Abrasive wear resistance of thermal surfacing materials for soil tillage applications

Senad Dizdar* and Barbara Maroli,

Höganäs AB, Global Development, Höganäs, Sweden *E-mail: senad.dizdar@hoganas.com

Abstract

A number of common thermal surfacing materials for soil till-age applications were ranked for abrasive wear by dry–sand–rubber–wheel testing according to ASTM G65. Focus was on plasma transferred arc (PTA) and powder welding (PW) deposi-tion consisting of a nickel based self-fluxing matrix with and without additions of 50 or 60% tungsten carbide. For compari-son PTA coated M2 tool steel and the quenched and tempered wrought spring steels EN C75S and EN30MnB5 were also test-ed. PTA and PW deposition techniques produced coatings with a similar level of abrasive wear resistance. Hardfacing with M2 and a 60 HRC nickel based self-fluxing grade showed ~30% and ~15% wear respective compared to the reference steels while the grades with additions of 50% carbide phase showed ~5% wear compared to the reference steels. By increasing the amount of tungsten carbide from 50 to 60 wt% abrasive wear resistance was increased by 25%.

Introduction

Sustainable agriculture demands cost effective farming opera-tions where fuel and tooling costs are of major concern. Tooling costs include not only the cost of tillage parts such as plough-shares but also production time loss due to unnecessary stops for tool change or the inability to reach a target tillage operation at the proper time point. A substantial increase of tillage tool life is gained by applying hard coatings to the wear exposed tool regions. Investigations performed in Turkey (Bayhan Ref. 1) illustrate tillage tolling costs for the area of cultivated land in Turkey of 18.5·106 ha for one year. For example, the average tool weight loss for chisel ploughshares was 23 to 40 g/ha, for ploughshares 90 to 210 g/ha, for cultivator sweeps 60 to 120 g/ha and for har-row tines 30 to 70 g/ha. The total amount of wear was estimated to 5365 tons corresponding to a total loss of ~ 4.4 million USD.

This investigation ranks the abrasive wear resistance of some common wrought Q&T (quenched and tempered) alloys and MMC coatings containing tungsten carbide embedded in a nick-el based matrix, according to ASTM G65. Fused and crushed tungsten carbides (FTC) as well as recycled WC-7Co carbides were investigated. The focus is on powder mixes deposited by plasma transferred arc (PTA) and powder welding (PW)/puddle torch.

Abrasive wear testing of soil tillage tools

Abrasive wear Soil tillage tools are individual soil engaging elements often in the form of a blade, a wing or a shank. They are exposed to low–stress abrasive wear when the sand in the soil sweeps the tool surface during a soil tilling operation but the sand itself will not break down (Moore Ref. 2 and Horvat Ref. 3). To illustrate tribological loading of the soil tillage tools, it may be advised to start with Holm-Archard wear equation (Jacobson et al. Ref. 4) (Eq. 1) �

�= �

��

� (Eq. 1)

where V stands for the worn volume (mm3), S the sliding dis-tance (mm), K the wear coefficient (dimensionless), Fn the nor-mal load and H the hardness (MPa). The Eq.1 was primarily an adhesive wear equation but it was found to be valid even for abrasive wear (Jacobson et al. Ref. 4), with the wear coefficient K described as in Eq. 2:

� = � ∙ �� (Eq. 2)

where kR stands for the removal factor and kF the form factor. The former is zero for micro-ploughing, one for micro-cutting and exceeds one for micro cracking. The latter shows sharpness of the abradant and is a ratio of the scratching cut area and the load bearing area. As seen in Eq. 1, wear of a tillage tool is proportional to load and sliding distance but inversely proportional to hardness. Ap-plied to field farming operations it may be seen as follows. A

© 2013 ASM International®. This paper was published in Proc. Int. Therm. Spray Conf., May 13-15, 2013, Busan, Republic of Korea, and is made available as an electronic re-print with the permission of ASM International®. One print or electronic copy may be made for personal use only. Systematic or multiple reproduc-tion, distribution to multiple locations via electronic or other means, duplications of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited.

