graphite flakes in the matrix. It has been reported that GI has high machinability, high vibration adsorption, and high castability. In industry, GI has been widely utilized in the production of gears, guide rails, and cylinder liners for diesel engines [1–3]. These GI engineering parts frequently are required to withstand direct surface contacts with applied pressure and relative movement. Superior wear resistance is required to ensure good working performance and long service life. Thus far, surface-hardening treatments and heat treatments are considered as the common strengthening methods to improve the anti-wear performance of ferrous mechanical components [4–6]. Austempering heat treatment has received considerable attention in the past two decades due to the formation of a unique ausferritic structure with high strength–weight ratio, fracture toughness, ductility, and good tribological performance [7–10]. In the austempering heat treatment, fully austenitized cast iron is quickly transferred to a salt bath furnace and maintained for a certain time period. The austempering temperatures should be controlled within the range between the temperatures at which pearlite and martensite form. Quench-tempering heat treatment is a conventional heat treatment to increase the strength and hardness of cast irons and steels. During the quench-tempering process, the as-quenched martensite decomposes to tempered martensite with the precipitation of cementite particles or islands. Additional tempering can decrease the amount of retained austenite and release the internal residual stress caused by the rapid cooling rate in the quenching step [11,12]. The wear properties of GI have been significantly improved by the use of austempering and quench-tempering heat treatments. Balachandran et al. [8] investigated the wear resistance of hypereutectic gray cast iron after receiving austempering and quench-tempering heat treatments. The austempered gray cast iron (AGI) had the best wear performance, which was related to the Transformation induced plasticity (TRIP) effect, and the quench-tempered gray cast iron (QTGI) had moderate wear resistance, which was attributed to fine carbides. Sarkar et al. [10] determined the effects of austempering heat treatment on the microstructure, wear, and mechanical properties of AGI alloyed with copper. The amount of retained austenite increased, the hardness and tensile strength decreased, and the ductility and wear depth increased with higher austempering temperature. Laser surface hardening treatment is an effective approach to lower the wear loss and extend the fatigue life of cast iron and steel components. During the laser-hardening process, the top surface of cast irons or steels is heated above Acm or the eutectic critical temperature to transform the original microstructure into an unstable austenite or liquid state. Then, the top surface in the fully austenitized or liquid state is cooled by air and self-quenches to the adjacent cold substrate. Currently, laser surface hardening treatment is often applied to small components with complicated geometry [13–16]. Joshi et al. [17] applied laser glazing treatment to GI. The surface hardness was significantly improved due to the far-from equilibrium phase transformation. It was experimentally observed that the central fusion regions containing dendritic structure and cementite plates, and the heat-affected region containing bainite with graphite flakes was formed after laser glazing. Liu et al. [18] investigated the effects of surface temperature and scanning speed on the characteristics of the hardened layers of gray cast iron such as microstructure and hardness profile by using a high-power diode laser. It was found that a homogeneous microstructure with hardness ranging from 700 HV to 800 HV was formed in the laser-hardened zone, which was not dependent on the temperature and scanning speed. In addition, a deep hardened zone was produced when using high temperature and low scanning speed. In addition, they pointed out that overlapping areas around 16.7%–20.8% of the hardened track width was found, which would degrade the surface hardness because the martensite was tempered. Since the austempering, quench-tempering, and laser surface hardening treatments were frequently utilized to strengthen the cast irons and steels, it is vital to understand the coupling influence of traditional heat treatment and surface treatment on the phase transformation and wear properties of GI specimens. In the present study, the roles of laser surface-hardening treatment on the microstructure and wear resistance of AGI and QTGI specimens were investigated. A reciprocating sliding wear test Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 12

Appl.the Sci.microstructure2020, 10, 3049 and wear resistance of AGI and QTGI specimens were investigated.3 of 12A reciprocating sliding wear test configuration was utilized to determine the wear performance. The wear performance of AGI and QTGI with and without laser-hardening treatment was compared configuration was utilized to determine the wear performance. The wear performance of AGI and QTGI under equivalent macro-hardness values. The phase transformations in the laser-hardened regions with and without laser-hardening treatment was compared under equivalent macro-hardness values. and heat-affected regions were determined using an optical microscope. The macro-hardness and The phase transformations in the laser-hardened regions and heat-affected regions were determined micro-hardness profiles were measured by using a Rockwell hardness tester and a Vickers hardness using an optical microscope. The macro-hardness and micro-hardness profiles were measured by using tester. The worn surfaces were analyzed by utilizing a scanning electron microscope (SEM) to find a Rockwell hardness tester and a Vickers hardness tester. The worn surfaces were analyzed by utilizing possible wear mechanisms. The phase transformations in the matrix of AGI and QTGI after receiving a scanning electron microscope (SEM) to find possible wear mechanisms. The phase transformations laser hardening treatment can be utilized as useful references for other related GI research studies. In in the matrix of AGI and QTGI after receiving laser hardening treatment can be utilized as useful addition, a comparison of wear resistance between AGI and QTGI with and without laser surface- references for other related GI research studies. In addition, a comparison of wear resistance between hardening treatment has importance in developing future GI engineering components with better AGI and QTGI with and without laser surface-hardening treatment has importance in developing operational performance and longer service life. future GI engineering components with better operational performance and longer service life. 2. Materials and Methods 2. Materials and Methods

2.1.2.1. As-Received Gray Cast Iron TheThe percentage of chemical chemical elements elements of of the the GI GI was was analyzed analyzed by by a carbon–sulfur a carbon–sulfur analyzer analyzer and and an anoptical optical spectrometer, spectrometer, as aspresented presented in inTable Table 1.1 .The The microstructure microstructure of of the originaloriginal GIGI consistedconsisted ofof graphitegraphite ﬂakes,flakes, ferrite,ferrite, andand pearlite;pearlite; seesee FigureFigure1 1..

