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DEGREE PROJECT IN TECHNOLOGY, FIRST CYCLE, 15 CREDITS

STOCKHOLM, SWEDEN 2019

Evaluation of material properties

after laser welding on ductile cast

iron

OLIVIA TAIVALKOSKI

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Abstract

Scania wants to lower the weight of their trucks, including the goal to reduce the carbon dioxide emissions, and one way to do that is to use laser welding instead of fastenings.This bachelor thesis work is about laser welding of ductile cast iron, or spheroidal graphite cast iron or nodular cast iron, to QT-steels and case hardening steel and evaluation of the mechanical properties of the weld. Also laser welding of cast steel to the same two materials are being evaluated in this work. Tests are done to evaluate the effect on the material from laser welding. The tests are tensile tests and Vickers hardness test; both across and along the weld and in some areas of interest. EDS (Energy Dispersive X-Ray Spectroscopy) is used to analyze the composition in the weld and light optical microscope is used to look at the fusion zone (FZ) and the heat affected zone (HAZ). The results shows that the hardness is high in the heat affected zone due to the formation of martensite and that the materials mix more towards the root of the weld. The materials also mix more if the weld depth is deeper. The width of the heat affected zone seems to be longer if the heat input is higher. It is also clear that welding of cast steel is less complicated than the welding of ductile cast iron. That is because ductile cast iron gets a hard and brittle heat affected zone (HAZ) while the cast steel does not. The cast steel can also be welded without filler wire without getting to hard or to brittle. If laser welding is to be used in the future the component should be constructed in such a way that the fusion zone is not carrying the main load. Tests on fatigue strength should also first be done on a finished component as it cannot be tested on the samples in this work. This work was conducted at Scania AB and the royal institute of technology, KTH, in Sweden.

Sammanfattning

Scania vill sänka vikten på sina lastbilar, bland annat för att minska utsläppen av koldioxid, och ett sätt att göra det är att lasersvetsa istället för att använda bultar. Detta kandidatexamensarbete handlar om lasersvetsning av segjärn, eller nodulärt gjutjärn som det också kallas, till

seghärdningsstål och sätthärdningsstål samt utvärdering av svetsens mekaniska egenskaper. Även lasersvetsning av gjutstål till samma stålsorter som ovan utvärderas i detta arbete.Tester görs för att utvärdera effekten på materialet från lasersvetsningen. Testerna är dragprov och Vickers

hårdhetstestning; både tvärs över och längs med svetsen samt även i vissa områden av särskilt intresse. EDS (Energy Dispersive X-Ray Spectroscopy) används för att analysera sammansättningen i svetsen och ljusoptiskt mikroskop används för att se svetsgodset och den värmepåverkade

zonen.Resultaten visar att hårdheten går upp i den värmepåverkade zonen på grund av martensit bildning och att materialen blandar sig mer närmare svetsroten. Materialen blandar sig också mer om svetsdjupet är djupare. Den värmepåverkade zonens bredd verkar vara större om sträckenergin är hög. Det står också klart att svetsning av gjutstål är mindre komplicerat än svetsning av segjärn eftersom segjärnet får en hög hårdhet i den värmepåverkade zonen medan det inte alls blir så för gjutstålet. Gjutstålet kunde också svetsas utan tillsatsmaterial utan att få ett för hårt eller sprött svetsgods. Om man vill använda lasersvetsning i framtiden ska komponenter konstrueras så att svetsen inte bär huvudvikten eftersom resultatet visar att svetsgodset får lägre brottgräns.

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Table of content

1. Introduction ..………..…1

1.1 Aim………...1

2. Background …..……….…..2

2.1 Laser welding………...2

2.2 Heat affected zone………3

2.3 Ductile cast iron……….….6

2.4 Cast steel……….…6

2.5 Quenched and tempered steel………....6

2.6 Case hardening steel……….…..8

2.7 Filler wire material……….…..8

3. Experimental method……….…9

3.1 Laser welding and the samples………..….9

3.2 Metallurgical testing………...10

3.3 Mechanical testing………...12

4. Results………...15

4.1 Microstructural analyzes ……….15

4.2 Hardness results……….18

4.3 Tensile tests………...23

4.4 Heat input and carbon equivalent………..…23

5. Discussion……….……..25

5.1 Weld depth………..……….…….25

5.2 HAZ length……….…….25

5.3 The hardness profiles ……….…..25

5.4 Ethical and environmental aspects ……….…….26

6. Conclusions………27

7. Recommendations for the future………..28

8. Acknowledgments………...…29

9. References……….…30

Appendix A: Welding data………..32

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1. Introduction

The United Nations, UN, have made the sustainable development goals consisting of seventeen (17) goals for the future that range from human rights to protection of natural resources and the

environment [1].The emissions of carbon dioxides must go down in the world and Sweden has a national goal set by the government to reduce carbon dioxide emissions [2].

