Erosive and abrasive wear resistance of overlay coatings

Vacuum 83 (2009) 166–170

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Erosive and abrasive wear resistance of overlay coatings Tadeusz Hejwowski* Chair of Materials Engineering, Lublin University of Technology, 36 Nadbystrzycka Street, 20-618 Lublin, Poland

a b s t r a c t Keywords: Erosion Abrasion PTA TIG Fuseweld coating Microstructure

In the paper, the erosion and abrasion resistance of PTA, TIG and flame deposited coatings was investigated. Hardness of coatings has almost no effect on erosion resistance and incubation period. Microstructure of coatings has significant effect on erosive wear of coatings. No significant correlation was found between results of abrasive and erosive tests. Statistically significant correlation was found between erosive wear intensities determined in tests carried out at similar angles, the total content of B and C correlates with mass loss in abrasion test and erosive wear intensity at normal incidence. Laboratory tests with model abrasives cannot be used as a guide for material selection in industry. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Overlay coatings are mostly used to change an existing wear situation from a destructive to a permissible type. Correct selection of wear resistant materials can cut downtime and considerably reduce maintenance costs. Wear should be regarded as a system property as its intensity depends on environmental conditions, applied materials and operating conditions. Numerous industrial components are subject to erosive or abrasive wear. The examples are the blades and casings of fans transporting gases containing solid particles. These fans are used, for instance, to provide gas flow in a rotary kiln used for cement production. Another example are also the blades of carbon dust blow-in fan used to feed a kiln burner with fuel, the service lives of which, according to maintenance reports, are about six months [1]. The wear of blades is caused by fine particles, whose size is contained in a wide range. Earlier trials showed that heat treatment of blade steel has only marginal effect on erosion resistance and that it is justifiable to use steels of higher hardness in a normalized condition. Another potential method of increasing durability of components subject to this type of wear is an application of hardfacings to protect the surface. A number of alloys are used for this purpose and the most important fall into three categories: Co-based, Ni-based or Fe-based. Hardfacing with Co–Cr–W–C or Ni–Cr–Si–B satisfies most of requirements specified by the user. Highly alloyed materials containing large volume fractions of carbides or borides are not necessarily the most wear resistant materials, more durable microstructure contains evenly distributed fine carbides. The brittle and continuous carbide network is considered undesirable as it provides paths of easy crack propagation throughout the structure, small, relative to abrasive grit size, hard * Fax: þ48 81 525 08 08. E-mail address: [email protected] 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.03.029

particles are easily dug-out from the microstructure, whereas large carbides provide good abrasion resistance. The following features of the microstructure have major impact on wear resistance of overlay coatings: (i) the size, shape and distribution of hard phase precipitates, (ii) the volume content of the hard phase, (iii) the hardness of microstructural constituents, properties and texture. Although a large number of papers have been published on abrasive wear of weld overlay coatings, only few published investigations dealt with erosive wear [2,3]. The data on wear resistance of coatings are usually generated in laboratory tests using standard abrasives and test rigs. In industry, such data are frequently of questionable applicability as laboratory tests usually oversimplify working conditions of industrial components. The aim of this paper is to determine erosive and abrasive wear resistance of coatings deposited by means of different methods, to find the effect of some test variables on wear resistance of materials and, finally, to explore the effect of microstructure and chemistry of overlay coatings on their behavior. 2. Experimental procedure There are a number of methods used to produce hardfacings, in the present study, to deposit overlay coatings, the PTA, fuseweld and TIG methods were applied. Coatings were produced on flat mild steel coupons by means of the SPT 100 fusewelder oxyacetylene powder torch, commercial TIG device and PTA deposition system of NP 1-250 type, produced by Institute of Welding, Poland. Table 1 lists PTA weld deposition parameters and Table 2 presents nominal chemical compositions of materials used to perform coating. Microhardness measurements were performed under the load of 40 gf by means of Hanemann’s hardness tester mounted on the Neophot 2 optical microscope. Supplementary measurements were done by means of nano- and microhardness tester Micron Gamma.

