DEGREE PROJECT IN TECHNOLOGY,FIRST CYCLE, 15 CREDITSSTOCKHOLM, SWEDEN 2019

Experimental and Theoretical Investigation of Selective Laser Melted Uddeholm Dievar ®

SANJIN PEPIĆ

OTTO RIDEMAR

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

Abstract

Themain problem encountered in this thesis is the lack of research and knowledge

of selective laser melted-printing with Uddeholm Dievar®. This absence of

information could cause issues regarding quality and properties of the alloy as

well as uncertainty regarding an appropriate heat treatment cycle.

This thesis mainly focuses on observing the changes that occur in the

microstructure when Uddeholm Dievar® is manufactured through the additive

manufacturing (AM) method known as selective laser melting (SLM). The SLM-

method consists of a high-power laser that melts together thin layers of powder,

one layer at a time, until a three-dimensional product is created according to

selected drawings.

The methodology on which this thesis is based on is the execution of a theoretical

study, scientific experiments and thermodynamic calculations. Analysis of

the microstructure is performed using a scanning electron microscope with

techniques such as Energy-dispersive X-ray spectroscopy (EDS) and Electron

backscatter diffraction (EBSD). The purpose of the methods are to map

the constituent elements of the alloy and observe the orientation of the

crystallographic phases in the atomic lattice respectively.

The results show that the powder, both before and after printing, mainly consists

of martensite with a low amount of residual austenite. The amount of primary

carbides is relatively low and has been classified as MC (V-rich) and/or M6C (Mo-

rich) type. The remaining residual austenite could be explained by the segregation

of constituent alloying elements, where the carbon content is a dominant factor

to why the MS-temperature lowers significantly causing the presence of retained

austenite even though SLM has a cooling rate that varies between 103 and 108

[K/s].

Keywords

Uddeholm Dievar®, Additive manufacturing, AM, Selective laser melting, SLM,

Gas-atomization, Hot-work tool steel

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Sammanfattning

Det huvudsakliga problemet som denna avhandling behandlar är bristen

på forskning och kunskap inom selective laser melting (SLM) 3D-printing

med Uddeholm Dievar®. Avsaknaden kan leda till sämre kvalité och

produktegenskaper hos legeringen. Det kan även leda till ovisshet gällande val

av lämplig värmebehandling.

Arbetet fokuserar på att dokumentera utformningen av stålets mikrostruktur

när Uddeholm Dievar® tillverkas med den additiva tillverkningsmetoden SLM.

Tillverkningsprocessen består av en högeffektslaser som detaljerat smälter

samman tunna lager pulver, ett lager i taget, tills att en tredimensionell produkt

skapats utefter valda ritningar.

Använda metoder är; utförandet av en teoretisk studie, vetenskapliga experiment

och thermodynamiska beräkningar. Analys av mikrostrukturen genomförs

med hjälp av svepelektronmikroskåp där teknikerna Energy-dispersive X-ray

spectroscopy (EDS) och Electron backscatter diffraction (EBSD) används. Syftet

med EDS är att kartlägga de ingående elementen i legeringen, syftet med EBSD

är att se orientering av de kristallografiska faserna i atomgittret.

Resultaten visar på att legeringen, både före och efter printing, till största del

består av martensit med en låg mängd restaustenit. Mängden primärkarbider

är relativt låg och har klassifiserats som typen MC (V-rik) och/eller M6C (Mo-

rik). Den kvarstående restausteniten kan möjligen förklaras av segringen av

ingående legeringsämnen där kolhalten är en dominerande faktor som sänker

MS-temperaturen. Detta gör att restaustenit förekommer trots den höga

kylhastigheten som varierar mellan 103 och 108 [K/s] i SLM.

Nyckelord

Uddeholm Dievar®, Additiv tillverkning, AM, Selective laser melting, SLM, Gas-

atomisering, Varmarbetsverktygsstål

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Authors

Sanjin Pepić andOtto Ridemar Department of Material Science and EngineeringSchool of Industrial Engineering and Management (ITM)KTH Royal Institute of Technology

Place for Project

Department of Material Science and EngineeringSchool of Industrial Engineering and Managment (ITM)KTH Royal Institute of TechnologyStockholm, Sweden

Examiner

Anders EliassonDepartment of Material Science and Engineering, Applied Process MetallurgySchool of Industrial Engineering and Management (ITM)KTH Royal Institute of Technology

Supervisors

Greta Lindwall, Assistant ProfessorDepartment of Material Science and EngineeringSchool of Industrial Engineering and Management (ITM)KTH Royal Institute of Technology

Dr. Niklas Holländer PetterssonDepartment of Material Science and EngineeringSchool of Industrial Engineering and Management (ITM)KTH Royal Institute of Technology

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Contents

1 Introduction 11.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.3 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.4 Goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.5 Benefits, Ethics and Sustainability . . . . . . . . . . . . . . . . . . . 3

1.6 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.7 Stakeholders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Theoretical Study 52.1 Selective Laser Melting (SLM) . . . . . . . . . . . . . . . . . . . . . 5

2.2 Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.1 Martensite . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.2 Segregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.3 Carbides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3 Uddeholm Dievar® . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.3.1 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.3.2 Heat Treatment . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.4 Gas Atomization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3 Experiments 163.1 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.1.1 Powder samples . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.1.2 SLM printed samples . . . . . . . . . . . . . . . . . . . . . . 17

3.2 Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . . 17

3.2.1 Energy-dispersive X-ray spectroscopy . . . . . . . . . . . . . 18

3.2.2 Electron backscatter diffraction . . . . . . . . . . . . . . . . 19

3.3 Thermodynamic simulations . . . . . . . . . . . . . . . . . . . . . . 20

4 Result 224.1 Scanning Electron Microscope . . . . . . . . . . . . . . . . . . . . . 22

4.1.1 Powder samples . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.1.2 SLM as-printed Uddeholm Dievar® . . . . . . . . . . . . . . 23

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4.2 Thermodynamic simulations . . . . . . . . . . . . . . . . . . . . . . 25

5 Discussion 285.1 Powder samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.2 As-built samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.3 Heat treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.3.1 Microsegregation . . . . . . . . . . . . . . . . . . . . . . . . . 32

6 Conclusions 33

7 Future work 35

8 Acknowledgements 36

9 References 37

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1 Introduction

This is a bachelor thesis for the Degree Programme in Materials Design and

Engineering, School of Industrial Engineering and Management (ITM) at KTH

Royal Institute och Technology.

