Microstructure and cavitation behaviour of alloyed ...

Taylor&Francis | e.Proofing

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Microstructure and cavitation behaviour of alloyed austempered ductile irons Running heads: International Journal of Cast Metals Research O. Eric Cekic et al. Eric Cekic O. a,, *

AQ1

Dojcinovic M. b Rajnovic D. c Sidjanin L. c Balos S. c aInnovation Centre, Faculty of Mechanical Engineering, University of Belgrade, Belgrade, Serbia bFaculty of Technology and Metallurgy, University of Belgrade, Belgrade, Serbia AQ2 cFaculty of Technical Sciences, University of Novi Sad, Novi Sad, Serbia *Corresponding author. Email: [email protected]

Received 30 Apr 2017; Accepted 23 Feb 2018 © 2018 Informa UK Limited, trading as Taylor & Francis Group

Abstract In this paper, the study of cavitation behaviour of austempered ductile iron (ADI) alloyed with copper, as well as copper and nickel with a fully ausferritic microstructure, is presented. The ADI materials used were austenitized at 900 °C and austempered at 350 °C having an ausferrite microstructure with 16 and 19% of austenite, respectively. The experimental investigations were conducted using the ultrasonically induced cavitation test method. The results show that the cavitation damage was initiated at graphite nodules, as well as in the interface between a graphite nodule and an ausferrite matrix. The cavitation rate revealed that the ADI material alloyed with Cu + Ni austempered at 350 °C/3 h has a higher cavitation resistance in water than ADI alloyed with Cu. An increased cavitation resistance of the ADI material alloyed with Cu and Ni is due to the matrix hardening by stress assisted phase transformation of austenite into the martensite (SATRAM) phenomenon. Keywords


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Ccavitation austempered ductile iron cast iron austenite-to-martensite phase transformation

1. Introduction Cavitation is a process of the formation and collapse of bubbles in liquid, with a considerable local reduction in the pressure at a given temperature [1]. When the liquid is exposed to a higher hydrostatic pressure, the bubbles can collapse, producing shock waves and microjets, which are the main cause of plastic deformation and erosion of the material surface [23]. This phenomenon occurs in hydraulic machinery parts that are usually produced from ferrous alloys. It was found that the cavitation resistance of these alloys depends on mechanical properties (hardness, tensile strength) and microstructure [4]. The strength of the matrix as well as size, shape and distribution of graphite influences the cavitation resistance of cast iron [5678]. Austempered Ductile Iron (ADI) is a special type of cast iron produced by austempering of the ductile iron where an excellent combination of mechanical properties is obtained due to its ausferrite microstructure [9101112]. The ausferrite is a mixture of ausferritic ferrite and carbon enriched reacted austenite [9101112]. It is well established by several authors [912] that the austempering reaction in ductile iron occurs in two stages. The first stage is the initial transformation when the primary austenite (γ0) transforms into ausferrite. During first stage, ausferritic ferrite (αAF) free of carbides is nucleated. Consequently, remaining adjacent austenite might become carbon enriched (γHC). Depending on the carbon enrichment, three types of austenite can be distinguished: (a) unreacted metastable austenite with less than 1% carbon, which did not participate in the austempering reaction, did not increase in carbon content compared to γ0, and therefore upon cooling to room temperature transforms into martensite; (b) reacted metastable austenite with carbon content between 1·2 and 1·6%; and (c) reacted stable austenite with carbon content 1·8–2·2% [13]. Accordingly, the metastable reacted and stable reacted austenite are carbon enriched during the austempering reaction. The stable reacted austenite is more preferable, because metastable reacted austenite can be transformed into martensite by stress or strain [141516] When the austempering treatment is prolonged, the second stage of austempering reaction begins. At that point, the austenite cannot hold carbon any longer, and the carbide nucleation starts extensively. Eventually, all carbon enriched austenite transforms into bainitic ferrite and carbides, i.e. microstructure becomes bainitic. This leads to significant reduction of mechanical properties, especially ductility [9101112].


