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MASTER'S THESIS

Laser Welding of Boron and Bainitic

Steels

Waael Alqhadafi

Master of Science (120 credits) Materials Engineering

Luleå University of Technology

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Laser Welding of Boron and Bainitic Steels Page 1

Laser Welding of Boron and Bainitic Steels

Master Thesis

Waael Alqhadafi

Division of Materials Science

Luleå University of Technology

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Laser Welding of Boron and Bainitic Steels Page 2

Abstract

This work was concerned with the development of welding procedures for joining of thin ultra high strength sheets, using an ytterbium fiber laser with a power of 4KW. Sheet

thicknesses of 4 and 5 mm were tested for boron steels and the bainitic steels had a thickness of 5 mm. The sheet materials have a tensile strength of 1500 MPa and an average hardness of

480HV0.5 . The softening and hardening behavior respectively and hot and cold cracks were determined. Influence of quenching and partitioning heat-treatment with induction-heater in Laser welding with constant parameters was discussed. The steel in this work had carbon contents of 0.22%, 0.24% and 0.3% and Boron content of 0.0032% for increase of strength and hardenability of the materials. The fusion zone shows hardness values up to 587HV0.5 with martensitic microstructures if it is welded without any pre-or post-weld heat treatments. Different post weld heat treatment will decrease the hardness values. The results show that the microstructure of HAZ and weld metal consists of low carbon martensite and bainite. Although the grain size of the heat affected zone and weld metal has increased but the steels still have excellent strength.

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Aknowledgement

First of all I would like to thank my supervisor Esa Vuorinen who had the idea for the project and was support me throughout the whole semester.

I also want to thank Greger Wiklund and Tore Sliver who is in charge of the laser experiments at the University and I am grateful to Johnny Grahn for his support and patient in my work too. I would further like to thank the organisation and administration of the AMASE Program. I thank students in materials department in LTU University for their advice and support. The support from Accra Linde-Wieman and the CHS (Center for High Performance Steels) at Lulea University of Technology is highly appreciated.

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Table of contents

Page

Abstract...2

Aknowledgement…..………...3

List of Tables………...6

List of Figures………..7

Chapter 1-Background

1.1 Introduction ………...9 1.2. Laser processing ……….10 1.3. Laser welding………..10 1.4. Laser mode………..11

1.5. Gas laser CO2...………..12

1.6. Fiber laser ...………12

1.7. Welding defects ……….12

1.8. Marangoni effect………..………...13

1.9. Effect of alloying elements…..………...14

1.10. The quenching and partitioning process………...16

1.11. Scanninig electron microscope……….16

Chapter 2- Materials

2.1. Boron steel sheets... ……..……….17

2.2. Bainitic steel sheets……….……….………..19

2.3. Hardenability………...20

Chapter 3- Experimental procedure

3.1.Sample preparations……….21

3.2. Microhardness Testing………21

3.3. Microstructure Observation………23

3.4.Bainitic steels Manufacturing process……….25

3.5. Lab Experiments ………26

3.5.1. Laser welding………...…………...….26

3.6. Three point bending Test...……….28

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3.7. Tensile Testing………....34

Chapter 4-Results and discussions

4.1. The first group of welding tests………..………...………….31

4.2. Microstructures of the 1st group of welding tests……….……..………...32

4.3. The 2:nd group of welding tests………..34

4.4. Microstructures of the 2:nd group of welding tests……….……….…..35

4.5. The 3:rd group of welding tests………..………37

4.6. Hardness measurement of the 3:rd group of welded samples………...……….……40

4.7. Three point bending test……….42

4.8. Tensile test results………...46

4.9. Fracture surfaces..………...49

Chapter 5-Conclusions..………...50

Chapter 6-Future work………...……….51

References………..52

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List of Tables

Page

Table 1: Laser Parameters used in lab tests at LTU...26

Table 2: The 1st group of welding tests...31

Table 3: The 2nd group of welding tests...34

Table 4: Welding parameters used in the 3rd group of welding tests...37

Table 5: The 3:rd group of welding tests...37

Table 6: The 3:rd group of welding tests...38

Table 7: Bending strength 3:rd cycles group bainitic steel...44

Table 8: Bending strength 3:rd cycles group hardened boron steel …...46

Table 9: Tensile test and elongation results 3:rd cycles group bainitic steels …...47

Table 10: Tensile test and elongation results 3:rd cycles group hardened boron steels …………..…..48

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List of figures

Page

Figure 1Polarisation effects……….…….10

Figure 2 Conduction mode and keyhole mode laser welding…...12

Figure 3 Flow pattern in weld pool...………..………...13

Figure 4 Schematic illustration of Q&P microstructures and temperatures………….………....16

Figure 5 The effect of carbon on Ms and Mf temperatures..………...17

Figure 6 CCT and TTT diagrams of boron steel...…..………..18

Figure 7Microhardness values for bainitic microstructures …...20

Figure 8 Microhardness tester Matsuzawa MXT‐CX………..…….22

Figure 9 Microhardness values of boron Steel 22MnB5 hardened and unhardened..……..…....22

Figure 10 Microhardness values of hot rolled and hardened 22MnB5 steel……...……….23

Figure 11 Optical microscope and SEM microscope………...23

Figure 12Optical microstructures of not unhardened boron steel………...……….24

Figure 13 SEM microstructures of boron steel………….………...25

Figure 14 Optical and SEM bainitic microstructures ...……….………….26

Figure 15 Laser lab equipment………..27

Figure 16 Bending machine at the company and bended samples……....………...29

Figure 17Bending machine at the University……….…..29

Figure 18 Tensile test machine……….30

Figure 19 Experimental quenching and partitioning heat treatment 1:st group cycles…..…….32

Figure 20 Microstructures for 1:st group cycles………...………...…...…..……....33

Figure 21 SEM 1:st group cycles…..………...……….34

Figure 22 Experimental quenching and partitioning heat-treatment 2nd group cycles …….…...35

Figure 23 Microstructures of 2:nd group cycles………..…………..…..….36

Figure 24SEM 2:nd group cycles………..………...36

Figure 25 Experimental time-temperature curves of 3:rd group cycles……….……...38

Figure 26 Experimental time-temperature curves of 3:rd group cycles ………...…....39

Figure 27 Microstructures for 3:rd group cycles………..………....39

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Figure 29 Hardness profile across welds………..41

Figure 30 Hardness of the 3:rd group welding tests on bainitic steel……...41

Figure 31 Hardness values for 1:st group of welded samples………...…..………42

Figure 32 Bending test results for the 1:st group of welded samples…………...…..…..…....43

Figure 33 SEM of Fracture surfaces after bending test...………..………….…………..43

Figure 34 Bending strength for bainitic steels 5mm thick…...……….……45

Figure 35 Tensile test results for bainitic steels...……….47

Figure 36 Tensile test results for hardened boron steels………...………48

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1.1 Introduction

This work is performed in cooperation with Linde-Wieman ACCRA in Öjebyn. Their main products are high strength continually shaped and hardened profiles. The production includes also welding of the shaped profiles before the handling operation. Strength requirements for structural steel components has been increased for vehicles because of the goal to reduce the weight of cars while preserving their safety characteristics. Boron steels alloyed with manganese

gives strength values greater than 1500N/mm2 together with high hardness and good formability.

Boron as an alloying element improves the hardenability by heterogeneous precipitation of boron carbides at the grain boundaries of the austenite by suppressing the austenite to ferrite

transformation. The effects of different important alloying elements are described in detail later.

Thus is the goal of this work to investigate the cause of the crack formation after the CO2 laser

welding and also to test a novel welding methodology with the ytterbium fiber Laser welding at LTU, on existing boron steel plates and on a new bainitic steel quality. The specific production process at Accra consists of cold shaping –laser welding –austenitisation-quenching. The weld is allowed to cool to room temperature before austenitisation and the quenching is done with water.

