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Increase the capacity of continuous annealing furnaces

at Ovako

Viktor Dahlqvist

December 17, 2012

School of industrial Engineering and Management

Department of Materials Science and Engineering

Royal Instityte of Technology

SE-100 44 Stockholm

1

Abstract

The capacity of soft annealing of low alloyed tubes at Ovako’s continuous anneal-

ing furnaces have been evaluated by comparing how it is done today with information

from published and internal articles in the subject. It was found that it is possible

to reduce the cycle time by 30 % for one furnace, 55 % for one furnace and 72 % for

two furnaces. Two separate full scale tests were made to assess whether the faster

soft annealing procedure was feasible. The tests were performed without any recon-

struction of the furnace and were made by continuously vary the speed of the batch

inside the furnace. The temperature in the batch was measured and compared with

results from computer simulations of the heating/cooling sequences. The computer

simulations were performed in COMSOL.

The soft annealing was evaluated according to the SEP-1520 standard, which

means evaluating the microstructure and hardness. The results show that the faster

heat treatment could yield lower grades than today but still meet it’s requirements.

In order to achieve this increase a reconstruction of the furnaces is needed and the

reconstruction is brifly treated in the report. Ideas to further increase the speed of

the soft annealing procedure are also presented.

Keywords: soft annealing, spheroidization, heat treatment, OVAKO, capac-

ity increase, productivity, continuous furnace, normalization, isothermal annealing,

quench and tempering, tube mill, COMSOL, simulation.

ii

Abstract

Kapaciteten för mjukglödgning av låglegerade rör i Ovakos kontinuerliga glödgn-

ingsugnar har utvärderats genom att jämföra dagens körsätt med information från

publicerade- och interna artiklar på området. Det har konstaterats att det finns

möjligheter att reducera värmebehandlingstiden med 30 % för en ugn, 55 % för en

ugn och 72 % för två ugnar. Två separata test i full produktionsskala gjordes för att

se huruvida den snabbare cykeln var genomförbar och gav tillräckliga bra resultat.

Testet gjordes utan ombyggnation av ugnen genom att istället kontinuerligt variera

hastigheten på lasten som var inuti ugnen. Temperaturen i lasten mättes under ex-

perimentet och jämfördes med resultat från datorsimuleringar av samma ugn för att

se hur väl ugnens värmning- och kylningskapacitet gick att simulera. Simuleringarna

utfördes i COMSOL.

Mjukglödgningen utvärderades i enlighet med SEP-1520 standarden vilket bety-

der att mikrostrukturen betygssätts och hårdheten testas. Resultaten från den

utvärderingen visar att den snabbare värmebehandlingen ger något sämre resultat

än vad som erhålls idag, men fortfarande inom kravspecifikation. För att åstad-

komma denna kapacitetsökning krävs en ombyggnation av ugnen. Ombyggnationen

avhandlas något i rapporten.

Nyckelord: Mjukglödgning, sfäroidisering, värmebehandling, OVAKO, kapacitet-

sökning, produktivitet, kontinuerlig ugn, normalisering, etappglödgning, seghärd-

ning, rörvalsning, COMSOL, simulering.

iv

Preface

This master thesis is performed at Ovako AB, Tube & Ring Department located in Hofors,Sweden. The range of this work is 30 hp, which correspond to 20 weeks of study. Thethesis is the final part of a master degree in materials design and engineering at theMaterials Science and Engineering Department at the Royal Institute of Technology,Stockholm.

The main purpose of this work was to increase the capacity of the continuous annealingfurnaces at Ovako. It has been an instructive project and I have learned that there can bequite big differences between the laboratory and the large scale production environment.It is a good experience to do the thesis in the industry that I would recommend foreveryone, even those going for an academic career.

Viktor Dahlqvist

2012-12-17

vi

Table of content

Contents

1 Introduction 1

1.1 Project motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Ovako AB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.3 Tube milling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.4 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.4.1 The heat treatment cycle . . . . . . . . . . . . . . . . . . . . . . . 2

1.4.2 Charging/loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.4.3 New heat treatments . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.5 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Theoretical Background 4

2.1 Continuous furnaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1.1 Loading the furnaces . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Furnace atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3 Soft annealing/spheroidization . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3.1 Soft annealing today . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3.2 Soft annealing tomorrow . . . . . . . . . . . . . . . . . . . . . . . . 14

2.4 Furnaces’ current set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.4.1 Today’s capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.5 Possible new heat treatments . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.5.1 Isothermal annealing . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.5.2 Normalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.5.3 Quench and tempering . . . . . . . . . . . . . . . . . . . . . . . . . 20

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3 Increased Capacity 22

3.1 Proposal 1 - equalize the rate . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.1.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.2 Proposal 2 - maximize the rate . . . . . . . . . . . . . . . . . . . . . . . . 23

3.2.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4 Experimental procedure 29

4.1 The material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.2 Simulating a faster heat treatment without reconstruction of furnace . . . 29

4.3 Computer simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.4 Analysis of microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5 Results 33

5.1 Loading of furnaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33


5.3 Computer Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6 Discussion 39


6.2 Computer Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

6.3 How to use the capacity increase . . . . . . . . . . . . . . . . . . . . . . . 40

6.4 Possibilities for further capacity increase . . . . . . . . . . . . . . . . . . . 41

7 Conclusions 43

8 Future work 44

9 Appendix 48

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1 Introduction

1.1 Project motivation

Keeping Ovako in a leading position and competitive in Europe with long special steelproducts require constant research and development of new application areas. But itdoesn’t matter how good the products are if they can’t be manufactured in a financiallysustainable way. Therefore it is as important to maintain a constant process developmentand strive to always have lower production costs than the competitors.

In the Ovako group there are a large number of annealing furnaces with similar set upand potential. It is believed that there is a potential for capacity increase in the combinedsystem of furnaces that can be used for cust cutting and/or capacity increase, which inturn can be used for implementing new heat treatments. To clarify, this project have apotential for both process development and development of new application areas.

1.2 Ovako AB

Ovako produces engineering steel used in the bearing, transport and manufacturing indus-tries. Production comprises bars, tubes, rings and other pre-components in low-alloyedcarbon steels. The market is mainly long products for demanding applications such as ballbearings and rock drills. Ovako has 11 production sites located in Sweden and Finlandand customers are found mainly in Europe but also in North America and Asia.

Ovako has its roots in small steel manufacturers in Sweden, founded over 300 yearsago. Since then, Ovako has been owned by SKF and Rautaruukki amongst others andmost recently by Pampus Indistrie Beteiligungen until August 2010 when private equityinvestor Triton acquired the bar, bright bar and tube & ring business areas, excluding awire division, forming the Ovako as it is today.

In 2011 Ovako generated sales of EUR 1121 million and had 3239 full time equivalentemployees at year end. They have an annual production capacity of about 1.3 milliontonnes and the current CEO is Tom Erixon. This thesis was carried out at the Tube andRing - department located in Hofors, Sweden.

1.3 Tube milling

Ovako in Hofors has a scrap based steelmaking and melts the scrap in an electric arcfurnace. After a series of treatments the liquid steel is cast as ingots and when completelysolidified, transported to a rolling mill. In the rolling mill the ingots are rolled to round

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bars and then cut into suitable lengths. The cut up round bars are put in a rotating kilnand heated to 1100-1250 oC in order to get soft enough for penetration. Penetration ismade in the tube mill by having the round bar forced by cone formed rolls up over a plug.The rolls press on the tube, which creates a tensile tension in the center that makes thepenetration go easier. When the tubing is complete and the penetration plug is removedthe wall thickness is formed in an assel mill. The next step is the reduction mill wherethe tube gets its final outer diameter. After reduction the tube is straightened in a tworoller cross roll straightener before ending up on a cooling bed.

1.4 Objectives

The main objectives for this project was to increase the capacity of the continuous an-nealing furnaces. This were done by evaluating the heat treatment cycle time and tem-perature and the batch loading. Also possible new heat treatments were evaluated. Themain outputs are listed below.

1.4.1 The heat treatment cycle

1. What quantified capacity increase can be achieved in the targeted 4 continuousannealing furnaces by minimizing the cycle time?

2. What changes to be done to achieve the maximum capacity increase?

1.4.2 Charging/loading

1. What quantified capacity increase can be achieved by optimize the charging?

2. What changes to be done to achieve the maximum capacity increase?

1.4.3 New heat treatments

1. What new heat treatment cycles can be added by utilizing the freed capacity?

1.5 Limitations

• The study was not focused on changing the temperatures in the temperature cyclesince this require extensive evaluation.

