| 1. |
- Jakobsson, Erik, 1960-
(författare)
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Mot en ny vuxenutbildningspolitik? : Regional utveckling som policy och praktik
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Avhandlingen beskriver och analyserar ett regionalt utvecklingsinitiativ i nätverksform, genom vilket de kommunala lärcentrumen i Örebro län samverkar för att utveckla en gemensam infrastruktur för vuxnas lärande. Detta innebär, exempelvis, att utveckla former för samverkan mellan olika regionala aktörer inom området, öka samordningen och samverkan inom den kommunala vuxenutbildningen och utveckla metoder för lärande med fokus på arbetet och arbetsplatsen.Den övergripande forskningsfrågan kan formuleras på följande sätt: Vilka är förutsättningarna för att genom regional samverkan, med en horisontell organisering i nätverksform, utveckla politiken, planeringen och organiseringen för vuxnas lärande, och med vilka teoretiska och metodologiska perspektiv kan vi förstå såväl problem som möjligheter?Två övergripande teoretiska perspektiv presenteras i avhandlingen, som relateras till det studerade fallet. Det gäller å ena sidan ett traditionellt planeringsperspektiv och å andra sidan ett evolutionärt lärandeperspektiv. Dessa perspektiv utgör både kontraster och komplement till varandra. Planeringsperspektivet har ’policysändaren’ som utgångspunkt, medan lärandeperspektivet har ’policymottagaren’ som utgångspunkt. Diskussionen i avhandlingen behandlar också de möjligheter som finns att kombinera de båda övergripande perspektiven, både som analytiska och strategiska perspektiv.Sammantaget kan studien av nätverket beskrivas som en kvalitativt inriktad fallstudie, där mitt kontinuerliga deltagande över tid i nätverkets sammankomster är ett centralt inslag. Intervjuer, en enkätundersökning, diskussioner och dokumentstudier utgör också material till studien. En interaktiv forskningsansats har tillämpats, som syftar till en gemensam kunskapsbildning och lärprocess mellan deltagarna och forskaren. Metodologiskt hämtar studien inspiration från den vetenskapsfilosofiska inriktning som kallas kritisk realism.En hastig överblick över studiens resultat ger vid handen att mycket av det som diskuterats i nätverket har stannat vid intentioner och har inte blivit omsatt i en praktik. Nätverket har också förblivit ganska internt, med en krets av lärcentrum- och vuxenutbildningschefer. Lite tillspetsat kan man säga att nätverket varken arbetat sig ’nedåt’ i respektive lokal organisation eller ’uppåt’ mot den politiska sfären, eller ’utåt’ mot det omgivande samhället och arbetslivet.Nätverket har onekligen ändå skapat nya och bättre förutsättningar för samarbete mellan lärcentrumen. Det finns nu betydligt mer av en gemensam förståelse och någonting att bygga vidare på, regionalt och lokalt. Det är också hoppfullt att se att nätverket fortsätter sitt arbete trots att det inte längre har externt stöd. Nätverket har, förefaller det, etablerat sig som en gemensam utvecklingsorganisation för deltagarna, och har nått ett stadium där mer konkreta och påtagliga former av samverkan börjar äga rum.