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1.2 Laser Processing

Laser beam is a light source and heat source in material processing; it has useful intensity heat source when focused, because it is monochromatic, coherent, and has low divergence properties compared with normal light source. One characteristic its high energy beam density in welding with absence of filler wire. The high welding speeds gives high solidification and cooling rates and result in a narrow molten zone and heat affected zone. Due to low heat input will the deformation of parts be limited and at the same time is there no direct contact between the work piece and the energy source.

1.3 Laser welding

Welding is one means for joining parts together. The zone affected by the welding process is characterised by the fusion zone where a portion of the joint melts and a non fusion zone called heat affected zone. Parameters affecting the laser welding process are:

1. Beam power and speed. Microstructure is affected by the heat and with an increase of the speed will the time for penetration decrease and the depth of penetration will be decrease and it affects the keyhole form.

2. Beam characteristics (mode, stability, polarization, form pulsed or CW )

The wave length of the beam affects the absorption from the materials. A short wave length light is absorbed more efficiently by the material and at the same time the size of the focused spot is reduced which increases the power density. There is a decrease in plasma formation with short wave lengths. The bead penetration increases with increase of the beam power for a given welding speed. For penetration mode or keyhole mode is the power density required 104W/mm2 [1]. The beam mode has big effect on the weld bead shape and beam stability which influences the weld quality. The depth of penetration is found to be depending on the direction of the beam polarization, being better when the same direction with the welding speed. The parallel polarization has a more narrow welding bead than the vertical as shown in figure 1 with an increase of the heat distribution at the keyhole walls, will the bead width increase.

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3. Shielding gas is used in laser welding in order to avoid plasma formation and enable the beam to reach the materials surface, to provide protective environment for the weld pool and protect the focusing lens. The orientation of the shielding gas flow in high power diode laser affects the weld bead dimension, microstructure and the penetration depth. For example when the shielding gas flow used is in coaxial direction, it causes destabilization in the weld pool and the plasma formation is not completely stopped which gives low penetration depth and hard microstructures such as martensite. Transversely directed shielding gas flow leads to the formation of microstructures less sensible for crack formation and with higher welding quality. [3]

Argon gas has high density, is often used in low and medium power laser welding applications and it is cheaper than helium and gives better shielding. The weld surface will be smoother than with helium shielding, because its density is lower and requires higher flow rates for good shielding.

The angel of inclination of plasma gas nozzle with the horizontal does not show great different effect on the process in the range of 30°-60°. Forward or backward welding at 10 or -10 degree along the welding direction was performed to reduce underfilling, spattering and number of pores. It shows very high penetration in high power fiber laser with different laser beams and welding speeds. [4]

4. Location of focal point relative to the sample surface determines the beam size and it affects the depth of penetration and the bead width, which optimum position result in maximum penetration depth and minimum bead with.

5. Beam power and density at the focal point , which intensity is defined as I=

Where q is the power of laser in watt, is the beam radius in mm

6. Wavelength and focusing system influence the welding process in ways such as absorption

of the beam and the extend of focusing, where focused radius defined as = ( and 1

is lens of focal length, wavelength, and beam radius.

From that equation the greater the original beam diameter, the smaller the focused beam radius and increase the power density of the focused beam. If the focal length is shorter, the smaller the size the focused beam, and thus the greater the power density. The use of shorter focal length gives deeper penetration for the same beam power.

1.4 Laser mode

There are two laser welding modes as shown in fig 2, one of them conduction welding which

occur at lower power density than 106 w/cm2 with low metal vaporization. The resulting weld is

shallower and it has wide heat affected zone. The other mode is penetration welding in which deep penetration occurs. The vapor cavity has diameter close to the laser beam that formed by internal reflection in the deep hole. The energy is absorbed by inverse bremsstrahlung in ionized plasma, above the keyhole and by Fresnel absorption with reflections on the walls of the

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and forms a cavity hole, which is surrounded by molten metal. The molten metal around the keyhole fills the cavity as the laser beam is moved along the weld. It is recognized that conduction mode has lower efficiency than keyhole welding in which the energy is reflected away. This mode show welding results in depth to width of about 3:1 compared to about 10:1 or higher in keyhole.

A) B)

Fig. 2 A) schematic of conduction mode welding and B) keyhole mode welding, ref [7]

1.5 Gas Laser CO2

CO2 lasers with power between 5-10 kw are often been used in different applications because they are simple, reliable and available with output powers of up to 50KW. The gas in CO2 lasers contains Nitrogen and Helium gas in addition to CO2 [5]. The CO2 molecule provides the transition that generates the laser beam. He has high thermal conductivity which conducts heat away to the walls and keeps the CO2 gas at low temperature [2]. Applications of the CO2 lasers are found in materials processing, spectroscopy and surgery.

1.6 Fiber laser

A fiber laser was used in the laser lab at Lulea University for the welding experiments; A fiber laser has an optical fiber with core doped with active elements such as erbium(Er), ytterbium (Yb) and neodymium (Nd). They have output powers from 100 watts to 50 Kilo Watts. The fiber core can be single mode or multimode, they consist of glassy fibers of diameter about 125µm and length up to kilometers, and they operate in the temperature -250°C to 500°C. It has good high beam quality which results by using single-mode fiber in low output power but multimode result in high power and low quality, ability for generation ultrashort pulse laser beams and high output efficiency of about 50% compared to about 10-30% for CO2 and 2% for Nd:YAG lasers. [4] This research shows the fiber laser welding with different laser beam spot diameters 130,200,360 and 560 µm with full penetration. The laser power density increase in weld penetration with higher welding speed without defects such as porosity and humping.

1.7 Welding defects

Hot cracks appear usually in the fusion zone during or at the end of solidifications in the

temperature range between 200-300 C° from the melting temperatures, due to forming low melting temperatures constituents which affect the grain boundaries such as boron,

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Laser Welding of Boron and Bainitic Steels Page 13

sulfur and phosphorus. This segregation causes weakness along the grain boundaries and fracture occurs in the material when stress is greater than fracture stress. Hot cracks are exposed to the surface and identified by oxide coating which discolors the crack surfaces.

Cold cracks appear due to high cooling rates which result in brittleness in fusion zone,

heat-affected zone and martensite formation. Dissolved hydrogen from water present in ambient air and in the welding gas affects crack formation in steels. It occurs when the materials is cooled to low temperatures. It is usually transgranular but sometimes intergranular. Cold cracks are induced due to martensite formation, residual stresses or embrittlement by hydrogen. [6]

1.8 Marangoni effect

Fluid flow in the welding molten pool influence the properties of the fused zone and the structure such as surface smoothness (ripples), the geometry (penetration and bead width), lack of fusion, solidification structure, porosity and macrosegregation. So the flow factors that affect the process quality are the flow velocities and mode of solidification for the process in the melting. These forces are surface tension and the buoyancy forces. The surface tension in the molten pool induces thermocapillary flow on the pool surface which induces a surface tension gradient on the surface as a function of with temperature. This result in flow along the surface from low surface tension region to higher surface tension region is called Marangoni convection. The bead shape depends on surface tension gradient which is determined by surface active elements such as oxygen and sulfur in the base materials. These two forms of surface tension gradients are shown in fig 3. Pure metal or low surface active elements result in a negative surface tension gradient which result in the induces outward flow in the molten pool. As the flow reaches the edge of the molten pool, it is moved downward to the pool center in recirculation pattern and results in wider and shallower pool. In positive surface tension gradient, the surface tension forces will increase with temperature which is cold at the edges and hot in the center of the molten pool. This high surface tension in the center pulls the material from external surface toward the center which carry the hot material to the bottom of the pool and result in deeper penetration. [2]

