Abrasive impact wear resistance of hardfacings P. Kulu*, A. Surženkov, R. Tarbe, T. Simson Department of Materials Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia

ABSTRACT To reduce wear at extreme abrasive wear conditions (high abrasivity, high velocities and elevated temperatures), materials with optimal hardness-toughness properties are preferable. In composites structures it is possible to combine high hardness of reinforcement (for example of carbides) with high toughness of a metal matrix (nickel or iron based). Recent study is devoted to the technology and properties of powder metallurgy produced composite hardfacings. As a matrix material, Fe- and Ni-based self-fluxing alloys powders and as a reinforcement recycled WC-Co hardmetal powder were used. Centrifugal type universal accelerator and abrasive impact wear tester were used for the study of the wear resistance at abrasive erosive and at abrasive impact wear. The influence of reinforcement size and abrasive particles velocity to wear of studied materials was investigated. As reference materials, steel Hardox 400 and composite wear plate CDP112 (Castolin Eutectic®) were studied. The relative wear resistance was found to be dependent as on hardfacings' composition, as on wear parameters: at abrasive erosive wear the wear resistance of best hardfacings exceeded of that of reference steel Hardox 400 up to 3 times; at abrasive impact wear – up to 4 times. Key words: hardfacing, powder metallurgy, abrasive erosive wear, abrasive impact wear.

INTRODUCTION To reduce wear at extreme wear conditions (high abrasivity, high impact energy, elevated temperatures) various composite materials and hardfacings are available. In composite structures it is possible to combine high hardness of reinforcement and high toughness of matrix. The wear behaviour of the above-mentioned structures depends as on their microstructure, as on the wear conditions. The wear mechanism of a material at abrasive wear depends on its hardness: if it is lower than the hardness of the abrasive, the wear mechanism will be the microcutting; if it is higher, the wear of the material will occur due to the direct fracture or fatigue [1–3]. If the erosive wear finds place, composite materials with a hard reinforcement (carbides, nitrides, etc.), embedded in a relatively hard matrix, such as hardmetals and cermets, are advised [4,5]. At mixed wear conditions, where abrasive wear is accompanied by the impact, composite structures with multimodal reinforcements, so-called „double cemented structures“ [3,6,7] (Fig. 1), are preferred [5].

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Fig. 1. Recommended structure with multimodal reinforcements (double cemented) for mixed wear conditions. Toughness is an important mechanical characteristic of a hardfacing, as it determines its resistance to impacts. As a result of an impact, nucleation and propagation of cracks find place. These cracks reduce hardfacing's shear strength and therefore lessen its resistance to erosion wear at oblique angles [8]. It is recommended [3] that at high impact angles (α > 60º), higher toughness and lower hardness are required, at low impact angles (α < 30º) higher hardness and lower toughness are acceptable (Fig. 2). Therefore depending on the wear conditions, different composite materials are recommended.

Fig. 2. Hardness-toughness criteria of materials' selection for different erosive wear conditions: 1, 2 – for low impact angles, 2 to 4 – for mixed abrasive-erosion, 5 – for close to normal impact angles [9]. The present paper studies the influence of composition of the hardmetal reinforced hardfacings, as well as abrasive wear parameters (size and velocity of the abrasive particles) to the performance of hardmetal reinforced powder metallurgy hardfacings with Ni- or Fe-alloy based matrixes. EXPERIMENTAL Hardfacings and materials to be studied The hardfacings studied were produced from commercial Fe- and Ni-based self-fluxing alloy powders and recycled WC-Co hardmetal powders, made from hardmetal scrap by mechanical

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disintegrator milling [10]. The powder mixtures of the Fe- or Ni-based powders (respectively FeCrSiB or NiCrSiB) and WC-12Co hardmetal powders were processed by the powder metallurgy (PM) technology – vacuum sintering (1100 °C, 30 min). During sintering, the self-fluxing alloy melted and solidified, when the hardmetal particles remained solid through the whole process. As the final result, a composite structure with hardmetal particles, embedded in a self-fluxing alloy matrix, formed. Composite wear plate CDP112 (Castolin Eutectic®) , as well as steel Hardox 400 were taken as the reference materials. The detailed information about the tested hardfacings is carried out in Table 1. Table 1. Studied hardfacings. Hardfacing type and Designation WC-Co particle Hardness, HV composition size, mm Macrohardness Low-force hardness HV0.3 HV30 matrix / reinforcement 50 wt% FeCrSiB1) + PM-Fe (f) 50 wt% WC-12Co

