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The use of materials in increasingly severe service conditions raises concerns about structural safety with respect to cleavage fracture. There are three main material-related challenges that structures face under harsh environments: 1) the trade-off between strength and toughness; 2) the ductile-to-brittle transition behaviour of BCC high strength steels; 3) the inhomogeneous microstructures found in multiphase steels, thick-section steels, and welded structures. Therefore, the objective of this research is to systematically investigate the cleavage fracture micromechanisms in high strength steels considering diverse microstructures (e.g., as-received commercial steel, thermally simulated heat-affected zones, and grain refined microstructure) and experimental conditions (e.g., plastic constraint and temperature). Thereby, this study provides a thorough understanding of the effect of the microstructural details on cleavage fracture behaviour of high strength steel structures allowing for failure control and improvement of cleavage-resistant steel’s design...

High Strength Steels (HSS) are widely used in the automotive industry to reduce the vehicle’s weight and improve fuel efficiency. The press hardening process is used, for instance, to form and harden low-alloyed steel simultaneously. A deep understanding of the interfacial phenomena and the friction behavior at high temperatures is significant in describing the process, especially when considering Finite Elements (FE) analysis. In this paper, the results of a series of tensile friction tests carried out with aluminum-silicon coated low alloyed steel 22MnB5 for different values of drawing speed, temperature, and die pressure are investigated. All tests were conducted by a special test rig designed by the authors. Following the Surface Response Methodology approach, a Central Composite Design was used to identify the best fitting friction model that approximates the friction coefficient behavior depending on the main testing parameters. The identified model can explain up to 88% of the variability of the response variable and predict the friction coefficient with acceptable error. In conclusion, a FE multi-physical model of the tensile friction test, which combines a structural and a thermal analysis, was created and validated by LS Dyna software simulations.

The use of ultra-high-strength steels (UHSS) has been growing in recent years, mainly in the automotive industry. Since these steels have high strength and hardness, more applied stresses are required to deform them, probably also impacting friction behaviour. In this article, a variation in the process parameters commonly observed in sheet-metal forming, such as contact pressure, sliding speed, lubrication and working temperature was performed. The material used was TRIP1000. These process parameters were varied, aiming to investigate the friction-coefficient behaviour; however, it was observed that there were no significant variations, indicating that the steel hardness may have contributed to this. Another finding is that, even if the lubricant did not change the average value of the friction coefficient, it contributed to a more stable process, favouring the absence of premature wear of the tools.

The heat-affected zone (HAZ) is an unmelted region of a welded joint that has changed in material properties because of high temperatures during the welding process. HAZ has a lower strength than the base (parent) material (BM) and the weld metal (WM). The lower material strength is more significant if BM is made of high strength steel and the undermatching filler metal is used. Therefore, the constitutive model of HAZ is essential for predicting the mechanical behaviour of the welded joint. In this paper, a method for determining the true stress–strain relationship of HAZ is proposed. The effect of the transverse constraint on the longitudinal deformation of HAZ imposed by BM and/or WM is eliminated by a linear modification factor correlating to the true strain. Standard tensile coupon tests were used to obtain the constitutive model of HAZ using digital image correlation (DIC). The modification factor proposed for reducing the true stress is calibrated based on finite element analysis (FEA). The modified constitutive model of HAZ is validated against the experimental results obtained by DIC. The validated HAZ material property can be used in advanced numerical simulation of welded joints. publishedVersion

Hydrogen absorption into steel during atmospheric corrosion has been of a strong concern during last decades. It is technically important to investigate if hydrogen absorbed under atmospheric exposure conditions can significantly affect mechanical properties of steels. The present work studies changes of mechanical properties of dual phase (DP) advanced high strength steel specimens with sodium chloride deposits during corrosion in humid air using Slow Strain Rate Test (SSRT). Additional annealed specimens were used as reference in order to separate the possible effect of absorbed hydrogen from that of corrosion deterioration. Hydrogen entry was monitored in parallel experiments using hydrogen electric resistance sensor (HERS) and thermal desorption mass spectrometry (TDMS). SSRT results showed a drop in elongation and tensile strength by 42% and 6%, respectively, in 27 days of atmospheric exposure. However, this decrease cannot be attributed to the effect of absorbed hydrogen despite the increase in hydrogen content with time of exposure. Cross-cut analysis revealed considerable pitting, which was suggested to be the main reason for the degradation of mechanical properties.

Lazer uygulamaları, gün geçtikçe kullanımı yaygınlaşan, mühendislikten tıbba kadar geniş yelpazede kendine yer bulan, yüksek teknoloji içeren işlemlerdir. Metal endüstrisinde ise lazer; kesme, yüzey işleme, sertleştirme, kaplama ve kaynak işlemlerinde sıklıkla kullanılmaktadır. Bu çalışmada, lazer kaynağının diğer kaynak türlerine göre önemli avantajlarını ortaya koymak için literatürdeki mevcut çalışmalar incelenmiş ve analiz edilmiştir. Günümüzde otomotiv sanayi için büyük önem arz eden yüksek mukavemetli çeliklerin birleştirilmesi işleminde lazer kaynağı kullanılmaktadır. Yüksek mukavemetli çeliklerin birleştirme işlemi esnasında yaşanan bazı sorunlar vardır ve bu sorunları gidermek için lazer kaynak parametrelerinin optimizasyonu oldukça önemlidir. Bu çalışmada, önceki araştırmacıların yapmış olduğu yüksek mukavemetli çeliklerin kaynak işleminde işlem parametreleri, işlemde oluşan kusurlar, içyapılar, deneysel çalışmalar irdelenmiştir. Laser applications are high-tech processes that are becoming widespread day by day and find a place in a wide range from engineering to medicine. Laser in metal industry; It is frequently used in cutting, surface treatment, hardening, coating and welding processes. In this study, in order to reveal the important advantages of laser welding over other welding types, published researchs in the literature have been examined and analyzed. Laser welding is used in the assembly of high strength steels, which are of great importance for the automotive industry today. There are some problems experienced during the joining process of high strength steels and optimization of laser welding parameters is very important to solve these problems. In this study, the process parameters in the welding process of the high strength steels made by the previous researchers, the defects in the process, microstructure of material, and experimental studies are reviewed. Durmazlar Makine A.Ş.

The integration of forming in the fatigue modelling of cold-formed components significantly improves the predictive accuracy of the estimated life. The current study investigated the fatigue behaviour of a bent specimen made from a 5 mm thick, S900MC high strength steel plate. Because of its superior static and dynamic strength, this grade is progressively used for hollow cold-formed sections in mobile applications. However, it exhibits a strong stress saturation as well as limited formability. In this regard, a finite element modelling framework was adopted from previous research and further developed to integrate bending in the fatigue modelling and life estimation procedure. However, this framework currently ignores the possible influence of kinematic hardening and associated Bauschinger effect. For this reason, a numerical study was performed that compares isotropic with kinematic hardening for this specific application. First, the characteristic behaviour of these models was verified in a virtual tension-compression test. Subsequently, they were implemented in forming simulation followed by fatigue loading. Herein, the stress-strain evolution was investigated and a multi-axial fatigue criteria was used to map the sensitivity of the estimated life to the type of hardening. In general, the stress that entered the fatigue calculation was at least 21% lower for the kinematic model. As a result, a significant increase of 65% was observed for the estimated fatigue life, yielding a better comparison with experimental data.