Fig.3 Flow pattern for (a) a negative surface tension gradient in the melt pool (b) a positive

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1.9 Effect of alloying elements

Most of the alloying elements which in solid solution in austenite decreases the martensite start Ms and finish Mf temperatures. Carbon and nitrogen have larger effect than the metallic substitutional Solutes. One wt% carbon lowers Mf by over 300°C. The effect of alloying elements on austenite transformation depends on the free energy of the phase. It increases the hardenability by delaying transformations, shift CCT curve towards longer time and it lowers the Ms and Mf temperatures, which depends on austenitizing temperature too. The carbon has higher solubility in austenite than in ferrite and stabilizes austenite which leads to retardation of reactions. The amount of carbides increases with the carbon concentration, thus in order to get good mechanical properties must the carbon concentration be kept below 0.4%. Alloying elements in addition to C and N that pushes the C-curve to the right for longer time and lowers the martensite start and finish temperatures (Ms) and (Mf) are silicon, boron, vanadium, manganese, copper, chrom, calcium, aluminum, titanium and sulfur. [7]

Silicon (Si) If Si content is below 0.3%, it is dissolved in ferrite which improves strength

without decreasing ductility, wear resistance, elasticity or hardness. If the Si amount is larger than 1.5% is the carbide formation retarded. It is also used as deoxidizer in steel making and less effective than Mn in strength and Hardness as rolled [8]

Boron

Boron retards the nucleation of ferrite and pearlite at the austenite grain boundaries, Small amount (0,001-0,003%) increases the hardenability equivalent to about 0.5% of elements such as Mn, Cr and Mo and pushes the C curve to the right for long time, which causes heterogeneous precipitation of boron carbide at the grain boundaries. Boron is limited to steels containing less than 0.4% carbon contents. Boron increases the susceptibility to solidification cracking of carbon steels. It promotes a strong hardening and a quite good wear and abrasion resistance after quenching. [8] [9] [10]

Vanadium

Vanadium forms carbides and nitrides and gives a grain refinement in high manganese steels up to 0.04% with vanadium. Vanadium is excellent for steels with high strength and toughness such as axles and connecting rods. Due to air cooling followed by tempering treatment can precipitation hardening giving fine carbides be performed. [11]

Manganese

Manganese is present in all steels in amount of 0.3% or more. It is a deoxidizer, desulfurizer and gives less macrosegregations. It gives good surface quality, improves the strength and hardness in solid solution. Increasing carbon content with decrease of manganese content

decreases the ductility and weldability. Large amount than 2% increased tendency toward

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Cobalt

Cobalt inhibits grain growth at high temperatures and improves high temperature strength

Copper

Copper increases hardenability of low carbon steels and improves also the corrosion resistance. Small amount 0.25% of copper is beneficial in avoiding hydrogen induced cracking. More than 0.3% of copper gives the possibility for creating precipitates.

Chromium

Chromium increases the strength of steels without a lot of changes in strain and hardness values. The necessary cooling velocity for quenching is decreased. It is very good for the corrosion resistance and Cr-contents over 12% and improves high temperature oxidation resistance. [8]

Calcium

It is useful addition to control the shape of inclusions and to improve machinability and surface quality. [11]

Aluminum and titanium

Titanium promotes the formation of acicular ferrite, through the formation of TiN or TiO2 nuclei. Under the grain coarsening temperature, grain boundary movement is impeded by the presence of certain particles such as aluminum nitride, this particle above 1200°C goes into solution and grain growth is unimpeded. So Al and Ti are used for impeding grain growth during thermal cycles. [8][9]

Sulfur

High amount of S has a detrimental effect on ductility, impact toughness, weldability and surface quality, but less effect on longitudinal tensile properties. It causes reduction in hot working properties due to low melting sulfide surrounding the grains. More than 0.05% of S results in a structure susceptible to quench cracking. Sulfur is present in form of inclusions because it has a greater segregation tendency than other elements. It causes coarse grains for better machinability which increases brittleness and promotes quench cracking. Sulfur improves fatigue life of bearing steels because MnS inclusions produce compressive stresses in the surrounding matrix caused by high thermal coefficient of expansion. [8][9][12]

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1.10.The quenching and partitioning process

A new heat-treatment process has been used for creating retained austenite in steel microstructures as shown in fig 4. It consists of quenching from austenite range to a temperature below the martensite start temperature, followed by a partitioning treatment in which carbon atoms diffuses to the retained austenite, which becomes stable at room temperature. A new metastable equilibrium condition has been proposed that the end point of the partition of carbon between retained austenite and martensite. It has been called constrained paraequilibrium (CP) in which only carbon atoms will be partitioned over a wide range. As a contrast to paraequilibrium the short range diffusion of substitutional elements and iron is hindered and leads to fixed grain boundaries.[13]

Fig. 4 Schematic illustration of Q&P microstructures and temperatures Ref [13]

The fraction of austenite that transforms to martensite at the quench temperature can be calculated based on the undercooling below martensite start temperature ( Ms) with Koistinen-Marburger relationship:

fm=1- ………..(1)

where fm is the fraction of austenite that transforms to martensite , Ms is the martensite start temperature and Qt is the Quenching temperature.

1.11. Scanning electron microscope

Scanning electron microscope is used to image the sample surfaces by scanning it with high energy (0.5-40 KeV) thin electron beam (diameter 0.01μm) in vacuum. The specimen is scanned line by line, the interaction between material surface and primary electrons will remove secondary electrons. An electron detector collects the electrons and the signal is amplified. It shows information about the local distribution of secondary electrons on screen played and shows a bright area if it is hit by a lot of secondary electrons and they diffracted dark if deep from the surface. SEM can produce very high resolution images of sample surfaces and can be down to about 5nm, a magnification about 250 times better than the best light microscopes. The depth of field is 100nm-5μm into the surface, and this interaction depends on electron energy, the atomic number of the specimen and material density. SEM is also used for investigation of fracture surfaces, other damages and failure analysis. [14]

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2. Materials

2.1.Boron steel sheets

Boron steel sheets with a thickness of 2.5 mm that were tested contained 0.22 % C, 0.29 Si and 1.19% Mn. For simplifications, the base metal and weld metal are supposed to have the same physical properties. Martensite start and finish transformation temperatures for this metal are usually lowered by the concentration of solutes such as carbon, nickel and manganese, but increases with the addition of cobolt and aluminium. An empirical relationship was used for the determination of Ms.

The relationship in HKDH Bhadeshia [7] is:

Ms(C%)=539-423(%C)-30.4(%Mn)-17.7(%Ni)-12.1(%Cr)-7.5(%Mo)……….(2) from calculations Ms is 407°C.

But from Ms and Mf diagram without alloyingelements effect as shown in fig 5, it is found that Ms and Mf at C%=0.22 has 445°C and 279°C respectivity. The difference between alloyed and unalloyed is 38°C.

Fig 5 The effect of carbon content in steel on Ms and Mf temperatures [7]

Mf was defined as the temperature below which no more martensite forms, it is taken as 215°C below the Ms temperature [8].

Different Ms and Mf values can be found in different papers for 22MnB5 which depends on different alloying elements and amounts. For example in this one work about quenching and partitioning was Ms higher than 320°C and Mf lower than 280°C [11]. In another work was Ms defined at 410°C and Mf at 310°C [16], and in hot press forming Ms at 438°C and Mf at 256°C.

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The continuous cooling transformation (CCT) diagram and time temperature transformation (TTT) for 22MnB5 Boron steel has been calculated with software by EWI as shown in fig 6 [17].

