Tempered Gray Cast Irons Under Similar Hardness

Metals 2019, 9, 1329 x FOR PEER REVIEW of 14 13 33 of Table 1. Alloy content for gray cast iron. Table 1. Alloy content for gray cast iron. Elements Elements Carbon, C Carbon, C Si Silicon, Silicon, Si Manganese, Mn Manganese, Mn Chromium, Cr Chromium, Cr Copper, Copper, Cu Cu Sulfur, Sulfur, S S Phosphorous, P P Phosphorous, Iron, Fe Iron, Fe Percentage Percentage 3.53% 3.53% 2.71% 2.71% 0.74% 0.74% 0.12% 0.12% 0.94% 0.94% 0.03% 0.03% 0.08% 0.08% Remainder Remainder 2.2. As-Cast Gray Cast Iron The original microstructure of the as-cast GI is shown in Figure 1. The main components in the matrix were graphite flakes, ferrite, and and pearlite. pearlite. Original microstructure of as-cast gray cast iron. Figure 1. Original 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 Figure 2a and Table 2. Metals 2019, 9, x FOR PEER REVIEW 4 of 14 water. The austempering process diagram is displayed in Figure 2a and Table 2. 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. Figure 2. Heat treatment processes: (a) austempering heat treatment; (b) quenching and tempering Figure 2. Heat treatment processes: (a) austempering heat treatment; (b) quenching and tempering treatment. heatheat 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 oxide 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,

Metals 2019, 9, 1329 4 of 13 Table 2. Details of heat treatment designs and experiments. Heat Treatment Designs Austempering Heat Treatment (AGI) Temperature: 832 C; Time: 20 min; Medium Temp Salt Bath Furnace Austenitizing Process: Temperatures: 232 C, 260 C, 288 C, 316 C, 343 C, 371 C; 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 Austempering Process: 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; Oil Cool; Electrical Heating Furnace Tempering Process: Experiments Metallurgical Evaluations AGI and QTGI Coupons (15 mm 15 mm 15 mm) 3% Nital for 2 s or 3 s Optical Microscopy with 500 Magnification 16 Sample Etching Test Facility Sample Size Hardness Measurements Sample Test Facility Repetition Sample Size AGI and QTGI Sample Coupons/Disks Rockwell Hardness Tester 3 Times 234 Wear Tests Upper Sample Lower Sample Normal Load Rotational Speed Lubricant Test Duration Test Facility Repetition Sample Size Alumina Ball (Diameter: 7.94 mm; Hardness: 75 HRC; Surface Roughness (Ra): 10 nm) AGI and QTGI Disks (Diameter: 64 mm; Thickness: 11 mm; Surface Roughness (Ra): 300 nm) 300 N 240 rpm PAO4 Base Oil 30 min Rotational Ball-on-Disk Sliding Configuration 3 Times 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 Figure 2b, and Table 2 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

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 surface using Si-carbide sandpaper from 240 grit to 1200 grit and polishing cloths with 0.3 µm alumina mounted using Diallyl Phthalate powder. The coupons were ground and polished to a mirror-like oxidesurface suspension. Then, coupons werefrom thoroughly water etchedcloths by 3% nital using Si-carbide sandpaper 240 grit rinsed to 1200by grit and and polishing with 0.3solutions μm for 2alumina s to 3 s.oxide Metallurgical evaluation was were carried out using optical microscopy (PME-3, suspension. Then, coupons thoroughly rinsed by water and etched by 3%Olympus, nital Tokyo, Japan).for 2 s to 3 s. Metallurgical evaluation was carried out using optical microscopy (PME-3, solutions 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, Sliding tests wereball-on-disk conducted onconfiguration a universal mechanical (UMT-3, Billica, MA, USA) withwear a rotational at roomtribometer temperature. An Bruker, alumina ball was 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 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 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 speed was 240speed rpm.was In 240 sliding tests,wear the tests, sample into PAO4 base oil rotational rpm. wear In sliding the disks samplewere diskssubmerged were submerged into PAO4 C). The test duration was 30 min. Each test was repeated three (kinematic viscosity of 16.8 cSt at 40 base oil (kinematic viscosity of 16.8 cSt at 40 C). The test duration was 30 min. Each test was repeated times, andtimes, the averages were reported. After the wear an SEM (JSM-6510, JEOL, Tokyo, Japan) three and the averages were reported. After the tests, wear tests, an SEM (JSM-6510, JEOL, Tokyo, equipped EDS with was EDS usedwas to observe the worn for potential mechanisms. TheThe details of the Japan)with equipped used to observe thetracks worn tracks for potential mechanisms. details of the wear tests are summarized in Table 2. wear tests are summarized in Table 2. Figure 3. Sectional ball-on-disksliding sliding wear fixture. Figure 3. Sectionalview viewofofthe therotational rotational ball-on-disk wear testtest 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 (Table 2). 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 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