Table 1.1. Chemical Percentage ofof GrayGray CastCast Iron.Iron.

ElementsElements Percentage Percentage (%) (%) Carbon, C 3.53 Carbon, C 3.53 Silicon, Si 2.71 Silicon, Si 2.71 Copper,Copper, Cu Cu 0.94 0.94 Manganese,Manganese, Mn Mn 0.74 0.74 Chromium,Chromium, Cr Cr 0.12 0.12 Phosphorous,Phosphorous, P P 0.08 0.08 Sulfur,Sulfur, S S 0.03 0.03 Iron, Fe Remainder Iron, Fe Remainder

FigureFigure 1.1. Microstructure ofof thethe As-ReceivedAs-Received GrayGray CastCast IronIron (Graphite(Graphite FlakeFlake ++ Pearlite).

2.2. Heat Treatment Designs 2.2. Heat Treatment Designs 2.2.1. Austempering Heat Treatment 2.2.1. Austempering Heat Treatment The original GI specimens were ﬁrst austenitized at a temperature of 832 ◦C (1530 ◦F) with a The original GI specimens were first austenitized at a temperature of 832 °C (1530 °F) with a holding duration of 20 min in a high-temperature salt-bath furnace. The initial pearlite was converted holding duration of 20 min in a high-temperature salt-bath furnace. The initial pearlite was converted to unstable austenite, and the alloy elements were distributed uniformly in the matrix. Next, the to unstable austenite, and the alloy elements were distributed uniformly in the matrix. Next, the austenitized GI specimens were quickly transferred to another pre-heated medium temperature salt austenitized GI specimens were quickly transferred to another pre-heated medium temperature salt bath furnace for the austempering step. Austempering temperatures of 232 ◦C (450 ◦F), 288 ◦C (550 ◦F), bath furnace for the austempering step. Austempering temperatures of 232 °C (450 °F), 288 °C (550 and 343 ◦C (650 ◦F) with a constant holding duration of 120 min were used in this study. Then, the Appl. Sci. 2020, 10, 3049 4 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 12

°F), and 343 °C (650 °F) with a constant holding duration of 120 min were used in this study. Then, AGI specimens were quenched to room temperature by water. The austempering heat treatment is the AGI specimens were quenched to room temperature by water. The austempering heat treatment sketched in Figure2a. is sketched in Figure 2a.

