DEGREE PROJECT, IN , SECOND LEVELMATERIAL DESIGN
STOCKHOLM, SWEDEN 2014
Wear of Grate Plates
JOHAN MARTINSSON
KTH ROYAL INSTITUTE OF TECHNOLOGY
DIVISION OF MICRO-MODELLING AND EXPERIMENTAL KINETICS
Foreword
This Master Thesis has been carried out with the Department of Mining and Logistic Technology ofLKAB, in Kiruna, in collaboration with the Division of Mico-Modelling and Experimental Kineticsof the Royal Institute of Technology (KTH), Stockholm, Sweden. It is directed to people with someexpertise in materials and process science. For questions, �nd contact details in table below.
Student Supervisor LKAB Supervisor KTH Examinator KTH
Johan Martinsson Lars Pettersson Bjoern Glaser Du [email protected] [email protected] [email protected] [email protected]
Abstract
This report describe the wear of the protruding areas on the sides of the grate plates in the grate-kilnprocess in the hematite pellet production in LKAB, Kiruna. The steel plates are exposed to a hostileenvironment with heat cycles and corrosive atmosphere. An evaluation of the plates was made inco-operation with LKAB Metlab in Luleå and LKAB mechanical workshop in Kiruna. Instrumentsused are stereomicroscope, LOM, SEM, Spectroscope, Vickers Hardness and a surface �nish meter.Results show the protruding areas of the plates are exposed to a tribochemical wear, where tops of therough areas are torn down.
A coating test is carried out at Tribolab, LTU in Luleå, using an SRV. Samples with a wear andcorrosion resistant coating called Diamalloy 4276 abrade against eachother at high temperature andpressure. The coating do help to resist wear, but the environment of the test is to unrealistic to sayby certain that it will help in the grate.
A FEM-model in COMSOL Multiphysics 4.4 is made to calculate thermal stresses between coatingand metal, the result show stresses up to 1 GPa will occur, this can be explain by the big di�erence inthermal expansion coe�cients of the materials. It will probably create cracks in the coating surface.
Two solutions are presented, a coating is not recommended. The tribochemical wear is decreasedby using a better surface �nish. Therefore one can either machine the areas by drilling or milling, orone can change the casting method. Today sand casting is used, by using shell casting or precisioncasting, for example Shaw process, the surface �nish is better over the whole plate, which also is betterfor corrosion resistance since less initiation points exist.
Sammanfattning
Denna rapport presenterar en undersökning av slitaget på sidorna av grateplattor som används i grate-kilnprocessen i hematitpelletstillverkningen på LKAB i Kiruna, även lösningar presenteras. Plattornabe�nner sig i en svår miljö med termiska cykler och korrosiva substanser. Utvärderingen av slitagetgjordes i samarbete med LKAB Metlab i Luleå och LKAB mekaniska verkstad i Kiruna. Utrust-ning som användes var Stereomikroskop, LOM, SEM, Spektroskop, Vickers hårdhetsmätare och yt�n-hetsmätare. Resultatet visar att sidorna av plattorna utsetts för ett tribokemiskt slitage där topparav den grova utan slits ner.
Ett ytbeläggningstest utfördes på Tribolab, LTU i Luleå, med en SRV. Prover med en beläggninghar gnidits mot varandra under tryck och hög temperatur och jämförts med prover utan beläggningsom utsattes för samma test. Ytbeläggningen som används står främst emot korrosion, men ävenslitage, den kallas Diamalloy 4276. Resultatet visar att beläggningen skyddar bra mot slitage, menmiljön under testet var för orealistiskt för att med säkerhet kunna säga att det kommer hjälpa i graten.
En FEM-modell gjordes med hjälp av COMSOL Multiphysics 4.4 för att beräkna de termiska spän-ningarna som uppstår mellan ytbeläggningen och metallen. Resultatet visar att spänningar på upp till1 GPa kommer uppstå, detta kan förklaras med den stora skillnaden i termisk utvidgningskoe�cientmellan de två materialen. De höga spänningarna kan skapa sprickor i ytan.
Två lösningar presenteras, en ytbeläggning rekommenderas inte i dagsläget. Det tribokemiska slitagetkan motverkas genom att förbättra yt�nheten. Detta kan antingen utföras genom att bearbeta ytan,med fräsning eller slipning, eller att man byter gjutningsmetod. Idag används manuell formtillverkning,om man istället skulle använda skalformsgjutning eller precisionsgjutning, till exempel Shawprocessen,skulle man få en bättre yt�nhet över hela plattan. Detta leder även till bättre korrosionsbeständighetdå färre initieringspunkter �nns.
