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Home , Austempering, Cast iron, Ductile iron, Gray iron, Quenching, Tempering (metallurgy)

Article Behavior of Austempered and Quenched and Tempered Gray Cast under Similar

1,2 2 2 2, , Bingxu Wang , Xue Han , Gary C. Barber and Yuming Pan * † 1 Faculty of Mechanical and Automation, Zhejiang Sci-Tech University, Hangzhou 310018, China; [email protected] 2 Automotive Tribology Center, Department of Mechanical Engineering, School of Engineering and Computer Science, Oakland University, Rochester, MI 48309, USA; [email protected] (X.H.); [email protected] (G.C.B.) * Correspondence: [email protected] Current address: 201 N. Squirrel Rd Apt 1204, Auburn Hills, MI 48326, USA. †  Received: 14 November 2019; Accepted: 4 December 2019; Published: 8 December 2019 

Abstract: In this research, an heat treatment was applied on gray cast using various austempering temperatures ranging from 232 ◦C to 371 ◦C and holding times ranging from 1 min to 120 min. The microstructure and hardness were examined using optical microscopy and a Rockwell hardness tester. Rotational -on-disk sliding wear tests were carried out to investigate the wear behavior of austempered gray samples and to compare with conventional quenched and tempered gray cast iron samples under equivalent hardness. For the austempered samples, it was found that acicular ferrite and saturated were formed in the matrix. The ferritic platelets became coarse when increasing the austempering temperature or extending the holding time. Hardness decreased due to a decreasing amount of in the matrix. In wear tests, austempered gray cast iron samples showed slightly higher wear resistance than quenched and tempered samples under similar hardness while using the austempering temperatures of 232 ◦C, 260 ◦C, 288 ◦C, and 316 ◦C and distinctly better wear resistance while using the austempering temperatures of 343 ◦C and 371 ◦C. After analyzing the worn surface, wear and wear with the presence of pits, spalls, voids, long cracks, and wear debris were the main mechanisms for austempered gray cast iron with a low austempering temperature. However, only small pits and short cracks were observed on the wear track of austempered gray cast iron with high austempering temperature. Furthermore, the flakes were exposed and ground by the counterpart surface during wear tests. Then, the graphite particles would form a tribo-layer to protect the contact surface.

Keywords: AGI; QTGI; abrasive wear; fatigue wear; graphite tribo-layer

1. Introduction Gray cast iron (GI) is one of the conventional iron-carbon alloys with a carbon content of 2.5–4% and a content of 1–3%. Its typical microstructure contains graphite flakes surrounded by or ferrite. In terms of morphology, size, and distribution, graphite flakes are divided into five patterns from A to E in the ASTM Standard A247 [1]. Due to its excellent machinability and damping capacity with low production cost, GI has been broadly used in the manufacturing of brake rotors, clutch discs, cylinder liners, and mounts. Most of the GI applications require superior resistance to retard wear loss on contact surfaces. Therefore, heat treatment processes such as austempering treatment and and treatment are expected to provide benefits for the tribological properties of GI. The austempering heat treatment was first proposed by Edgar C. Bain in the 1930s [2]. In this process, GI is austenitized above the Acm critical temperature to convert the ferrite or pearlite into

Metals 2019, 9, 1329; doi:10.3390/met9121329 www.mdpi.com/journal/metals Metals 2019, 9, 1329 2 of 13 unstable austenite. Then, the full austenitized GI is transferred and soaked in a salt bath furnace at a constant temperature for a specific period. The isothermal temperatures should be between the pearlite formation temperature and martensite formation temperature, which are similar to the formation temperatures of . The final microstructure of austempered gray cast iron (AGI) consists of acicular ferrite and carbon saturated austenite. In the austempering process, a salt bath furnace is typically used in order to eliminate or minimize surface oxidation and carburization. A quenching and tempering treatment is one common heat treatment process applied on cast irons and . Additional tempering is introduced on as-quenched materials to improve the and relieve some internal stress. The tempering temperature should be set below the eutectoid temperature. Precise control of tempering temperature and holding duration is vital for the desired mechanical properties. In the tempering process, the martensite formed during the quenching step is transformed into tempered martensite by carbon precipitation and diffusion. The tribological performance of GI has been studied by several researchers using different elements and heat treatment processes through various test configurations. Hassani et al. [3] studied the influence of hard elements such as and on the wear properties of GI by using a pin-on-disk rotational fixture. The presence of vanadium and chromium induced the formation of oxidative layers to reduce the wear loss. Sarkar et al. [4] investigated the wear behavior of alloyed AGI using six austempering temperatures by a block-on-roller tribometer. Low wear resistance was obtained for high austempering temperature due to the significant drop in hardness and tensile strength. Vadiraj [5] studied the wear resistance of quenched and tempered gray cast iron (QTGI) on a pin-on-disk test rig. It was found that the wear rate increased when increasing the tempering temperature. This was attributed to the softening of the martensitic matrix with the tempering reaction. Vadiraj et al. [6] also evaluated the wear performance of a series of alloyed AGIs using a pin-on-disk tribometer. The specific wear rate had a decreasing trend when increasing the graphite content since more graphite could be engaged into the contact interface as solid lubricant. Furthermore, the wear rate would increase when the ferritic laths became thick due to the low wear resistance of the soft ferrite phase. Balachandran et al. [7] found that the wear resistance of AGI was degraded after adding alloy since the presence of nickel would stabilize the austenite and inhibit the stress induced transformation. Some recent studies have reported better wear resistance of AGI compared with as-cast GI [8–10]. However, few research studies have paid attention to the comparison of the tribological characteristics between AGI and conventional QTGI. Since it has been well known that the wear resistance of cast irons and steels is often correlated with surface hardness, the comparison in wear resistance is reasonable under equivalent hardness [11–14]. In the current research, AGI samples were prepared by a wide range of austempering temperatures and holding times. Various tempering temperatures were applied on quenched GI to match the hardness of AGI samples for the comparison in wear performance. Wear resistance of AGI and QTGI samples was tested using a rotational ball-on-disk sliding rig. In addition, the microstructure of AGI and QTGI produced by the different heat treatment parameters was evaluated by optical microscopy, and the worn surface was examined through scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) to analyze the potential mechanisms. The results will be helpful for the possible substitution of traditional QTGI by AGI in existing and future applications.