Fig. 6 CCT and TTT diagrams of boron steel. [17]

Bainite formation depends on the carbon content which affects the temperature for forming lower and upper bainite. Following formula gives an approximation of the Bs.

Bs(°C)=830-270C-90Mn-37Ni-70Cr-83Mo………(3) [1] From calculations bainite starts to form at 648°C.

Boron steel with 0.24% C, 0.28% Si and 1.18% Mn with thickness of 4 mm was also used the

experiments. The samples dimension were 18.3 cm long and 12 cm wide.

So the alloying elements affects the transformations temperatures and will change the martensite start temperatures, martensite finish temperatures and bainite start transformations temperatures too.

From HKDH Bhadeshia [7] it will be calculated Ms and Bs

Ms(C%)=539-423(%C)-30.4(%Mn)-17.7(%Ni)-12.1(%Cr)-7.5(%Mo)….……(4) gives Ms at 395.6°C

Bs(C°)=830-270C-90Mn-37Ni-70Cr-83Mo ………..(5) [7] and bainite start formation at 640°C

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Factors that influences the microstructure of weld metals are:

 Cooling rate

 Composition

 The presence of non metallic nuclei

 Plastic strain

From cooling rate diffusion is limited and the grain size depends on heat input rate and composition which fine grains due to low heat input.

Boron steel with athickness of 5 mm was also tested and has the same composiotion as the 2.5 mm thick sheets and has the same physical properties such as Ms 395.6°C and Bainitic start at 640°C. The sheets were cutted to the dimension 16.3 [cm] long and 14 [cm] wide.

2.2- Bainitic steel sheets

The bainitic steel used in this work with a composition of 0.3% C, 0.5 Si, Al 1% and 1.9% Mn has been hot formed, air cooled and coiled. The sheets of this materials in the project were 180 [cm] long and 9-9.5 [cm] in wide. Samples were cutted in the workshop into pieces 9 [cm] in wide and 15 [cm] long for the welding tests.

After cutting were the samples heated and austenitised at 900°C for 30 min and then austempered by quenching in saltbath to get full bainitic transformation with moderate hardness, good ductility and toughness. It was found from measurement of the temperature with help of thermo-couple that the time needed to get these temperatures was about 120 seconds. Small test pieces were held at the austempering temperature for 5,15,30 and 60 minutes, one piece was quenched in water to get martensitic microstructure as reference. The heat-treatment was performed at Ms+20°C 320°C for 30 minutes close to the Martensite start temperature to get lower bainite microstructures which characterized with high ductility, good toughness and strength.

The microhardness of these test samples was measured with Matsuzawa microhardness measuring machine as shown in fig 8. The measurement show us that the martensitic microstructure and the hot rolled air cooled microstructure have the highest hardness values greater than 560HV0.5. The samples austempered for 5 and 15 minutes shows lower hardness of 512HV0.5 but for 30 and 60 minutes austempering times give maximum hardness of 470HV0.5. According to these results it was decided to run the heat-treatment to produce a bainitic microstrucure at 320°C for 30 minutes in order to prepare the samples for the laser welding for comparison with hardened Boron steels.

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Fig. 7 Microhardness values for bainitic microstructures austempered for 5,15,30,60 min and also for a martensitic microstructure after water quenching

2.3.Hardenability

Due to carbon content, alloying elements and high cooling rates can the material transform to martensite in the fusion zone. The ease with which the steel forms martensite is named hardenability. It does not show the hardness or strength of the steel. For example plain carbon steels have low hardenability and only at high cooling rates is martensite formed. In high carbon and alloy steels will martensite be formed more easily, One way to estimate the hardenability of 22MnB5 is to determine its carbon equivalent (CE) a relationship which depends on alloying elements and carbon content [2,6].

………..…..(6)

If CE 0.35 preheating or postweld heattreatment is not necessary If 0.55 preheating is required

If CE 0.55 pre-and post-heat treatments are important

From CE formula can found that CE=0.4709 for 2.5 mm 22MnB5 and CE=0.493 for 4mm and 5mm . Thus, they must be preheated before laser welding.

350 450 550 650 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Har

d

n

ess

HV

0

.5

Hot rolled +austenitizing at 900°C for 30 Min+ austempering at 320 °C for 5,15,30,60 Min and Water quenching

Austempering at 320C° for 5 Min Austempering at 320C° for 15 Min Austempering at 320C° for 60 Min Quenching in water Austempering at 320C° for 30 Min Distance on the samples surface in mm

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3. Experimental procedure

3.1. Sample preparations

The first welding tests were performed at ACCRA company. Steel sheets with thicknesses 1.3, 2.5 and 4 mm of boron steel 22MnB5 were welded and the microstructures were investigated on hardened as well as unhardened conditions as shown in appendix 1.

The sample preparation was performed according to:-

 Sample cutting

 Mounting

 Grinding to remove damage on the surface, by using SiC papers with 60-240-600-1200

mesh with rotation specimen 90 degree.

 It was used polishing wheels with rotation speed 250 rpm with rotate sample in circular

pattern.

 Final polishing.

 Etching with Nital for 5 seconds to reveal the grain boundaries

After laser welding the samples were cut perpendicular to the weld direction, mounted and metallographically prepared and etched with 2% Nital solution for 5 seconds for boron steels and 13 seconds for bainitic steels in order to investigate the microconstituents.

3.2. Microhardness Testing

Hardness test measures the resistance of the materials surface to penetration by a hard object. The depth of penetration of the indenter formed by diamond pyramid with different loads from 10g to 1kg are calculated and converted by the software connected to the Vickers hardness measuring device. Vickers hardness testing was conducted on etched samples with a load of 500g with dwell time of 15 s by means of Matsuzawa MXT-CX tester shown in fig 8. Fig 9 A and B shows the microhardness for boron steel sheets, 22MnB5, with thickness 2.5 mm which is unhardened and for hardened sheet with 1.3mm thickness. The microhardness is according to the different microstructures in the fusion zone. In the fusion zone were martensitic fine lathes formed with hardness from 520- 580 HV0.5. The heat afected zone showed hardness values between from 220 and 400HV0.5 and the base metal from 198 to 220HV0.5 unhardened conditions. In laser welding by the air cooling transform the molten pool very fast from liquid to solid martensitic microstructure in the fusion zone. In the heat affected zone the temperature gradient forms causes grain growth in a narrow zone to low heat input. The material from the company was analyse for different thicknesses, 1.3 mm ,2.5 mm ,3.5 mm,4 mm and 5 mm in hardened and unhardened. The hardened material in fig 9 C and D for 1.3 mm has the highest hardness with max 517HV0.5 and in fig 10A for 3.5 and 5 mm thick plates the hardness was max 470HV0.5. Hot rolled materials, 5 mm thick used to form bainitic microstucture has max hardness of 560HV0.5 as shown in fig 10 B and Boron steel 4 mm thick used for laser welding cycles without heat-treatment has max hardness of 520HV0.5.