1.1 Background

Uddeholm Dievar® is a hot work tool steel alloy developed by Uddeholm and

is commonly used in demanding application such as extrusion, forging and

die casting. Today, the alloy is mostly produced using conventional casting

methods which limits the geometry to rounds, slabs, blooms, etc. without further

machining. However, as technology continuously evolves, so does the steel

industry. With additive manufacturing (AM) on the up-rise and becoming more

accessible, the demand for powder alloys has increased. Therefore Uddeholm is

interested in producing Uddeholm Dievar® as a powder.

The powder will be used mainly for AM using the Powder Bed Fusion (PBF)

technology known as Selective Laser Melting (SLM). The goal for Uddeholm is

to be able to easily manufacture complex geometries and integrate secondary

processes directly in the printing stage. In order for this to become reality, the

high performance properties of the alloy must be retained. Due to the many

cycles of melting and solidification from the SLM process, the micro-structure,

and therefore the properties of the steel, changes.

Uddeholm has contacted the Materials Science and Engineering department at

KTH Royal institute of Technology to thoroughly analyze the micro-structure and

properties of the steel alloy when produced with SLM.

1.2 Problem

When printing with SLM, the alloy experiences rapid cooling-times and

large temperature gradients. These are factors that majorly influence the

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microstructure and in turn, the high performance properties of the tooling-

steel. This involves changes in carbide precipitation and the amount of

constituent phases. Due to the effects that the manufacturing process have on

the microstructure, the heat treatment could have to be altered. The products

manufactured using SLM experience an intense alteration in temperaturemaking

the production method itself similar to a internal heat-treatment step. Hence

fewer steps of heat-treatment afterwards could be required.

Going from several small separate powder particles to a single homogeneous piece

of steel is a challenge. The previous powder particles have to fuse into a larger

coherent one. This means that the so called melt pools created by the SLM have

to solidify and interact with the already printed parts of the component.

Several problems involving themicrostructure andheat treatment steps of the tool

steel alloy will be examined and discussed in this thesis.

1.3 Purpose

The purpose of the thesis is to examine the changes of the microstrcutre of

Uddeholm Dievar® through powder production and SLM printing in order to

gain knowledge of the effects of that AM has. Associated phenomenons that are

connected to said changes inmicrostructure and thereby the properties of the alloy

will be discussed in order to understand their effect. Suitable heat treatment steps

will also be noted.

1.4 Goal

The aim of the thesis is to compile information about the alloy UddeholmDievar®

when it ismanufactured first into powder using gas atomization and later onwhen

it is 3D-printed using SLM. This information will hopefully lead to an insight

of what heat treatment steps are necessary in order to receive the same high

performance properties as conventionally produced Uddeholm Dievar®.

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1.5 Benefits, Ethics and Sustainability

Research in AM will benefit the society in multiple ways. With more and more

components and alloys being compatible with the newAM-processes, thematerial

consumptionwill decrease. This enables for new, innovative lightweight solutions

with even greater bearing capacity.

One must also be aware of the energy consumption of the processes. The PBF

productionmethods are very energy costly in order to reach the high temperatures

tomelt the steel powder. From a sustainability point of view, this has to be altered

through research and innovation in order to reduce the energy consumption per

finished kilo of component.

Metallurgy and the industries linked to this subject have traditionally been

dominated by the male sex. With AM on the rising, more engineers have to

be educated in order to make it a reliable production method. Hopefully the

technology can be a fresh start without any stereotypes attached and be a more

attractive subject for a more diverse group of engineers.

This thesis will not examine nor discuss these aspects, mainly because of the

broadness of the subjects related to sustainability and ethics. Due to, among other

things, time constraints, the subjects that were examined in this thesis had to be

narrowed down, otherwise the quality would deteriorate. Therefore, these major

aspects were not taken into consideration.

1.6 Methodology

In order to determine how the microstructure will behave during SLM printing, it

has to be examined using a Scanning Electron Microscopy (SEM) in combination

with thermodynamical simulations. Firstly, the powder will be analyzed to further

understand the effects that gas atomization have on themicrostructure. Secondly,

a SLM-printed sample will be observed. Energy-Dispersive X-ray Spectroscopy

(EDS) and Electron Backscatter Diffraction (EBSD) will also be used to see the

effects of segregation in the structure and facilitate the identification of different

phases.

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The composition will also be analyzed from a thermodynamic point of view using

the Thermo-Calc software to be able to foresay the formation of phases and

carbides. This will complement the SEM-imaging to get a better understanding of

the microstructure, thus explaining the properties.

A theoretical literature-study will support the experimental results and connect

scientific facts with our results. Prior literature and research of the subject will

be examined and compared to the results in this thesis. Both gas-atomization and

SLM are regarded as themain subjects and will be observed asmuch as necessary,

as long as it is relevant to the purpose of the thesis.

1.7 Stakeholders

This project is written for the Department of Material and Science Engineering on

behalf of Voestalpine High Performance Metals Sweden AB (former Uddeholm

Svenska AB).

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2 Theoretical Study

In order to comprehend the experimental results, a literature study describing the

relevant technologies that are included in this thesis are presented. This includes

the production method of the Uddeholm Dievar® powder and also the properties

of the alloy when conventionally produced.

2.1 Selective Laser Melting (SLM)

SLM is one of many AM-methods. More specifically it is found under the

PBF subgroup. According to ASTM International, a PBF method is defined as:

”an additive manufacturing process in which thermal energy selectively fuses

regions of a powder bed” [1].

The process starts with setting the machine parameters. This involves layer

thickness, laser power, scan speed, morphology and size of the powder, etc.

Powder size usually varies between 20-45 µm and layer thickness between 30-100

µm. The component is designed using computer aided design (CAD) and later on

converted into a STL-file which is compatible with the printer.

