Abrasive wear of hard faced ground engaging tool

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Abrasive wear of hard faced ground engaging tools

DAVID EUGENIO CANTU GOMEZ

Master of Science Thesis Stockholm, Sweden 2017

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Abrasive wear of hard faced ground engaging tools

David Eugenio Cantú Gómez

Master of Science Thesis MMK 2017:193 MKN 202 KTH Industrial Engineering and Management

Machine Design SE-100 44 Stockholm

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Examensarbete MMK 2017:193 MKN 202

Abrasiv nötning av ythärdade jordbearbetningsverktyg

David Eugenio Cantú Gómez

Godkänt

2017-10-31

Examinator

Ulf Sellgren

Handledare

Stefan Björklund

Uppdragsgivare

Senad Dizdar

Kontaktperson

Senad Dizdar

SAMMANFATTNING

Jordbearbetningsverktyg är väldigt viktiga komponenter i maskiner för jordbruksapplikationer, såsom jordbearbetning. Det är skärbladet på plogen, den så kallade plogbillen, som står för kontakten mellan plogen och de hårda mineralerna i jorden. Ett av de största problemen som dessa verktyg möter är abrasivnötning, som gör att verktygen efter en tid blir ineffektiva och bland annat orsakar frekventa stopp, erosion av marken och låg jordkvalité på grund av den försämrade jordbearbetningen samt ökar traktorns bränsleförbrukning. I denna undersökning testades provbitar av två olika slags stål runt i kvarts- respektive granitsand, och därefter utfördes mätningar på den nötning som skett samt även ytprofilering och mikroskopi gjordes. Testerna utfördes i kvartsand och i granitsand. Provbitar var två kommerciellt tillverkad härdade skärblad gjorda i stål EN 22MnB5. Ett tredje skärblad var också en komerciellt gjord i EN 22MnB5 stål men ohärdad och laserpåsvetsad med Ni-bas + 50% karbid pulverblandning (Höganäs 1559-40 +50% 4590). Provkörningarna utfördes i en speciellt utvecklad karusell-tribometer. Resultatet från provningarna visade att i fallet med laserpåsvetsade bitarna så var nötningen endast 30%

relativt nötningen som EN 22MnB5-provet uppvisade under samma förutsättningar.

Nyckelord: Jordbearbetning, laserpåsvetsning, nötningsbeständig, plog

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Master of Science Thesis MMK 2017:193 MKN 202 Abrasive wear of hard faced ground engaging tools

David Eugenio Cantú Gómez

Approved

2017-10-31

Examiner

Ulf Sellgren

Supervisor

Stefan Björklund

Commissioner

Senad Dizdar

Contact person

Senad Dizdar

ABSTRACT

Ground engaging tools are very important components of machinery for agricultural applications, such as soil tillage. Ploughshare points serve as the first point of contact between ploughs and the hard minerals in the soil. One of the biggest problems that these tools encounter is abrasive wear, which decreases tillage quality, causes frequent tillage stops, increases fuel consumption of the tractor, and results in soil erosion. During this investigation, wear measurement, surface profiling and microscopic analysis were performed on three share point samples running in silica and granite sand – two points were commercial ones made of steel EN 22MnB5 and hardened. They served as commercial references. A third share point was also a commercial EN 22MnB5 one, but not hardened and laser cladded by a Ni-base + 50% carbide powder mix (Höganäs 1559–40 + 50% 4590). The abrasive wear testing was performed in an especially designed carousel tribometer. The laser cladded sample suffered only 30% of the wear shown in the EN 22MnB5 reference sample running under the same conditions.

Keywords: Ground engaging tool, laser cladding, plough, wear resistance

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FOREWORD

I would like to extend my most sincere gratitude to Höganäs AB for the opportunity of developing this master thesis with them, and for the financial and operational support that they provided during the project. A very special and sincere thanks to my supervisor at Höganäs, Dr.

Senad Dizdar, whose expertise in the field of tribology and his exceptional guidance, were key for the proper development of this thesis. Thank you, Senad, for the knowledge you shared with me both inside and outside the laboratory, and for your invaluable advice! I extend my gratitude to the team at R&D Lab at Höganäs AB for their aid, especially to Henrik Holmqvist for his kind support in the laboratory, and to Björn Skårman for this aid with the XRD analysis of the sand.

I’d also like to thank KTH - Royal Institute of Technology, and the staff at the Machine Design department, for their academic and logistic support, and for the physical space provided in the laboratory at KTH. A very sincere thanks to my supervisor at KTH, Dr. Stefan Björklund, for the expertise and the knowledge of the field of tribology and mechanical design that he shared with me, and for his aid in setting up and carrying out the experiments inside the KTH laboratory.

Your contribution to Academia and to this project is extremely valuable and much appreciated!

Special thanks to Tomas Östberg and Ramtin Massoumzadeh for their assistance in manufacturing of components for the tribometer tests. Very warm thanks to Elin Skoog for her help in the Swedish translation of the abstract on this thesis.

My gratitude also goes to Consejo Nacional de Ciencia y Tecnología (CONACYT) in Mexico, for providing the financial support that allowed me to pursue a Master’s Degree at KTH.

Last, but not less important, immense thanks from the bottom of my heart to my family for their care and support during this (and all) stages of my life. My mother, Susana Gómez, and my grandmother, Josefina Gómez: your strength, courage, kindness, and everlasting love have pushed me this far, and have always made me a better person. May the success of this thesis project and this Master’s degree be shared with you!

Una dedicatoria especial y un agradecimiento desde el fondo de mi corazón a mi familia por su cariño y apoyo durante esta (y todas) las etapas de mi vida. Mi mamá, Susana Gómez Martínez, y mi abuela, Josefina Gómez Martínez: su fuerza, coraje, valentía, bondad y amor incondicional me han llevado lejos, y siempre me han hecho una mejor persona. La conclusión de esta tesis y la obtención de este grado de maestría es un éxito compartido entre nosotros tres. ¡Las quiero más que a nada!

David Eugenio Cantú Gómez Stockholm, October 2017

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1. INTRODUCTION

This chapter explains the background and requirements behind this thesis project. The importance for both the company, the science and the environment are stated here, as well as the objectives, scope, delimitations and methods to follow.

1.1 Background 1.1.1 GETs

Ground-engaging tools (GETs) are the first point of contact between a machine and a work soil.