Scania states on their website that they are: “taking on global challenges. Climate change, population growth, and urbanization are all intimately tied to transport. Scania doesn’t shy away from

challenges like this. We are driving the shift towards a sustainable transport system, to create a world of mobility that is better for business, society and the environment.”[3]

Developing a sustainable transport system is not as simple as only one thing, but one thing that is included is the carbon dioxides emissions. There are some ways to reduce emissions but the main way is to lower the weight of the truck and therefor the amount of gas needed. A lower weight of the truck means either less emission per trip or less emission per goods when more goods can be loaded and fewer trips needed.

One way to lower the weight is to optimize geometry and material usage of components. New joining methods enable lighter components, for example changing from fastening to laser welding. Fastening requires steel bolts and a lot of material to achieve the right clamping length [4]. Using laser welding might also make more room for other parts of the truck and/or make other smart solutions possible.

In the thesis Laser welding of tool steels, A. Lundstjälk tested the weldability of different high strength metallic materials. [4]

This is a continuation on his work, now with focus on the weldability of ductile cast iron to QT-steels and case hardening steels.

Ductile cast iron has replaced forged steel in some applications and in that way reduced the cost since fewer parts for one component was needed. In the transport industry ductile cast iron is used for high load carrying components [6]. So if it could be laser welded it would be even more useful. Cast steel is also tested since it could be a good alternative to ductile cast iron considering both the materials can be casted and have similar mechanical properties [7]. The cast steel advantage could be that, even if it is more expensive, it is/may be easier to weld and thus the production of the laser welded part could be easier and less expensive.

1.1 Aim

The aim with this work is to evaluate the mechanical properties after laser welding ductile cast iron or cast steel to QT-steels and case hardening steel to provide design guidelines and support

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2. Background

Here is the result from the literature study concerning laser welding, the heat affected zone, ductile cast iron, cast steel, QT-steels and case hardening steel. Also a little about filler wire and SEM is found here.

2.1 Laser welding

Laser welding has high energy density and gives a narrow weld with low material distortion [8]. A narrow weld means a narrow fusion zone (FZ) and a narrow heat affected zone (HAZ) (see figure 1). Laser welding forms a melt pool where the materials somewhat blends together (see figure 2) [4] and when the melt solidifies it forms what is called the fusion zone (FZ). Sometimes a filler material can be used to give the fusion zone (FZ) better mechanical properties [9]. The heat affected zone (HAZ) is around the fusion zone where the material has been heated from the welding enough to change its properties without melting the material [10]. The unaffected material is called the base material (BM) and is the same as the material before it is welded. The parts of the weld can be seen in figure 1. Two metal plates can be welded together using I-joints, with or without gap, or single- V-joints (see figure 3) [11].

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Figure 2: Schematic figure showing the meltpool and flow in it. On the right is a weld cross-section with the resulting hour-glass shape of the fusion zone (the white in the middle). [4]

Figure 3: The different weld joints used in this work.

2.2 Heat affected zone

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Since martensite is such a brittle phase it would be best to avoid it, and especially the hardest and more brittle martensite. Preheating could make the heat affected area wider but less dangerous, meaning less brittle. That is because the preheating would make the base material hotter and then the cooling rate would not be as fast. Post-heating would have a similar effect on the heat affected zone [12]. The cooling rate is also affected by the heat input; the higher the heat input the lower the cooling rate (it cools slower), at least for arc welding [14]. The heat input may not have the same effect on the cooling rate for laser welding because the weld geometry is different [15]. The equation to calculate the heat input for laser welding can be seen in equation 1 [15]. A thicker material also affects the cooling rate as a thicker material has a better chance to lead the heat away and makes it cool faster.

Heat input equation for laser welding. Heat input in [kJ/mm]. Where W is the laser power in [kW] and v is the travel speed in [mm/min]. [15]:

(eq. 1)

The higher the carbon content the harder and the more brittle is the martensite [16], but it is not only the carbon that affects the hardness of the martensite; other alloying materials also do. To calculate the effect of other alloying materials and to predict the weldability of the material, some carbon equivalent equations are used. There are different kinds of carbon equivalent equations where some takes in to a count more materials than the others (see equation 2-5). [17]

Carbon equivalent equations [17]:

(eq. 2)

(eq.3)

(eq. 4)

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Figure 4: Fe C phase diagram, showing the austenitic temperature. [18]

Figure 5: The TTT-diagram for 0,45wt% C iron-carbon alloy, only to show that rapid cooling of

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2.3 Ductile cast iron

Ductile cast iron, or spheroidal graphite cast iron and nodular cast iron as it is also called, is

characterized by its spherical graphite which enables the materials good mechanical properties. The material behaves more ductile because the spherical graphite does not induce cracks since it doesn’t build up tension (see figure 6) [19]. Ductile cast iron exists in two forms; one with spherical graphite and only ferritic structure and the other with spherical graphite and both ferritic and pearlitic structure [17]. The latter is used in this work. Ductile cast iron has high carbon content and most of the carbon is in the graphite.