When printing starts, the SLM machine disposes a layer of the powder onto the

building plate and the laser with a maximum power of usually 1kW completely

melts the disposed layer and fuses it together with underlying layers. The laser

beam has a spot-size of typically 80 µm in diameter that produces the ”melt zone”.

After the layer has solidified together with its previous layer, the build platform

descends one layer in thickness and the process is repeated until the build is

finished.

The final temperature delivered to the powder layer is dependent on what process

parameters are used, with the most important ones being laser power and scan

speed. This has to matched to each alloy in order to fully melt the powder and

receive a component with good surface quality and minimal porosity [2].

Due to the large surface area of the melt, the solidification is very rapid. This is

crucial to the process, enabling each layer to be deployed with minimal wait. The

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cooling rate varies between 103 and 108 [K/s] in SLM [3].

The build volume varies frommachine to machine, one of the largest commercial

printers has a build volume of 80x40x50 cm3. In cases of complex geometries

and large building angles compared to the laser, support structures will need to

be printed as well.

When the print is completed, the component will be surrounded by excess powder

that has not been melted. The remaining powder is brushed and blown off and

then later reused if in adequate condition. This refers to a common issue when

producing products using PBF technologies. While printing, oxidation of the

powder and product occurs. This is usually avoided by flowing an inert gas

through the building chamber. Nevertheless, a powder cannot be reused an

unlimited number of times due to the risk of contamination.

2.2 Microstructure

2.2.1 Martensite

When hardening a steel product, a desired structure is sought after. This structure

is named martensite and is what gives the steel its properties regarding hardness

and thermodynamic resistance. It is a body-centered tetragonal (BCT), non-

equilibrium structure of ironwith a lowpercentage of dissolved carbon. The phase

transforms out of the equilibrium-state austenite when quenched. During this

transformation the material experiences a volume expansion. This is because the

BCT structure has a lower density than the FCC structure in austenite [4].

There are mainly two types of martensitic micro-structure, either as-quenched

or tempered structures. As-quenched martensite is characterized by higher

hardness, lesser toughness and a larger grain size [5], shown in fig. 2.1a,

compared to the tempered alteration. Tempered martensite has finer grains,

higher precipitation of secondary carbides and a higher toughness [6], as seen

in fig. 2.1b.

Depending on the amount of dissolved carbon in the austenite, the morphology

of the martensite will be different. If the amount of carbon is below 0.6wt%, the

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(a) Grain-size in quenched-onlymartensite [7]. (b) Grain-size in tempered martensite [8].

Figure 2.1: Structural differences in (a) quenched-only and (b) temperedmartensite.

appearance of the structure is named lath-martensite, whereas if the amount of

carbon is greater than 1wt%, the martensite is seen as plate-like. In the area 0.6 -

1wt% carbon, the martensite will appear as a combination of both types [9].

The hardness of the martensitic structure continuously increases until it reaches

0.6wt% carbon, but begins to decrease if the percentage of C increases even further

[9]. Furthermore, with an carbon fraction over 1wt%, retained austenite is likely

to be present in the grain-boundaries among the plate-martensite .

Retained austenite is commonly found within the martensitic structure. Its

unique properties, such as high toughness, can when combined with the brittle

characteristics ofmartensite create an alloy that has benefits fromboth structures.

In most applications, the presence of retained austenite often causes problems in

the structure and therefore controlling the amount of retained austenite is crucial

for the qualities of the alloy. The affected properties are several; dimensional

instability, fatigue, low impact strength, etc. Most of the qualities are affected

due to the incoherent FCC-structure in the BCT-martensite [10].

2.2.2 Segregation

The term segregation (also known as coring) is a phenomenon where an alloy has

a difference in composition either locally or globally. Local segregation or micro-

segregation, as it is commonly refereed to, is a deviation of the composition in a

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grain. This occurs during solidification of an alloy where the cooling rate does

not allow the microstructure to reach equilibrium. During perfect conditions, the

alloying elements diffuse through the solid phase and equalizes any heterogeneity

in the composition. This however is almost never the case due to solid phase

diffusion being considerably time consuming which makes micro-segregation a

large issue during casting [11].

Microsegregation can be avoided through very rapid cooling in order to receive

a fine structure and short diffusion distances. It is however possible to reverse

themicrosegregation after solidification through aprocess called homogenization.

This is a heat treatment where the alloy is heated close to the solidus line in order

to even out the composition throughout the grain [12].

A global segregation in composition is called macrosegregation where differences

in composition canbe seen in the ingot as awhole. Commonly seen in ingot casting

where the first solidification takes places at the medium between the molten alloy

and the casting mold. Throughout the solidification towards the center of the

ingot, the composition varies in the way the the elements with the highest melting

points will be the last fraction to solidify [13].

2.2.3 Carbides

The presence of carbides in tool steel is crucial in order to receive a high strength,

therefore carbide-forming elements are added to the composition. In the case

of Uddeholm Dievar® these are Cr, Mo and V. The strength in the material

comes from the effect knownas precipitation hardening, where a secondary phase,

usually carbides, interrupts the homogeneity in the atomic lattice and hinders

the movement of dislocations. In order for the dislocation to continue it has

two options. Particle cutting, which is common for smaller particles, where the

dislocation cuts the particle and travels through. This is shown in fig. 2.2a. Or

through the Orowanmechanism, where the dislocation wraps around the particle

in order to continue. This is more common for larger particles and is visualized

in fig. 2.2b. These mechanisms empower the yield strength of the material

[14].

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Figure 2.2: a) Orowan mechanism. b) Particle cutting [15].

Usually carbides are divided into two groups. Primary and secondary carbides,

where the first are carbides produced during the solidification of the alloy and

the latter includes carbides formed in solid state. Secondary carbides can also be

divided into two separate groups known as equilibrium and tempering carbides,

where the equilibrium carbides are formed during high temperatures and during

a longer period enabling the composition of the carbide to reach equilibrium.

Tempering carbides are usually in a meta-stable phase and precipitate during

lower temperatures, not allowing the carbide to reach its equilibrium [13].

