Heat Treatment Evaluation for the Camshafts Production of ADI Low Alloyed with Vanadium

Article Heat Treatment Evaluation for the Camshafts Production of ADI Low Alloyed with Vanadium

Eduardo Colin García 1, Alejandro Cruz Ramírez 1,* , Guillermo Reyes Castellanos 1, José Federico Chávez Alcalá 1, Jaime Téllez Ramírez 2 and Antonio Magaña Hernández 2

1 Metallurgy and Materials Department, Instituto Politécnico Nacional—ESIQIE, UPALM, Ciudad de México 07738, Mexico; [email protected] (E.C.G.); [email protected] (G.R.C.); [email protected] (J.F.C.A.) 2 R&D ARBOMEX S.A de C.V., Calle Norte 7 No. 102, Cd. Industrial, Celaya, Guanajuato 38010, Mexico; [email protected] (J.T.R.); [email protected] (A.M.H.) * Correspondence: [email protected]; Tel.: +52-55-5729-6000 (ext. 54202)

Abstract: Ductile iron camshafts low alloyed with 0.2 and 0.3 wt % vanadium were produced by one of the largest manufacturers of the ductile iron camshafts in México “ARBOMEX S.A de C.V” by a phenolic urethane no-bake sand mold casting method. During functioning, camshafts are subject to bending and torsional stresses, and the lobe surfaces are highly loaded. Thus, high toughness and wear resistance are essential for this component. In this work, two austempering ductile iron heat treatments were evaluated to increase the mechanical properties of tensile strength, hardness, and toughness of the ductile iron camshaft low alloyed with vanadium. The austempering process was held at 265 and 305 ◦C and austempering times of 30, 60, 90, and 120 min. The volume fraction of Citation: Colin García, E.; high-carbon austenite was determined for the heat treatment conditions by XRD measurements. The Cruz Ramírez, A.; Reyes Castellanos, G.; ausferritic matrix was determined in 90 min for both austempering temperatures, having a good Chávez Alcalá, J.F.; Téllez Ramírez, J.; Magaña Hernández, A. Heat Treatment agreement with the microstructural and hardness evolution as the austempering time increased. The Evaluation for the Camshafts mechanical properties of tensile strength, hardness, and toughness were evaluated from samples Production of ADI Low Alloyed with obtained from the camshaft and the standard Keel block. The highest mechanical properties were ◦ Vanadium. Metals 2021, 11, 1036. obtained for the austempering heat treatment of 265 C for 90 min for the ADI containing 0.3 wt % https://doi.org/10.3390/ V. The tensile and yield strength were 1200 and 1051 MPa, respectively, while the hardness and the met11071036 energy impact values were of 47 HRC and 26 J; these values are in the range expected for an ADI grade 3. Academic Editor: Anders E. W. Jarfors Keywords: camshaft; ductile iron; vanadium; as-cast; ADI; microstructure; mechanical properties

Received: 26 May 2021 Accepted: 24 June 2021 Published: 28 June 2021 1. Introduction

Publisher’s Note: MDPI stays neutral Austempered ductile iron or ADI is a family of ductile iron (DI) that has been treated with regard to jurisdictional claims in by austempering (isothermal heat treatment) [1] that results in nodules immerses in an published maps and institutional afﬁl- ausferritic matrix composed of acicular ferrite (αAc) and high-carbon austenite (γHC)[2]. iations. The ADI microstructure provides good ductility and fracture toughness, high strength, good wear resistance, high fatigue strength, as well as rolling contact resistance, and a density lower than steel. The minimum characteristics in ductile iron that must be taken into consideration to obtain the best mechanical properties in ADIs are (a) minimum nodule count of 100 nodules/mm2 with uniform distribution, (b) 85% nodularity, (c) 1.5% Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. maximum of the combined content of carbides, non-metallic inclusions, micro-shrinkage, This article is an open access article and porosity, and (e) homogenous chemical composition [3]. distributed under the terms and The complete austempered heat treatment is a set of processes used to obtain ADIs. conditions of the Creative Commons The heat treatment starts with the austenitizing step of ductile iron in the range of tem- ◦ Attribution (CC BY) license (https:// peratures of 850–950 C for 1 h, or longer residence times to ensure transformation from creativecommons.org/licenses/by/ the as-cast matrix to austenite [4]. After austenitizing, the sample is quenched in a salt ◦ 4.0/). bath to the austempering temperatures in the range of 250–450 C with enough holding

Metals 2021, 11, 1036. https://doi.org/10.3390/met11071036 https://www.mdpi.com/journal/metals Metals 2021, 11, 1036 2 of 23

time to obtain the ausferritic matrix and finally cooled to room temperature [5]. In the ADI process, two stages of austempering have been identified; in the first stage represented by reaction (1), the austenite unstable (γ) transforms into acicular ferrite (αAc) and high-carbon austenite (γHC). γ → αAc + γHC (ausferrite) (1) Longer austempering times are required for the second stage of austempering that proceeds according to Reaction (2). In this stage, the high-carbon austenite (γHC) transforms into ferrite and carbides of the type Fe3C or ε. The occurrence of the second stage is not desired because promotes brittleness, thus bringing down the properties of the casting [6].

γHC → α + carbide (Fe3C or ε ) (2)

In the ADI microstructure, ferrite is called commonly acicular or bainitic; however, acicular ferrite is a product from the ﬁrst stage, while bainitic ferrite is a product formed from the second stage [7]. The maximum ausferrite amount is obtained between the two stages of austempering; this is at the end of the ﬁrst stage and the onset of the second stage. This period is called the process window (PW), and it is represented by Reaction (3) [8].

PW : αAc + γHC (stable structure) (3)

The amount and morphology of the high-carbon austenite and acicular ferrite depend on the austempering parameters, temperature, and holding time. Fine ausferrite is obtained by austempering in the range of 260–316 ◦C, while coarser and feathery ausferrite is formed in the range of 316–450 ◦C. Lower austempering temperatures result in higher yield and tensile strength and hardness but with lower ductility, while higher ductility and fracture toughness are obtained when the austempering temperature is higher than 316 ◦C with a corresponding decrease in the yield and tensile strength [9,10]. The mechanical properties that can be achieved by ADIs are referenced in the standard ASTM A 897, where six ADI grades are classified. Compared to steel, ADI has low material and production cost, low density, good processing ability, and a high vibration damping ability. These advantages make ADI attractive for industrial applications. For the automotive industry, ADI has an important task as a structural material that should have a good wear resistance and tensile strength, in such applications as camshafts [11,12]. Some forged steel components have been replaced by austempered ductile iron (ADI), mainly in automotive applications as camshafts. A camshaft is a critical component required to enable a combustion engine to work. It is constituted by a shaft with shaped lobes (cam lobes) positioned along with it. When the shaft is rotated, the profile of the lobe allows it to act upon a valve or switch to a degree matching with the speed of rotation controlling the rate of action. The camshafts are connected via a timing belt or chain to the turning of the crankshaft—which is directly moving the pistons inside the cylinder [13]. During the engine functioning, the camshaft is subject to different mechanisms of degradation such as multiaxial stresses, corrosion, abrasion, creep, and wear as a result of contact stresses and temperature operations that are conducive to crack or failure [14]. Wear is developed at the top of the cams, causing changes in the design contour [15]. In this sense, there is a scarcity of research focused on increasing the hardness of the lobes. Chills were used on the cams of gray cast iron to increase the cooling rate, promote directional solidification, and obtain a hard ledeburitic structure [15,16]; however, a black line composed of pearlite and graphite was formed inside the chilled area of the lobe, decreasing the hardness [17]. Kumruo˘glu[18] studied the mechanical and microstructure properties of chilled cast iron camshaft. As a result of the strong cooling effect of chill, top lobes are rapidly solidified, obtaining a hard ledeburitic phase and fine pearlite, increasing the hardness. Karaca [19] studied the combined heat treatment of induction hardening and austempering on GGG60 class ductile iron for camshafts production. They found that the surface microstructure of the camshaft consists of nodule graphite, fine martensite, some untransformed austenite, and some needles Metals 2021, 11, 1036 3 of 23

of ferrite. The surface hardness reached a maximum value of 62.4 HRC. Laser surface hardening is an effective process used to increase the working characteristics of product surfaces of high load components such as camshafts lobes, crankshafts necks, and gears, among others. Recently, hard facings introduced by melting and alloying via high energy are new trends in the surface strengthening of steel and ductile iron [20–22]. Alloying elements are used to improve the mechanical properties or modify the austemperability of ADI [23,24]. Given the effects of vanadium on the transformation of steels, it was expected that the beneﬁcial effects of microalloying elements may be exploited on ductile iron and ADI production [24]. Since vanadium is a carbide stabilizer, its addition promotes the formation of eutectic carbide that appears as small white inclusions in the microstructure. The addition of vanadium to the ductile cast iron increases the strength and hardness by increasing the pearlite amount; however, elongation is decreased [25]. Colin [26] studied the microstructural features and the mechanical properties of ductile iron camshafts low alloyed with 0.2 and 0.3 wt % of vanadium. In both ductile irons, the highest carbide formation (less than 1 wt %) was located principally in the middle region of the lobes due to the inverse chill behavior. The mechanical properties of hardness, tensile, and yield strength were increased with the addition of 0.3 wt % V, while the highest values of toughness and ductility were obtained for the ductile iron containing 0.2 wt % V. Both ductile irons fulﬁll the minimum requirements to produce ADIs [3]. This work aims to determine the process window (PW) of two austempering temperatures (265 and 305 ◦C) to identify the optimum austempering parameters that increase the mechanical properties of the ductile irons alloyed with 0.2 and 0.3 wt % V for the camshaft manufacturing. The microstructural evolution of the austempering heat treatment was evaluated in three regions located near the top of the lobes. The mechanical properties of tensile strength, hardness, and toughness by the Charpy impact test were evaluated to the austempering time where the highest high-carbon austenite value was reached.