Figure 6: Tension lines around lamellar graphite (on the left) and spherical graphite (on the right) [17].

2.4 Cast steel

Cast steel has lower carbon content than cast iron and has a bit different properties. For example cast steel shrinks more when it solidifies and is therefore a bit more complicated to cast than cast iron, but this problem is fixed using a riser that works as an extra melt reservoir that can fill the mold as the cast steel shrinks [7]. Cast steel may also be harder to machine depending on the composition and alloying materials [7].

2.5 Quenched and tempered steel

Quenched and tempered steels, also known as QT-steels, combine hardness and ductility, which is achieved by quenching and then tempering the steel [20]. QT-steels have many applications and are for example used in bridges, earthmoving buckets and gear wheels [20]. In this work two different quenched and tempered steels called 25CrMoS4 and 42CrMoS4 are used.

Quenching steel is done by heating the steel to over a specific temperature (the austenitic

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such as water or oil (see figure 7 and 8 of the time, temperature and transformation diagrams/TTT-diagrams). This causes the austenite to transform into a martensitic structure and gives the steel its hardness, but also its brittleness. To give the steel some ductility back it is then heated to a specific temperature and kept there for a specific amount of time before letting it cool slowly in air. This is called tempering and gives it a bit less hardness but makes it more ductile by formation of carbides which lowers the carbon content in the martensitie and thus lowering the hardness and brittleness of the martensite and the material.

Figure 7: TTT-diagram for 25CrMoS4.

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2.6 Case hardening steel

Case hardening steels have a tough core and a hard surface. The hard surface of the case hardening steels gives wear resistance while the tough core works against crack growth and fracture. This makes them suitable for example gear wheels and forging presses [21]. The hard surface is achieved by increasing the carbon content in the surface by heating the piece to the austenitic temperature in a carbon rich atmosphere which allows the carbon to diffuse in to the surface. The piece is then cooled rapidly and this gives a hard martensitic structure in the surface while the core is still unchanged [21]. In this work a case hardening steel called 17NiCrMoS6-4 is used.

2.7 Filler wire material

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3. Experimental method

Since this is a continuation of a master thesis work [4] the welding of the samples had already been done but will be shortly described in chapter 3.1 laser welding and the samples. Some tests had also already been done and only the data had to be summarized, such as the tensile tests and hardness across the weld. Also some calculations were made, such as the heat input and the carbon

equivalent.

3.1 Laser welding and the samples

To make the samples two metal plates, 27 mm deep, 100.8 mm long and of either 10 or 8.5 mm thickness, were welded together at Trumpf in Germany using laser welding. Some welding

parameters were different for every weld, such as the laser power and feed rate and can be seen in table 1, the other parameters not stated in table 1 is the same for every weld and can be seen in the appendix (appendix A) and none of the samples were pre- or post-heated. All samples are with I-joints except I2 and J2 that are with V-shape (see figure 3 and table 1 column 6, Gap). The filler wire is a nickel base material called “UTP A 8051 Ti” and is the same for every weld with filler wire (see table 1, column 8; Filler wire used). The filler wire material composition can be seen in table 2. Every weld with the specified set of parameters and materials combinations was given a sample name (see table 1) and then cut into smaller pieces.

The materials used are ductile cast iron, cast steel, two quenched and tempered steels (QT-steels), 42CrMoS4 and 25CrMoS4, and one case hardening steel, 17NiCrMoS6-4.

Table 1: Sample names, materials and set of welding parameters for every sample. Here “dci” stands for ductile cast iron. QT25 stand for QT-steel 25CrMoS4, QT42 stands for QT-steel 42CrMoS4, “chs” stand for the case hardening steel (17NiCrMoS6-4), “CS” stands for cast steel.

Sample name Material no1 Material no2 Thickness (mm) Laser power (W) Feed rate (m/min) Gap (mm) Filler wire used Wire feed rate (m/min) G1 dci chs 8,5 6000 1,0 0,3 yes 4,0 G5 dci chs 8,5 6600 1,0 0,7 yes 8,8 H4 dci chs 10 9000 1,5 0,3 yes 7,5 H6 dci chs 10 8000 1,0 0,5 yes 9,0