The present study reports on results of a completed research project, which investigated amongst others the fatigue strength of weldments made of structural steels S355J2+N, S690QL and S960QL and treated with the post-weld treatment process High Frequency Mechanical Impact (HFMI). Reference tests on as-welded (AW) specimens, which were manufactured from the same material batches, were carried out in order to enable a direct comparison. Main goal of the present tests series was, apart from the revalidation of the higher efficiency of HFMI on high strength steels, to investigate possible scale effects, when the post-weld treatment method is applied on real size structural components. Therefore, usual fatigue test specimens and real size structural beams HEA 260 were tested. The welded structural detail of transverse stiffeners was investigated in all cases. All results are evaluated with the nominal stress approach and they are presented in S-N curves along with the respective FAT classes, which are proposed by the design guidelines of Eurocode and the recommendations of International Institute of Welding (IIW). Beach mark tests were carried out on HFMI-treated small scale fatigue test specimens, in order to investigate the influence of HFMI on the fatigue crack growth behaviour (FCG). In all cases, a significant increase in fatigue strength was achieved by the application of HFMI with the fatigue life extension being more significant for the high strength steels. No scaling effect was documented regarding the application of the method on the structural beams having similar plate thicknesses as the small scale specimens but more complex geometries and significantly longer weldments. Finally, the evaluation of the beach mark tests and their comparison with reference curves found in literature did not allow a clear assessment of the HFMI influence on the fatigue crack propagation (FCP).

Due to very favourable combination of properties, high strength steels are of great importance for numerous industrial applications where lightweight designs are of interest, such as automotive/mobility industry. Material modelling and simulation are an integral part of fast and integrated product development in these areas, so that detailed knowledge of materials’ cyclic stress-strain and fatigue behavior and corresponding properties/parameters, as early in the product design process as possible, becomes ever more important. Consequently, various existing methods for estimation of cyclic/fatigue parameters from monotonic properties are used, and new ones are developed. With rare exceptions such as method of variable slopes and modified uniform material law where high strength steels and sheet steels are treated separately, most estimation methods have been developed and evaluated using data on wide range of metallic materials or on rather general groups such as unalloyed, low-alloy and high-alloy steels. As previous evaluation of estimation methods performed on a representative HSLA steel has shown, this may result in significant results averaging and estimation errors. In this work, several methods for estimation of cyclic and fatigue parameters have been analyzed and evaluated with regard to their applicability to high strength steels. Using error criterion i.e. fraction of data points falling within a predefined scatter bands, performance of individual methods has been evaluated in low-cycle and high-cycle fatigue regime separately. Comparison is provided based on estimation results obtained for more general groups of steels (all steels, unalloyed, low-alloy, high-alloy steels). Notable differences could be observed and are reported regarding the applicability of analyzed methods for low- and high-cycle fatigue regimes and for selected subgroup of high strength steels.

This paper reports an experimental and numerical study of the cross-sectional behaviour of cold-formed high-strength steel (HSS) square and rectangular hollow section (SHS and RHS) members. Six stub column tests, six 3-point and three 4-point bending tests have been carried out, followed by the development of accurate finite element (FE) models and a parametric study. Based on the test and FE results, the design rules for slender cross-sections in European, American, Australian and Chinese standards and the GSRM method have been assessed via reliability analyses. A new equation for deriving the cross-sectional slenderness of SHS and RHS considering the influence of corner radius and new area reduction factors for designing slender cold-formed S900 (and above) SHS and RHS have also been proposed

Son dönemlere dayanımı daha yüksek ve hafif olan yüksek mukavemetli çelik malzemelerin otomotiv sektöründeki kullanımı yükselen bir trenle artış göstermektedir. Bu nedenle düşük mukavemetli malzemelerde yaşanılan kaynak problemlerinden yola çıkarak yüksek mukavemetli malzemelerde aynı sorunları yaşamamak adına araştırmalar yapılmıştır. Bu çalışmada 980MPa ve 1180MPa yüksek mukavemetli ve çift fazlı olan iki çeşit çelik sac kullanılmış ve robot teknolojisi kullanılarak direnç spot kaynağı uygulamaları yapılmıştır. Malzemelerin öncelikle mikro analizleri yapılarak malzeme karakterizasyonları, mekanik ve kimyasal özellikleri incelenmiştir. Kaynak kalitesini arttırmak için az deneme ile daha iyi sonuçlar almayı hedefleyen Taguchi metodu kullanılarak kaynak parametrelerinin optimizasyonu amaçlanmıştır.

The industrial application of high-strength low-alloyed steel (HSLA) in welded structures has increased the demand for understanding fracture behavior and structural integrity assessment of this type of steel and produced welded joints. The aim of this paper is to simulate the experimental evaluation of the fracture mechanics specimens by using the micromechanical model. The investigation is performed on two standard single edge notch bend (SENB) specimens with imposed crack in the central region. Numerical analysis was carried out by using Simulia Abaqus software package on 2D models used to simulate the damage development on the local level. The comparison between numerical and experimental results is presented through measured values of J-integral, load-line displacement νLL and crack growth resistance (J-Δа) curves. This paper shows that numerical simulations are promising in respect to their accuracy. The application of this model enables to decrease the amount of expensive experiments for determination of the load level that causes crack propagation.

This paper demonstrates that the instrumented indentation test (IIT), together with a trained artificial neural network (ANN), has the capability to characterize the mechanical properties of the local parts of a welded steel structure such as a weld nugget or heat affected zone. Aside from force-indentation depth curves generated from the IIT, the profile of the indented surface deformed after the indentation test also has a strong correlation with the materials&rsquo, plastic behavior. The profile of the indented surface was used as the training dataset to design an ANN to determine the material parameters of the welded zones. The deformation of the indented surface in three dimensions shown in images were analyzed with the computer vision algorithms and the obtained data were employed to train the ANN for the characterization of the mechanical properties. Moreover, this method was applied to the images taken with a simple light microscope from the surface of a specimen. Therefore, it is possible to quantify the mechanical properties of the automotive steels with the four independent methods: (1) force-indentation depth curve, (2) profile of the indented surface, (3) analyzing of the 3D-measurement image, and (4) evaluation of the images taken by a simple light microscope. The results show that there is a very good agreement between the material parameters obtained from the trained ANN and the experimental uniaxial tensile test. The results present that the mechanical properties of an unknown steel can be determined by only analyzing the images taken from its surface after pushing a simple indenter into its surface.

This paper demonstrates that the instrumented indentation test (IIT), together with a trained artificial neural network (ANN), has the capability to characterize the mechanical properties of the local parts of a welded steel structure such as a weld nugget or heat affected zone. Aside from force-indentation depth curves generated from the IIT, the profile of the indented surface deformed after the indentation test also has a strong correlation with the materials’ plastic behavior. The profile of the indented surface was used as the training dataset to design an ANN to determine the material parameters of the welded zones. The deformation of the indented surface in three dimensions shown in images were analyzed with the computer vision algorithms and the obtained data were employed to train the ANN for the characterization of the mechanical properties. Moreover, this method was applied to the images taken with a simple light microscope from the surface of a specimen. Therefore, it is possible to quantify the mechanical properties of the automotive steels with the four independent methods: (1) force-indentation depth curve; (2) profile of the indented surface; (3) analyzing of the 3D-measurement image; and (4) evaluation of the images taken by a simple light microscope. The results show that there is a very good agreement between the material parameters obtained from the trained ANN and the experimental uniaxial tensile test. The results present that the mechanical properties of an unknown steel can be determined by only analyzing the images taken from its surface after pushing a simple indenter into its surface.