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Laser Welding of Boron and Bainitic Steels Page 22

Fig.8 Microhardness tester Matsuzawa MXT‐CX

0 100 200 300 400 500 600 0 0.5 1 1.5 2 2.5 Ha rd ne ss V H (5 00 g)

Distance From weld Centreline (mm)

Hardness Hardness distribution of cross section weld metal for Boron steels

0 100 200 300 400 500 600 0.5 2 3.5 5 6.5 8 9.5 11 12.5 14 15.5 17 18.5 20 21.5 Ha rd ne ss H V 5 00 g

Hardness Harden Boron steel flat surface with thickness 1.3mm

Distance from Edge (mm)

0 100 200 300 400 500 600 700 0 0.5 1 1.5 2 Vi ck er s h ar dn es s H V (5 00 g)

Distance from weld center (mm)

Series1 Hardness distribution of flat surface weld metal for Boron steels

22MnB5 0 100 200 300 400 500 600 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Hardness Boron … Hardness Harden Boron steel crosssection 1.3 mm

Ha rd ne ss H V 50 0g

Distance from Edge (mm) A

D C

B

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Laser Welding of Boron and Bainitic Steels Page 23 300 320 340 360 380 400 420 440 460 480 500 1 3 5 7 9 11 13 15 17 Ha rd ne ss H V 50 0g Distance in mm

Hardened Boron steel hardness

Harden Boron steel Flat surface 3.5 mm Harden Boron steel cross section 3.5 mm Harden Boron steel cross section 5 mm A 500 510 520 530 540 550 560 570 1 3 5 7 9 11 13 15 17 Ha rd ne ss H V5 00 g Distance in mm

Hot rolled steel 5mm thick

Hot rolled steel 5mm thick

B

A B

Fig. 10 Microhardness values of boron steel 22MnB5 A) hardened cross section and flat surface for 3.5 mm and 5 mm and B) hot rolled sheet of bainitic steel

3.3. Microstructure observations

The obtained weldments were cut into specimens via electro-discharge machining for subsequent handling. The specimens were ground from 60 to 1200 mesh, and polished with diamonds of 9 µm to 0.25µm size , mounted as metallografic samples using standard metallografic procedure, and then etched with 2% Nital for 5 seconds. Microstructure was analysed by means of optical microscope Olympus Vanox-T and scanning electron microscope (SEM) JSM 6460 LV SEM as shown in fig 11.

A

B

A B

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Laser Welding of Boron and Bainitic Steels Page 24

The initial microstructure by optical microscopy of the boron steel 22MnB5 exhibited in the FZ martensitic microstructure with fine laths as observed with the optical microscope fig 12.

A 10µm 10µm 10µm 10µm 10µm 10µm 10µm B C

D

E

F G

Fig. 12 Optical microstructures in laser welding with CO2 for unhardened 22MnB5 A) Fusion zone cross section , B)HAZ in cross section, C) Base metal in cross section, D) Fusion zone in flat surface, E)

HAZ in flat surface, F) Base metal in flat surface, G)Hardened boron steel for cross section

As shown in fig 13 A,B,D and E has hardened boron steel homogeneous in the microstructure with small laths due to formation of a fine martensitic microstructure from the austenitizing and quenching process in fusion zone, heat-affected zone and base metal. Figure 13 C show pearlitic microstructure in base metal that is not affected by temperature. Figure 13 F shows martensitic

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Laser Welding of Boron and Bainitic Steels Page 25

microstructure in heat-affected zone and pearlite blocks, bainite and martensite in the heat affected zone of G

Hardened boron steel cross section base

metal

Hardened boron steelcross section fusion zone

Hardened boron steel flat surface base metal

Hardened boron steel flat surface fusion zone

unhardened boron steel cross section base metal

unhardened boron steel cross section fusion zone (martensitc microstructure)

unhardened boron steel cross section heat-affected zone

A

B

C

D

E

F

G

Fig. 13 Scanning electron micrographs; A,B,D and E for hardened boron steel; C,F and G for unhardened boron steel

3.4. Manufacturing process of bainitic steels

The bainitic microstructure was produced by heat-treatment in austenitic range at 900°C, higher than AC1 for 30 min to be sure that all carbides are decomposed, after that quenched in salt bath at constant temperature 320°C (Ms+20°C) and after that quenched in water. Fig 14 shows bainitic microstructures after austempering for 5, 15, 30 and 60 minutes. According to

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Laser Welding of Boron and Bainitic Steels Page 26

austempering time was bainitic and martensite microstructure formed. For example gives 5 min austempering high hardness due to high amount of martensite and low amount of bainite.

500X 5 Min 500X 60 Min 500X 15 Min 500X 30 Min 2000X 5 Min 2000X 15 Min 2000X 30 Min 1000X 60 Min

Austempering for 5,15,30 and 60 Min

Fig. 14 Optical and Scanning electron micrographs of bainitic microstructures

3.5.Lab Experiments

3.5.1. Laser welding Samples of boron Boron steel 22Mn5B, 4 mm thick,18.6 cm long and 12

cm wide were welded using same power, speed, focal length and heat input but with different preheating and post- weld heat-treatments. The laser welding parameters used in the test of first and second groups are listed in Table 1.

Table 1 Laser parameters used in lab tests at LTU

Weld Type Keyhole pentration Mode

Laser Ytterbium Fiber laser(YLR-15000,IPG)

Max Power 4KW

Focal length 200mm

Shielding gas Ar

Shielding gas flow rate 18 l/min

Spot point 267µm

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Laser Welding of Boron and Bainitic Steels Page 27 Laser and Gas nozzle Induction heater Thermocouple wire A rigid clamping device

Spot welding on the Samples surface Boron steels weld

Samples A B C D Voltage and Current button Voltage and Current gage

Fig. 15 A- rigid clamping device gas nozzle and laser output, B- Device for spot welding for the thermocouple wire, C- Induction heater and D- Yitterbium Fiber Laser

The sheets were prepared in the workshop by cutting and after that machining to get flat and fine surface for the laser welding. The ambient temperature was 20C°. A rigid clamping device as shown in fig 15 A was used to maintain equal fixture parameters for test samples. The ytterbium Fiber Laser with max power 15 KW and 10.6 µm wavelength was used in the experiments. The Q&P heat-treatment was used after laser welding with help of induction generator (with max.power 40 KW and 18-25 kHZ) in order to avoid a quick decrease of the temperature and thereby the formation of martensitic microstructure. Thus in order to avoid full martensitic structure, it is quenched to a temperature lower than Ms which is called QT and after that using induction heater giving heat input on both sides of the weld with help of two-armed induction coils to reduce the stress formations in the solidified increase of the partitioning temperature (PT) to get tempered martensite in order to avoid cracks. The induction heating E(Power in watt)=U(Voltage) (Current) was steered by adjusting only the Voltage while the induction current was kept at max level as shown in fig 15 C. The thermocouple was spot welded 1.5 mm away from the welding track on the plate surface as shown in fig 15 B and connected to Data logger RS-232 to control the temperature during welding and to switch on the induction heater after QT to get the partitioning temperature by adjusting the voltage in short time to 300 U for 1 second and after that to 180 U for some seconds.

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Laser Welding of Boron and Bainitic Steels Page 28

3.6. Three point bending test

Three-point bending test consists of 2 parallel supports for the sample and a single loading pin in the middle, between the supports where the force acts. Bending ductility tests determine the smallest radius around which the specimen can be bent without cracks forming on the outer surface. It is used to test the ductility of welds. Bending strength tests is used for determining the modulus of elasticity in bending and bending strength for flat samples such as sheets or plates. From test curve, physical examination and calculations it is possible to get the material resistance to bending and its ductility. It is also a method for controlling and modifying of manufacturing processes. In this Master Thesis were bending ductility tests performed. These tests were performed at company Linde-Wieman, ACCRA in Öjebyn. The tests were done by applying a force ( Max Force 100 KN by the test machine) until the material is deformed, from the bending test cycles we get the max force to cause the plastic deformation and from the dimensions can the bending strength be calculated. Bending test samples of boron steels in the first group of test samples had a thickness =4 mm, length L= 150 mm and width = 40 mm.