2. Materials and Methods

2.1. Chemical Composition The percentage of main alloy elements of the GI was measured by a carbon- analyzer (CS-200, LECO, San Jose, MI, USA) and an optical spectrometer (3460, Applied Research Laboratories ARL, Austin, TX, USA), as shown in Table1. More specific details of the GI used in this research are available on the supplier’s website at www.mcmaster.com/8928k79. Metals 2019, 9, 1329x FOR PEER REVIEW 33 of of 14 13

Table 1. Alloy content for gray cast iron. Table 1. Alloy content for gray cast iron. Elements Percentage ElementsCarbon, C 3.53%Percentage Carbon,Silicon, C Si 2.71% 3.53% ,Silicon, Si Mn 0.74% 2.71% Manganese, Mn 0.74% Chromium,Chromium, Cr Cr 0.12% 0.12% Copper,Copper, Cu Cu 0.94% 0.94% Sulfur,Sulfur, S S 0.03% 0.03% Phosphorous,Phosphorous, P P 0.08% 0.08% Iron,Iron, Fe Fe Remainder Remainder

2.2. As-Cast Gray Cast Iron The original microstructure of the as-cast GI is shown in Figure1 1.. TheThe mainmain componentscomponents inin thethe matrix were graphite flakes,flakes, ferrite, and pearlite.pearlite.

Figure 1. Original microstructure of as-cast gray cast iron. 2.3. Austempering Heat Treatment 2.3. Austempering Heat Treatment The as-cast GI samples were austenitized at a temperature of 832 C for 20 min in a medium The as-cast GI samples were austenitized at a temperature of 832 ◦°C for 20 min in a medium temperature salt bath furnace (50% KCl + 20% NaCl + 30% CaCl2). The pearlite was transformed into temperature salt bath furnace (50% KCl + 20% NaCl + 30% CaCl2). The pearlite was transformed into unstable austenite, and the alloy elements were distributed uniformly. Then, the fully austenitized unstable austenite, and the alloy elements were distributed uniformly. Then, the fully austenitized GI samples were quickly transferred to another pre-heated low temperature salt bath furnace (50% GI samples were quickly transferred to another pre-heated low temperature salt bath furnace (50% KNO3 + 50% NaNO3) for the austempering process at various austempering temperatures (232 ◦C, KNO3 + 50% NaNO3) for the austempering process at various austempering temperatures (232 °C, 260 C, 288 C, 316 C, 343 C, and 371 C) and holding times (1 min, 2 min, 3 min, 6 min, 10 min, 260 °C,◦ 288 °C,◦ 316 °C,◦ 343 °C,◦ and 371 °C)◦ and holding times (1 min, 2 min, 3 min, 6 min, 10 min, 20 20 min, 30 min, 60 min, 90 min, and 120 min). The above parameters were selected in terms of the min, 30 min, 60 min, 90 min, and 120 min). The above parameters were selected in terms of the previous related research [6,10,15–19]. Then, the AGI samples were cooled to room temperature by previous related research [6,10,15–19]. Then, the AGI samples were cooled to room temperature by water. The austempering process diagram is displayed in Figure2a and Table2. water.Metals The 2019 austempering, 9, x FOR PEER REVIEW process diagram is displayed in Figure 2a and Table 2. 4 of 14

2.4. Quenching and Tempering Heat Treatment The as-cast GI samples were first austenitized at a temperature of 832 °C for 20 min in a medium temperature salt bath furnace (50% KCl + 20% NaCl + 30% CaCl2) to obtain unstable austenite. After that, the fully austenitized GI samples were quenched by oil. Then, different tempering temperatures (316 °C, 371 °C, 399 °C, 454 °C, 482 °C, 510 °C) with a constant holding time of 60 min were applied on quenched GI samples to match the hardness of the AGI samples, respectively. The tempering process was conducted using an electrical heating furnace under air atmosphere (Lindberg-M, Thermo Scientific, Waltham, MA, USA). Finally, the tempered samples were cooled to room temperature in oil. The quenching and tempering process diagram is displayed in Figure 2b, and Table 2 gives the details. FigureFigure 2. Heat 2. Heat treatment treatment processes: processes: (a(a)) austemperingaustempering heat heat treatment; treatment; (b) ( bquenching) quenching and and tempering tempering heatheat treatment. treatment.