In this work, the objective was to correlate anisotropy with the mechanical and microstructural behavior of two high strength steels used in vehicle production in order to obtain variables to alter future manufacturing processes in order to obtain steels with a lower elastic return, known as springback effect, which means geometric changes suffered by the part at the end of the plastic deformation process, after the release of the forces applied by the stamping tool and this causes dimensional failures that compromise the production. In this research, biphasic steel and one low carbon steel were evaluated, being Docol_DL800 and LC200 their trade names, respectively. Tensile tests and three-point bending in air were performed to determine the mechanical properties and behaviors and these results were compared with those obtained by computational simulation using the finite element method and were also correlated with microstructural data from the backscattered electron technique. The results indicate that biphasic steel has a higher hardening allowing a greater springback effect due to its microstructure with ferrite and martensite, high grain refinement, greater amount of elastic residual energy and a lower degree of disorientation after mechanical conformation, creating the effect Bauschinger. Already the steel LC200 presented a smaller degree of springback thanks to the more isotropic hardening due to the increase in the degree of grain disorientation after the conformation. Thus, it was concluded that the manufacturing processes of these steels should seek to combine high mechanical strength with a more isotropic behavior. En este trabajo, el objetivo era correlacionar la anisotropía con el comportamiento mecánico y microestructural de dos aceros de alta resistencia utilizados en la producción de vehículos para obtener variables que alteren los procesos de fabricación futuros con el fin de obtener aceros con un retorno elástico más bajo, conocido como efecto de recuperación elástica, que significa cambios geométricos sufridos por la parte al final del proceso de deformación plástica, después de la liberación de las fuerzas aplicadas por la herramienta de estampado y esto causa fallas dimensionales que comprometen la producción. En esta investigación, se evaluaron el acero bifásico y uno de bajo contenido de carbono, siendo Docol_DL800 y LC200 sus nombres comerciales, respectivamente. Se realizaron pruebas de tracción y flexión de tres puntos para determinar las propiedades y comportamientos mecánicos y estos resultados se compararon con los obtenidos por simulación computacional utilizando el método de elementos finitos y también se correlacionaron con datos microestructurales de la técnica de electrones retrodispersados. Los resultados indican que el acero bifásico tiene un mayor endurecimiento permitiendo un mayor efecto de recuperación elástica debido a su microestructura con ferrita y martensita, alto refinamiento de grano, mayor cantidad de energía residual elástica y un menor grado de desorientación después de la conformación mecánica, creando el efecto Bauschinger. Ya el acero LC200 presentaba un menor grado de recuperación elástica gracias al mayor endurecimiento isotrópico debido al aumento en el grado de desorientación del grano después de la conformación. Por lo tanto, se concluyó que los procesos de fabricación de estos aceros deberían tratar de combinar una alta resistencia mecánica con un comportamiento más isotrópico. Neste trabalho, o objetivo foi correlacionar a anisotropia com o comportamento mecânico e microestrutural de dois aços de alta resistência utilizados na produção de veículos a fim de obter variáveis para alterar futuros processos de fabricação de forma a obter aços com um menor retorno elástico, conhecido como efeito springback, que significa alterações geométricas sofridas pela peça ao final do processo de deformação plástica, após a liberação das forças aplicadas pela ferramenta de estampagem e isso ocasiona falhas dimensionais que comprometem a produção. Nesta pesquisa, foram avaliados um aço bifásico e um aço baixo carbono, sendo Docol_DL800 e LC200 os seus nomes comerciais, respectivamente. Foram realizados ensaios de tração e flexão de três pontos ao ar para determinar as propriedades e comportamentos mecânicos e tais resultados foram comparados com os obtidos por simulação computacional utilizando o método de elementos finitos e foram também correlacionados com os dados microestruturais provenientes da técnica de elétrons retroespalhados. Os resultados indicam que o aço bifásico tem um maior encruamento permitindo um maior efeito springback devido à sua microestutura com ferrita e martensita, alto refinamento dos grãos, maior quantidade de energia residual elástica e um menor grau de desorientação após a conformação mecânica, criando o efeito Bauschinger. Já o aço LC200 apresentou um menor grau de springback graças ao encruamento mais isotrópico devido ao aumento no grau de desorientação dos grãos após a conformação. Assim, concluiu-se que os processos de fabricação desses aços devem buscar aliar alta resistência mecânica com um comportamento mais isotrópico.

In the last decades, significant improvements regarding the design, materials and technology of rock drills have been made. Likewise, in sheet metal forming, forming tools experience very high contact pressures when processing high strength steel sheets. In both applications components operate under extremely tough contact conditions that result in an accelerated component failure. Enhancements on mechanical properties of components material subjected to extreme contact conditions are highly required in order to withstand the application loads and prevent severe wear. The present thesis was focused on understanding of machinery component damage mechanisms under severe contact conditions. A case study of worn components used in rock drilling and sheet metal cold work was carried out. Thread joints from rock drilling and punches from sheet metal pressing were selected for the investigation. For these components, sliding contact under high contact pressure is a common load condition under the components usage. Then, to understand and quantify the influence of contact parameters, load and surface quality on material performance, laboratory simulations were performed. The results were used for a comparative analysis of the typical damage mechanisms observed in the tests and the case study of the components. The case study results showed that the threaded surfaces underwent severe plastic deformation due to the high-pressure sliding contact. The microstructure beneath the worn surface was altered and surface cracks and delamination were frequently observed at the worn surface. The dominant damage mechanism found on the investigated punches was adhesive wear. Material transfer adds friction stresses at the punch surface and ultimately, with repeated punch strokes, it leads to initiation and propagation of fatigue cracks. Wear process in thread joint and punch wear was simulated using the SOFS. The worn specimens tested experimentally showed similar wear mechanisms obtained in the case study. The thread joint wear simulation showed that the total damage at the worn surface was a result of adhesive wear, plastic deformation, and initiation and propagation of fatigue cracks. In addition, the results showed that the type of motion had a significant influence on the worn volume and crack initiation, and more severe wear was observed at reciprocal motion. The punch wear simulation showed that the friction quickly increased as work material from metal sheets transferred to the disc surface. The rate of the material transfer was strongly dependent on the combination of sheet material and tool steel. Further, the present experimental simulations were applicable to characterize and predict wear of components in the application. Components used in rock drilling and sheet metal forming operate under harsh contact conditions that result in an early-life component failure. Wear and fatigue are considered as the most common damage mechanism for these components. Commonly, the service life of a component is designed based on its fatigue life. However, wear might have a significant effect on the components life too. Wear results in a surface damage that in turn may cause a fatigue crack initiation. Therefore, knowledge about wear of materials and components is a key factor in design and prediction of the lifetime of the components. In order to predict wear of a certain component, a thorough understanding of the component with regards to its material properties, application loads and working environment, and damage mechanisms is required. The overall aim of the present work was to define the typical wear mechanisms occurred on machinery components used in rock drilling and sheet metal forming. A comparative analysis of the case studies and results from performed laboratory tests simulated wear mechanisms in the applications highlighted wear mechanisms and factors influencing severity of wear in the applications. Obtained information is crucial for ranking and selection of the best material in the applications. The presentation will will be via zoom. PhD student will together with the supervisors will be in Karlstad while the opponent is in Luleå.