0.35 – 1.0

975 ± 105

725 ± 165 / 1445 ± 310

50 wt% FeCrSiB1) + PM-Fe (c) 50 wt% WC-12Co

1.0 – 2.5

1025 ± 85

945 ± 115 / 1455 ± 270

50 wt% NiCrSiB2) + PM-Ni (f) 50 wt% WC-12Co

0.35 – 1.0

425 ± 75

310 ± 35 / 1300 ± 350

50 wt% NiCrSiB2) + PM-Ni (c) 50 wt% WC-12Co

1.0 – 2.5

535 ± 75

320 ± 40 / 1280 ± 45

Castodur Plate DP04023)

–

550 ± 40

525 ± 110 / 1345 ± 510

Diamond CDP112 CDP112

Steel Hardox 4004) Hardox 400 – 425 ± 45 – 1) 6AB, Höganäs AB; 13.7 wt% Cr, 2.7 wt% Si, 0.3 wt% Mn, 2.1 wt% C, 3.4 wt% B, 6.0 wt% Ni, bal. Fe 2) EuTroLoy 16221 G, Castolin Eutectic®; 4 wt% Cr, 2.5 wt% Si, 0.2 wt% C, 1 wt% B, max 2 wt% Fe, 1 wt% Al, bal. Ni 3) Castolin Eutectic®; substrate – steel S235 (0.23 wt% C, 1.50 wt% Mn, 0.05 wt% P and S, 0.60 wt% Cu; 4 mm), wear resistant layer – spray-fused 12112 (NiCrSiB + 35 wt% WC; 2 mm), 4) SSAB Öxelösund AB; 0.25 wt% C, 0.70 wt% Si, 1.60 wt% Mn, 1.00 wt% Cr, 0.70 wt% Ni, 0.80 wt% Mo, 0.004 wt% B, Characterization of the hardfacings' structure The microstructure of the PM hardfacings was studied under the optical microscope Axiovert 25 (Carl Zeiss). Distribution of the chemical elements inside them was investigated by the energy dispersive spectroscopy (EDS). Vickers macrohardness was measured at the surfaces of the hardfacings and the reference materials applying the load 30 kgf (294.3 N), Vickers low-force hardness – at the cross-sections of the hardfacings and some reference materials, applying the load 0.3 kgf (2.9 N). The results of hardness measurements are given in Table 1. Abrasive wear characterization The sintered PM hardfacings and the reference materials were tested at abrasive erosive wear (AEW) [12] and at abrasive impact wear (AIW) [13] conditions. The schemes and parameters of both tests are brought in Table 2. Three specimens were utilized for each test, and average weight

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loss was found. On its basis, volumetric wear rate was found as the volume wear loss of tested hardfacing or material per 1 kg of the abrasive, mm3/kg. Relative wear resistance ε as a ratio of volumetric wear rate of a hardfacing or a material to the volumetric wear rate of reference material – steel Hardox 400 – was determined. Table 2. Parameters of wear tests. Designation of wear test Scheme of wear test setup

Used abrasive parameters

and

wear

AEW

Granite sand (0.1 – 0.3 mm) Hardness of the abrasive – 930 ± 20 HV0.1 Velocities 40, 60 and 80 m/s

AIW

Granite gravel (3 – 5 mm) Hardness of the abrasive – 930 ± 20 HV0.1 Velocities 40, 60 and 80 m/s

RESULTS AND DISCUSSION Microstructure analysis As it follows from the study of the microstructure of the hardfacings, they had a composite structure, where hardmetal particles were embedded and uniformly distributed in the self-fluxing alloy matrix (Fig. 3).