A large range of carbides are anticipated to form during heat treatments. The

carbides that form usually fit into the atomic lattice of the micro structure that

is present. Common carbides in hot-work tool steels are MC(Face-centered

cubic, V- andNb-rich), M2C(Hexagonal, Mo-rich), M6C(Face-centered cubic, Mo-

rich), M7C3(Hexagonal, Cr-rich) and M23C6(Complex face-centered, Mo- and Cr-

rich). [13, 14] These carbides are prone to precipitate during different stages

of the manufacturing process of the alloy which alters their size and therefore

functionality.

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2.3 Uddeholm Dievar®

Uddeholm Dievar® is a high-performance tool steel engineered specifically to

handle demanding working environments during high temperatures. It has a

high resistance towards plastic deformation, fracture, thermal elongation and

wear. The metal is characterized by its isotropic ductility, strength and great

dimensional stability throughout a large temperature-range. These unique

properties that makes up for Dievars® great application in tooling are obtained

partly because of the several alloying elements included, as shown in table 2.1

[16].

Table 2.1: Composition of Uddeholm Dievar®

Elements C Si Mn Cr Mo V Fe

Percentage 0.35 0.2 0.5 5.0 2.3 0.6 Balance

It is a hot-work tooling-steel that offers great resistance towards heat checking,

that can be further enhanced in the case of no occurring cracking. Regardless,

Dievar® offers qualities that are above market-standard which allows for longer

lasting and more sustainable tools.

2.3.1 Properties

Uddeholm Dievars® great tooling-properties are obtained by a manufacturing

process named electro-slag remelting. Thismethod gives Dievar® a homogeneous

composition which, in combination with the included elements, results in

[16]:

• High isotropic toughness and ductility.

• Good temper resistance

• High temperature strength

• Dimensional stability

• High hardness.

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Dievars® great resistance towards thermal elongations is obtained from the

constituent components of the alloy that can be correlated to the density,

temperature and the homogeneity of the metal, as presented in table 2.2 [16].

Table 2.2: Variation of density with respect to temperature

Temperature [°C] 20 400 600

Density [Kg/m3] 7800 7600 7400

As seen in table 2.2, it is a small span of change in density throughout a wide range

of temperatures.

2.3.2 Heat Treatment

The main purpose of heat treating alloys is to change the microstructure so that

desired phases and carbides form and thus the wanted properties can be obtained.

By heat treating a metal, internal stresses can be relieved, the hardness can be

adjusted and the ductility can be altered [17].

Heat treatment of a conventional hot-working tool steel consists of several steps,

as follows [13]:

1. Stress relieving.

2. Hardening.

I Heating.

II Austeniting.

III Quenching.

3. Tempering.

Stress-relieving should be done after the machining of the metal. Due to the

relatively low process-temperature at about 650°C, no structural phase-changes

occur during this treatment. The aim is to reduce internal stresses that are residual

in the alloy so that when the steel is used in warm applications, the geometrical

alterations are nominal.

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Figure 2.3: Typical hardening and tempering cycle for a hot-work tool steel [18].

As mentioned in list above, hardening consists of three separate main steps

that together enhance the mechanical properties by reorganizing the constituent

elements for a more homogeneous micro-structure, achieving a harder material.

The process involves the material being heated up so that the dissolving ferritic

phase can be transformed into austenite.

This allows for a higher solubility-rate of carbon and also the dissolution of

carbides which enhances the composition of the FCC phase. The retention

of some carbides are crucial in order to inhibit abnormal grain growth. The

material is then quickly quenched, allowing for a fast phase-transformation

from austenite to martensite. The quenching also prohibits the carbides from

coarsening excessively.

The three main steps when hardening are as follows:

1. Heating.

2. Austenizing.

3. Quenching.

The procedure of heating is done differently depending on the sample size

and heat-conducting properties. Larger samples with low heat-conductivity

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tend to generate thermal stress, therefore they are heated gradually in steps.

Smaller parts with adequate heat-conductivity can instead be heated quicker, and

continuously, without gathering internal stresses [13, 18].

When heated to the desired hardening-temperature, the following step is to

keep the temperature constant during a period of time. This stage is called

austenitization. During this phase, growth of grains and carbides occur. It is

important that the temperature is not excessive nor the period of time is to long,

as it might lead to overgrowth of the carbides and grains which gives undesirable

properties [18].

Lastly, the final step when hardening, namely quenching, is the process of rapidly

cooling down the metal from the austenitic temperature. By quenching, the

temperature is promptly lowered producing a large enough driving force making

it thermodynamically beneficial for the microstructure to transform into a meta-

stable phase referred to as martensite.

There are two main approaches when quenching; direct- and step quenching.

Direct quenching implies that the cooling is continuous without interruptions

in the process. Step-quenching is instead a periodical process. When cooling,

it is important to keep in mind how different cooling-rates affect the micro-

structure differently. If quenched to fast, cracks might erupt, whereas if too slow,

carbides and grains can change attributes and affect the final properties of the

alloy [13].

The final phase of heat-treatment is called tempering, which has the essential

function of improving the toughness of a tool steel [18]. The process consists of

heating up the tool to about 600°C so that the internal stresses, created by the

quenching, are relieved and secondary carbides precipitate into the martensitic

structure. Usually two or three cycles of tempering is performed in order to obtain

the desired amount of carbides [13].

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2.4 Gas Atomization

There are several production methods in powder-manufacturing, with the most

common one being gas atomization. This process takes place in a environment

usually filled with inert gas in order to avoid oxidation or contamination of the

particles. It starts at the top of the atomization-tank where the melt is stored.

The liquid alloy then falls down and meets a number of gas-jets of said inert gas

that breaks up the melt causing small particles to form and quickly solidify with a

cooling-rate of 1.0×105 to 4.8×106 [K/s] [19].

The quickly expanding gas delivers energy to the molten metal and forms a

spherical shape. The gases that are used are usually nitrogen or argon due to their

inert character but nitrogen is more common due to it being the less expensive

alternative. The powder falls down to the bottom of the tank where the collection

chamber is located. Usually yielding a powder size

achieved when the powder size throughout the batch is homogeneous, when the

particles are close to perfect spheres andwhenminimal satelliting is present.