Contents
1 Introduction 10
2 Background 12
2.1 Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.2 Grate plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.3 Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3.1 Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.3.2 Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.3.3 Thermal Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.3.4 Abrasion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.4 Problematic consequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.5 Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.6 Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3 Experimental Procedure 18
3.1 Grate Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.1.1 Old Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.1.2 New Plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2 Coating test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4 Modelling 20
4.1 Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5 Results 24
5.1 Grate Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245.1.1 Ocular Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.1.2 Stereoscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.1.3 LOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.1.4 SEM Pictures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.1.5 SEM EDS Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285.1.6 Vickers Hardness test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315.1.7 Optical emission Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325.1.8 Ra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
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5.2 Coating test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325.3 COMSOL Multiphysics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
6 Discussion 36
6.1 Study of Grate plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366.2 Study of coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376.3 Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376.4 Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
7 Conclusions 40
8 Future Work 42
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Chapter 1
Introduction
Loussavaara-Kiirunavaara AB (LKAB) is a Swedish mining company producing mainly pellets ofhematite, Fe2O3. The iron ore that is extracted is however mostly magnetite, Fe3O4, this is very ben-e�cial in a sustainability perspective in the production of hematite pellets, since in the sinter process,the reaction of magnetite to hematite is exothermic and produces heat. LKAB uses this heat to reduceexternal heating and thereby lowering the total energy consumption. They promote this method bycalling their product �Green Pellets�. [1]
In the production of pellets in Kiruna, the pellets are sintered in a Grate-Kiln process. It consistof a preheating furnace called Grate, a sinter furnace called Kiln and ends with a cooling system. TheGrate is a belt furnace consisting numerous small steel plates that moves a bed of pellets forward. [1]
The steel plates in the Grate are degrading fast in a problematic environment and need to be changedonce every year. This is very expensive and prevents other necessary maintenance work. Therefore, theDivision of Mining and Logistic Technology of LKAB has worked to develop a solution that increasesthe lifetime of the steel plates. The degradation is suspected to be due to a combination of corrosion,erosion, abrasion and thermal stresses. [2]
The aim and purpose of this project is to determine the wear on the sides of the plates, and �nda solution for the problem. This will be carried out by investigating old, used plates, and to makean experiment to compare new plates with and without a wear and corrosion resistant coating calledDiamalloy 4276, and see how it might help at high temperatures. Also a model in COMSOL Multi-physics 4.4 will be designed to simulate thermal stresses and see how the adhesion of the coating willbe a�ected by the severe temperature changes.
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Chapter 2
Background
2.1 Process
When the pellets arrive to the Grate, they have been formed to spheres with a diameter of around10 mm and contain magnetite, binders, additives and water. The pellets are evenly distributed in a200 mm thick layer on the belt of steel plates. The Grate process' purpose is now to dry and pre-heatthe pellets, which make them harder and well prepared for the upcoming sinter process in the Kiln.The steel plates building up the Grate belt is shown in Figure 2.1. These plates go through fourdi�erent temperature zones in the Grate, with isolating walls in between. In the process, as mentionedin the introduction, the pellets generate a large amount of energy through the exothermic oxidation.This allows LKAB pellet production in Kiruna to use 60% less energy than standard hematite-basedmanufacturing.[1]
Figure 2.1: Grate plate
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2.2 Grate plate
The plates are produced by sand casting and sandblasting, leaving a rough surface. The plates aremade by a company called Shakespeare Foundry LTD. The material used is called HH type-II, namedby Alloy Casting Institute, covered by ASTM 447 speci�cation. It is a high-alloyed casted steel withfully austenite structure, where the alloy composition can be seen in Table 2.1. The material is designedto resist corrosion, creep and stress-rupture in applications where service temperature is above 650 oC.The yield strength is then 150MPa, and it may work up to temperatures as high as 1100 oC where ayield strength of 65MPa applies. [3][4]
C Mn Si Cr Ni P S
min% 0.20 - - 24.0 11.0 - -max% 0.50 2.0 2.0 28.0 14.0 0.04 0.04
Table 2.1: Alloy composition of HH type II
2.3 Degradation
The plates are suspected to be exposed to four types of degradation; Corrosion, Erosion, Thermalstress and Abrasion. [2]
2.3.1 Corrosion
The environment is very hostile, consisting for example oxygen, carbon oxides, sulphur and chlorine.A previsous study of Grate plates, [5], have discussed the possibility of carburization, metal dusting,chromium vaporization, intergranular attack, hydrogen assisted cracking, stress corrosion cracking,halogens and sul�dation corrosion, that study was made on the whole grate plate.
2.3.2 Erosion
Erosion may occur due to dust and particles in the atmosphere, which are created by crushed pellets.These particles can scratch and plow the surface and degrade the material. The properties of theseparticles are of great interest for understanding erosion; a combination of size, hardness, velocity andangle of impingement have to be considered [6]. An evaluation of the erosion of the grate plates is partof a PhD project that LKAB is �nancing.
2.3.3 Thermal Stresses
As mentioned before, the Grate consist of di�erent temperature zones, this exposes the plates to athermal cycle. The temperatures the plates obtain in each temperature zone in the grate are notcertain since they are protected by the bed of pellet and therefore not achieving the same controlled
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atmosphere temperature which is found above the pellet bed. A smaller investigation has althoughtbeen carried out, using thermo elements to examine the temperature change of di�erent plates duringthe grate process, the test were made by Swerea Mefos, the results are con�dential but used in theCOMSOL model. In an earlier work made in co-operation with Swerea Swecast[7], a FEM-simulationresulted in a change of design which will lowering the thermal stresses in the whole plate. The newdesign will be used in four plate rows from next year. It is the new design that is used as model inthis project.