Their main function is to initiate the removal of soil matter by sharply cutting through the surface of the ground and leading bigger plows or buckets into the ground at different depths and rates, depending on the application of the tool. As the tools interact with the ground, they undergo high loads, and high friction and wear phenomena during operation. GETs are generally found in applications where a structural component interacts with soil, sand or hard minerals. For example, the oil and gas industry as drilling tools, in construction and mining industries in shapes of blades, cutters, edge protections, adapters, tips, teeth or ripper boots.

This thesis project focuses on the analysis of the wear suffered by a particular GET used for tillage operations in the agricultural industry. Tillage is a widely used agronomical method of soil preparation by which the soil can be loosened, in order to incorporate crop residues and manure, thus allowing the creation of a seedbed for germination and growth of crops [1].

The GET to be analyzed is called ploughshare (see Figure 1), and is used to perform the first cut through the soil. Ploughshares are mounted at the front of each share on a moldboard plough which will work displacing the soil. Figure 2 shows a schematic of how the plough is assembled.

Figure 1: Kverneland reversible ploughshare point, model 063090 (courtesy of Kverneland AS)

Moldboard ploughs have been used for soil tillage for hundreds of years, but their popularity has been increasing lately due to its advantages in the agricultural industry. They allow larger work areas and reduced operation times. Avoiding erosion of the ground is also extremely important for the environment: inversion and sowing must be performed on the same day to avoid it [2].

Worn or damaged ploughshares will cause frequent tillage stops and degraded tillage quality, making the ground more exposed to erosion.

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Figure 2: Plough assembly [3]

1.1.2 Abrasive wear on GETs

Wear is essentially the removal of surface material from solids, and there are many different definitions of it (i.e. Zmitrowicz, 2006). Wear plays a major role in failure of tools. Abrasive wear, where one harder body abrades another, stands for roughly 50% of all wear problems [4], and it’s the most common failure mechanism for GETs [5].

Table 1: Mohs scale of hardness

Mohs

hardness Mineral Mineral type and formula

Scratch test Scratchable with…

Vickers hardness

Similar hardness level

1 Talc Silicate Mg3Si4O10(OH)2 Fingernail 2,4

2 Gypsum Sulfate CaSO4·2H2O Fingernail 36

3 Calcite Carbonate CaCO3 Copper coin 109

4 Fluorite Calcium fluoride CaF2. Steel knife (easily) 189

5 Apatite Phosphate

Ca5(PO4)3(F,Cl,OH Steel knife (still) 536

6 Orthoclase

(feldspar) Silicate KAlSi3O8 Steel file 795

7 Quartz (silica) Oxide SiO2 Topaz 1120 Silica sand

Granite sand

8 Topaz Silicate Al2SiO4(FOH)2 Corundum 1427 Granite sand

9 Corundum Oxide Al2O3 Diamond 2060 Tungsten carbide

Granite sand

10 Diamond C 10600

GETs in agriculture are normally made of low-alloy structural steel such as EN 22MnB5, which endures orthoclase particles (level 6 in Table 1) for a reasonable amount of time. However,

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ploughshares are continuously in contact with dry soil that is normally composed of silica or granite (the latter is mainly composed of quartz and feldspar) located higher in the Mohs resistance to scratching scale. This decreases GET’s lifespan considerably.

Abrasive wear causes components to fail, which will then have to be fixed or replaced. It also affects production schedules, the quality of the tillage decreases, and the power required to run a tractor and perform tillage increases [4, 6]. Wear has large repercussions on machinery performance and, ultimately, in economy and on the environment. As precedent, an estimation of yearly monetary losses in Canada in 1986, due to wear in agricultural tools alone, reached $940 million USD, which surely is a much higher cost if adjusted to 2017’s schemes [7]. Another study performed in Turkey shows that an area of approximately 18,500,000 ha of land is cultivated in that country, and the amount of expected wear is close to 5365 tons of plough equipment, which finally translates into $4.4 million USD per year [8]. This shows the necessity and importance of enhancing the surface properties of the tools used in agriculture, in a manner that is still cost-effective and environmentally-friendly.

It has been shown that tool abrasion depends directly on parameters such as surface hardness, roughness and microstructure [4, 5], all of which can be controlled or enhanced by means of surface treatments. The main advantages of reducing the effects of abrasive wear on the tooling include: direct monetary benefits from not having to replace tools as often, increased machinery efficiency, or less tillage pauses. Environmental benefits include reduced steel re-process (melting damaged ploughshares) or re-manufacturing, and reduced erosion on the ground.

1.1.3 Höganäs AB, the customer

This project is developed in cooperation between KTH Royal Institute of Technology in Stockholm, and Höganäs AB, a Swedish company founded in the town that bears the same name in 1797, who is the worldwide leader in production of metal power, and a leading company in the development of metal powder applications for a wide variety of industries [9]. Their research and development efforts in powder metallurgy serve a variety of industries, such as automotive, agriculture, chemical, energy production, glass molding, heavy duty machinery, marine and petrochemical, plastic and steel manufacturing.

One of Höganäs’s key business areas is thermal surfacing, a cost-effective method to achieve high surface-related performance characteristics of metallic components [10]. In principle, a component can be coated with a layer created from metal powder and different deposition methods to protect its surface from wear and corrosion. This thesis work intends to investigate how laser cladding with precious metal powder/mixes, one of Höganäs’s advanced coating techniques, affects the wear on GETs.

1.1.4 Tests categorization

Studying the tribological properties of the materials selected to create modern tools has become essential in the machine design process, if the designer aims at maximizing components’

lifespan. Standardized tests have been created by several organizations to replicate and study these tribological phenomena on materials, lubricants and coatings in a controlled way. An example of a standardized method for testing abrasion on metallic samples is the ASTM G65 test [11], where an abrasive sand flow is fed onto a sample’s surface by pressing the sample against a rotating rubber lined wheel. The sample’s surface is scratched by the abrading sand, and the result of this test is a volume loss, reported in cubic millimeters [11].

However, companies can also make use of internal methods to perform more realistic tribological tests [12], which more thoroughly completes the understanding of the properties of

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their components. These methods sometimes consider testing under real operation conditions (field research), which requires many more resources and is far more complex or difficult to control and repeat; but sometimes, laboratory tests and real working parts (specimens) can be mixed to achieve an acceptable balance between replication of real conditions and the cost of carrying out the test. A classification of how close to reality a test is, in terms of operating factors and control conditions, was created within the DIN 50322 norm, as seen in Figure 3.