2.5. Metallurgical Evaluation Fifteen millimeter cubic coupons were used for metallurgical evaluation. Coupons were hot mounted using Diallyl Phthalate powder. The coupons were ground and polished to a mirror-like surface using Si-carbide sandpaper from 240 grit to 1200 grit and polishing cloths with 0.3 μm alumina suspension. Then, coupons were thoroughly rinsed by water and etched by 3% nital solutions for 2 s to 3 s. Metallurgical evaluation was carried out using optical microscopy (PME-3, Olympus, Tokyo, Japan)

2.6. Rotational Ball-on-Disk Sliding Wear Test Sliding wear tests were conducted on a universal mechanical tribometer (UMT-3, Bruker, Billica, MA, USA) with a rotational ball-on-disk configuration at room temperature. An alumina ball was used as the counterpart to simulate the ceramic ball bearings. The dimensions of the AGI and QTGI sample disks are shown in Figure 3. The AGI and QTGI sample disks were ground and polished to approximately 300 nm (Arithmetic Roughness/Ra), which was measured by a 3-dimensional surface profilometer (ContourGT-K, Bruker, Billica, MA, USA). The normal load was 300 N, and the rotational speed was 240 rpm. In sliding wear tests, the sample disks were submerged into PAO4 base oil (kinematic viscosity of 16.8 cSt at 40 °C). The test duration was 30 min. Each test was repeated three times, and the averages were reported. After the wear tests, an SEM (JSM-6510, JEOL, Tokyo, Japan) equipped with EDS was used to observe the worn tracks for potential mechanisms. The details of the wear tests are summarized in Table 2.

Figure 3. Sectional view of the rotational ball-on-disk sliding wear test fixture.

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Table 2. Details of heat treatment designs and experiments.

Heat Treatment Designs Austempering Heat Treatment (AGI) Temperature: 832 ◦C; Austenitizing Process: Time: 20 min; Medium Temp Salt Bath Furnace

Temperatures: 232 ◦C, 260 ◦C, 288 ◦C, 316 ◦C, 343 ◦C, 371 ◦C; Austempering Process: Times: 1 min, 2 min, 3 min, 6 min, 10 min, 20 min, 30 min, 60 min, 90 min and 120 min; Water Cool; Low Temp Salt Bath Furnace Quenching and Tempering Heat Treatment (QTGI) Temperature: 832 ◦C; Time: 20 min; Austenitizing Process: Oil Cool; Medium Temp Salt Bath Furnace

Temperatures: 316 ◦C, 371 ◦C, 399 ◦C, 454 ◦C, 482 ◦C, 510 ◦C; Time: 60 min; Tempering Process: Oil Cool; Electrical Heating Furnace Experiments Metallurgical Evaluations Sample AGI and QTGI Coupons (15 mm 15 mm 15 mm) × × Etching 3% Nital for 2 s or 3 s Test Facility Optical Microscopy with 500 Magnification × Sample Size 16 Hardness Measurements Sample AGI and QTGI Sample Coupons/Disks Test Facility Rockwell Hardness Tester Repetition 3 Times Sample Size 234 Wear Tests Alumina Ball (Diameter: 7.94 mm; Hardness: 75 HRC; Upper Sample Surface Roughness (Ra): 10 nm) Lower Sample AGI and QTGI Disks (Diameter: 64 mm; Thickness: 11 mm; Surface Roughness (Ra): 300 nm) ≈ Normal Load 300 N Rotational Speed 240 rpm Lubricant PAO4 Base Oil Test Duration 30 min Test Facility Rotational Ball-on-Disk Sliding Configuration Repetition 3 Times Sample Size 36

2.4. Quenching and Tempering Heat Treatment

The as-cast GI samples were first austenitized at a temperature of 832 ◦C for 20 min in a medium temperature salt bath furnace (50% KCl + 20% NaCl + 30% CaCl2) to obtain unstable austenite. After that, the fully austenitized GI samples were quenched by oil. Then, different tempering temperatures (316 ◦C, 371 ◦C, 399 ◦C, 454 ◦C, 482 ◦C, 510 ◦C) with a constant holding time of 60 min were applied on quenched GI samples to match the hardness of the AGI samples, respectively. The tempering process was conducted using an electrical heating furnace under air atmosphere (Lindberg-M, Thermo Scientific, Waltham, MA, USA). Finally, the tempered samples were cooled to room temperature in oil. The quenching and tempering process diagram is displayed in Figure2b, and Table2 gives the details.

2.5. Metallurgical Evaluation Fifteen millimeter cubic coupons were used for metallurgical evaluation. Coupons were hot mounted using Diallyl Phthalate powder. The coupons were ground and polished to a mirror-like Metals 2019, 9, x FOR PEER REVIEW 4 of 14