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 and feather-like was formed in the matrix the C austempering temperature wastemperature, formed in the matrix at the ferrite austempering temperature of at 371 with the holding time ofof120 min. 371 C with the holding time of 120 min. In Figure 4b,d,f,h,j, the light areas are carbon saturated In Figure 4b,d,f,h,j, the light areas are carbon saturated austenite, which were stable at room temperature. austenite,some which were stablereported at roomthat temperature. Insaturated addition, austenite some researchers reported that theinto In addition, researchers the carbon would be decomposed 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 austempering temperature and longest holding duration were applied, which austenite suggestedhad thatnot thebeen carbon holding duration were applied, which suggested that the carbon saturated saturated austenite not beenphases. decomposed into equilibrium phases. decomposed intohad equilibrium Metals 2019, 9, x FOR PEER REVIEW 7 of 14 Figure 4. Metallurgicalevaluation evaluationof of AGI AGI Samples austempering temperatures Figure 4. Metallurgical SamplesProduced Producedbybydifferent different austempering temperatures C, C,120 holding durations: 232 C,2020min; min;(b) (b)232 232 C, C,120 120min; min;(c) (c) 288 288 C, andand holding durations: (a)(a) 232 C, 3 min; (d) (d) 288 288 C, 120min; min; (e) C, C, C, C, 316 C, 2 (f) min; (f) C, 316120 C, min; 120 min; (g) 343 1 min; C, 120min; min;(i)(i)371 371 C, min; (j) (j) 371 316(e) 2 min; 316 (g) 343 1 min; (h)(h) 343343 120 C, 11 min; 371 C, C, 120 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 cementite 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 Figure 5. When increasing the tempering temperatures with the same holding time, the cementite particles continuously developed. Finally, coarse cementite particles could be Metals found2019, within thePEER ferritic matrix. 9, x FOR REVIEW 8 of 14 Figure 5. Metallurgical evaluation QTGI samplesproduced producedbybydifferent differenttempering temperingtemperatures: temperatures:(a) Figure 5. Metallurgical evaluation ofof QTGI samples C; C; C; C; C; C. C. 316 (b) C; 371(c) C;399 (c) 399 C;454 (d) 454 (e) 482 316(a) (b) C; 371 C; (d) (e) 482 (f) (f) 510510 3.2.3.2. Hardness Measurement Hardness Measurement The hardness measurements are plotted plottedininFigures Figures6 6and and7.7.AtAt The hardness measurementsof ofAGI AGIand and QTGI QTGI samples samples are thethe same austempering samples decreased decreasedfirst firstwhen whenextending extending same austemperingtemperature, temperature,the thehardness hardness of of AGI samples thethe holding time and becamealmost almostconstant constantafter after aa critical critical point point in holding holding time and became in time. time.When Whenusing usingthe thelong long holding time, more unstableaustenite austenitewas wastransformed transformed into into acicular acicular ferrite time, more unstable ferriteand andcarbon carbonsaturated saturatedaustenite austenite rather than martensite,which whichwould wouldresult result in in the the decrease decrease in AGI under rather than martensite, in hardness. hardness.The Thehardness hardnessofof AGI under each austempering temperature becoming gradually indicated that most acicular ferrite each austempering temperature becoming gradually flatflat indicated that most of of thethe acicular ferrite and and carbon saturated austenite were still maintained in the matrix. Otherwise, the hardness would carbon saturated austenite were still maintained in the matrix. Otherwise, the hardness would vary vary the saturated carbon saturated austenite was decomposed This suggested that the longest once theonce carbon austenite was decomposed [24,25].[24,25]. This suggested that the longest holding holding time utilized in the present work was within the “processing window”, which was defined time utilized in the present work was within the “processing window”, which was defined as 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 carbides 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 acicular and carbon saturateddecreased austenite. with For QTGI samples, it couldtemperature. be seen that the it could ferrite be seen that the hardness increasing tempering Thehardness slight decreased increasing tempering temperature. slight increase in hardness under tempering increase with in hardness under tempering temperaturesThe of 371 C was probably caused by the effects of temperatures 371 C was probably caused bythe the tempering effects of tempered brittleness [27–29]. Increasing tempered of brittleness [27–29]. Increasing temperature would promote the thedecomposition tempering temperature would promote the decomposition of martensite into dispersive cementite of martensite into dispersive cementite particles and ferrite, which facilitated the particles andrate. ferrite, which facilitated the softening rate. softening