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| 2. |
- Nilsson, Erik A. A., 1986-
(författare)
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Degradation Mechanisms of Heat Resistant Steel at Elevated Temperatures : In an Iron Ore Pelletizing Industry
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on the different degradation mechanisms of the stainless steel in a travelling grate in a Grate-Kiln iron ore pellet indurator. The travelling grate is a conveyor belt that transports green-body pellets to a rotary kiln while the pellets are being dried and pre-heated to a temperature of 900-1100 °C by recycled hot air. After unloading of the pellets to the rotary-kiln for further sintering, the travelling grate is cooled in room temperature while returning to the loading zone of the wet pellets.The steel was tested during thermal cycling in a test-rig, in order to simulate the influence of thermo mechanical fatigue and oxide spallation. The influence of erosion-deposition was investigated in a modified horizontal industrial combustion kiln at 800 °C, with slag and coal from production used as erosive media and combustion fuel, respectively. The influence of minor alloying additions of Mn, Si and Ti on the microstructure was explored by eight different casted alloy compositions. Isothermal heat treatments were performed at 800 °C during 200 hours on steel immersed in deposits recovered from a travelling grate in production.The three main degradation mechanisms found in this work are thermal spallation, erosion-deposition and deposit induced accelerated corrosion (DIAC). Thermal spallation of the oxide layer is caused by the thermal expansion difference between the oxide and the metal during heating and cooling. It has been found that Ti improves the spallation resistance while Si reduces it. Spallation of deposits is another cause believed to increase the degradation. Erosion-deposition appears due to simultaneous erosion and deposition of particles on the travelling grate that causes erosion or deposition depending on the amount of alkali metals in the environment. The velocity of the particles also influences erosion and deposition in the way that higher velocities increase erosion. DIAC is proposed to form on the travelling grate due to the concentration of chloride- and sulphate containing alkali metals in the deposits. Other than these major degrading mechanisms, minor degradation mechanisms such as internal oxidation, sigma formation, carburization and sensitization towards inter-granular attack have been found inside the steel during heating. Thermo mechanical fatigue (TMF) causes intergranular cracks in the material of the travelling grate. Casting issues such as micro-segregation have also been addressed in this thesis.A few different ways to improve degradation resistance have been proposed, such as homogenization heat treatments, optimization of process parameters and inhibitor solutions.
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| 3. |
- Rehan, Arbab
(författare)
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Effect of heat treatment on microstructure and mechanical properties of a 5 wt.% Cr cold work tool steel
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work presents investigations related to phase transformations occurring inthe 5 wt.% Cr cold work tool steel Caldie during hardening and tempering treatments. The influence of austenitisation temperature, cooling rate, sub-zero cooling, isothermal treatment during cooling, tempering temperature and holding time on the microstructure and mechanical properties were investigated.The hardened microstructure of the investigated steel consisted of a mixture ofplate and lath martensite, minor amounts of bainite, blocky and thin retained austenite and M7C3 carbides. Increasing austenitisation temperature from 1020°Cto 1050°C was found useful as it provided higher hardness, good compressive strength and sufficient toughness. However, a further increase to 1075°C resulted in large prior austenite grains which produced coarse martensite containing somewhat increased carbon content. This was found to reduce the impacttoughness of the steel. Significant amounts of retained austenite were present after tempering for 2x2 h between 200°C and 500°C while tempering at 525°C or higher, reduced retained austenite content to below 2%. During holding at tempering temperature carbides precipitated in martensite and possibly in retained austenite. The retained austenite was thereby destabilised and transformed to martensite on cooling. This fresh martensite was tempered by following tempering treatments. It was concluded that tempering at 525°C for 2x2 h was suitable to achieve a good combination of hardness, compressive strength and impact toughness. Retained austenite was also found to transform during holding at 600°C for longer times. Initially, carbides formed in the austenite and after some time transformation of retained austenite to ferrite and carbides took place. Results were used to discuss alternative heat treatment procedures for the 5wt.% Cr cold work tool steel Caldie and some changes of current heat treatment recommendations were suggested.
