The effect of microstructure on abrasive wear of hardfacing alloys

The effect of microstructure on abrasive wear of hardfacing alloys

Wear 259 (2005) 52–61 The effect of microstructure on abrasive wear of hardfacing alloys M.F. Buchely, J.C. Gutierrez, L.M. Le´on, A. Toro ∗ Tribolog...

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Wear 259 (2005) 52–61

The effect of microstructure on abrasive wear of hardfacing alloys M.F. Buchely, J.C. Gutierrez, L.M. Le´on, A. Toro ∗ Tribology and Surfaces Group, National University of Colombia, Medell´ın, Colombia Received 2 August 2004; received in revised form 1 March 2005; accepted 3 March 2005 Available online 23 May 2005

Abstract Hardfacing is one of the most useful and economical ways to improve the performance of components submitted to severe wear conditions. A study was made to compare the microstructure and abrasion resistance of hardfacing alloys reinforced with primary chromium carbides, complex carbides or tungsten carbides. The hardfacing alloys were deposited onto ASTM A36 carbon steel plates by a shielded metal arc welding (SMAW) method. Three different commercial hardfacing electrodes were employed to investigate the effect of the microstructure. The abrasion tests were carried out in a dry sand–rubber wheel abrasion machine according to the procedure A of ASTM G65 standard. Microstructure characterization and surface analysis were made using optical and scanning electron microscopy. The results showed that the wear resistance is determined by the size, shape, distribution and chemical composition of the carbides, as well as by the matrix microstructure. The best abrasion resistance was obtained in microstructures composed of eutectic matrix and primary M7 C3 or MC carbides, while the higher mass losses were measured in completely eutectic deposits. The main wear mechanisms observed at the surfaces included micro-cutting of the matrix and brittle fracture of the carbides. © 2005 Published by Elsevier B.V. Keywords: Hardfacing alloys; Abrasion resistance; Microstructure characterization; Wear mechanisms

1. Introduction Hardfacing is a commonly employed method to improve surface properties of agricultural tools, components for mining operation, soil preparation equipments and others [1,2]. An alloy is homogeneously deposited onto the surface of a soft material (usually low or medium carbon steels) by welding, with the purpose of increasing hardness and wear resistance without significant loss in ductility and toughness of the substrate. A wide variety of hardfacing alloys is commercially available for protection against wear. Deposits with a microstructure composed by disperse carbides in austenite matrix are extensively used for abrasion applications [3] and are typically classified according to the expected hardness. Nevertheless, the abrasion resistance of a hardfacing alloy depends



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0043-1648/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.wear.2005.03.002

on many other factors such as the type, shape and distribution of hard phases, as well as the toughness and strain hardening behavior of the matrix [4]. Chromium-rich electrodes are widely used due to low cost and availability; however, more expensive tungsten or vanadium-rich alloys offer better performance due to a good combination of hardness and toughness. Complex carbides electrodes are also used; especially when abrasive wear is accompanied by other wear mechanisms [5]. Several welding techniques such as oxyacetylene gas welding (OAW), gas metal arc welding (GMAW), shielded metal arc welding (SMAW) and submerged arc welding (SAW) can be used for hardfacing. The most important differences among these techniques lie in the welding efficiency, the weld plate dilution and the manufacturing cost of welding consumables [6]. SMAW, for example, is commonly used due to the low cost of electrodes and easier application. The present investigation aims to study three commercial electrodes in terms of their chemical composition, microstructure, hardness and abrasive wear resistance.

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Table 1 Nominal chemical composition of ASTM A36 steel plates (wt%) C

Si

S

Cu

P

Fe

0.25

0.40

0.05

0.20

0.04

Balance

Fig. 2. Schematic of the dry sand–rubber wheel testing machine, ASTM G65 standard. Fig. 1. Schematic of welding layers deposition. Each layer is approximately 5 mm in thickness.