Satelliting is a phenomenon in powder production when smaller particles attach

to larger ones, as seen in fig. 2.5 [22]. This happenswhen said smaller particles get

caught by turbulence caused by the gas-flow and returns close to the gas-nozzles

where they can attach to larger particles [23].

Figure 2.5: The occurence of satellites impairs the flowability of the powder [24].

Gas atomization is known as a versatile powder production method and through

altering process parameters, a large extend of different metal alloys can be used.

The operating variables are gas type, gas pressure, gas flow rate and velocity, melt

temperature etc.

Besides powder homogenity, flowability and packing ability, an important

property to be able to alter is powder size. This is controlled through the

energy input to the melt with the dominating factors being gas velocity and melt

temperature. The melt is usually superheated above the melting point in order

to lower the viscosity. This prolongs the solidification time of the melt droplet

allowing it more time to shrink which facilitates the spheriodization [23].

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3 Experiments

Themajority of experiments in this project involves looking at themicro-structure

of both the Uddeholm Dievar® powder as well as the SLM-printed samples. This

is done by using SEM as well as through different types of thermodynamical

simulations that have been conducted to confirm findings in the SEM-imaging,

further supporting the scientific facts found in the literature.

3.1 Sample Preparation

3.1.1 Powder samples

Mounting a steel powder is quite similar to mounting a larger sample, although

increasingly difficult to achieve a acceptable cross-section of the powder particles

due to the size of the powder.

Three different powder sizes were mounted and examined.

• Powder size 1:

3.1.2 SLM printed samples

The samples of SLM printed Dievar® are as-built, meaning that the samples have

not been heat treated but come directly from the SLM. They have been mounted

in Bakelite and processed through sanding and polishing, ending with a liquid

diamond polish and finally an oxid polishing suspension (OPS).

Two samples have been studied. These have been mounted differently in aspect

towards the build/printing direction. Sample XX is mounted perpendicular

towards the building direction while sample XY is mounted parallel with the

building direction. In other words, sample XX views a cross section of multiple

powder layers while regarding sample XY, only one layer is viewed.

3.2 Scanning Electron Microscopy

Experiments were done using the JOEL JSM-7800F Schottky Field Emission

Scanning Electron Microscope. When observing small particles such as powder

size 1 it is important to choose the right voltage due to the occuring pear-shaped

excitation volume caused by the electron beam. With a lower voltage, the electrons

will not travel as deep but will be spread out. On the contrary, with a higher

voltage, the electrons will continue deeper in the sample and perhaps exceed the

size of particle while being more concentrated.

When using the backscatter detector it is important to be aware of the imaginging

technique known as Electron Channeling Contrast Imaging (ECCI). This is used

to gather information of the orientation of the grains in the crystalline structure.

Normally when using back scatter detection, a beam of electrons interact with

the lattice of the sample and the heavier the atoms are the more prone they

are to backscatter electrons. I.e. appear brighter on the imaging. However, if

the electron beam aligns with the crystalline lattice and fulfills the Bragg angle

as seen in fig. 3.1, the electrons will travel deep into the sample and minimal

backscattering occurs. This will appear as dark areas on the imaging [25].

Furthermore, ECCI is very convenient when localizing defects in the lattice due to

the fact that any disturbance in the lattice (dislocations, stacking faults etc.) will

17

Figure 3.1: Illustration of ECCI on lattices with different orientations [26].

be prone to strong backscattering, thus appearing as light areas in the imaging. A

lot of information about the lattice can be collected and analyzed through ECCI

[25].

3.2.1 Energy-dispersive X-ray spectroscopy

This analytically method is used whenmapping elements in themicrostructure or

specific particles. With this information, conclusions can bemade regarding what

phases and carbides are present in the imaging.

When incident electrons travel from the SEM power source to make contact with

electrons from the atoms in the observedmaterial, an electron from an inner shell

becomes excited and travels to a higher energy level. This results in a ”hole” in the

inner shell which forces an electron from a higher energy level to transit to a lower

energy level and fill this gap. When this happens, an X-ray is emitted with the

corresponding energy difference between the two energy levels. The characteristic

X-ray is absorbed by the detector and through identifying the peak energy from

the X-ray, it can be corresponded to an element [27].

In order to receive data-content of each element. The intensity of the peak-

energy is analyzed. This data is visualized through a spectrum of different energy

peaks where each peak represents an element. Commonly a specific spot in the

microstructure is analyzed. It can be difficult to get a fair reading from smaller

spots due to the fact that the electrons travel into the material in a pear shaped

fashion as seen in fig. 3.2. This means that there will be readings of background

elements which may cause an inaccurate spectrum.

18

Figure 3.2: Pear shaped interaction volume of incident electrons in a sampleduring EDS [28].

3.2.2 Electron backscatter diffraction

This is a crystallographic characterization technique that uses the backscatter

electrons in order to provide data about the crystal orientation and the constituent

phases in the microstructure of a sample. The incident electrons are emitted

at an angle of 70° towards the surface of the sample and the diffraction pattern

of the backscatter electrons are projected onto a phosphorous-screen which is

analyzed with a camera where the received pattern can be corresponded to a

unique crystalline orientation [29].

The results of an EBSD analysis can be view through a crystal orientation map

(COM) where different crystal orientation are colour coded on a SEM image,

making it easy to understand the results and to draw conclusions from.

19

Figure 3.3: Components and methodology of an EBSD analysis [30].

3.3 Thermodynamic simulations

Simulations regarding phase diagrams andmicrosegregation are performed using

the software Thermo-Calc version 2019a. This simulation software calculates and

presents phase-diagrams and Scheil-solidification simulations based on empirical

data and theoretical thermodynamic calculations that are based on Gibbs free

energy.

This is known as the CALPHADmethod where experimental data is collected and

stored in different data bases. The calculation gather information from these data

bases in order to receive results based on reality. Due to this, the results should

be reviewed cautiously and should be viewed as guidelines. Thermo-Calc displays

the perfect scenariowhere everything is in equilibrium, while it is rarely the case in

reality, especially when heat treating an alloy. The results are however considered

sufficient in most applications.