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| 4. |
- Avby, Gunilla
(författare)
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Evidence in practice : On knowledge use and learning in social work
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Against the backdrop of the transformations in the entire framing of professional work, social work has come under close scrutiny in many countries, including Sweden. Doubts have been raised about practitioners’ existing knowledge base, and the importance of practitioners engaging in learning and the renewal and extension of professional capacities has been emphasized.The present thesis concerns knowledge use and learning in the daily practices of child investigation work. The aim is to explore processes of knowledge use and learning in practice. The study is based on a mix of qualitative approaches, basically from ethnography, comprising methods such as participant observations, interviews, reflective dialogues and documentary analysis of case data.The main findings demonstrate that investigation work is characterized mainly by the use of practice-based knowledge. Research-based knowledge is predominantly used as a means of explaining a client’s situation or to underpin and legitimize one’s own beliefs and decisions made on other grounds. Professional learning is largely adaptive in character, as the social workers learn to handle tasks in a fairly routinized way on the basis of rules or procedures that draw on existing knowledge in the practice setting.Two conclusions are drawn: First, the use of knowledge in child investigation work bears little resemblance to principles of evidence-based practice. Second, the reproduction of professional knowledge is largely implicit and taken for granted. The study offers insight into the much-discussed topic of putting knowledge into practice, which is of importance to strategies for organizing profes sional learning and knowledgeable practice.
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| 5. |
- Chen, Zhuo Yuan, 1970-
(författare)
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The role of particles on initial atmospheric corrosion of copper and zinc : lateral distribution, secondary spreading and CO2-/SO2-influence
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The role of sodium chloride (NaCl) particles and ammonium sulfate ((NH4)2SO4) particles on the initial atmospheric corrosion of copper and zinc was investigated under in situ and ex situ conditions using microgravimetry, FTIR spectroscopy, ion chromatography, scanning electron microscopy with x-ray microanalysis and the scanning Kelvin probe. For the first time, in situ infrared spectra were collected on a micron level during particle induced atmospheric corrosion using a recently developed experimental set-up for in situ FTIR microspectroscopy. Lateral distribution of corrosion and reaction products on copper and zinc surfaces was determined and could be connected with the mechanisms of the initial particle induced corrosion. The recently discovered secondary spreading effect from NaCl electrolyte droplets on metal surfaces was studied under in situ conditions and the effect of CO2 on the spreading process was elaborated. The ambient level of CO2 (350 ppm, 1 ppm = 10-6 volume parts) results in a relatively low secondary spreading effect, whereas the lower level of CO2 (<5 ppm) causes a much faster secondary spreading effect over a large area. At low CO2 concentration alkaline conditions will prevail in the cathodic area, leading to large changes in the surface tension at the oxide/electrolyte interface in the peripherical parts of the droplet. This induces a surface tension driven convective flow of electrolyte from the NaCl droplet. The continuous growth of the secondary spreading area at low CO2 concentration is possible due to the galvanic coupling with the droplet leading to transport of sodium ions to this region and maintenance of the alkaline conditions. At 350 ppm CO2, carbonate formation in the secondary spreading area results in lowering of the pH, increasing the surface tension of the oxide/electrolyte interface and inhibiting the secondary spreading. CO2 strongly affects the NaCl-induced atmospheric corrosion rate of copper. The overall influence of CO2 and NaCl depends on at least three identified mechanisms. At low NaCl particle density, CO2 affects the secondary spreading effect from the electrolyte droplet. This leads to a larger effective cathodic area at low CO2 concentration and a higher corrosion rate. The more alkaline surface electrolyte present at low CO2 concentration also affects the formation of corrosion products and the amount of soluble copper chloride. Whereas the presence of larger amounts of soluble chloride tends to increase the corrosion rate, the formation of CuO results in a more protective surface film which decreases the corrosion rate. This effect was observed at higher NaCl particle densities, where the secondary spreading areas overlapped with adjacent NaCl particle clusters. The formation of CuO leads to lower corrosion rates compared to ambient CO2 concentration in which this phase was not formed. For zinc, the formation of a more protective corrosion product layer was not observed and the corrosion rate is generally higher for low than for ambient CO2 concentration. The presence of NaCl particles on the metal surfaces strongly affects the SO2 interaction with the metal surfaces. The oxidation of S(IV) turned out to be fast at the area of the NaCl-containing electrolyte droplet, both for copper and zinc. On copper surfaces, both sulphate (SO4 2-) and dithionate (S2O6 2-) ions formed which is consistent with a copper catalysed reaction route for sulfite oxidation including the formation of a Cu(II)–sulfito complex as an important step. For zinc, a surface mediated sulfite