Figure 3: Tribological tests categories [12]

Research has been previously conducted at Höganäs AB in Levels V and VI of the classification under the ASTM G65 standard [13], where a correlation between abrasive wear and amount of tungsten carbides added by powder welding and plasma transfer arc was found. It’s important to mention that Levels V and VI can be “merged”, as the ASTM G65 test provides a well-modelled operating condition compared to normal conditions, yet it’s still a laboratory test. The experiment’s results can be used to anticipate the lifespan of a component surface treated with either one of both methods, however, they do not provide results tailored to a specific component’s performance.

Figure 4 specifically categorizes ploughing components’ testing stages. This thesis project will be allocated in Level IV given that:

 Actual ploughshares will be used to test wear upon, not standardized probes.

 Assembly and position of the ploughshares, and the abrasive sand used for testing, will resemble real world environment and operating conditions.

 An especially designed test rig will be used inside a laboratory with controlled conditions.

 Wear and metallographic data will be gathered before and after each test on each specimen.

 The test does not use tractors or vehicles whatsoever, and is not carried out in the field.

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The advantage of working under Level III of the testing categories is that a more realistic prediction of the wear can be obtained from the experiment, while keeping costs relatively low.

Figure 4: Plough abrasive wear testing categorization based on DIN 5032 [14]

1.1.5 Carousel tribometer

In order to test the specimens’ resistance to wear, an especially designed test rig will be used, called carousel tribometer. This machine was built for Höganäs AB at KTH Royal Institute of Technology in the frame of the Advanced Machine Design course within the Machine Design department.

The tribometer consists of a drum filled with sand, built on a steel frame. The powertrain driving the specimen through the sand rotationally is composed of a 4kW motor rated at 1464 RPM and torque of 24 Nm. The gearbox has a reduction of 23, yielding a calculated torque of 552 Nm at 63 RPM. A shaft with a diameter of 50 mm will rotate the specimen through the sand at a speed of 2.8 m/s [15].

Inside the drum, and attached to the shaft, there is a holder designed to support the ploughshare.

This holder must be redesigned to fulfill the requirement of this thesis project and the type of ploughshare to be used for the tests.

Figure 5: Carousel tribometer [15]

Abrasive wear testing

Field testing , complete equipment

Lowcost, acc.testing, hightesting skills Highcost, lower demands ontesting skills Lab testing, complete equipment

Lab testing, plough assembly

Lab testing, plough share point

Lab model testing, plate test sample I

II III

IV

V ^S.

Dizdar

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1.2 Objectives and purpose

This thesis project builds upon Höganäs’s research mentioned previously: Abrasive wear resistance of thermal surfacing materials for soil tillage applications [13], whose ranking results will be compared to this project’s findings. The machine that resulted from the project Development of an Abrasive Wear Tribometer [15] will be used to perform the test, and the work will be divided into two main stages.

Table 2: Materials to be tested

Material

designation Type Description ASTM G65 testing,

abrasive wear

EN 22MnB5 Wrought steel reference

- Low alloyed through hardening steel with micro- alloyed with B

- Hardened to 60 HRC

63 mm³, Proc. B (proc. A/3

sliding distance) Höganäs

1559-40 +50% 4590

Metal powder + carbide powder mix

- Hard face: Ni-base with admixed relatively coarse carbides, sieve cut 53-150 µm.

- 60+ HRC

4 to 10 mm³

A type of sand called 0/2 Kabelmjöl from the supplier NCC Arlanda will be used. Its properties can be found in Table 4, and the certificate can be found in APPENDIX B: NCC Arlanda Kabelmjöl specification. All materials have high hardness, but type, size and dispersion of the hard phase will be different.

Table 3: Silica sand properties

Sand name Silica AFS 50/70

Nominal particle size range 0.212 – 0.300 mm

Sand type Quartz / SiO2

Sand density (1000 kg/m³) 2.65

Hardness Mohs: 7 (see Table 1), 1070 HV

Sand particle shape Roundish (natural sand)

Sieve size (mm) 0.212 0.300 0.425

Passing (%) 2 99 100

Table 4: Kabelmjöl sand properties [16]

Sand name NCC 0/2 Kabelmjöl

Nominal particle size range 0-2 mm

Sand type Granite

Sand density (1000 kg/m³) 2.67-2.77

Hardness Mohs: approx. 7 (see Table 1) Sand particle shape Angularish (crushed sand)

Sieve size (mm) 0.063 0.25 1 2 4

Passing (%) 13.2 30 57 86 100

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 Stage 1: Abrasive wear testing at KTH (surface profiling and worn volume).

One reference material (EN 22MnB5 wrought low-alloy steel) blade, and one carbide surface-coated blade (see Table 2) will be analyzed. The coating is applied by means of laser cladding deposition. This stage aims at answering the following:

 Will the abrasive wear resistance ranking (Table 2) be the same between the carousel tribometer and the ASTM G65 testing?

 Will the hard phase particle size be the strongest factor again, compared to the previous study [13]?

 Stage 2: Analysis of wear testing results (microscopy and metallography)

The blades will be analyzed before and after the test in the carousel tribometer to compare the wear and final states of the worn areas. This stage aims at answering the following:

 How does the wear pattern look like?

 How does the wear pattern develop with time?

 How do the hard phase and matrix wear look like, and what is the hard phase particle size?

 Is there a possibility to improve the ploughshare design?

 What sections of the ploughshare’s surface should be hard faced?

1.3 Scope and delimitations

This project’s deliverables include:

 Redesign of the ploughshare’s holder in the tribometer for better recreation of the tool’s position in the real-world application.

 Wear, expressed in volume and mass loss, for the reference material (EN 22MnB5 steel) ploughshare and a hard-faced ploughshare.

 Daily surface profile data of certain selected areas of each ploughshare.

 Metallographic analysis, microscopy and photography of the worn ploughshares.

 Recommendation of ploughshare surface area to be coated based on wear.

 Geometrical improvement opportunities of the tool based on visual inspection of the wear.

What is out of scope on this project:

 Redesign of structure or powertrain of the tribometer.