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a

b

250 μm

250 μm

Fig. 3. Microstructure of the sintered hardfacings: a – PM-Fe (f), b – PM-Ni (f). All the studied hardfacings exhibited porosity in the matrix, whereas two types of pores could be distinguished. The pores of the first type had a large size (up to 400 μm) and complex non-spherical shape. The pores of the second type were of a significanlty smaller dimensions (10–15 μm) and were of a near-spherical shape; their presense was more remarkable in the Fe-based hardfacing (Fig. 3b) than in the Ni-based one (Fig. 3b). The pores of the first type were identified as shrinkage porosity [11], which formed during the solidification of the melt. The pores of the second type most probably originated from the gas, entrapped between powder particles, which could not escape during vacuumizing. A presense of small amounts of tungsten (up to 3.1 wt% in the Ni-based alloy and up to 6.8 wt% in the Fe-based alloy) was found in the self-fluxing alloy matrix, what was an evidence of a dissolution of WC particles in the molten metal. A larger proportion of tungsten in the Fe-based self-fluxing alloy is in a good agreement with the results from other researchers, reporting higher dissolution of tungsten carbides in the Fe-based alloys [14]. Apart from that, the formation of the interdiffusion zone at the 'reinforcement-matrix' boundary [11] was observed, where cobalt from the hardmetal diffused into the matrix and, on the other hand, nickel or iron replaced it in the hardmetal particles. Similarly to the above-described situation the concentration of nickel in binder of the hardmetal particles in this zone did not exceed 9.7 wt%, while the concentration of the iron reached the value of 26.3 wt%. Abrasive wear resistance Abrasive erosive wear (AEW) The study of the hardfacings with fine (0.35 – 1 mm) and coarse (1 – 2.5 mm) hardmetal particles at different velocities of abrasive particles demonstrated a notable dependence of the wear rate on these parameters (Table 3, Fig. 4).

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Table 3. Wear rates of the PM hardfacings at different velocities and impact angles of abrasive particles. Wear rate, mm3/kg at velocities and impact angles

Designation of hardfacing

40 m/s

60 m/s

80 m/s

30º

90º

30º

90º

30º

90º

PM-Fe (f)

7.9

5.4

8.9

18.0

19.9

59.5

PM-Fe (c)

5.4

8.9

6.0

13.3

11.0

25.2

PM-Ni (f)

11.0

3.5

11.3

15.3

23.5

27.4

PM-Ni (c)

5.1

6.2

4.9

9.5

21.1

28.4

-

-

15.8

30.7

27.6

54.7

8.1

7.2

-

-

32.6

21.7

CDP112 Hardox 400 (reference)

Relative wear resistance of the studied hardfacings at AEW is given at Fig. 4. 40 m/s

80 m/s

Relative wear resistance ε

Relative wear resistance ε

40 m/s

3,5 3

2,5 2 1,5

1 0,5 0 PM-Fe (f) PM-Fe (c) PM-Ni (f) PM-Ni (c) CDP112

80 m/s

2,5

2 1,5 1 0,5

0 PM-Fe (f) PM-Fe (c) PM-Ni (f) PM-Ni (c) CDP112

a b Fig. 4. Relative wear resistance of the studied hardfacings at AEW (reference material – steel Hardox 400); impact angles: a – 30º, b – 90º. As it follows from the wear results (Table 3, Fig. 4), Fe-based hardfacings with a coarser reinforcement demonstrate a higher wear resistance at the low impact angle (30º). This fact can be explained by a higher hardness of the matrix (Table 1) and smaller loss of the reinforcement particles [11]. At the normal impact angle (90º), several trends can be seen. At the low velocity of abrasive particles (40 m/s), both Fe- and Ni-based hardfacings with a finer reinforcement exhibit lower wear, however, with the increase of the velocity values the situation changes to the opposite in the case of the Fe-based hardfacings. At the low impact angle, except for the PM-Fe (f) and PM-Ni (f) hardfacings, the ohter had 1.5 – 3.0 times higher wear resistance, than the reference steel Hardox 400 (Fig. 4). However, at the normal impact angle the hardfacings exibited lower wear resistance (except for the PM-Ni (c) hardfacing), what is a sign of a higher brittleness of the obtained hardfacings in comparison with the reference steel [15]. It is also worth noting that the studied hardfacings had up to 2.6 times and 3.2 times lower wear to be compared with the CDP112 composite wear plate at the low and normal impact angles, respectively. In the first case, a higher wear resistance of the experimental hardfacings can be probably explained by a higher reinforcement content and in the second – by a higher toughness of the reinfocement [16].