oxidation process leads to rapid formation of sulphate in the electrolyte droplet area. The presence of SO2 strongly inhibits the secondary spreading due to the decrease in pH induced by absorption of SO2 in the cathodic areas. The presence of gaseous oxidants, such as NO2 and O3, has previously been considered as an important prerequisite for the oxidation of sulfite on copper. The results obtained here suggest that the formation of local electrochemical cells induced by deposited NaCl particles could be another important route for S(IV)- oxidation to sulfate formation. On copper, SO2 was also found to promote the formation of less soluble copper chlorides, such as paratacamite (Cu2(OH)3Cl) and nantokite (CuCl). The electrolyte droplet was dried after 24 hours of exposure due to the formation of less soluble paratacamite (Cu2(OH)3Cl) and nantokite (CuCl) and led to a decrease in the corrosion rate. Thus, SO2 alone promotes the corrosion rate of copper, whereas in the presence of NaCl particles the corrosion rate of copper may slow down due to the formation of insoluble copper chloride compounds. The lateral distribution of corrosion products after exposure of NaCl contaminated copper and zinc surfaces to humid air with gaseous pollutants is a result of the formation of local electrochemical cells at the particles and concomitant differences in chemical composition and pH. For (NH4)2SO4 deposited copper and zinc surfaces the corrosion effects increase with the amount of pre-deposited particles and with the exposure time. On copper, the size of the particles affects the corrosion rate, smaller particles resulting in a higher corrosion rate than larger particles at equal amount of deposition. The formation of Cu2O was the dominant corrosion product after exposure longer than 10 days. (NH4)2SO4 particles result in enhanced Cu2O formation on copper due to a reaction sequence involving catalysis by NH3. The corrosion of copper by (NH4)2SO4 particles was much larger than that induced by NaCl particles. However, for zinc, the (NH4)2SO4 particles lead to smaller corrosion effects than those of NaCl particles. For both particles, significant corrosion attack was observed at relative humidity (RH) lower than the deliquescence point of the salts.
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| 6. |
- Elm Svensson, Erik, 1976-
(författare)
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Nanotemplated High-Temperature Materials for Catalytic Combustion
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Catalytic combustion is a promising technology for heat and power applications, especially gas turbines. By using catalytic combustion ultra low emissions of nitrogen oxides (NOX), carbon monoxide (CO) and unburned hydrocarbons (UHC) can be reached simultaneously, which is very difficult with conventional combustion technologies. Besides achieving low emission levels, catalytic combustion can stabilize the combustion and thereby be used to obtain stable combustion with low heating-value gases. This thesis is focused on the high-temperature part of the catalytic combustor. The level of performance demanded on this part has proven hard to achieve. In order to make the catalytic combustor an alternative to the conventional flame combustor, more stable catalysts with higher activity have to be developed. The objective of this work was to develop catalysts with higher activity and stability, suitable for the high-temperature part of a catalytic combustor fueled by natural gas. Two template-based preparation methods were developed for this purpose. One method was based on soft templates (microemulsion) and the other on hard templates (carbon). Supports known for their stability, magnesia and hexaaluminate, were prepared using the developed methods. Catalytically active materials, perovskite (LaMnO3) and ceria (CeO2), were added to the supports in order to obtain catalysts with high activities and stabilities. The supports were impregnated with active materials by using a conventional technique as well as by using the microemulsion technique. It was shown that the microemulsion method can be used to prepare catalysts with higher activity compared to the conventional methods. Furthermore, by using a microemulsion to apply active materials onto the support a significantly higher activity was obtained than when using the conventional impregnation technique. Since the catalysts will operate in the catalytic combustor for extended periods of time under harsh conditions, an aging study was performed on selected catalysts prepared by the microemulsion technique. The stability of the catalysts was assessed by measuring the activity before and after aging at 1000 C in humid air for 100 h. One of the most stable catalysts reported in the literature, LMHA (manganese-substituted lanthanum hexaaluminate), was included in the study for comparative purposes. The results showed that LMHA deactivated much more strongly compared to several of the catalysts consisting of ceria supported on lanthanum hexaaluminate prepared by the developed microemulsion method. Carbon templating was shown be a very good technique for the preparation of high-surface-area hexaaluminates with excellent sintering resistance. It was found that the pore size distribution of the carbon used as template was a crucial parameter in the preparation of hexaaluminates. When a carbon with small pores was used as template, the formation of the hexaaluminate crystals was strongly inhibited. This resulted in a material with poor sintering resistance. On the other hand, if a carbon with larger pores was used as template, it was possible to prepare materials with hexaaluminate as the major phase. These materials were, after accelerated aging at 1400 C in humid air, shown to retain surface areas twice as high as reported for conventionally prepared materials.