 Modification of operation parameters or control implementation in the tribometer.

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1.4 Methods

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2. FRAME OF REFERENCE

In this chapter, the theory surrounding the topic of tribology and coatings will be presented to explain the abrasive wear mechanisms.

Engineering design comprises all aspects of the product development process, from conception and design to production and end-of-life. Part of the work done by design engineers is to optimize the lifetime of a component, which includes predicting how and when it is expected to fail due to outside factors and interaction with the environment it will work under. In the case of agriculture, a lot of interaction between machines and the environment happens directly with several types of soil, for instance excavating, crushing, tilling, or ploughing. Granulates from the soil act as abrasive or erosive particles on machine components, especially when the soil is dry.

The design process of these ground engaging tools should consider how the environment will degrade the tool’s material. Tribology is the science that studies the interaction of surfaces in relative motions, as well as friction wear and lubrication [17]. Several different types of tribological mechanisms affect the performance and lifetime of the components, where heat generation, power consumption or friction are factors directly related to the mechanical contact between two of the surfaces in the analyzed system.

Understanding which tribological mechanisms directly affect the GET during tillage leads to suggesting ways to increase the lifespan of the component, and this chapter intends to shortly provide the theoretical foundations of these phenomena.

2.1 Mechanisms of wear

In tribology, wear mechanisms occur at the interface of at least two contacting bodies. At least one of the bodies needs to be a solid, the other one can be a solid or a surrounding medium containing hard particles. Wear is accountable for most of the material damage and mechanical performance loss during operation and, from the economical point of view, reducing wear can lead to major monetary savings [18].

Figure 6: The process of wear

Wear mechanisms can be classified into several types [17, 18], and wear phenomena occurs often as a combination of these mechanisms, where one of the occurring mechanisms is dominant. The mechanisms are presented below.

Lack of lubrication or

coating Lack of lubrication or

coating

Increase in friction Increase in

friction

Wear increases and performance

drops Wear increases and performance

drops

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2.1.1 Fatigue wear

Surface fatigue results from cyclical or repetitive stresses applied on a material, which reaches its elastic limit and suffers permanent plastic deformations after many cycles. This leads to the formation of microcracks and fissures. Commonly, it happens during repetitive sliding, rolling, impact or bending. Per ASTM, fatigue can be classified into high-cycle fatigue (more than 10⁵ cycles) and low-cycle fatigue [19].

Figure 7: Development of fatigue wear [18]

As depicted in Figure 7, the damage begins with the formation of a crack nucleation on the surface, which extends along the weakest plane, for example, cell boundaries where dislocation exists [18]. As the main crack forms, secondary (smaller) subsurface cracks form as well, until the microcracks reach another point of the surface, or reach each other and cause the entire particles to detach from the body (ultimate ductile fracture). Fracture can be surface-originated or subsurface-originated, which results in debris detaching from the tool. The debris often is shaped as flakes, and therefore, the process is called flaking failure.

Figure 8: Flake caused by fatigue wear [18]

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ASTM states that for fatigue to appear, three factors need to be present: high value of tensile stress, large variations of applied stress, and a large amount of cycles of applied stress [19]. As none of the last three conditions are met in this project’s application, this wear mechanism is not considered to be the cause of failure of the ploughshares.

2.1.2 Corrosive wear

Corrosive wear presents itself as a condition in which metallic components undergo the process of oxidation in a corrosive environment. Corrosive environments can be liquids, gases, and even air, where metallic tools react to the oxygen in the environment [17]. From there, an oxide layer appears. This type of wear can happen in the presence of lubricant (corrosive wear), and without it during sliding when there is oxygen or air present (oxidative wear) [18].

The formation of an oxide layer on the substrate (the tool) is beneficial up to some point. This layer will act as protection against adhesive wear, but the film can be worn out and cause further wear problems. The film’s degradation model is described in Figure 9. A durable, thin protective layer is formed by either oil, water, and other liquid or solid lubricants to avoid high friction and adhesive contact between two surfaces. Due to the normal wear phenomena on the tool, and the way the lubricant film behaves, the protective layer can slowly regrow on a worn part of the surface, and cover it, while some debris of the film is dragged during the sliding process. After the protective layer is worn, corrosion starts to affect the substrate and adhesive wear begins to remove material from the tool, causing severe damage.

Figure 9: Process of corrosion wear [18]

As mentioned before, the ploughshare point works under dry and humid conditions, depending on the weather in the operation field. Water in the soil is often slightly acidic or alkaline [20] and can corrode the blade whenever a non-stainless steel is used, like the AISI 300 and 400 series, but normally wrought steel (such as EN 22MnB5) is used in agricultural applications, making the tools more prone to corrosion. Since high temperature environments generate more chemical corrosion, reducing the temperature at which the tools work can also help in reducing some wear [21].

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Figure 10: Evidence of corrosive wear [17]

2.1.3 Adhesive wear

Adhesive wear is a severe consequence of the adhesive bonding between two similar (metallurgically compatible) metallic surfaces due to the action of atomic or inter-molecular forces [17]. This means that friction will be higher when rubbing two similar metallic materials with smooth finishing.

Figure 11: Material transfer between asperities [17]

Sliding of the surfaces will be severely affected by adhesive wear, and a very unstable coefficient of friction will be present in the system [18]. The result is micro-welding between the asperities of both surfaces, which ends with a transfer of material from one surface to the other. Therefore, the use of metallurgically compatible metallic materials must be avoided, and lubrication should be present between two sliding surfaces.

Figure 12: Development of adhesive wear [18]

The damage caused by adhesive wear can be reduced by adding lubricants (solid or liquid), by creating a strong oxide layer (which can also cause corrosive wear if out of control), and by carburizing or nitriding the surfaces [17].

This type of wear might be rarely seen during the tillage operations that concern this project, where the only contact is between particles and the tool. Therefore, it will not be considered as a cause for wear on the ploughshare.

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2.1.4 Abrasive wear

Abrasive wear is the damage on a body that occurs when particles or bodies of equal or greater hardness apply a load on it [17], and penetrate the body [4], causing irreversible plastic deformation (in form of cutting or ploughing). Another definition for abrasive wear is the loss of material by the passage of hard particles over a surface [18]. This type of wear can be modelled as a two-body or a three-body system [17]. As mentioned before, abrasive wear is accountable for up to 90% of the total wear on machine components [22], and is present in 50% of industrial operations [4].