T. Hejwowski / Vacuum 83 (2009) 166–170 Table 1 PTA deposition parameters

167

Table 3 Results of microhardness measurements

Arc current Main argon flow Auxiliary argon flow Powder feed rate Torch travel speed Swing range of torch Swing frequency

80 A 3 l/min 2.5 l/min 3 cm3/min 1 mm/s 20 mm 1 1/s

Volume fractions of some microstructural constituents and morphology of precipitates were estimated by means of the qualitative metallography method using the Microscan 1.5 and Image J software. Wear scars were examined by means of SEM. Test plates were sectioned into 30  30 mm specimens used in both erosion and abrasion test. The erosion test was done by means of the sand-blast type apparatus. Pressurized air was applied as the carrier gas. The abrasive used in erosion and abrasion tests was crushed silica with grain size below 0.1 mm and grade 120 alumina. The abrasive was fed to the jet by means of vibratory feeder. The particle velocity was measured by means of a double disc method [4]. The angles of incidence, measured between the particle velocity vector and the specimen surface were 30 , 50 , 70 and 90 . Erosive wear was assessed by mass loss measurements. Prior to weighing with 0.1 mg accuracy, specimens were blasted with clean air to remove any residue from the surface and washed in an ultrasonic bath. In the abrasion test, the tester of rubber-wheel type was applied. The counter-specimen was in the form of 50 mm diameter roll and was pushed to the specimen surface with the force of 44 N. The roll performed 1800 revolutions during the test, the control specimen was made of normalized grade 45 steel. 3. Results and discussion Majority of performed coatings belong to the Ni-base family, their structure consists of Ni-solid solution forming a dendritic Table 2 Chemical compositions of coatings No.

Coating

Chemical composition

Hardness HV30

Deposition method

1

Colmonoy 237

379

Flame

2

Buildup 22

289.5

Flame

3

Deloro alloy 60

795

Flame

4

Deloro alloy 50

734

Flame

5

Deloro alloy 40

618

Flame

6

Deloro alloy 56

796

Flame

7

Stellite grade 6

391.5

TIG

8

El-Co 1

529

TIG

9

Deloro Alloy 35

329.5

Flame

10

Colmonoy 43

348

Flame

11

PMCo45

618

PTA

12

PMNi30

223

PTA

13

PMFeCr-60/P

496

PTA

14

PMNiCr55P

B ¼ 1.3%, Cr ¼ 4%, Si ¼ 2.8%, others – 5.15%, Ni-bal. C ¼ 0.1%, B ¼ 1.25%, Si ¼ 3.15%, Fe ¼ 0.75%, Ni-bal. C ¼ 0.9%, Si ¼ 4.3%, B ¼ 3.3%, Fe ¼ 4.2%, Cr ¼ 16.3%, Ni-bal. C ¼ 0.45%, Si ¼ 3.9%, B ¼ 2.3%, Fe ¼ 2.9%, Cr ¼ 11%, Ni-bal. C ¼ 0.25%, Si ¼ 3.5%, B ¼ 1.8%, Fe ¼ 2.5%, Cr ¼ 7.5%, Ni-bal. C ¼ 0.6%, Si ¼ 4.2%, B ¼ 3.6%, Fe ¼ 3%, Cr ¼ 16.2%, Mo ¼ 2.6%, Cu ¼ 2.6%, Ni-bal. C ¼ 1.2%, Si ¼ 1.2%, Ni ¼ 3%, Mo ¼ 0.5%, Fe ¼ 3.0%, Cr ¼ 28%, W ¼ 4%, Co-bal. C ¼ 2.33%, Si ¼ 1.16%, Cr ¼ 30.6%, Ni ¼ 2.73%, W ¼ 12.2%, Fe ¼ 2.71%, Co-bal. C ¼ 0.17%, Si ¼ 3.1%, B ¼ 1.5%, Fe ¼ 1.6%, Cr ¼ 4.7%, Ni-bal. C ¼ 0.4%, Cr ¼ 10%, B ¼ 2.1%, Si ¼ 2.3%, Fe ¼ 3%, Ni-bal. C ¼ 0.7%, B ¼ 0.8%, Si ¼ 1.3%, Cr ¼ 30%, W ¼ 4%, Co-bal. C ¼ 0.1%, B ¼ 1.4%, Si ¼ 3%, Fe ¼ 1.5%, Ni-bal. C þ B þ Si ¼ 7%, Cr ¼ 12%, Fe ¼ 80% C ¼ 0.4%, B ¼ 2.6%, Cr ¼ 12%, Fe ¼ 2.6%, Ni-bal.