Figure 2. HeatHeat Treatment Treatment Processes: ( a)) Austempering; Austempering; ( b) Quench-Tempering. 2.2.2. Quench-Tempering Heat Treatment 2.2.2. Quench-Tempering Heat Treatment First, the austenitizing temperature of 832 ◦C (1530 ◦F) with a holding duration of 20 min First, the austenitizing temperature of 832 °C (1530 °F) with a holding duration of 20 min was was used to fully transform the pearlite into unstable austenite in the matrix of the as-received GI used to fully transform the pearlite into unstable austenite in the matrix of the as-received GI specimens. Then, oil quenching was applied to cool the austenitized GI specimens to room temperature specimens. Then, oil quenching was applied to cool the austenitized GI specimens to room to minimize internal stresses and avoid internal cracks due to the quick cooling rate. Afterwards, temperature to minimize internal stresses and avoid internal cracks due to the quick cooling rate. tempering temperatures of 316 ◦C (600 ◦F), 399 ◦C (750 ◦F), and 482 ◦C (900 ◦F) were applied for 60 min Afterwards, tempering temperatures of 316 °C (600 °F), 399 °C (750 °F), and 482 °C (900 °F) were to duplicate the macro-hardness of the above AGI specimens. The tempering step was completed using applied for 60 min to duplicate the macro-hardness of the above AGI specimens. The tempering step electric furnaces. Finally, the tempered GI specimens were quenched again by oil to room temperature. was completed using electric furnaces. Finally, the tempered GI specimens were quenched again by The quench-tempering heat treatment is sketched in Figure2b. oil to room temperature. The quench-tempering heat treatment is sketched in Figure 2b. 2.3. Metallurgical Evaluation 2.3. Metallurgical Evaluation Specimen cubes (10 mm 10 mm 10 mm) were used in the metallurgical evaluation using an × × opticalSpecimen microscope. cubes The (10 surfacesmm × 10 of mm specimen × 10 mm) cubes were were used ground in the andmetallurgical polished usingevaluation sandpapers using an of optical microscope. The surfaces of specimen cubes were ground and polished using sandpapers of 240–1200 grit and polishing pads with 0.05 µm Al2O3-deagglomerated suspension. Then, the polished surfaces240–1200 were grit and etched polishing with a pads 3% nital with solution 0.05 µm for Al2 aO time3-deagglomerated period of 2 s or suspension. 3 s. Then, the polished surfaces were etched with a 3% nital solution for a time period of 2 s or 3 s. 2.4. Hardness Measurement 2.4. Hardness Measurement The macro-hardness was determined using a Rockwell hardness tester. The micro-hardness was determinedThe macro-hardness using a Vickers was hardness determined tester. using The a normal Rockwe loadll hardness was 9.8 tester. N, and Th testinge micro-hardness time was 10 was s in thedetermined micro-hardness using a measurements.Vickers hardness The tester. test surfaces The normal of GI load specimens was 9.8 were N, and ground testing and time polished was 10 prior s in tothe the micro-hardness hardness measurement. measurements. The The macro test andsurfaces micro-hardness of GI specimens measurements were ground were and repeatedpolished threeprior timesto the onhardness each GI measurement. specimen, and The the averagemacro and values micr wereo-hardness included. measurements were repeated three times on each GI specimen, and the average values were included. 2.5. Ball-On-Flat Reciprocating Wear Tests 2.5. Ball-on-FlatThe reciprocating Reciprocating sliding Wear wear Tests tests were conducted using a mechanical tribometer with a ball-on-flatThe reciprocating configuration. sliding The wear upper tests specimen were conducted was a ceramic using a ballmechanical with a 4tribometer mm diameter with a and ball- a surfaceon-flat configuration. finish (Ra) of 10 The nm. upper The hardnessspecimen of was the a ceramic ceramic ball ball was with approximately a 4 mm diameter 75 HRC, and which a surface was significantlyfinish (Ra) of higher 10 nm. than The that hardness of the GI of specimens. the ceramic Using ball hard was ceramic approximately balls eliminated 75 HRC, the which possibility was ofsignificantly any wear scarshigher on than the that upper of specimens.the GI specimens. The dimensions Using hard of ceramic the GI platesballs eliminated were 45 mm the possibility20 mm × × 10of any mm wear (length scarswidth on the upperheight). specimens. The tests The were dimensions conducted withof the a GI submerged plates were lubricating 45 mm × condition 20 mm × × × with10 mm PAO (length4, which × width is a × common height). baseThe tests oil with were no conducted additives. with The a parameterssubmerged lubricating in the wear condition tests are shownwith PAO in Table4, which2, which is a common were selected base oil basedwith no on additives. our preliminary The parameters experiments in the and wear previous tests are studies. shown Thein Table orientation 2, which of slidingwere selected motion based was 90 on degrees our preliminary to the laser-hardened experiments tracks, and previous and the starting studies. point The fororientation the wear of test sliding was at motion the side was of the90 degrees laser-hardened to the laser-hardened track, as shown tracks, in Figure and3. Beforethe starting and after point each for the wear test was at the side of the laser-hardened track, as shown in Figure 3. Before and after each wear test, the lubricant container and upper and lower specimens were submerged in acetone and Appl. Sci. 2020, 10, 3049 5 of 12

Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 12 wear test, the lubricant container and upper and lower specimens were submerged in acetone and cleanedcleaned using using an an ultrasonic ultrasonic cleaner cleaner for for 5 5 min. min. After After each each wear wear test, test, the the upper upper specimen specimen and and lubricant lubricant werewere changed changed in in each each wear wear test test in in case case of of the the pres presenceence of of wear debris. Afte Afterr the the wear wear test, test, the the mass mass lossloss of of GI GI specimens specimens was was measured measured using using a a precise precise analytical analytical scale. scale. Additionally, Additionally, the the worn worn areas areas were were analyzedanalyzed using using SEM SEM to to obtain obtain the the possible possible wear wear mechanisms. mechanisms. Each Each wear wear te testst was was carried carried out out three three times,times, and and the the averages averages were were calculated. calculated.

TableTable 2. 2. ParametersParameters in in the the Ball-on-Plate Ball-on-Plate Reciprocating Reciprocating Sliding Sliding Wear Wear Tests. Tests. GI: GI: Gray Cast Iron.

FrictionalFrictional Pair Pair Ceramic Ball—GI Plate Plate LubricantLubricant PAO 4 NormalNormal Load Load 400 N FrequencyFrequency 2 Hz Displacement 10 mm Displacement 10 mm Total Sliding Distance 120 m Total Sliding Distance 120 m

FigureFigure 3. 3. (a)( Ball-on-Platea) Ball-on-Plate Reciprocating Reciprocating Sliding Sliding Wear Wear Test Configuration; Test Conﬁguration; (b) Laser-Hardened (b) Laser-Hardened Track Distribution.Track Distribution. 2.6. Laser Surface-Hardening Process 2.6. Laser Surface-Hardening Process Grinding and polishing processes were applied on the surfaces of AGI and QTIGI specimens to Grinding and polishing processes were applied on the surfaces of AGI and QTIGI specimens to achieve a surface roughness (Ra) within a range of 200–400 nm before applying the laser-hardening achieve a surface roughness (Ra) within a range of 200–400 nm before applying the laser-hardening treatment. The surface topography was determined using a 3D surface tracer. The parameters in treatment. The surface topography was determined using a 3D surface tracer. The parameters in the the laser-hardening treatment are shown in Table3. A thin graphite coating was applied to the test laser-hardening treatment are shown in Table 3. A thin graphite coating was applied to the test surface of GI specimens to improve the absorptivity. Since some researchers have reported that the surface of GI specimens to improve the absorptivity. Since some researchers have reported that the back-tempering issue in the overlapped laser-hardened tracks would signiﬁcantly degrade the wear back-tempering issue in the overlapped laser-hardened tracks would significantly degrade the wear resistance [19–21], a 1-mm distribution spacing interval between two adjacent laser-hardened tracks resistance [19–21], a 1-mm distribution spacing interval between two adjacent laser-hardened tracks was utilized in this study (see Figure3). was utilized in this study (see Figure 3).