Figure 13: Modes of abrasive wear [17]

Two-body systems are present in its majority when dealing with mineral treatment or transport of granulates, as the particles abrade a tool’s surface by cutting and ploughing. Three-body systems can be found when particles in between two surfaces scratch the softer surface when a force is applied on it.

Figure 14 shows the two-body system, which could better describe the behavior of abrasive wear in the agricultural industry. Dry granulates from the soil will act as very small ploughs on the surface of steel blades as they work on the tillage, slowly and continuously removing material from the tool. The plastic damage induced on the blade’s surface decreases its life expectancy (thus, its reliability) by causing premature failure.

Figure 14: Abrasive wear model [4]

Other authors define the two-body wear system as a rough surface (with firmly attached granulates on it) which abrades another softer surface, while defining the three-body system as free-rolling particles between two surfaces, which can abrade either one or both boundary surfaces, as shown in Figure 15.

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Figure 15: Two and three-body abrasive systems [18]

Figure 16 depicts four failure modes of abrasion, explained by Stachowiak & Batchelor [18].

a) Cutting or ploughing represent the case when a hard particle cuts the surface of the tool. This happens when the tool is subjected to a load while being displaced through an abrasive environment, such as soil or sand. It is interesting to mention that the presence of lubricant can cause a higher rate of cut by the abrasive particles on the tool.

b) Fracture results from the development of subsurface cracks which accumulate and then converge, causing the surface to break and form debris. This is particularly more common in brittle (low toughness) materials which undergo large loads.

c) Fatigue is the result of a cyclic load on the surface of the tool. The cyclic load causes repeated strain, so the material displacement to the sides leads to mild abrasive wear. When loads are not very high, the process is slow and does not damage the tool as much as cutting or fracture.

d) Grain pull-out is mostly present in ceramics, where the bond between grains is weak.

Figure 16: Abrasion failure modes [18]

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The result of abrasion, as stated, can be visually noted as scratches on the surface, or as fractured grains off the surface.

Figure 17: Scratched surface due to abrasion [21]

Figure 18: Ploughing and cutting due to abrasion [21]

Erosive wear has some similarities with abrasive wear, but the particle mass and velocity are very different. Here, relatively small abrasive particles, commonly less than 100 µm, impact a surface at a relatively high velocity, as common higher than 30 m/s as per ASTM G76 [23] , and cause material loss from the surface (wear) [21]. This type of wear is occurring, for instance, in pipelines for pneumatic transport of granular materials, fans, propellers, etc. As the particles involved in this process are very small, micro-cutting, micro-ploughing, micro-fatigue and micro-cracking could be mechanisms acting on the tool at the same time [24].

Figure 19: Erosive wear [21]

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2.2 Abrasivity of particles

Three parameters of the particles affect their abrasivity: hardness, size and shape [25]. When running a tool through an abrasive environment, such as ploughshares digging into the soil during tillage, the particles of the soil will scratch the surface of the tool causing irreversible plastic deformation and fractures.

For abrasive wear to damage a material, the material’s hardness must be 0.8 or less of the particle’s hardness [18]. Figure 20 explains this graphically. However, one problem with designing a tool that endures abrasive particles is that particles’ hardness is very difficult to estimate, as soil is normally a mixture of different minerals. Minerals that compose soil are classified in Mohs hardness, and are also measured in Vickers scale (Table 1).

Figure 20: Relative wear resistance vs hardness ratio [18]

Particle size has influence on abrasivity as well. There is evidence to suggest that the larger the particles, the higher the wear rate on the surfaces they impact, which means they damage the tools faster [25]. When the particle size is below 50 µm, wear rate on steels increases in a non- linear way [18], and reaches its limit at 100 µm. Thereafter, the wear rate stabilizes (see Figure 21 and Figure 22).

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Figure 21: Particle diameter vs wear rate [18]

Figure 22: Particle size vs wear rate in two-body abrasion [25]

Particle shape is the third parameter that affects particle abrasivity. A particle shape is normally defined by using parameters such as sphericity, aspect ratio and parameters that describe roughness or sharpness of the particle surface – periphery.

Figure 23: Particle shape [25]

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The aspect ratio RA (Eq. 1) is defined as the ratio between maximum diameter and its smallest orthogonal diameter of a particle:

𝑅

_𝐴

=

^𝐵_𝐻^𝑚𝑎𝑥

𝑚𝑎𝑥 (1)

Sphericity RS (Eq. 2) is defined as the ratio between the surface area of a sphere with the same volume as the particle, to the surface area of the particle (see Figure 23):

𝑅

_𝑆

=

^4𝜋𝐴_𝑃₂ (2)

Particle perimeter ratio refers to the particle perimeter to the perimeter of the convex bounding polygon.

There are also a few mathematical ways to define a sharpness of a particle surface, such as perimeter ratio or convexity parameter, the spike parameter with linear fit (SP), or with quadratic fit (SPQ). The SP was the first parameter to be introduced, that included sharpness in its analysis, and it’s based on representing the particle’s protrusions as a set of triangles [25].

Figure 24: Particle shape, SP approach [18]

Another way to describe the shape is by using the spike parameter with quadratic fit (SPQ), where a circle is drawn with origin in the particle’s centroid. The maximum local radius of each spike is marked as the apex and the sides of the spike are fit with a quadratic polynomial function [18].

Figure 25: Particle shape, SPQ approach [18]

Depending on the toughness and the shape of the particle, the particles themselves can be worn out during operation and can either break apart to form new, smaller particles, or their surface can be smoothed, as shown in Figure 26. Therefore, to maintain the same initial conditions for all the tests in this project, the sand used to simulate the soil will be changed before every test.

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Figure 26: Brittleness of particles [18]

2.3 Characterization and testing of wear

2.3.1 The ASTM G65 test

A standardized test for measuring the volume loss of metallic components by scratching (abrasion) of hard particles on their surfaces was created by ASTM with the designation G65-16 [11]. The standard defines the test as a rubber wheel applying a load on a metallic specimen while a steady flow of sand passes between the wheel and the specimen’s surface, as depicted in Figure 27. The test’s parameters can be found in the standard.

Figure 27: ASTM G65 schematic [11]

At Höganäs, the ASTM G65 test is currently used to validate and rank the wear resistance of different coatings. In order to understand its functioning better, a test was performed.