618

PTA

No

Coating

Microhardness Solid solution, eutectics

Volume fraction Carbides, borides

1

Colmonoy 237

321–629

47% – solid solution 41.9% – solid solution

2

Buildup 22

253–424, 1110–1285

3 4 5

Deloro alloy 60 Deloro alloy 50 Deloro alloy 40

629–713 713–822, 925–1119 331–571, 983–1904

1611–3863 1490–2285 4100

6 7 8 9

Deloro alloy 56 Stellite grade 6 El-Co 1 Deloro alloy 35

966–1048, 1611–1904 822–925 822–983, 871–1285 280–629, 1119–1198

2518–4100

10

Colmonoy 43

279–437, 1119

2285–4100

11 12 13

PMCo45 PMNi30 PMFeCr-60/P

629–983 333–359, 498–850 882,1285

1644

14

PMNiCr55P

713–967, 1611

2174–3935

38% – solid solution 11% – carbides

1904–4100

50.9% – solid solution 43.11% – solid solution 29.1% – eutectics 31.9% – primary carbides

pattern whose hardness depends on amount of C, Si and Fe atoms dissolved, interdendritic carbides, borides and silicides, various boride/silicide/Ni phase eutectics, namely: Ni/Ni3B, Ni/NiSi, Ni/CrB binary eutectics. Cr forms hard phases – (Fe,Cr)7(C,B)3 type carbides and (Fe,Cr)(B,C) borides. These hard precipitates act as obstacles hindering sliding of abrasive grains during wear. B and Si not only provide self fluxing characteristics to the deposit, but also create low melting point eutectic phases; these secure adequate coherency between carbides, borides and the matrix. Among the produced coatings, these flame deposited had the highest number of pores and inclusions. Table 3 presents results of microhardness measurements and volume fractions of some microstructural constituents. Microstructure of coatings Nos. 1, 2, 5, 9, 13 and 20 contain coarse and easy to differentiate, in optical microscopy examinations, precipitates of solid solution, eutectics or primary carbides. Coating No. 1 contains elongated grains of Nisolid solution forming a dendritic pattern, the diameter of these precipitates is in the range of 11–100 mm, the eutectic with the average grain size of constituents of about 2 mm. Microstructure of

Fig. 1. Microstructure of No. 14 coating, etched with HCl–HNO3, magn. 200.

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Fig. 4. Coating No. 6 eroded with silica, magn. 200. Fig. 2. Microstructure of No. 13 coating, etched with HCl–HNO3, magn. 200.

No. 2 coating is composed of grains of Ni-solid solution with average grain size of 20 mm and an eutectic. Microstructure of No. 3 coating is finer compared with the former ones, no paths of easy crack propagation is seen, precipitates of Ni primary and secondary solid solution have the average size of about 16 mm, carbide and boride size is about 2 mm. Microstructure of No. 4 coating is similar to that of No. 3 coating. Co-based coatings revealed dendritic structure and contained precipitates of Co-solid solution of f.c.c. structure and interdendritically placed fine carbides. Coating No. 13 contains large, elongated precipitates of M7C3 carbides whose size exceeds even 150 mm. Microstructures of coatings No. 14 and No. 13 are shown in Figs. 1 and 2, respectively. It is acknowledged that angular abrasive grains are effective in removing material being abraded or eroded. Silica hardness lies in the range of 900–1100 HV and this abrasive can be considered soft relative to hard precipitates. As seen from Table 3, majority of coatings contains hard precipitates whose hardness exceeds that of silica. Silica was not classified after crushing, which means that this abrasive contains grains of various size and angular shape. Relatively soft erodents cause less erosion damage compared to hard abrasives and it is rationalized in terms of fragmentation and crushing of abrasive grains during erosion upon impact on hard