Bending test samples of bainitic and hardened boron steels in 3rd group of test samples had a

thickness of 5mm, length=15cm and width=8mm. The bending strength is given by equation 7

σ = MY/I

……….…… (7)

For rectangular beams is I=bd3 /12 where I is the second moment of area,M is the maximum

bending moment, Y is the distance from the neutral axis, b is width, d is the thickness, F is

applied load and

σ

is the bending strength M= 3FL/2bd2

The test machine and samples shown in fig 16 are used only for first group of samples at the company.

Bending tests for bainitic and hardened boron steels for the 3rd group of samples were done at

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Laser Welding of Boron and Bainitic Steels Page 29 A B D C E 61.7mm D=47mm

Fig. 16 Schematic illustration for A) 3-Point bending test B) Dimensions of test machine used by ACCRA C),D) and E) Samples after different bending test cycles

Fig. 17 Bending test machine at the solid mechanics lab at the University

The bending tests for bainitic and hardened boron steels were done at the solid mechanics lab at Lulea University which has another pin form but the same support dimension as shown in fig 16 A and B. The distance between the two supports is 61.7mm and the single loading pin has 10mm diameter. From fig 17 C,D,E and F can a brittle fracture without necking and very bad laser

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Laser Welding of Boron and Bainitic Steels Page 30

welding in cycle 12 be seen. This is due to the hardened boron sheets misalignment at the laser welding which caused bad weld bead due to limited penetration and only partial melting.

3.6.1. Factors affecting the bendability

Bendability depends on surface finishing of the sheet such as surface roughness, which increases the stress concentration and decreases the bendability. The amount, shape, and hardness of inclusions present in the sheet metal and amount of cold working during shearing are very important factors in edge cracking. The resistance to edge cracking during bending can be improved by removing the cold worked regions by annealing to improve its ductility.

3.7. Tensile test

Tensile test samples were machined from the sheets and cut in the rolling direction to get maximum tensile strength. It gives information about strength, strain, elasticity and elongation. Elongation is measure of the ductility; it is defined as the increase in length after fracture of the tensile sample. The samples prepared for tensile tests were cut with water-jet-cutting. It was done for bainitic and hardened boron steels. The max load of the tensile test machine is 250 KN and the velocity used was 0.5mm/s, see figure 18; according to the Europe standard dimensions [21] because the sheet thickness is thick sheets 5 mm in cross section, will the dimension for the tensile test samples be:

L0 (Original gauge length) =50mm , Lc (Parallel length) = 86mm

Lf (total length of test piece) =300mm B (width of the parallel length) =10mm Gripped ends =60mm

The cross section is rectangular with width 9.87-10 mm and thickness of 4.76-5mm. This project examined the material properties of bainitic and hardened boron steel materials at different quenching and partitioning temperatures for the 3rd group.

Fig. 18 Tensile test machine

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Laser Welding of Boron and Bainitic Steels Page 31

ϵ=

=

………..……… (8)

is the initial length of the sample and Lf is the final length

In calculation of YS and UTS, it is assumed that E= Modulus of elasticity is taken as constant,

σ = F/A

where σ is stress, F is the load applied and A is the original cross section.

4-Results and discussions

The first group of welding tests considered of 10 Quench and Partitioning cycles and one martensitic (M1) for a boron steel. They were quenched after laser welding to temperatures below Ms to form martensite laths in the microstructures. After that they were heated again with induction heater for partitioning above Ms at different temperatures .

4.1.The first group of welding tests

In cycle 2 and 3 as explained in Table 2 has moderate hardness from 346 and 367HV been formed in comparison with martensitic microstucture (M1) 525HV. In cycle 2 and 3 has lower bainitic microstructures that gives high strength, ductility and toughnes been formed. In cycles 5,6 and 10 has not martensite been formed but bainitic microstructures with hardness from 380 HV to 317HV has been formed.

Table 2, The lst group of welding tests. In the table describes fm the theoretical koistinen-Marburger equation. Cycle 1 2 3 4 5 6 7 8 9 10 M1 QT °C 336 317 350 300 426 435 350 359 340 408 PT °C 433 443 415 600 589 577 450 518 418 521 P TIME (S) 6 6 10 2 6 6 3 6 6 10 fm 48% 58% 40% 65% 0% 0% 40% 33% 46% 0% 100% HV50 0g Max 432 FZ C 346 FZ C 367 HAZ C 486 FZ C 380 FZ C 317 FZ C 370 HA Z C 339 HAZ C 458 HAZ C 347 HAZ C 524 HAZ C

Group1 boron steel 4mm, martensite start temperature 396°C and bainite start temperature 640°C

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Laser Welding of Boron and Bainitic Steels Page 32 0 50 100 150 200 250 300 350 400 450 500 11:15:50 11:15:59 11:16:08 11:16:17 11:16:26 11:16:35 11:16:44 11:16:53 11:17:02 11:17:11 11:17:20 11:17:29 11:17:38 G1 Cycle2 0 50 100 150 200 250 300 350 400 450 13:06:59 13:07:11 13:07:23 13:07:35 13:07:47 13:07:59 13:08:11 13:08:23 13:08:35 13:08:47 13:08:59 G1 Cycle3 0 100 200 300 400 500 600 700 13:21:39 13:21:45 13:21:51 13:21:57 13:22:03 13:22:09 13:22:15 13:22:21 13:22:27 G1 Cycle5 0 100 200 300 400 500 600 13:46:32 13:46:44 13:46:56 13:47:08 13:47:20 13:4 7: 32 13:47:44 13:47:56 13:48:08 13:48:20 13:48:32 13:48:44 G1 Cycle8 0 100 200 300 400 500 600 13:52:10 13:52:19 13:52:28 13:52:37 13:52:46 13:52:55 13:53:04 13:53:13 13:53:22 13:53:31 G1 Cycle9 0 100 200 300 400 500 600 13:58:51 13:59:06 13:59:21 13:59:36 13:59:51 14:00:06 14:00:21 Series1 G1 Cycle10

Fig. 19 Boron steel experimental time-temperature-curves of the quenching and partitioning heat treatment measured 1.5 mm from the fusion zone for group1 Cycles 2, 3,5,8,9 and 10 The temperature characteristics of the quenching and partitioning cycles 2,3,5,8,9 and 10 shown in fig 19 and table 2 formed small amount of martensite and the partitioning was performed at temperatures where lower bainitic microstructures which have good mechanical properties.

4.2 Microstructures of the 1st group of welding tests

The microstructures that resulted from different welding experiments with different QT &PT are shown in fig 20 with optic microscope and fig 21 with scanning electron micrographs. In cycles 4 and 8 were bainite formed at higher partitioning temperatures. Cycle 4 has a hardness of 486 HV due to high martensite content 65% and cycle 8 has 339HV due to low martensite content 33% and longer partioning time. It can be seen that the microstructures have different intensity according to different chemical reactions with the 2% Nital, which can be seen from different colors in fusion zone and heat affected zone compared to the base material. Fig 19 shows the experiment cycles and fig 20 show the microstructures for cycles 2, 5, and M1 by optical microscope without cracks and pores due to good structure material microstructures and high weldability sheet steels. The base material structure was ferritic pearlitic as shown in fig 20. Cycle 2 and M1 and SEM pictures in fig 21 pearlite blocks. Heat affected zone in fig 20 cycle 2 shows tempered martensite, martensitic blocks that are not transformed and lower bainitic microstructures due to moderate hardness 346HV and the same can be seen in fig 21 with SEM microscope.