I2 dci chs 10 7000 1,0 V-shape yes 6,3

J1 dci chs 8,5 6000 1,0 V-shape yes 5,0

K1 dci QT25 8,5 6000 1,0 0,3 yes 4,0

L1 dci QT25 10 9000 1,5 0,3 yes 7,5

Z1 QT42 QT42 8,5 6000 1,0 0,3 yes 4,0

Q1 CS chs 8,5 6500 1,5 - no -

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Table 2: Showing the composition of the steels and the filler wire. The balance is iron (Fe). (Note that the table continuous with the same materials but different alloying elements to fit in the page)

wt% C Si Mn P S Cr Filler wire 0.1 3.5 17NiCrMoS6-4 0.153 0.28 0.86 0.009 0.032 1 25CrMoS4 0.22-0.29 0.4 0.6-0.9 0.035 0.02-0.04 0.9-1.2 42CrMoS4 0.447 0.31 0.78 0.01 0.026 1.09 wt% Ni Mo Ti Nb Cu Al Filler wire 55 0.5 17NiCrMoS6-4 1.22 0.12 0.002 0.003 0.11 0.035 25CrMoS4 0.15-0.30 42CrMoS4 0.12 0.17 0.003 0.002 0.22 0.016

3.2 Metallurgical testing

Every sample was cut and mounted in Bakelite so that the cross section of the weld was visible. The samples were grinded to a specific fineness (of 3 or 1 micrometer) and then etched with Nital 2% which is a common etchant for carbon steel and ductile cast iron. Three samples, G1, H4 and Z1, were also ultrasound cleaned before they were analyzed in the SEM (with EDS).

SEM, scanning electron microscope, is a microscope that works by scanning a small area with electrons and since electrons has such a small wavelength it gets very high resolution. With a SEM both the topography and composition can be analyzed. When analyzing the composition EDS (Energy Dispersive X-Ray Spectroscopy) is used. The electrons are excited when the material is “bombarded” with electrons and when they return to their ground state a characteristic x-ray are emitted. The X-ray is then analyzed and information about what atoms the material contains is attained. [23] The composition of the fusion zone (FZ) were examined using the EDS in the SEM machine, Zeiss sigma 300 VP, on four areas in the fusion zone on H4 and G1 (see figure 9) and on the top and bottom of the fusion zone on Z1 (see figure 10).

The samples were also analyzed in a light optical microscope, OPTICA B-380. Photographs were taken with the integrated digital camera Zeiss AxioCam MRc and analyzed with Axiovision. The whole weld and close up of the fusion zone (FZ) and the heat affected zone (HAZ) were photographed. The length of the heat affected zone (HAZ) in to the base material (BM) was measured using the same

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Figure 9: A map over the areas (the squares) that were analyzed in the EDS. This picture is of sample G1 but it is fairly the same for sample H4.

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3.3 Mechanical testing

The hardness was tested with Vickers since the areas that are being tested are very small, the fusion zone (FZ) and the heat affected zone (HAZ). The hardness was tested for every sample across the fusion zone (FZ) on three different positions; top, middle and root (see figure 11). On two samples, G1 and H4, the hardness was also tested along the fusion zone (FZ) (see figure 11 again).Vickers hardness test was also done on some sites of interest; that is in the heat affected zone (HAZ) both near and further away from the weld on those two samples (G1 and H4, see figure 12 and 13) and in the fusion zone on Z1 (see figure 14). The Vickers hardness tests were done using the qness and Matsuzawa MXT30 hardness testing machines.

The tensile tests were done using “the tensile apparatus MTS 810” [4] with test bars cut out from the plates so that the weld was in the middle where it would (hopefully) break. The test bars had the dimensions 10 x 56 x thickness (either 8.5 or 10) mm without reduced midsection.

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Figure 12: This figure shows where the extra hardness tests were taken on sample G1.

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4. Results

Gathered here are the results of the microstructural analyze, EDS analyze, hardness tests, tensile tests and the calculations.

4.1 Microstructural analyses

The measured HAZ length into the base material is shown in figure 15, note that the plate thickness of the sample is denoted with color. Note also that the HAZ length is much shorter for sample K1 and H4. Also note that sample H6, I2 and L1 have the longest HAZ.

The composition down the weld is shown for H4 in figure 16 and for G1 in figure 17. Note that the plates for G1 are 8.5 mm thick and for H4 it is 10 mm. The iron content goes up as the nickel content goes down further down in the fusion zone toward the root (to the right in the graph) for both sample (see figure 17 and 16).

The composition in the top of Z1 is shown in figure 18 and 19. Note that the dendritic structure is more visible in the middle and that the material is harder there, the marks from the HV test are smaller (see figure 18).

Figure 15: length in micrometer of the heat affected zone in the ductile cast iron. (The light green bars are for samples with plate thickness of 8.5 mm, and the blue bars are for plate thickness of 10 mm).

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Figure 16: Composition of Iron (red line) and nickel (blue line) in the fusion zone (FZ) in sample H4, ductile cast iron to the right of FZ (in the top of the picture) and case hardening steel to the left of FZ (bottom in the picture), welded with Ni-base filler wire material.