Fig. 3. Coating No. 6 eroded with alumina, magn. 200.

precipitate. Crushed fragments are usually more angular, which changes their efficiency as erodents. Crushed fragments on impact slide outward from the collision point causing only little wear. The mechanism of erosive wear of hard precipitates is a formation of saucer-shaped lateral cracks formed on unloading after impact. The pattern of cracks formed during erosion of brittle material resembles that formed in quasistatic indentation with diamond indenter. In case of soft abrasives, an incubation period is necessary, during which damage of hard precipitates is accumulated and this stage ends when crack forms. Large precipitates are liable to cracking as they can contain flaws. Small, hard precipitates are easily removed from the structure of material. Crushed silica is an interesting abrasive for erosion test because of its grain size distribution, these grains effectively penetrate intercarbide spaces removing matrix, which finally causes a removal of carbide. Due to grain size distribution it should not create a pattern of preferential erosion where some precipitates remain intact and protrude from the surface. In the erosion tests, the curves of cumulative mass loss versus mass of abrasive spent in the test were drawn. It was found that in test with silica incubation period lasts not more than 0.5 g of abrasive mass compared to even 15 g of alumina abrasive mass which must be spent to commence steady state erosion state of coating No 5. Coating No. 6 revealed a zig-zag curve, it was found

Fig. 5. Coating No. 1 eroded with silica, magn. 200.

T. Hejwowski / Vacuum 83 (2009) 166–170

169

Fig. 8. Erosion resistance versus hardness of coatings for test with alumina, þ 30 , 50 ,  70 , B 90 . Fig. 6. Erosive wear intensity for different impact angles, the leftmost bar corresponds to 30 , the rightmost to 90 , alumina.

that during erosion recesses are formed in the surface which can be filled up with abrasive, which causes obviously mass gain of the specimen and protects from further impacts of abrasive grains. Laboratory erosion tests are almost always carried out with model abrasives like corundum of chosen grade, crushed silica seems, however, similar to some dusts find in industry. Results of SEM examinations of surfaces eroded at normal incidence and particle velocity of 75 m/s, are shown in Figs. 3–5. Fig. 3 shows the surface of coating No. 6 eroded with alumina, the selective erosion is seen. Brittle cracking of coating material and also plastic deformation of coating material are seen. Fig. 4 shows the same coating after erosion with silica, the observed structure is finer

Fig. 7. Erosive wear intensity for different impact angles, the leftmost bar corresponds to 30 , the rightmost to 90 , silica.

compared to the former case, some dimples in the surface are also seen. Fig. 5 shows coating No. 1 after erosion with silica, no dimples are seen. Figs. 6 and 7 present the dependence of erosion rate on the angle of impact, it is seen that most of materials behave unlike brittle material with the maximum of erosion intensity at 90 . Although alumina (corundum) particles have higher hardness and larger size than silica grains, erosive wear intensities are for these abrasives comparable. Angular silica grains effectively removed coating material. Rankings of coatings depended of the abrasive used. Fig. 8 shows the dependence of erosion intensity on coating hardness at a number of angles for test with alumina (75 m/s). Similar pattern is found in tests with silica, Fig. 9. It is seen that for a given impact angle there is no optimum hardness which would secure erosion resistance.

Fig. 9. Erosion resistance versus hardness of coatings for test with silica, þ 30 , - 50 ,  70 , B 90 .

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T. Hejwowski / Vacuum 83 (2009) 166–170

used in the test were angular, which intensified wear, and some of silica grains were small enough to penetrate between constituents of fine eutectics. Results of wear test with silica abrasive are depicted on Fig. 10. Wear resistance of multiphase materials can be described by various rules of mixtures which take into account volume fractions of phases and their wear resistances [5,7], in the studied case, precipitates of hard phase are frequently pulled and dug-out, intense wear of neighbouring matrix makes them protruding from the surface and vulnerable to cracking. Cr atoms are present in a solid solution, which cause a strengthening effect, and in carbides and borides. Boron forms only borides or substitutes carbon in carbides, it is interesting whether there is a correlation between Cr or total C þ B content and wear test results. To analyze the obtained results, the Statistica 7 package was used. Statistically significant simple linear correlations were found at p < 0.05:

Fig. 10. Relative mass loss of coating material versus hardness for silica abrasive.