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Nowadays, the research in this field has revealed that cavitation energy absorption causes stress assisted phase transformations of low-carbon, metastable austenite into martensite (SATRAM phenomenon) that affect cavitation erosion resistance [1718]. The studies of these materials were focused on the austempering temperature influence on microstructure and cavitation behaviour of unalloyed ADI. It was shown that ADI austempered at 400 °C, has higher cavitation resistance in water than ADI austempered at 300 °C [17]. The base iron chemistry and the alloy additions in ductile iron play an important role in obtaining ADI materials. In many cases, ADI materials need to be alloyed to achieve adequate austemperability and subsequent improvement in mechanical properties. In previous studies [11121920], investigations of ADI were concerned with the effect of the alloying elements on the microstructure and mechanical properties. As an alloying element [19], copper widens the austenite zone of the phase diagram, hence increasing the transformation time during the austenitising process and the initial carbon content (Cγ0) in the matrix at austenitisation temperature. On the other side, during the austempering process, copper may restrain carbide formation. Nickel also plays a significant role in the property development of ADI. Nickel decreases the strength of ADI material; however it increases ductility to a considerable extent [19]. It was found [20] that a combination of alloying elements such as Cu and Ni (austenite stabilisers), can also influence the transformation rate of austenite into martensite. However, relatively few studies were performed on the effect of different alloying elements on cavitation resistance behaviour of the ADI materials. For that reason, in this paper, the cavitation erosion behaviour of the ADI Cu and ADI Cu + Ni materials was analysed as a function of microstructure, mechanical properties, and alloying elements.

2. Experimental procedures 2.1. Materials In this paper, two alloyed ductile cast irons (designated as SG Cu and SG Cu + Ni), produced in an electric foundry furnace, were used. Ductile irons were produced by a sandwich spheroidisation treatment: the melt was poured into the green sand moulds and 25-mm-thick Y block castings were obtained. Base irons were treated with 2·2 mass% Fe-Si-Mg5 magnesium ferrosilicon nodularizer (containing 5÷5·5 mass% of Mg) for spheroidisation followed by post-inoculation with 0·3 mass% FeSi particles (containing 75 mass% of Si). The chemical composition of the ductile irons used in the study are given in Table 1 . Table 1. Chemical composition of the ductile cast irons (in mass%).

Material SG Cu



C

Si

3·64

2·49

Mn 0·30

P 0·014

S 0·014

Cu 0·46

Ni –

Mg 0·042

Fe Balance



 

Taylor&Francis | e.Proofing Material SG Cu+Ni

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C

Si

3·47

2·15

Mn 0·26

P

S

0·020

0·012

Cu 1·6

Ni

Mg

1·5

0·040

Fe Balance

The implemented heat treatments consisted of: (i) austenitisation in a furnace with a protective argon atmosphere at 900 °C for 2 h; (ii) austempering in a salt bath of 50–50% NaNO2-KNO3 at the austempering temperature of 350 °C, holding there for 2 h in case of ADI Cu and 3 h for ADI Cu + Ni, and finally air–cooled to room temperature. The stated austempering parameters were chosen according to the results of previous research [1221], as parameters that produce ADI material with the highest ductility and impact energy. For the produced ADI materials, the following abbreviations were used: ADI Cu austempered at 350 °C for 2 h, and ADI Cu + Ni material austempered at 350 °C for 3 h. The mechanical properties of alloyed ADI are shown in Table 2 , where it can be seen that the ADI alloyed with copper is less ductile, but has a higher yield strength (YS), ultimate tensile strength (UTS), and hardness, while elongation of the copper and nickel alloyed ADI is higher. The effect of alloying ADI with copper or copper and nickel on the impact energy of the specimens austempered at 350 °C is also shown in Table 2 . The impact energy of ADI alloyed with copper (110 J) is achieved after austempering for 2 h, whereas alloying with copper and nickel postpones the maximum energy to 3 h (122 J in this case). The fact that the maximum values of elongation and impact energy corresponds to the maximum value of the austenite in both cases, suggests that the ductility of ADI materials strongly depends on the amount of austenite [111221]. Table 2. Mechanical properties of alloyed ADI.