Figure 2. Heat treatment processes: (a) austempering heat treatment; (b) quenching and tempering heat treatment. Metals 2019, 9, 1329 5 of 13 2.5. Metallurgical Evaluation Fifteen millimeter cubic coupons were used for metallurgical evaluation. Coupons were hot µ surfacemounted using using Si-carbide Diallyl sandpaper Phthalate frompowder. 240 The grit coupons to 1200 gritwere and ground polishing and polished cloths with to a 0.3mirror-likem alumina oxidesurface suspension. using Si-carbide Then, coupons sandpaper were from thoroughly 240 grit rinsedto 1200 by grit water and andpolishing etched cloths by 3% with nital 0.3 solutions μm for 2alumina s to 3 s. oxide Metallurgical suspension. evaluation Then, coupons was were carried thorou outghly using rinsed optical by water microscopy and etched (PME-3, by 3% Olympus, nital Tokyo,solutions Japan). for 2 s to 3 s. Metallurgical evaluation was carried out using optical microscopy (PME-3, Olympus, Tokyo, Japan) 2.6. Rotational Ball-on-Disk Sliding Wear Test 2.6. Rotational Ball-on-Disk Sliding Wear Test Sliding wear tests were conducted on a universal mechanical tribometer (UMT-3, Bruker, Billica, MA, USA)Sliding with wear a rotational tests were ball-on-disk conducted on configuration a universal mechanical at room tribometer temperature. (UMT-3, An Bruker, alumina Billica, ball was usedMA, as the USA) counterpart with a rotational to simulate ball-on-disk the ceramic configuration ball bearings. at room The temperature. dimensions An ofalumina the AGI ball and was QTGI sampleused disks as the are counterpart shown in to Figure simulate3. The the ceramic AGI and ball QTGI bearings. sample The disks dimensions were groundof the AGI and and polished QTGI to sample disks are shown in Figure 3. The AGI and QTGI sample disks were ground and polished to approximately 300 nm (Arithmetic Roughness/Ra), which was measured by a 3-dimensional surface approximately 300 nm (Arithmetic Roughness/Ra), which was measured by a 3-dimensional surface profilometer (ContourGT-K, Bruker, Billica, MA, USA). The normal load was 300 N, and the rotational profilometer (ContourGT-K, Bruker, Billica, MA, USA). The normal load was 300 N, and the speedrotational was 240 speed rpm. was In 240 sliding rpm. wearIn sliding tests, wear the te samplests, the diskssample were disks submerged were submerged into PAO4into PAO4 base oil (kinematicbase oil viscosity(kinematic of viscosity 16.8 cSt of at 16.8 40 ◦cStC). at The 40 °C). test Th duratione test duration was 30was min. 30 min. Each Each test test was was repeated repeated three times,three and times, the averages and the averages were reported. were reported. After theAfter wear the tests,wear tests, an SEM an SEM (JSM-6510, (JSM-6510, JEOL, JEOL, Tokyo, Tokyo, Japan) equippedJapan) with equipped EDS with was EDS used was toobserve used to observe the worn thetracks worn tracks for potential for potential mechanisms. mechanisms. The The details details of the wearof tests the wear are summarized tests are summarized in Table 2in. Table 2.

FigureFigure 3. Sectional3. Sectional view view of of the the rotationalrotational ball-on-disk ball-on-disk sliding sliding wear wear test test fixture. fixture.

2.7. Rockwell C Hardness Measurement The hardness of the GI samples under different heat treatments was measured by a Rockwell hardness tester (R-260, LECO, St. Joseph, MI, USA) under the ASTM Standard E18-16 [20]. The sample surfaces were first ground flat and then polished by Si-carbide sandpaper with 240 grit before each hardness measurement. Each sample was measured three times and then averaged (Table2).

3. Results

3.1. Metallurgical Evaluation of AGI and QTGI As compared with the microstructure of as-cast GI in Figure1, no changes could be found related to the characteristics of graphite flakes after receiving the austempering heat treatment. The original pearlitic structure was transformed into acicular ferrite and carbon saturated austenite, as shown in Figure4. Figure4a,c,e,g,i shows the microstructure of AGI samples at the beginning of the transformation reaction under each austempering temperature. It could be seen that the amount of acicular phases became more with decreasing austempering temperature because of the high degree of supercooling. It was also observed that most of the thin needle-like ferrite initiated around graphite flakes where the potential energy was high. After extending the holding duration, thin needle-like ferrite grew coarse since more carbon atoms diffused into adjacent austenitic areas, as is evident in Figure4b,d,f,h,j. In addition, ferritic sheaves became thicker after increasing the austempering temperature, and feather-like ferrite Metals 2019, 9, x FOR PEER REVIEW 6 of 14

3. Results

3.1. Metallurgical Evaluation of AGI and QTGI

As compared with the microstructure of as-cast GI in Figure 1, no changes could be found related to the characteristics of graphite flakes after receiving the austempering heat treatment. The original pearlitic structure was transformed into acicular ferrite and carbon saturated austenite, as shown in Figure 4. Figure 4a,c,e,g,i shows the microstructure of AGI samples at the beginning of the transformation reaction under each austempering temperature. It could be seen that the amount of acicular phases became more with decreasing austempering temperature because of the high degree of supercooling. It was also observed that most of the thin needle-like ferrite initiated around graphite flakes where the potential energy was high. After extending the holding duration, thin needle-like Metals 2019, 9, 1329 6 of 13 ferrite grew coarse since more carbon atoms diffused into adjacent austenitic areas, as is evident in Figure 4b,d,f,h,j. In addition, ferritic sheaves became thicker after increasing the austempering temperature, and feather-like ferrite was formed in the matrix at the austempering temperature of was formed in the matrix at the austempering temperature of 371 ◦C with the holding time of 120 min. In Figure371 °C4b,d,f,h,j, with the the holding light areastime areof 120 carbon min. saturated In Figure austenite, 4b,d,f,h,j,which the light were areas stable are at carbon room temperature.saturated In addition,austenite, some which researchers were stable reported at room thattemperature. the carbon In saturated addition, austenitesome resear wouldchers bereported decomposed that the into carbon saturated austenite would be decomposed into the equivalent ferrite and carbide once the the equivalent ferrite and carbide once the holding time was too long. The formation of ferrite and holding time was too long. The formation of ferrite and carbide would degrade the mechanical carbide would degrade the mechanical properties of austempered cast irons [21–24]. In the present properties of austempered cast irons [21–24]. In the present research, no carbidic particles and islands research, no carbidic particles and islands could be found among the ferritic plates when the highest could be found among the ferritic plates when the highest austempering temperature and longest austemperingholding duration temperature were applied, and longest which holding suggested duration that the were carbon applied, saturated which austenite suggested had that not the been carbon saturateddecomposed austenite into hadequilibrium not been phases. decomposed into equilibrium phases.

