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

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 high-carbon austenite was determined for the heat treatment conditions by XRD measurements. The ausferritic matrix was determined in 90 min for both austempering temperatures, having a good agreement with the microstructural and hardness evolution as the austempering time increased. The mechanical properties of tensile strength, hardness, and toughness were evaluated from samples obtained from the camshaft and the standard Keel block. The highest mechanical properties were obtained for the austempering heat treatment of 265 °C for 90 min for the ADI containing 0.3 wt % V. The tensile and yield strength were 1200 and 1051 MPa, respectively, while the hardness and the energy impact values were of 47 HRC and 26 J; these values are in the range expected for an ADI grade 3.

1. Introduction

Austempered ductile iron or ADI is a family of ductile iron (DI) that has been treated by austempering (isothermal heat treatment) [1] that results in nodules immerses in an ausferritic matrix composed of acicular ferrite (α_Ac) and high-carbon austenite (γ_HC) [2]. 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% maximum of the combined content of carbides, non-metallic inclusions, micro-shrinkage, and porosity, and (e) homogenous chemical composition [3].

The complete austempered heat treatment is a set of processes used to obtain ADIs. The heat treatment starts with the austenitizing step of ductile iron in the range of temperatures of 850–950 °C for 1 h, or longer residence times to ensure transformation from the as-cast matrix to austenite [4]. After austenitizing, the sample is quenched in a salt bath to the austempering temperatures in the range of 250–450 °C with enough holding 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 (α_Ac) and high-carbon austenite (γ_HC).

$$\mathsf{\gamma }\to {\mathsf{\alpha }}_{\mathrm{Ac}}+{\mathsf{\gamma }}_{\mathrm{HC}} \left(\mathrm{ausferrite}\right)$$

(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 (γ_HC) 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].

$${\mathsf{\gamma }}_{\mathrm{HC}}\to \mathsf{\alpha }+ \mathrm{carbide} ({\mathrm{Fe}}_3\mathrm{C} \mathrm{or} \mathsf{\epsilon } )$$

(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].

$$\mathrm{PW}: {\mathsf{\alpha }}_{\mathrm{Ac}}+{ \mathsf{\gamma }}_{\mathrm{HC}} \left(\mathrm{stable} \mathrm{structure}\right)$$

(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]. Kumruoğlu [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 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 [20,21,22]. 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. Figure 1 shows the tooling used for the preparation of the camshaft molds; every camshaft contains 16 lobes.

The nominal chemical composition in the camshafts was analyzed by an OBLF GS 1000 II emission optic spectrograph (OBLF Gesellschaft für Elektronik und Feinwerktechnik mbH, Witten, Germany), and the reported values are the average of three measurements on each cast alloy. Carbon and sulfur content was determined by combustion analysis using a Leco C/S 744 analyzer (LECO Corporation, St. Joseph, MI, USA).

2.2. Austempering Heat Treatment

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 austempering 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), middle, and bottom (base circle) for the microstructural characterization.

Two austempering heat treatments were 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 decarburization) 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.

2.3. Microstructural Characterization

In terms of microstructural examinations, standard metallography was employed using an optical microscope Olympus PMG-3 model according to the 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 the as-cast condition. The austempered ductile iron samples were etched with nital 3% to reveal the phases. Carbides were revealed by etching for 80 s with a water solution of ammonium persulfate (10% vol) [27,28]. The volume fraction of vanadium carbides in ADIs was obtained with the Image J software. The reported results for the optical microscopy analysis were the average of three different regions in each sample. The ADI phases and the high-carbon austenite of the heat-treated samples were analyzed by X-ray diffraction measurements using an X-ray Bruker D8 Focus (Bruker, Billerica, MA, USA) with monochromatic Cu Kα₁ radiation working in θ/2θ configuration. Data were collected in an angular range from 35 to 100° with a step size of 0.02° and a counting time of 2°/min. The method reported by Miller [29] was utilized to determine the volume fraction of high-carbon austenite (%Vγ_HC) based on Equation (4) and integrated intensities of the peak of ferrite and austenite for each sample.

$${\%\mathsf{\gamma }}_{\mathrm{HC}}=\frac{1.4{\mathrm{I}}_{\mathsf{\gamma }}}{{\mathrm{I}}_{\mathsf{\alpha }}+1.4{\mathrm{I}}_{\mathsf{\gamma }}}$$

(4)

where I_γ and I_α are the intensities of the (hkl) reflections in the α and γ phases, as determined with Equations (5) and (6), respectively.

$${\mathrm{I}}_{\mathsf{\gamma }}=\frac{{\mathrm{I}}_{\mathsf{\gamma }220}+{\mathrm{I}}_{\mathsf{\gamma }311}}{2}$$

(5)

$${\mathrm{I}}_{\mathsf{\alpha }}={\mathrm{I}}_{\mathsf{\alpha }211}$$

(6)

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 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 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 the camshaft was used to obtain samples for Charpy and tensile tests. Figure 4b shows the as-cast keel block used to obtain samples mainly for the tensile test. The samples were austempered according to the procedure reported in Figure 3, and the soaking time was chosen according to the highest high-carbon austenite value obtained by XRD measurements. The mechanical results show the average and standard deviation for each ADI produced.