Figure 17: Composition of Iron (red line) and nickel (blue line) in the fusion zone (FZ) in sample H4, Casehardening to the right of FZ (in the top of the picture) and ductile cast iron to the left of FZ (bottom in the picture), welded with Ni-base filler wire material.

0 10 20 30 40 50 60 70 80 90 100 0 2 4 6 8 10 12 C o m po sit io n, m ass -% Distance, mm

Composition in weld, top to root, H4

Ni Fe 0 10 20 30 40 50 60 70 80 90 100 0 2 4 6 8 10 C o m po sit io n w e ig th% Distance, mm

Composition in weld, top to root, G1

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Figure 18: Picture of sample Z1 top (se figure # for reference), to show where the spectrums were taken in the EDS.

Figure 19: Results from the EDS showing the nickel content in sample Z1 top of the weld. 0 2 4 6 8 10 12 14 16 18

Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 4 Spectrum 5

N

i w

t%

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4.2 Hardness results

The results from the hardness tests across the weld can be seen here of four samples, H6, Z1, Q1 and R1.

The graph for sample H6 (figure 20) is a typical graph of the hardness across the weld using ductile cast iron (in this work) but all the graphs from the other samples can be seen in appendix B. Note the spike in hardness beside the fusion zone (FZ) on the ductile cast iron side (the left side) where the hardness goes above 800.

The hardness graph for Z1 has a bit different result than the others since the hardness does not go down in the fusion zone (FZ) on the root side (compare with H6, see figure 20 and 21).

Figure 20: HV across the weld, top, center and root. To the left in the graph is ductile cast iron and to the right is the case hardening steel. The middle (where the hardness goes down) is the fusion zone.

0 100 200 300 400 500 600 700 800 900 0 1 2 3 4 5 6 7 H ar d n e ss H V1

Distance from first mark (mm)

Hardness across the weld, sample H6

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Figure 21: HV across the weld, top, center and root. To the left and to the right in the graph is a case hardening steel (42CrMoS4), welded with filler wire. Note that the root line (dotted) is different in the

middle (FZ) than the other two lines.

For Q1 and R1 the results are similar to each other, they are both welded without filler wire and with cast steel on the one side. The difference between the two is the steel on the right side of the weld; for Q1 it is the case hardening steel and for R1 it is a QT-steel (see figure 22 and 23).

Figure 22: HV across the weld, top, center and root. To the left in the graph is cast steel and to the right is the case hardening steel (17NiCrMoS6-4), no filler wire. Note also that the highest hardness

here does not exceed 500 HV. 0 100 200 300 400 500 600 700 800 0 1 2 3 4 5 6 7 H ar d n e ss H V1

Distance from first mark (mm)

Hardness across the weld, sample Z1

Top side Centre Root side 0 100 200 300 400 500 600 0 1 2 3 4 5 6 7 H ar d n e ss H V1

Distance from first mark (mm)

Hardness across the weld, sample Q1

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Figure 23: HV across the weld; top, center and root. To the left in the graph is cast steel and to the right is the QT-steel (25CrMoS4), no filler wire.

The results from the hardness tests along the fusion zone in H4 and G1 (see figure 24 and 25). Note that the hardness gets slightly higher/harder further down toward the root of the weld. Here is also the results from the extra hardness tests in the heat affected zone (HAZ) on sample G1 and H4 (see figure 26 and 27). Note that the hardness is lower further away from the fusion zone (FZ). The hardness is higher in the martensite closes to the weld than in the other phases.

0 100 200 300 400 500 600 700 0 1 2 3 4 5 6 7 H ar d n e ss H V1

Distance from first mark (mm)

Hardness in weld/fusion zone and heat

affected zone, sample R1

Top side Centre Root side 0 50 100 150 200 250 300 350 0 2 4 6 8 10 12 H ar d n e ss, H V0. 3

Distance from fist mark, mm

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Figure 24: The hardness in the fusion zone vertically down the cross section on sample H4. H4 was welded with filler wire and has 10 mm thick plates, ductile cast iron and case hardening steel. The

graph goes beyond 10 mm since the weld has a bump on the root side that makes it longer.

Figure 25: The hardness in the fusion zone vertically down the cross section on sample G1. G1 was welded with filler wire hmmm and has 8 mm thick plates, ductile cast iron and case hardening steel.

The graph goes beyond 8 mm since the weld has a bump on the root side. 0 50 100 150 200 250 300 350 400 0 2 4 6 8 10 H ar d n e ss, H V 0.3

Distance from first mark, mm

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Figure 26: The heat affected zone (HAZ) in sample G1 with hardness and phases marked, the white furthest to the right is the fusion zone (FZ). See figure 12 for reference on where this is on the sample G1. (This picture is a composition of several micrographs).

Figure 27: The heat affected zone (HAZ) and around it in sample H4 with hardness and phases

marked. The white furthest to the left is the fusion zone (FZ) and the black in the top is the plastic that the sample I imbedded in. (See figure 13 for reference on where this is in sample H4). (This picture is a composition of several micrographs).