Metals 2019, 9, x FOR PEER REVIEW 7 of 14

FigureFigure 4. Metallurgical4. Metallurgical evaluation evaluation ofof AGIAGI Samples Produced Produced by by different different austempering austempering temperatures temperatures

andand holding holding durations: durations: (a )(a 232) 232◦C, °C, 20 20 min; min; (b (b)) 232 232◦ C,°C, 120 120min; min; ((cc)) 288288 ◦°C,C, 3 min; ( d)) 288 288 °C,◦C, 120 120 min; min; (e) 316(e◦)C, 316 2 min;°C, 2 (min;f) 316 (f)◦ C,316 120 °C, min;120 min; (g) 343(g) 343◦C, °C, 1 min; 1 min; (h )(h 343) 343◦C, °C, 120 120 min; min; ( i()i) 371 371◦ °C,C, 11 min;min; (j) 371 ◦C, 120°C, min. 120 min.

In the microstructure analysis of QTGI samples, graphite flakes were retained, and as-quenched martensite and retained austenite were transformed into tempered martensite containing the cementitic and ferritic phases, as shown in Figure 5. When increasing the tempering temperatures with the same holding time, the particles continuously developed. Finally, coarse cementite particles could be found within the ferritic matrix.

Metals 2019, 9, 1329 7 of 13

In the microstructure analysis of QTGI samples, graphite flakes were retained, and as-quenched martensite and retained austenite were transformed into tempered martensite containing the cementitic and ferritic phases, as shown in Figure5. When increasing the tempering temperatures with the same holding time, the cementite particles continuously developed. Finally, coarse cementite particles could beMetals found 2019 within, 9, x FOR the PEER ferritic REVIEW matrix. 8 of 14

FigureFigure 5. Metallurgical5. Metallurgical evaluation evaluation of of QTGI QTGI samples samples produced produced by by di differentfferent tempering tempering temperatures: temperatures: (a)

316(a◦)C; 316 (b °C;) 371 (b)◦ C;371 ( c°C;) 399 (c)◦ 399C; (°C;d) 454(d) 454◦C; °C; (e) 482(e) 482◦C; °C; (f) ( 510f) 510◦C. °C.

3.2.3.2. Hardness Hardness Measurement Measurement TheThe hardness hardness measurements measurements ofof AGIAGI andand QTGIQTGI samples are are plotted plotted in in Figures Figures 66 and and 7.7. At At the the samesame austempering austempering temperature, temperature, thethe hardnesshardness ofof AGI samples decreased decreased first first when when extending extending the the holdingholding time time and and became became almost almost constant constant afterafter aa criticalcritical point in in time. time. When When using using the the long long holding holding time,time, more more unstable unstable austenite austenite was was transformed transformed intointo acicular ferrite ferrite and and carbon carbon saturated saturated austenite austenite ratherrather than than martensite, martensite, which which would wouldresult result inin thethe decreasedecrease in hardness. hardness. The The hardness hardness of ofAGI AGI under under eacheach austempering austempering temperature temperature becoming becoming gradually gradually flat flat indicated indicated that that most most of of the the acicular acicular ferrite ferrite and carbonand carbon saturated saturated austenite austenite were stillwere maintained still maintained in the in matrix. the matrix. Otherwise, Otherwise, the the hardness hardness would would vary oncevary the once carbon the saturatedcarbon saturated austenite austenite was decomposed was decomposed [24,25]. [24,25]. This suggested This suggested that the that longest the longest holding timeholding utilized time in theutilized present in the work present was withinwork was the within “processing the “processing window”, window”, which was which defined was as defined the time as the time interval between where 3% martensite existed and where 10% stable austenite was interval between where 3% martensite existed and where 10% stable austenite was decomposed [26]. decomposed [26]. The hardness measurements also demonstrated that there were no found The hardness measurements also demonstrated that there were no carbides found in the matrix, as in the matrix, as mentioned in Section 3.1. For the same holding time, AGI samples became softer mentioned in Section 3.1. For the same holding time, AGI samples became softer when increasing the when increasing the austempering temperature. This is because the high carbon diffusion rate austempering temperature. This is because the high carbon diffusion rate accelerated the transformation accelerated the transformation of acicular ferrite and carbon saturated austenite. For QTGI samples, of acicularit could ferritebe seen and that carbon the hardness saturated decreased austenite. with For QTGIincreasing samples, tempering it could temperature. be seen that theThe hardness slight decreasedincrease within hardness increasing under tempering tempering temperature. temperatures The of 371 slight °C was increase probably in hardness caused by under the effects tempering of temperaturestempered ofbrittleness 371 ◦C was [27–29]. probably Increasing caused bythe the temperin effects ofg tempered temperature would [27 –promote29]. Increasing the thedecomposition tempering temperature of martensite would into promote dispersive the decompositioncementite particles of martensite and ferrite, into which dispersive facilitated cementite the particlessoftening and rate. ferrite, which facilitated the softening rate.