2. Materials and Methods 2.1. Ductile Iron Castings Two hyper-eutectic ductile irons containing 0.2% and 0.3 wt % V were produced in an Induction Furnace of medium frequency (300 Hz), Inductotherm of 3500 kW potency capacity, and 6 tons per hour of furnace capacity by ARBOMEX S.A de C.V., which is a Mexican company located at Celaya and Apaseo el Grande Guanajuato, México that specializes in camshafts manufacturing. The base iron was produced using 30 wt % low- carbon steel, 30 wt % iron burrs from the machining area, and cast-iron scrap as balance. All the materials were melted and homogenized at 1400–1440 ◦C. The chemical composition of the base iron was adjusted in a preheated ladle by adding the ferroalloys: FeSi (70%), high-purity carbon, and FeV (61.5%). The ductile iron alloyed was poured into a tundish ladle where 1.05 wt % of MgFeSi (45% Si, 7.5% Mg, 0.8% Al, 2.6% Ca, 2.48% rare earth) was added as a nodulizing agent. Later, the melt was poured into a ladle and inoculated with the inoculant FeSi (70% Si + 0.8% Ca, 3.9% Al) by the ladle inoculation method. Each of the two cast alloys was poured at 1385–1420 ◦C into the phenolic urethane no-bake sand molds casting method, which was previously obtained by a prototype tooling with four cavities of camshafts (intake and exhaust lobes) for a V8 engine to obtain about 100 camshafts for each alloy. Figure1 shows the tooling used for the preparation of the camshaft molds; every camshaft contains 16 lobes. MetalsMetals 2021, 202111Metals, x ,FOR 112021, xPEER, 11FOR, 1036 REVIEW PEER REVIEW 4 of 234 of 21 4 of 21

Figure 1. Camshaft tooling containing four camshafts with 16 lobes.

TheFigureFigure nominal 1. Camshaft1. Camshaft chemical tooling toolingcomposition containing containing four camshaftsin the four camshafts with camshafts 16 lobes. was with analyzed 16 lobes. by an OBLF GS 1000 II emission optic spectrograph (OBLF Gesellschaft für Elektronik und Feinwerktech‐ nik mbH, Witten,The nominal Germany), chemical and composition the reported in the values camshafts are was the analyzed average by of an three OBLF measure GS ‐ 1000 II emissionThe nominal optic spectrograph chemical (OBLF composition Gesellschaft fürin Elektronikthe camshafts und Feinwerktechnik was analyzed by an OBLF GS ments onmbH,1000 each Witten, II cast emission Germany),alloy. Carbon optic and thespectrograph and reported sulfur values content (OBLF are thewas average Gesellschaft determined of three by measurements für combustion Elektronik on anal und‐ Feinwerktech‐ ysis usingeachnik a cast mbH,Leco alloy. C/S Witten, Carbon 744 analyzer and Germany), sulfur (LECO content and Corporation, was the determined reported St. by Joseph, combustion values MI, are analysisUSA). the usingaverage a of three measure‐ Leco C/S 744 analyzer (LECO Corporation, St. Joseph, MI, USA). 2.2. Austemperingments on Heat each Treatment cast alloy. Carbon and sulfur content was determined by combustion anal‐ 2.2.ysis Austempering using a Leco Heat Treatment C/S 744 analyzer (LECO Corporation, St. Joseph, MI, USA). Four camshafts were randomly selected from each alloy and the lobes were sectioned Four camshafts were randomly selected from each alloy and the lobes were sectioned on the cross‐section with a metallographic fine cutter disc and liquid cooling for the aus‐ on2.2. the Austempering cross-section with Heat a metallographic Treatment ﬁne cutter disc and liquid cooling for the temperingaustempering heat treatment. heat treatment. Figure 2a Figure shows2a shows the samples the samples taken taken from from the lobes the lobes heat heat treated, and Figuretreated, 2bFour andshows Figure camshafts the2 bthree shows wereregions the three randomly analyzed regions from analyzedselected the fromtop from area the each top (nose area alloy of (nose the and lobe), of the midlobes‐ were sectioned dle, andlobe),on bottom the middle, cross (base and‐ sectioncircle) bottom for (basewith the circle) microstructurala metallographic for the microstructural characterization. fine characterization.cutter disc and liquid cooling for the aus‐ tempering heat treatment. Figure 2a shows the samples taken from the lobes heat treated, and Figure 2b shows the three regions analyzed from the top area (nose of the lobe), mid‐ dle, and bottom (base circle) for the microstructural characterization.

(a) (b)

Figure 2.Figure Lobe 2. takenLobe taken from from camshaft camshaft showing showing (a ()a )the the sample sample heatheat treated treated and and (b) ( theb) threethe three regions regions analyzedanalyzed from top from to top bottom. to bottom.

Two austempering heat treatments were carried out in two electric furnaces with a Two austempering◦ heat treatments were carried out in two electric furnaces with a heatingheating rate of rate 10 of°C/min 10 C/min based based on onthe the austempering austempering heat treatmenttreatment cycles cycles of Figureof Figure3. 3. The samples taken from the lobes(a) were coated with carbon paint (to avoid decarburization)(b) The samples taken from the lobes were coated with carbon paint (to avoid decarburiza‐ tion) duringFigure austenitizing 2. Lobe taken held from at 900camshaft ± 5 °C showingwith a residence (a) the sample time of heat180 min.treated Then, and the (b ) the three regions samplesanalyzed were quickly from toptransferred to bottom. to a second furnace containing a salt bath melt (50% KNO3 and 50% NaNO3) at 265 or 305 ± 5 °C. The soaking time was set at 30, 60, 90, and 120 min, andTwo then, austemperingthe samples were heat water treatments‐cooled at roomwere temperature. carried out in two electric furnaces with a heating rate of 10 °C/min based on the austempering heat treatment cycles of Figure 3. The samples taken from the lobes were coated with carbon paint (to avoid decarburiza‐ tion) during austenitizing held at 900 ± 5 °C with a residence time of 180 min. Then, the samples were quickly transferred to a second furnace containing a salt bath melt (50% KNO3 and 50% NaNO3) at 265 or 305 ± 5 °C. The soaking time was set at 30, 60, 90, and

120 min, and then, the samples were water‐cooled at room temperature.

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during austenitizing held at 900 ± 5 ◦C with a residence time of 180 min. Then, the samples were quickly transferred to a second furnace containing a salt bath melt (50% KNO3 and Metals 2021, 11, x FOR PEER REVIEW ± ◦ 5 of 21 50% NaNO3) at 265 or 305 5 C. The soaking time was set at 30, 60, 90, and 120 min, and then, the samples were water-cooled at room temperature.

(a) (b)

Figure 3. Austempering heat treatment cycles forfor temperaturestemperatures ofof ((aa)) 265265 andand ((bb)) 305305 ◦°C.C.