Metals 2019, 9, 1329 Metals 2019, 2019, 9, 9, xx FOR FOR PEER PEER REVIEW REVIEW Metals 8 of 13 of 14 14 99 of Figure 6. Rockwell C hardness AGIunder undervarious various austempering andand holding times. Figure 6. Rockwell C hardness ofofAGI austemperingtemperatures temperatures holding times. Figure 7. Rockwell hardnessof ofQTGI QTGI under under various temperatures. Figure 7. Rockwell CC hardness varioustempering tempering temperatures. 3.3. Ball-on-Disk Rotational Wear Tests 3.3. Ball-on-Disk Rotational Wear Tests In rotational the rotational ball-on-disk slidingwear wear tests, tests, fully AGI samples with a 120 min min In the ball-on-disk sliding fullytransformed transformed AGI samples with a 120 holding time at each austempering temperature were utilized and compared with corresponding holding time at each austempering temperature were utilized and compared with corresponding samples under equivalent hardness,as asshown shown in The upper error barsbars QTGIQTGI samples under equivalent hardness, in Table Table33and andFigure Figure8. 8. The upper error represent maximum wearloss, loss, and and lower bars represent the minimum wear loss of AGI represent the the maximum wear lowererror error bars represent the minimum wear lossand of AGI QTGI samples under each heat treatment condition. In Groups 1, 2, and 3, AGI samples had higher 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 dominated thewas wear resistance. A similar behavior was on QTGI samples even though the hardness improved slightly in Group 2 since it hasalso beenfound reported that the tempered brittleness hardness was improved slightly in Group 2 since it has been reported that the tempered would significantly reduce the toughness and promote wear loss [27,30]. In Groups 4 and 5, brittleness higher would significantly reduce thecould toughness andthe promote lossand [27,30]. In Groups 4 and More 5, higher austempering temperature enhance carbon wear content austenitic percentage. austempering temperature could enhance the carbon austenitic percentage. austenite austenite with a high percentage of carbon wouldcontent provideand superior fracture toughnessMore to inhibit removal, which could compensate for the reduction in hardness. Similar results were with material a high percentage of carbon would provide superior fracture toughness to inhibit material reported by Yang, J et al. [31] in the study of tensile toughness and fracture toughness in dual

Iron, Fe Remainder 2.2. As-Cast Gray Cast Iron The original microstructure of the as-cast GI is shown in Figure1. The main components in the matrix were graphite flakes, ferrite, and pearlite. Metals 2019, 9, x FOR PEER REVIEW 3 of 14 Figure 2. Table 1. Alloy content for gray cast iron. Elements Percentage Carbon, C 3.53% Silicon, Si 2.71%