Search

Environmental impact awareness in civil engineering is nowadays an important concern. The last message from the IPCC (Intergovernmental Panel on Climate Change) was clear: significant actions are mandatory and urgent to achieve the objective to limit global warming to 1.5°C. The building sector is one of the most polluting industrial sectors for which economically viable solutions must be found to cut the emissions. The development of new production technologies can contribute to this aim by creating innovative and more sustainable materials. Amongst the new materials that appear on the steel market, high strength steels are a nice example as they offer higher strength to weight ratio of structural steel elements, resulting in material savings, lighter foundations, easier transportation, and erection. These multiple advantages explain why the use of high strength steels could lead to both significant carbon emission and cost savings. However, the production of such steels uses more alloying elements and sometimes they require more production energy than for regular grades. The aim of this paper is to estimate the relative prices and relative carbon emissions of high strength steels to evaluate whether they constitute sustainable materials. This paper demonstrates that, in many cases, the increase of relative prices and carbon emissions as function of yield strength can be negligible by contrast to the weight savings induced by using high strength steels. [ABSTRACT FROM AUTHOR]

A comprehensive numerical investigation into the cross-sectional behaviour and ultimate capacity of non-slender welded I-sections, made of both normal and high strength steels (NSS and HSS), under combined compression and uniaxial bending is presented. Finite element (FE) models were initially established and validated against test results collected from the literature. Subsequently, parametric studies were conducted using the validated FE models to generate extensive numerical data considering different steel grades, cross-section geometries and loading combinations. The obtained numerical data, together with the test results collected from the literature, were utilised to assess the accuracy of the traditional European (EC3) and North American (AISC) design provisions, as well as the Continuous Strength Method (CSM), for NSS and HSS welded I-sections under combined loading. The assessment indicated that the CSM was able to provide more accurate and consistent resistance predictions than the current EC3 and AISC design provisions owing to its ability to capture the spread of plasticity and strain hardening in a systematic, mechanics-based manner. Finally, the reliability levels of the different design methods were statistically evaluated in accordance with EN 1990:2002. [ABSTRACT FROM AUTHOR]

Purpose: It is an important problem that the machines become unusable due to the deformation of the machine elements produced before the planned time. In this study, it was aimed to determine the KIC value of X210Cr12, which is a high strength material. In this way, more accurate load values can be used in the design. Methods: In the experimental study to determine the KIC value, the sample geometry, the crack depth, the load-dependent parameters and calculations and some conformity checks were carried out. Experimental system; KIC consists of 3-point flexure specimens, a press, an electronic circuit capable of detecting the change in load crack opening, and a logger. Findings: In the study, the loads were determined from experimental graphs. Subsequently, load-crack opening values were determined. With these data, the KIC values were calculated as 719,7 and 839,7 MPa.mm1/2 as the minumum and maximum values, respectively. Conclusion: The experimental graphs are in the form of curves that break abruptly with unstable crack propagation without showing plastic deformation. There is no study in the literature on the KIC value of X210Cr12 and it has been determined that it has a low KIC value compared to the high-strength steels studied. It is valuable to determine the KIC value, as fracture problems may occur in designs prepared with the material. The results of the study are data at room temperature.

Purpose: It is an important problem that the machines become unusable due to the deformation of the machine elements produced before the planned time. In this study, it was aimed to determine the KIC value of X210Cr12, which is a high strength material. In this way, more accurate load values can be used in the design. Methods: In the experimental study to determine the KIC value, the sample geometry, the crack depth, the load-dependent parameters and calculations and some conformity checks were carried out. Experimental system; KIC consists of 3-point flexure specimens, a press, an electronic circuit capable of detecting the change in load crack opening, and a logger. Findings: In the study, the loads were determined from experimental graphs. Subsequently, load-crack opening values were determined. With these data, the KIC values were calculated as 719,7 and 839,7 MPa.mm1/2 as the minumum and maximum values, respectively. Conclusion: The experimental graphs are in the form of curves that break abruptly with unstable crack propagation without showing plastic deformation. There is no study in the literature on the KIC value of X210Cr12 and it has been determined that it has a low KIC value compared to the high-strength steels studied. It is valuable to determine the KIC value, as fracture problems may occur in designs prepared with the material. The results of the study are data at room temperature. [ABSTRACT FROM AUTHOR]