2.2. Hardness measurements The bulk hardness of the hardfacing deposits was measured by the Rockwell hardness (HRC) method, while a micro-hardness tester allowed measuring the hardness of the

2. Experimental procedure 2.1. Materials and welding conditions Three commercial hardfacing electrodes were applied onto ASTM A36 steel plates according to the manufacturer’s directions. 150 mm × 50 mm × 12 mm coupons were used in all cases. The nominal chemical composition of ASTM A36 steel can be seen in Tables 1 and 2 show the main features and composition of each electrode. The deposition was carried out in flat position, the current and travel speed were fixed in all the tests and no buffer layers were used. The welding process parameters for each electrode are shown in Table 3. After deposition, the samples were cooled in air. Fig. 1 shows the location of the different welding layers. Only one layer of hardfacing 2 was deposited to avoid cracking due to excessive internal stresses.

Fig. 3. Morphology of quartz particles used in abrasion tests.

Table 2 Expected chemical composition of the hardfacing deposits Nominal chemical composition (wt%)

Hardfacing 1 (Cr-rich) Hardfacing 2 (W-rich) Hardfacing 3 (complex carbides)

C

Cr

W

Mn

Nb

Mo

Si

V

Fe

4.3 4.5 4.2

35 – 23

– 26 3.5

1.1 2.1 –

– – 5.4

– – 4.1

– – 1.5

– – 0.8

Balance Balance Balance

Table 3 SMAW process parameters for hardfacing deposition Electrode

Current (A)

Voltage (V)

Travel speed (mm min−1 )

Number of layers

Hardfacing 1 (Cr-rich)

130

20–23

190–200

1 2

Hardfacing 2 (W-rich)

175

20–23

180–200

1

Hardfacing 3 (complex carbides)

200

25–30

180–200

1 2 3

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phases in the microstructure by using a Vickers indenter with a load of 0.3 kgf.

Table 4 Abrasive wear test conditions, dry sand–rubber wheel testing machine Procedure

Load (N)

Velocity (rpm)

Wear distance (m)

2.3. Microstructure analysis

A

130

200

4309

Optical (OM) and scanning electron microscopes (SEM) were used to analyze the microstructure of the specimens. Secondary electron imaging allowed morphologic description of the worn surfaces, while backscattered electron imag-

ing and EDS compositional maps were used to qualitatively describe chemical variations in the microstructure. Cross sections of the weld were polished and etched with Kalling’s (hardfacings 2 and 3) and Nital 2% (hardfacing 1). Different

Fig. 4. Hardfacing microstructures: (a) Cr-rich, first layer; (b) Cr-rich, second layer; (c) W-rich, first layer; (d) complex carbides, first layer; (e) complex carbides, second layer; (f) complex carbides, third layer. Images (a), (b), (d) and (e) were acquired using optical microscope (OM) and images (c) and (f) were acquired using scanning electron microscope (SEM).

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Fig. 5. EDS compositional map for Cr-rich hardfacing. Cr-rich carbides are revealed.

212 and 300 ␮m were used (Fig. 3). The normal load and duration of the tests are shown in Table 4 for each hardfacing tested. Before the tests, all the specimens were cleaned in ultrasonic bath and rinsed with warm air. The abrasive wear resistance was determined from the mass loss results, which were measured with 0.01 mg resolution, converted to volume losses and corrected by considering changes in the diameter of the rubber wheel.

types of carbides present in the microstructures were first identified on the basis of their morphologies and confirmed by micro-hardness measurements. 2.4. Abrasive wear tests Abrasive wear tests were carried out in a dry sand–rubber wheel testing machine (Fig. 2) according to ASTM G65 standard. Rounded quartz particles with mean diameter between Table 5 General results from dry sand–rubber wheel abrasion tests Hardfacing

Cr-rich, 1st layer

Cr-rich, 2nd layer

W-rich, 1st layer

Complex carbides, 1st layer

Complex carbides, 2nd layer

Complex carbides, 3rd layer

Mass loss (mg) Volume lossa (mm3 ) Abrasive wear resistanceb (mg m−1 )−1 Abrasive wear resistanceb (mm3 m−1 )−1

292.38 37.5 14.7 114.9

151.9 19.5 28.4 221.3

177.9 22.8 24.2 188.9

385.4 49.4 11.2 87.2

278.5 35.7 15.5 120.7

147.1 18.8 32.3 228.5

a b

Average density estimated: 7.8 g cm−3 . Defined as volume loss/sliding distance.

