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

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Heat Treatment Evaluation for the Camshafts Production of ADI Low Alloyed with Vanadium metals 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 ". 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 " ) (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.
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