The paper focuses on the submerged arc welding (SAW) process in application to structures for Arctic conditions. One of the technical challenges for modern Arctic structures is to produce high-quality welds since a weld is usually the weakest part of any structure. Welding is especially difficult for the high strength steels (HSS), which are used in structures for weight-reduction purposes. The objective of the study is to explore the usability, development possibilities and parameters of SAW process for welding of thick cold-resistant HSS plates. Meeting this objective required in-depth understanding of the welding and material science background, which includes the quality requirements of weld joints intended for Arctic service as described in various standards, properties of cold-resistant HSS and description of testing methods used to validate welding joints for low temperature conditions. The study describes experimental findings that improve understanding of SAW process of thick quenched and tempered (QT) and thermo-mechanically processed (TMCP) HSS plates. Experiments were conducted to develop SAW procedures to weld several thick (exceeding 25 mm) high strengths (580–650 MPa tensile strength) cold-resistant (intended operational temperature at least −40 ℃) steel grades. The welds were evaluated by a wide range of industrial tests: analyses of chemical, microstructural and mechanical properties; hardness tests; and cold resistance evaluation tests: the Charpy V-notch impact test and the Crack tip opening displacement (CTOD) test. Acceptable welding parameters and recommendations were developed, and the results of the experiments show that high quality welds can be obtained using heat input up to 3.5 kJ/mm.

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.

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. [ABSTRACT FROM AUTHOR]

The paper focuses on the submerged arc welding (SAW) process in application to structures for Arctic conditions. One of the technical challenges for modern Arctic structures is to produce high-quality welds since a weld is usually the weakest part of any structure. Welding is especially difficult for the high strength steels (HSS), which are used in structures for weight-reduction purposes. The objective of the study is to explore the usability, development possibilities and parameters of SAW process for welding of thick cold-resistant HSS plates. Meeting this objective required in-depth understanding of the welding and material science background, which includes the quality requirements of weld joints intended for Arctic service as described in various standards, properties of cold-resistant HSS and description of testing methods used to validate welding joints for low temperature conditions. The study describes experimental findings that improve understanding of SAW process of thick quenched and tempered (QT) and thermo-mechanically processed (TMCP) HSS plates. Experiments were conducted to develop SAW procedures to weld several thick (exceeding 25 mm) high strengths (580-650 MPa tensile strength) cold-resistant (intended operational temperature at least -40 °) steel grades. The welds were evaluated by a wide range of industrial tests: analyses of chemical, microstructural and mechanical properties; hardness tests; and cold resistance evaluation tests: the Charpy V-notch impact test and the Crack tip opening displacement (CTOD) test. Acceptable welding parameters and recommendations were developed, and the results of the experiments show that high quality welds can be obtained using heat input up to 3.5 kJ/mm. [ABSTRACT FROM AUTHOR]

Numerous researchers conducted work on the fatigue strength improvement of high strength steels welding joint by applying different post-weld treatment on it, in the last decade. These research works based on different approaches to compare the post-weld treated welding joints fatigue strength with the as welded condition or with another post-weld treatment processes. These methods are limitedly applied based on practical experiences, however significantly different results were presented in research studies. To compare the different post-weld treatment fatigue strength improvement results on high strength steels, experimental program should be performed. The aims of this article are summarizing the most relevant information for our experimental work based on analysed case studies, and proposing program setup method for evaluation different post-weld treatment process results. [ABSTRACT FROM AUTHOR]

The International Institute of Welding (IIW) published a recommendation on methods for improving the fatigue strength of welded joints in 2013, valid for maximum specified yield strength of 900 MPa. In this time was consider that by using these methods is reasonable to expect that fatigue performance of welded higher strength will also improve. Since the publication several studies investigate different improvement techniques effects on high strength steels fatigue strength. The aim of this paper is to review post treatment techniques by improving the weld geometry consider the application of these techniques in high-strength steels. The post treatment techniques by residual stress methods will be reviewed in the second part of our article. [ABSTRACT FROM AUTHOR]

A review of sacrificial Zn–Ni electroplating coatings on high-strength steels is presented. These steels are used for heavy structural applications such as landing gears, etc., that are subjected to high stresses and corrosive environments in service. The electroplating process involving aqueous electrolytes invariably produces hydrogen. The emitted hydrogen can diffuse into substrate steel, contributing to the delayed failures by hydrogen embrittlement. Microstructural inhomogeneities arising from the heat treatments and defects produced during coatings and those inherently present in the steels can trap hydrogen emitted during plating. Dissolved and trapped hydrogen can slowly diffuse to the stress concentrations or crack tips, contributing to the delayed structural failures. Baking after plating helps to eliminate hydrogen to some extent, though it may introduce some thermomechanical stresses at the bimaterial interfaces. This review discusses a) the current state of sacrificial Zn–Ni coatings, b) their protection against corrosion of the substrate, c) the associated hydrogen embrittlement predominately under cyclic loads, and d) recent advances in terms of the compositionally modulated coatings for enhanced protection. [ABSTRACT FROM AUTHOR]

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.