Table 3. Parameters in the Nd: YAG Laser Surface-Hardening Process. Table 3. Parameters in the Nd: YAG Laser Surface-Hardening Process. Wavelength 1064 nm Wavelength 1064 nm Power Distribution Gaussian-like Proﬁle Power Distribution Gaussian-like Profile Laser Power 800W Laser SpotLaser Diameter Power 800W2 mm ScanningLaser Spot Speed Diameter 2 2 mmmm/ s FrequencyScanning Speed 2 6mm/s Hz Pulse Time 8 ms Frequency 6 Hz Pulse Time 8 ms

3. Results

3.1. Microstructure

3.1.1. Laser-Hardened Austempered Gray Cast Iron (LHAGI) Appl. Sci. 2020, 10, 3049 6 of 12

3. Results

3.1. Microstructure

3.1.1. Laser-Hardened Austempered Gray Cast Iron (LHAGI) Figure4 shows the microstructure of LHAGI with various austempering temperatures. There were fourAppl. diverse Sci. 2020, zones10, x FOR produced PEER REVIEW under the laser-hardened surface. First, Zone 4 was the6 substrate of 12 of

AGI, whichFigure indicates 4 shows that the the microstructure original pearlitic of LHAGI structure with various has been austempering converted temperatures. into acicular There ferrite and stable austenitewere four with diverse high zones carbon produced content under after the receivinglaser-hardened the austemperingsurface. First, Zone heat 4 treatmentwas the substrate and is called ausferriteof AGI, when which produced indicates in that austempered the original pearlitic ductile structure iron (ADI) has been [22– 24converted]. The ferriteinto acicular plates ferrite grew from needle-likeand stable into feather-likeaustenite with with high increasingcarbon content austempering after receiving temperaturethe austempering from heat 232 treatment◦C to 343and ◦isC, since more carboncalled atomsausferrite diff whenused produced with a high in austempered diffusion rate duct [25ile]. iron Then, (ADI) Zone [22–24]. 1 was The the ferrite central plates laser-hardened grew from needle-like into feather-like with increasing austempering temperature from 232 °C to 343 °C, region, which contained the ledeburite. In Zone 1, the graphite flakes were dissolved during the since more carbon atoms diffused with a high diffusion rate [25]. Then, Zone 1 was the central laser- laser-hardeninghardened region, process which due contained to the high the localledeburite. temperature. In Zone 1, The the graphite morphology flakes ofwere ledeburite dissolved had no differencesduring among the laser-hardening LHAGI specimens process due with to variousthe high local austempering temperature. temperatures. The morphology In of addition ledeburite to Zone 1, therehad were no twodifferences heat-a amongffected LHAGI zones, specimens which were with causedvarious austempering by the thermal temperatures. radiation In from addition the central laser-hardenedto Zone 1, region. there were In two the heat-affected matrix of the zones, upper which heat-a were ffcausedected by zone the thermal (Zone 2),radiation because from the the heating temperaturecentral was laser-hardened above the region. eutectoid In the temperature matrix of the during upper heat-affected the laser-hardening zone (Zone process, 2), because pearlite the was heating temperature was above the eutectoid temperature during the laser-hardening process, transformed into unstable austenite, and austenitized regions were quickly cooled by air and the pearlite was transformed into unstable austenite, and austenitized regions were quickly cooled by air neighboringand the substrate. neighboring Therefore, substrate. the Therefore, main component the main component in Zone 2in was Zone martensite, 2 was martensite, which which was coarsewas near Zone 1 andcoarse thin near near Zone Zone 1 and 3 duethin near to the Zone decrease 3 due to in the temperature. decrease in temperature. In addition, In the addition, heating the temperature heating in the lowertemperature heat-affected in the zone lower (Zone heat-affected 3) wasmuch zone (Zone lower 3) than was thatmuch in lower Zone than 2, which that in resulted Zone 2, inwhich tempering of acicularresulted ferrite in andtempering the carbon of acicular saturated ferrite austenite. and the carbon Table 4saturated displays austenit the dimensionse. Table 4 displays of laser-hardened the dimensions of laser-hardened regions and heat-affected regions of LHAGI with various regions and heat-affected regions of LHAGI with various austempering temperatures. The width and austempering temperatures. The width and depth of Zone 1, Zone 2, and Zone 3 were independent depth ofof Zone the austempering 1, Zone 2, andtemperature. Zone 3 were independent of the austempering temperature.