The velocity exponents in the power equation for erosion intensity were calculated for particle velocities contained in the range from 65 to 82 m/s. The derived values were about 4, i.e. higher than theoretically predicted for either plastic or brittle materials and higher than majority of published data [5]. It is presumably due to the inherent inaccuracy of the double disc method, however, in case of such heterogeneous materials as those studied further departure from actually used test conditions could change the operating erosion mechanism. Erosive wear intensities of normalized grade 45 steel, for normal incidence, was 0.2644 mg/ g for corundum and 0.5157 mg/g for silica, whereas for oblique impact intensities were 0.35248 mg/g and 0.598 mg/g, respectively. Coatings No. 5 and 7 which contain solid solution grains of relatively high hardness and a moderate size with fine carbides distributed interdendritically outperformed remaining coatings. It was found in the test carried out with ashes collected from coal fired industrial boiler that durability of coatings studied in this paper was significantly longer than that of mild steel [6]. In abrasion, mass loss experienced by material is due to microcutting, microploughing, microfatigue when material ploughed aside repeatedly ultimately breaks off by low cycle fatigue mechanism and microcracking when stresses imposed by abrasive grit on brittle surface are sufficient [7,8]. The wear situation studied in the paper can be referred to as three-body low stress abrasion. The chemistry and microstructure of studied materials was optimized on the basis of tests carried out in accordance with ASTM Standard G65 which models wear of groundworking machines. In that test, rounded quartz sands of large size in the range 212–300 mm are used. In case of studied coatings, rounded abrasive grains are larger than characteristic dimension of microstructural constituents and grains could roll over the coating surface causing very little wear. Material can be detached by microcutting when the attack angle of the abrasive is greater than a critical value. Abrasives

– for corundum abrasive, erosive wear intensities at 50 and 70 (coefficient of determination r2 ¼ 0.4874); the content of C þ B and resistances to abrasion and erosion intensity at 90 , – for silica abrasive, erosive wear intensities at 70 and 90 (r2 ¼ 0.6941), the hardness and abrasive wear intensity; the content of C þ B and abrasive wear resistance, erosive wear intensities at 70 , 90 .

4. Conclusions The obtained results can be summarized as follows: (1) Microstructure of coatings has significant effect on erosive wear of coatings. No significant correlation was found between results of abrasive and erosive tests. Statistically significant correlation was found between erosive wear intensities determined in tests carried out at similar angles. (2) The total content of boron and carbon correlates with mass loss in abrasion test and erosive wear intensity at normal incidence. (3) Hardness of coatings does not correlate with erosive wear resistance. Laboratory tests with model abrasives cannot be used as a guide for material selection in industry.

Acknowledgement Financial support of Ministry of Science and Higher Education in years 2006–2008, grant PB 1056/T02/2006/30, is acknowledged. References [1] Hejwowski T, Weron´ski A. J Mater Process Technol 1995;54:144–8. [2] Hejwowski T, Szewczyk S, Weron´ski A. J Mater Process Technol 2000;106: 54–67. [3] Levin BF, DuPont JN, Murder AR. Wear 1995;181–183:810–20. [4] Ruff AW, Ives LK. Wear 1975;35:195–9. [5] Hejwowski T. Treatise on wear and thermal fatigue of machine components and fabrication of structures with advantageous properties. Lublin University of Technology Press; 2003. [6] Hejwowski T. Weld J 2007;10:60–3 [in Polish]. [7] Zum Gahr K-H. Microstructure and wear of materials. Elsevier; 1987. [8] Kotecki DJ, Ogborn JS. Weld Res Suppl 1995:269–78.