2.3.4 Abrasion
The Department of Mining and Logistic Technology of LKAB has believes that the plates slide againsteach other and abrasion occurs. The theoretical movement of the belt is described in Figure 2.2 and2.3. When the belt moves forward, movements are possible both vertically and horizontally. In Figure2.2, it is shown how the belt may move sideways, while in Figure 2.3, one can see the belt of platesmoves forward as waves, up and down, due to placement of the big wheels that pushes the belt forward,this �gure is very exaggerative.[2]
Figure 2.2: Side movements of Grate belt
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Figure 2.3: Vertical movements of Grate belt
A picture from the side of one plate is illustrated in Figure 2.4. The blue areas are surfaces thatprotrude a little bit from the other surfaces and therefore the areas where wear most likely occur. Thepurpose of the protrusion is to create an air gap between the plates for the air to �ow through tofacilitate the heat treatment process of the pellets. The exposed areas must, according to blueprints,have a surface �nish of 6.3 Ra.
Figure 2.4: Areas where abrasion most likely occur
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2.4 Problematic consequences
After a couple of months, big air gaps between the steel plates occur, see description in Figure 2.5.This allows hot air to �ow between the plates and accelerate the degradation. When the gap is bigenough, pellets can fall down between, or get stuck between the plates which may cause erosion andaccelerate the degradation even more. Today, this is �xed with a maintanance shutdown and the platerows are pushed together and a spacer plate is placed on the sides of the steel bars holding up thegrate plates [2].
Figure 2.5: Air gap between plates
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2.5 Coating
The coating that is used as wear and corrosion resistance in the experiments in this project is calledDiamalloy 4276 and is manufactured by Sulzer Metco, and the composition can be seen in Table 2.2. Itresists crevice and pitting corrosion in sulfuric acid and chlorine environments to a service temperatureof 875oC, and oxidation and wear resistance to a service temperature of 980oC [8]. Manufactures cannot give any garanties of lifetime since the application is new. The coating is mainly protective againstcorrosion.
Alloy Ni Cr W Mo Fe
wt% Balance 15.5 4.5 16.0 4.0
Table 2.2: Alloy composition of Diamalloy 4276
The coating is applied by ABRATEC AB with a method called High Velocity Oxygen Fuel (HVOF).The thickness of the coating is 0.2 mm and the idea is that if the experiments succeed and the resultsof the examination of the used plates tells a coating is necessary, it will be applied on the protrudingareas shown in Figure 2.4.
2.6 Modelling
Computerizing models are more and more common in material and process development. Therefore,in this project, a model was made using COMSOL Multiphysics. It is a software developed by COM-SOL AB, which was founded 1986 in Stockholm, Sweden. Their purpose has been to create a userfriendly software for modelling and simulating real world system, for example in heat conduction, �uidmechanics, structural mechanics, electromagnetism and chemical engineering. The company targetsresearchers and engineers working for leading technical enterprises, research labs, and universities andthey wishes to accelerate the development with virtual prototypes. The software solves partial di�er-ential equations using �nite element method. [9, 10].
In this project COMSOL Multiphysics is used to calculate the thermal stresses between coating andmetal to see if there will be any problems with the adhesion of the coating.
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Chapter 3
Experimental Procedure
3.1 Grate Plates
Both old and new plates were examined. Old plates were examined to determine the wear of theprotruding areas, and a new plate were examined to be used as reference to the old ones, but also tocheck the surface �nish.
3.1.1 Old Plates
Two old, used plates were examined. First an ocular examination was made and pictures were takenwith a digital camera. Then parts of the exposed areas were cut out, from which three parts weremounted in plugs of Polyfast using compression-type molding, these samples were grounded and pol-ished for cross-section examination. An other part was put in ethanol for a couple of hours for removalof oxidation layer which facilitate the examination of wear mechanism on the surface, and the partsleft were just let to be.
The plugs for cross-section examination were �rst used for a Vickers Hardness test, and then analysedwith Light Optical Microscope (LOM) and Scanning Electron Microscope (SEM). All samples wereanalysed with and without etch, etchant used was a solution of 60 ml Oxalic acid and 40 ml distilledwater, which exposes carbides. All SEM-pictures were taken by a Hitachi S3400N with 20.0 kV, bothback scattered electrons (BSE) and Secondary electrons (SE) were used, the result �gures show whichone. Also Energy Dispersive X-ray Spectrometry (EDS) analysis were made in SEM, with the purposeto �nd which elements that were present in a speci�c area of the samples.
The sample that was put in ethanol and the ones let to be were examined in a Stereo microscope,LOM and SEM. The same SEM settings were used as for cross-section samples.