In the following simulation experiments, the database TCFE9: Steels/Fe-alloys

v9.0 has been used and the element input corresponds to the composition of

Uddeholm Dievar® with the exception of a small amount of nitrogen (0,05 wt%)

in order to compensate for the nitration of the powder duringmanufacturing with

gas atomization.

When performing Scheil segregation calculations in Thermo-Calc, the assumption

20

that diffusion in solid phases is very slow is made leading it to being completely

ignored in the simulation if not specified that diffusion in some solid elements

should be included. In liquid phase, the diffusion is however assumed to be rapid,

hence being completely homogeneous at all times [31].

The calculations are based on the Scheil-Gulliver equation as follows:

CS = kCO(1− fs)k−1 and k =CSCL

(1)

Where CS and CL are fractions collected from the phase diagram at the solidus

and liquidus lines respectively and CO is the composition of the alloy. The sought

after value is fs which is the fraction of solid phase [12]. Thermo-Calc does this

calculation after each temperature step and uses the new composition of the liquid

for the next iteration [31].

21

4 Result

The results below are explained and examined to the extent of what is clearly

visible from the experiments.

4.1 Scanning Electron Microscope

4.1.1 Powder samples

(a) 7000x of a non-martensitic powderparticle.

(b) 7000x of a martensitic powder particle.

Figure 4.1: BSE SEM images of powder size 1 (

(a) SEM image of the powder particle that wasexamined using EBSD.

(b) EBSD phase-map of the powder particle in(a). Red is martensite and blue is retainedaustenite.

Figure 4.2: EBSDphase-map showing the fractions of different phases in a singlepowder-grain.

4.1.2 SLM as-printed Uddeholm Dievar®

When manufacturing components of Uddeholm Dievar® using SLM, a fair

amount of microsegregation was seen in fig. 4.3. This is illustrated through the

difference in shading on the SEM image where the lighter areas include heavier

atoms i.e. more backscattering electrons.

Figure 4.3: Microsegregation i XY-sample, as-built SLM printed UddeholmDievar® observed with SEM using a magnification of 10,000X.

23

(a) SEM image of the area examined with EBSDin the as-printed direction.

(b) EBSD phase-map with coloured fractions.Red ismartensite and blue is retained austenite.

Figure 4.4: EBSD phase-map showing the fractions of different phases in the as-printed sample’s XY-direction.

AnEBSD-analysis was performed, which clarified the presence of different phases

in the microstructure, as seen in fig. 4.4b. The constituent phases in the

structure can be seen as coloured fractions wheremartensite is represented by the

dominating red colour, whereas the retained austenite can be seen in blue.

An EDS was performed on the microstructure comparing two specific areas. One

area that was clearly segregated (Point 11) and the other which is determined to

be part of the nominal structure i.e. the first area of solidification (Point 12) and

should have the composition of the alloy. Both points are shown in fig. 4.5b.

The EDS spectrum in fig. 4.6b reveals that point 11 shows a clearly greater content

of Mo and some higher readings involving Cr content.

An EDS analysis was also performed on a possible carbide in the microstructure.

The composition of the point was compared with the composition of a point in

nominal martensitic structure. These are points 4 and 5 respectively as seen in

fig. 4.5a and the corresponding EDS spectra is shown in fig. 4.6a. The results

show an increase in Mo as well as a decrease of Fe on the point of interest (point

4). If studied closely it can also be seen that the results yield a very slight increase

in V. The peaks close to Fe are hard to distinguish if they are V or Fe.

24

(a) Points 4 and 5 (b) Points 11 and 12

Figure 4.5: SEM imaging of the four points that were evaluated using EDS.Results are seen in fig. 4.6.

(a) EDS spectra of points 4 and 5 shown in fig.4.5a

(b) EDS spectra of points 11 and 12 shown in fig.4.5b

Figure 4.6: EDS Spectra results from the points shown in fig. 4.5.

4.2 Thermodynamic simulations

When studying a specific point of the property diagram as shown in fig. 4.7 for

the alloy it is noted that several carbides have the possibility to precipitate in the

microstrcture during tempering temperatures. It can also be seen that the high-

temperature carbides that precipitate should be V-rich MC carbides. Carbides

such as M7C3 could also be present but are not visible in the diagram, this is

because it is not a thermodynamically stable phase at the shown temperatures but

could still nucleate during cooling and become stable at room temperature.

During cooling, different elements will segregate at separate rates. This may

locally affect which carbides are formed during solidification. Some elements are

25

Figure 4.7: Enlarged part of the phase diagram for Dievar® using the TCFE9database in Thermo-calc shows percipitation of several carbides during lowertemperatures.

more prone to segregate in the liquid phase than others, this can be seen clearly

in fig. 4.8a as the amount of Mo and Cr increases in the liquid phase as the

temperature lowers. This will continue until it is thermodynamically beneficial to

form carbides like the Cr-richM7C3 which will deplete the Cr in the liquid.

When examining the general segregation of the alloying elements, simulations

based on the Scheils segregation equation were conducted. This is visualized in

fig. 4.8where themicrosegregation is clear. This allows carbide prone elements to

enrich in the liquid phase resulting in formation of corresponding carbides.

The temperature where martensite begins to from, called MS-temperature, in

relation to the mass percent of Mo can be seen in fig. 4.9a. Relating the MS-

temperature to the segregation seen in fig. 4.8a, it becomes clear that the MS-

temperature is higher for Uddeholm Dievar® at lower temperatures due to the

increased amount of segregated Mo in the alloy. At 2.3wt% Mo, which is the

included amount ofMo inUddeholmDievar®, theMS-temperature will be 541.8K

according to Thermo-Calc simulations displayed in fig. 4.9a.

26

(a) Microegregation of different elements in theliquid phase.

(b) Due to microsegregation, liquid phase ispresent at lower temperatures where otherphases are prone to form.

Figure 4.8: The effects of microsegregation during solidification. Grafs obtainedusing the Scheil calculator in Thermo-Calc.

(a) MS-temperature in relation to a varyingcomposition of Mo in Uddeholm Dievar®.

(b) MS-temperature in relation to a varyingcomposition of C in Uddeholm Dievar®.