common tractor draws the tillage implement with a speed of 10 to 15 km/h. The tractor pulling load is divided between the tools compacting, shearing and inverting of the soil. The sand parti-cles slide (and/or roll) against the tool surface with a sliding ve-locity corresponding 2.8 to 4.2 m/s. Energy intensive primary tillage operations, such as plowing of previously non–cultivated land or plowing after harvesting by a moldboard plow includes deeper soil preparation and exposes the plow to relatively higher loads and a relatively long period of time in contact with the soil. In contrast, less energy intensive secondary tillage opera-tions such as soil stirring by a harrow, after a primary tillage, is a shallower soil preparation and exposes the harrow teeth to moderate loads and a relatively short period of time in contact with the soil. Finally, soil is considered a solid medium in engineering appli-cations. Thus, soil is a mixture of sand, slit and clay, after re-moval of particles exceeding 2 mm (gravel and stones) by siev-ing and organic matter – humus (Figure 1) (see unified soil classification system, Ref. 5). The prevailing mineral in the earth or soil is quartz or silica sand, SiO2, with hardness Mohs 7 or between 750 and 1200 HV. Depending on the geographic lo-cation, the sand particles differ in chemical purity, size distribu-tion and morphology. The latter is considered through particle roundness, sphericity, aspect ratio and a parameter showing sharpness, or concavity of the particle peripheral surface (Pabst Ref. 6 ). The concavity was shown to describe abrasivity of the sand (Swanson Ref. 7). Silica sand, AFS 50/70, mined in Otta-wa, IL, USA, showed the highest abrasivity among a dozen mined or crushed silica and soil samples.

Figure 1: Soil considered for engineering applications (Ref. 5) Wear testing methods Abrasive wear testing is done on various levels (Swanson Ref. 1). The most realistic and expensive level is of course field test-ing. Soil bin testing level offers a lower testing cost, includes real tillage tools and a soil complexity but puts strong demands

on the characterization of the soil or sand samples (Figure 1). Laboratory wear testing level by dry/wet-sand–rubber–wheel tribometers is a fast accelerated testing, relatively low cost but includes a high grade of abstraction and modeling where only one or a few single well-defined parameters can be tested. Per-sonnel highly skilled in tribology are also a demanded. All three levels of abrasive wear testing have their place and importance in the development of materials for tillage tools. The laboratory wear testing cannot give the answers generated in the field test-ing but can rank material wear resistance and narrow the num-ber of materials aimed for the soil bin and/or field testing. Laboratory wear testing performed here is DSRW testing ac-cording to ASTM G65 (Ref. 9). It is a de facto industry standard for ranking of low–stress abrasive wear resistance of engineer-ing materials. Tool material samples are exposed to abrasive ac-tion by the Ottawa silica sand brought into contact by a chloro-butyl rubber lined steel wheel, see Figure 2. The virtual sliding velocity is ~4.8 m/s which is quite close to sliding velocities in soil tillage operations. Test load is 130 N and sliding distance is 4309 m for procedure A – aimed for materials with high abra-sive wear resistance. Coating by thermal surfacing for high wear resistance Spring steels such as EN C75S or micro alloyed EN 30MnB5 steel in quenched and tempered, Q&T, condition are often a de-fault material for tillage tools. However, their abrasive wear re-sistance may not be sufficient for soils with relatively higher sand content and the abrasivity of primary tillage operations. Surface coating techniques such as tungsten inert gas welding (TIG), metal inert gas welding (MIG), laser cladding, plasma transferred arc (PTA) and powder welding (puddle torch) can be used to multiply the abrasion wear resistance of the tillage tools. The tool main body or substrate can be made of a weldable low carbon structural steel to withstand the stresses while the wear resistance is achieved by overlay welding of hard materials to abrasive wear exposed regions of the tool.

Figure 2: Sketch of the abrasive tribometers for dry sand rubber wheel wear testing according to ASTM-G65.