At last, the oxide layer of a part were cut o� and an alloy composition was measured using opti-cal emission spectroscopy, �ve tests were made on the part, and an average value for each elementwere calculated.
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3.1.2 New Plate
The protruding areas of a new plate were cut to pieces and mounted in plugs of Polyfast and etchedwith same etchant as for old plates. SEM and LOM were used for examination with same settings asfor old plates.
A surface roughness test of Ra was made on the protruding areas. The tests were carried out bysta� at LKAB Mechanical Workshop.
3.2 Coating test
The wear resistance experiments were made at Tribolab, at Luleå Technology University (LTU), withan instrument called SRV, made by Optimol Instruments. It's designed to make tribological studieson lubrication and studies of friction and wear between solids at elevated temperatures. Two sizes ofcylindrical pellets were cut out from an unused grate plate, one disc with dimensions Ø10mm withheight 10mm, and a pin with dimensions Ø24mm with height 7.5mm. Machining was made by amechanical workshop of LTU. Half of the cylindrical pellets were then coated on one of the �at sideswith Diamalloy 4276, using HVOF.
During the test, the SRV preheat the disc, then pusches the pin on to the disc and stroke the pinon the disc with constant force during a preset time with a preset frequency.
Tests were made both with and without coating, friction coe�cient and weight loss of the tests werecalculated. The friction coe�cient were measured by test equipment, and the weight loss were mea-sured by a precition weight using �ve decimals of one gram.
The parameters of the experiment can be seen in Table 3.1
Force Stroke length Frequency Temperature Time236 N 4 mm 5 Hz 600 degC 15 min
Table 3.1: Parameters of coating test
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Chapter 4
Modelling
Software version used for modelling and simulation is COMSOL Multiphysics 4.4. The material pa-rameters used in the model are presented in Table 4.1.
Thermal Conductivity, [W/mK] Heat Capacity, [J/kgK] Density, [kg/m^3]Coating 11.26[11] 385[12] 8941[11]Metal 17.5[3] 450[12] 7722[4]
Thermal Expansion Coe�cient, [1/K] Young's Modulus, [GPa] Poisson's ratio [1]Coating 11.34e-6[11] 180[11] 0.3[13]Metal 17.1e-6[4] 187[4] 0.3[13]
Table 4.1: Material parameters for COMSOL Multiphysics 4.4
COMSOL needs a unique solution when calculating solid mechanics. This may create di�culties withhigh local stresses where one set speci�c mechanical conditions to lock the geometry, therefore di�erenttests with di�erent solid mechanical conditions were made.
4.1 Approach
• 3D-component as chosen
• The geometry was imported from �le, it's half of the total geometry due to symmetry. A coatingwas extruded 0.2mm on the areas where abrasion most likely occur, which could be seen in Figure2.4
• Two new materials were chosen and given parameters, as can be seen in Table 4.1, and set torespectively domain
• A Parameter was added, cycle length, t_cyc = 1840 s
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• An interpolation function was added with the name Tplate to describe the temperature of thegrate plate top during the cycle, representing the pellet bed, values used are the con�dentialvalues from the Swerea Mefos evaluation
• Another interpolation function was added with name Tair, to describe the temperature of theair surrounding the plate, these temperatures are also con�dential
• Thermal stress in the mechanical structure branch was chosen as physics, and a solid mechanicsand a heat transfer in solids-interface automatically occurred
• In the solid mechanics, a symmetry node was set on the area representing the middle, prescribeddisplacement was set on line 1 in Figure 4.1 with setting 0 m displacement in x-axis. Anotherprescribed displacement was set on line 2 and 3 in Figure 4.1 with setting 0 m displacement iny-axis
◦ A second model was made with di�erent solid mechanical contition settings. Instead ofusing prescribed displacement of three di�erent lines, a node of Rigid Connector was set onblue area in Figure 4.2
• In the Heat transfer interface, a symmetry node was set on same area as in solid mechanicinterface, a surface-to-ambient radiation node was added for whole geometry, A Convective heat�ux node was set with External forced convection with Plate length = 0.3 m, Velocity external�uid = 1 m/s, External �uid = air, Absolute pressure = 1 atm, and External temperature =TGrate(mod(t,t_cyc)). A Temperature node was at last set on the top area of the plate with thetemperature Tplate(mod(t,t_cyc)). The mod-command creates a loop, which allow the thermalcycle to continue to wanted time
• A free tetrahedral mesh with normal prede�ned mesh size was added
• A Time Dependent Study was added with time range: start at 0 s, time step 1840/15 s andstop at 6*1840 s, where 6 represent number of cycles
Figure 4.1: Lines with Prescribed Displacement
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Figure 4.2: Area of Rigid connector condition
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23
Chapter 5
Results
This chapter is divided into three sections; Grate plates, Coating test and COMSOL Multiphysics.Overall the experiments was succeeded and results seem fair.
5.1 Grate Plates
Figure 5.1-5.6 show a severe corrosion of the plate, but very little abrasion. The surface is still veryrough and uneven with many peaks and valleys. On the triangular protruding part seen in Figure5.5, only the peaks of the rough surface has been exposed to abrasion. The large scratch marks seenin Figure 5.6 come from machining after casting process at foundry and can be observed over severalareas on the plate.