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Laser Welding of Boron and Bainitic Steels Page 33 Cycle2 FZ 500x Cycle2 HAZ 500x Cycle2 BM 500x Cycle2 25x Cycle5 FZ 500x Cycle5 FZ 500x Cycle5 HAZ 500x Cycle5 HAZ 25x CycleM1 FZ 500x Cycle M1 HAZ 500x Cycle M1 BM 500x Cycle M1 25x

Fig. 20 Microstructures for 1st group cycles 2,5 and martensite cycle in Fusion zone (FZ), Heat

affected zone (HAZ) and Base metals (BM)

Another laser experiment is cycle 5 in fig 20 which shows tempered bainite, No martensite was formed but only lower bainite at 426°C after that heating again to 589°C to get tempered bainite which had a hardness of 380HV . Due to boron in boron steels increase the hardenability which induction heater increases the time to get partition temperatures. M1 cycle (without induction heater) microstructure in fusion zone in fig 20 shows fine martensitic laths with 524HV . No pre or post weld heat-treatment was used. Heat affected zone has martensite blocks with high hardness. The martensitic microstructure is very hard, brittle and has very bad toughness which affects the formability in different applications.

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Laser Welding of Boron and Bainitic Steels Page 34

G1 Cy3 FZ 1000X G1 Cy3 HAZ

1000X G1 Cy3 BM 1000X G1 Cy2 BM 1000X G1 Cy2 FZ 1000X Bainitic Blocks perlite G1 Cy7 HAZ 1000X G1 Cy7 BM 1000X G1 Cy7 FZ 1000X G1 Cy2 HAZ 1000X G1 M1 FZ 1000X G1 M1 HAZ 1000X

Fig. 21 Scanning electron micrographs for 1st group Cycles JSM 6460 LV SEM for Cy2, Cy3 and Cy7 for Fusion zone (FZ), Heat affected zone (HAZ) and Base Metal (BM).

SEM graph in fig 21 show cycles 2, 3 and M1. Table 2 shows the amount of martensite formed, quenching and partitioning temperatures and the resulting hardness due to different amounts of bainitie and martensite formed in cycles

4.3. The 2:nd group of welding tests

In cycles 1 and 2 were tempered martensite formed in the microstructures in order to study the effect of residual stresses on the hardness and hot and cold crack formation in HAZ and FZ. The other cycles 3,4,8,13 and 14 were performed with the concept of quenching and partitioning. These cycles were performed in the laser lab and the quenching and partitioning temperatures were kept for 6 seconds as shown in fig 22 and appendix3

Table 3 The 2:nd group of welding tests

Cycle 1 2 3 4 8 13 14 QT °C 351 193 300 292 313 384 365 PT °C 388 363 451 437 443 449 464 P Time (s) 6 6 6 6 6 6 6 fm 39% 89% 65% 68% 60% 12% 29% Max HV0,5 412 HAZ

423 HAZ 358 HAZ 417 HAZ 403 HAZ 362 HAZ 337

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Laser Welding of Boron and Bainitic Steels Page 35 0 100 200 300 400 500 600 700 10:04:49 10:04:58 10:05:07 10:05:16 10:05:25 10:05:34 10:05:43 10:05:52 10:06:01 G2Cycle1 0 100 200 300 400 500 600 10:45:53 10:46:05 10:46:17 10:46:29 10:46:41 10:46:53 10:47:05 10:47:17 10:47:29 10:47:41 G2 Cycle 2 0 50 100 150 200 250 300 350 400 450 500 10:57:54 10:58:09 10:58:24 10:58:39 10:58:54 10:59:09 10:59:24 10:59:39 10:59:54 11:00:09 G2 Cycle 3 0 50 100 150 200 250 300 350 400 450 500 13:25:01 13:25:16 13:2 5: 31 13:25:46 13:26:01 13:26:16 13:26:31 13:26:46 13:27:01 13:27:16 13:27:31 G2 Cycle 8 0 50 100 150 200 250 300 350 400 450 500 14:24:49 14:25:04 14:25:19 14:25:34 14:25:49 14:26:04 14:26:19 14:26:34 14:26:49 14:27:04 G2 Cycle 13 0 50 100 150 200 250 300 350 400 450 500 14:33:52 14:34:07 14:34:22 14:3 4: 37 14:34:52 14:35:07 14:35:22 14:35:37 14:35:52 G2Cycle 14

Fig. 22 Experimental time-temperature-curves of the quenching and partitioning heat treatments measured 1.5 mm from the fusion zone for 2:nd group cycles 1, 2, 3,8,13 and 14

4.4. Microstructures of 2:nd group of welding tests

From results of cycle 2 in Table 3 and as shown in fig 23 from optical microscope and SEM from fig 24, 89% martensite is formed at QT 193°C. Tempering at 363°C below martensite start (Ms) gave tempered martensite with low hardness 423HV in comparison to full martensitic microstructures 524HV. The toughness will increase with dispersions of fine iron carbide and formation of ferrite, also there is no segregations on the grain bounderies which cause formations of cracks on the fusion zone or heat affected zone. SEM pictures show tempered martensite in heat affected zone and fusion zone. Cycle 4 formed 68% martensite at 292°C and after partitioning at 437°C was bainitic microstructure and tempered martensite formed. Cycle 13 was quenched to 384°C to form very low martensite amount 12% after that it was heated with the induction heater to 449°C to form fine bainitic microstructure with hardness of 362HV.

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Laser Welding of Boron and Bainitic Steels Page 36

G2 Cycle 2 FZ 500x G2 Cycle 2 HAZ 500x

G2 Cycle 2 HAZ 25x

G2 Cycle 4 FZ 500x G2 Cycle 4 HAZ 500x

G2 Cycle 4 HAZ 25x

G2 Cycle 13 FZ

500x G2 Cycle 13 HAZ 500x G2 Cycle 13 25x

Fig. 23 Microstructures for 2:nd group of welding tests cycle 4 and cycle 13 for Fusion zone (FZ),Heat affected zone (HAZ) and Base metals (BM).

G2 Cycle 2 BM 1000X G2 Cycle 2 FZ 1000X G2 Cycle 2 HAZ 1000X G2 Cycle 4 FZ 2500X G2 Cycle 4 BM 2500X G2 Cycle 4 HAZ 2500X G2 Cycle 13 HAZ 2500X G2 Cycle 13 FZ 2500X G2 Cycle 13 BM 2500X

Fig. 24 Scanning electron micrographs for 2nd group of welding tests JSM 6460 LV SEM for cycle 2, 4 and 13 for Fusion zone (FZ), Heat affected zone (HAZ) and Base Metal (BM). As shown in fig 24 with SEM there are fine martensitic plates and bainitic blocks in fusion zone and heat affected zone too. Cycle 2 has tempered martensite, cycle 4 has in FZ and HAZ

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Laser Welding of Boron and Bainitic Steels Page 37

tempered martensite and bainitic blocks which gives moderate hardness but cycle 13 has bainitic microstructure with small amount of fine martensite laths.

4.5. The 3rd group of welding tests

The laser welding parameters used in the 3rd group of welding tests with 5mm thick sheets are listed in Table 4.

Table 4 Welding parameters used in the 3rd group of welding tests.

Weld Type Keyhole penetration Mode

Laser Ytterbium Fiber laser(YLR-15000,IPG)

Max Power 4KW

Focal length 300mm

Shielding gas flow rate Ar 18 l/min

Welding speed 2m/min

These tests were applied on bainitic and boron steel samples with 5 mm thickness as shown in fig 25 and 26. The sample dimensions were 15cm long and 9.2cm width. The heat-treatment in lab to get the bainitic microstructures was done by austenitisation at 900°C for 30 min and after that austempering in salt bath for 30 min at 320°C (Ms+20°C). Max hardness 470HV as explained in fig 7 earlier.