2.4.1. Hardness

Rockwell C hardness measurements were made on the polished surfaces of the as-cast cross-section of 4 camshaft lobes by a Wilson 3T TBRB hardness tester (Buehler, Lake Bluff, IL, USA). The hardness test was carried out at room temperature and an applied load of 150 kg under the standard specification ASTM E 18. The average of the measurements is reported for each ADI heat-treated to the four austempering times evaluated.

2.4.2. Tensile Test

Tensile testing was carried out at room temperature using a universal testing machine (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan) Shimadzu of 100 kN with 10 mm/min cross-head speed. The size and geometry of the specimens were following the specifications of ASTM E 8. Four specimens from each ADI were tested for tensile test, and the average of the measurements is reported for the ADIs heat-treated to 90 min.

2.4.3. Charpy Impact Test

A Charpy unnotched bars impact test was machined based on the specifications of ASTM A 327. The maximum energy of the machine was 220 J, and the impact velocity was 4.5 m/s. Four specimens from each cast alloy were taken from the camshaft and were tested for impact test in a Tinius Olsen Charpy impact testing machine tensile test (Tinius Olsen TMC, Hursham, PA, USA). The average of the measurements is reported for the ADIs heat-treated to 90 min.

3. Results and Discussion

The ductile irons unalloyed and alloyed with 0.2 and 0.3 wt % V manufactured and used as raw material for the ADI production were wholly characterized in the as-cast condition and previously reported by Colin [26]. 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. 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].

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 (particles/mm2)	155 ± 28.67	210 ± 46.34	203 ± 42.58
Nodule size (µm)	32.49 ± 3.69	27.32 ± 1.57	29.46 ± 1.15
Porosity, inclusions and micro-shrinkages (%)	0.27 ± 0.08	0.78 ± 0.08	0.95 ± 0.12
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

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].

It is observed from Table 2 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.

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

shows the mechanical properties evaluated for the ductile irons manufactured in the as-cast condition.

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 Figure 3, 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. Figure 5 and Figure 6 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 Figure 2b. The microstructural evolution of ADIs containing 0.3 wt % V heat-treated at 265 and 305 °C at different times are found in Appendix A.

The microstructures in Figure 5 and Figure 6 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.

The phase transformations were similar for both alloys and austempering temperatures; however, a microstructural change was observed between ADIs heat-treated to low and high austempering temperature. Figure 7 shows the microstructure of the ADIs containing 0.2 and 0.3 wt % V for the austempering temperatures of 265 and 305 °C and 90 min of soaking time.

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 blocks of high-carbon austenite. When the austempering temperature was increased to 305 °C, the microstructure changed to coarse ausferrite with a higher volume fraction of high-carbon austenite. During the austempering heat treatment, the acicular ferrite is formed due to its nucleation and growth from unstable austenite in the solid state. This phenomenon is aided by the cooling rate; at lower austempering temperature (265 °C), the cooling rate is highly promoting the nucleation of a high amount of acicular ferrite into an ausferritic matrix. When the austempering temperature is higher (305 °C), the cooling rate is decreased, forming an ausferritic matrix constituted by high-carbon austenite with a lower amount of ferrite [36]. This behavior was reported by Dojcinovic [37], where ADIs were heat-treated to 300 and 400 °C; they found that when the temperature is increased, 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 different 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 carbide were not observed because of their very low concentration in both alloys. The volume fraction of high-carbon austenite (%Vγ_HC) was determined by using Equations (4)–(6) [29] and the XRD pattern results.

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

Sample	%VγHC
	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

shows the results of the XRD analysis and the application of Equations (4)–(6) to determine the influence of the austempering time on the high-carbon austenite formation.

Figure 9 shows the %Vγ_HC behavior as the austempering time increased. The %Vγ_HC increases first, reaching a maximum value and then decreasing. The maximum value of %Vγ_HC 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 (1)–(3) occurs as follows: The first stage of the PW is presented to the austempering times of 30 and 60 min where Reaction (1) proceeds; afterwards, the highest amount of ausferrite is obtained at 90 min, reaching the PW (reaction 3), and then, the second stage of the PW window has been reached at the austempering time of 120 min, proceeding Reaction (2) [38]. It is observed from Table 4 that the highest values of %Vγ_HC are reached for the highest austempering temperature; this behavior is related to the carbides amount in the ausferritic matrix [39].