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| 7. |
- Fahlström, Karl, 1986-
(författare)
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Laser welding of ultra-high strength steel and a cast magnesium alloy for light-weight design
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- There is a strong industrial need for developing robust and flexible manufacturing methods for future light-weight design. Better performing, environmental friendly vehicles will gain competitive strength from using light weight structures. In this study, focus has been on laser welding induced distortions for ultra-high strength steel (UHSS) where trials were performed on single hat and double hat beams simulating A-pillar and B-pillar structures. Furthermore, also laser welding induced porosity in cast magnesium alloy AM50 for interior parts were studied. For UHSS, conventional laser welding was done in a fixture designed for research. For cast magnesium, single-spot and twin-spot welding were done. Measurements of final distortions and metallographic investigations have been performed. The results show that the total weld metal volume or the total energy input were good measures for predicting the distortions within one steel grade. For comparing different steel grades, the width of the hard zone should be used. The relation between the width of the hard zone, corresponding to the martensitic area of the weld, and the distortions is almost linear. Additionally, compared with continuous welds, stitching reduced the distortions. For cast magnesium, two-pass (repeated parameters) welding with single-spot gave the lowest porosity of approximately 3%. However, two-pass welding is not considered production friendly. Twin-spot welding was done, where the first beam provided time for nucleation and some growth of pores while reheating by the second beam should provide time for pores to grow and escape. This gave a porosity of around 5%. Distortions and porosity are the main quality problems that occur while laser welding UHSS and cast magnesium, respectively. Low energy input seems to generally minimize quality issues. Laser welding shows high potential regarding weld quality and other general aspects such as productivity in light-weight design for both high strength steel and cast magnesium.
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| 8. |
- Gould, Rachael, 1982-
(författare)
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The individual human side of supporting sustainable design beginners
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Starting to include sustainability considerations in a design project is a transition requiring a change in how things are done, that is, a change in behaviour. Furthermore, this transition takes place in the midst of the usual pressures of product design. Prior research on sustainable design has mostly explored the so-called technical side – identifying what tasks should be performed, such as specifics of including sustainability criteria when analysing product concepts. However, this has not been enough. These tasks are not being performed to the extent that they could, or that is needed. Recent studies have advocated the consideration of the human nature of the people who are to execute these ‘technical’ tasks. In other words, there is a need to work with the socio-psychological factors in order to help sustainable design beginners to adopt new mindsets and practice (their usual way of doing design).My aim was therefore to investigate how to support individual product design team members with the human aspects of transitioning to executing sustainable design. In particular, I focused on supporting good individual decision-making and individual behaviour change. This aim was addressed through multiple research projects with four partner companies working with the early phases of product design. Given a focus to change practice, I followed an action research approach with a particular emphasis on theory building. This action research approach comprised two phases: understanding the challenge and context, and then iteratively developing solutions through a theorise–design-act-observe-reflect cycle.Through the research projects, my colleagues and I found that there are challenges related to behaviour change and decision-making that are hindering execution of sustainable design. In order to help organisations to overcome or avoid these challenges, we found that it may be beneficial for those developing sustainable design tools and methods to (i) use techniques to mitigate for cognitive illusions, (ii) provide individuals with the opportunity to implement sustainable design while helping those individuals to increase their motivation and capability to execute sustainable design, and (iii) communicate with these individuals in such a way as to avoid triggering psychological barriers (self-defence mechanisms). I combined these points into two models.Together with the partner organisations, we applied the two models to design some actions that we then tested. The actions included integrating behaviour change and decision-making considerations into sustainable design tools as well as stand-alone interventions in the culture.Given the findings of these studies, I urge developers of sustainable design tools to see implementation of their tool as a learning journey. The beginning of the journey should comprise small steps supported by handrails, which then increase in size and decrease in support as the journey continues. Especially in the beginning, tool developers will also need to help travellers to avoid the decision-making errors that occur due to being in unfamiliar territory.