2

• The furnaces chosen to be studied were furnace 9, 10, 13 and 14, which are contin-uous furnaces.

• The heat treatment studied was soft annealing since these furnaces almost exclu-sively perform that heat treatment.

• Ovako have many steel grades but this study focused on one steel grade only, namelyOvako 803, which is a low alloyed high carbon steel made mostly for bearings.

3

2 Theoretical Background

2.1 Continuous furnaces

At Ovako in Hofors the soft annealing is performed in four different continuous furnacesthat all use a protective gas atmosphere. The furnaces are called furnace 9, 10, 13 and 14in the report. In a continuous furnace the batch is loaded in one end of the furnace. Thebatch then moves along a conveyor that consists of rotating rolls in parallel, and comesout on the other end, where the batch is unloaded. A schematic 2D view from the sideof a continuous furnace can be seen in figure 1.

The furnaces are 39 m or 47 m long and have 12 or 18 different temperature zones inside.There are gates both before and after the actual furnace body, each about 15 m long.Their function is to exchange the air atmosphere back and forth for a protective gas, inthis case N2. In the gate by the furnace entrance an overpressure is made, which is keptthroughout the furnace to avoid any other gas leaking into the furnace.

The temperature is continuously measured inside the furnace by thermocouples near theinside walls. Due to the location of the thermocouples the measured temperature does notnecessarily reflect the temperature of the material being heat treated. The temperaturedata is stored in databases, which enables evaluation in retrospect. If the temperaturedifference between two zones is significant, a temperature barrier can be installed to makesure the heat stays in the warmer zone. The design of a temperature barrier vary but ascreen of brick inside the furnace is one way to do it.

As can be seen in figure 1, the material to be heat treated is inserted to the left andtransported by rotating rolls into the furnace, where it gets heated. The orange colouredcircles above and beneath the material inside the furnace are the combustion tubes thatconsist of SiC. Furnace 9 and 10 run solely on propane gas while furnace 13 and 14run on propane gas in the first 10 zones and electricity in zones 11 to 18. The propaneis combusted inside combustion tubes and the combustion gas then is transported outthrough the combustion tubes again in order to avoid contamination of the protectivegas atmosphere inside the furnace. More information about protective gas atmosphereand why combustion gas can contaminate it is given in section 2.2. The last 8 zones infurnace 13 and 14 are heated by having electricity is sent through coils mounted on thefurnace inside walls, which get heated due to electrical resistance.

The soft annealing cycle requires an intermediate cooling step, which is done in a ’coolingzone’, in the middle of the furnace. The cooling is done by letting room tempered airflow through the combustion tubes in that zone. The air will absorb heat and transportit out from the furnace by cooling the combustion tubes. This way, the protective gasatmosphere is unaffected.

4

In a large scale production the margin of error is higher compared to a controlled lab,especially for heat treatments. The furnaces at Ovako are built in the 60’s or 70’s, whichwas before the revolution of computerized control systems. Even though the furnaceshave been continuously upgraded to have most functions computer controlled, there stillhave to be margins of error when trying to optimize the soft annealing.

This description of a continuous furnace regards the furnaces at Ovako in Hofors. Thereare many other designs of continuous furnaces.

Figure 1: Illustrates schematically how a continuous furnace is designed. The tube isinserted to the left and transported by a conveyor line consisting of rotating rolls into thefurnace. The orange dots are combustion tubes that generates the heat. The tube thenexits the furnace to the right. The gates can be seen as the smaller areas before and afterthe heating zone.

2.1.1 Loading the furnaces

The charging of the furnaces is done by operators on the floor by lifting the tubes with anoverhead crane. The tubes are picked up from either the cooling bed or from temporarystorage pallets and placed on a queue-bed, which is connected to the conveyor line thatruns through the furnace.

The size of the charging is limited by a number of parameters. The width and heightof the furnace, load capacity of conveyor rolls, load capacity of the overhead crane andof the heat treatment. A matlab program that calculates how many tubes fit per batchdepending on these limitations has been written and can be found in appendix A.

2.2 Furnace atmosphere

When performing heat treatments on steel it can sometimes be important to have a spe-cific atmosphere inside the furnace [1, 2]. Specific atmosphere means that the compositionof the gas inside the furnace is chosen to have specific thermodynamic properties. The

5

atmosphere can be either inert or active where inert means no reactions will take placewhile active means reactions will take place. When it comes to steel, inert atmospheresare used to maintain the chemical composition of the surface by not having any reactionstake place. The most commonly used gas for this application is N2, but sometimes Ar orHe is used, see table 1. However, it is difficult and expensive to upgrade Ovako’s furnacesto be completely impenetrable, which means that other gases, such as moist (water) orCO/CO2 from the combustion gas, can leak into the furnace. An active gas can thereforebe used to maintain a reducing/oxidizing state and/or a certain C activity.

A reducing state is required when oxidation of the surface of the steel is to be avoided.Reducing state can be seen as that the atmosphere oxidize faster than the steel and thus,the steel will not be oxidized. This can be done by adding components to the atmospherethat tend to react with O2, for instance CO or H2.

Table 1: How different gases affect the properties of the atmosphere[2].Neutral gas Active gas

Reducing Oxidizing Decarburizing CarburizingAr H2 H2O H2O CON2 CO CO2 CO2

He O2 O2 CnHm

Steel contains from low levels of C, up to two percent, and the mechanical properties ofthe steel are strongly connected to the C concentration[3], see figure 2. Some gases havethe tendency to steal C from the surface (decarburize the surface) during heat treatment,why it is important to have an atmosphere with a certain C activity. By having the sameC activity on the surface and in the atmosphere one can make sure that no decarburizationwill take place. If the atmosphere is internally equilibrated then its C activity is relatedto the concentration ratio of CO2/CO where a lower ratio increases the C activity [2], seeeq 1.

ac = K·P 2CO

PCO2

(1)

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Figure 2: Illustrates hardness for steel with different C concentration after different typesof heat treatments. The Y-axis to the left is HV (vickers hardness) and the right is HRC(rockwell hardness). The x-axis is carbon concentration in weight-percent. The threelines represent from the top, hardened, annealed and spheroidized heat treatments. [3]

Active gases can also be used to on purpose affect the chemical composition of the surface.Sometimes different properties in the core and the surface are desired, which can be doneby affecting the chemical composition of the surface during the heat treatment. Oftenit is desired to have a tough core and a very hard surface. This can be achieved bycarburization, nitridization or carbonitridization, which means that the activity of Cand/or N2 is higher in the atmosphere than on the surface, which will lead to an increaseof dissolved C and/or N in the surface, of a low carbon steel. This results in a core of asofter low C steel with a hard surface with high concentration of C and/or N.

The chemical properties of gases are highly temperature dependent, which can be relatedto the thermodynamic quantity Gibbs energy, see eq 4, where ∆Go, R and K are constants(at constant pressure and temperature). By analyzing the value of the Gibbs energy ofthe system one can determine what phases or components are stable, i.e. what reactionsthat are likely to take place in a system [3]. For instance if the piece that is to be heattreated is wet when placed in the furnace, the furnace atmosphere will be affected by theevaporation of water. Whether the gas then has oxidizing or reducing properties can be

7

determined by the Gibbs energy. The term Rln(K) stems from the entropy where R isthe gas constant and K is defined by eq. 3 below. Its value at equilibrium is called theequilibrium constant and can be evaluated by setting up the reaction, see equation 2-3.

n1A+ n2B ⇔ n3C + n4D (2)

Introducing K;

K =an1A

·an2B

an3C

·an4D

(3)

where aij

are the activities, we may write the change in Gibbs energy for reaction 2 as

∆G = ∆Go + RTln(K) (4)

At equilibrium ∆G = 0 and the equilibrium constant thus is exp(-∆Go/RT) .

At Ovako in Hofors, most furnaces have no controlled atmosphere, which means theyrun in air. Although they have four furnaces where N2 serves as neutral gas and crackedmethanol serves as active gas to steer the carbon potential upwards. These four furnacesare used for soft annealing of low alloyed high carbon steels that easily get decarburizedand oxidized. Equations 5-8 describes how to calculate the ratio of P 2

CO/PCO2 required

to not decarburize the surface depending on the amount of carbon in the steel, Xc. Thereactions are assumed to be in Equilibrium (∆G = 0).