Material

Yield strength [MPa]

Ultimate tensile strength [MPa]

Elongation [%]

Impact energy K0 [J]

Hardness HV10

ADI Cu

995 ± 17·7

1110 ± 25·7

7·9 ± 1·3

106·1 ± 5·5

373 ± 6·2

ADI Cu+Ni

901 ± 4·5

1073 ± 32·4

11·1 ± 0·7

122 ± 7·8

308 ± 6·1

2.2. Methods For ultrasonic cavitation testing, a Branson Ultrasonics JP40022 device was used according to the standard test procedure [22]. The frequency of the vibration and the peak-to-peak displacement amplitude of the horn were 20 ± 0·5 kHz and 50 μm, respectively, with separation of 0·5 mm between the specimens and the horn tip. The cavitating liquid was water at 25 ± 0·5 °C. In order to obtain the erosion curve, the mass loss measurements were performed after each exposure interval. Before the test and after each test interval, the specimens were cleaned and dried with hot air. Measurements of the


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mass loss were taken after 0·5, 1, 2, 3, and 4 h of cavitation damage, in order to obtain the erosion curves. Each mass loss point reported is the average of three cavitation damage tests. Mass losses of the tested specimens were measured using an analytical balance with an accuracy of ± 0·1 mg. Tensile properties of the ADI (Rm-UTS, Rp0·2%-proof strength, A5-elongation) were performed in accordance to ISO 6892-1, impact energy (K0) to ISO 148-1 and Vickers hardness (HV10) to ISO 6507-1 standard. For each test at least three specimens were measured. All mechanical properties were determined at room temperature. Furthermore, Vickers hardness (HV10) tests were performed after cavitation on the cross-sections of the tested specimens in order to verify the existence of work hardened subsurface layers affected by cavitation. Light microscopy (LM) was used to observe the microstructure in an as-cast condition, as well as before and after cavitation. Standard metallographic preparation techniques (grinding, polishing and etching in 3% nital solution) were applied. It should be noted that light microscopy (LM) results were obtained from the sample cross-section of approximately 50 μm under the cavitation surface. Furthermore, in this study, the heat-tinting technique [23] was also used to reveal the different behaviours of the phases in the specimens of ADI material alloyed with copper, and the copper and nickel after 4 h cavitation. Heat tinting is a type of etching technique, leading to distinct colours of different phases in the ADI. Specimens etched by 3% nital were heated in a furnace without a protective atmosphere at a temperature of 260 °C for a period of 6 h and then air cooled to room temperature. After this heat treatment, the volume fraction of low-carbon, reacted metastable austenite appears as light-blue to blue; high-carbon, reacted stable austenite as purple to red (the higher the carbon, the darker the purple colour); ausferritic ferrite as beige; carbides as white; while the martensite turns lightblue with distinctive lenticular (lens-shaped) morphology. The morphology of the cavitated specimen surfaces was observed by a SEM JEOL JSM-5800 in order to assess the development of the cavitation damage. To determinate the volume fraction of austenite (Xγ

), all samples were characterized at room

temperature by X-ray diffraction (XRD) using an Ultima IV Rigakudiffractometer, equipped with Cu Kα1,2 radiation, using a generator voltage (40 kV) and a generator current (40 mA), without a monochromator. At least three XRD patterns were obtained for every sample. The range of 35–90° 2θ was used in a continuous scan mode with a scanning step size of 0·02° and at a scan rate of 5°/min. The diffractograms were analysed applying the direct comparison method as performed by Cullity [24]. In addition, the carbon content of the austenite (Cγ) was calculated using the following equation:

C = (a − 0 3548)/0 0044

(1)


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where aγ is the lattice parameter of austenite [nm] and Cγ is the carbon content of austenite in [mass.%]. The [111] and [200] planes of austenite were used to estimate the lattice parameter aγ. It should be pointed out that the XRD results were obtained from a relatively thin surface layer affected by cavitation, as it is known that x-rays penetrate only few microns into the surface [24] and the results represent the microstructure transformation only in that region.