USDA particle sizes:

- Clay < 0.002 mm

- Silt 0.002-0.05 mm

- Sand 0.05-2 mm

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Silty clay

Silt

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Clay

Sandyclay loam Loam

(ideal garden soil)Sandy

loamLoamysand

90100

LoaLoasansansansansanSand

CIIR-lined wheel

(228,6 x 12.7 mm, 9”x 1/2”)

Sand hopper

Ottawa silica sand AFS 50/70

ASTM G65 DSRW testing, procedure A

Load arm

SpecimenTotal

6000 wheel rev.

(4309 m sl. dist.)

(130 N)

200 RPM Weights

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Table 1: Nominal composition and description of thermal surfacing powder grades (Höganäs Ref. 14) and wrought materials used. Designation Particle

range (µm) Composition (wt %)

Typical hardness C Co W Si B Fe Cr Ni Mn V

EN C75S (AISI 1075) NA 0.75 - - - Bal. - - <1 - 45 HRC EN 30MnB5 (*AISI 15B35H) NA 0.30 - - - 0.005 bal. - - 1.2 - 49 HRC M2 53-150 1.0 6.2 0.3 - bal. 4.0 0.3 2.0 63 HRC Surfit 1560 Surfit 1060

53-150 20-106

0.75 - - 4.3 3.1 3.7 14.8 bal. - - 62 HRC/ 810 HV30

1559-40 53-150 ≤0.06 - - 3.0 2.9 - - bal. - - 49 HRC PA2 45-106 5.7 7.5 bal. Recycled WC-7Co powder, angular form 2000 HV0.1 4070 4570

36-106 53-150

4 - bal. Fused and crushed W2C-WC powder, angular form

(FTC) 2300 HV0.1

*Closest match. Table 2: Description of the test samples and their manufacturing routes

Material composition Method Coat. thick.

(mm) Last forming or machin-

ing / heat treatment

Vol. % added carbide phase Hardness**

Powder mix Welded HV30 HRC EN C75S Wrought steel - Cold rolling, Q&T NA NA 429 45 EN 30MnB5 Wrought steel - Grinding, Q&T NA NA 609 49 M2 PTA 2 mm Air cooling, grinding NA NA 761 62 Surfit 1560 PTA 2 mm Air cooling, grinding NA NA 809 62 Surfit 1060+ 50wt% PA2 PW 1 mm Air cooling, grinding 37 41 996 66 Surfit 1060+ 50wt% 4070 PW 1 mm Air cooling, grinding 40 40 1033 66 1559-40+50wt% 4570 PTA 2 mm Annealing in vermiculite,

grinding 40 32 730 59

1559-40+60wt% 4570 PTA 2 mm 49 41 814 60 ** Values with 50% probability according lognormal statistic distribution

Experiment Powders Commercial powder mixes consisting of Ni- based self-fluxing alloys and different types and amount of tungsten carbide were selected for the investigation. Two different nickel based pow-ders were tested depending on the deposition technique used, one with a typical hardness of 62 HRC and one with a typical hardness of 49 HRC. Fused and crashed WC/W2C carbides (FTC) and recycled WC-7Co carbides were admixed to the nickel based alloys. Powder properties are summarized in Table 1 and Table 2. For comparison the following materials were tested: a PTA deposited tool steel, M2, a PTA coated nickel based self-fluxing grade with no additions of tungsten carbide and a typical hardness of 60HRC and the wrought spring steels EN C75S and EN 30MnB5. Thermal surfacing of the test specimen blanks The powder mixes were deposited on test specimens blanks made of “killed” structural mild carbon steel EN S235JRG2 (ASTM A570 Gr.36) by PW and PTA techniques. The PW specimen blanks, 50x60x10 mm plates, were overlay welded by using a Super Jet Eutalloy oxyacetylene torch. Prior to welding the specimen surface was blasted with alumina sand to facilitate bonding to the substrate and then it was covered with a thin layer of coating to minimize oxidation of the sub-strate during the subsequent pre-heating step. Pre-heating until the surface achieved a light red colour corresponding to ~600ºC, was carried out to ensure better bonding to the substrate materi-al and to reduce the risk for cracks. Cooling was done in air.