2.3. Microstructural Characterization In terms terms of of microstructural microstructural examinations, examinations, standard standard metallography metallography was was employed employed us‐ usinging an an optical optical microscope microscope Olympus Olympus PMG PMG-3‐3 model model according according to to the the standard standard ASTM A247 and the Image J software to evaluate the nodule count, average nodule size, nodularity, and the volume fraction of graphite, ferrite, and pearlite of the ductile iron in thethe as-castas‐cast condition. The austempered ductile iron samples were etched with nital 3% to reveal thethe phases. CarbidesCarbides werewere revealed revealed by by etching etching for for 80 80 s with s with a water a water solution solution of ammonium of ammonium per- sulfatepersulfate (10% (10% vol) vol) [27, 28[27,28].]. The The volume volume fraction fraction of vanadium of vanadium carbides carbides in ADIs in ADIs was obtained was ob‐ withtained the with Image the Image J software. J software. The reported The reported results results for the for optical the optical microscopy microscopy analysis analysis were thewere average the average of three of differentthree different regions regions in each in sample. each sample. The ADI The phases ADI phases and the and high-carbon the high‐ austenitecarbon austenite of the heat-treated of the heat‐treated samples samples were analyzed were analyzed by X-ray by X diffraction‐ray diffraction measurements measure‐ usingments an using X-ray an Bruker X‐ray Bruker D8 Focus D8 (Bruker, Focus (Bruker, Billerica, Billerica, MA, USA) MA, with USA) monochromatic with monochromatic Cu Kα1 radiation working in θ/2θ conﬁguration. Data were collected in an angular range from 35 Cu Kα◦1 radiation working in θ◦/2θ configuration. Data were◦ collected in an angular range tofrom 100 35with to 100° a step with size a step of 0.02 sizeand of 0.02° a counting and a timecounting of 2 time/min. of The 2°/min. method The reported method re by‐ γ Millerported [ 29by] Miller was utilized [29] was to determine utilized to the determine volume fraction the volume of high-carbon fraction of austenite high‐carbon (%V ausHC‐) based on Equation (4) and integrated intensities of the peak of ferrite and austenite for tenite (%VγHC) based on Equation (4) and integrated intensities of the peak of ferrite and each sample. austenite for each sample. 1.4Iγ %γ = (4) HC +. %γIα 1.4Iγ (4) . where Iγ and Iα are the intensities of the (hkl) reﬂections in the α and γ phases, as determinedwhere Iγ and with Iα are Equations the intensities (5) and of (6), the respectively. (hkl) reflections in the α and γ phases, as deter‐ mined with Equations (5) and (6), respectively. + Iγ220Iγ311 Iγ =I (5)(5) 2

Iα = Iα211 (6) I I (6) It should be noted that Equation (4) gives an insight into the high-carbon austenite It should be noted that Equation (4) gives an insight into the high‐carbon austenite quantiﬁcation and it has been used successfully [30]; however, high-accuracy methods quantification and it has been used successfully [30]; however, high‐accuracy methods must be applied for the phase quantiﬁcation that involves the texture effects on the peak must be applied for the phase quantification that involves the texture effects on the peak intensities due to the thermal stability of the ausferrite, which depends on the chemical intensities due to the thermal stability of the ausferrite, which depends on the chemical composition and the heat treatment [31,32]. composition and the heat treatment [31,32].

2.4. Mechanical Properties Samples were obtained from the camshafts of both alloys for the tensile strength, hardness, and impact properties by the Charpy test. Keel‐block castings based on the standard specification ASTM A 536 were also used to obtain samples for mechanical tests to ensure process quality. Figure 4a shows the camshaft produced containing 16 lobes. Hardness measurements were taken from the cross‐section of the lobes while the shaft of

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2.4. Mechanical Properties Samples were obtained from the camshafts of both alloys for the tensile strength, hard- Metals 2021, 11, x FOR PEER REVIEW 6 of 21 ness, and impact properties by the Charpy test. Keel-block castings based on the standard specification ASTM A 536 were also used to obtain samples for mechanical tests to ensure process quality. Figure4a shows the camshaft produced containing 16 lobes. Hardness measurementsthe camshaft was were used taken to fromobtain the samples cross-section for Charpy of the and lobes tensile while tests. the shaft Figure of the4b shows camshaft the wasas‐cast used keel to obtainblock used samples to obtain for Charpy samples and mainly tensile tests.for the Figure tensile4b showstest. The the samples as-cast keelwere blockaustempered used to obtainaccording samples to the mainly procedure for the reported tensile in test. Figure The 3, samples and the were soaking austempered time was accordingchosen according to the procedure to the highest reported high in Figure‐carbon3, austenite and the soaking value timeobtained was chosenby XRD according measure‐ toments. the highest The mechanical high-carbon results austenite show value the obtained average by and XRD standard measurements. deviation The for mechanical each ADI resultsproduced. show the average and standard deviation for each ADI produced.

(a) (b)

FigureFigure 4. 4.Samples Samples obtained obtained for for mechanical mechanical tests tests from from the the ( a(a)) camshaft, camshaft, ( b(b)) keel-block keel‐block casting. casting.

2.4.1.2.4.1. Hardness Hardness RockwellRockwell C C hardness hardness measurements measurements were were made made on on the the polished polished surfaces surfaces of the of as-cast the as‐ cross-sectioncast cross‐section of 4 camshaft of 4 camshaft lobes lobes by a Wilson by a Wilson 3T TBRB 3T TBRB hardness hardness tester (Buehler,tester (Buehler, Lake Bluff, Lake IL,Bluff, USA). IL, TheUSA). hardness The hardness test was test carried was outcarried at room out at temperature room temperature and an applied and an loadapplied of 150load kg of under 150 kg the under standard the standard specification specification ASTM E ASTM 18. The E average 18. The ofaverage the measurements of the measure is‐ reportedments is forreported each ADI for each heat-treated ADI heat to‐treated the four to austempering the four austempering times evaluated. times evaluated. 2.4.2. Tensile Test 2.4.2. Tensile Test Tensile testing was carried out at room temperature using a universal testing machine Tensile testing was carried out at room temperature using a universal testing (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan) Shimadzu of 100 kN with 10 mm/min machine (Shimadzu Corporation, Nakagyo‐ku, Kyoto, Japan) Shimadzu of 100 kN with cross-head speed. The size and geometry of the specimens were following the specifications 10 mm/min cross‐head speed. The size and geometry of the specimens were following the of ASTM E 8. Four specimens from each ADI were tested for tensile test, and the average specifications of ASTM E 8. Four specimens from each ADI were tested for tensile test, of the measurements is reported for the ADIs heat-treated to 90 min. and the average of the measurements is reported for the ADIs heat‐treated to 90 min. 2.4.3. Charpy Impact Test 2.4.3. Charpy Impact Test A Charpy unnotched bars impact test was machined based on the specifications of ASTMA A Charpy 327. The unnotched maximum bars energy impact of test the machinewas machined was 220 based J, and on thethe impactspecifications velocity of wasASTM 4.5 A m/s. 327. Four The specimensmaximum fromenergy each of the cast machine alloy were was taken 220 J, from and the impact camshaft velocity and were was tested4.5 m/s. for Four impact specimens test in a Tiniusfrom each Olsen cast Charpy alloy impact were taken testing from machine the camshaft tensile test and (Tinius were Olsentested TMC, for impact Hursham, test in PA, a Tinius USA). Olsen The averageCharpy impact of the measurementstesting machine is tensile reported test for (Tinius the ADIsOlsen heat-treated TMC, Hursham, to 90 min. PA, USA). The average of the measurements is reported for the ADIs heat‐treated to 90 min. 3. Results and Discussion 3. ResultsThe ductile and Discussion irons unalloyed and alloyed with 0.2 and 0.3 wt % V manufactured and usedThe as raw ductile material irons for unalloyed the ADI and production alloyed with were 0.2 wholly and 0.3 characterized wt % V manufactured in the as-cast and conditionused as raw and material previously for reported the ADI by production Colin [26]. Thewere main wholly results characterized as chemical in composition, the as‐cast microstructuralcondition and features,previously and mechanicalreported by results Colin are [26]. presented. The main results as chemical composition, microstructural features, and mechanical results are presented.

3.1. Ductile Iron Characterization The chemical composition of the standard unalloyed ductile iron and the ductile iron alloyed with 0.2 and 0.3 wt % V are shown in Table 1.

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3.1. Ductile Iron Characterization The chemical composition of the standard unalloyed ductile iron and the ductile iron alloyed with 0.2 and 0.3 wt % V are shown in Table1.

Table 1. Chemical composition of the unalloyed and alloyed camshafts with 0.2 and 0.3 wt % V.

Sample C Si Mn P S Mg V Ni Al Cu Cr Mo CE 1 DI-0V 3.61 2.36 0.83 0.015 0.008 0.046 0.008 0.103 0.013 0.879 0.043 0.03 4.40 DI-0.2V 3.61 2.49 0.96 0.016 0.013 0.045 0.2 0.117 0.016 0.943 0.20 0.098 4.44 DI-0.3V 3.58 2.48 0.94 0.016 0.012 0.041 0.3 0.115 0.016 0.968 0.13 0.092 4.41 1 Balance Fe. Carbon equivalent = %C + 1/3(%Si + %P).