The results from the hardness test in the root of the fusion zone (FZ) on sample Z1 can be seen in figure 28. Note that the hardness correlates to where the etching has affected the material the most (it is harder where the dendritic microstructure is more visible). For the hardness tests results in the top of Z1 see figure 18 in the previous part chapter.

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4.3 Tensile tests

The results from the tensile tests are the ultimate tensile strength (UTS) in the fusion zone (FZ) for the welds (see table 3). The ultimate tensile strength is also listed in table 3 for the base material (BM) to be compared. Note that the ultimate tensile strength is lower in the fusion zone (FZ) than the base metal (BM) for the steels.

Table 3: Result from the tensile test. Listed below is the ultimate tensile strength of the welded material and of the base metals. (Data of the base metals is from material standards and they differ for the steels depending on different heat treatments,for the ductile cast iron (here dci) the lowest value is used)

4.4 Heat input and carbon equivalent

See table 4 for the calculated heat input, plate thickness and HAZ length on the ductile cast iron side. See table 5 for the calculated carbon equivalents for the QT-steels and case hardening steel, using equation 2-5 with different denominations, CEV, CET, Ceq, Cew, for the carbon equivalent

depending on what equation is used. In figure 29 is a graph of the calculated heat input for every sample for easier comparison. The heat input is calculated using equation 1.

Table 4: Calculated heat input and measured maximum hardness in the heat affected zone (HAZ). Sample name Heat input (kJ/mm) Plate thickness (mm) Average HAZ length (μm)

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24 Table 5: Carbon equivalent for the steels

Steel type CEV CET Ceq Cew

17NiCrMoS6-4 case hardening steel 0,61 0,34 0,36 0,48

25CrMoS4 QT-steel 0,74 0,47 0,50 0,50

42CrMoS4 QT-steel 0,85 0,61 0,65 0,85

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5. Discussion

In this chapter the results are discussed as well as the ethical and environmental aspects. Here also the conclusions from this work.

5.1 Weld depth

The varying composition along the depth of the weld for sample G1 and H4 looks fairly the same, the iron content goes up and the nickel content goes down towards the root, but H4 has a longer part of the weld with low nickel content (see figure 16). So the thickness of the plate has a big effect on how the weld metal is diluted by the base material, the thicker it is the more the filler material is diluted.

5.2 HAZ length

In general the heat affected zone (HAZ) is broader for the samples with thicker plates. Sample H4 has narrower HAZ but the same calculated heat input as G1 (see figure 15) but since H4 is thicker it gets lower per material volume and also the thicker the material is the faster the heat is transferred away from the weld. Sample L1 has the same parameters as H4 (same heat input and same thickness (see table 1)) except for what material it is welded to, and has a much broader heat affected zone. H6 and I2 have a bit higher heat input and longer HAZ length.

So a reliable conclusion is hard to draw from only these results, but it would seem as though the heat affected zone (HAZ) gets broader if the heat input is higher, which is also suggested in other reports [14].

5.3 The hardness profiles and composition

As can be seen for sample Z1 the nickel makes a softer weld, the lower the nickel content the harder the weld (see figure 18 and 19). The hardness in Z1 top is harder in the middle where the dendritic structure is more prominent because of the difference in composition and segregation in the fusion zone. Note that the nickel content is lower in the spectrum taken where the dendritic microstructure is more prominent, spectrum 4 and 5, (see figure 19). So the nickel filler wire makes the fusion zone “softer” and that can also be seen in the results from the hardness test across the weld where the hardness goes down in the fusion zone (see figure 20 and 21). So the chosen filler wire material is a good filler material for welding.

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But in sample R1 the hardness spike up in the HAZ, which indicates that a hard structure has formed in the HAZ of the case hardening steel, but it does not go above 608 HV so it is not too much. That indicates that cast steel and QT-steel is the best materials to weld together out of these two combinations. But it is only the result from one sample from each of the two material combinations so it might not be a reliable result. The carbon equivalent is, over all, the lowest for the case

hardening steel (see table 5) and that indicates that the case hardening steel would be more suitable for welding.

Never the less it is clear that cast steel gives a softer and more ductile weld than with ductile cast iron.

5.4 Ethical and environmental aspects

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6. Conclusions

It is seen that the weld depth affects the dilution of the filler material and the base material. When designing the weld joint, it is important to minimize the weld depth both to achieve low dilution but also to minimize the use of filler wire. Components with laser welding of ductile cast iron should be constructed so that the load is mostly carried by the base material and not by the weld since the ultimate tensile strength is lowered in the weld.