Table 5 The 3rd group of welding tests

Cycle 1 2 3 4 5 6 7 8 9 10 11 QT °C 288 271 277 277 381 283 297 282 285 Martensitic 253 PT °C 332 319 353 351 436 440 423 407 403 422 R time (s) 6 6 6 6 10 6 6 6 6 6 Fm% 12% 27% 22% 22% 0 17% 3% 18% 15% 100% 40%

Table 5 shows the QT, PT and P-time for the 11 bainitic steel cycles and table 6 shows the boron steel cycles 12, 14, 15 and cycle 13.

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Laser Welding of Boron and Bainitic Steels Page 38 0 200 400 600 9:41:02 9:41:46 9:42:29 9:43:12 9:43:55

Bainitic Steels Cycle 1 QT 288C° PT 332C° 5S Bainitic Steels Cycle 1 0 200 400 600 800 9:50:24 9:51:07 9:51:50 9:52:34 9:53:17

Bainitic Steels Cycle 2 QT 271C° PT 319 C° 6S Bainitic Steels Cycle 2 0 100 200 300 400 500 10:09:50 10:10:34 10:11:17 10:12:00 10:12:43

Bainitic Steels Cycle 3 QT 277C° PT 353C° 6S Bainitic Steels Cycle3 0 200 400 600 10:24:5810:26:2410:27:5010:29:1710:30:43

Bainitic Steels Cycle 4 QT 277C° PT 351C° Bainitic Steels Cycle 4 0 200 400 600 800 11:06:4311:07:2611:08:1011:08:5311:09:36

Bainitic Steels Cycle 5 QT 381C° PT 434C° Bainitic Steels Cycle 5 0 100 200 300 400 500 12:57:3612:59:0213:00:2913:01:5513:03:22

Bainitic Steels Cycle 6 QT 283C° PT 440C° 6S Bainitic Steels Cycle 6 0 100 200 300 400 500 600 13:10:34 13:12:00 13:13:26

Bainitic Steels Cycle7 QT 297C° PT 423C° 6S Bainitic Steels Cycle7 0 100 200 300 400 500 13:16:1913:17:4613:19:1213:20:38

Bainitic Steels Cycle 8 QT 282C° PT 407C° 6S Bainitic Steels Cycle 8 0 100 200 300 400 500 600 13:20:38 13:23:31 13:26:24

Bainitic Steels Cycle 9 QT 285 C° PT 403C°

Bainitic Steels Cycle 9

Fig. 25 Experimental time-temperature-curves of 3: rd group measured 1.5 mm from the fusion zone cycles 1 to 9 for the 3:rd group of welding tests for bainitic steels.

Table 6 The 3rd group of welding tests

Cycle 12 13 14 15 16

QT C° 263 Martensitic 382 344 385

PT C° 431 432 440 421

P Time (s) 6 6 6 10

Fm% 77% 100% 14% 44% 11%

The boron steels used were 16.3 cm long and 14 cm wide in laser welding but they were hardened before laser welding by austenitising at 900C° for 30 min, after that quenched in water. After laser welding was the heat-treatment concept with induction heating to form bainitic microstructures utilized. Experimental time-temperature-curves of the quenching and partitioning heat- treatment measured 1.5 mm from the fusion zone for cycles 10, 11 bainitic steels and boron steel cycles 12, 14 and 15 are shown in figure 26. Cycle 16 performed on boron steel bar with 3.5 mm thickness showed same results.

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Laser Welding of Boron and Bainitic Steels Page 39 0 50 100 150 200 250 300 350 13:40:0513:40:4813:41:3113:42:1413:42:5813:43:4113:44:24

Bainitic Steels Cycle 10 Martensitic

Bainitic Steels Cycle Martensitic 0 100 200 300 400 500 600 13:50:53 13:51:36 13:52:19 13:53:02 13:53:46 13:54:29

Bainitic Steels Cycle 11 QT 253C° PT 422C° 6S

Bainitic Steels Cycle 11 0 100 200 300 400 500 600 700 800 900 14:15:2214:18:1414:21:0714:24:0014:26:53

Boron Steels Cycle 12 QT 263C° PT 431C° Cycle 12 Boron Steels 0 100 200 300 400 500 600 15:11:31 15:12:58 15:14:24 15:15:50

Boron Steels Cycle 14 QT 382C° PT 432C° 6S Cycle 14 Boron Steels 0 100 200 300 400 500 600 15:27:22 15:28:48 15:30:14 15:31:41

Boron Steels Cycle 15 QT 344C° PT 440C° 6S

Cycle 15 Boron Steels

Fig. 26 Experimental time-temperature-curves of 3: rd group cycles 10 and 11 bainitic steel; 12, 14 and 15 for boron steel.

Cycle1 FZ 50µm Cycle1 HAZ 50µm

Cycle1 50X Cycle11 HAZ 50µm Cycle11 FZ 50µm Cycle3FZ 50µm Cycle3 HAZ 50µm Cycle3 50X Cycle11 50X

Fig. 27 Microstructures for the 3:rd group of welding test cycles 1,3 and 11 in fusion zone (FZ), heat affected zone (HAZ) and base metals (BM)

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Laser Welding of Boron and Bainitic Steels Page 40 Cycle1 FZ 1000 Cycle1 HAZ 1000 Cycle2 HAZ 1000 Cycle2 FZ 1000 Cycle3 FZ 1000 Cycle3 FZ 1000 Cycle4 HAZ 1000 Cycle4 FZ 1000 Cycle4 FZ 1000

SEM microscope for cycle 1,2 and 3

Fig. 28 Scanning electron micrographs for group3 bainitic steel Cycle 1,2,3 and 4

4.6. Hardness measurement of the 3:rd group of welded samples

The 3rd group of welded samples was bainitic steels heat treated in LTU materials lab. The obtained samples were cut into specimens by electro-discharge machining for subsequent handling.To avoid an increase in martensite amount formation can heat-treatment with Q&P concept be used for bainitic steels. The comparison of the cycles in fig 29 and 30 shows that heat treatment formed softening in the heat affected zone and lower hardness than in the base metal, but also higher hardness than the fusion zone.. Hardness depends on grain size; it can be affected by low amount of alloying elements or absence of some alloying elements such as V, Ti and Al which decreases grain growth from high heat input. Hardness diagrams in fig 29 shows that cycle 4, 7 and 8 increases in the width of the heat affected zone due to higher heat input and the hardness decreases in comparison with martensitic hardness.

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Laser Welding of Boron and Bainitic Steels Page 41 300 350 400 450 500 550 -3 -2 -1 0 1 2 3 Ha rd ne ss H V Distance in mm

Bainitic steels Cycle 8

Bainitic steels Cycle 8 300 350 400 450 500 550 -4 -2 0 2 4 Ha rd ne ss H V Distance in mm

Bainitic steel cycle 7

Bainitic steel cycle 7 300 350 400 450 500 550 600 650 -4 -2 0 2 4 Ha rd ne ss H V Distance in mm

Bainitic steel cycle 10

Bainitic steel cycle 10 300 350 400 450 500 550 -3 -2 -1 0 1 2 3

Bainitic steel cycle 4

Bainitic steel cycle 4

Fig. 29 Hardness profiles across welds

Fig. 30 Hardness of the 3rd group welding tests on bainitic steel.

This softening affects the mechanical properties, and fracture can occur in this fusion zone because this region has lower hardness than the base metal, but the tensile and bending tests showed that fracture did not happen in the heat affected zone but in the fusion zone due to lack

1 2 3 4 5 6 7 8 9 10 11 460 454 448 444 440 390 410 430 432 493 415 372 379 373 343 314 362 337 373 360 398 373 551 534 514 532 550 484 505 522 490 616 494 0 100 200 300 400 500 600 700 1 2 3 4 5 6 7 8 9 10 11

Cycles Hardness HV500g Hardness FZ Low Hardness HAZ High Hardness HAZ

C ycl es Har d n ess HV5 0 0 g

References

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