The chemical composition is in the range expected for hyper-eutectic ductile iron and fulfills the standard specifications of ARBOMEX S.A. de C.V. for the camshafts production. The manganese and copper contents are required to obtain a high volume fraction of pearlite. It is expected that these elements influence the austempering heat treatment. Content higher than 0.8 wt % Mn increased the volume fraction of high-carbon austenite in the ausferritic matrix, but it delays the first and second stages of austempering, narrowing the process window [25]. On the other hand, copper increases the volume fraction of high-carbon austenite for contents lower than 1 wt % Cu and can delay the first stage of the austempering process, which also decreases the carbide formation [25]. The vanadium content is the principal difference between both camshafts alloyed, vanadium is a strong carbide promoter [33,34], and it is well known that it does not have an influence on the austempering kinetics of the ausferrite matrix [25]. Table2 shows the graphite nodules features and the phases formed during solidification for the ductile irons manufactured. These results represent the average of the whole lobe for four lobes distributed alongside the camshaft [26].

Table 2. Graphite features and volume fraction of phases formed for camshafts unalloyed and alloyed with 0.2 and 0.3% V.

Characteristics DI-0V DI-0.2V DI-0.3V Nodularity (%) 85.17 ± 2.64 81.33 ± 6.33 82.85 ± 6.41 Nodule count 155 ± 28.67 210 ± 46.34 203 ± 42.58 (particles/mm2) Nodule size (µm) 32.49 ± 3.69 27.32 ± 1.57 29.46 ± 1.15 Porosity, inclusions and 0.27 ± 0.08 0.78 ± 0.08 0.95 ± 0.12 micro-shrinkages (%) Graphite (%) 12.84 ± 0.55 10.60 ± 2.22 13.36 ± 1.49 Ferrite (%) 5.3 ± 0.20 2.99 ± 0.51 1.33 ± 0.35 Pearlite (%) 81.43 ± 0.20 85.14 ± 2.19 83.39 ± 1.56 Carbides (%) 0.156 ± 0.04 0.49 ± 0.12 0.97 ± 0.16

It is observed from Table2 that the graphite features are in the range recommended for ADI production [3]. It seems that the nodularity for the DI alloyed with vanadium is slightly below the recommended values; however, if the top (nose of the lobe) and bottom regions of the lobes are only considered, the nodularity reaches values greater than 85%. Therefore, it is considered that the as-cast DIs low alloyed with vanadium are optimal for the ADI heat treatment. Table3 shows the mechanical properties evaluated for the ductile irons manufactured in the as-cast condition. Metals 2021, 11, 1036 8 of 23

Table 3. Mechanical properties for camshafts unalloyed and alloyed with 0.2 and 0.3 wt % V.

Mechanical Properties DI-0V DI-0.2V DI-0.3V Hardness (HRC) 28 ± 2.31 37.05 ± 2.09 36.54 ± 1.99 Yield Strength (MPa) 528 ± 29 559 ± 21 588 ± 25 Tensile Strength (MPa) 735 ± 32 775 ± 28 782 ± 26 Elongation (%) 5.42 ± 0.63 4.5 ± 0.6 3.6 ± 0.4 Impact Energy (J) 9.3 ± 1.4 14.8 ± 1.2 11.0 ± 1.4

It was found that vanadium addition increased the yield and tensile strength; however, the elongation and toughness were decreased due to an increase in the carbide particles formation in the as-cast condition [26].

3.2. Microstructural Characterization of ADIs Based on the austempering heat treatment cycles shown in Figure3, four ADIs were produced for the ductile irons alloyed with 0.2 and 0.3 wt % V, which were heat-treated to 265 and 305 ◦C. The ADIs obtained were designated based on the vanadium content and the austempered temperature as ADI-0.2V-265, ADI-0.3V-265, ADI-0.2V-305, and ADI-0.3V- 305. The microstructural characterization was carried out for the four ADIs produced, and a similar microstructural evolution was observed for the two alloys. Figures5 and6 show the microstructural evolution for the ADIs containing 0.2 wt % V, which were heat-treated at 265 ◦C and 305 ◦C, respectively at different times (ADI-0.2V-265 and ADI-0.2V-305) and the three regions analyzed based on Figure2b. The microstructural evolution of ADIs containing 0.3 wt % V heat-treated at 265 and 305 ◦C at different times are found in AppendixA. The microstructures in Figures5 and6 show a mixture of dark needles constituted by ﬁne acicular ferrite and high-carbon austenite, which was observed as little white blocks. The evolution of the samples shows that at an austempering time of 30 min, the microstructure is composed mainly of martensite with a small amount of ausferrite because it is not possible to oversaturate large regions of austenite with carbon in a short amount of time. When the time was increased to 60 min, the microstructure is mainly constituted by a mixture of ausferrite and martensite. The unstable austenite has a longer time to transform into acicular ferrite, so this phase rejects carbon atoms due to the low solubility of carbon in ferrite, oversaturating large regions of high-carbon austenite, increasing the amount of ausferrite, and decreasing the martensite in the matrix. At the austempering time of 90 min, the microstructure is composed principally by ausferrite. In this case, the matrix had enough time to form acicular ferrite, and therefore, the microstructure is saturated with the highest amount of high-carbon austenite reaching Reaction (3); during cooling, only remainder martensite is formed. Hence, the austempering times of 30 and 60 min are considered in the ﬁrst stage of the austempering. At the longest austempering time of 120 min, the microstructure consists of ferrite plus precipitated carbides based on the phase transformation of the high-carbon austenite shown in Reaction (2). It was observed from the microstructural evolution that for the austempering time of 90 min, the microstructure is mainly constituted of ausferrite, so this time was considered to carry out the heat treatments to evaluate the mechanical properties. Ch. F. Han [35] reported that an austempering time of 90 min is required to obtain a fully ausferritic matrix during the austempering heat treatments of ductile iron alloyed with vanadium in the range from 0.24 to 0.71% V and for austempering temperatures in the range from 250 to 320 ◦C. Metals 2021, 11, x FOR PEER REVIEW 8 of 21

It was found that vanadium addition increased the yield and tensile strength; how‐ ever, the elongation and toughness were decreased due to an increase in the carbide par‐ ticles formation in the as‐cast condition [26].

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Figure 5. Microstructural evolution for the ADI-0.2V-265 sample at the austempering times of 30, 60, 90, and 120 min.

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Figure 5. Microstructural evolution for the ADI‐0.2V‐265 sample at the austempering times of 30, 60, 90, and 120 min.

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Figure 6. Microstructural evolution for the ADI‐0.2V‐305 sample at the austempering times of 30, 60, 90, and 120 min. Figure 6. Microstructural evolution for the ADI-0.2V-305 sample at the austempering times of 30, 60, 90, and 120 min.

TheThe microstructures phase transformations in Figures were 5 and similar 6 show for both a mixture alloys of and dark austempering needles constituted temper- byatures; fine however,acicular ferrite a microstructural and high‐carbon change austenite, was observed which betweenwas observed ADIs heat-treatedas little white to blocks.low and The high evolution austempering of the temperature.samples shows Figure that 7at shows an austempering the microstructure time of of 30 the min, ADIs the microstructurecontaining 0.2 and is composed 0.3 wt % V mainly for the of austempering martensite with temperatures a small amount of 265 and of 305ausferrite◦C and be 90‐ causemin of it soaking is not possible time. to oversaturate large regions of austenite with carbon in a short amount of time. When the time was increased to 60 min, the microstructure is mainly constituted by a mixture of ausferrite and martensite. The unstable austenite has a longer time to transform into acicular ferrite, so this phase rejects carbon atoms due to the low solubility of carbon in ferrite, oversaturating large regions of high‐carbon austenite, in‐ creasing the amount of ausferrite, and decreasing the martensite in the matrix. At the aus‐ tempering time of 90 min, the microstructure is composed principally by ausferrite. In this case, the matrix had enough time to form acicular ferrite, and therefore, the microstructure

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is saturated with the highest amount of high‐carbon austenite reaching Reaction (3); dur‐ ing cooling, only remainder martensite is formed. Hence, the austempering times of 30 and 60 min are considered in the first stage of the austempering. At the longest austem‐ pering time of 120 min, the microstructure consists of ferrite plus precipitated carbides based on the phase transformation of the high‐carbon austenite shown in Reaction (2). It was observed from the microstructural evolution that for the austempering time of 90 min, the microstructure is mainly constituted of ausferrite, so this time was considered to carry out the heat treatments to evaluate the mechanical properties. Ch. F. Han [35] reported that an austempering time of 90 min is required to obtain a fully ausferritic matrix during the austempering heat treatments of ductile iron alloyed with vanadium in the range from 0.24 to 0.71% V and for austempering temperatures in the range from 250 to 320 °C. The phase transformations were similar for both alloys and austempering tempera‐ tures; however, a microstructural change was observed between ADIs heat‐treated to low and high austempering temperature. Figure 7 shows the microstructure of the ADIs con‐ Metals 2021, 11, 1036 taining 0.2 and 0.3 wt % V for the austempering temperatures of 265 and11 of 305 23 °C and 90 min of soaking time.