Vickers hardness test results are shown in Figures 5.4-5.6. They tell there have been no harden-ing on the surface, which indicate the plates do not grind to each other with pressure high enough tocreate deformation hardening.
From the LOM-pictures without etch, one can see there are many small cracks on the surface. Thesecracks are analysed with SEM EDS Analysis, and in Figure 5.15 and Table 5.3 one can see a crackpropagate along the grain boundary and contains sulphur all the way. This is a recurring phenomenonin many cracks that were found. In the LOM-pictures where the surface has been etched, one can seeaustenite that has grown in dendrite structure, with chromium carbide in between them.
In SEM-pictures Figure 5.11 and 5.12, a peak is investigated where �akes were observed and anal-ysed in SEM EDS. This can be seen in Figure 5.13 with Table 5.1 and Figure 5.14 with Table 5.2.
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5.1.1 Ocular Evaluation
A severe corrosion can be seen, motsly on the bars in the middle of the plate. The protruding areasshow some abraded areas where shiny metal can be observed.
Figure 5.1: Used Plate from aboveFigure 5.2: Protruding Area
Figure 5.3: Abraded area around hole Figure 5.4: Abraded triangular area
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5.1.2 Stereoscope
Abraded peaks can be seen in Figure 5.5.
Figure 5.5: Abraded peaks Figure 5.6: Scratch lines
5.1.3 LOM
Some cracks were found, mostly smaller but every sample had a few longer ones, Figure 5.7. Themicrostructure of a used plate did not di�er much from a new one, a betters grain size homogeneitycould be seen in some parts of the surface, a comparison can be seen in Figure 5.9 and 5.10.
Figure 5.7: Crack Figure 5.8: Two abraded peaks with cracks
26
Figure 5.9: Microstructure of Old Pate Figure 5.10: Microstructrure of New Plate
5.1.4 SEM Pictures
Peaks were located and examined and many �akes were found.
Figure 5.11: Abraded peak Figure 5.12: Oxide �ake on abraded peak
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5.1.5 SEM EDS Analysis
Two abraded peaks on the surface were located, marked with orange rings, and a point analyse wasmade. The results show that the �at areas are mostly main material while there are oxides on thesides.
Figure 5.13: Point analysis of two peaks
Mass Percent (wt%)
Element O Al Si S Cl Cr Fe NiP 64 2.52 - 1.05 0.88 1.28 25.89 51.36 12.42P 65 10.81 0.20 1.30 0.74 0.57 28.87 47.02 5.70P 66 1.56 2.01 0.80 0.61 1.33 23.40 54.18 12.60P 67 1.48 - 0.79 0.35 0.74 26.41 53.80 11.44P 68 10.70 0.16 0.52 0.41 0.23 11.30 66.35 6.38P 69 29.29 - 53.77 - - - 2.78 -
Table 5.1: Values in points on peaks
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On one of the big abraded peak that was shown in Figure 5.13, a �ake was located, see �gure 5.14,and a point analyse was made. The results show the �ake is an oxide and the bright areas besides itare the main material.
Figure 5.14: Point analysis of a �ake on a peak
Mass Percent (wt%)
Element O Si S Cl Cr Fe NiP 80 9.95 0.79 0.75 0.89 20.40 53.14 7.27P 81 9.56 0.83 0.64 0.69 13.76 59.93 7.27P 82 0.68 1.05 - - 25.40 56.22 12.35P 83 1.07 0.87 - - 24.50 56.40 12.52
Table 5.2: Values in point of a �ake
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This analyse show the cracks contain sulphur and chlorine, P55 in Figure 5.15 with Table 5.3.
Figure 5.15: Point analysis of cross-section
Mass percent (wt%)
Element O Si S Cl Cr Fe NiP 52 8.28 1.14 2.44 0.91 28.87 46.61 7.80P 53 10.86 1.24 1.98 0.55 14.14 56.52 5.17P 54 10.58 1.19 2.03 0.72 22.41 51.92 6.49P 55 7.03 0.89 6.27 2.43 22.83 40.61 13.90P 56 - 0.86 - - 22.67 40.61 12.05P 57 - 0.75 - - 23.29 60.86 11.75
Table 5.3: Values in points of cross-section
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5.1.6 Vickers Hardness test
The test show no di�erence in Vickers Hardness between old plates, Table 5.4 and 5.5, compared tohardness in a new plate, Table 5.6.