2CO ⇔ CO2 + C (5)

∆G = ∆Go +RT ·ln(aC ·PCO2

P 2CO

) = 0 (6)

aC = γC ·XC = exp

�-∆Go

RT

�P 2CO

PCO2

(7)

For low C concentrations, Henry’s law can be assumed to be valid, which means thatγC → γo

C(becomes constant).

XC =exp

� -∆Go

RT

�

γo

C

P 2CO

PCO2

= Const·P 2CO

PCO2

(8)

8

Equations 9-11 describes how to calculate the partial pressure of oxygen inside the furnacedepending on the ratio of (PCO2/PCO)2.

CO2 ⇔ CO +1

2O2 (9)

∆G = ∆Go +RT ·ln(�

PO2 ·PCO

PCO2

) = 0 (10)

PO2 = exp

�-2·∆Go

RT

��PCO2

PCO

�2

(11)

Equations 12-14 describes how to calculate what partial pressure of oxygen is required toavoid oxidation of the surface.

Fe+1

2O2 ⇔ FeO (12)

∆G = ∆Go +RT ·ln(aFeO

aFe·�

PO2

) = 0 (13)

If Fe and FeO are assumed to be pure solids, one may set aFeO = aFe = 1.

1�PO2

= exp

�-∆Go

RT

�= Const (14)

By having a lower PO2 than the equilibrium reaction above states, the reaction in eq 12tend to go to the left, which means that oxidation is avoided.

It is also possible to do this by put in values in eq 4, and if ∆G > 0 the reaction willnot happen, if ∆G = 0, the reaction is in equilibrium and if ∆G < 0 the reaction will bespontaneous until ∆G becomes zero.

2.3 Soft annealing/spheroidization

2.3.1 Soft annealing today

Soft annealing is a way of heat treating steel to make it softer [1, 2, 4, 5] . Since softannealing increases machinability it is a common heat treatment at steel production sites

9

because steels often are to be worked on in some way after leaving the steel plant. Aftera material is machined and the shape is near end-product it is often heat treated againto get mechanical properties more suitable for a specific application. For instance, Ovakoproduce rings for ball-bearings. Ball-bearings need to have a carved groom to keep theballs in position and that is done by machining. However, ball-bearings need to be hardand tough in order to have a long lifetime but such a material is very difficult to machine.Therefore Ovako soften the rings and the ball-bearing producer harden them, all by heattreatment.

Soft annealing is performed on tool steels and low alloyed high carbon (roughly > 0.35wt% C) steels. If the steel have a low carbon concentration it will get too soft after a softannealing, which actually decreases machinability because the material only will smearout instead of breaking into small chips during machining.

During soft annealing of steels with a pearlitic structure, the cementite lamellae willspheroidize, hence the process is sometimes called spheroidization. The spheroidizedcementite will be embedded in a soft ferritic matrix, which is the key to get a softermaterial. Figure 3 illustrates schematically the microstructural development during softannealing. At Ovako in Hofors, the spheroidization procedure is mainly performed onhigh carbon steels.

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Figure 3: SEM micrographs of pearlite (a) before and after spheroidization at 700 °C for(b) 10 min, (c) 60 min and (d) 360 min.[8]

Spheroidization can be performed in numerous ways [1, 2, 4, 6, 7]. However, the basic ideais to heat the material to just below or above the austenitization temperature, dependingon the C content, for about an hour and then cool the material down to about 650 oC

with a cooling rate of 10 oC/h. Due to the slow cooling rate spheroidization is a timeconsuming process and can easily become a bottle neck in the production.The combination of high temperature and long heat treatment times can make decarbur-ization become a problem, which was the case at Ovako in Hofors for about 20 years ago.The eutectoid point in the phase diagram for Fe - C, see figure 5, is at about 0.8 wt-% C,which means that if the depleted surface contain less than 0.8 wt-% C it will transforminto austenite/ferrite instead of austenite/cementite during heating. Therefore no cemen-tite nuclei will be present at top temperature in the surface region and if there is no nucleipresent, the surface material will transform into hard lamellar pearlite during cooling. A

11

new heating cycle was then introduced[9], that differs somewhat from the conventionalcycle. This is the cycle used today and it is illustrated schematically in figure 4.

The hypereutectoid carbon steels at Ovako are soft annealed by initial partial austeniti-zation just above the A1 line in the Fe - C phase diagram, see figure 5. The main reasonfor this is to dissolve the extensive amount of carbide network that form during the slowcooling that follow the tube milling, i.e. the as-rolled structure consist of thick carbidesthat need to be dissolved. At this temperature, austenite and cementite are stable, whichleads to initial spheroidization of cementite and dissolution of carbon into the austenite.The holding time is 1-1.5 h. The material is then cooled down to 690 oC in 1 h in orderto get a fully pearlitic structure at the surface. Then the material is heated again, thistime to 770 oC in order to spheroidize the pearlite at the surface as well. Finally thematerial is cooled down to 690 again with a cooling rate of about 20 oC/h to get furtherspheroidization.

Figure 4: A schematic illustration of the soft annealing cycle used at Ovako.

Ovako follow the SEP-1520 standard for soft annealing results, The analysis refers to as-sessing hardness and three different microstructural properties by the SEP 1520 standard.The properties to be measured are carbide size (CG), pearlite amount (PA) and carbide

12

network (CN) where CG measures the size of the spheroidized cementite, PA measuresthe remaining amount of pearlite and CN measures the remaining carbide network in thesample.

The SEP 1520 standard includes pictures of microstructures that are graded regardingCG, PA and CN. CG is graded from 2.0 to 2.5 where 2.0 have the smallest grains and2.5 the largest. PA is graded from 3.0 to 3.3 where 3.0 are abscense of pearlite and3.3 have a substantial amount. CN is graded from 4.0 to 4.6 where 4.0 are abscence ofcarbide network and 4.6 are substantial amount. The microstructure to be evaluated arethen graded by using a light optical microscope (LOM), to find the most correspondingpictures to the microstructure. The average result of today can be seen in table 2.

Table 2: The average results of Hardness, Carbide size, Pearlite amount and CarbideNetwork today.

Hardness [HV] CG PA CNValue 190 2.3 3.0 4.0

The times and temperatures in the soft annealing procedure is not carved in stone butan effect of the specification by the customer. Some customers have high demands thatmay require longer times while others have lower demands. In the production however,it is more difficult to have specific times and temperatures for each specification than tohave a specific time and temperature for each steel grade, which might lead to overdoingthe soft annealing in some cases, but the bar have to be set to meet the highest demands.However, as the quality of the steel gets better and better, the properties of the machiningtools get better, which tend to decrease the demands on the soft annealing. Therefore it isimportant to evaluate the time and temperature of soft annealing now and then to avoidoverdoing and thereby be as cost effective as possible. It is also very important that theprocess is stable over time so that the soft annealed material have the same propertiesregardless when soft annealed due to the customers optimizing their processes to fit anarrow range in the material’s properties.

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Figure 5: Phase diagram of iron-carbon.[14]

2.3.2 Soft annealing tomorrow

Numerous ideas of how to spheroidize carbon steels more effectively are presented in theliterature [1, 4, 7, 10, 11, 12]. The most promising results applicable to bulk productionare achieved by either manipulating the as-rolled structure or by using multiple heating-cooling cycles.

The results from [1] show that the as-rolled structure, i.e. the initial structure, have ahigh influence on the soft annealing results. It reveals that the finer the pearlite the betterresults, where a fully martensitic structure gave the best results. The underlying causesof this are not discussed in the report.

The results from [7, 12] show that by using a cyclic heat treatment the soft annealing timescan be reduced. A cyclic heat treatment means that the temperature cycles around theaustenitization temperature several times, schematic illustrations of cyclic heat treatmentscan be seen in figure 6. Saha et al [7] concludes, regard using a cyclic heating-coolingcycle, for a 0.6 wt-% C steel:

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1. “The fragmentation of lamellae is augmented by the dissolution of cementite throughatomic diffusion from preferred sites of ‘lamellar fault’ in cementite to the adja-cent austenite during short-duration holding above Ac3 temperature. The non-equilibrium forced air cooling generates more lamellar fault regions that act as thepotential sites for spheroidization.“

2. “With increasing number of heat treatment cycles the proportion of ferrite andspheroidized cementite increases, the proportion of lamellar pearlite decreases andmicroconstituents (pearlite and ferrite) become finer.”