Metals 2019, 9, 1329 8 of 13 Metals 20192019,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 99 ofof 1414

FigureFigure 6. Rockwell 6. Rockwell C hardnessC hardness of of AGI AGI under under variousvarious au austemperingstemperingstempering temperaturestemperatures temperatures andand and holdingholding holding times.times. times.

FigureFigure 7. Rockwell 7. Rockwell C C hardness hardness of of QTGI QTGI underunder various various tempering tempering temperatures. temperatures.

3.3. Ball-on-Disk3.3. Ball-on-Disk Rotational Rotational Wear Wear Tests Tests In theInIn rotationalthethe rotationalrotational ball-on-disk ball-on-diskball-on-disk sliding slidingsliding wear wearwear tests, testtests,s, fully fully transformedtransformed transformed AGIAGI AGI samplessamples samples withwith with aa 120120 a 120minmin min holdingholding time time at each at each austempering austempering temperature temperature werewere utilized and and compared compared with with corresponding corresponding QTGIQTGI samples samples under under equivalent equivalent hardness, hardness, asas shownshown inin TableTable Table 33 andand FigureFigure Figure 8.8.8 .TheThe The upperupper upper errorerror error barsbars bars representrepresentrepresent the thethe maximum maximummaximum wear wearwear loss, loss,loss, andand lowerlower lower errorerror error baba barsrsrs representrepresent represent thethe minimumminimum the minimum wearwear lossloss wear ofof AGIAGI loss andand of AGI QTGI samples under each heat treatment condition. InIn GroupsGroups 1,1, 2,2, andand 3,3, AGIAGI samplessamples hadhad higherhigher and QTGI samples under each heat treatment condition. In Groups 1, 2, and 3, AGI samples had wear volume loss when increasing the austempering temperature since the softening effect higher wear volume loss when increasing the austempering temperature since the softening effect dominated the wear resistance. A similar behavior was also found on QTGI samples even though the dominatedhardness the was wear improved resistance. slightly A similar in Group behavior 2 since it was has also been found reported on QTGIthat the samples tempered even brittleness though the hardnesswould was significantly improved reduce slightly the intoughness Group 2and since promote it has wear been loss reported [27,30]. that In Groups the tempered 4 and 5, brittlenesshigher wouldaustempering significantly temperature reduce the could enhance and the promote carbon wearcontent loss and [27 ,austenitic30]. In Groups percentage. 4 and More 5, higher austemperingaustenite with temperature a high percentage could enhance of carbon the carbon would content provide and superior austenitic percentage. toughness More to inhibit austenite withmaterial a high percentageremoval, which of carbon could compensate would provide for the superior reduction fracture in hardness. toughness Similar to inhibitresults were material removal,reportedreported which byby Yang,Yang, could JJ compensateetet al.al. [31][31] inin thethe for studystudy the reduction ofof tetensile toughness in hardness. and fracture Similar toughnesstoughness results were inin dualdual reported stepstep by Yang,austempered J et al. [31] inductile the study iron with of tensile high austenitic toughness content and fracture and carbon toughness content. in The dual QTGI step samples austempered in ductileGroups iron with4 and high 5 also austenitic showed lower content wear and volume carbon loss, content. which could The QTGI be associated samples with in Groups the presence 4 and of 5 also a significant amount of dispersive coarse cementite particles. Dong, C et al. [32] found that coarse showed lower wear volume loss, which could be associated with the presence of a significant amount granular cementite phases could have a large bindinging force,force, whichwhich couldcould slowslow downdown thethe cleavagecleavage of dispersive coarse cementite particles. Dong, C et al. [32] found that coarse granular cementite phases separationsseparations insideinside thethe matrixmatrix ofof temperedtempered steel.steel. InIn GroupGroup 6,6, scuffingscuffing withwith highhigh vibrationvibration andand noisenoise couldoccurred have a largeon both binding AGI and force, QTGI which samples could because slow down of the the low cleavage hardness. separations Overall, AGI inside samples the matrix had of tempered steel. In Group 6, scuffing with high vibration and noise occurred on both AGI and QTGI samples because of the low hardness. Overall, AGI samples had slightly lower wear volume loss than QTGI samples under similar hardness, approximately 6%, 6.2%, 8.4%, and 6.5% while using the Metals 2019, 9, x FOR PEER REVIEW 10 of 14 Metals 2019, 9, 1329 9 of 13 slightly lower wear volume loss than QTGI samples under similar hardness, approximately 6%, 6.2%, 8.4%, and 6.5% while using the austempering temperatures of 232 °C, 260 °C, 288 °C, and 316 °C and austempering temperatures of 232 C, 260 C, 288 C, and 316 C and distinctly better wear resistance distinctly better wear resistance ◦while using◦ the ◦austempering◦ temperatures of 343 °C and 371 °C. while using the austempering temperatures of 343 C and 371 C. The best result with a wear loss The best result with a wear loss of around 0.44 mm◦ 3 was obtained◦ using AGI samples with the 3 of aroundaustempering 0.44 mm temperaturewas obtained of 343 using °C, which AGI was samples 21.7% with lower the than austempering that of QTGI temperature samples. In all of wear 343 ◦ C, whichtests, was the21.7% upper lowerceramic than balls that were of much QTGI harder samples. than InGI all disks wear and tests, had no the detectable upper ceramic wear scars. balls were much harder than GI disks and had no detectable wear scars. Table 3. Average hardness of AGI and QTGI sample groups in rotational ball-on-disk sliding Tablewear 3. Averagetests. hardness of AGI and QTGI sample groups in rotational ball-on-disk sliding wear tests.