Hardness test 1
Impact Hardness Distance fromsurface
Average 192
1 200 250 Max 2032 189 530 Min 1833 183 850 Std 8.64 187 11505 203 1450
Table 5.4: Hv values nr 1 of old plate
Hardness test 2
Impact Hardness Distance fromsurface
Average 200
1 220 250 Max 2202 199 550 Min 1923 198 850 Std 11.44 193 11505 192 1450
Table 5.5: Hv values nr 2 of old plate
Hardness test of new Plate
Impact Hardness Distance fromsurface
Average 199
1 222 250 Max 2222 199 550 Min 1853 193 850 Std 13.84 198 11505 185 1450
Table 5.6: Hv values of new plate
31
5.1.7 Optical emission Spectroscopy
The alloy composition in the material was measured �ve times in two samples, an average value wascalculated and the results can be seen in Table 5.7
Element C Mn Si Cr Ni P Swt% 0.436 0.63 0.93 24.59 11.86 0.025 0.35
Table 5.7: Alloy composition of old Grate Plate
5.1.8 Ra
The value was measured on the protruding areas of a new, unused plate. Due to a very rough surface,the test was di�cult to perform, but a smoother area was found and a value of Ra= 61 m was measured.
5.2 Coating test
Four tests are presented. Sample 1 and 2 have no coating. Sample 3 and 4 have coating.
The results of the friction coe�cient measurement show the samples 3 and 4, Figure 5.18 and 5.19,get a relatively constant value of 0.8, while samples 1 and 2, Figure 5.16 and 5.17, are very irregularwith a base value of around 0.7-0.8.
The weight loss measurement show a relativly constant mass loss of the disc, while the loss of thepin is more irregular. Sample 3 and 4 have a lower mass loss than Sample 1 and 2. Results of weightloss can be seen in Table 5.8.
Figure 5.16: Friction Coe�cient Sample 1Figure 5.17: Friction Coe�cient Sample 2
32
Figure 5.18: Friction Coe�cient Sample 3 Figure 5.19: Friction Coe�cient Sample 4
Before g After g Before g After g Weight loss g Percentage weight loss %Disc Pin Disc Pin Disc Pin
Sample 1 27,13185 27,07972 6,03532 6,0345 0,052133 0,00082 -0,192518 -0,0135885Sample 2 27,27002 27,22153 6,04901 6,04547 0,04849 0,00354 -0,1778265 -0,0585219Sample 3 27,54893 27,54640 6,10280 6,10113 0,00253 0,00167 -0,0091845 -0,0274737Sample 4 27,52858 27,52723 6,10977 6,10854 0,00135 0,00123 -0,0049161 -0,0201862
Table 5.8: Weight measurements coating test
33
5.3 COMSOL Multiphysics
The model show that high stresses occur, up to 1 GPa. The di�erent solid mechanic conditions usedin the approach gave no big di�erenties in results, see Figure 5.20 and 5.21
Figure 5.20: Thermal stresses, Prescribed displacement
Figure 5.21: Thermal stresses, Rigid connector
34
The mechanical conditions do create an error. High stresses occur at the points where conditionparameters take place, but the error do not seem to disturb the calculated stresses on the abradingareas, see Figure 5.22 and 5.23 for error.
Figure 5.22: Error from Prescribed displace-ment
Figure 5.23: Error from Rigid connector
35
Chapter 6
Discussion
6.1 Study of Grate plate
The main purpose of this thesis was to examine the wear of the exposed protruding areas seen in Figure2.4, and to �nd a solution. The results of the examination of old used plates compared to a new unusedplate show that the abrasion is not that severe. In the ocular examination, Figures 5.1 - 5.6, it seemsthat on some areas abrasion do not even occur, only corrosion which is extreme. When looking closerwith LOM and SEM, it is shown that the peaks on the rough area are torn and sometimes completelygone, but never is the whole protruding areas exposed for abrasion. When several peaks in a smallerarea of the exposed areas are torn down completely, the abrasion seems to stop, which is naturallysince the pressure of the abrasion equals force per area, and the area is much larger when the peaksare torn o� to a smooth surface.
In Figures 5.13 and 5.14 with respectively Tables 5.1 and 5.2, one can see abraded peaks with surfacesconsisting exposed basic metal and �akes of iron oxides. P65, 68 in Figure 5.13 and P80, 81 in Figure5.14 show content of oxide �akes, while P66, 67 in Figure 5.13 and P82, 83 in Figure 5.14 displaysarea that lack of oxides. Knowing this and the fact that the atmosphere is very complex with heat,oxygen and corrosive ions, one can assume that tribochemical wear takes place, mostly oxidative. Thisis very common on rough surfaces with peaks that are exposed to abrasion in combination with acorrosive environment; a layer of oxides is created on the peaks that are easily torn o� due to abrasion,after which the metal is exposed on the peak again and a new oxide layer may be created and thentorn o� again. Since there are totally 30 steel plate sides that are exposed to this mechanism, it caneasily be the reason for the unwanted gap of around 20 mm that occur between the rows of steel plates.
At the same time the tribochemical wear takes place, and metal is exposed to the atmosphere, ions ofsulphur and chloride may react and di�use into the material through grains boundaries of chromiumcarbides and create cracks that propogate from the surface. Cracks can be seen in Figures 5.7 and5.8, and the content of a crack in P55 in Figure 5.15 with Table 5.3 proves the present of sulphur andchlorine. This will most certainly enhance the abrasion of peaks and make the surface more brittle.