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3. Results and discussion 3.1. Microstructure The typical microstructure of the studied hardfacing alloys is shown in Fig. 4. The first and second layers of Cr-rich deposits have a eutectic matrix with proeutectic M7 C3 -type chromium carbides (1800 HV micro-hardness), which were identified by EDS-SEM due to their large size. In the second layer, both the volume fraction and the mean size of chromium carbides are higher than in the first one. The volume fraction of chromium carbides in the second layer, calculated by using an empirical relation from literature [7], is circa 52%. The microstructure of the W-rich deposit is composed by proeutectic MC-type carbides (2500 HV micro-hardness) surrounded by the eutectic structure containing M6 C-type (fishbone-type) carbides (1600 HV micro-hardness) and some martensite.

The first layer of the complex carbides deposit is mainly eutectic with finely dispersed (Nb, Mo)-rich hard particles, while the second and third deposit layers have a similar microstructure which includes chromium-rich M7 C3 , Nb-rich MC, Mo-rich M2 C and W-rich WC carbides. Figs. 5–7 show EDS compositional maps of the deposits, in which it is possible to identify the carbide types present in the microstructure. In Fig. 8, localized spectra are taken to identify M2 C (Mo-rich), M7 C3 (Cr-rich) and MC (Nb-rich) carbides. 3.2. Abrasive wear resistance and mass removal mechanisms Table 5 and Fig. 9 present the general mass loss and abrasive wear resistance results from the dry sand–rubber wheel tests. The best abrasion resistance was obtained in the third layer of the complex carbides deposit, in

Fig. 6. EDS compositional map for W-rich hardfacing. W6 C-type carbides can be observed.

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which the elevated volume fraction of coarse M7 C3 carbides provided a barrier against indentation, grooving and cutting. This beneficial effect is probably reinforced by the NbC particles, which prevent the detachment of M7 C3 carbides due to their finely dispersed distribution in the matrix and their mechanical properties as well, as shown by Roda V´azquez et al. [3] and Chen and Chang [8]. Another important factor in abrasion resistance is carbide orientation. Carbides in the second layer of Cr-rich and third layer of complex carbides hardfacings have no especial orientation (Fig. 10a), while the first layer of both deposits showed carbides elongated in direction normal to the interface between the hardfacing and the substrate (Fig. 10b). As a consequence of their hardness and ability to deform plastically up to a certain extent, the M6 C tungsten carbides (fishbone-type) also contributed to prevent the cutting effect of abrasive particles in the W-rich deposit, as can be seen in Fig. 11a. The carbide sinks in the ma-

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trix, with no evidence of brittle failure. The “stopper” action of M7 C3 carbides is shown in Fig. 11b, in which an abrasive particle was blocked after it was cutting the matrix. Fig. 12 shows the typical aspect of the worn surfaces. Micro-ploughing and micro-cutting were the main abrasive micro-mechanisms observed in Cr-rich hardfacing alloys. In these materials, lips and prows caused by microplugging eventually covered previous welding cracks, as can be seen in Fig. 12b. Tungsten carbides in W-rich deposits had different response as a function of their shape and size: M6 C carbides absorbed high amounts of plastic deformation as shown in Fig. 11a, while MC carbides were broken by the abrasive particles (Fig. 12c). In complex carbides, hardfacing alloys intense micro-cutting was observed when only one layer was applied, due to the absence of massive hard second phases in the microstructure (Fig. 12d). In specimens with two layers brittle fracture of NbC carbides was observed, together with minor crack formation in M7 C3 phase (Fig. 12e). When three layers were

Fig. 7. EDS compositional map for complex carbides hardfacing. Carbides rich in different elements can be identified.

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Fig. 8. EDS spectra showing the different carbides in the microstructure of the complex carbides deposit.

Fig. 9. Abrasive wear resistance of the studied hardfacing alloys.