ADI‐0.2V

ADI‐0.3V

(a) (b)

Figure 7. 7.Microstructure Microstructure of ADIof ADI alloyed alloyed with with 0.2 and 0.2 0.3 and wt 0.3 % V wt heat-treated % V heat‐ totreated (a) 265 to◦C (a and) 265 °C and (b) (305b) 305 °C◦ forC for 90 90 min. min.

Figure7a shows a microstructure of ﬁne ausferrite for the ADIs low alloyed with Figure 7a shows a microstructure of fine ausferrite for the ADIs low alloyed with vanadium heat-treated to 265 ◦C; it is observed mainly ﬁne acicular ferrite and a few blocksvanadium of high-carbon heat‐treated austenite. to 265 When °C; theit is austempering observed mainly temperature fine wasacicular increased ferrite to and a few 305blocks◦C, theof high microstructure‐carbon austenite. changed to When coarse the ausferrite austempering with a higher temperature volume fraction was increased to of305 high-carbon °C, the microstructure austenite. During changed the austempering to coarse heatausferrite treatment, with the a acicular higher ferritevolume is fraction of formed due to its nucleation and growth from unstable austenite in the solid state. This high‐carbon austenite. During the austempering heat treatment, the ◦acicular ferrite is phenomenonformed due is to aided its nucleation by the cooling and rate; growth at lower from austempering unstable temperature austenite (265in theC), solid the state. This cooling rate is highly promoting the nucleation of a high amount of acicular ferrite into anphenomenon ausferritic matrix. is aided When by the the austempering cooling rate; temperature at lower is austempering higher (305 ◦C), temperature the cooling (265 °C), ratethe iscooling decreased, rate forming is highly an ausferritic promoting matrix the constituted nucleation by high-carbonof a high amount austenite of with acicular a ferrite lowerinto an amount ausferritic of ferrite matrix. [36]. ThisWhen behavior the austempering was reported bytemperature Dojcinovic [is37 higher], where (305 ADIs °C), the cool‐ wereing rate heat-treated is decreased, to 300 and forming 400 ◦C; an they ausferritic found that whenmatrix the constituted temperature isby increased, high‐carbon the austenite ausferritewith a lower is coarser, amount and of the ferrite morphology [36]. This of ferrite behavior changes was from reported needle-like by Dojcinovic (acicular) to [37], where plate-like (feathery). In addition, the volume fraction of high-carbon austenite is increased.

3.3. Volume Fraction of High-Carbon Austenite The X-ray diffraction patterns of the samples austempered at 265 and 305 ◦C for different soaking times and both alloys are shown in Figure8. It is observed that the planes (111), (200), (220), and (311) correspond to high-carbon austenite, while the planes (110), (200), (211), and (310) refer to the acicular ferrite. Planes corresponding to the vanadium carbide were not observed because of their very low concentration in both alloys. The volume fraction of high-carbon austenite (%VγHC) was determined by using Equations (4)–(6) [29] and the XRD pattern results. Metals 2021, 11, x FOR PEER REVIEW 11 of 21

ADIs were heat‐treated to 300 and 400 °C; they found that when the temperature is in‐ creased, the ausferrite is coarser, and the morphology of ferrite changes from needle‐like (acicular) to plate‐like (feathery). In addition, the volume fraction of high‐carbon austenite is increased.

3.3. Volume Fraction of High‐Carbon Austenite The X‐ray diffraction patterns of the samples austempered at 265 and 305 °C for dif‐ ferent soaking times and both alloys are shown in Figure 8. It is observed that the planes (111), (200), (220), and (311) correspond to high‐carbon austenite, while the planes (110), (200), (211), and (310) refer to the acicular ferrite. Planes corresponding to the vanadium Metals 2021, 11, 1036 carbide were not observed because of their very low concentration in both alloys.12 of 23 The volume fraction of high‐carbon austenite (%VγHC) was determined by using Equations (4)–(6) [29] and the XRD pattern results.

(a) (b)

FigureFigure 8. X‐ray 8. X-ray diffraction diffraction patterns patterns corresponding corresponding to thethe samples samples (a )(a ADI-0.2V-265,) ADI‐0.2V‐265, (b) ADI-0.2V-305, (b) ADI‐0.2V (c‐)305, ADI-0.3V-265, (c) ADI‐0.3V and‐265, and (d()d ADI) ADI-0.3V-305.‐0.3V‐305.

TableTable 4 4shows shows the the results of of the the XRD XRD analysis analysis and and the applicationthe application of Equations of Equations (4)–(6) (4)– (6) toto determine the the inﬂuence influence of the of austemperingthe austempering time on time the high-carbonon the high austenite‐carbon formation.austenite for‐ mation. Table 4. The volume fraction of high-carbon austenite to 265 and 305 ◦C for different austempering times.

%VγHC Sample Austempering Time (min) 30 60 90 120 ADI-0.2 V-265 6.83 8.26 9.53 8.96 ADI-0.3 V-265 5.83 7.63 8.83 8.36 ADI-0.2V-305 7.73 9.76 10.93 9.7 ADI-0.3V-305 8.13 10.6 12.93 11.33 Metals 2021, 11, x FOR PEER REVIEW 12 of 21

Table 4. The volume fraction of high‐carbon austenite to 265 and 305 °C for different austempering times.

%VγHC Sample Austempering Time (min) 30 60 90 120 ADI‐0.2 V‐265 6.83 8.26 9.53 8.96 ADI‐0.3 V‐265 5.83 7.63 8.83 8.36 ADI‐0.2V‐305 7.73 9.76 10.93 9.7 ADI‐0.3V‐305 8.13 10.6 12.93 11.33

Figure 9 shows the %VγHC behavior as the austempering time increased. The %VγHC Metals 2021, 11, 1036 increases first, reaching a maximum value and then decreasing.13 of 23 The maximum value of %VγHC in all cases was obtained for the austempering time of 90 min. The maximum ausferrite amount referred to as a process window (PW) and represented in Equations Figure9 shows the %V γHC behavior as the austempering time increased. The %VγHC increases(1)–(3) ﬁrst, occurs reaching as a follows: maximum value The and first then stage decreasing. of the The maximumPW is presented value of to the austempering times %VγHC in all cases was obtained for the austempering time of 90 min. The maximum aus- ferriteof 30 amount and 60 referred min to where as a process Reaction window (PW) (1) and proceeds; represented afterwards, in Equations (1)–(3) the highest amount of ausferrite occursis obtained as follows: at The 90 ﬁrst min, stage ofreaching the PW is presented the PW to the(reaction austempering 3), timesand of then, 30 the second stage of the PW and 60 min where Reaction (1) proceeds; afterwards, the highest amount of ausferrite is obtainedwindow at 90 min,has reaching been thereached PW (reaction at 3),the and austempering then, the second stage time of the of PW 120 win- min, proceeding Reaction (2) dow[38]. has It been is observed reached at the from austempering Table time 4 that of 120 the min, highest proceeding valuesReaction (2) of[38 %V]. γHC are reached for the high‐ It is observed from Table4 that the highest values of %V γHC are reached for the highest austemperingest austempering temperature; this temperature; behavior is related this to the carbidesbehavior amount is in therelated ausferritic to the carbides amount in the matrixausferritic [39]. matrix [39].

FigureFigure 9. Effect 9. Effect of the austempering of the austempering time on the high-carbon time austenite on the formation. high‐carbon austenite formation. 3.4. Carbides The austenitizing parameters (900 ◦C, 180 min) were chosen to decrease the number of carbides3.4. Carbides in the ductile iron matrix reported in Table2. In general, during the austenitizing, higher temperatures,The austenitizing and large residence parameters times promote (900 that °C, the 180 phases min) and microcon-were chosen to decrease the number stituents presented in the as-cast microstructure can be partially or dissolved in the matrix toof form carbides austenite [40in]. the ductile iron matrix reported in Table 2. In general, during the austen‐ Figure 10 shows the etched microstructure with ammonium persulfate of the ADIs alloyeditizing, with higher 0.2 and 0.3temperatures, wt % V, heat-treated and to 265large and residence 305 ◦C for 90 min,times and promote the that the phases and mi‐ threecroconstituents regions analyzed in presented the lobes according in the to Figureas‐cast2b. The microstructure etching with ammonium can be partially or dissolved in the persulfatematrix darkens to form the ausferriticaustenite matrix [40]. and reveals the carbides as white regions. Figure 10 shows the etched microstructure with ammonium persulfate of the ADIs alloyed with 0.2 and 0.3 wt % V, heat‐treated to 265 and 305 °C for 90 min, and the three regions analyzed in the lobes according to Figure 2b. The etching with ammonium per‐ sulfate darkens the ausferritic matrix and reveals the carbides as white regions.