Figure 4.9: Effects of microsegregation in the melt on the the MS-temperatureduring solidification.

When simulating a varying amount of C in the alloy, an increase of C content

yielded a rapid decrease of the MS-temperature. The MS-temperature drops

below room temperature (298K) at 1,1wt% C. Fig. 4.9b shows a graph with the

results from the simulation.

27

5 Discussion

5.1 Powder samples

While evaluating the powder, an anomaly was spotted in fig. 4.1a. This is thought

to be due to the nitrogen gas used in the gas atomization process. The small

particle is believed to have solved a small margin of N which in turn should

lower the MS-temperature causing it to be thermodynamically disadvantageous

to transform into martensite.

Small amounts of retained austenite was observed in numerous powder particles

and it is considered that this is also on account of the nitrogen content in the alloy

or the microsegregations affect on the MS-temperature. Moreover, the powder-

grains are almost of fully martensitic character.

It is thought that the microstructure of the powder has little affect on the

microstructure of the built SLM component. The powder is completely

melted during printing and therefore the prior micro structure is considered

to be inessential. With this said, individual powder particles can affect the

microstructure and therefore the mechanical properties of the component. If the

powder is heavily reused and oxidized, it will yield poor characteristics towards

the mechanical properties.

Contaminated powder particles can still be present in new powder however. This

is seen in fig. 4.1a where an anomaly is detected. How this affects the final

microstrucutre is difficult to tell. The difference in composition that caused this

particle to behave differently will most likely disappear through diffusion in the

liquid state during the SLM process.

The largest effect that the characteristics of the powder have on a SLM printed

product is the morphology and the actual size of the particles. This will affect the

flowability of the powder which is a main parameter that affects the quality of the

printed component.

28

5.2 As-built samples

When analyzing fig. 4.3a, micro-segregation is evident and the heterogeneous

distribution of alloying elements is a common problem which can easily be

avoided by homogenization of the sample.

Some of the elements segregate more than others as seen in fig. 4.8a. This is

helpful when trying to confirm what type of carbide is visible in fig. 4.5a. From

the EDS results in fig. 4.6a we have concluded that the carbide seems to be

Mo and/or V rich, indicating a M6C (Mo-rich) or MC (V-rich) carbide. Both of

these carbides have an FCC structure which furthermore confirms the fact that

it is indeed a primary carbide which has precipitated during solidification in the

austenitic phase. It is favorable for the carbides to precipitate in a phase with the

same crystalline lattice as the carbides themselves.

It could also be discussed regarding the brightness of the potential carbide in SEM

BSE imaging. It is seen in fig. 4.5a that the particle is quite dark in colour when

comparing it to the surrounding area. This means that the atoms in the dark spot

are lighter than the atoms in the matrix around it. V, Cr and Fe all have about the

same atomic weight with atomic numbers 23, 24 and 26 respectively. Mo however

has the atomic number 42 which almost doubles the atomic weight. This would

suggest that the carbide is not Mo-rich but the surrounding should be. This is

confirmed by looking at fig. 4.8a where Mo clearly segregates substantially.

When taking the channeling effect into account, this dark spot would suggest

a local homogeneous crystalline lattice that differs from the area around. This

could further confirm that at least a different phase is present in the examined

area.

Examination and comparison of the results obtained by theEBSD, seen in fig. 4.2b

and fig. 4.4b, it becomes evident that in both cases, the structure mainly consists

of martensite with traces of residual austenite. However, several differences are

identifiable. Firstly, the fraction of residual austenite is significantly higher in

the as-printed sample while being nearly undetectable in the powder-grain. Why

the fraction of martensite differs before and after printing can be explained by

several different explanations. For instance, the disparities can be related to

29

the different processes of manufacturing. The powder is manufactured by gas

atomization. This process involves a gas that quickly solidifies themelt so that the

MS-temperature that is needed for the formation of martensite is reached. Due to

the high cooling-rate, the probability of reaching close to the M99-temperature is

high, thus prohibiting a large amount of residual austenite.

Meanwhile, the greater fraction of austenite in the as-printed sample could

be explained through similar reasoning. The energy produced by the high

powered laser in the SLM melts a small layer of grains in a gas-filled chamber.

After melting, solidification takes place. The temperature rapidly drops to

approximately 25°C, which is due to the building plate not being heated during

the process while at the same time only a small area is exposed to the energy

from the laser, therefore not heating up the surrounding air in the cabin. This

might lead to an insufficient rate of cooling in order to reach close to the M99-

temperature, causing a greater amount of retained austenite to be present in the

microstructure.

However, observations of the as-built sample seen in fig. 4.4b indicates that

the amount of retained austenite compared to the powder-sample (viewed in fig.

4.2) is approximately the same and the fraction of martensite nearly identical,

therefore it could be argued that the cooling rate of 103-108 [K/S] combined with

the relatively lowMS-temperature for Dievar® (observed in fig. 4.9) is ample even

in the case of the as-printed sample.

The retained austenite that can be found in the as-printed sample could however,

in this case, be beneficial to the component. Due to the internal tensions arising

by the SLM-printing, the probability of crack-formations is high. The greater

toughness of the retained austenite in the grains counteracts the formation of

cracks by gathering all the internal tension in the austenitic grain boundaries,

where toughness, and therefor, crack-resistance, is reaching a maximum.

These residual stresses incorporated in the SLM-printed product result from

the large temperature gradient, causing the material to thermally expand

during heating and then decrease in size during cooling and solidification.

The component also experiences a volume expansion during the martensitic

transformation due to the fact that crystal lattice BCT (martensite) has a lower

30

density than FCC (austenite).

How the sub-cooling is obtained in the SLM-printer can depend on several factors,

one being the size of the melt. As a result of the microscopic size of the grains a

thin, small layer of melted grains are quickly exposed to the ambient temperature

after being energized by the laser. The difference in temperature in combination

with the thermal conductivity of the alloy and the great cooling-rate creates a fast

enough cooling of the thin layer of melt, thermodynamically favouring the growth

of the martensitic structure.