This thesis is a part of Ascometal project, which aims at guiding the development of new high strength steels used foroffshore drilling parts in oil and gas domain. The aim of the project is to predict the ductile-to-brittle transition curve in Charpyimpact tests by considering microstructural parameters. The main two objectives of this study are: improving the knowledge of the link between the microstructure and fracture properties and building a microstructurally-based predictive tool for the impacttoughness. The chosen scientific approach includes four steps described as follows: 1) building of a database of six microstructures. These microstructures were quantitatively characterized at different scales. 2) The elastic-plastic behavior was studied by varying the test temperature, the strain rate, the sampling direction and the tempering temperature. 3) The fracture behavior was studied by using an instrumented Charpy impact test in a test temperature range of -196°C to +60°C. 4) Establishment of the links between microstructure and fracture properties. Mechanical analysis by the finite element method was used in some cases in order to determine the local stress. In this work we discuss the effect of strain rate, the test temperature, the sampling direction and the tempering temperature on the elastic-plastic behavior of steels containing martensite and upper bainite. Fractography was used to characterize the fracture mechanism in the ductile region, in the transition and in the brittle domains. The effect of tempering of ductile fracture damage is also discussed. The local approach to fracture is adopted for the purpose of determining the critical cleavage stress at -196°C. A quantitative link is established between the critical fracture stress and the carbide size. The effect of the upper bainite on the cleavage fracture stress is also discussed. Finally, a microstructurally-based global model for impact toughness is proposed in this thesis. The model takes the intercarbide spacing and the fracture appearance transition temperature into account to describe the instrumented Charpy impact curve. Complementary studies have to be made in order to deeply understand the dependence of the fracture appearance temperature on the austenite grain size, the packet size and the upper bainite fraction; they will make the global model for impact toughness more robust.; Cette thèse s’inscrit dans un projet dans lequel Ascometal souhaite améliorer le développement de nuances d’aciers à haute résistance destinées à la fabrication des pièces de forage offshore. Le projet vise notamment la prédiction, au moyen de la microstructure, de la courbe de transition ductile - fragile en résilience. Les objectifs de la thèse sont doubles à savoir : améliorer la compréhension du lien entre microstructure et propriétés à rupture, puis mettre au point un modèle prédictif de la résilience basée sur la microstructure. Nous avons adopté une démarche scientifique en quatre étapes: 1) la construction d’une base de données constituée de six microstructures. Les microstructures ont été caractérisées quantitativement à différentes échelles. 2) La caractérisation du comportement élastoplastique en variant la température d’essai, la vitesse de sollicitation, la direction de prélèvement des échantillons et le revenu. 3) La caractérisation du comportement à rupture au moyen des essais Charpy instrumentés dans une gamme de température allant de -196°C à +60°C. 4) L’établissement des liens quantitatifs entre chacune des microstructures et leurs propriétés à rupture. Dans certains cas, le recours aux outils numériques a été adopté pour accéder aux contraintes locales et les relier aux entités microstructurales. Les résultats ont permis d’aborder les discussions autour des effets : de la vitesse de sollicitation, du revenu, de la température d’essai et la direction de sollicitation sur le comportement élastoplastique des aciers de microstructure martensite et bainite supérieure. Pour les essais à rupture, l’analyse fractographique a permis d’accéder aux mécanismes de rupture dans les domaines de rupture ductile, de transition et de rupture fragile. Les observations de l’endommagement en post-mortem sur coupe polie ont permis de discuter l’effet du revenu sur l’endommagement ductile des aciers trempés-revenus à haute résistance. Au niveau du plateau ductile, un lien quantitatif est proposé entre la distance intercarbures et l’énergie totale à rupture, il permet d’expliquer l’augmentation de l’énergie à rupture. Au niveau du plateau fragile, l’approche locale de la rupture sur des éprouvettes axisymétriques entaillées est mise à contribution pour déterminer les contraintes critiques de clivage à -196°C. Un lien quantitatif a été établi entre la contrainte critique de clivage à -196°C et la taille des carbures. L’effet de la bainite supérieure sur la contrainte critique de clivage est également abordé. Enfin, un modèle global de la résilience basé sur la microstructure est proposé. Il s’appuie sur la distance intercarbures et la température de transition en cristallinité pour prédire la courbe de transition ductile-fragile des aciers à haute résistance de l’étude. Il semble enfin indispensable d’étendre l’approche proposée sur d’autres nuances de même grade afin de disposer d’une base de données plus large, ce qui devrait rendre le modèle global robuste.

The phase composition and portion of individual phases in advanced high-strength steels (AHSS) CP1000 and DP1000 was studied by complementary microscopic and diffraction techniques. CP1000 and DP1000 steel grades have a high strength-to-density ratio and they are used in many applications in the automotive industry. The microstructure of the CP1000 &ldquo, complex phase&rdquo, steel consists of ferrite, bainite, martensite and a small amount of retained austenite. DP1000 is a dual phase steel, which has a structure of a ferritic matrix with islands of martensite and a minor amount of retained austenite. The influence of selected etchants (Nital, LePera, Beraha I, Nital followed by metabisulfite, Nital followed by LePera, and Nital followed by Beraha I) on the microstructure image is described. X-ray diffraction, neutron diffraction and light optical, scanning and transmission electron microscopy were used in this work for advanced characterization of the microstructure and phase composition. The information provided by each technique is critically compared.

Development of high strength or even ultra-high strength steels is mainly driven by the automotive industry which strives to reduce the weight of individual parts, fuel consumption, and CO2 emissions. Another important factor is to improve passenger safety. In order to achieve the required mechanical properties, it is necessary to use suitable heat treatment in addition to an appropriate alloying strategy. The main problem of these types of treatments is the isothermal holding step. For TRIP steels, the holding temperature lies in the field of bainitic transformation. These isothermal holds are economically demanding to perform in industrial conditions. Therefore new treatments without isothermal holds, which are possible to integrate directly into the production process, are searched. One way to produce high-strength sheet is the press-hardening technology. Physical simulation based on data from a real-world press-hardening process was tested on CMnSi TRIP steel. Mixed martensitic-bainitic structures with ferrite and retained austenite (RA) were obtained, having tensile strengths in excess of 1000 MPa.

V moderní éře strojírenského průmyslu neodmyslitelně patří zpracování materiálů na plechové polotovary pomocí válcování. Největším odběratelem plechových polotovarů z ocelí je automobilový průmysl. Ten se v posledních desetiletí zabývá snižováním emisních zplodin dopravních prostředků. Jednou z možností snižování emisních zplodin je využití pokročilých vysokopevných ocelí, které díky svému chemickému složení a mechanismům zpevňovaní umožňují redukovat tloušťku plechových přístřihů budoucích dílů karoserií. Na dvou moderních vysokopevných oceli s obsahem uhlíku 0,2 hm. % a legovaných manganem, křemíkem a rozdílným podílem hliníku bylo odzkoušeno převálcování ingotů na plechy o tloušťce 1,8 mm kombinací válcování za tepla a za studena. Bylo zjištěno, že je možné dosáhnout meze pevnosti přesahující 1000 MPa s tažností přes 15%. One of the essential branches of today’s engineering production involves production of sheet stock by rolling. The majority of steel sheet stock is used in the automotive industry. In recent decades, this sector has been striving to reduce vehicle emissions. One of available solutions involves the use of advanced high-strength steels whose chemical composition and strengthening mechanisms make it possible to build the car body with thinner sheet blanks than before. Two advanced high-strength steels containing 0.2 wt. % carbon and additions of manganese, silicon and different levels of aluminium were used for rolling trials in which ingots were converted into 1.8 mm sheet by combined hot and cold rolling. This procedure was found to produce strengths in excess of 1000 MPa combined elongation of more than 15%.

The use of high strength steels is increasing steadily due to the cost, weight and performance advantages they offer. As a drawback, these materials are sensitive to environmental hydrogen embrittlement processes that reduce drastically the working loads they can resist, what limits their spreading to many applications. Since this embrittlement is catalyzed by corrosion prevention strategies involving cathodic protection, seawater exposed structures are specially concerned for the attack of hydrogen. Thus, high strength steel use for wet applications must be carefully assessed. In this context, this thesis deals with the study of the hydrogen embrittlement susceptibility of industrial high strength steels for immersed service in mooring chains and bolted joints. Based on the slow strain rate tensile testing method, the influence of cathodic protection and water temperature has been studied for submerged mooring chain steel grades R4, R5 and bolt steel class 10.9. A finite difference modeling strategy for predicting the cracking pattern observed during the experimental work is proposed. El empleo de aceros de alta resistencia está en constante aumento debido a las ventajas que ofrecen en coste, peso y rendimiento. Como contrapartida, estos materiales son sensibles a procesos de fragilización ambiental por hidrógeno que reducen de forma drástica las cargas que pueden resistir en servicio, lo que limita su extensión a muchas aplicaciones. Teniendo en cuenta que esta fragilización se cataliza en presencia de estrategias de protección catódica contra la corrosión, las estructuras en contacto con agua marina están especialmente expuestas al ataque por hidrógeno. En consecuencia, el uso de aceros de alta resistencia en contacto con agua debe ser analizado con cautela. En este contexto, esta tesis estudia la susceptibilidad a la fragilización por hidrógeno de aceros de alta resistencia de uso industrial para cadenas de fondeo y uniones atornilladas sumergidas. Se ha estudiado la influencia del tipo de protección catódica y la temperatura del agua en aceros para cadena de grados R4, R5 y acero de tornillería de clase 10.9, aplicando el método de ensayo de tracción a baja velocidad de deformación. Los patrones de agrietamiento observados en el trabajo experimental se han modelizado mediante una estrategia de cálculo por diferencias finitas.