With the addition of Al, the type, amount and precipitation temperature of inclusions in high strength steel for automobile will change.The characteristics of inclusions including amount, species and precipitation temperature in three typical automotive high strength steels with different Al contents were investigated by oxygen and nitrogen analysis, scanning electron microscopy (SEM) , and thermodynamic calculation.The results show that the mass fraction of Al in steel increases from 0.054%to 1.35%, the mass fraction of T[O]in steel decreases from 0.00150%to 0.00075%, and the number of inclusions decreases from 38.28 mm-2to 28.12mm-2, the number of inclusions in steel tends to decrease.With the increase of Al mass fraction in the steel, the evolution route of main oxide inclusions is Al2O3·MnO→MgAl2O4, the main aluminum inclusion changes from Al2O3to Al N;and the evolution route of sulfide inclusions is MnS to MgS.The thermodynamic calculations indicate that the formation of Al N inclusions is related to the Al content in the steel, and the Al N inclusions in steel with high Al content are easier to be formed.The precipitation temperature of Al N increases with the increase of Al content, and the precipitation temperatures are 1375, 1528 and 1561 K, respectively.

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.

Galvanizing of high‐strength MSH sections Batch hot‐dip galvanizing of steel components has been known and established for decades to ensure permanent corrosion protection. This process is also advantageous for hot‐rolled tubes and hollow sections, as by immersion in the molten zinc, both the outer and the inner surface of the component is completely coated with a zinc layer and thus protected. The impact and interactions occurring during the galvanizing process are well known with regard, to the steel structure and the steel itself, which up to now has usually been normal strength steel. With the increasing spread of high‐strength steels, the question arises, whether the available findings with regard to the influence of the galvanizing process can be transferred and confirmed to these steels. In the course of an extensive test campaign, the effect of hot‐dip galvanizing on the mechanical material properties and the formation of the zinc layer was examined on six steel materials. The results obtained are presented in this paper. [ABSTRACT FROM AUTHOR]

The main aim of the present work is to investigate, through the strain energy density method, the fatigue behaviour of high strength welded joints realised employed in hydraulic runner blades. The geometries, found in literature, present the welding bead machined in order to have no geometrical discontinuities in the specimen realising a wide fitting radius between the two welded plates; the only critical geometrical discontinuity in the specimen is given by the lack of penetration that leads to an internal crack‐like defect. The specimens presented failure both from the weld toe and from the weld root depending on the amount of welding penetration. The results, summarised in this work with the strain energy density method, show clearly the possibility to consider a unique master curve for this kind of joints regardless of the failure initiation point. Acquiring, through a finite element model, the strain energy density value both at the weld toe and at the weld root and comparing them, the method is proved to be adequate to detect the most critical area of the joint. A first fatigue master curve in terms of cyclic averaged strain energy density value is provided for the fatigue design of high strength welded joints. [ABSTRACT FROM AUTHOR]

Experimental and numerical analysis of formability behavior for two types of steel sheets, namely E335D and DP600, was conducted. The former is a high strength low alloy (HSLA) steel, while the latter is an advanced high strength steel (AHSS). Initially, mechanical tests including uniaxial tensile test and the Erichsen cupping test were performed. Tensile test revealed that AHSS steel had higher yield and tensile strength, higher uniform elongation, as well as smaller non-uniform elongation. Then, forming limit diagrams (FLD) were determined via both experimental and numerical approach. Numerical modeling of the forming limit diagrams was carried out by means of the finite element analysis (FEA) ABAQUS/CAE software, using Hill's 1948 yield criterion considering maximum thickness strain as the point of necking. The resulting forming limit diagrams were compared with the experimental results and showed a good agreement between the two, suggesting capability of the Hill's 1948 yield criterion for predicting formability of the tested materials. [ABSTRACT FROM AUTHOR]

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.

Flame straightening of normal and high strength steels – part 2 Due to many parameters and transient temperature and strain distributions, the process of flame straightening appears to be rather complex. In practice, flame straightening actually bases on experience rather than on engineering approaches. In contrast, regarding the impact on material, technical rules are existing deemed to exclude strength and toughness reduction. However, compared to mild steels, high strength structural steels require more detailed or even additional guidance. For the achievable remaining deformations are getting smaller the higher the yield strength is. To compensate this effect, one often chooses higher temperatures and longer holding times. This however increases the risk of material degradation. Consequently, if we strive at an easy and efficient flame straightening also for high strength steels, suitable parameter constellations need to be investigated leading to safe results as before. The paper consists of two parts and explains how to achieve this by first explaining the material related mechanisms of flame straightening and then showing application corridors. In addition, it suggests improved guidance when using high strength steels. In the first part [1], the principles of flame straightening of normal and high strength steels and the interrelation of thermodynamics, metallurgy and mechanics are presented. Furthermore, the determination of heat input and temperature by experimental and numerical means are shown. The second part deals with the deformations and forces arising from the flame straightening process, the influence on the material and the prediction of relevant parameters of the flame straightening process. [ABSTRACT FROM AUTHOR]