3. Results 3.1. ADI microstructure The microstructures of ADI materials alloyed with Cu austempered at 350 °C/2 h and Cu + Ni austempered 350 °C/3 h in polished and etched condition are shown in Figures 1 and 2 . The morphology of the graphite nodules in the microstructure of the both ductile irons (ADI Cu and ADI Cu + Ni) is spherical with the comparable nodule count of 60 ± 5 and 58 ± 6 nodules mm−2, respectively (Figure 1 ). The graphite spheroidisation in all specimens was more than 90% with an average nodule size 48 ± 4 μm (Figure 1 ). The ADI microstructures are shown in Figure 2 . In Figure 2 (a), which refers to the ADI austempered at 350°C for 2 h (ADI Cu), a fully ausferritic microstructure is observed, consisting of a mixture of ausferritic ferrite needle and high carbon enriched austenite (stringer type austenite). However, alloying the ADI with Cu + Ni, the acicular appearance of the microstructure obtained in ADI (Figure 2 (b)) changed to a wider and more separated ausferritic ferrite plates, i.e. more plate-like (feathery) morphology. Furthermore, it can be seen that the microstructure of ADI Cu + Ni (Figure 2 (b)) has a larger percentage of bright areas than in ADI Cu (Figure 2 (a)), corresponding to a some extent greater amount of austenite. The morphologies of austenite in the matrix of ADI could be also referred to as stringer type and island–like (blocky) [18]. The Figure 2 (b) and (d), shows that the volume fraction of island-like austenite (BA – ‘blocky’ austenite) for sample ADI Cu + Ni is greater than for the ADI Cu (Figure 2 (a)). Further microstructure examination by heat tinting revealed that island-like austenite (BA) corresponded to low-carbon, reacted metastable austenite, Figure 2 (c) and (d).

Figure 1. Graphite nodules in ADI (polished): (a) ADI Cu; (b) ADI Cu + Ni.


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Figure 2. Microstructure of ADI, etched with 3% nital: (a) ADI Cu, and (b) ADI Cu + Ni; heat tinting: (c) ADI Cu, and (d) ADI Cu + Ni (BA – ‘blocky’ austenite).

3.2. Cavitation erosion behaviour The SEM micrographs of ADI after 0·5 h of cavitation testing are shown in Figure 3 (a) and (b) for ADI Cu and ADI Cu + Ni, respectively. At the beginning of cavitation testing, the separation of the graphite nodules from the metal matrix occurs. In this stage, the metal matrix of both the ADI materials


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was slightly attacked by the peeling of the weakly bonded particles. It can be seen, that in both ADI materials, the microcracks were nucleated at the edge of the pits formed by separating the nodules, as well as in the metal matrix.

Figure 3. SEM micrograph of alloyed ADI after 0·5 h of cavitation testing in water: (a) ADI Cu; (b) ADI Cu + Ni.

After 1 h of cavitation testing, a greater degree of damage in relation to the previous test period may be observed, Figure 4 . By comparing two tested materials, it can be noticed that the damage is caused by peeling in the matrix area around the nodule pit rim. There are several new connected microcracks on the left side of the damaged zone in the ADI Cu, Figure 4 (a). Areas of metal matrix between these cracks will be removed by their development during prolonged effects of cavitation. This behaviour was not present in the ADI Cu + Ni, Figure 4 (b). The damages after 4 h of cavitation testing are shown in Figure 5 . It can be seen that a large part of the surface became damaged in both ADI materials. However, on the damaged surface of the ADI Cu + Ni, the pits created by removing the graphite nodules can still be seen clearly, Figure 5 (b). This suggests that the thickness of the removed surface layer during testing of the ADI Cu + Ni is less in comparison with the ADI Cu and indicates a higher cavitation resistance of the ADI Cu + Ni.

Figure 4. SEM micrograph of alloyed ADI after 1 h of cavitation testing in water: (a) ADI Cu; (b) ADI Cu + Ni.