The PTA specimen blanks, 200x60x20 mm plates, were overlay welded by using a 200A Hettiger PTA unit. Prior to PTA coat-ing, the specimen blanks were blasted with alumina sand to re-move surface oxides and roughen the surface to ~6 µm in aver-age roughness, Ra, to enable better bonding to the substrate. The blanks were then pre-heated to 400ºC to reduce the risk for cracking. After overlay welding the samples were cooled in a bath containing vermiculite granules. Finally, the blanks were cut and ground to 25x58x10 mm blocks to fit the specimen holder in the abrasive tribometer. The overlay welded surface of the specimens was ground to average roughness Ra of 0.3 µm. Evaluation Five specimens were tested and average volume loss, AVL, was evaluated following ASTM G65. Then the five AVL data points were analyzed by lognormal probability plot statistic technique and AVL with 50% probability level estimated. The samples were examined with respect to hardness HV30, HRC and microstructure. Hardness was evaluated by doing be-tween 8 and 12 indentations on each specimen per test material composition. The hardness data was then analysed by using the lognormal probability plot statistic technique. The values shown are with 50% probability. Microstructure of the coatings was analyzed in a light optical microscope, LOM, and scanning elec-tron microscope, FEG-SEM equipped with a SDD-EDS detec-tor.

Results Results of the wear testing according to ASTM-G65 are shown in Figure 3. All coatings by grades containing carbide powder achieved average volume wear, AVL, between 5 and 9 mm3, in comparison with 31 mm3 for grade 1560, 58 mm3 for grade M2 and 200 and 189 mm3 respectively for the references EN C75S and EN 30MnB5. Wear loss was due to abrasion removal of the matrix followed by carbides when they were no longer sur-rounded by the matrix. For the reference steels, ploughing was evident, see Figure 4. The PW coatings with grades containing carbide powder achieved slightly lower wear in comparison to the PTA. This could be due to the lower temperature of the PW process com-pared to PTA, and/or the different matrices used in the PW and PTA coatings. The lower temperature of the powder welding process caused less dissolution of the carbide particles at their periphery. This is illustrated in Figure 4 showing that the FTC has smoother edges after PTA coating than after PW. Typical hardness of the matrix used in the PW coatings was 62 HRC while that used in the PTA coatings was 49 HRC. Abrasive wear is a function of the hardness only for similar types of microstructure i.e. matrix and hard phase addition. For PTA coatings with 1559-40 grade AVL decreased by about 10% when the carbide content increased from 50% to 60%.

Figure 3: ASTM G65 average volume loss for five specimens with 50% probability by lognormal statistic and hardness HV30 for the test samples.

a) 1559-40 + 50% 4570, PTA

b) Surfit 1060 + 50% 4070, PW

c) 30MnB5 Figure 4: SEM photographs of the worn track of the specimens as above. Overview of the coating or clad, as observed in a light optical microscope, is shown in Figure 5. The carbides were evenly dis-tributed in the samples used for wear testing. The PW samples contained slightly more pores than the PTA ones, due to the lower temperature of the process and more oxidizing environ-ment. When adding the recycled WC-7Co carbides somewhat higher porosity was observed. This is a known phenomenon as the recycled carbides are less pure than the WC/W2C FTC car-bides and therefore more prone to gassing.