U ovom radu obrađuje se tema visokočvrstih čelika STRENX, proizvoda švedsko-američke tvrtke SSAB. Bit će spomenute i ukratko opisane pojedine vrste čelika, te nakon toga svaki visokočvrsti čelik sa njegovom primjenom i najvažnijim svojstvima. Bit će ukratko opisana i tvrtka SSAB te neke najvažnije obrade u tvrtci SSAB poput zavarivanja i rezanja. Zahvaljujem se roditeljima i sestri na pruženoj podršci i razumijevanju! This paper deals with the topic of STRENX high strength steels, products of the Swedish-American company SSAB. There will be a brief description of each type of steel, followed by each high-strength steel with its application and the most important properties. SSAB will be briefly described and some of the most important workings in SSAB such as welding and cutting. I thank my parents and sister for their support and understanding!

Delo obravnava problem vpliva protikorozijskih zaščit na visoko trdnostna jekla pri dinamičnem obremenjevanju. Opisani so tipi korozije in njihovi vplivi ter najpogostejše protikorozijske zaščite jekel v avtomobilski industriji. Cilj je bil ugotoviti ali protikorozijska zaščita z galvanskim cink-nikljanjem in barvanje s kataforezo vpliva na dobo trajanja zvarjencev. Z utrujanjem vzorcev smo ugotovili, da obstaja vpliv Zn-Ni zaščite, medtem ko je vpliv kataforeze zanemarljiv. Dobo trajanja jekel z različnimi protikorozijskimi zaščitami smo napovedovali tudi s programom FEMFAT. This master thesis discusses an impact of the anti-corrosive protection on dynamically loaded high-strength steel. The main types of corrosion, its effects, and most common anti-corrosive protection of steel in the automotive industry are described. The main objective was to see if the zinc-nickel electrodeposited coating and the cataphoresis coating affect the dynamically loaded welded specimens. The conclusion was that there is an effect of the zinc-nickel coating, however, the cataphoretic coating has no effect. The duration of the steel with various anti-corrosive protection has been predicted by the FEMFAT software.

Son dönemlere dayanımı daha yüksek ve hafif olan yüksek mukavemetli çelik malzemelerin otomotiv sektöründeki kullanımı yükselen bir trendle artış göstermektedir. Bu nedenle düşük mukavemetli malzemelerde yaşanılan kaynak problemlerinden yola çıkarak yüksek mukavemetli malzemelerde aynı sorunları yaşamamak adına araştırmalar yapılmıştır. Bu çalışmada 980MPa ve 1180MPa yüksek mukavemetli ve çift fazlı olan iki çeşit çelik sac kullanılmış ve robot teknolojisi kullanılarak direnç spot kaynağı uygulamaları yapılmıştır. Malzemelerin öncelikle mikro analizleri yapılarak malzeme karakterizasyonları, mekanik ve kimyasal özellikleri incelenmiştir. Kaynak kalitesini arttırmak için az deneme ile daha iyi sonuçlar almayı hedefleyen Taguchi metodu kullanılarak kaynak parametrelerinin optimizasyonu amaçlanmıştır. In recent times, the use of higher strength and higher strength steel materials in the automotive sector has increased with a rising trend. Therefore, researches have been carried out in order not to repeat the same problems in high-strength materials, starting from the welding problems experienced in lowstrength materials. In this study, two types of high strength and dual phase steel sheets of 980 MPa and 1180 MPa were used and resistance spot welding applications were carried out using robot technology. First of all, material characterization, mechanical and chemical properties of materials were analysed by micro analysis. Optimization of welding parameters is aimed by using Taguchi method which aims to obtain better results with fewer experiments in order to increase the welding quality.

International audience; Two 3$^{rd}$ generation steels were studied an austenitic-ferritic MnAl steel and an austenitic-martensitic quenched and partitioned (QetP) steel. Hydrogen absorption during elaboration was simulated by exposing the steels to deuterium atmosphere during their final thermal treatment. Nitrogen-exposed specimens were also deuterium cathodically charged in order to simulate hydrogen entry in the steels during an optional electrogalvanization process. Thermal desorption mass spectrometry (TDS), combined with aging at room temperature and cold rolling, was used to study diffusion and trapping. The evolution of different TDS spectral contributions with aging and cold rolling allowed educated guesses on their origin. Peaks specific to martensitic transformation were identified.

International audience; Third generation steels are multiphase steels that contain high amounts of residual austenite that can transform into martensite during deformation and provides the steel a better ductility/formability. During their final elaboration steps, they are generally heat-treated in reducing atmospheres and potentially submitted to zinc electroplating, processes during which they may absorb a significant amount of hydrogen. Hydrogen embrittlement susceptibility of these steels is therefore to be quantified. Two 3rd generation steels have been studied an austenitic-ferritic MnAl steel and an austenitic-martensitic quenched and partionned (QetP) steel. Both were elaborated and finally heat treated under controlled reducing atmosphere (high temperature HT), using deuterium as isotopic tracer for hydrogen. Hydrogen introduction in the material during zinc plating was simulated by cathodic charging (low temperature LT). Thermal desorption mass spectrometry (TDMS) was used to study diffusion and trapping evolution in such complex materials; the localization of deuterium in the different phases was studied by nanoSIMS. The effect of hydrogen/deuterium on mechanical properties was investigated by crossed tensile tests and TDMS. Aging after deuterium charging followed by TDMS evidenced for MnAl and QetP alloys two contributions (570 K and 630 K) assigned to low energy trap sites. Besides, the QetP alloy exhibits an extra contribution around 730 K, whose assignment will be discussed with respect to nanoSIMS analysis. Modelling of the TDMS spectra, taking into account the microstructure of the materials and their history in terms of hydrogen exposure and aging, will give elements for the discussion.Finally, based on tensile tests and fractography analysis, the effect of deuterium or hydrogen on the mechanical behavior of these materials will be discussed in terms of hydrogen origin (HT or LT), concentration and localization.