36
Results of Vickers hardness test tell there has been no hardening. The new plate, Table 5.6, hasthe same values as the old plates, Tables 5.4 and 5.5.
Comparing microstructure of old and new plates, one can see the grains have grown together in the oldones. In Figure 5.10 one can see the microstructure of a new plate, with small grains at the surface,followed by columnar grains and in the middle normal grains. While in Figure 5.9, the microstruc-ture of an old plate is shown, here one can see a more homogeneous size distribution of the grains.This has nothing to do with wear, but the heat treatment the plates are exposed to in the Grate process.
The composition from the optical emission spectroscopy, Table 5.7, show that all elements are incorrect range according to HH type II standard.
The surface roughness are according to blueprints supposed to be maximum 6.3 µm on the protrudingareas, but no surface treatment has been made and a roughness of 61 µm in Ra has been calculated.Whether the surface roughness has been discussed between LKAB and manufactures, and if a decisionto ignore the surface �nish has been made is unknown.
6.2 Study of coating
The test do not represent the real environment of the grate plate, but the results do show the coatinghelp to lower the wear, Table 5.8. What the test do not take into consideration is the corrosive atmo-sphere and the thermal cycle. Also, due to time constraints only eight tests were made, and only fourof them gave results worth evaluating. More tests have to be made to get better statistics and morecertain results.
The friction coe�cients, Figure 5.16 - 5.19, tell the samples with coating slides smoother againsteachother than the ones without coating. A theory of the irregularity of the friction coe�cient thatcan be seen in Figure 5.16 and 5.17, is that adhesion could occur between the two steel surfaces forcingthe surface to rip of material and disturb the friction. This can create a mass transfer between thesamples, which could explain the irregularity of the mass loss of the pin without coating, Table 5.8.
Since the coating is designed to primary resist corrosion, a coating of this kind is most suitable,but a more severe investigation and an economic evaluation have to be made.
6.3 Modelling
The results from COMSOL Multiphysics show there will be high thermal stresses, this is mostly dueto the di�erence in thermal expansion coe�cient, the metal has a 50 % higher value than the coating,Table 4.1.
Stresses are calculated up to 1 GPa, this does not mean there will be problem with the adhesionbetween coating and the metal, but cracks will probably occur on the surface and in combination withthe corrosive environment and the abrasion that occur, the lifetime of the coating may not be as longas one may hope.
37
An error in the model is that the coating lays on the casted steel as a di�erent domains, while inreality, the coating has di�used in to the steel. This is lowering the gradient of property di�erencesbetween the two di�erent materials, which in turn lowers the thermal stresses.
6.4 Solutions
Since most wear is tribochemical wear, with peaks of the rough surface with oxide layers being torno�, a better surface �nish is to be recommended. The roughness in blueprints is recommended to Ra=6.3 µm, this should be applied. The surface �nish can be improved in several ways, one simple way isto grind or mill the surface after the manufacture, another way is to change the casting method. Usinga process that gives a better surface �nish over the whole grate plate will also lower the corrosion overthe plate since there will be fewer initiation points. Many casting methods gives better surface �nish,but for a relativley low production cost for small series sizes either shell casting or precision castingis recommended. Shell casting give high dimensional accuracy, good reproduction of the shape of thecomponent, good surface smoothness and no burnt sand sticking to the surface, but the initial costfor the model equipment is high and must be made of cast iron. Precision casting gives roughly thesame measure of precision as the shell mould casting method but is protable to use for smaller seriesand single castings because the pattern of the mould can be made of wood or gypsum. A more severeevaluation should be made to deside which is more reliable and pro�table in this case.
Another solution that could be considered is the possibility of manufacturing wider steel plates. Withwider steel plates, one get fewer surfaces that have to abrade against each other. Problem with thissolution is the alignment of the circular holes in the plates; without perfect alignment it will be di�cultto assemble the plates on the steel bars. If a change of casting method is made to get better dimensionaccuracy, this solution can be worth evaluating.
The coating tests show the corrosion resisting coating do increase wear resistance, but the thermalstress calculation made in COMSOL are very high and it is uncertain if the coating will prolong thelifetime of the grate. The coating is also a very expencive.
In a longer time range, LKAB should consider designing a Grate belt where ceramic are possibleto use, and still use same environment and conditions. This will most probably prolong the lifetime ofthe Grate. If one would use ceramic plates instead of HH type II in today's design, cracks would mostlikely occur and break the plates, but even if ceramic plates would hold, the steel bars holding themup, would probably prohibit a much longer lifetime.
38
39
Chapter 7
Conclusions
• The protruding areas are exposed to tribochemical wear.
• COMSOL model show there will be thermal stresses up to 1 GPa.
• The coating is lowering the wear, but can not be recommended until furthur research has beenmade using more realistic environment.
• The recommended solution is to improve the surface �nish. This could be applied by milling orgrinding the protruding surfaces, or using an other casting method, for example shell casting orprecision casting. An economic and reliability comparison of these alternatives should be made.