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7. Recommendations for the future

The mechanical properties are (of course) affected by laser welding but one must take in to consideration that the alternative fastening neither keeps all the same properties as if the whole component was in one piece. But with laser welding the risk for cracks increases and testing the finished component might be a lot harder when it is laser welded. Therefore it would be best if a maximum hardness could be found that correlates to all the other mechanical properties (such as brittleness/martensite transformation, HAZ length and ultimate tensile strength) so that the Vickers hardness can be tested in FZ and HAZ on the finished component to see if it is a good weld and an approved component to use.

More testing with welding of a finished component would be needed to determine the weldability of ductile cast iron and to say if welding is a good option. That is since it is not possible to test the fatigue strength on the sample plates but that has to be done on the finished component.

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8. Acknowledgements

I would first like to say a big thank you to my supervisor at Scania AB Claes Morin, Master of science at CM Materials Consulting AB, for all his help with the experiments and this work.

I also want to thank Thomas Hammarlund, Test engineer at Scania AB, for his help with the hardness testing machines at Scania and Christian Öberg, phD student at Scania AB, for his help on sample preparation with cast irons in regard to how fast it starts to corrode.

I also want to say thank you to my supervisor at KTH Nils Andersson, researcher at the department of Material Science and Engineering at KTH, for his help and support on how to write a technical report. Also a thank you to Anders Tilliander, Associate Professor at the department of Material Science and Engineering at KTH, teacher and course coordinator for this course, for supporting me and all the other students doing their bachelor thesis within the material and process design area.

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9. References

[1] UN,2015, Sustainable Development Goals, knowledge platform, Available:

https://sustainabledevelopment.un.org/?menu=1300, (accessed 2019-05-08)

[2] Regeringskansliet, 2018, Övergripande mål och svenska mål inom Europa 2020, regeringen.se, Available: https://www.regeringen.se/sverige-i-eu/europa-2020-strategin/overgripande-mal-och-sveriges-nationella-mal (accessed 2019-05-08)

[3] Scania, 2016, Partnership solutions for a more sustainable future, Scania.com, Available:

https://www.scania.com/uk/en/home/partnership-solutions.html (accessed 2019-05-08) [4] A. Lundstjälk, 2017, Laser welding of tool steels, master thesis, Kungliga tekniska högskolan. [5] lock group, 2017, The experts: hoe to optimize a bolted joint through clamped length, Nord-lock international AB, Available: https://www.nord-lock.com/insights/bolting-tips/2017/the-experts-how-to-optimize-a-bolted-joint-through-clamped-length/ (accessed 2019-05-08)

[6] Gjuteri föreningen Swedish foundry association, Gjuteriteknisk handbok, kap 3.4 Segjärn,

Available: http://gjuterihandboken.se/handboken/3-gjutna-material/34-segjaern (accessed 2019-05-08)

[7] Reliance foundry, 2019, Cast Iron vs Cast Steel, Available: https://www.reliance-foundry.com/blog/cast-iron-vs-cast-steel/ (accessed 2019-05-09)

[8] Altair technologies, 2017, Laser welding & advantages, available: http://www.altairusa.com/laser-welding-advantages/ (accessed 2019-05-09)

[9] U. Dilthey et.al, 1995, laser welding with filler wire, Technischen Hochschule, Available:

https://link.springer.com/article/10.1007/BF00326474 (accessed 2019-05-09)

[10] Fotovvati, Behzad & Wayne, Steven & Lewis, Gladius & Asadi, Ebrahim. (2018). A Review on Melt-Pool Characteristics in Laser Welding of Metals. Advances in Materials Science and Engineering. 2018. 1-18. 10.1155/2018/4920718. Available:

https://www.researchgate.net/publication/324184351_A_Review_on_Melt-Pool_Characteristics_in_Laser_Welding_of_Metals (accessed 2019-05-09) [11] world auto steel, 2018, AHSS and laser welding, AHSS insights, Available:

https://www.ahssinsights.org/news/ahss-and-laser-welding/ (accessed 2019-05-09)

[12] Industrial metallurgists, LLC, Residual stress, available: https://www.imetllc.com/training-article/residual-stress/ (accessed 2019-05-09)

[13] WelderDestiny, 2016-2019, Welding without cold cracks, Available:

https://www.welderdestiny.com/cold-cracking.html (accessed: 2019-05-09)

[14] G.Turchin et.al., 2018, Influence of heat input and preheating on the cooling rate, microstructure and mechanical properties at the hybrid laser-arc welding of API 5L X80 steel, Available:

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[15] TWI ltd, What is the difference between heat input and arc energy ,Available https://www.twi- global.com/technical-knowledge/faqs/faq-what-is-the-difference-between-heat-input-and-arc-energy (accessed 2019-05-21)

[16] B. Capudean, 2003, Metallurgy Matters: Carbon content, steel classifications, and alloy steels, FMA, Available: https://www.thefabricator.com/article/metalsmaterials/carbon-content-steel-classifications-and-alloy-steels (accessed: 2019-05-09)

[17] TWI ltd, 2019, What is the difference between the various carbon equivalent formulae used in relation to hydrogen?, Available: https://www.twi-global.com/technical-knowledge/faqs/faq-what-is- the-difference-between-the-various-carbon-equivalent-formulae-used-in-relation-to-hydrogen-cracking (accessed 2019-05-09)

[18] Chegg study, 2003-2019, Question: Reproduced below are the Fe-C phase diagram and the isothermal…, expert Q&A, available: https://www.chegg.com/homework-help/questions-and- answers/reproduced-fe-c-phase-diagram-isothermal-transformation-diagram-hypoeutectoid-steel-contai-q9550801 (accessed: 2019-05-08) Picture reference.