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Figure 10. Microstructures etched with ammonium persulfate for the ADIs alloyed with 0.2 and 0.3% V heat‐treated at 265 Figure 10. Microstructures etched with ammonium persulfate for the ADIs alloyed with 0.2 and 0.3% V heat-treated at 265 and 305 °C for 90 min and the three regions analyzed of the lobe. and 305 ◦C for 90 min and the three regions analyzed of the lobe.

ItIt was was reported reported [26] [26] in thethe as-castas‐cast conditioncondition for for these these alloys alloys used used in in the the manufacture manufacture of ofcamshafts camshafts that that the the highest highest carbide carbide formation formation is locatedis located in in the the middle middle of of the the lobes lobes instead instead of ofthe the external external parts parts of the of the lobes lobes due due to the to inverse the inverse chill, wherechill, where there is there a segregation is a segregation of carbide- of carbideforming‐forming elements elements to the middle to the middle zone of zone the camshaft, of the camshaft, increasing increasing the concentration the concentration of these ofelements these elements in the last in liquidthe last to liquid solidify, to promotingsolidify, promoting eutectic iron eutectic carbide iron formation carbide formation [41]. After [41].the ADI After heat the treatment ADI heat to treatment 90 min, it wasto 90 observed min, it was that observed the highest that carbides the highest concentration carbides is concentrationprincipally located is principally in the bottom located region in the of bottom the lobes. region Smaller of the carbide lobes. particlesSmaller carbide are found par in‐ ticlesthe top are and found middle in the region top and of the middle lobes homogeneouslyregion of the lobes distributed homogeneously in the analyzed distributed region. in the analyzedTable5 shows region. the volume fraction of vanadium carbides of ADIs heat-treated to 265 andTable 305 ◦ C5 shows for 90 minthe volume and the fraction three regions of vanadium analyzed. carbides In addition, of ADIs Tableheat‐treated5 also shows to 265 andthe carbides305 °C for formed 90 min in and the the camshaft three regions for the analyzed. as-cast condition. In addition, It should Table be5 also noted shows that the the carbidesas-cast results formed were in the obtained camshaft from for the the whole as‐cast lobe condition. [26], while It should the results be noted of the that ADIs the only as‐ castrepresent results the were external obtained part from of the the lobe, whole from lobe the [26], nose while of the the lobe results to the of upper the ADIs part of only the represent the external part of the lobe, from the nose of the lobe to the upper part of the

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camshaft (base circle), as can be observed in Figure2a. Therefore, the volume fraction of carbides presented in the as-cast condition is signiﬁcantly larger than that obtained after the austempering heat treatment; however, during austenitizing, a high amount of carbides was dissolved or partially dissolved [40–42].

Table 5. The volume fraction of carbides for the ADIs heat-treated to 265 and 305 ◦C to 90 min and the three regions analyzed.

Carbides (%) Sample As-Cast Top Middle Bottom ADI-0.2V-265 0.49 0.1 0.18 0.2 ADI-0.3V-265 0.97 <0.1 <0.1 0.15 ADI-0.2V-305 0.49 <0.1 0.20 0.4 ADI-0.3V-305 0.97 <0.1 0.14 0.2

Owing to the low volume fraction of carbides in the ADIs, their presence was not detected by the XRD technique; however, Rezvani [41] reported that fine particles of V4C3 are formed when vanadium is added to ductile iron, and these particles are uniformly distributed in the ferrite zone in the as-cast condition. Therefore, the carbide particles identified correspond to vanadium carbide particles. A low volume fraction of carbides was observed from the external parts of the lobe (top and middle regions) for the ADIs produced, while the bottom region showed the highest amounts of carbides. Dymek [43] reported that the vanadium carbides are partially dissolved during the austenitizing process, and they form again with better distribution and little size at an isothermal temperature at 640 ◦C. This behavior is observed from Figure 10 where fine carbide particles are homogeneously distributed in the matrix, especially for the top and middle regions analyzed.

3.5. Mechanical Properties Samples of the ductile irons alloyed with 0.2 and 0.3 wt % V were obtained from the camshafts and the keel-block casting for the mechanical tests. The samples were heat- treated based on the speciﬁcations of Figure3 for the austempering time of 90 min where the highest amount of ausferrite was obtained. The effect of the austempering temperature and time on the Rockwell C hardness for ADIs low alloyed with vanadium are shown in Table6.

Table 6. Hardness Rockwell C as a function of the austempering parameters.

Hardness (HRC) Sample Austempering Time (min) As-Cast 30 60 90 120 ADI-0.2V-265 37.05 ± 2.09 56 ± 0.5 48 ± 0.5 44 ± 0.7 45 ± 0.7 ADI-0.3V-265 36.54 ± 1.99 55 ± 0.9 48 ± 1.2 47 ± 0.8 48 ± 0.5 ADI-0.2V-305 37.05 ± 2.09 50 ± 0.5 44 ± 0.5 43 ± 0.5 44 ± 0.6 ADI-0.3V-305 36.54 ± 1.99 52 ± 0.5 50 ± 0.9 44 ± 1.2 47 ± 0.5

It is evident that hardness obtained by austempering heat treatment is higher than in the as-cast condition because hardness obtained in as-cast depends directly on phases and microconstituents such as pearlite, ferrite, and carbides formed during the camshaft solidi- ﬁcation process; in this case, the camshaft microstructure is constituted mainly of pearlite with lower amounts of ferrite and carbides (Table2)[ 38,44]. However, the austempering heat treatment applied to the ductile iron promotes phase transformations forming harder phases as ausferrite and martensite with a low volume fraction of vanadium carbides homogeneously distributed in the camshaft microstructure. Metals 2021, 11, x FOR PEER REVIEW 15 of 21

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Figure 11 shows the hardness evolution as the austempering time was increased. The hardnessFigure 11 shows behavior the hardness is directly evolution as related the austempering to the time microstructural was increased. The evolution shown in Figures 4 hardness behavior is directly related to the microstructural evolution shown in Figures4 andand5 and 5 alsoand with also the %V withγHC asthe was %V previouslyγHC as reported was [ 38previously,45]. reported [38,45].