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Abrasive impact wear (AIW) Similarly to AEW, hardfacings were subjected to abrasive impact wear (AIW) applying different velocities of abrasive particles, varying from 40 to 80 m/s. The test results are brought in Table 4 and Fig. 5. Table 4. Wear rates of the PM hardfacings at AIW at different velocities of abrasive particles. Designation of hardfacing

Wear rate, mm3/kg at velocities 40 m/s

60 m/s

80 m/s

PM-Fe (f)

24.1

17.8

30.8

PM-Fe (c)

13.6

13.5

29.3

PM-Ni (f)

16.8

24.3

59.3

PM-Ni (c)

14.8

23.1

40.3

CDP112

21.8

22.4

32.0

Hardox 400 (reference)

31.7

53.1

94.5

As follows from Table 4, the wear rate of hardfacings depends on the particles' size of the reinforcement. For example, in the case of PM-Fe hardfacings, at the low impact velocity the difference in wear rates is about 2 times to compare the hardfacings with fine and coarse reinforcement, although at the higher velocities the difference is minimal. Also the PM-Fe hardfacings generally showed smaller wear rates. A better resistance to wear of the hardfacings with the FeCrSiB matrix can be explained by its higher hardness in comparison with the NiCrSiB matrix, as can be seen from Table 1. A better performance of the hardfacings, reinforced with coarser hardmetal particles, is most probably due to a higher load bearing capacity, provided by a coarser hardmetal. The Fe- and Ni-based hardfacings exhibit different tendencies with the increase of the velocity of abrasive particles (Fig. 5). In the case of Fe-based hardfacings, the largest difference in wear rates for fine and coarse particles reinforced hardfacings (1.8 times) is observed at the low velocity, in the case of Ni-based hardfacings, vice-a-versa, at the high velocity (1.5 times). The relative wear resistance at the AIW of all the studied hardfacings exhibited a better wear resistance than the reference steel (Fig. 5), moreover, the experimental PM-Fe hardfacings showed 1.1 – 1.5 lower wear to be compared with the commercial composite wear plate CDP112, except for the PM-Fe (f) hardfacing at 40 m/s. However, with the increase of the velocity and, consequently, the kinetic energy of abrasive particles the difference became less remarkable. Better results, demonstrated by the experimental hardfacings, can be accounted for the higher toughness of the hardmetal reinfocement in comparison with the carbide reinforcement of the studied reference wear plate, allowing to withstand higher impact loads [16].

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40 m/s

80 m/s

Relative wear resistance ε

3,5 3 2,5 2

1,5 1 0,5 0 PM-Fe (f)

PM-Fe (c)

PM-Ni (f)

PM-Ni (c)

CDP112

Fig. 5. Relative wear resistance at AIW of the studied hardfacings (reference material – steel Hardox 400). In general, it can be said that for extreme mixed wear conditions, enclosing high hardness and high velocity, as well as normal angle of impingement of the abrasive, wear is directly proportional to the kinetic energy of abrasive particles and inversely proportional to the hardness of the material and function of the angle of impingement. For such conditions, metal-matrix composites or hardfacings are recommended (Fig. 6).

Fig. 6. Choice of materials in case of different impact kinetic energy and abrasive particles impact angles. CONCLUSIONS 1. At abrasive erosive wear, Fe-based hardfacings with a coarser reinforcement are preferable at the low impact angle conditions. At the normal impact angle at the low velocity of the abrasive hardfacings with a finer reinforcement exhibit higher wear resistance, with increasing the velocity of abrasive particles, hardfacings with coarser reinforcement become favourable from the point of higher resistance to wear. 2. At abrasive impact wear, hardfacings with a coarser reinforcement should be preferred. Febased hardfacings become more advantageous with the growth of the velocity of abrasive particles.

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