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Figure 5. SEM micrograph of alloyed ADI after 4 h of cavitation testing in water: (a) ADI Cu; (b) ADI Cu + Ni.

The materials resistance to cavitation is represented by cavitation rate which correspond to mass loss during testing time. The measured mass losses are shown in Figure 6 . The calculated cavitation rate for the ADI Cu is 0·03625 mg min−1, while the cavitation rate of the ADI Cu + Ni is 0·02875 mg min−1. Based on the results presented in the Figure 6 , it can be seen that after each test a higher values of mass loss were obtained for the ADI Cu. The difference between measured mass loss is the lowest after 0·5 and 1 h, which corresponds to the separation of the graphite nodules and the subsequent peeling of the matrix around the pit rims. The most significant difference in measured mass loss was noticed after 2 h cavitation.

Figure 6. Cavitation rate trendlines of the ADI Cu and ADI Cu + Ni.


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3.3. ADI microstructure after cavitation The microstructures of the ADI Cu and ADI Cu + Ni obtained by light microscopy (LM) after 4 h cavitation are given in Figures 7 and 8 , respectively. The change in the morphology of ausferrite (Figure 7 ) compared to the un-cavitated samples of ADI Cu material (see Figure 2 (a) and (c)) could be noticed. The interface between the acicular ferrite and austenite becomes less pronounced (dimmed), Figure 7 (a). After heat tinting, it could be noticed that the high carbon enriched austenite gradually decomposed into a more thermo-mechanically stable bainite microstructure (Stage II of transformation), Figure 7 (b) (light blue area with small white particles). Furthermore, a small fraction of martensite (Figure 7 (a)) is also visible in the island-like austenite.

Figure 7. The microstructure of ADI Cu after 4 h of cavitation: (a) standard etching with 3% nital; (b) heat tinting (AF – ausferrite, M – martensite, B – bainite).


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Figure 8. The microstructure of ADI Cu + Ni after 4 h of cavitation: (a) standard etching with 3% nital; (b) heat tinting (AF – ausferrite, M – martensite, B – bainite).

In the microstructure of the ADI Cu + Ni, a higher martensite (lenticular, high carbon) presence could be seen after 4 h of cavitation (Figure 8 ) than in the ADI Cu material (Figure 7 ). Also, in the ADI Cu + Ni, evidence of the austenite decomposition into a more thermo-mechanically stable ferrite and carbides, namely bainite (Stage II of transformation), is noticed. However, the amount of decomposition as a result of the cavitation process is lower compared to the transformation into martensite. Vickers hardness (HV10) tests at the cross-sections of the cavitated specimens in the region under cavitated surface (up to 50 μm) showed an increase of hardness to 480 HV10 in the case of ADI Cu, and to 402 HV10 for ADI Cu + Ni material. This increases in hardness supports aforementioned observations of microstructure transformation during cavitation and the existence of work hardened subsurface layers.


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In that way, the presented results show the affinity of the microstructure to transform into martensite or to decompose into bainite. Additionally, the LM results were confirmed by the X-ray diffraction patterns of the ADI alloyed with Cu and Cu + Ni.

3.4. X-ray diffractometry The volume fraction of austenite, the carbon content in austenite, and the total austenitic carbon content for the ADI Cu and ADI Cu + Ni are given in Table 3 , while the corresponding XRD patterns are shown in Figure 9 . The volume fraction of austenite for the ADI Cu is 16%, while the ADI Cu + Ni has a higher amount of 19%. On the other hand, the carbon content of austenite is higher in the ADI Cu then in the ADI Cu + Ni. Thus, the austenite of ADI Cu with 2·18% carbon content is more stable than that of the ADI Cu + Ni with 1·91%C. Table 3. The volume fraction of the austenite, the carbon content in the austenite and total austenitic carbon content for ADI Cu and ADI Cu + Ni material.