FigureFigure 11. Austempering 11. Austempering time inﬂuence ontime the Rockwell influence C hardness. on the Rockwell C hardness. Figure 11 shows that hardness is sharply increased from the as-cast condition to the ﬁrst austempering temperature; then, hardness values decrease as the austempering time is increased,Figure reaching 11 the shows lowest valuethat for hardness the austempering is sharply time of 90 increased min, followed from the as‐cast condition to the byfirst a slight austempering hardness increase totemperature; the highest austempering then, time.hardness The lowest values Rockwell decrease C as the austempering time hardness values are found at 90 min, where the highest volume fraction of high-carbon austeniteis increased, (Table4) was reaching obtained for the all thelowest ADIs evaluated. value for Therefore, the austempering the hardness time of 90 min, followed by behaviora slight is directly hardness related to increase the %VγHC andto matchthe highest in the process austempering window determination. time. The lowest Rockwell C hard‐ Following the sample ADI-0.2V-265, at the austempering time of 30 min, the microstructure isness constituted values mainly are of martensite found with at a90 low min, amount where of ausferrite, the these highest allow obtaining volume fraction of high‐carbon aus‐ the highest hardness value of 56 HRC with the lowest value of %VγHC = 6.83. When thetenite austempering (Table time 4) waswas increased obtained to 60 min, for theall ausferrite the ADIs amount evaluated. was increased Therefore, the hardness behavior inis the directly microstructure; related thus, to the the value %V of %VγγHCHC andincreased match to 8.26, in and the the process martensite window determination. Follow‐ decreased. Consequently, the hardness decreased to 48 HRC. The microstructure is mainly constituteding the by sample ausferrite with ADI a small‐0.2V amount‐265, of martensite at the foraustempering 90 min of austempering; time this of 30 min, the microstructure is microstructureconstituted promotes mainly the highest of martensite value of %VγHC =with 9.53 and a thelow lowest amount hardness valueof ausferrite, these allow obtaining (44 HRC). For the longest austempering time of 120 min, the second stage of the process windowsthe highest occurs, which hardness means that value the microstructure of 56 HRC is composed with of ferritethe andlowest carbides; value of %VγHC = 6.83. When the thus,austempering the hardness increased time to 45 was HRC andincreased the %VγHC decreasedto 60 min, to 8.96. the Similar ausferrite behavior amount was increased in the is carried out for the ADI alloyed with 0.3% V, and when the austempering temperature wasmicrostructure; increased. Han [35] reported thus, hardness the value values of 51 %V andγ 48HC HRC increased for ADIs alloyed to with8.26, and the martensite decreased. 0.2% V austempered to 280 and 320 ◦C, respectively, and these are in good agreement with theConsequently, results obtained in Table the4. hardness decreased to 48 HRC. The microstructure is mainly consti‐ tutedStress–strain by ausferrite curves were obtainedwith a for small the ADIs amount alloyed with of 0.2martensite and 0.3 wt % Vfor 90 min of austempering; this for the austempering temperatures of 265 and 305 ◦C. The results of yield and tensile strengthmicrostructure are shown in Figure promotes 12 for the ADIsthe produced.highest It value is observed of that%V theγHC yield = and9.53 and the lowest hardness value tensile(44 HRC). strength are For remarkably the longest increased from austempering the as-cast to the austempered time of condition120 min, for the second stage of the process both vanadium contents. The ADI-0.3V-265 reached the highest yield and tensile strength windows occurs, which means that the microstructure is composed of ferrite and carbides; thus, the hardness increased to 45 HRC and the %VγHC decreased to 8.96. Similar behavior is carried out for the ADI alloyed with 0.3% V, and when the austempering temperature was increased. Han [35] reported hardness values of 51 and 48 HRC for ADIs alloyed with 0.2% V austempered to 280 and 320 °C, respectively, and these are in good agreement with the results obtained in Table 4. Stress–strain curves were obtained for the ADIs alloyed with 0.2 and 0.3 wt % V for the austempering temperatures of 265 and 305 °C. The results of yield and tensile strength are shown in Figure 12 for the ADIs produced. It is observed that the yield and tensile strength are remarkably increased from the as‐cast to the austempered condition for both vanadium contents. The ADI‐0.3V‐265 reached the highest yield and tensile strength val‐ ues of 1051 and 1200 MPa, respectively, whereas the ADI‐0.3V‐305 showed 999 and 1176 MPa for the yield and tensile strength, respectively. The ADI alloyed with 0.2 wt % V showed the highest yield and tensile strength of 1032 and 1107 MPa for the austempered temperature of 265 °C, while 781 and 989 MPa were obtained when the austempering temperature was increased to 305 °C, respectively. Padan [24] evaluated an ADI contain‐ ing 0.1 wt % V austempered to 335 °C for 1.5 h; the yield and tensile strength values were 978 and 1088 MPa, which match with the trend shown in Figure 11. In addition, Han [35]

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values of 1051 and 1200 MPa, respectively, whereas the ADI-0.3V-305 showed 999 and 1176 MPa for the yield and tensile strength, respectively. The ADI alloyed with 0.2 wt % V showedreported the highest a constant yield and tensile value strength of 1050 of 1032 MPa and 1107 of MPa tensile for the austemperedstrength for ADIs alloyed with 0.2 wt % temperature of 265 ◦C, while 781 and 989 MPa were obtained when the austempering temperatureV austempered was increased to to 305250◦C, and respectively. 320 Padan°C. A [24 ]slight evaluated increase an ADI containing in the tensile strength of 1060 MPa 0.1 wt % V austempered to 335 ◦C for 1.5 h; the yield and tensile strength values were 978was and reported 1088 MPa, which for match an ADI with thealloyed trend shown with in Figure 0.4% 11 V. In austempered addition, Han [35] to 250 °C. In general, the tensile reportedstrength a constant decreased value of 1050 when MPa of the tensile vanadium strength for ADIs content alloyed with was 0.2 wt higher % V than 0.4% V and with the in‐ austempered to 250 and 320 ◦C. A slight increase in the tensile strength of 1060 MPa was reportedcrease for of an the ADI austempering alloyed with 0.4% V austemperedtemperature. to 250 ◦ C.The In general, results the in tensile Figure 12 show that by increasing strengththe vanadium decreased when content the vanadium from content 0.2 to was 0.3 higher wt than% V, 0.4% the V andyield with and the tensile strength are the highest increase of the austempering temperature. The results in Figure 12 show that by increasing theat vanadiumboth austempering content from 0.2 to 0.3 temperatures. wt % V, the yield and The tensile ADI strength containing are the highest 0.3 wt % V showed that increas‐ ating both the austempering austempering temperatures. temperature, The ADI containing the 0.3 wt tensile % V showed properties that increasing are slightly decreased because of the austempering temperature, the tensile properties are slightly decreased because of a coarsera coarser ausferritic ausferritic matrix and a highermatrix amount and of thea higher high-carbon amount austenite content.of the high‐carbon austenite content.

FigureFigure 12. Inﬂuence 12. Influence of vanadium content of vanadium and austempering content temperature and in yield austempering and tensile strength. temperature in yield and tensile strengthFigure 13. shows the impact energy and elongation results for the ADIs low alloyed with vanadium. It is observed that the impact energy increases linearly for the ADI alloyed with 0.3 wt % V from the as-cast condition to the higher austempering heat temperature while theFigure ADIs containing 13 shows 0.2% Vthe increase impact considerably energy from and the as-cast elongation condition to results for the ADIs low alloyed the ADI heat-treated to 265 ◦C; then, a slight increase occurs when the austempering temperaturewith vanadium. was increased It to is 305 observed◦C. The elongation that showsthe impact an opposite energy behavior increases to linearly for the ADI al‐ impactloyed energy. with The 0.3 ADIs wt containing% V from 0.2 wtthe % as V shows‐cast an condition elongation decrease to the from higher austempering heat temper‐ the as-cast condition (4.5%) to the ADIs heat-treated to both austempering temperatures (3ature and 3.5% while for the the austempered ADIs containing temperature of 2650.2% and V 305 increase◦C, respectively), considerably while from the as‐cast condition the ADIs containing 0.3 wt % V keep a similar elongation value with 3.7% on average withto the a slight ADI increase heat for‐treated the austempering to 265 temperature °C; then, of 305 a ◦C.slight Both behaviors increase are occurs when the austempering relatedtemperature to the volume was fraction increased of carbides in to the microstructure.305 °C. The The elongation samples in the as-cast shows an opposite behavior to im‐ conditionpact energy. contain a higher The volume ADIs fraction containing of vanadium 0.2 carbides, wt % which V affectsshows the matrixan elongation decrease from the as‐ cast condition (4.5%) to the ADIs heat‐treated to both austempering temperatures (3 and 3.5% for the austempered temperature of 265 and 305 °C, respectively), while the ADIs containing 0.3 wt % V keep a similar elongation value with 3.7% on average with a slight increase for the austempering temperature of 305 °C. Both behaviors are related to the volume fraction of carbides in the microstructure. The samples in the as‐cast condition contain a higher volume fraction of vanadium carbides, which affects the matrix continu‐ ity acting as crack initiation sites [41]. Thereby, the ADIs evaluated presented higher im‐ pact energy values than ductile iron alloys because the ADIs showed a low content of fine vanadium carbide particles homogeneously distributed in the matrix, which allows in‐ creasing the impact energy, keeping almost constant or even showing a slight ductility increase [46]. A microstructure constituted by a mixture of coarse ausferrite and a high‐ carbon austenite content was obtained in the austempering heat treatment carried out at 305 °C; this microstructure allows obtaining the higher impact energy values for both ADIs; this behavior has been previously reported [36,47]. The ADIs alloyed with 0.2 and 0.3 wt % V heat‐treated to 305 °C showed the highest impact energy values of 31 and 40 J respectively, while the ADIs heat‐treated to 265 °C show impact energy values of 29 and 26 J for the ADIs alloyed with 0.2 and 0.3 wt % V, respectively. The results for impact energy are almost equal to those reported by Han [35] for the same vanadium contents and austempering temperatures of 250 and 320 °C.

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continuity acting as crack initiation sites [41]. Thereby, the ADIs evaluated presented higher impact energy values than ductile iron alloys because the ADIs showed a low content of ﬁne vanadium carbide particles homogeneously distributed in the matrix, which allows increasing the impact energy, keeping almost constant or even showing a slight ductility increase [46]. A microstructure constituted by a mixture of coarse ausferrite and a high-carbon austenite content was obtained in the austempering heat treatment carried out at 305 ◦C; this microstructure allows obtaining the higher impact energy values for both ADIs; this behavior has been previously reported [36,47]. The ADIs alloyed with 0.2 and 0.3 wt % V heat-treated to 305 ◦C showed the highest impact energy values of 31 and 40 J respectively, while the ADIs heat-treated to 265 ◦C show impact energy values of 29 Metals 2021, 11, x FOR PEER REVIEWand 26 J for the ADIs alloyed with 0.2 and 0.3 wt % V, respectively. The results for impact 17 of 21 energy are almost equal to those reported by Han [35] for the same vanadium contents and austempering temperatures of 250 and 320 ◦C.