Vysoce pevné oceli označované zkratkou TRIP (tranformation-induced plasticity) se pro své jedinečné vlastnosti (vysoká pevnost a tažnost, relativně nízká hmotnost) staly vyhledávaným materiálem zejména v kontrukci deformačních zón automobilů. Struktura TRIP ocelí obsahuje metastabilní zbytkový austenit, který v průběhu plastické deformace tranformuje na martenzit a přispívá tak k deformačnímu zpevnění a rovnoměrnému prodloužení. Právě množství a stabilita zbytkového austenitu má zásadní vliv na výsledné mechanické vlastnosti těchto materiálů. Stabilita austenitu závisí na jeho chemickém složení, předchozí deformaci, velikosti zrna a morfologii. V minulosti bylo hodnocení stability austenitu prováděno prostřednictvím makroskopických tahových zkoušek, které však mohly poskytnout informaci pouze o vlastnostech materiálu jako celku. V poslední době vznikají vědecké práce zaměřené na testování jednotlivých austenitických zrn, případně ostatních fází obsažených v TRIP ocelích pomocí nanoindentačního měření, které bývá obvykle doplněno elektronovou mikroskopií a difrakční (EBSD) nebo spektrometrickou (EDS) analýzou. Tímto způsobem lze detailněji zmapovat a vysvětlit jednotlivé procesy, ke kterým ve struktuře v průběhu deformace dochází. Tento text si klade za cíl shrnout základní aspekty nanoindentace TRIP ocelí. PING 2018 Junior je pořádána s podporou prostředků na specifický vysokoškolsky výzkum projektu SVK2-2018004 TRIP steels are high strenght steel used mainly in automotive industry for their remarkable properties (hight strength and elongation, relatively light weight). The microstructure of TRIP steels contains retained austenite which transforms to martensite during plastic deformation. The transformation improves work hardening properties and contributes to uniform elongation. The amount and stability of retained austenite determine the mechanical propetries of the steel. In the past tensile test used to be the dominant method of testing mechanical stability of austenite. In recent years nanoindentation has become widely used technique of TRIP steel research which is capable of testing very small volume of material and provides us detailed information on properties of particular phases. In combination with other experimental techniques (microscopy, EDS, EBSD) nanoindentation is an advantageous method to study and explain the mechanisms of deformation. The aim of this article is to summarize the basics of TRIP steels nanoindentation.

High strength steels (HSS) used in highly abrasive environments, such as in mining and crushing, must endure high stress abrasion. To properly understand the wear behavior of materials under such circumstances, the connection between surface loading, work hardening, and material removal has first to be determined. In this study, wear resistant steels with initial hardness ranging from 400 to 750 HV were investigated in single-grit abrasion. In the cyclic abrasion experiments, the abrasion resistance of the steels was improved noticeably from the initial state due to surface hardening. However, the highest surface hardening rate did not result in the highest wear resistance. Moreover, when the surface loading was sufficiently increased, the transition to a high wear rate mechanism was observed. acceptedVersion

The axial behaviour and design of hot-finished high strength steel tubular elements with internal prestressing cables, representing the chord members in prestressed trusses, are examined in this paper. A comprehensive programme of experiments and numerical modelling was performed, the results of which were combined to develop resistance expressions for the design of prestressed high strength steel members. A total of 12 tensile and 10 compressive member tests were carried out, with the key variables examined being the steel grade (S460 and S690), the initial prestress level and the presence of grout. Numerical models were developed to replicate the structural response of the compressive member tests and subsequently used to generate parametric results, where the member slenderness, size of prestressing cable, applied prestress level, steel grade (S460 or S690), and grout condition (grouted and non-grouted), were varied. The presence of cables was shown to enhance the tensile capacity of the members, while the addition of prestress resulted in extended elastic range. In compression, the effect of prestress was detrimental, and a modified Perry-Robertson method, developed in [1], was extended to hot-finished high strength steel members. The research leading to these results has received funding from the Research Fund for Coal and Steel (RFCS) under grant agreement No. RFSR CT 2012-00028. V&MDEUTSCHLAND GMBH is acknowledged for the supply of the test specimens.

It is well-known from the literature that an interaction effect on crack propagation arises when a specimen or a component is subjected to variable amplitude loading. In dependence of the applied load sequence, a certain amount of retardation or acceleration onto fatigue crack growth rate can then be observed if compared to the constant amplitude case. In the case of structural ductile materials, the interaction phenomenon is mainly addressed by the local plasticity at the crack tip and can be explained, from a global point of view, by adopting the crack closure concept. Considering the applicative case of railway axles, a good correlation between crack growth interaction effects under variable amplitude loading and the amount of plasticity-induced crack closure has been previously derived by the authors, relatively to the standardized EA1N steel. The other standardized European steel for railway axles, a 25CrMo4 grade named EA4T, is instead considered in the present research, as well as an high strenght steel grade. An experimental campaign was carried on this material, using SE(T) specimens, in order to understand and quantify the interaction effects arising from relevant load sequences derived from service. Firstly, tests were performed directly applying the acquired load time history. Eventually, the load history was transformed into an equivalent block loading sequence and applied to different specimens varying the number of cycles of each single block. Finally, the experimental outcomes were modeled adopting both a strip yield model and a simple no-interaction approach, in order to quantify the possible interaction effects. The modeling was carried out considering different experimental techniques for deriving the crack growth and threshold behaviors of the material, i.e. the traditional ΔK-decreasing technique and the compression pre-cracking one.

Car manufacturing was always the key industry behind sheet metal forming thus the requirements and developments in car manufacturing have decisive role in the development of sheet metal forming, too. The automotive industry faces with very contradictory demands and requirements: better performance with lower consumption and lower harmful emissions, more safety and comfort that are hardly available simultaneously with conventional materials and conventional manufacturing processes. These requirements are the main driving forces in the automotive industry and in the material and technological developments in sheet metal forming, as well. During the recent years significant developments can be observed in the application of high strength steels. In this respect the application of various dual-phase steels is one of the best examples to meet these contradictory requirements. However, the application of these high strength steels often leads to formability problems. Among them the springback occurring after sheet metal forming should be mentioned first. In this paper, some recent results will be given obtained by using the new device together with a sensitivity analysis done by numerical simulation.

Failure in sheet metal structures of ductile material is usually caused by one of, or a combination of, ductile fracture, shear fracture or localised instability. In this paper the failure of the high strength steel Docol 600DP and the ultra high strength steel Docol 1200M is explored. The constitutive model used in this study includes plastic anisotropy and mixed isotropic-kinematic hardening. For modelling of the ductile and shear fracture the models presented by Cockroft–Latham and Bressan–Williams have been used. The instability phenomenon is described by the constitutive law and the finite element (FE) models. For calibration of the failure models and validation of the results, an extensive experimental series has been conducted including shear tests, plane strain tests and Nakajima tests. The geometries of the Nakajima tests have been chosen so that the first quadrant of the forming limit diagram (FLD) were covered. The results are presented both in an FLD and using prediction of force–displacement response of the Nakajima test employing element erosion during the FE simulations. The classical approach for failure prediction is to compare the principal plastic strains obtained from FE simulations with experimental determined forming limit curves (FLCs). It is well known that the experimental FLC requires proportional strains to be useful. In this work failure criteria, both of the instability and fracture, are proposed which can be used also for non-proportional strain paths.

Чукин Михаил Витальевич, д-р техн. наук, профессор, заведующий кафедрой технологий обработки материалов, первый проректор – проректор по научной и инновационной работе, Магнитогорский государственный технический университет им. Г.И. Носова, г. Магнитогорск; m.chukin@mail.ru. Полецков Павел Петрович, д-р техн. наук, профессор кафедры технологий обработки материалов, Магнитогорский государственный технический университет им. Г.И. Носова, г. Магнитогорск; pavel_poletskov@mail.ru. Набатчиков Дмитрий Геннадьевич, канд. техн. наук, заместитель директора ООО «Инжиниринговый центр Термодеформ-МГТУ, г. Магнитогорск; nabat4ikov@mail.ru. Емалеева Динара Гумаровна, канд. техн. наук, доцент кафедры технологий обработки материалов, Магнитогорский государственный технический университет им. Г.И. Носова, г. Магнитогорск; emaleevadg@mail.ru. Гущина Марина Сергеевна, магистрант кафедры технологий обработки материалов, Магнитогорский государственный технический университет им. Г.И. Носова, г. Магнитогорск; marina.mgn.89@mail.ru. Кузнецова Алла Сергеевна, научный сотрудник научно-исследовательского сектора, Магнитогорский государственный технический университет им. Г.И. Носова, г. Магнитогорск; allakuznetsova.mgtu@ mail.ru. M.V. Chukin, m.chukin@mail.ru, P.P. Poletskov, pavel_poletskov@mail.ru, D.G. Nabatchikov, nabat4ikov@mail.ru, D.G. Emaleeva, emaleevadg@mail.ru, M.S. Gushchina, marina.mgn.89@mail.ru, A.S. Kuznetsova, allakuznetsova.mgtu@mail.ru Nosov Magnitogorsk State Technical University, Magnitogorsk, Russian Federation Выполнен анализ технических требований, предъявляемых к ультрахладостойкому листовому прокату по выбранным направлениям наукоемких отраслей промышленности. Определены возможные потребители разрабатываемой продукции и конкуренты, предлагающие аналогичную продукцию. Проведен анализ показателей качества, установленных для аналогичной продукции в технических документах российских и зарубежных организаций. Установлен достигнутый уровень свойств данного вида проката в России и за рубежом. The paper presents an analysis of technical requirements for ultra-low temperature resistant steels, which are used in high-tech industry areas. The potential consumers of the developed products are determined and alternative manufacturers are described. Metal quality characteristics that are demanded by technical documents of Russian and foreign organizations for similar products are analyzed. Currently achieved level of properties of this kind of rolled products is also described. Работа проведена при финансовой поддержке Минобрнауки России в рамках реализации комплексного проекта по созданию вы- сокотехнологичного производства, выполняемого с участием российского высшего учебного заведения (договор 02.G25.31.0235).