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Figure 28: ASTM G65 test rig at Höganäs AB in Höganäs

2.3.2 Profilometry

A profilometer is a device used to measure a surface’s profile with all its features: peaks and valleys. The acquired profile is then mathematically processed and expressed as standardized, two-dimensional roughness parameters (as per ISO 4287). The following list shows frequently- used surface roughness parameters [17]:

 Ra: average roughness – arithmetical mean deviation of the assessed profile (most commonly used)

 Rq: root mean square roughness

 Rt: largest peak-to-valley sample

 Rp: maximum peak-to-mean sample

 Rz: maximum roughness – “ten point height”

Profilometry can be performed with optical devices or with mechanical (contact) devices. A contact profilometer will be used during this research to investigate the surface’s shape before, during and after the testing phase.

2.3.3 Archard’s equation

The wear rate of a tool or a product depends on parameters like applied or reacting load, sliding velocity of the tool against another surface, operating temperature, and environment conditions [17]. Wear rate is normally not constant. During running-in, increased wear might occur.

Thereafter, the wear rate stabilizes, and a way to predict it is more plausible. Archard’s equation [18] is developed for sliding adhesive wear but can be used to some extent even for abrasive wear.

𝑉 =

^{𝐹∙𝑘∙𝑠}

𝐻 (3)

Where:

 V: worn volume [mm³]

 F: load [N]

 k: specific wear rate [constant]

 s: sliding distance [mm]

 H: Vickers hardness of the soft surface [MPa]

It is worth mentioning that the total volume worn from a surface is inversely proportional to its hardness [22]. This means that the harder a surface is, the less wear it will suffer.

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2.4 Increasing resistance to abrasive wear

It is known that wear is a mechanical phenomenon that cannot be avoided, but the effects of those mechanical interactions on tools can be minimized in several ways [17, 18]:

 Increasing the hardness of the softest material until a difference between both materials is less than 10% of the hardness.

 Increasing the hardness of the material to at least 80% of the abrasive particles’ hardness.

 Lower the roughness on the harder surface.

 Coating the surface or embedding hard particles in the surface of the softer material (carbide- forming metals when dealing with steels). Surface coatings can be applied by means of several methods of which the relatively thick sprayed and overlay welded coatings are often in use.

2.4.1 Laser cladding

Laser cladding is an advanced direct metal deposition process which creates a metallic (metallurgical) bonding between the substrate and the cladding by means of intense heat, created by a high-energy laser beam [26]. As explained before, the bonding of two dissimilar metals can provide the substrate with a hard, stronger layer that endures abrasive and corrosive wear further.

Generally, the machinery required for this process to happen is composed of two main systems:

1. A powder feeder that delivers metallic powder to the substrate via a lateral nozzle and a motion control system.

2. A laser generator attached to the three-dimensional positioning system (CAM-controlled) and focusing optics which (vertically) direct the energy of the laser towards the substrate sample.

Figure 29: Laser cladding process [27]

The way the material is supplied onto the substrate can vary, since it can happen in a single-step or in a two-step process [28]. The two-step technique requires that a powder be firstly pre-placed onto the substrate, and then molten by the laser beam. This process takes longer time, but requires a less complex delivery system. In the one-step technique, the coating material (commonly powder metal) is delivered continuously into a laser-generated melt pool. That means that both the laser and the delivery system’s nozzle move together.

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The laser cladding process has several advantages [29]. It produces very low dilution of the substrate material into the cladding, normally between 2% and 5% in a single layer. Compared to a TIG/MIG overlay welding, the latter produces a dilution typically from 10% to 40%. In other words, the target material composition can be achieved in a single layer, where a TIG/MIG welding could require two or three layers. Wear and corrosion are effectively reduced by this process, and that more complex, near-net-shape geometries can be coated [28]. Commercially, applications of this process on near-net-shape pieces save considerable amounts of time and resources, making the final product more competitive in the market.

This project deals with the plough point component mentioned in 1.1.1 GETs, where wear will occur in the contact point with the ground at engagement, making laser cladding a great choice of a process for coating the blade. Other advantages include low deformation of the substrate, microstructure improvement through faster cooling rate, and the lack of cracks and porosity on the product after finishing the process. Table 2 shows the materials which will be applied by laser cladding on the substrate for testing of this project.

The increase in a tool’s resistance to abrasive wear (and thus total lifetime) will vary, depending on the type of material which is deposited on the substrate. As mentioned, there is a wide range of materials that can be deposited by means of laser cladding [28], which makes this process a very versatile one. Höganäs’s equipment can reach a deposition rate of 8 kg/hour, a deposition thickness of up to 4 mm, and a deposition hardness of 60+ HRC. This shows that laser cladding can be a quick, automated process for wear protection of metals.

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3. IMPLEMENTATION

This chapter explains which materials will be tested in the tribometer, under which conditions, and the method to gather the information before and after the tests.

The work done during this project will be divided into two stages, consistently with the objectives explained before in 1.2 Objectives and purpose. In total, three different material configurations will be tested in the tribometer.

Table 5: Experimental setup Material

designation Type Description Sand type

EN 22MnB5

Wrought steel reference

W.-Nr.1.5531, Fe- 0.3 C- 1.25 Mn- 0.005 B - 60 HRC

Silica

EN 22MnB5

Wrought steel reference

W.-Nr.1.5531, Fe- 0.3 C- 1.25 Mn- 0.005 B - 60 HRC

Kabelmjöl

Höganäs 1559-40 +50% 4590

Metal powder +

carbide powder mix

- Hard face: Ni-base with admixed relatively coarse carbides

- Ni+<0.06 C- 3 Si- 2.9 B-0.2 Fe + 50% 4590 (WCx) - 60+ HRC

Kabelmjöl

1. The reference steel (EN 22MnB5) will be run in silica sand, and then in Kabelmjöl sand. The abrasive wear suffered on the same material under two different types of sand will be compared. The wear caused on the blade by the reference material test will show which areas shall be laser-cladded to run the following test. The carbide-overlaid material will only be run in Kabelmjöl sand.