Figure 4.FigureMicrostructure 4. Microstructure of Di ofﬀerence Difference Zones Zones in thein the Matrix Matrix of of Laser-Hardened Laser-Hardened Austempered Gray Gray Cast Iron (LHAGI)Cast Iron Specimens (LHAGI) Specimens with Various with AustemperingVarious Austempering Temperatures Temperatures (a) 232(a) 232◦C; °C; (b )(b 288) 288◦ C;°C; ( c(c)) 343 ◦C. 343 °C. Appl. Sci. 2020, 10, 3049 7 of 12

Table 4. Width and Depth of the Laser-Hardened, Upper Heat-Aﬀected, and Lower Heat-Aﬀected Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 12 Zones for LHAGI Specimens with Various Austempering Temperatures. Table 4. Width and Depth of the Laser-Hardened, Upper Heat-Affected, and Lower Heat-Affected Austempering W D W D W D 1 1 2 Zones 2for LHAGI3 Specimens3 with Various Austempering Temperatures. Temperature (mm) (mm) (mm) (mm) (mm) (mm)

232 ◦C 2.171 0.755 2.823 1.085 3.555 1.529

288 ◦C 2.168 0.716 2.726 1.071 3.445 1.613

343 ◦C 2.187 0.745 2.851 1.058 3.483 1.575

Austempering W1 D1 W2 D2 W3 D3 Temperature (mm) (mm) (mm) (mm) (mm) (mm) 3.1.2. Laser-Hardened Quench-Tempered Gray Cast232°C Iron (LHQTGI)2.171 0.755 2.823 1.085 3.555 1.529 Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 12 288°C 2.168 0.716 2.726 1.071 3.445 1.613 Figure5 shows the microstructure of LHQTGI343° specimensC 2.187 with0.745 various2.851 tempering1.058 3.483 temperatures.1.575 Three different regions399 °C were2.216 found beneath0.735 the3.062 laser-hardened 1.089 surface in the matrix of LHQTGI. 482 °C 2.219 0.728 2.947 1.082 Zone 3 was the substrate containing the tempered3.1.2. Laser-Hardened martensite, Quench-Tem whichpered is a typical Gray Cast phase Iron (LHQTGI) component Figure 5 shows the microstructure of LHQTGI specimens with various tempering temperatures. for QTGI.3.2. The Hardness tempered martensite became coarse when increasing tempering temperature from 316 C to 482 C. Similar to the LHAGIThree specimens, different theregions laser-hardened were found beneath region the laser-hardened (Zone 1) was surface also in the matrix of LHQTGI. ◦ The◦ macro-hardness values in HRCZone are 3given was thein Tablesubstrate 6. The containing AGI and the QTGI tempered specimens martensite, had which is a typical phase component full of ledeburiticsimilar macro-hardness structures in without each test thegroup.for presence FoQTGI.r AGI The ofspecimens, tempered graphite the martensite flakes,macro-hardness whichbecame decreased indicatescoarse when with that increasing Zone tempering 1 temperature from experiencedincreasing a solid–liquid–solid austempering temperature phase transformation because316 °C tothere 482 was°C. during Similar more theacicular to the laser-hardening LHAGI ferrite specimens,and stable process. theaustenite laser-hardened Zone 2 was region (Zone 1) was also full a heat-affwithected high zone, carbon which content contained produced martensitein theof matrixledeburitic than due structuresmartensite. to the e ffwithout ectsLikewise, of the self-quenching. the presence macro-hardness of graphite The of graphiteflakes, which indicates that Zone 1 flakes remainedQTGI specimens with no decreased changes when in morphologythe temperingexperienced temp or size.aerature solid–liquid–solid The was martensite increased phase due around totransformati the decomposition Zoneon 1during was the coarser laser-hardening process. Zone 2 of martensite and growth of cementite phases.was a Inheat-affected Table 7, the zone, micro-hardness which contained of different martensite regions due in to the effects of self-quenching. The than thatthe around matrix Zone of LHAGI 3 due and to LHQTGI the diff erentspecimensgraphite temperature is re flakesported. remained distributions. For LHAGI with nospecimens, changes Table5 inledeburitedisplays morphology in the Zone or dimensions size. The martensite around Zone 1 was of the laser-hardened1 and martensite areas in Zone and 2 heat-a had highffected hardnesscoarser regions thanabove inthat 67 the aroundHRC. matrix Since Zone of the3 LHQTGIdue acicular to the ferritedifferent specimens and temperature stable with various distributions. Table 5 displays the temperingaustenite temperatures. with high carbon The widthcontent andweredimensions depthtempered of in of Zone Zone the laser-hardened 13, andit has Zonea lower areas 2 hardness in and LHQTGI heat-aff than thatected specimens of regions the in werethe matrix of LHQTGI specimens similar tosubstrate. those in For LHAGI LHQTGI specimens specimens, dueZonewith to 1 theand various sameZone tempering 2 parameters were similar temperatures. beingin hardness used The with inwidth the the and laser-hardeningLHAGI depth of Zone 1 and Zone 2 in LHQTGI specimens because of the formation of ledeburitespecimens and were martensite. similar to Sincethose thein LHAGI solid–liquid–solid specimens due phase to the same parameters being used in the treatment. The lower heat-affected zone was not produced in the matrix of LHQTGI specimens due to transformation in Zone 1 and solid–solidlaser-hardening phase transformation treatment. in ZoneThe lower 2 occurred heat-affected during zonethe laser- was not produced in the matrix of LHQTGI the higherhardening thermal process, stability the ofhardness tempered of these martensitespecimens two regions due as did compared to notthe dependhigher with thermalon the acicular original stability ferrite heat of temperedtreatment and high martensite carbon as compared with acicular content austenite.designs. ferrite and high carbon content austenite.