3. “After 8 heat treatment cycles the microstructure mostly contains spheroidized ce-mentite particles in very fine ferrite matrix ... with an excellent combination ofstrength and ductility properties.”

Figure 6: A schematic illustration of potentially faster soft annealing. Red line cyclesbetween two temperatures while the blue line continuously decreases temperature witheach cycle. At present it is unknown which is most effective.

2.4 Furnaces’ current set up

The schemes for soft annealing in furnace 9, 10, 13 and 14 can be seen in table 3-5. Noticethat the specifications only consider the actual furnace body and not the gates in front ofand after. This means that the soft annealing procedure takes approximately three hourslonger in reality than what is stated in the tables, however the actual heat treatment ismade within the time specified in the tables.

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Table 3: The specifications of furnace 9 and how it is run during soft annealing.Furnace 9

Zone Temperature [oC] Zone length [m] Length [m] Time at 4 m/h [h]1 810 3.4 3.4 0.8392 820 3.1 6.5 1.623 820 2.2 8.6 2.164 820 2.2 10.8 2.75 670 4.3 15.1 3.786 800 3.6 18.7 4.687 800 3.6 22.3 5.588 750 3.6 25.9 6.489 720 2.9 28.8 7.210 710 2.9 31.7 7.9211 700 2.9 34.6 8.6412 690 4.3 38.9 9.72

Table 4: The specifications of furnace 10 and how it is run during soft annealing.Furnace 10

Zone Temperature [oC] Zone length [m] Length [m] Time at 4 m/h [h]1 810 4.4 4.4 1.12 820 2.2 6.6 1.643 820 2.2 8.8 2.184 670 4.3 13.1 3.265 740 2.9 16.0 3.986 800 5.8 21.8 5.427 800 5.8 27.6 6.868 750 2.9 30.5 7.589 720 5.0 35.5 8.8410 710 5.0 40.5 10.111 700 4.3 44.8 11.1812 690 3.0 47.8 11.92

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Table 5: The specifications of both furnace 13 and 14 since they are identical, and howthey are run during soft annealing.

Furnace 13 and 14Zone Temperature [oC] Zone length [m] Length [m] Time at 3.6 m/h [h]

1 815 3 3 0.832 820 3.4 6.4 1.783 820 1.7 8.1 2.254 820 4 12.1 3.365 670 3.2 15.3 4.256 750 1.3 16.6 4.617 770 5.7 22.3 6.198 770 2 24.3 6.759 735 1.3 25.6 7.1110 730 2.5 28.1 7.8111 725 1.7 29.8 8.2812 720 2.4 32.2 8.9413 - 2.3 34.5 9.5814 - 2.5 37 10.2815 - 2.4 39.4 10.9416 - 2.3 41.7 11.5817 - 2.4 44.1 12.2518 - 2.9 47.1 13.06

2.4.1 Today’s capacity

The capacity will be calculated by multiplying the maximum weight allowed per length(kg/m), which is set by the conveyor rolls’ mechanical properties, with the speed of thefurnace (m/h). The outcome is [kg/h] and the capacities can be seen in table 6.

Table 6: Displays the maximum capacity of the furnaces.Furnace 9 10 13 14

Maximum weight [kg/m] 750 750 750 750Speed [m/h] 4 4 3.6 3.6

Capacity [kg/h] 3000 3000 2700 2700

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2.5 Possible new heat treatments

2.5.1 Isothermal annealing

Low alloyed, often case hardened, low carbon steels (< 0.3 wt% C) are frequently used inthe automotive industry for instance for cogwheels or other transmission details[2]. Theseapplications require material with high homogenity and good machinability, which canbe achieved with isothermal annealing.

Isothermal annealing means initial austenitization above the A1 line, followed by quench-ing or at least fast cooling down to the transition temperature below the A1 line where thematerial is maintained for several hours. Isothermal annealing leads to a ferrite-pearlitestructured material with good chip breaking properties, which is good for machinability.A schematic curve of isothermal annealing can be seen in figure 7.

A continuous furnace suits well for isothermal annealing since it requires a rapid coolingbetween two high temperatures. Isothermal annealing can be done at Ovako today, how-ever the furnace it is done in have no cooling abilities. It is a continuous furnace that issimilar to furnaces 9, 10, 13 and 14 but have only two zones; one high temperature (HT)zone and one low temperature (LT) zone. The material is austenitized in the HT zone andthen brought out to air cool before placed in the LT zone to have phase transformationtake place. This makes the procedure very operator dependent, which tend to create bigvariations in the process.

The furnace doing isothermal annealing today has no ability to control the atmosphere.This is a problem since austenitization means high temperatures, sometimes up to around960 oC, which means the surface get decarburized and oxidized fast.

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Figure 7: A schematic illustration of an isothermal annealing.

2.5.2 Normalization

Normalization is a heat treatment performed mainly on carbon steels and low alloyedsteels[2]. The idea is to get smaller grains and a more homogeneous grain size, whichmakes the material harder and enable for more homogeneous properties. Material thathas been worked on will not have a homogeneous mictrostructure, but will consist of bothlarge and small grains and can have a variation of structures such as carbide precipitatesand bainite. A normalization heat treatment removes these inconveniences and therebylead to better mechanical properties as well as increased machinability.

Initially the material is austenitized in 800 - 920 oC with a short holding time. Thisgenerates newly formed homogeneous austenite grains that dissolves carbides and otherunwanted structures. The austenitization is followed by a controlled cooling to roomtemperature. The cooling rate will determine the size of the newly formed ferrite-pearlitegrains and also the thickness of the lamellae in the pearlite. A schematic illustration of anormalization procedure can be seen in figure 8.

A continuous furnace might be considered overqualified for normalization, but it is goodto have the ability to vary the cooling rate depending on the specification of the customer.It is also good to have the ability to use a protective gas atmosphere in order to not affectthe chemical composition of the surface.

Normalization is done at Ovako today, in the same furnace as the isothermal annealingand it thus yields the same difficulties and limitations.

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Figure 8: A schematic illustration of a normalization heat treatment.

2.5.3 Quench and tempering

It is of great interest for steel producers to find applications for their products where nointermediary is needed. The intermediaries are is most often subcontracting engineeringcompanies performing machining operations. If the intermediary can be cut out there ismore profit to gain. In order to do so the steel producer must be able to perform finaliz-ing heat treatments and since most applications require materials with good mechanicalproperties (high hardness, toughness etc.) the heat treatment must be hardening. Quenchand tempering is such a hardening heat treatment.

There are several ways to harden steels, but for low alloyed steels the alternatives arefewer where the easiest and most common is to create a martensitic or bainitic structure.Martensite is created by initial austenitization followed by quenching while bainite iscreated by initial austenitization followed by rapid cooling. Often these structures areso hard that the material becomes brittle. To get a higher toughness the material istempered after quenching in about 550-700 oC for a few hours. This procedure enablesa hardness of around 250 HB - 350 HB[2]. A schematic illustration of a quench andtempering procedure can be seen in figure 9.

Quench and tempering are sometimes used on semi-finished products that are to bemachined. Materials that have been quench and tempered often undergo some kind ofcase-hardening, for instance nitridization, carburization or the combination nitrocarbur-ization.

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Today, quench and tempering can not be done at Ovako in Hofors, but it can be done atOvako Bright Bar at Hällefors.

Figure 9: A schematic illustration of quench and tempering.

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3 Increased Capacity

Evaluation of todays set-up and running scheme enabled finding a potential for capacityincrease. In this report, two ideas for capacity increase will be presented. The firstproposal will increase the capacity by making all 4 furnaces run equally fast comparedto their length. The second proposal sets as high speed as possible and have greaterpotential to increase the capacity substantially.

3.1 Proposal 1 - equalize the rate

As mentioned earlier furnace 9 is shorter than furnace 10, 13 and 14. However, furnace9 and 10 run of the same speed while 13 and 14 run on lower speed. The soft annealingprocedure require only a certain time in the furnace and since furnace 9 is shorter the timefor each batch in that furnace are shorter than in the other furnaces. Since all furnaceshave the same heating/cooling capabilities, logically it should be possible to run faster infurnace 10, 13 and 14 compared to furnace 9.