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PTA

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Abrasive wear testing - ASTM G65, procedure A

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a)1559-40+4570, PTA b)1060+4070, PW c)1060+4070, PW Figure 5a, b, c: LOM micrographs showing an overview of the investigated coatings at 50% WC/W2C FTC

In the case of the PTA welded samples the metal matrix selected had no chromium and no carbon to avoid the formation of rela-tively soft chromium carbides and limit the risk for crack for-mation. The microstructure of the nickel based matrix, when welded without additions of primary carbides is shown in Figure 6 and consists of austenitic primary dendrites (γ), γ-nickel bo-ride eutectic and nickel boride-nickel silicide eutectic (Ref. 10). When tungsten carbides were added, the microstructure of the nickel based powder was modified. Due to the heat generated by the PTA welding process partial dissolution of the primary WC/W2C FTC took place (area 1, Figure 7a), followed by the re-precipitation of tungsten rich carbides (area 2, Figure 7a) mainly at the surface of the primary carbides. EDS analysis was used to estimate the chemistry of the interface area of the sec-ondary carbides and of the matrix. The results, presented in Ta-ble 3, showed that the secondary carbides (2) were rich in W but also contained Ni indicating it could be ƞ-W3Ni3C carbides. The matrix chosen for the PW coatings, shown in Figure 6, con-taining both chromium and carbon was harder, typically 62 HRC, than the one used for PTA hardfacing (typically HRC). The microstructure of the matrix was complex and consisted of primary austenitic dendrites, nickel borides, nickel silicides, chromium carbides and chromium borides (Ref. 10).

a) 1559-40, PTA b) Surfit 1060, PW Figure 6 a,b): LOM micrographs of the matrix etched in Ni-1.

Figure 7: SEM-BSE photos showing different degree of degra-dation of the tungsten carbides in a)1559-40+50%4570, PTA; b)Surfit1060+50%4070, PW; c)Surfit1060+50%PA2, PW

Similarly to the PTA welded samples partial dissolution of the primary WC/W2C FTC, occurred at their periphery, however due to the lower heat input of the PW process, when compared to PTA, the affected area was smaller as shown in Figure 7b. Re-precipitation of secondary carbides containing W took place at the border of the primary FTC carbides and in the matrix. Two types of W-containing carbides were observed, light grey (2) and black (4). EDS analysis of the light grey carbides showed they contained mainly W while the black ones (4) con-tained primarily Cr. The W-rich carbides were overall coarser than in the PTA welded samples, due to the lower cooling rate

of the powder welding process and the different chemistry of the matrix containing Cr and higher C and B content. When us-ing recycled WC-7Co carbides their periphery looked only mar-ginally affected. However, EDS analysis showed that both co-balt and tungsten were dissolved in the matrix and re-precipitated as light grey W-rich (2) and black Cr-rich second-ary carbides (Ref. 10). Overall the secondary carbides were less in number and smaller in size than in the samples admixed with the FTC carbides. Further EDS analysis showed the presence of Ni in the original WC-Co primary carbides. Table 3: Chemical composition of areas in Figure 7. Coating Area wt%

Si Cr Fe Co Ni W

1559-40 + 50% 4570 (FTC)

1-Interface - - - - 15 85 2-Grey carb - - - - 12 88 3-Matrix 1 2 - 81 16

Surfit 1060 +50% 4070(FTC)

1-Interface - 3 1 - 14 83 2-Grey carb 2 17 1 - 26 53 3-Matrix 4 3 3 - 89 0 4-Black carb 2 47 1 - 15 37

Surfit 1060 +50% PA2 (recycled.)

2- Grey carb - 12 1 5 14 67 3- Matrix 3 6 4 9 78 1 4-Black carb - 71 4 6 12 7 5-WC-7Co - 1 - - 7 92

Discussions

Tungsten carbide between 40 and 70 wt% are commonly added to Ni-based thermal surfacing alloys aimed to reduce abrasive wear on exposed components. This investigation showed ~25 % less wear, AVL 9 to 7 mm3, when tungsten carbide was in-creased from 50 to 60 wt%. Polak et al. (Ref. 12. R) investigat-ed abrasive wear of laser cladded Ni-based coatings with 40, 60 and 70 wt% carbides with different particle size. The results cannot be directly compared since the matrix alloy compositions were not listed, but samples with 40 wt% carbide achieved AVL of 17 mm3, in comparison to other samples which achieved AVL of 4.7 to 6.5 mm3. This supports an optimal carbide con-tent of 50 to 70 wt% and the wear loss absolute level in this in-vestigation.