FigureFigure 13. Inﬂuence13. Influence of vanadium of content vanadium and austempering content temperature and austempering on the impact energy temperature on the impact energy and and elongation. elongation. Two austempering temperatures were evaluated at different austempering times to determine the highest ausferrite content that enables obtaining the highest mechanical propertiesTwo and an austempering adequate balance between temperatures strength and toughness were for evaluated the ADI camshaft at different austempering times to production. The highest mechanical properties of tensile and yield strength of 1200 and 1051determine MPa, respectively the were highest obtained ausferrite for the austempering content heat treatment that ofenables 265 ◦C for theobtaining the highest mechanical ADIproperties containing 0.3 and wt % V.an However, adequate when the balance ADI containing between 0.3 wt % Vstrength was austempered and toughness for the ADI camshaft to 305 ◦C, the highest impact energy of 40 J was obtained with a slight decrease in the tensileproduction. and yield strength The ofhighest 1000 and 1176 mechanical MPa, respectively. properties Both ADIs showed of tensile a high and yield strength of 1200 and hardness1051 MPa, value of respectively 47 HRC; these mechanical were results obtained are in the for range the expected austempering for an ADI heat treatment of 265 °C for grade 3 based on the standard ASTM A 897, and they are an attractive option for obtaining optimumthe ADI performance containing from the 0.3 camshaft. wt % V. However, when the ADI containing 0.3 wt % V was aus‐ tempered to 305 °C, the highest impact energy of 40 J was obtained with a slight decrease in the tensile and yield strength of 1000 and 1176 MPa, respectively. Both ADIs showed a high hardness value of 47 HRC; these mechanical results are in the range expected for an ADI grade 3 based on the standard ASTM A 897, and they are an attractive option for obtaining optimum performance from the camshaft.

4. Conclusions Camshafts alloyed with 0.2 and 0.3 wt % V were austempered to 265 and 305 °C for the austempering times of 30, 60, 90, and 120 min. The microstructural evolution and the mechanical properties of the ADIs were determined. The following conclusions were ob‐ tained. 1. A microstructure constituted by fine acicular ferrite and a few blocks of high‐carbon austenite (fine ausferrite) was obtained for both ADIs austempered to 265 °C, while coarse ausferrite was obtained when the austempering temperature was increased to 305 °C. 2. The process window was obtained for the austempering time of 90 min for both ADIs alloyed with 0.2 and 0.3 wt % V, while the highest %VγHC was obtained for the high‐ est austempering temperature. 3. The austempering heat treatment parameters allow obtaining a low volume fraction of fine vanadium carbides particles homogeneously distributed in the top zone (nose of the lobe) of the camshaft lobe, improving the toughness of the camshaft. 4. The evolution of hardness matched with the high‐carbon austenite results and the microstructural evolution in the process window determination. 5. The ADI alloyed with 0.3 wt % V austempered at 265 °C meets the requirements to be considered as ADI grade 3, obtaining a tensile and yield strength of 1200 and 1051 MPa, respectively, a hardness of 47 HRC (442 HB), and energy impact of 25.9 J; how‐ ever, the elongation (3.6%) is a bit lower than that specified in the standard ASTM A 897. 6. An optimum balance between strength and toughness is found in the ADI alloyed with 0.3 wt % V austempered at 305 °C. The highest energy impact was obtained

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4. Conclusions Camshafts alloyed with 0.2 and 0.3 wt % V were austempered to 265 and 305 ◦C for the austempering times of 30, 60, 90, and 120 min. The microstructural evolution and the mechanical properties of the ADIs were determined. The following conclusions were obtained. 1. A microstructure constituted by fine acicular ferrite and a few blocks of high-carbon austenite (fine ausferrite) was obtained for both ADIs austempered to 265 ◦C, while coarse ausferrite was obtained when the austempering temperature was increased to 305 ◦C. 2. The process window was obtained for the austempering time of 90 min for both ADIs alloyed with 0.2 and 0.3 wt % V, while the highest %VγHC was obtained for the highest austempering temperature. 3. The austempering heat treatment parameters allow obtaining a low volume fraction of fine vanadium carbides particles homogeneously distributed in the top zone (nose of the lobe) of the camshaft lobe, improving the toughness of the camshaft. 4. The evolution of hardness matched with the high-carbon austenite results and the microstructural evolution in the process window determination. 5. The ADI alloyed with 0.3 wt % V austempered at 265 ◦C meets the requirements to be considered as ADI grade 3, obtaining a tensile and yield strength of 1200 and 1051 MPa, respectively, a hardness of 47 HRC (442 HB), and energy impact of 25.9 J; however, the elongation (3.6%) is a bit lower than that specified in the standard ASTM A 897. 6. An optimum balance between strength and toughness is found in the ADI alloyed with 0.3 wt % V austempered at 305 ◦C. The highest energy impact was obtained (40.45 J) with a tensile and yield strength of 1176 and 999 MPa respectively. In addition, the hardness and elongation were set in 42 HRC (390 HB) and 3.8%, respectively.

Author Contributions: Data curation, E.C.G., G.R.C., and A.M.H.; Formal analysis, E.C.G. and A.C.R.; Funding acquisition, J.T.R.; Investigation, E.C.G., A.C.R., G.R.C., J.F.C.A., and A.M.H.; Methodology, J.F.C.A.; Project administration, J.T.R.; Supervision, J.T.R.; Validation, A.C.R., J.T.R., and A.M.H.; Visualization, A.C.R. and A.M.H.; Writing—original draft, E.C.G.; Writing—review and editing, A.C.R. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: No additional data. Acknowledgments: The authors wish to thank the enterprise ARBOMEX S.A. de C.V. for the facilities given for the trial’s development. A. Cruz, F. Chávez, G. Reyes, and E. Colin wish to thank Institutions CONACyT, SNI, COFAA, and SIP-Instituto Politécnico Nacional for their permanent assistance to the Process Metallurgy Group at ESIQIE-Metallurgy and Materials Department. Conﬂicts of Interest: The authors declare no conﬂict of interest. Metals 2021, 11, x FOR PEER REVIEW 18 of 21

(40.45 J) with a tensile and yield strength of 1176 and 999 MPa respectively. In addi‐ tion, the hardness and elongation were set in 42 HRC (390 HB) and 3.8%, respectively.

Author Contributions: Data curation, E.C.G., G.R.C., and A.M.H.; Formal analysis, E.C.G. and A.C.R.; Funding acquisition, J.T.R.; Investigation, E.C.G., A.C.R., G.R.C., J.F.C.A., and A.M.H.; Methodology, J.F.C.A.; Project administration, J.T.R.; Supervision, J.T.R.; Validation, A.C.R., J.T.R., and A.M.H.; Visualization, A.C.R. and A.M.H.; Writing—original draft, E.C.G.; Writing—review and editing, A.C.R. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: No additional data. Acknowledgments: The authors wish to thank the enterprise ARBOMEX S.A. de C.V. for the facil‐ Metals 2021, 11, 1036 ities given for the trial’s development. A. Cruz, F. Chávez, G. Reyes, and E. Colin wish to 20thank of 23 Institutions CONACyT, SNI, COFAA, and SIP‐Instituto Politécnico Nacional for their permanent assistance to the Process Metallurgy Group at ESIQIE‐Metallurgy and Materials Department. Conflicts of Interest: The authors declare no conflict of interest. Appendix A Appendix A

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FigureFigure A1. A1. MicrostructuralMicrostructural evolution evolution for for the the ADI ADI-0.3V-265‐0.3V‐265 sample sample at the austempering times of 30, 60, 90, and 120 min.

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Figure A2. Microstructural evolution for the ADI‐0.3V‐305 sample at the austempering times of 30, 60, 90, and 120 min.

References 1. Blackmore, P.A.; Harding, R.A. The effects of metallurgical process variables on the properties of austempered ductile irons. J. Heat Treat. 1984, 3, 310–325.

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Figure A1. Microstructural evolution for the ADI‐0.3V‐265 sample at the austempering times of 30, 60, 90, and 120 min.

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Figure A2. Microstructural evolution for the ADI‐0.3V‐305 sample at the austempering times of 30, 60, 90, and 120 min. Figure A2. Microstructural evolution for the ADI-0.3V-305 sample at the austempering times of 30, 60, 90, and 120 min.

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