Capitulo de Libro, Advanced High Strengh Steel (AHSSs) are mateials with carefully designed chemical compositions and multi-phase microstructures resulting from precisely controlled cold Rolling and annealing processes. Mircrostrucures can be tailored to meet specific requirements of automotive parts, giving rise to the diffeerent AHSSs generations. The first generation of AHSSs includes steels with ferrite-based microstructures, i.e., dual phase (DP), transformation induced plasticity (TRIP), complex-phase (CP) and martensitic (MS) steels. The second generation…

Bu çalışmada, ileri teknoloji yüksek mukavemetli çeliklerde uygulanan ön deformasyon ve fırında sertleştirme işlemlerinin çeliklerin mekanik özellikler üzerindeki etkilerine yönelik araştırmalar gerçekleştirildi. Gerçekleştirilen çalışmalar neticesinde elde edilen verilerden yüksek mukavemetli çeliklerin gerilme birim deformasyon eğrileri incelendiğinde akma eğrisinin deformasyon oranına bağlı olduğu görülmektedir. Deformasyon oranının artmasıyla malzemenin akma dayanımı artarken şekillendirilebilme kabiliyeti azalmıştır. Ön deformasyonla birlikte bekleme süresi ve farklı sıcaklık parametrelerde gerçekleştirilen fırında sertleştirme işlemiyle birlikte çift fazlı numunelerin akma ve çekme dayanımlarında artma görülürken, % uzama değerlerinde azalma olmuştur. Elde edilen analiz sonuçları ve verilen şekiller incelendiğinde genel itibari ile yüksek mukavemetli çeliklere (DP 600/TRIP 800/TWIP 900) ait deformasyon oranı hassasiyeti uygulanan deformasyonla birlikte farklılık göstermiştir. Aynı zamanda farklı şekillendirme sıcaklıklarında elde edilen akma eğrilerinden deformasyon hız oranını hassasiyetinin (m) değiştiği gözlemlenmiştir. Anahtar Sözcükler: Yüksek mukavemetli çelikler, fırında sertleştirme, ön deformasyon, DP 600, TRIP 800, TWIP 900, In this study, advanced high strength steels process applied to pre-deformation and bake hardening, the microstructure of the steel and research on the effect on the mechanical properties were carried out. From the data gained as a result of research carried out, when stress strain rates of high strength steels are analyzed, it has been found out that flow curve depends on the strain rate. In conjunction with the increase in the strain rate, the capability of material's formability has decreased while its creep strength has increased. In conjunction with pre-strain, the strengths of waiting time have increased and in conjunction with bake hardening carried out on the various temperatures, creep and pull-off strengths of dual phase samples have increased while elongation values (%) has decreased. When test results and images given are analyzed, the sensibility of strain rate that belongs to the high strength steels (DP 600/TRIP 800/TWIP 900) has altered in general together with the strain practiced. It has been also observed in the lights of flow curves gained at the different formalizing temperatures that the sensibility of strain velocity has changed. Keywords: High strength steels, bake hardening, pre-strain, DP 600, TRIP 800, TWIP 900

S960 and S700 are two types of high strength low alloy steels (minimum yield strengths at 960 MPa and 700 MPa, respectively) developed recently by Tata Steel. These steels are typically used in heavy lifting equipment. This research examines the feasibility and characteristics of single pass autogenous laser welding (ALW), multi-pass ultra-narrow gap laser welding (NGLW) of 8 mm thick S960 and 13 mm thick S700 high strength low alloy (HSLA) steels and compared the characteristics of the welds with those of gas metal arc welding (GMAW). The work aims to understand the development of welding induced residual stresses, microstructures, microhardness, tensile properties, bending properties and Charpy impact toughness at different temperatures as produced by different welding techniques (ALW, NGLW and GMAW).Design of experiments and statistical modelling were used to predict and optimise laser welding parameters of S960 and S700 HSLA steels. The contour method was used to measure the 2D distribution of residual stresses of the welded specimens. X-ray diffraction was carried out to measure the surface residual stresses of the welded specimens. The main novel contributions include:(1) Development of welding procedures for ultra-NGLW of HSLA steels. The ultra-NGLW process was successfully applied to the welding of 8 mm thick S960 and 13 mm thick S700 HSLA steels with a very narrow groove (1.2-1.4 mm wide) using a moderate laser power (2-3 kW).(2) Resolving the melt sagging problem for single pass autogenous laser welding of thick section materials. Horizontal (2G) welding position was applied to successfully resolve the melt sagging problem when single pass flat (1G) position ALW was applied to welding a 13 mm thick S700 steel plate. Computational fluid dynamic (CFD) modelling was carried out to understand the dynamic force interactions in the weld pool and the factors affecting the formation of the weld bead profile.(3) Understanding the effects of heat input on the microstructures evolution and mechanical properties of the welded high strength steel joints. The much lower heat input for ALW of 8 mm thick S960 steel and ultra-NGLW of both 8 mm thick S960 and 13 mm thick S700 steels results in the generation of hard martensite in the narrow fusion zone (FZ) and heat affected zone (HAZ), which strengthened the welded joints but deteriorated the toughness of the welded joints. The strengthened narrow FZ and HAZ for both the ALW and ultra-NGLW of 8 mm thick S960 steels demonstrated almost the same tensile strength and elongation as the base material. A relatively high heat input for the ALW of 13 mm thick S700 steel results in the generation of bainite in the FZ, which has almost the same microstructure and hardness as the base material.(4) Understanding the effect of solid-state phase transformation on the residual stresses of the welded specimens. It was demonstrated that the solid-state phase transformation from austenite to ferrite, bainite and martensite changes the magnitude of residual stress in the fusion zone for the welded S700 steel plates. In addition, it also changes the yield strength of the FZ, which also has a significant effect on the welding residual stress.In summary, this work has resulted in a significantly enhanced understanding of the way in which the choice of welding process affects the properties of welded joints in high strength steels. Laser welding was found to offer strengthened welded joints. However, the laser welded joints presented low impact toughness. If the toughness of the laser welded joints can be improved, laser welding will be a promising technique for joining high strength steels.

paper provides an overview of published experimental data on the fatigue strength of welded joints by high frequency mechanical impact (HFMI) treatment methods, In total, 414 data points from four specimen types are available,tests were performed using constant amplitude R = 0.1 axial tension fatigue, but some data for other R rations, variable amplitude testing and bending fatigue are also reported. An S-N slope of m = 5 gives a very good description of both individual data sets and of the composite data Design curve recommendations for the four joint types and for the structural stress-based design curve are given. HFMI treated specimens generally follow the same trend as experimental data for hammer peened specimens, but the degree of improvement is better. Data for large structures, at stress ratios other than R=0.1 and for variable amplitude loading are still needed in order to update the IIW guideline for post-weld improvement. There is a general trend for increasing fatigue strength improvement as a function of steel yield strength but this influence needs further study in order to develop guidelines. Quality assurance measures for HFMI treatment methods must also be defined.