5.3 Heat treatment

Conventionally produced hot work tool steels undergo a series of heat treatments

in order to receive their wanted properties. The sought after microstructure

is completely martensitic with both primary- and secondary carbides. After

the primary carbides have precipitated and controlled the grain growth, it is

beneficial for the alloy to dissolve said carbides during tempering in order to create

secondary carbideswith the carbide prone elements instead. This is due to the fact

that the secondary carbides contribute a great deal of themicrostructures strength

by prohibiting dislocation movement.

In order for the secondary carbides to precipitate, multiple cycles of tempering

are necessary. These cycles could also be seen as a homogenization treatment,

but it is not certain that the temperatures would be high enough to remove the

microsegregation.

The internal residual stresses in the SLMprinted components can be counteracted

through stress-relief annealing at about 650°C. Fortunately, the temperature for

tempering is only slightly lower at 600°C hence that these two heat treatment

processes could theoretically be done simultaneously.

Austenization of the as-built component could be conducted to homogenize the

composition in the microstructure i.e. reverse the microsegregation. The high

temperature will however also benefit coarsening of the primary carbides which

is unwanted later on when tempering the alloy in order to receive secondary

31

carbides.

Conventionally produced hot work tool steel has to go through an austenization

cycle and later on, a rapid cooling rate in order to receive a martensitic structure.

Components printed with SLM are already martensitic begging the question of

whether or not the austenitizing-cycle is important or could be skipped. Further

research has to be conducted in order to answer this.

5.3.1 Microsegregation

The segregation that occurs while printing evidently has a great impact on the

alloy, in particular on the MS-temperature, as seen in fig. 4.8 and 4.9. With

an increase of Mo in the melt, it is simulated that the MS-temperature slightly

increases to about 290°C from the nominal MS-temperature of 269°C. However

when the carbon-amount varies, theMS-temperature rapidly decreases as seen in

fig. 4.9b. When taking into account the amount of C that increases in the melt

during solidification, the transformation from austenite to martensite becomes

increasingly more difficult due to the low MS-Temperature in these areas with an

enriched carbon amount.

This is thought to explain the appearance of retained austenite in the

microstructure even though the cooling rate of both SLM and gas atomization

is so immense. However there are elements that are prone to microsegregation

that have the opposite effect towards the MS-temperature. A consequence of a

increased Cr-amount is a decrease of the MS-temperature at a proportional rate

compared to the segregation of Mo, evidently canceling the change of the MS-

Temperature caused by these two elements.

32

6 Conclusions

After deliberation in the previous section where scientific facts were compared

with thermodynamical calculation and our results from the experiments, the

following can be concluded:

• When manufacturing Uddeholm Dievar® with SLM technology, a almost

purely martensitic structure is received with a small content of retained

austenite. The martensitic structure is obtained through the rapid cooling

rates in SLM that varies between 103 and 108 [K/s].

• Percipitation of primary carbides in SLM printed Dievar is observed. These

are thought to be of MC and/or M6C type due to its composition (V and Mo

respectively) and crystalline structure (FCC), which fits in the lattice of the

austenite. The observed carbide is suggested to be ofMC type due to it being

thermodynamical stable during higher temperatures causing it to percipitate

during solidification.

• As a result of the martensitic structure of the as-built samples, the

austenetization heat treatment cycle normally done on conventionally

produced Uddeholm Dievar® could be avoided for the sole purpose of

receiving martensite. The heat treatment could however be required for

other purposes such as homogenization and stress relief.

• SLM products often have residual stresses incorporated in the component

after completion due to thermal expansion throughout the several heating

cycles during printing. Common temperature for Stress-relief annealing is at

about 650°C. This means that it could theoretically be done simultaneously

as the tempering heat treatment cycles that tool steel undergoes in order to

receive strengthening secondary/tempering carbides.

• Microsegregation of alloying elements will affect the MS-temperature

differently depending on which element segregates in the liquid phase.

The most impactful element being C which at only 1,1 wt% brings the

MS-temperature down to room temperature, making it thermodynamically

unfavorably to form martensite and instead stays as retained austenite.

33

• It is believed that the microstructure of the powder has little affect on the

microstructure of the final SLM printed product for the reason that the

powder is completely melted during printing and therefore the powders

microstructure becomes irrelevant. The effect that the powder can have on

the finished SLM product is instead related to the powders flowability and

morphology.

34

7 Future work

In order to examine the full effects that the SLM manufacturing process has on

the alloy Uddeholm Dievar®, mechanical testing should be carried out to confirm

the findings of this thesis. This should be done with several samples that have

beenheat treateddifferently. Preferably as-built, only austenitized, only tempered

and finally austenitized and tempered. This would truly describe the mechanical

properties and also confirm what the preferred heat treatment should be.

These mechanical experiments should also be conducted on conventionally

produced Uddeholm Dievar® and then compared with the SLM built samples in

order to measure eventual differences that may occur in the differently produced

and heat-treated samples.

Characterization of the amount of phases could be done using X-ray diffraction

(XRD) in order to receive the fraction of retained austenite in the matrix. This

can be correlated to the microsegregation of different alloys that influence the MS

temperature.

The data received from simulations in this thesis is based on the database TCFE9:

Steels/Fe-alloys v9.0 from Thermo-Calc. More accurate simulations could be

performed if databases developed for these steel systems available at Uddeholm

were applied.

More computational studies should be conducted in order to receive more data

and confirm the findings. A diffusion simulation where different cooling rates

can be used in order to receive the changes in composition over a distance from

the first solidification. I.e. the exact microsegregation of the different alloying

elements in the dendritic arms.

Different heat treatments can also be simulated in order to analyze the

homogenization and also both primary and secondary carbide precipitation

simulations should be conducted.

35

8 Acknowledgements

We would first like to thank our supervisor assistant professor Greta Lindwall at

the Department ofMaterials Science and Engineering, KTH, for providing us with

great knowledge and support during this thesis. For always being there when

needed and giving advice which kept us in the right direction throughout every

phase in this thesis.

Not to be forgotten is Dr. Niklas Holländer Pettersson at the department of

Materials Science and Engineering, KTH, who has been there for the entire

thesis helping us not only with sample preparation and SEM imaging but also

with quality discussions about the results and learning us about metallurgic

phenomenons.

36

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