3.4. Carbides

The austenitizing parameters (900 °C, 180 min) were chosen to decrease the number of carbides in the ductile iron matrix reported in Table 2. In general, during the austenitizing, higher temperatures, and large residence times promote that the phases and microconstituents presented in the as-cast microstructure can be partially or dissolved in the matrix to form austenite [40].

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 persulfate darkens the ausferritic matrix and reveals the carbides as white regions.

It was reported [26] in the as-cast condition for these alloys used in the manufacture of camshafts that the highest carbide formation is located in the middle of the lobes instead of the external parts of the lobes due to the inverse chill, where there is a segregation of carbide-forming elements to the middle zone of the camshaft, increasing the concentration of these elements in the last liquid to solidify, promoting eutectic iron carbide formation [41]. After the ADI heat treatment to 90 min, it was observed that the highest carbides concentration is principally located in the bottom region of the lobes. Smaller carbide particles are found in the top and middle region of the lobes homogeneously distributed in the analyzed region.

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.

Sample	Carbides (%)
	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

shows the volume fraction of vanadium carbides of ADIs heat-treated to 265 and 305 °C for 90 min and the three regions analyzed. In addition, Table 5 also shows the carbides formed in the camshaft for the as-cast condition. It should be noted that the as-cast results were obtained from the whole lobe [26], while the results of the ADIs only represent the external part of the lobe, from the nose of the lobe to the upper part of the camshaft (base circle), as can be observed in Figure 2a. 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 [40,41,42].

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 Figure 3 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

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

Sample	Hardness (HRC)
	As-Cast	Austempering Time (min)
		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 solidification process; in this case, the camshaft microstructure is constituted mainly of pearlite with lower amounts of ferrite and carbides (Table 2) [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.

Figure 11 shows the hardness evolution as the austempering time was increased. The hardness behavior is directly related to the microstructural evolution shown in Figure 4 and Figure 5 and also with the %Vγ_HC as was previously reported [38,45].

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, reaching the lowest value for the austempering time of 90 min, followed by a slight hardness increase to the highest austempering time. The lowest Rockwell C hardness values are found at 90 min, where the highest volume fraction of high-carbon austenite (Table 4) was obtained for all the ADIs evaluated. Therefore, the hardness behavior is directly related to the %Vγ_HC and match in the process window determination. Following the sample ADI-0.2V-265, at the austempering time of 30 min, the microstructure is constituted mainly of martensite with a low amount of ausferrite, these allow obtaining the highest hardness value of 56 HRC with the lowest value of %Vγ_HC = 6.83. When the austempering time was increased to 60 min, the ausferrite amount was increased in the microstructure; thus, the value of %Vγ_HC increased to 8.26, and the martensite decreased. Consequently, the hardness decreased to 48 HRC. The microstructure is mainly constituted by ausferrite with a small amount of martensite for 90 min of austempering; this microstructure promotes the highest value of %Vγ_HC = 9.53 and the lowest hardness value (44 HRC). For the longest austempering time of 120 min, the second stage of the process windows occurs, which means that the microstructure is composed of ferrite and carbides; thus, the hardness increased to 45 HRC and the %Vγ_HC 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 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 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 containing 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] reported a constant value of 1050 MPa of tensile strength for ADIs alloyed with 0.2 wt % V austempered to 250 and 320 °C. A slight increase in the tensile strength of 1060 MPa was reported for an ADI alloyed with 0.4% V austempered to 250 °C. In general, the tensile strength decreased when the vanadium content was higher than 0.4% V and with the increase of the austempering temperature. The results in Figure 12 show that by increasing the vanadium content from 0.2 to 0.3 wt % V, the yield and tensile strength are the highest at both austempering temperatures. The ADI containing 0.3 wt % V showed that increasing the austempering temperature, the tensile properties are slightly decreased because of a coarser ausferritic matrix and a higher amount of the high-carbon austenite content.

Figure 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 the ADIs containing 0.2% V increase considerably from the as-cast condition to the ADI heat-treated to 265 °C; then, a slight increase occurs when the austempering temperature was increased to 305 °C. The elongation shows an opposite behavior to impact energy. The ADIs containing 0.2 wt % V shows an 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 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 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.

Two austempering temperatures were evaluated at different austempering times to determine the highest ausferrite content that enables obtaining the highest mechanical properties and an adequate balance between strength and toughness for the ADI camshaft production. The highest mechanical properties of tensile and yield strength of 1200 and 1051 MPa, respectively were obtained for the austempering heat treatment of 265 °C for the ADI containing 0.3 wt % V. However, when the ADI containing 0.3 wt % V was austempered 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 obtained.

• 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.
• 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 %Vγ_HC was obtained for the highest austempering temperature.
• 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.
• The evolution of hardness matched with the high-carbon austenite results and the microstructural evolution in the process window determination.
• 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.
• 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.