GroupGroup Number Number 1 1 22 33 4 5 5 6 6

AustemperingAustempering Temperature Temperature (◦ C)(°C) 232 232 260 260 288 288 316 343 343 371 371 HardnessHardness (HRC) (HRC) 42.3 42.3 42 42 37.4 37.4 28.9 28.1 28.1 26.8 26.8 TemperingTempering Temperature Temperature (◦ C)(°C) 316 316 371 371 399 399 454 482 482 510 510 HardnessHardness (HRC) (HRC) 41.1 41.1 41.841.8 38 38 29.5 28.7 28.7 27.2 27.2

Figure 8. Wear volume loss of AGI and QTGI samples under equivalent hardness. Figure 8. Wear volume loss of AGI and QTGI samples under equivalent hardness. 3.4. Worn Surface Analysis 3.4. Worn Surface Analysis SEM analysis was conducted to evaluate the worn surface of AGI samples; see Figure9. It can be SEM analysis was conducted to evaluate the worn surface of AGI samples; see Figure 9. It can seen that cracks spread out on the wear track. In Group 1, which was also representative of Groups 2 be seen that cracks spread out on the wear track. In Group 1, which was also representative of Groups and 3 (Figure9a), some parallel grooves along the sliding direction were found on the wear track, which 2 and 3 (Figure 9a), some parallel grooves along the sliding direction were found on the wear track, suggested abrasive wear was one of the main mechanisms for these AGI samples with relatively high which suggested abrasive wear was one of the main mechanisms for these AGI samples with hardness.relatively Furthermore, high hardness. some Furthermore, spalls were observed, some spalls which were were observed, caused bywhich the linkages were caused of cracks by onthe the surfacelinkages and sub-surface,of cracks on the which surface introduced and sub-surface, fatigue wear which as anotherintroduced main fatigue mechanism. wear as Itanother is well main known thatmechanism. the tips of graphiteIt is well known flakes actthat as the stress tips concentrationof graphite flakes sites. act Cracksas stress can concentration nucleate around sites. Cracks graphite flakescan and nucleate grow around towards graphite the adjacent flakes graphite and grow flakes towards and thethe outeradjacent surface graphite easily flakes due toand the the brittleness outer of AGIsurface samples. easily due During to the the brittleness wear tests, of AGI voids samples. were During produced the wear by the tests, graphite voids were flakes produced being peeled by outthe from graphite the surface. flakes being Similar peeled findings out from were the also surfac reportede. Similar by Sarkar,findings T were et al. also [4, 10reported]. In addition, by Sarkar, they concludedT et al. [4,10]. that adhesive In addition, wear they with concluded plastic flow that andadhesive oxidative wear wear with wereplastic also flow important and oxidative mechanisms wear forwere AGI. However,also important plastic mechanisms flow and oxidationfor AGI. areasHowever, were plastic not detected flow and in theoxidation current areas research. were Asnot the weardetected test continued, in the current large research. scale debris As wasthe wear broken test up continued, into small large wear scale debris. debris Then, was the broken small up particles into wouldsmall plough wear debris. the AGI Then, surfaces the small under particles the high would Hertzian plough contact the AGI stress; surfaces see Figureunder the 10 a.high Hertzian contactIn Group stress; 5, whichsee Figure was 10a. also representative of Group 4 (Figure9b), more darks spots were seen than in Group 1. These dark spots were identified as carbon by using EDS; see Figure9c. On the wear track, only small pits and cracks could be found, and the crack length was shorter than that in Group 1. These findings could be explained as follows: More stable austenite in Group 5 with high carbon content could withstand severe plastic on the surface and sub-surface under the high shearing force. The resulting higher would retard the nucleation and propagation of cracks. Therefore, no spalls were observed. Under high normal load and shear force, the graphite flakes on the Metals 2019, 9, 1329 10 of 13 surface and sub-surface would be deformed with the substrate along the sliding direction. Graphite flakes were then exposed and ground by the ceramic ball to produce graphite powder. The graphite MetalsMetals 2019 2019, 9, ,x 9 ,FOR x FOR PEER PEER REVIEW REVIEW 11 11of of14 14 powder resulted in a tribo-layer which lowered the wear, as shown in Figure 10b.