The new speed can be calculated by:

• Furnace 9 runs in 4 m/h and is 38.9 m long. This makes a total soft annealing timeof 38.9 m /4 m/h = 9.7 h.

• Furnace 10 run in 4 m/h and is 47.7 m long. This makes a total soft annealing timeof 47.7 m /4 m/h = 12 h.

• Furnace 13 and 14 runs in 3.6 m/h and are 47.1 m long. This makes a total softannealing time of 47.1 m /3.6 m/h = 13 h.

If it is assumed that furnaces 10, 13 and 14 require only as long time as in furnace 9 then:

• Furnace 10 can be run in 47.7 m /9.7 h = 4.9175 m/h ≈ 5 m/h

• Furnace 13 and 14 can be run in 47.1 m /9.7 h = 4.86 m/h ≈ 5 m/h

If this would work, then the speed increases are:

• For furnace 10: 5 m/h /4 m/h = 1.25, i.e. 25 % increase.

• For furnaces 13 and 14: 5 m/h /3.6 m/h = 1.39, i.e. 39 % increase.

And the total maximum capacity increaes are:22

• For furnace 10: (5 m/h · 750 kg/m) - (4 m/h · 750 kg/m) = 750 kg/h

• For furnace 13 and 14: (5 m/h · 750 kg/m) - (3.6 m/h · 750 kg/m) = 1050 kg/h

• Total capacity increase: 750 kg/h + 2·1050 kg/h = 2850 kg/h

• Percentage increase: 2850 kg/h /(2·3000 kg/h + 2·2700 kg/h) = 0.25 = 25 %increase

3.1.1 Requirements

In order to achieve this increase the furnaces have to be reconstructed. The main thingis that the cooling zone have to be moved backwards approximately 4-5 m in furnace10 and about 1 m in furnace 13 and 14, in order for the initial austenitization to becomplete. The new zone lengths, temperatures and times can be seen in tables 7-9 andan illustration of the heating cycle can be seen in figure 10.

Moving the cooling zone and remake zone lengths means moving combustions tubes andcooling tubes. This means new holes have to be drilled in the furnace. The exact numberof new holes and the details of the new combustion/cooling tube set-up require must beevaluated further. Also the last 6 zones that run on electricity in furnace 13 and 14 haveto be started up.

3.2 Proposal 2 - maximize the rate

Different test results combined with how the furnace are run today are used to optimizethe cycle in proposal 2, which set to be as fast as possible.

Soft annealing requires, according to [15]:

• 1.5 h heating time to reach 820 oC (from measurements made in furnace 13)

• 1.5 h holding time at 820 oC (to dissolve carbide networks)

• 2 h cooling after second heating, from 770 oC to 690 oC. This makes 40 oC/h

(report states maximum 50 oC/h)

According to current set-up, soft annealing requires:

• 1 h at the intermediate cooling step, from 820 oC to 670 oC.

• 1.5 h heating after intermediate cooling step, from 670 oC to 770 oC. Holding timeat 770 oC included.

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This gives a theoretical cycle of total 7.5 h. The speed can then be calculated by dividingthe furnace length by the cycle time, 39 m /7.5 h = 5.2 m/h for furnace 9, 47.7 m /7.5 h= 6.36 m/h for furnace 10 and 47.1 m /7.5 h = 6.28 m/h for furnace 13 and 14. 6.2 m/his chosen for furnace 10, 13 and 14.

If this speed is used, then the speed increases are:

• For furnace 9: 5.2 m/h /4m/h = 1.3, i.e. 30 % increase

• For furnace 10: 6.2 m/h /4 m/h = 1.55, i.e. 55 % increase.

• For furnaces 13 and 14: 6.2 m/h /3.6 m/h = 1.72, i.e. 72 % increase.

And the total maximum capacity increaes are:

• For furnace 9: (5.2 m/h · 750 kg/m) - (4 m/h · 750 kg/m) = 900 kg/h

• For furnace 10: (6.2 m/h · 750 kg/m) - (4 m/h · 750 kg/m) = 1650 kg/h

• For furnace 13 and 14: (6.2 m/h · 750 kg/m) - (3.6 m/h · 750 kg/m) = 1950 kg/h

• Total capacity increase: 900 kg/h + 1650 kg/h + 2·1950 kg/h = 6450 kg/h

• Percentage increase: 6450 kg/h /(2·3000 kg/h + 2·2700 kg/h) = 0.57 = 57 %increase

3.2.1 Requirements

In order to achieve this increase the furnaces have to be reconstructed. The main thing isthat the cooling zone have to be moved backwards approximately 10 m in furnace 10 andabout 6 m in furnace 13 and 14, in order for the initial austenitization to be complete. Thenew zone lengths, temperatures and times can be seen in tables 10-12 and an illustrationof the heating cycle can be seen in figure 11.

Moving the cooling zone and remake zone lengths means moving combustions tubes andcooling tubes. This means new holes have to be drilled in the furnace. Exactly how manymust holes that must be drilled and how the new combustion/cooling tube set-up shouldbe must be further evaluated. Also the last 6 zones that run on electricity in furnace 13and 14 have to be started up.

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Table 7: Specifications of how thereconstruction will affect the zoningin furnace 10.Zone Length [m]

4 m/h

Length [m]5 m/h

Length [m]6.2 m/h

1 4.4 4.4 4.42 2.2 2.2 2.23 2.2 2.2 2.24 4.3 6.3 4.35 2.9 5.5 5.56 5.8 3.2 6.27 5.8 4.8 5.88 2.9 2.9 2.99 5.0 4.0 2.010 5.0 5.0 5.011 4.3 4.3 4.312 3.0 3.0 3.0

Table 8: Specifications of how thereconstruction will affect the timesper zone in furnace 10.Zone Time [h]

4 m/h

Time [h]5 m/h

Time [h]6.2 m/h

1 1.10 0.88 0.712 1.64 1.31 1.063 2.18 1.75 1.414 3.26 3.01 2.105 3.98 4.11 2.996 5.42 4.75 3.997 6.86 5.71 4.938 7.58 6.29 5.399 8.84 7.10 5.7210 10.10 8.11 6.5211 11.18 8.97 7.2212 11.92 9.55 7.70

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Table 9: Specifications of how thereconstruction will affect the tem-perature in furnace 10.Zone

Temp.[oC]

4 m/h

Temp.[oC]

5 m/h

Temp.[oC]

6.2 m/h

1 810 810 8102 820 820 8203 820 820 8204 670 820 8205 740 670 8206 800 800 6707 800 800 8008 750 760 8009 720 725 78010 710 710 73011 700 700 70012 690 690 690

Table 10: Specifications of how thereconstruction will affect the zoningin furnace 13 and 14.Zone Length [m]

3.6 m/h

Length [m]5 m/h

Length [m]6.2 m/h

1 3 3 32 3.4 3.4 3.43 1.7 1.7 1.74 4 4 45 3.2 1.2 2.56 1.3 5.3 47 5.7 2.7 38 2 2 39 1.3 2.3 1.310 2.5 2.5 2.511 1.7 1.7 2.212 2.4 2.4 1.913 2.3 2.3 2.814 2.5 2.5 215 2.4 2.4 216 2.3 2.3 217 2.4 2.4 218 2.9 2.9 2

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Table 11: Specifications of how thereconstruction will affect the timesper zone in furnace 13 and 14.Zone Time [h]

3.6 m/h

Time [h]5 m/h

Time [h]6.2 m/h

1 0.83 0.60 0.482 1.78 1.28 1.033 2.25 1.62 1.314 3.36 2.42 1.955 4.25 2.66 2.356 4.61 3.72 3.007 6.19 4.26 3.488 6.75 4.66 3.979 7.11 5.12 4.4510 7.81 5.62 4.8511 8.28 5.96 5.2112 8.94 6.44 5.5213 9.58 6.90 5.9714 10.28 7.40 6.2915 10.94 7.88 6.6116 11.58 8.34 6.9417 12.25 8.82 7.2618 13.06 9.42 7.58

Table 12: Specifications of how thereconstruction will affect the tem-perature in furnace 13 and 14.Zone Temp. [oC]

3.6 m/h

Temp. [oC]5 m/h

Temp. [oC]6.2 m/h

1 815 820 8152 820 820 8203 820 820 8204 820 820 8205 670 820 8206 750 670 8207 770 800 6708 770 800 6709 735 800 80010 730 780 80011 725 760 80012 720 745 80013 - 730 76014 - 720 73515 - 710 72016 - 705 71017 - 700 70018 - 690 690

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!