This work describes the experimental study of the friction drilling process in dual phase (DP) steels, with yield strengths from 600 to 800 Mpa. The optimal machining conditions with different thicknesses was assessed through controlled tests at different rotation speeds and feed rates. On one hand, the torque and the thrust force were computed and monitorized. On the other hand, the dimensional tolerances of the holes were evaluated. Another topic of interest inherent to this special technique is the temperature level reached during the friction process which is crucial when it comes to development of microstructural transformations. This thermal condition can transform the mechanical properties of material near the hole and the burr.En este trabajo se expone un estudio del proceso de taladrado por fricción en chapas de diferentes aceros de doble fase (DP), con límites elásticos variables desde 600 hasta 800 MPa. Se realiza una evaluación de las condiciones óptimas de mecanizado en función del material y el espesor empleado, variando las velocidades de giro y los avances de penetración. Al mismo tiempo se monitoriza el par y la fuerza axial resultante, analizando además el espesor de la rebaba en distintas profundidades, es decir, las tolerancias dimensionales generadas por el proceso. Otro punto de interés es la medición de la temperatura alcanzada durante el proceso como consecuencia de la fricción, ya que la cantidad de calor absorbido por la pieza puede influir en las transformaciones microestructurales que el material experimenta. Esta afectación térmica puede transformar las propiedades mecánicas del material en las cercanías del agujero y en la rebaba.

Mechanical clinching can be used to joining different metallic materials. The only restriction are their plastic properties. However some plastic materials, with good ductility, do not conform strong clinch joint, e.g. materials, featured by high strain hardening phenomena are difficult to clinching and do not create durable clinch joint. In case of others materials with limited ductility clinch forming generates the process-induced defects such as cracks. So, there are material’s features which are very important for the clinch forming process and among them the strain hardening properties seem to be in special importance. The clinch joints of different materials with diversified plastic and strength properties were tested. A single overlap clinch joints with one clinch bulge were realized in the tests. The joints were tested in the pull test. The obtained results showed the relation of the clinch joinability to the materials’ strain hardening exponent. The good quality and good strength joints, were obtained for materials with low value of strain hardening exponent below n = 0,22.

The Volvo XC60 car body contains numerous parts in Ultra High Strength Steels (UHSS) in order to guarantee the structural integrity of the car in the event of a crash situation. Most of the parts are manufactured in a hot-forming process, so called presshardening, resulting in component tensile strength in the range of 1,500 MPa. As this type of material also presents fairly high carbon content (∼0.22%) it brings a challenge when it comes to welding. The Volvo XC60 car body is at the same time to a large extent assembled by laser welding technology. In early development stages of the project (Y413), it was observed that laser welding of hot-formed components presented a number of challenges due to the unique conditions offered by this welding method. The presentation will thoroughly describe the modes of procedure how to avoid crack inducement during the welding operation. A variable analysis approach was used based on the present circumstances at the production facility in the Gent plant. Crucial variables at laser welding such as gap between sheets, focal point position, welding speed and laser weld position relative to the flange edge were included in a test matrix and welding trials were carried out accordingly in the Pilot Plant in Gothenburg. The paper will discuss those welding results, the subsequent analysis and plausible theoretic explanations. From the lessons learnt in this research, the optimum laser welding parameters were then transferred to the laser welding stations in the Gent plant. There it has been proven, that also at high volume automotive manufacturing, it is possible to provide an outstanding weld quality also at such difficult pre-conditions. The presentation ends with some facts and figures and experiences from high volume series production, which also includes aspects on quality assurance.

The article deals with two middle alloyed high strength cast steels and the possibility of their mechanical properties improvement with modification by titanium. The presented study has two main topics. The former is the evaluation of basic mechanical properties of modified and unmodified samples in normalizing and quenched-and-tempered states. The latter is the effect of modification exhibited on the nitridation process and parameters evaluated after nitridation as the course of micro hardness across nitridation layer, the depth of the layer and hardness of nitride surface.

In most forming processes based on tailored blanks, the tool material remains the same as that of sheet metal blanks without tailored properties. A novel concept of lightweight construction for deep drawing tools is presented in this work to improve the forming behavior of tailored blanks. The investigations presented here deal with the forming of tailored blanks of dissimilar strengths using tailored dies made of two different materials. In the area of the steel blank with higher strength, typical tool steel is used. In the area of the low-strength steel, a hybrid tool made out of a polymer and a fiber-reinforced surface replaces the steel half. Cylindrical cups of DP600/HX300LAD are formed and analyzed regarding their formability. The use of two different halves of tool materials shows improved blank thickness distribution, weld-line movement and pressure distribution compared to the use of two steel halves. An improvement in strain distribution is also observed by the inclusion of springs in the polymer side of tools, which is implemented to control the material flow in the die. Furthermore, a reduction in tool weight of approximately 75% can be achieved by using this technique. An accurate finite element modeling strategy is developed to analyze the problem numerically and is verified experimentally for the cylindrical cup. This strategy is then applied to investigate the thickness distribution and weld-line movement for a complex geometry, and its transferability is validated. The inclusion of springs in the hybrid tool leads to better material flow, which results in reduction of weld-line movement by around 60%, leading to more uniform thickness distribution., Materials;9(2)

Hot stamping of high strength automotive body parts is a key technology to fulfill safety requirements and CO2-emmision limit values which are made by the EU. In the last ten years, high effort was made to improve the hot stamping process. Besides optimizing workflow and heating of the sheet metal blanks, new tools steel grades were developed which show beneficial properties with respect to hot stamping and hard cutting of high strength body parts. Furthermore, engineering of cooling systems results in different cooling strategies which all shall improve the cooling efficiency of hot stamping tools. In combination, these improvements lead to a reduction of cycle time and maintenance work and therefore increases productivity of hot stamping processes. This paper gives a short summary of the latest developments with respect to tool steels for hot stamping and hard cutting tools including concepts for active cooled hot stamping tools., International Conference on Stone and Concrete Machining (ICSCM), Vol 3 (2015)

Welding modern high-strength steel with low carbon and impurity contents, preheat may be dictated by cracking sensitivity of the weld metal instead of the HAZ. Standard EN 1011 does not provide the user with any unified methodology for the calculation of safe preheat for weld metal. The few calculation formulae that apply to multipass welds can give greatly varying predictions. This article studies controlling factors that govern transverse hydrogen cracking in high-strength multipass weld metal. The experiments comprised heavily restrained Y- and U-groove multipass cracking tests of SMAW and SAW welds. The objectives were the assessment of hydrogen cracking risk by defining the Crack — No Crack boundaries in terms of safe line description giving the desired lower-bound estimates, and to derive predictive equations capable of giving reliable estimates of the required preheat/interpass temperature T 0/T i for the avoidance of cracking. Equations were derived to assess the weld critical hydrogen content H cr corresponding to the Crack — No Crack conditions as a function of either weld metal P cm, yield strength R p0.2 or maximum hardness HV 5(max). For the calculation of safe T 0/T i estimates, a formula incorporating weld metal strength as linear functions of either CET or weld HV 5(max), weld build-up thickness a w in the form of tanh expression and weld diffusible hydrogen H d in terms of combined [In / powerlaw] expression, was found descriptive.