Table 5. Width and Depth of Laser-Hardened and Heat-Affected Zones for LHQTGI Specimens with Various Tempering Temperatures.

Tempering W2 D2 W1 (mm) D1 (mm) Temperature (mm) (mm) 316 °C 2.237 0.751 3.022 1.118

Figure 5.FigureMicrostructure 5. Microstructure of Di ﬀoference Difference Zones Zones in thein the Matrix Matrix of of Laser-Hardened Laser-Hardened Quench-Tempered Quench-Tempered Gray Gray Cast Iron (LHQTGI) Specimens with Various Tempering Temperatures (a) 316 °C; (b) 399 °C; Cast Iron (LHQTGI) Specimens with Various Tempering Temperatures (a) 316 ◦C; (b) 399 ◦C; (c) 482 ◦C. (c) 482 °C.

Table 6. Macro-Hardness of AGI and QTGI Specimens with Various Heat Treatment Designs.

Group 1 Group 2 Group 3 Austempering Temperature 232 °C 288 °C 343 °C AVE SD AVE SD AVE SD Macro-Hardness (HRC) 41.6 0.6 37.5 0.3 27.7 0.5 Tempering Temperature 316 °C 399 °C 482 °C Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 12

Table 4. Width and Depth of the Laser-Hardened, Upper Heat-Affected, and Lower Heat-Affected Zones for LHAGI Specimens with Various Austempering Temperatures.

Austempering W1 D1 W2 D2 W3 D3 Temperature (mm) (mm) (mm) (mm) (mm) (mm) 232°C 2.171 0.755 2.823 1.085 3.555 1.529 288°C 2.168 0.716 2.726 1.071 3.445 1.613 343°C 2.187 0.745 2.851 1.058 3.483 1.575

3.1.2. Laser-Hardened Quench-Tempered Gray Cast Iron (LHQTGI) Figure 5 shows the microstructure of LHQTGI specimens with various tempering temperatures. Three different regions were found beneath the laser-hardened surface in the matrix of LHQTGI. Zone 3 was the substrate containing the tempered martensite, which is a typical phase component for QTGI. The tempered martensite became coarse when increasing tempering temperature from 316 °C to 482 °C. Similar to the LHAGI specimens, the laser-hardened region (Zone 1) was also full of ledeburitic structures without the presence of graphite flakes, which indicates that Zone 1 experienced a solid–liquid–solid phase transformation during the laser-hardening process. Zone 2 was a heat-affected zone, which contained martensite due to the effects of self-quenching. The graphite flakes remained with no changes in morphology or size. The martensite around Zone 1 was coarser than that around Zone 3 due to the different temperature distributions. Table 5 displays the dimensions of the laser-hardened areas and heat-affected regions in the matrix of LHQTGI specimens with various tempering temperatures. The width and depth of Zone 1 and Zone 2 in LHQTGI specimens were similar to those in LHAGI specimens due to the same parameters being used in the Appl. Sci. 2020, 10, 3049 8 of 12 laser-hardening treatment. The lower heat-affected zone was not produced in the matrix of LHQTGI specimens due to the higher thermal stability of tempered martensite as compared with acicular Table 5. Width and Depth of Laser-Hardenedferrite and high and carbon Heat-A ﬀcontentected Zones austenite. for LHQTGI Specimens with Various Tempering Temperatures. Table 5. Width and Depth of Laser-Hardened and Heat-Affected Zones for LHQTGI Specimens with Tempering W (mm) D (mm)Various Tempering W (mm) Temperatures. D (mm) Temperature 1 1 2 2

316 ◦C 2.237 0.751 3.022 1.118

399 ◦C 2.216 0.735 3.062 1.089

482 ◦C 2.219 0.728 2.947 1.082

Tempering W2 D2 W1 (mm) D1 (mm) 3.2. Hardness Temperature (mm) (mm) The macro-hardness values in HRC are316 given °C in Table2.2376. The AGI0.751 and QTGI3.022 specimens 1.118 had similar macro-hardness in each test group. For AGI specimens, the macro-hardness decreased with increasing austempering temperature because there was more acicular ferrite and stable austenite with high carbon content produced in the matrix than martensite. Likewise, the macro-hardness of QTGI specimens decreased when the tempering temperature was increased due to the decomposition of martensite and growth of cementite phases. In Table7, the micro-hardness of di ﬀerent regions in the matrix of LHAGI and LHQTGI specimens is reported. For LHAGI specimens, ledeburite in Zone 1 and martensite in Zone 2 had high hardness above 67 HRC. Since the acicular ferrite and stable austenite with high carbon content were tempered in Zone 3, it has a lower hardness than that of the substrate. For LHQTGI specimens, Zone 1 and Zone 2 were similar in hardness with the LHAGI specimens because of the formation of ledeburite and martensite. Since the solid–liquid–solid phase transformation in Zone 1 and solid–solid phase transformation in Zone 2 occurred during the laser-hardening process, the hardness of these two regions did not depend on the original heat treatment designs.