2. The worn blades will be transported to the Höganäs R&D laboratory for microscopic and metallographic investigations.

3.1 Stage 1: performed at KTH

3.1.1 Carousel tribometer’s tool holder design

The tribometer designed by Anslin, Ferlin and Johansson [15] is located physically at KTH - The Royal Institute of Technology, where the tribological tests will be performed. The machine intends to simulate operating conditions of tractor ploughing, and its design parameters are shown in Table 6. A very important component is the tool holder, which fixes the ploughshare in

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the appropriate position, so that it’s dragged through the sand inside the drum to replicate actual wear. The original holder is shown in Figure 30.

Figure 30: Original tool holder [15]

The tool holder, however, requires a more adequate design to simulate the real position and depth of the ploughshare point during operation. The ploughshare point has a very particular shape, which can be seen in Figure 31. The bending and the twisting angle must adapt to the tool holder for the assembly to be closer to real-life mounting.

Figure 31: Kverneland ploughshare, complex bending

The actual tool positioning instruction, which was obtained by courtesy of Kverneland AS, is as shown in Figure 32:

 The long side of the ploughshare is set at a downward angle of 25° with respect to the ground.

 The short side of the ploughshare is tilted sideways by 15° with respect to the ground.

Figure 32: Actual ploughshare position (courtesy of Kverneland AS)

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The data provided by Kverneland yielded the design shown in Figure 33, which holds the tool at the precise depth of 20 cm below the surface of the sand, at the correct tilting and downward angles with respect to the ground. The design requires three low-carbon steel parts to be welded together, which should withstand the forces acting on the tool due to the weight of the granulate.

Figure 33: New tool holder design (left: frontal view, right: lateral view)

The new design was validated by means of an ANSYS simulation of the forces acting on the surface of both the ploughshare and the holder. The input forces for simulation were taken from the analysis performed by the team that built the tribometer [15], and a safety factor of 2 for yield stress was used, which resulted in an applied force of 300 N on the tool. The simulation yielded an approximate Von Mises equivalent stress of 170 MPa in the welded areas. To keep the mass of the holder low, and to prevent elastic deformation of the thin plate on top of the holder, the plate was reinforced with two welded ribs, as can be observed in the red circle on Figure 33.

Figure 34: Stress simulation on new tool holder

Direction of movement

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3.1.2 Setup and pre-processing of specimens

Correct positioning of the tool in the machine using the new tool holder is required for consistent results. The tool is manually adjusted with screws until tight fastening is achieved, as in Figure 35.

Figure 35:Assembled tool inside tribometer

The amount of sand to be used during each test shall bury the tip of the ploughshare point under 20 cm of sand. Similar studies have used depths ranging from 15 to 30 cm to simulate tillage operations [30, 31, 32]. This roughly translates into approximately 400 kg of sand, as per Figure 36. The pressure on the tool creates a drag force which the motor can easily overcome.

Figure 36: Sand mass calculations

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3.1.3 Tribometer’s test parameters

As explained before, the motor installed on the tribometer has a fixed speed which allows movement of the test blade through the sand. All the samples shall be run with the parameters depicted on Table 6.

Table 6: Tribometer's operation parameters Angular speed of the arm 63 RPM

Torque 552 Nm

Simulated tillage speed 10.13 km/hr Run time for each day 22 hrs Testing time for each day 2 hrs Distance travelled each day 223 km

Daily runs are set to start approximately at 18:00 hrs. The tribometer then runs for 22 hrs without interruption. The machine is to be stopped 22 hrs later (at 16:00 hrs on the next day), and some minutes should be given for the dust to settle down inside the drum, to avoid contamination of the surrounding environment by loose sand particles. Profilometry and weighing is performed during 2 hours.

After measuring, the blade is fastened to the tool holder and then to the tribometer again. The lid must always be fastened correctly to the tribometer, and masking tape or other types of sealing should be placed in the gap between the lid and the drum to avoid any dust from escaping during the test. This ensures a clean environment around the machine, and a consistent level of sand throughout the test.

3.1.4 Method for surface profiling

Roughness measurements taken on the specimen’s surface, as explained in 2.3.2 Profilometry, will allow to track how abrasive particles slowly wears the specimen. A high-precision profilometer, model Taylor Hobson Form Talysurf, will be used for all measurements. A tool holder was fabricated to firmly assemble the blade and to have a way to position the tool on the profilometer’s table with certain precision, so all measurements are taken as close to each other as possible. Five measurements along the Z axis, and six measurements along the X axis will be taken from the working side of the blade, as shown in Table 7.

Method for measuring the surface roughness:

1. Remove all dust and rust particles from the blade’s surface by mild scrubbing with a brush and cleaning with ethanol, then completely dry the blade.

2. Firmly screw the blade onto the blade holder, always in the same orientation.

3. Calibrate the stylus gage 1836 as per the machine’s calibration parameters.

4. Position the blade holder on the profilometer’s table as per Table 7.

5. Measure roughness profile and extract the graphs and results from the profilometer.

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The selected amount of measurements and the displacement distance of the stylus aim at having enough representative roughness data to track the development of the abrasive wear.

Table 7: Profilometry parameters

Figure 37: Taylor Hobson Form Talysurf surface profiler

3.1.5 Method for mass measurement

The mass of the blade will be measured daily, and its wear rate will be obtained from the measurements. The instrument to be used is a precision scale, model Mettler Toledo XP2003S with a precision of up to 0.001 grams.

Method for measuring mass:

1. Set the scale to zero grams.

2. Place the blade in a static position on the scale. The scale is to be set upside down for larger area of contact with the plate, and it should be centered as much as possible for precision.

3. Repeat the measurement 3 times and obtain an average of the mass.

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Figure 38:Mettler Toledo XP2003S equipment

3.2. Stage 2: performed at Höganäs AB 3.2.1 Method for cutting and sampling

Now that the ploughshare specimens have been weighed and their surface has been profiled, small, observable samples from the tip of the ploughshare can be obtained by means of cutting, grinding and polishing. The red square in Figure 39 shows the location of the sample. The idea is that the microstructure and the carbides on the surface of the cladded sample can be observed.

Figure 39: Sample obtained from ploughshare specimens

The following method for obtaining the samples was performed following Höganäs’s norms:

1. One 5 x 10 mm sample will be cut by means of water jet from the side of the blade that was exposed to wear. Cutting with water jet helps maintaining the sample cool, thus preserving its microstructure. Additionally, the quality of water-jet cut is good enough to provide a sample that must be only lightly grinded and polished without much effort.