Volume fraction of austenite Xγ [%]

Carbon content in austenite Cγ [%]

Total austenitic carbon content XγCγ [%]

Before cavitation ADI Cu

16 ± 1·8

2·18 ± 0·09

0·350

ADI Cu+Ni

19 ± 2·1

1·91 ± 0·11

0·36

After cavitation ADI Cu

4 ± 2·4

2·15 ± 0·07

0·086

ADI Cu+Ni

3 ± 1·3

1·97 ± 0·05

0·059

Figure 9. XRD patterns for alloyed austempered ductile iron before and after cavitation: (a) ADI Cu; (b) ADI Cu + Ni.


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




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After the cavitation process, the volume fraction of the austenite for both ADI materials decreased; namely, from 16 to 4% for the ADI Cu, and from 19 to 3% for the ADI Cu + Ni. The change in the austenite amount is clearly visible from the X-ray diffractogram where the peak intensities of the austenite (111γ and 200γ) are quite high before cavitation, while they are reduced after cavitation. This is because of the transformation of the face-centred cubic austenite to body-centred tetragonal martensite. According to Figure 9 , the increased integrated area under ferrite peaks is due to martensite formation where martensite peaks appear at almost the same angular positions as do the ferrite. This indicates that the austenite has transformed largely into martensite in a thin surface layer as a result of cavitation.

4. Discussion The results obtained in this study had shown that the ADI material alloyed with Cu and Ni possesses a higher cavitation resistance than the ADI material alloyed only with Cu in spite of a higher hardness of the ADI Cu and comparable other properties (YS, UTS, elongation, and impact energy). The obtained cavitation erosion behaviour could be attributed to the stability of austenite under the cavitation damage. The stability of the austenite during deformation is influenced by several parameters, such as: the carbon content, and the amount and morphology of the austenite [152526]. The main factor that controls the stability of the austenite is its carbon content [26]. It was suggested that the austenite transforms into martensite. However, results from the studies [1718] had further revealed that in ADI, after cavitation testing, the high carbon enriched austenite, besides transforming into martensite, might also decompose into a more thermo-mechanically stable ferrite and carbides (Stage II of transformation), as microjet impact and shock waves might locally raise temperatures of the matrix and maintain the diffusion of carbon on the ausferritic ferrite/austenite interface [18]. The formation of carbides on the ausferritic ferrite/austenite interface reduces the materials cavitation resistance [17] by increasing crack nucleation potential through the mechanism of plastic zone confinement [27]. In this study, the decrease of the austenite volume fraction of ADI Cu + Ni might be attributed to martensite formation by a SATRAM (Stress Assisted Transformation from Austenite to Martensite) mechanism [1718282930]. The martensite formation has a positive effect on the cavitation rate, as described by Yang and Putatunda [31], as well as Daber et al. [28], who determined that the martensite formation increases the strain hardening exponent (n).