Figure 10: Illustration of the soft annealing cycles at different speeds in furnace 10 com-pared to furnace 9 with speed of 4 m/h.

Figure 11: Illustration of the soft annealing cycles at different speeds in furnace 13 and14 compared to furnace 9 with speed of 4 m/h.

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4 Experimental procedure

The main focus of the experiments was to see if the faster heat treatment could meet themicrostructural requirements. Computer simulations were made as a side-project to seehow well the furnace’s heating/cooling capabilities can be simulated by using COMSOLsoftware.

Since proposal 1 is derived from looking at an already successful soft annealing procedurein furnace 9, it was decided that it does not need any other experimental verification.Instead the experiments were focused on the faster heat treatment of proposal 2.

4.1 The material

The material investigated for soft annealing in this report is of type 100Cr6 and is calledOvako 803. Ovako 803 is a group of through hardening bearing steels intended for rollingcontact and other high fatigue applications. 100Cr6 is mainly used for small and mediumsized bearing components but it is also regularly used for other machine components thatrequire high tensile strength and high hardness. The steel exists in several modificationsbut the soft annealing is assumed to be unaffected by this and therefore the modificationsare not considered. A list of the chemical composition of Ovako 803 can be seen in table13.

Table 13: The chemical composition of steel grade Ovako 803.Grade C% Si% Mn% P% S% Cr% Ni% Mo%803 min 0.95 0.20 0.20 0.00 0.00 1.35 0.00 0.00

max 1.00 0.35 0.40 0.025 0.015 1.60 0.25 0.08

4.2 Simulating a faster heat treatment without reconstruction offurnace

In order to see if the faster heat treatment was feasible, two experiments were conductedin furnace 10. In furnace 10, it is possible to set the speed manually and also to change itduring on-going heat treatment. The current zoning does not fit the increased speed; thecooling zone is too close to the entrance, which means the material will not be completelyaustenitized before cooling. By manipulating the speed at different zones it was possibleto simulate the faster heat treatment without any reconstruction.

A furnace speed of 6.2 m/h was simulated. The background of that speed is treated insection 3. The speed was varied as described in table 14. The two samples that underwent

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the faster heat treatment are called sample A and B. The specifications of the dimenionsand number of tubes can be seen in table 15.

The temperature was measured in the center of the batch by attaching a thermocouplethat came with the batch through the furnace. Thereby a plot similar to the one achievedby the computer simulation could be achieved and the results could easily be compared.Since the speed was varied during the heat treatment and the fact that the furnace hasdifferent zones with different temperature, the front and end part experienced a differentheat treatment. Therefore only the results from the front part was evaluated, in bothtemperature and microstructure.

Table 14: The schedule of how the speed were varied during simulation of 6.2 m/h.Speed [m/h] 3 4.3 10.3 8.8 Total

Time [h] 3 1 1.4 2.3 7.7

Distanse [m] 9 13.3 27.6 47.7 47.7

Table 15: Specification of samples that underwent the simulated faster heat treatment.Sample Outer Diameter Inner Diameter Wall-thickness Tubes Load

A 73.70 mm 47.00 mm 13.35 mm 28 pcs 556.36 Kg/mB 174.70 mm 123.30 mm 25.70 mm 6 pcs 566.64 Kg/m

4.3 Computer simulation

The computer simulations were made in COMSOL Multiphysics, which is a softwarefor simulating, as the name suggests, multiple physical phenomena. This was done tosupplement the test results and to see how well a computer model can correspond to thefurnaces actual heating/cooling capabilities.

The models are time-dependent and made in 2D. An illustration of the models can be seenin figure 12, where the orange layer represents the furnace wall and the cirkular objectsin the middle are the tubes to be heated. The rest is atmosphere, which consist only ofN2 gas. The tubes are assumed to be of steel and the walls of brick. The specificationsof each materials’ properties can be found in appendix B.

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Figure 12: The model built in Comsol in order to simulate the heat treatment. The scale is [m]in both x- and y-axis. The left is model M1 and the right is model M2.

The dimension and number of tubes were set to match the experiments performed infurnace 10 since experimental data is required for comparison. The specification of theset-ups can be seen in table 16. The exact specification of for instance zone lengths, times,temperatures and speeds can be seen in appendix B.

Table 16: Specifications of set-ups regarding tube dimensions, number of tubes and num-ber of layers.

Model Layers Outer Diameter Inner diameter Wall thickness TubesMI 2 73.90 mm 46.50 mm 13.70 mm 28 pcsM2 1 174.90 mm 122.70 mm 26.10 mm 6 pcs

The heat was generated by setting a fix temperature, Twall, at the inside of the furnacewalls including top and bottom. The function was time-dependent and is enclosed inappendix B. The inside walls and the outer surface of the tubes were set to radiate withit’s actual temperature through any transparent medium. The solid parts of the modelwere set to be opaque, which enabled radiation only through the atmosphere. The heattransfer were set to heat transfer in fluids for the gas and heat transfer in solids forthe solids. The outer sides of the wall was cooled by convective cooling with externaltemperature 25 oC and the heat transfer coefficient h = 10, the high value to compensatefor other heat-losses, such as when opening gates to charge or discharge the furnace.Three different initial temperatures were set. The initial temperature of the walls wereset to 100 oC and the atmosphere were set to initial temperature 400 oC. The initialtemperature in the tubes and in the gas inside and between the tubes were set to 25 oC.

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The convective contribution were evaluated during the project and found to be negligible.To be sure, forced convection using a fan was tested with and without turbulent flow(k-ε model) but this too was found to be negligible. All possible combinations couldnot be made since the computational fluid dynamics (CFD) required large amounts ofcomputing power, which in turn required long computing times. Therefore the convectiveheating/cooling is neglected in this experiment.

The outputs from the simulations are plots of temperatures at the same position in thebatch as in the temperature measurement experiment in furnace 10, over time.

4.4 Analysis of microstructure

Smaller samples were cut out from the tubes. The samples were then polished and etchedusing picric acid whereafter pictures of the microstructure were taken. Then the hardnessin HB were measured in an automatic hardness testing machine. The testing followed theSEP-1520 standard.

The limitations of each property are listed in table 17.

Table 17: The limitations of Hardness, Carbide size, Pearlite amount and Carbide Net-work.

Hardness [HV] CG PA CNMin - 2.1 3.0 4.0Max 210 2.3 3.1 4.2

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5 Results

5.1 Loading of furnaces

The charging of the furnaces was found to have been optimized recently. The limitationswere found to be set by the furnace volume and maximum load capacity. The distancebetween batches were as short it could be since the furnace have gates with locks atinlet and outlet that require a minimum space between the batches. The batches areautomatically transported through the gate when the last batch has come far enough intothe furnace .


Both samples passed the requirements for soft annealing and the exact results can be seenin table 18. As can be seen in figures 13 and 14, the structure consist of ferrite matrixwith spheroidized cementite. Since the grading is done by comparing the microstructurewith graded pictures of microstructures the results can vary depending on whom doingthe evaluation.

Figure 13: Light optical images of the microstructure of sample A. The white areas areferrite and the dark dots are spheroidized cementite.

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Figure 14: Light optical images of the microstructure of sample B. The white areas areferrite and the dark dots are spheroidized cementite.

The experiments went roughly as planned, however there were some implications. SampleA, which were run first, experienced a much faster cooling rate in the end of the heattreatment than expected, which can be seen in figure 16. The temperature in the endzones are too cold for the combustion system because the low flow of combustible gas tendto build up carbon residue that clog the burners. This is not a problem during regularheat treatments since the continuous flow of hot material keep the end zones heated. Thisissue was solved by placing a batch ahead of sample B.

Table 18: Results of microstructure analysis according to standard SEP1520.Sample A

Placement HB CG PA CN

Left 202 2.1 3.1 4.1Center 200 2.1 3.0 4.1Right 203 2.1 3.0 4.1

Sample B

Placement HB CG PA CN

Left 203 2.1 3.0 4.1Center 200 2.1 3.0 4.2Right 201 2.1 3.0 4.1

The result of the temperature measurement can be seen in figure 15 and 16 where theformer is raw data that had to be formatted to make sense and the results of the formattingcan be seen in the latter figure. The temperature were not measured in sample B dueto broken equipment. However, the temperature is continuously measured inside thefurnaces and the data stored. A compilation of the temperatures in the furnace forsample A and B have been made and can be seen in figures 17, 18. These figures givesan indication about the temperature in the material for sample B.