Table 6. Macro-Hardness of AGI and QTGI Specimens with Various Heat Treatment Designs.

Group 1 Group 2 Group 3 Austempering 232 C 288 C 343 C Temperature ◦ ◦ ◦ AVE SD AVE SD AVE SD Macro-Hardness (HRC) 41.6 0.6 37.5 0.3 27.7 0.5

Tempering Temperature 316 ◦C 399 ◦C 482 ◦C AVE SD AVE SD AVE SD Macro-Hardness (HRC) 41.2 0.3 38.9 0.5 26.1 0.7 AVE: Average Hardness, SD: Standard Deviation. Appl. Sci. 2020, 10, 3049 9 of 12

Table 7. Micro-Hardness Proﬁles of the LHAGI and LHQTGI Specimens with Various Heat Treatment Designs.

Group 1 Group 2 Group 3

Austempering Temperature 232 ◦C 288 ◦C 343 ◦C AVE SD AVE SD AVE SD Zone 1 67.5 0.4 67.8 0.5 68.2 0.2 Micro-Hardness Proﬁle AVE SD AVE SD AVE SD Zone 2 (HRC) 67.8 0.7 68.4 0.7 67.7 0.6 AVE SD AVE SD AVE SD Zone 3 38.3 0.4 33.5 0.4 24.6 0.5

Tempering Temperature 316 ◦C 399 ◦C 482 ◦C AVE SD AVE SD AVE SD Zone 1 68.1 0.4 68.6 0.6 68.4 0.2 Micro-Hardness Proﬁle (HRC) AVE SD AVE SD AVE SD Zone 2 66.5 0.6 66.2 0.3 66.7 0.8 AVE: Average Hardness, SD: Standard Deviation.

3.3. Wear Properties The mass loss due to wear of LHAGI, AGI, LHQTGI, QTGI, and untreated GI specimens is shown in Figure6. The untreated GI specimens had the highest wear loss due to the low surface hardness of approximately 215 HB. After receiving quench-tempering, the wear resistance was improved. The wear resistance decreased when raising the tempering temperature due to the lower hardness of the specimens. The wear performance of GI specimens was further enhanced by applying the austempering heat treatment. It is well known that the existence of acicular ferrite and stable austenite with high carbon content provides improved wear protection on surfaces. The metastable austenite can be converted into martensite to increase the surface hardness by the high localized strain and plastic deformation [26,27]. The wear resistance of AGI specimens also decreased when raising austempering temperature because of the lower hardness of the specimens. When the laser surface hardening treatment was utilized on AGI and QTGI specimens, the mass loss was signiﬁcantly reduced because high hardness ledeburitic and martensitic structures were formed in the matrix. The LHAGI specimens had higher wear resistance than LHQTGI specimens, although the microstructure and hardness of Zone 1 and Zone 2 were quite similar, which can be attributed to two reasons. (1) The acicular ferrite and stable austenite with high carbon percentage between two adjacent laser-hardened tracks had higher wear resistance than tempered martensite under equivalent macro-hardness. (2) The combined structures of tempered acicular ferrite and tempered austenite had higher fracture toughness than tempered martensite. Similar ﬁndings were reported in a previous study on the tribological properties of ADI and tempered ADI [28]. Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 12

Figure 6. FigureMass 6. Loss Mass inLoss the in the LHAGI, LHAGI, AGI,AGI, LHQTGI, LHQTGI, and QTGI and QTGISpecimens. Specimens.

3.4. Worn Surfaces The worn surfaces of representative LHAGI and LHQTGI specimens were analyzed by SEM, as shown in Figure 7. The main wear mechanism was crack formation. Under the external load in reciprocating sliding wear tests, cracks easily formed around the edge of graphite flakes on the surface or subsurface due to the high stress concentration. Then, the cracks propagated along the graphite flakes. These cracks were linked together to produce small-scale pits or large-scale spalls [29,30]. However, the graphite flakes in the laser-hardened region (Zone 1) dissolved during the laser- hardening process, and the high hardness ledeburitic and martensitic structures provided more resistance against surface fracture.

Figure 7. Worn Surfaces of LHAGI and LHQTGI Specimens: (a) 232 °C; (b) 316 °C.

4. Conclusions In the present research, the roles of laser surface-hardening treatment on the anti-wear performance of AGI and QTGI specimens were examined under equivalent macro-hardness. The microstructure, macro-harness, and micro-hardness values and wear tracks were evaluated to understand the phase transformations and possible wear mechanisms. Taken together, several conclusions can be drawn: • Four regions were found under the laser-hardened surface of LHAGI specimens. Zone 1 was the laser-hardened region containing the ledeburite and had a hardness of approximately 67 HRC. Zone 2 was an upper heat-affected zone containing martensite with a hardness of approximately 67 HRC. Zone 4 was the substrate containing the acicular ferrite and stable austenite with high Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 12

Appl. Sci. 2020, 10, 3049 10 of 12 Figure 6. Mass Loss in the LHAGI, AGI, LHQTGI, and QTGI Specimens.