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| 9. |
- Grinbergs, Johan, 1972-
(författare)
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Pratnerskap - Ett sätt att organisera regional tillväxt : Perspektiv på regional utveckling som fenomen och en analys av ett tillväxtprogram
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During the first decade of the 21st century, there has been a shift in the view of what role regions should play in the creation of growth. From being governed from a national level more and more of the responsibility of creating growth is being transferred to a regional level. One example of this development has been the Regional Growth Program (RGP), which were implemented in all regions in Sweden during the period 2004 to 2007. This dissertation studies the RGP in the county of Västmanland, where a number of actors within the program have been interviewed as well as meetings within the program have been monitored. The interviews have been analyzed through a phenomenological oriented method which has resulted in six empirical themes. They consist of the following themes: region, growth, entrepreneurship, cooperation and competition, results, and talk and action. The last theme which concerns talk and action is being examined more thoroughly through the observations of meetings in the RGP and interviews with central actors. This theme is viewed as the most central and essential theme of the study. The dominating view of the central actors is that there is “too much talk and too little action” in the RGP In the latter part of the dissertation the theme of talk and action is examined further using a organizational perspective. The RGP is organized in so called “partnerships”, where the differences become very clear when compared to organization in a more traditional form. In traditional organizations, output can be observed as manufactured units or the value created in services performed. From such a perspective, consensus, efficiency and action – becomes the rational to strive for to maximize the effectiveness. Such a traditional view of organization is less useful for RGP – but is still in use since they are the dominant perspectives on how we value “good” organizing. In the case of the RGP the partnership have being assigned to cooperate and find new, better ways to stimulate the regional growth. The success of the partnership is still measured by quantitative indicators, which is not very logical since it is almost impossible to see if the partnership has had anything to do with affecting these indicators. With the support of theories from Brunsson (2006) I therefore propose the “talk organization” as an alternative to the “action organization” – and also see the potential of redefining the partnership as a form of such a talk organization. The advantages of such an approach could be to view the talking as an important prerequisite to potential action, and furthermore we can revalue the importance of creating talk opportunities to occur in such partnerships. This creates a view full of conflicts in relating to the traditional view of organization that advocates consensus and order – but in the supposedly creative talking partnerships the issue may be to uphold conflict, and a wide range of different views to be able to create something new. The dissertation has a theoretical focus that highlights the connection, but also contradictions, between the structural and symbolic in organizing and also how it may affect the potential for development and change.