[19] European Association for Ductile Iron Pipe Systems, 2014, Ductile iron pipe systems, Chapter 2: Ductile cast iron as a material 2, Available: https://eadips.org/wp-content/downloads/handbuch-en/chap-02-material.pdf (accessed 2019-05-09)

[20] ShapeCUT steel, 2015, What is quenched and tempered steel?, Available:

https://www.shapecut.com.au/blog/what-is-quenched-and-tempered-steel/ (accessed 2019-05-09) [21] IMOA International molybdenum association, 2008, Case-hardening steel, Available:

https://www.imoa.info/molybdenum-uses/molybdenum-grade-alloy-steels-irons/case-hardening-steel.php (accessed 2019-05-09)

[22] M. A. Yescas-Gonzalez and H. K. D. H. Bhadeshia, Cast iron, welding of cast irons, university of Cambridge, Available: http://www.phase-trans.msm.cam.ac.uk/2001/adi/cast.iron.html (accessed 2019-05-09)

[23] NTS, Analysis via Scanning Electron Microscopy / Energy Dispersive X-Ray Spectroscopy

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Appendix

Appendix A: Welding data

Welding data that are the same for every sample: fiber diameter 300 μm, Collimation length 200 μm, focal length 200 μm, focus diameter 300 μm, focal position -3 μm, typre of gas (crossjet) Druckluft, gas flow crossjet 35 l/mm, type of nozzle Silvent 921, Wire diameter 1.0 mm. The surfaces were cleaned with alcohol before welding.

Appendix B: hardness profiles for the samples

Figure B1: Hardness profile for sample G1, ductile cast iron to the left and case hardening steel to the right. 0 100 200 300 400 500 600 700 800 900 0 1 2 3 4 5 6 7 H ar d n e ss H V0. 3 (e lle r H V1? ?) Distance (mm)

Hardness in weld/fusion zone and heat

affected zone, sample G1

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Figure B2: Hardness profile for sample G5, ductile cast iron to the left and case hardening steel to the right.

Figure B3: Hardness profile for sample H4, ductile cast iron to the left and case hardening steel to the right. 0 100 200 300 400 500 600 700 800 900 0 1 2 3 4 5 6 7 H ar d n e ss H V0. 3 8ell e r H V1? ) Distance (mm)

Hardness in weld/ fusion zone and heat

affected zone, sample G5

Top side Centre Root side 0 100 200 300 400 500 600 700 800 900 0 1 2 3 4 5 6 7 H ar d n e ss H V1 Distance (mm)

Hardness in weld/fusion zone and heat

affected zone, sample H4

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Figure B4: Hardness profile for sample H6, ductile cast iron to the left and case hardening steel to the right.

Figure B5: Hardness profile for sample I2, ductile cast iron to the left and case hardening steel to the right. 0 100 200 300 400 500 600 700 800 900 0 1 2 3 4 5 6 7 H ar d n ess H V1

Distance (mm) (from first mark)

Hardness in weld/fusion zone and heat

affected zone, sample H6

Top side Centre Root side 0 100 200 300 400 500 600 700 800 900 0 1 2 3 4 5 6 7 H ar d n e ss H V1 Distance (mm)

Hardness in weld/fusion zone and heat

affected zone, sample I2

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Figure B6: Hardness profile for sample J1, ductile cast iron to the left and case hardening steel to the right.

Figure B7: Hardness profile for sample K1, ductile cast iron to the left and QT-steel to the right. 0 100 200 300 400 500 600 700 800 0 1 2 3 4 5 6 7 H ar d n e ss H V1 Distance (mm)

Hardness in weld/fusion zone and heat

affected zone, sample J1

Top side Centre Root side 0 100 200 300 400 500 600 700 800 900 0 1 2 3 4 5 6 7 H ar d n e ss H V1 Distance (mm)

Hardness in weld/ fusion zone and heat

affected zone, sample K1

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Material Max HV in HAZ HV in BM

Ductile cast iron 846 210

Case hardening steel 549 165

QT-steel 25CrMoS4 608 215

QT-steel 42CrMoS4 688 297

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TRITA ITM-EX 2019:518

References

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