3.4. Worn Surfaces The worn surfaces surfaces of of representative representative LHAGI LHAGI an andd LHQTGI LHQTGI specimens specimens were were analyzed analyzed by bySEM, SEM, as asshown shown in inFigure Figure 7.7 .The The main main wear wear mechanism mechanism was was crack formation. Under the externalexternal loadload inin reciprocating slidingsliding wear wear tests, tests, cracks cracks easily easily formed formed around around the edgethe edge of graphite of graphite flakes onflakes the surfaceon the orsurface subsurface or subsurface due to the due high to the stress high concentration. stress concentration. Then, the Then, cracks the propagated cracks propagated along the along graphite the flakes.graphite These flakes. cracks These were cracks linked were together linked together to produce to produce small-scale small-scale pits or large-scalepits or large-scale spalls [ 29spalls,30]. However,[29,30]. However, the graphite the graphite flakes in flakes the laser-hardened in the laser-hardened region (Zone region 1) dissolved (Zone 1) dissolved during the during laser-hardening the laser- process,hardening and process, the high and hardness the high ledeburitic hardness and ledebu martensiticritic and structures martensitic provided structures more resistance provided against more surfaceresistance fracture. against surface fracture.

FigureFigure 7.7. Worn SurfacesSurfaces ofof LHAGILHAGI andand LHQTGILHQTGI Specimens:Specimens: ((aa)) 232232 ◦°C; (b)) 316316 ◦°C.

4. Conclusions In thethe present present research, research, the rolesthe ofroles laser of surface-hardening laser surface-hardening treatment treatment on the anti-wear on the performance anti-wear ofperformance AGI and QTGI of AGI specimens and QTGI were specimens examined were under examined equivalent under macro-hardness. equivalent macro-hardness. The microstructure, The macro-harness,microstructure, andmacro-harness, micro-hardness and values micro-hardne and wearss tracks values were and evaluated wear tracks to understand were evaluated the phase to transformationsunderstand the andphase possible transformations wear mechanisms. and possible Taken together,wear mechanisms. several conclusions Taken together, can be drawn: several conclusionsFour regions can be were drawn: found under the laser-hardened surface of LHAGI specimens. Zone 1 was the • • laser-hardenedFour regions were region found containing under the the laser-harden ledeburiteed and surface had a of hardness LHAGI specimens. of approximately Zone 1 67was HRC. the Zonelaser-hardened 2 was an upper region heat-a containingffected the zone ledeburite containing and martensite had a hardness with a hardnessof approximately of approximately 67 HRC. 67Zone HRC. 2 was Zone an 4upper was theheat-affected substrate containingzone containing the acicular martensite ferrite with and a hardness stable austenite of approximately with high carbon67 HRC. content Zone 4 and was had the asubstrate hardness containing of 41–27 HRC the acicular with austempering ferrite and stable temperatures austenite from with 232 high to 399 ◦C. Due to the tempering effect in Zone 3, the hardness decreased accordingly. The acicular ferrite became thick with the increasing austempering temperature. Three regions were found under the laser-hardened surface of LHQTGI specimens. Zone 1 was • the laser-hardened region containing the ledeburite and had a hardness of approximately 68 HRC. Zone 2 was the heat-affected zone containing the martensite and had a hardness of approximately 66 HRC. Zone 3 was the substrate containing the tempered martensite with a hardness ranging from 41 to 26 HRC with varying tempering temperatures from 316 to 482 ◦C. In reciprocating sliding wear tests, GI specimens without heat treatment had the lowest wear • resistance due to the low surface hardness. The wear resistance was improved by applying the quench-tempering heat treatment and further improved by applying the austempering heat treatment. Both AGI and QTGI specimens had lower wear resistance when raising the austempering temperature or tempering temperature as a result of the reduction in hardness. After introducing the laser-hardening treatment, the wear performance of LHAGI and LHQTGI specimens was significantly enhanced because of the formation of high hardness ledeburitic and martensitic structures. The LHAGI specimens had higher wear resistance than the LHQTGI Appl. Sci. 2020, 10, 3049 11 of 12

specimens. After analyzing the worn areas of the LHAGI and LHQTIGI specimens, it was found that cracks were the main wear mechanism. Overall, the use of LHQTGI to replace AGI in some GI parts can be considered since LHQTGI had • higher wear resistance than that of AGI. However, the substitution of AGI by LHQTGI needs to be further studied in regard to the other details in manufacturing and production processes and the specific mechanical properties. In addition, the wear loss of AGI could be reduced significantly after receiving laser surface-hardening treatment. The findings obtained in the present research provide value in the future designs of GI engineering components.

Author Contributions: Conceptualization, B.W. and G.C.B.; methodology, B.W., Y.P. and R.W.; validation, B.W., Y.P. and R.W.; formal analysis, B.W. and Y.P.; investigation, B.W. and Y.P.; resources, G.C.B.; data curation, B.W., Y.P. and R.W.; writing—original draft preparation, B.W. and R.W.; writing—review and editing, B.W. and G.C.B.; supervision, G.C.B.; project administration, G.C.B.; funding acquisition, B.W. and G.C.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the ACADEMIC AND RESEARCH DEVELOPMENT FUNDINGS at Zhejiang Sci-Tech University, grant number 11130131741904, grant number 11130231711923 and grant number 19022140-Y. Acknowledgments: This research was supported by the Tribology Laboratory, Department of Mechanical Engineering, Oakland University, Michigan, 48326, USA. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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