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Figure 15: The red represents the data measured by the thermo couple that were placedon the batch. The data was filtered and the remaining data is colored blue.

Figure 16: Illustrates how the temperature profile in sample A deviates from the plannedtemperature.

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Figure 17: Illustrates the furnace temperature in different zones at different times forsample A. The reason why the lines don’t stick together is because the temperature ismeasured continuously at different zones and the data is taken at different times fromseveral different thermo couples, to match the placement of the front of the batch.

Figure 18: Illustrates the furnace temperature in different zones at different times forsample B. The reason why the lines don’t stick together is because the temperature ismeasured continuously at different zones and the data is taken at different times fromseveral different thermo couples, to match the placement of the front of the batch.

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5.3 Computer Simulation

The results, illustrated in figures 19 and 20, show that the heating/cooling capabilitiesof furnace 10 very well can be described using COMSOL. In figure 19 the M1 model iscompared to the results from sample A from the experiments in furnace 10. As can beseen, the curves follow each other until the second heating, where they are separated dueto too low temperature during the experiment in furnace 10.

In figure 20, the results from M2 is included as well as the Twall function from thecomputer model. The Twall function (azur-blue line) and the planned temperature (redline) for the faster heat treatment follow each other in a satisfactory way. The sameapplies for M1 and M2 curves, however they might be considered too similar regardingtheir different appearence.

Figure 19: The same as figure 16 but the results from the COMSOL simulation is included.

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Figure 20: Illustrates results from M1 and M2 and the Twall function from COMSOL,the measured temperature in furnace 10 and the planned temperature cycle for a fastersoft annealing.

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6 Discussion


The results from the simulation of a faster heat treament indicates that a capacity increaseof 50-70 % for furnace 10, 13 and 14 is possible. The SEP-1520 evaluation indicates thata speed of 6.2 m/h will decrease the carbide size, introduce risk of small amounts ofpearlite and carbide networks. Although the results do meet the requirements, which atthis point is the only significant thing. However, the risk of not meeting requirementsneed to be evaluated carefully by doing more experiments similar to the ones conductedin this thesis and only endeavor being within specification. No extra revenue will comefrom overdoing the heat treatment.

According to figure 16, the first heating seemingly went as planned. The figure illustratesthat heating to 820 oC took approximately 2 hours and that heating rate was constant upto 750 oC when a phase transformation occurred (austenitization). In figures 17 and 18 itcan be seen that the furnace temperature is unaffected by the batch, which indicates thatthe heating rate is not limited by the furnace’s heating capabilities but the heat transferrate into the batch. This means that, besides increased convection, the only realistic wayto increase the initial heating rate inside the furnace is to increase the temperature in theheating zones.

By studying figures 15 and 16 it can be seen that something happened with the temper-ature data logger that made it log temperatures in a range from 0 to 750 oC at the sametime. The data from the cooling step and forward is therefore considered less reliable.

It seems as if the temperature decrease stops at approximtely 720 oC during the inter-mediate cooling, where it is maintained until the next heating step. The temperaturemight be held constant due to a phase transformation from austenite to cementite andpearlite. But according to the recipe of soft annealing at Ovako, the temperature shouldreach 690 oC in order to obtain a complete cementite/pearlite structure, which may haveresulted in having austenite left during the second heating. Remaining austenite couldhave enabled for growth of carbide networks. The cooling capability can be increased byinstalling more cooling tubes.

Table 18 show that both samples have the same grain size, which indicates that thefast cooling in the end for sample A did not have the negative effect it should havehad since the spheroidized cementite is expected to grow during the high temperaturethat is maintained during a slow cooling. By evaluating figure 16, one can see that thefinal temperature is achieved approximately 2 hours prior to what was planned and 500oC is achieved approximately 1 hour prior (diffusion rate decreases with temperature

39

and growth is assumed to cease below 500 oC). This opens for reflections regarding ifthe speed can be increased even further by increasing the cooling rate after the secondheating.

The fast cooling was only a problem for sample A and was successfully solved by placing abatch ahead of the batch to be tested. This is never a problem during regular productionsince the constant flow of batches help keeping temperature up.

No solid conclusions can be drawn with respect to how the dimension and number of tubesaffect the spheroidization. But logically, a smaller dimension and more tubes should beeasier to heat treat since the area to volume ratio is bigger, which means that heat transferinto the batch is faster. Unfortunately sample A and B had very similar weight-per-metervalue, which is a common measure of load regarding heating/cooling capabilities.

6.2 Computer Simulation

The computer simulation describes the heating/cooling capabilities of furnace 10 wellin half the furnace compared to temperature measurements, see figures 19, 20. Thisinformation can be interpreted as that the furnace have an unsufficient heating capabilityat the second heating as well as during the supposedly slow cooling in the end zones. Itmight therefore be of interest to install more burners in zones 5-7. The final cooling isnot controlled at all today and also not a problem during normal production conditions.However, installing some kind of heating capability would allow the ability of managingthe cooling more precise. The simulations also show that cooling is not as effectiveas desired and therefore more cooling tubes could be installed to increase the coolingcapability.

The computer model did not take phase transformations into consideration, which wouldhave affected the results by having a slower initial heating rate and also a slower inter-mediate cooling rate. Phase transformation in low alloyed steels are well described andcould be implemented in the model.

6.3 How to use the capacity increase

This increase is enough to cut the operational costs for soft annealing. A capacity increasecan be used in different ways to decrease expenses, for instance furnaces can be shut down,used for increased productivity or used for another heat treatment.

By shutting a furnace down the variable costs for that furnace will be cut. If the capacityincrease is used for increased productivity the fixed costs will be shared on larger quantitiesand thereby the production cost will be reduced.

40

The furnace can be reconstructed to handle other types of heat treatments. The marketchanges fast and it is difficult to know exaclty what new heat treatment would create newsales areas. However, the three discussed earlier in the report, normalization, isothermalannealing and quench and tempering are, according to the sales department at Ovako,the most likely to do that. Exactly which one, why and how it can be implemented cameoutside the framework of this thesis and deserve a separate investigation.

6.4 Possibilities for further capacity increase

By having a forced cooling at the cooling bed after the tube rolling, the initial structurecould be made more suitable for soft annealing. It is described in an internal report[13]that the thickness of the pearlite have a major influence on spheroidization time and amartensitic structure is the absolute fastest. A forced cooling on the cooling bed wouldresult in a finer pearlite.

Installing an induction heater before the furnace would reduce the initial heating timesubstantially. Induction is also a more energy efficient way to heat metallic materials.The tubes would have to be heated one by one to avoid welding tubes together due tothe increased electrical resistance between the tubes that could cause local melting. Ifthe tubes were austenitized before put in the furnace approximately 2 h could be saved.

Using the heat that is already in the tubes after tube rolling would also enable for adecrease in the initial heating. This could be done by connecting the cooling bed to thefurnaces queue bed. If the tubes were 500 oC instead of 25 oC when entering the furnace,the heating time could be reduced by approximately one hour (heating from 25oC to500oC takes roughly one hour according to figure 16). Today, this would not work dueto that the tube rolling are pickled before being heat treated.

Increasing temperature in the initial zones could probably be done and the magnitudeof the outcome can be evaluated theoretically by assuming that the heating is made upsolely by radiation and that the total power absorbed during heating can be described byeq 15.

dq = σεA(T 4 − T40 )·dt (15)

Where

T = furnace temperature

T0 = initial material temperature

σ = Stefan−Boltzmann constant

41

ε = Surface emissivity

A = Area of furnace walls

Integration gives.

Qtot = σεA(T 4 − T

40 )·t

The total heat absorbed should be constant between two temperatures.

Qtot

1 = Qtot

2 (16)And thereby.

σεA(T 41 − T

40 )·t1 = σεA(T 4

2 − T40 )·t2

Which gives.t2

t1=

σεA(T 41 − T 4

0 )

σεA(T 42 − T 4

0 )=

(T 41 − T 4

0 )

((T1 +∆T )4 − T 40 )

If it then is assumed that T1 = 1093 [K] and T0 = 298 [K] then it is possible to describehow much faster the heating should be. An illustration of the percentage increase can beseen in figure 21 and as can be seen, an 80 degree increase would decrease the heatingtime by 25 % or 30 min if heating time is 2 h. Enclosed in Appendix C is COMSOLsimulations that show how the model treats an increased temperature in the first zoneand in the first two zones.