Figure 9. Surface evaluation on wear track: (a) AGI sample in Group 1; (b) AGI sample in Group 5; Figure(c) elemental 9. Surface examination evaluation of on dark wear spot track: by EDS. (a) AGI sample in Group 1; (b) AGI sample in Group 5; (c) Figure 9. Surface evaluation on wear track: (a) AGI sample in Group 1; (b) AGI sample in Group 5; elemental examination of dark spot by EDS. (c) elemental examination of dark spot by EDS.

Figure 10. Cont.

MetalsMetals2019 2019, 9,, 13299, x FOR PEER REVIEW 12 of11 14 of 13

Figure 10. Potential failure mechanisms on AGI Samples in sliding wear tests: (a) production of wear Figure 10. Potential failure mechanisms on AGI Samples in sliding wear tests: (a) production of wear debris; (b) formation of graphite tribo-layer. debris; (b) formation of graphite tribo-layer. 4. Conclusions In Group 5, which was also representative of Group 4 (Figure 9b), more darks spots were seen thanIn in this Group research, 1. These AGI dark samples spots were were identified prepared as using carbon di byfferent using austemperingEDS; see Figure temperatures 9c. On the wear and holdingtrack, only durations. small pits Rotational and cracks ball-on-disk could be found, sliding and wear the crack tests length were carried was shorter out on than AGI that samples in Group and compared1. These withfindings conventional could be explained QTGI samples as follows: under More similar stable hardness. austenite In in addition, Group 5 microstructurewith high carbon and hardnesscontent were could evaluated withstand using severe optical plastic microscopy deformation and on a Rockwellthe surface hardness and sub-surface tester. Several under conclusions the high canshearing be drawn: force. The The original resulting pearlite higher was fracture transformed toughness into would acicular retard ferrite the and nucleation carbon saturatedand propagation austenite inof the cracks. austempering Therefore, heat no treatment.spalls were Most observed. of the Unde ferriticr high platelets normal nucleated load and around shear force, graphite the graphite flakes due toflakes the high on potential the surface energy. and Thinsub-surface needle-like would ferrite be deformed became coarse with afterthe substrate increasing along the austemperingthe sliding temperaturedirection. Graphite or extending flakes thewere holding then exposed time. The and hardnessground by of the AGI ceramic samples ball decreased to produce when graphite either increasingpowder. theThe austemperinggraphite powder temperature resulted in or a tribo-layer extending thewhich holding lowered time the and wear, became as shown nearly in Figure constant beyond10b. a critical time (mostly between 20 min to 30 min). This critical time occurred when the phase transformation had been fully completed in the matrix. The hardness drop was caused by the formation 4. Conclusions of soft acicular ferrite and stable austenite rather than hard martensite. The slight increase in hardness of QTGIIn samples this research, while AGI using samples the tempering were prepared temperature using ofdifferent 371 ◦C austempering was associated temperatures with the eff ectsand of temperedholding brittleness.durations. Rotational In rotational ball-on-disk ball-on-disk sliding sliding wear weartests were tests, carried the average out on wear AGI volumesamples lossand of AGIcompared samples with was conventional slightly lower QTGI than samples that of QTGI under samples similar hardness. under equivalent In addition, hardness, microstructure approximately and hardness were evaluated using optical microscopy and a Rockwell hardness tester. Several 6%, 6.2%, 8.4%, and 6.5% while using the austempering temperatures of 232 ◦C, 260 ◦C, 288 ◦C, and conclusions can be drawn: The original pearlite was transformed into acicular ferrite and carbon 316 ◦C, respectively. In addition, excellent wear resistance of AGI samples was found while using the saturated austenite in the austempering heat treatment. Most of the ferritic platelets nucleated around 3 austempering temperatures of 343 ◦C and 371 ◦C. The best result having a wear loss around 0.44 mm graphite flakes due to the high potential energy. Thin needle-like ferrite became coarse after was obtained using AGI samples with the austempering temperature of 343 ◦C, which was 21.7% lower increasing the austempering temperature or extending the holding time. The hardness of AGI than that of QTGI samples. In the analysis of worn tracks, AGI samples produced by low austempering samples decreased when either increasing the austempering temperature or extending the holding temperature showed abrasive wear and fatigue wear mechanisms with the presence of pits, spalls, time and became nearly constant beyond a critical time (mostly between 20 min to 30 min). This voids, and long cracks on the wear track. However, only small pits and short cracks could be detected critical time occurred when the phase transformation had been fully completed in the matrix. The on the wear track of AGI samples with a high austempering temperature. It was believed the graphite hardness drop was caused by the formation of soft acicular ferrite and stable austenite rather than flakeshard were martensite. ground The during slight the increase wear tests. in hard Then,ness the of graphite QTGI samples particles while would using form the a tribo-layertempering to protecttemperature the contact of 371 surface. °C was associated with the effects of tempered brittleness. In rotational ball-on- disk sliding wear tests, the average wear volume loss of AGI samples was slightly lower than that of Author Contributions: Conceptualization, B.W. and G.C.B.; methodology, B.W. and Y.P.; data curation, B.W.; formalQTGI analysis, samples B.W., under X.H. equivalent and Y.P.; investigation, hardness, approxim B.W., X.H.ately and 6%, Y.P.; 6.2%, resources, 8.4%, G.C.B.;and 6.5% writing, while originalusing the draft preparation, B.W. and Y.P.; writing, review and editing, B.W. and G.C.B.; project administration, G.C.B.

Metals 2019, 9, 1329 12 of 13

Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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