◦ Using a di�erent casting method with better dimensional accuracy allow a new grate platedesign with wider dimensions, which create fewer surfaces that abrade.
40
41
Chapter 8
Future Work
A discussion with Shakespeare Foundry should begin with the goal to �nd the most suitable solutionfor a better surface �nish. If the foundry can't change casting method, a change of foundry may benecessary to take into consideration.
42
Acknowledgment
I would like to thank the Department of Mining and Logistic Technology at LKAB for giving me thechance to live and work in Kiruna, it has been very interesting and I would certainly like to come backsome day. I would also like to thank Bjoern Glaser at KTH for the help during the project and withthe report. I would like to thank Jari and Greg at LKAB for helpful discussions and company duringthe days. At Metlab I would like to thank Lars Bohman and Lars-Olof Nordin who helped me with thestudy of the grate plates and the understanding of the results. At LTU, I would like to thank Tribolaband Sergej Mozgovoy who helped me with the coating tests. I would like to thank Per Backlund forthe help with modelling in COMSOL.
A special thanks goes to my LKAB supervisor Lars Pettersson, who guided me through the project,helped me get to know the city of Kiruna and showed me the art of waxing cross country skis.
43
Bibliography
[1] "Om oss", LKAB, [Online]. Available:http://www.lkab.com/sv/om-oss/
[2] Interview with Lars Pettersson, Development Engineer LKAB, 2014-03-20, Department of Miningand Logistic Technology at LKAB, Kiruna.
[3] ASM International, Heat-Resistant Materials, Chagrin Falls, Ohio: ASM International, 1999.
[4] Alloy Data Sheet HH II, Orilla, Ontario, Canada: Kubota Metal Corporation
[5] E. Nilsson, "Degradation of roaster plates", Project Course T7009T, Luleå University of Technol-ogy, 2011
[6] A.W. Batchelor, Loh Nee Lam and Margam Chandrasekaran, "Materials degradation and itscontrol by surface engineering", Imperial College Press, London, England, 2002
[7] S. Jidah, �Simulering rostfri Grateplatta,� Swerea Swecast, 2009
[8] "Material Product Data Sheet - Nickel Chromium Tungsten Molybdenum Superalloy Powders",Sulzer Metco, 2013
[9] �Company,� COMSOL Inc, 2014. [Online]. Available:www.COMSOL.com/company [Accessed 12-02-2014]
[10] �Nu lanseras multiphysics 4.3a�, COMSOL AB, 21 may 2012. [Online]. Available:http://www.COMSOL.se/press/news/article/865/. [Accessed 12-02-2014]
[11] �Stellite 6 Alloy Technical Data,� Deloro Stellite, [Online]. Available:http://stellite.co.uk/Portals/0/Stellite%206%20Final.pdf. [Accessed Februari 2014]
[12] D. R. Lide, �Heat Capacity of Selected solids,� in Handbook of Chemistry and Physics, BocaRaton, Florida, USA, CRC Press LLC, 2002, pp. 12-218
[13] �Dimensionering av konstruktioner av rostfritt stål", Euro Inox and The Steel ConstructionInstitute, 2006. [Online]. Available:http://www.stalforbund.com/Fagboker/Rustfritt/Dim_stainless/Recommend_SW.pdf. [Ac-cessed 12 february 2014]
44
List of Figures
2.1 Grate plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.2 Side movements of Grate belt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.3 Vertical movements of Grate belt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.4 Areas where abrasion most likely occur . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.5 Air gap between plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1 Lines with Prescribed Displacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.2 Area of Rigid connector condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5.1 Used Plate from above . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.2 Protruding Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.3 Abraded area around hole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.4 Abraded triangular area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.5 Abraded peaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.6 Scratch lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.7 Crack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.8 Two abraded peaks with cracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.9 Microstructure of Old Pate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.10 Microstructrure of New Plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.11 Abraded peak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.12 Oxide �ake on abraded peak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.13 Point analysis of two peaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285.14 Point analysis of a �ake on a peak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.15 Point analysis of cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305.16 Friction Coe�cient Sample 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325.17 Friction Coe�cient Sample 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325.18 Friction Coe�cient Sample 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335.19 Friction Coe�cient Sample 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335.20 Thermal stresses, Prescribed displacement . . . . . . . . . . . . . . . . . . . . . . . . . . 345.21 Thermal stresses, Rigid connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345.22 Error from Prescribed displacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355.23 Error from Rigid connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
45
List of Tables
2.1 Alloy composition of HH type II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2 Alloy composition of Diamalloy 4276 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1 Parameters of coating test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.1 Material parameters for COMSOL Multiphysics 4.4 . . . . . . . . . . . . . . . . . . . . . 20
5.1 Values in points on peaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285.2 Values in point of a �ake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.3 Values in points of cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305.4 Hv values nr 1 of old plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315.5 Hv values nr 2 of old plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315.6 Hv values of new plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315.7 Alloy composition of old Grate Plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325.8 Weight measurements coating test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
46
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