Figure 21: The left axis correspond to the blue line and illustrates the time reduction ofthe initial heating if it takes 2 hours at ∆T=0. The right axis correspond to the red line,which illustrates the percentage decrease in initial heating time.

42

7 Conclusions

The main objectives for this project was to increase the capacity of continuous annealingfurnaces. This were done by evaluating the heat treatment cycle time and temperatureand the batch loading. Also possible new heat treatments were evaluated. It can beconcluded that:

• A maximum increase of 6450 kg/h or 57 % can be achieved by implementing thefaster heat treatment

• The implementation requires some reconstruction

• The charging was found to be optimized already

• Proposals of new heat treatments are normalization, isothermal annealing andquench and tempering

43

8 Future work

Recommendations for future work are.

• Continue with experiments to

– completely prove that a faster heat treatment is feasible– gather information for the reconstruction

• Decide what to do with the increased capacity

• Investigate the possibilities of further capacity increase

• Conduct a similar project on other furnaces to find possibilities of capacity increases

44

ACKNOWLEDGEMENTS

I would like to thank all of them at Ovako who helped me during this thesis, it would’vebeen hard without your support. In particular my supervisor at Ovako, Mehmet Cengizfor continuous support during the thesis and Erik Claesson, who created this projectand gave me the opportunity to run it. I would like to thank the personnel at theheat treatment section at Ovako Tube who participated in discussions and helped withplanning and execution of experiments.

I would also like to thank my supervisor at KTH, John Ågren, for his support during thisproject.

45

References

[1] S.Ciftci, "Sfäroidisering av kolstål", Luleå Tekniska Universitet, 2006:318

[2] T. Holm, P. Olsson, E. Troell “Stål och Värmebehandling - En Handbok”, Mölndal,2010

[3] M. Hillert, J. Ågren, A. Borgenstam, “Mikro och nanostrukturer i materialdesign”,Institutionen för Materialvetenskap, Kungliga Tekniska Högskolan, Stockholm, 2005

[4] C-C. Chou, P-W. Kao, G-H. Cheng, “Accelerated Spheroidization of hypoeutectoidsteel by the decomposition of supercooled austenite”, Journal of materials science 21,p. 3339-3344, 1986

[5] L. Li, W. Yang, Z. Sun, “Mictrostructure evolution of a pearlitic steel during hotdeformation of undercooled austenite and subsequent annealing”, Metallurgical andmaterials transactions a vol. 39A, p. 624-635, 2008

[6] Ö.E. Atasoy, S. Özbilen, “Pearlite Spheroidization”, Journal of materials science 24,p. 281-287, 1989

[7] A. Saha, D. Kumar, J. Maity, “Effect of cyclic heat treatment on microstructure andmehcanical properties of 0.6 wt% carbon steel”, Materials Science and EngineeringA 527, p. 4001-4007, 2010

[8] Y-T. Wang, Y. Adachi, K. Nakajima, Y. Sugimoto, “Quantitative three-dimensionalcharacterization of pearlite spheroidization”, Acta Materialia Volume 58, August2010, p. 4849–4858

[9] J-E, Andersson, “New soft-annealing procedure for SKF grade 3”, Internal report atOVAKO STEEL, 1993-05-02

[10] S. E. Nam, D. N. Lee, “Accelerated spheroidization of cementite in high-carbon steelwires by drawing at elevated temperatures”, Journal of materials science 22, p.2319-2326, 1987

[11] S. Chattopadhayay, C. M. Sellars, “Kinetics of pearlite spheroidisation furing staticannealing and during hot deformation”, Acta Metallurgica vol. 30, p. 157-170, 1982

[12] P. Ölund, S. Larsson, “Soft annealing tests”, Internal report at OVAKO STEEL,1998-08-19

[13] T. Andersson, “Utgångsstrukturens inverkan på sfäroidiseringsförloppet vidmjukglödgning av kullagerstål”, Internal report at SKF Steel, 1991-01-03

46

[14] M. Andersson, T. Sjökvist, “Processmetallurgins Grunder”, Instutitionen för Materi-alvetenskap, Kungliga Tekniska Högskolan, Stockholm, 2002

[15] C. Fallqvist, “Flytt av U7 samt skrotning av U5 och 6”, Internal report Ovako AB,2011-02-04

47

9 Appendix

Appendix A

Matlab code

clear all; close all; clc;

%Villkor

%Max härdbelastning 750 kg/m; Max belastning (travers) 7500 kg

%Max bredd 1700 mm för VB00 - VB29 samt VB55 - VB56 samt VB60-VB99

%Max bredd 3500 mm för VB30 - VB54 samt VB57 - VB59

%Max antal lager 1 för YD > 150 mm 2 för YD > 101 mm 3 för YD < 101 mm

%Konstanter

rho = 7850*1e-6; %kg/m^3

maxLast = 750; %kg/m

YD = 160; %ytterdiameter

ID = 120; %innerdiameter

VB = 93; %värmebehandlingskod

viktPerMeter = pi*rho*((YD/2)^2-(ID/2)^2);

if YD > 150

antalRor = floor(3500/YD);

elseif YD < 150 && VB > 101

antalRor = floor(3500/YD*2);

else

antalRor = floor(3500/YD*3);

end

iter = antalRor + 1;

if YD >150

lager = 1;

while iter > antalRor && antalRor > 0

if VB >= 30 && VB <= 54 || VB >=57 && VB <=59

if antalRor*YD > 3500 || viktPerMeter*antalRor>maxLast

antalRor = antalRor - 1;

end

elseif antalRor*YD>1700 || viktPerMeter*antalRor>maxLast


end

iter = iter -1;

end

elseif YD < 150 && YD > 101

lager=2;


if VB >= 30 && VB <= 54 || VB >=57 && VB <=59



end



48

end

iter = iter -1;

end

else lager = 3;


if VB >= 30 && VB <= 54 || VB >=57 && VB <=59



end



end

iter = iter -1;

end

end

a = sprintf(’YD:\t%g mm\nID:\t%g mm\nVB:\t%d\nLager:\t%d \n’,YD,ID,VB,lager);

disp(a)

a = sprintf(’Antal rör:\t%d st \nKg/m:\t\t%g kg/m \nTotal bredd:\t%g

mm\n’,antalRor,antalRor*viktPerMeter,antalRor*YD);

disp(a)

a = sprintf(’Om du skulle lasta ett rör för mycket, dvs %d st:\n\nAntal rör:\t%d st \nKg/m:\t\t%g

kg/m \nTotal bredd:\t%g mm’,antalRor+1,antalRor+1,(antalRor+1)*viktPerMeter,(antalRor+1)*YD);

disp(a)

49

Appendix B

Comsol details

Variables:

Twall =

(t<=(t1-1/vConst))*(T1)+

((t1-1/vConst)<t&&t<=(t1+1/vConst))*(T1+(T2-T1)/(2/vConst)*(t-(t1-1/vConst)))+

((t1+1/vConst)<t&&t<=(t2-1/vConst))*(T2)+




















((t11+1/vConst)<t&&t<=(t12))*(T12)

Parameters:

50

Table 19: Parameters used in the COMSOL simulation.Parameter Value [m] Parameter Value [oC] Parameter Value [h]

Z1 4.4 T1 820 t1 0.71Z2 6.5 T2 820 t2 1.06Z3 8.7 T3 820 t3 1.41Z4 13 T4 820 t4 2.10Z5 18.5 T5 820 t5 2.99Z6 24.7 T6 670 t6 3.99Z7 30.5 T7 800 t7 4.93Z8 33.4 T8 800 t8 5.39Z9 35.5 T9 780 t9 5.72Z10 40.5 T10 730 t10 6.52Z11 44.7 T11 700 t11 7.22Z12 47.7 T12 690 t12 7.70

vConst 6.2/3600 [m/s]

Material parameters:

Table 20: Material parameters used in the COMSOL simulation.Brick Steel N2

Density [kg/m3] 2000 7850 From programHeat capacity [J/(kg·K)] 450 500 From program

Thermal conductivity [W/(m·K)] 2 30 From programEmissivity [1] 0.17 0.8 transparent

51

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