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The results from Table 3 indicate that the carbon content of the austenite in the ADI Cu was 2·18 and 2·15% before and after cavitation testing, respectively. This indicates that the stringer type austenite found in the ADI Cu contains a higher amount of carbon than the plate-like (feathery) morphology of the ADI Cu + Ni. Since, the high carbon enriched stringer type austenite is more stable and it is not highly sensitive to the SATRAM effect it could be assumed that this austenite tended to decompose into a more thermo-mechanically stable ferrite and carbides, that is, bainite (Stage II of transformation). According to the results published by S. Dhanasekaran et al. [32] the carbon in the ferrite needles diffuses into the surrounding austenite, during the first stage of austempering, resulting in an increase of the carbon in the austenite and its decrease in the ferrite. The austempering rate depends on the transformation driving force and the carbon diffusion rate. In the study by S. Dhanasekaran [32], the addition of copper decreases the austenite free energy, which results in a decrease of the driving force for the γ ( α transformation and hence the austempering transformation rate decreases too. On the other hand, the carbon content of the austenite in alloyed ADI with copper and nickel was about 1·91% before and 1·97% after cavitation testing. The lower amount of carbon compared to ADI Cu, makes the austenite less stable and more susceptible to transformation into martensite under stresses (SATRAM). As indicated from the previous studies [3334], nickel decreases the carbon content of both the parent austenite (Cγ0) and the high carbon austenite (CγHC). Also, it cannot change the driving force of the 1st stage reaction (Cγ0-CγHC) considerably, but only delays it slightly. However, it does increase the amount of the austenite in the ausferrite to some extent, and leads to the formation of blocky austenite. This blocky (metastable) austenite has a lower carbon content; furthermore, carbon content is not uniformly distributed and tends to induce the formation of martensite. The formation of martensite in the blocky austenite in the case of the ADI Cu + Ni is documented in Figure 8 . The quoted mass losses of 0·03625 mg min−1 (ADI Cu austempered at 350 °C for 2 h) and 0·02875 mg min−1 (ADI Cu + Ni austempered at 350 °C for 3 h) are lower than the values obtained in our previous studies: 0·0925 mg min−1 (unalloyed ADI austempered at 400 °C; ADI-400), and 0·1046 mg min−1 (unalloyed ADI austempered at 300 °C; ADI-300) [17]. This difference could be attributed to the various graphite nodule size and count, and to the rate of the austenite transformation to bainite or martensite. The ADI Cu and ADI Cu + Ni have fewer numbers of larger nodules (60 ± 5 and 58 ± 6 nodules mm−2, with average nodule size 48 ± 4 μm) compared to the unalloyed ADI-300 and ADI-400 (150–200 nodules mm−2 and average nodule size 25–30 μm [17]). The calculated average circumference of the surface nodules per 1 mm2 is 10·5 and 15·12 mm for the alloyed ADIs (ADI Cu and ADI Cu + Ni) and unalloyed ADIs (ADI-300 and ADI-400), respectively. Thus, the shorter edge length of the nodule pits (average circumference) offers fewer nucleation sites for microcracks


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initiation, which leads to the higher cavitation resistance of the alloyed ADIs. Furthermore, the higher cavitation resistance of the alloyed ADIs is also enhanced by a more pronounced SATRAM effect indicated by the higher transformation rate (total transformation: −12% in ADI Cu, and −16% in ADI Cu + Ni, Table 3 ) of the austenite into a more resistant martensite compared to the unalloyed ADIs (−10·5% in ADI-300, and −6·7% in ADI-400 [17]).

5. Conclusions In this study, the cavitation behaviour of ADI alloyed with Cu and Cu + Ni was analysed. The obtained conclusions can be summarized as follows: •

•Cavitation damage for both of the ADI materials was initiated by the separation of the graphite nodules from the metal matrix.

•

•During cavitation testing, the damage in both of the tested materials is caused by peeling in the matrix area around to the graphite nodule pit rim.

•

•In addition to peeling, the damage of ADI Cu occurs by developing groups of cracks arising from the pits in the metal matrix. This micro-cracking is not present in the ADI Cu + Ni due to its higher toughness.

•

•It was shown that the ADI Cu + Ni has a higher cavitation resistance than the ADI Cu having the cavitation rate of 0·02875 mg min−1 compared to 0·03625 mg min−1.

•

•The ADI Cu has a lower cavitation resistance due to the presence of carbides that are sensitive to separation during cavitation and the micro-crack development. Namely, the ADI Cu carbon enriched austenite (stringer-type) is more stable and is not highly sensitive to the SATRAM effect; consequently, it decomposes mainly into ferrite and carbides during cavitation.

•

•The higher cavitation resistance of the ADI Cu + Ni was related to the presence of nickel, which decreases the carbon content of the austenite and makes it more sensitive for transformation into martensite through the SATRAM phenomenon. The presence of a higher amount of martensite compared to the ADI Cu enhances the ADI Cu + Ni cavitation resistance.

Funding The authors gratefully acknowledge research funding from The Ministry of Education, Science and Technological Development of The Republic of Serbia [grant number TR34015]. The authors are also very grateful to Mr. Michael A. Maier for technical support.

Disclosure statement AQ3

No potential conflict of interest was reported by the authors.

Acknowledgements


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The authors gratefully acknowledge research funding from The Ministry of Education, Science and Technological Development of The Republic of Serbia under grant number TR34015. The authors are also very grateful to Mr. Michael A. Maier for technical support.

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