2. The cutout is then placed in a thermoset resin fixture.

3. The sample is grinded with 80 and a 220-grain paper, cooled by water.

4. The sample is washed with ultrasonic vibrations between disk changes.

5. The sample is polished with 3 µm and a 1 µm polishing clothes with diamond abrasive, until mirror finish is achieved.

6. The thermoset fixture is cracked open, and the sample is kept in a clean container for analysis.

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3.2.2 Method for electron microscope analysis

The scanning electron microscope (SEM) analysis will allow the observation of the

microstructure of the material to evaluate the size and distribution of the carbides from the laser cladding process. The samples must be as clean and as free from electrostatic charge as possible, and all security measures must be followed when using the microscope.

Method for analyzing the samples:

1. Each of the samples is to be cleaned with ethanol and a cotton to remove impurities and grease from the surface.

2. The sample is then cleaned with acetone, and dried with hot air directly on the surface of the sample.

3. All traces of cotton must be removed from the surface.

4. One sample at a time should be analyzed in the microscope.

5. The sample is placed on carbon tape onto a fixture, and the fixture is placed in the microscope.

6. Intensity of electron beam, and visualization parameters can be set as needed.

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4. RESULTS

This chapter presents the data gathered from the tribological tests performed in the custom made tribometer.

4.1 Mass, wear rate and surface profile

The mass of each specimen was measured daily following the method explained in 3.1.5 Method for mass. The data obtained from mass measurement yields the graphs from Figure 40 to Figure 42, which gives data for mass loss, wear rate and cumulative wear rate.

Table 8: Wear of the specimens - mass and volume loss summary

1 2 3 Ratio

3/2 Specimen material EN 22Mn B5 EN 22Mn B5 Höganäs

1559–40 + 50% 4590 - Specimen material specific

density (g/cm³) 7.75 7.75 13 1.7

Abradant Silica Kabelmjöl Kabelmjöl -

Initial mass (g) 1317.095 1301.860 1334.691 -

Final mass (g) 1307.549 1279.820 1286.105 -

Total mass loss (g) 11.493 22.040 48.586 2.1

Mass loss from 1120 km (g) 3.643 1.842 0.556 0.30

Wear rate from 1120 km

(g/day) 0.924 0.273 0.077 0.28

Total volume loss (mm³) 1483 2844 3737 1.31

Volume loss from 1120 km

(mm³) 470 238 43 0.18

Wear rate from 1120 km

(mm³/day) 119 35 6 0.17

 Mass loss is the subtraction of the final measured mass from the initially measured mass.

This subtraction may include the loss of paint and rust that may have already been found on the tool’s surface before the tests had begun.

 Mass loss from 1120 km is the subtraction of the final measured mass from the mass at 1120 km of travelled distance. This distance is the “stabilization point” after which the wear is observed to behave linearly on all three samples.

 Volume loss is obtained theoretically, dividing mass loss by specific density.

 Wear rate is the average loss of mass per day starting from the stabilization point of 1120 km of travelled distance.

 The last column of Table 8 presents the ratio between the results of Tests 3 and 2 on each row. It provides an easier way to interpret the results.

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From Table 8, the following important observation can be noted. It is further addressed in 5.1 Discussion.

 The laser-cladded sample, test 3, presents a specific density of approximately 13 g/cm³, since carbides’ density must be accounted for, and not just the density of the EN 22MnB5 steel.

 The 3/2 ratio of total mass loss is 2.1, and the ratio of total volume loss is 1.31.

 The 3/2 ratio of total mass loss after the stabilization point is only 0.3. The volume loss ratio is 0.18.

Figure 40 shows the mass of each specimen as travel distance increases. It is observed that a gradual loss of mass is achieved on all specimens. However, only the specimen running in silica sand (blue line) shows a continuously decreasing mass from beginning to end on all the days when mass was measured. The other two specimens, which ran in Kabelmjöl sand, show a strong loss of mass over the first 224 km. Thereafter, an increase in mass is evident from 224 km to 672 km of travelled distance. Finally, an almost linear mass loss is observed from 672 km.

It was observed that the loss of mass stabilizes on all three samples after 1120 km, since a linear regression with R² > 0.95 can be obtained.

The laser cladded sample is the one with the smallest loss of mass of all three samples after stabilization, with a linear regression slope of -0.069 and a loss of only 0.556 g.

The largest loss of mass after stabilization was suffered by the EN 22MnB5 specimen running in silica, with 3.643 g and a steeper slope of -0.923.

Abrasive wear of hard faced ground engaging tool

Abrasive wear of hard faced ground engaging tools

DAVID EUGENIO CANTU GOMEZ

Abrasive wear of hard faced ground engaging tools

David Eugenio Cantú Gómez

SAMMANFATTNING

ABSTRACT

FOREWORD

TABLE OF CONTENTS

1. INTRODUCTION

1.1 Background 1.1.1 GETs

1.1.2 Abrasive wear on GETs

1.1.3 Höganäs AB, the customer

1.1.4 Tests categorization

1.1.5 Carousel tribometer

1.2 Objectives and purpose

1.3 Scope and delimitations

1.4 Methods

2. FRAME OF REFERENCE

2.1 Mechanisms of wear

Lack of lubrication or

coating Lack of lubrication or

coating

Increase in friction Increase in

friction

Wear increases and performance

drops Wear increases and performance

drops

2.1.1 Fatigue wear

2.1.2 Corrosive wear

2.1.3 Adhesive wear

2.1.4 Abrasive wear

2.2 Abrasivity of particles

𝑅

=

𝑅

=

2.3 Characterization and testing of wear

2.3.1 The ASTM G65 test

2.3.2 Profilometry

2.3.3 Archard’s equation

𝑉 =

2.4 Increasing resistance to abrasive wear

2.4.1 Laser cladding

3. IMPLEMENTATION

3.1 Stage 1: performed at KTH

3.1.1 Carousel tribometer’s tool holder design

3.1.2 Setup and pre-processing of specimens

3.1.3 Tribometer’s test parameters

3.1.4 Method for surface profiling

3.1.5 Method for mass measurement

3.2. Stage 2: performed at Höganäs AB 3.2.1 Method for cutting and sampling

3.2.2 Method for electron microscope analysis

4. RESULTS

4.1 Mass, wear rate and surface profile