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| 10. |
- Harati, Ebrahim, 1986-
(författare)
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Improving fatigue properties of welded high strength steels
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In recent years a strong interest has been expressed to produce lighter structures.One possible solution to reduce the weight is to utilize high strength steels and use welding as the joining method. Many components experience fatigue loadingduring all or part of their life time and welded connections are often the prime location of fatigue failure. This becomes more critical in welded high strength steels as fatigue strength of welds does not increase by increasing the steel strength. A possible solution to overcome this issue is to use fatigue improvement methods.The main objectives of this project are, therefore, to increase understanding of the factors that control fatigue life and to investigate how the fatigue strength improvement methods; high frequency mechanical impact (HFMI) treatment and use of Low Transformation Temperature (LTT) consumables will affect fatigue properties of welds in high strength steels. In this regard, Gas Metal Arc Welding(GMAW) was used to produce butt and fillet welds using LTT or conventional fillers in steels with yield strengths ranging from 650-1021 MPa and T-joint weldsin a steel with 1300 MPa yield strength. The effect of HFMI on fatigue strength of the welds in 1300 MPa yield strength steels was also investigated. Butt and fillet welds in 650-1021 MPa steels were fatigue tested under constant amplitude tensile loading with a stress ratio of 0.1 while T-joints were fatigue tested under constant amplitude fully reversed bending load with a stress ratio of -1. The nominal stress approach was used for fatigue strength evaluation of butt and fillet welds whereas the effective notch stress approach was used in case of T-joints. Relative effectsof the main parameters such as residual stress and weld toe geometry influencing fatigue strength of welds were evaluated. Residual stresses were measured using X-ray diffraction for as-welded and HFMI treated welds. Neutron diffraction was additionally used to investigate the near surface residual stress distribution in 1300 MPa LTT welds.Results showed that use of LTT consumables increased fatigue strength of welds in steels with yield strengths ranging from 650-1021 MPa. For butt welds, the vii characteristic fatigue strength (FAT) of LTT welds at 2 million cycles was up to46% higher when compared to corresponding welds made with conventional fillermaterials. In fillet welds, a maximum improvement of 132% was achieved when using LTT wires. The increase in fatigue strength was attributed to the lower tensile residual stresses or even compressive stresses produced close to the weldtoe in LTT welds. Weld metals with martensite transformation start temperatures around 200 °C produced the highest fatigue strength. In 1300 MPa yield strength steel, similar FAT of 287 MPa was observed for LTT welds and 306 MPa for conventional welds, both much higher than the IIW FATvalue of 225 MPa. The relative transformation temperatures of the base and weldmetals, specimen geometry and loading type are possible reasons why the fatigue strength was not improved by use of LTT wires. Neutron diffraction showed that the LTT consumable was capable of inducing near surface compressive residual stresses in all directions at the weld toe. It was additionally found that there arevery steep stress gradients both transverse to the weld toe line and in the depth direction, at the weld toe. Due to difficulties to accurately measure residual stresses locally at the weld toe most often in the literature and recommendations residual stresses a few millimetre away from the weld toe are related to fatigue properties. However, this research shows that caution must be used when relating these to fatigue strength, in particular for LTT welds, as stress in the base materiala few millimetre from the weld toe can be very different from the stress locally at the weld toe.HFMI increased the mean fatigue strength of conventional welds in 1300 MPa steels about 26% and of LTT welds by 13%. It increased the weld toe radius slightly but produced a more uniform geometry along the treated weld toes. Large compressive residual stresses, especially in the longitudinal direction, were introduced adjacent to the weld toe for both LTT and conventional treated welds. It was concluded that the increase in fatigue strength by HFMI treatment is due to the combined effect of weld toe geometry modification, increase in surface hardness and introduction of compressive residual stresses in the treated region.It was concluded that the residual stress has a relatively larger influence than the weld toe geometry on fatigue strength of welds. This is based on the observation that a moderate decrease in residual stress of about 15% at the 300 MPa stress level had the same effect on fatigue strength as increasing the weld toe radius by approximately 85% from 1.4 mm to 2.6 mm, in fillet welds. Also, a higher fatigue strength was observed for HFMI treated conventional welds compared to as welded samples having similar weld toe radii but with different residual stresses.
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