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Fig. 10. (a) Distribution of chromium carbides in the second layer of hardfacing 1, with respect to the sliding direction, (b) preferential orientation of chromium carbides near the interface between the first welding layer and the substrate.

Fig. 11. (a) Plastic deformation of type-M6 C (fishbone) carbides in W-rich deposit, (b) “Stopper” effect of M7 C3 carbides in Cr-rich deposit.

deposited the increase in abrasion resistance was considerable, and virtually no continuous grooves were observed (Fig. 12f). 3.3. Relation between hardness and abrasion resistance of the deposits The average hardness of the hardfacing alloys is shown in Table 6. The lowest values were obtained in samples with eutectic microstructures, such as the first layer of the complex

carbides and Cr-rich deposits. In these samples the dilution with the substrate was responsible for a significant reduction in the chromium content of the deposit, which prevented the formation of proeutectic M7 C3 carbides. On the other hand, the third layer of the complex carbides hardfacing showed the highest hardness, although in a general way the relative differences between harder and softer deposits were not more than 10%. Fig. 13 shows that a relation between the hardness and abrasion resistance could be proposed for the whole group of tested materials.

Table 6 Hardness of the applied deposits

Hardness (HRC) Sample 1 Sample 2 Sample 3 Average

Hardfacing 1 (Cr-rich)

Hardfacing 2 (W-rich)

Hardfacing 3 (complex carbides)

1st layer

2nd layer

1st layer

1st layer

2nd layer

3rd layer

55.7 55.4 –

57.0 58.0 –

58.6 58.0 58.3

51.8 55.0 56.9

56.8 60.1 59.9

60.5 59.5 60.5

55.6

57.5

58.3

54.6

58.9

60.2

60

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Fig. 12. Typical aspect of worn surfaces. (a and b) Hardfacing 1; (c) hardfacing 2; (d, e and f) hardfacing 3.

4. Conclusions

Fig. 13. Relation between hardness and abrasion resistance of the tested hardfacing alloys.

Three-layer complex carbide deposits showed the best abrasive wear resistance of all the tested hardfacing alloys. Nevertheless, when only one layer was deposited the high dilution levels changed the microstructure and strongly reduced the wear resistance. W-rich hardfacing alloys showed a very good abrasive wear resistance with only one layer, due to their unique combination of tough M6 C and hard, massive MC carbides in a eutectic matrix. M7 C3 carbides played a crucial role in the abrasive wear resistance of all the deposits studied, since they act as effective barriers to cutting and ploughing by abrasive particles.

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The main mass removal mechanisms identified after examination of the worn surfaces were micro-cutting, ploughing and brittle fracture of carbides. Plastic deformation was also significant, especially in eutectic microstructures. Acknowledgement The authors thank COLCIENCIAS for financial support, contract N. 484-2003. References [1] P. Crook, Friction and wear of hardfacing alloys, in: ASM Handbook, Friction, Lubrication and Wear Technology, vol. 18 (1992) 758–765.

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[2] I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Cambridge, 1992, p. 133–171. [3] C. Roda V´azquez, A. Loureiro, J. Pita Cribeiro, Comportamiento frente al desgaste abrasivo de las aleaciones con tendencia a la formaci´on de carburos aplicados por soldadura, Mantenimiento 134 (2000) 78–89. [4] S. Chatterjee, T.K. Pal, Wear behavior of hardfacing deposits on cast iron, Wear 255 (2003) 417–425. [5] S.-H. Choo, C.K. Kim, K. Euh, S. Lee, J.-Y. Jung, S. Ahn, Correlation of microstructure with the wear resistance and fracture toughness of hardfacing alloys reinforced with complex carbides, Metall. Mater. Trans. A 31A (2000) 3041–3052. [6] W. Wo, L.-T. Wu, The wear behavior between hardfacing materials, Metall. Mater. Trans. A 27A (1996) 3639–3648. [7] F. Maratray, S. Bechet, Abrasion-Resistant High Chromium White Cast Irons, Climax Molybdenum SA Paris Publication, 1971. [8] H.-X. Chen, Z.C. Chang, Effect of niobium on wear resistance of 14% Cr with cast iron, Wear 166 (1993) 197–201.