Conclusions

Dry–sand–rubber–wheel abrasive wear testing according to ASTM G65 was performed for a selection of plate specimens made of reference wrought steels and coatings deposited by powder welding (PW) and plasma transferred arc (PTA). Wear expressed as average volume loss (AVL) was evaluated. Fol-lowing conclusions were made: • 200 respective 189 mm3 wear was achieved for specimens

made of the reference Q&T steels EN C75S and EN 30MnB5. • ~30% respective ~15% wear compared to nominal 100% ref-

erence for iron based M2 and nickel based 1560 coatings de-posited by PTA.

• ~5% wear compared to nominal 100% reference for PTA coated 1559-40 and PW Surfit 1060 with addition of 50% car-bide particles.

• By increasing the amount of carbides admixed to 1559-40 from 50 to 60 wt% abrasive wear decreased by ~25%.

• PW samples with carbide additions had a higher overall hard-ness; therefore a full comparison with the PTA coated samples cannot be done.

• PTA and PW deposition techniques produce coatings with similar abrasive wear resistance levels. PTA however appears to be an opportune choice for serial production and PW for maintenance on a smaller scale.

Acknowledgements

The authors wish to thank Mr. Lars-Åke Nilsson and Mrs. Patri-cia Jansson for their valuable comments in preparation of this manuscript.

References

1. Y. Bayhan, Reduction of wear via hardfacing of chisel ploughshare, Trib. Int. 39, 2006, p 570-574. 2. M.A. Moore, Abrasive wear by soil, Trib. Int. June, 1975, p 105-110. 3. Z. Horvat, D. Filipovic, S. Kosutic, R. Emert, Reduction of mouldboard plough share wear by a combination technique of hardfacing, Trib. Int. 41, 2008, p 778-782. 4. S. Jacobson, S. Hogmark, Tribologi – Friktion, smörjning och nötning, Liber Utbildning AB, Uppsala, 1996 (in Swedish). 5.”Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)” ASTM D2487 – 11, ASTM Book of Standards vol. 04.08, 2012 6. W. Pabst, E. Gregorová, Characterization of particles and particle systems, Dept. of An. Chemistry, ICT Prague, 2007. 7. P.A Swanson, A.F. Vetter, The measurement of abrasive par-ticle shape and its effect on wear, ASLE Trans. 28, 1985, p 25-230. 8. P. A. Swanson, Comparison of laboratory abrasion tests and filed tests of material used in tillage equipment, Tribology: Wear test selection for design and application, ASTM STP 1199 A.W: Ruff and R.G. Bayer (ed.), ASTM, Philadelphia, 1993. 9. “Standard Test Method for Measuring Abrasion Using the Dry Sand/Rubber Wheel Apparatus” G65 – 04, ASTM Book of Standards vol. 03.02, 2012. 10. S. Dizdar, L-Å Nilsson, B. Maroli, Abrasive wear of flame sprayed and fused NiCrBSi alloys, ITSC2011: 2011 Int. Therm. Spray Conf. & Exhib., 27-29 Sept. 2011, Hamburg, Germany. 11. A. Yazici, Investigation of the reduction of mouldboard ploughshare wear through hot stamping and hardfacing process-es, Turk. J. Agric. For. 35, 2011,p 461-468. 12. R. Polak, S. Ilo, E. Badisch, Relation between inter–particle distance (LIPD) and abrasion in multiphase matrix–carbide mate-rials, Trib. Lett. 33 (2009) p 29–35 13. C. Katisch, E. Badisch, Effect of carbide degradation in a Ni–based hardfacing under abrasive and combined im-pact/abrasive conditions, Surf. & Coat. Techn. 206, 2011, p 1062-1068. 14. Thermal Surfacing – Powder Choice With Ease, Product brochure by Höganäs AB (publ.), April 2012.