Flame straightening of normal and high strength steels – part 1 Due to many parameters, transient temperature and strain distributions, the process of flame straightening appears to be rather complex. In practice, flame straightening actually bases on experience rather than on engineering approaches. In contrast, regarding the impact on material, technical rules are existing deemed to exclude strength and toughness reduction. However, compared to mild steels, high strength structural steels require more detailed or even additional guidance. For the achievable remaining deformations are getting smaller the higher the yield strength is. To compensate this effect, one often chooses higher temperatures and longer holding times. This however increases the risk of material degradation. Consequently, if we strive at an easy and efficient flame straightening also for high strength steels, suitable parameter constellations need to be investigated leading to safe results as before. The paper consists of two parts and explains how to achieve this by first explaining the material related mechanisms of flame straightening and then showing application corridors. In addition, it suggests improved guidance when using high strength steels. In the first part the principles of flame straightening of normal and high strength steels and the interrelation of thermodynamics, metallurgy and mechanics are presented. Furthermore, the determination of heat input and temperature by experimental and numerical means are shown. The second part deals with the deformations and forces arising from the flame straightening process, the influence on the material and the prediction of relevant parameters of the flame straightening process. [ABSTRACT FROM AUTHOR]

Abstract The existing studies on thin-walled structures have focused on structural optimization mainly for enhancing crashworthiness and lightweighting, whilst relatively little attention has been paid to analysis of cost efficiency of an optimized structure. How to develop cost-effective products has always been a primary goal pursued by enterprises in different ways. To address this issue, this study aims to elucidate a systematic approach for exploring the effects of various material grades and structural dimension (e.g. wall thickness) on cost efficiency relative to the crashworthiness performance by taking the double-hat (DH) thin-walled structure as an example. First, a series of drop-weight tests for the DH structure are carried out under axial and lateral impacts. The experimental results show that different material grades and dimensions have different effects on the crashworthiness under different impact velocities. Second, the finite element (FE) models are established and validated with the experiments to analyze the effects of the three factors (wall thickness, material grade, and impact velocity) on the crashworthiness criteria. Third, the surrogate models for each crashworthiness performance indicator and cost efficiency (namely, specific energy absorption – SEA , bending moment – M b , SEA /cost and M b /cost) are constructed by using the regression technique. Finally, the combined effects of material grade and structural dimension on cost and performance are analyzed and compared respectively; and the results show that there is room to significantly reduce production cost without sacrificing the crashworthiness performance. For mass production implemented in modern automotive industry, the proposed design methodology allows to better balance performance and cost. Highlights • HSS double hat structures under axial and lateral impacts are experimentally investigated. • Mathematical relationships between crashworthiness indicators and cost efficiency are established. • The effect of material grade and structural dimension on cost and performance are analyzed. • Cost efficiency can provide useful design guidance. [ABSTRACT FROM AUTHOR]

An experimental and numerical investigation into the flexural buckling behaviour of cold-formed (CF) high-strength steel (HSS) square hollow section (SHS) members is reported in this paper. A series of 12 flexural buckling tests were conducted on S700 and S900 CF SHS columns. Finite element (FE) models were developed to simulate the experimental observations, which were utilized in a parametric study encompassing a broader range of steel grades and specimen geometries. Subsequently, the current buckling curves in European, North American, and Australian codes was evaluated through reliability analyses. Meanwhile, the effective width method (EWM) and the modified generalized slenderness-based resistance method (GRSM) were compared for slender cross-sections. According to the results of reliability analyses, the modified buckling curves were proposed for CF HSS SHS columns. • Experimental tests on cold formed high strength steel columns and tensile coupons have been carried out. • Numerical modelling validation and subsequent parametric study has been performed. • Effect of corner radius on the column strength has been quantified. • Design proposals have been made and modifications to current design codes have been proposed. [ABSTRACT FROM AUTHOR]

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.

Mechanical clinching can be used to joining different metallic materials. The only restriction is 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 increased attention ultrafine grained (UFG) materials have received over the last decade has been inspired by their high strength in combination with a remarkable ductility, which is a promising combination for good fatigue properties. In this paper, we focus on the effect of different carbide morphologies in the initial microstructure on the fatigue behavior after high pressure torsion (HPT) treatment of SAE 1045 steels. The two initial carbide morphologies are spheroidized as well as tempered states. The HPT processing increased the hardness of the spheroidized and tempered states from 169 HV and 388 HV to a maximum of 511 HV and 758 HV, respectively. The endurance limit increased linearly with hardness up to about 500 HV independent of the carbide morphology. The fracture surfaces revealed mostly flat fatigue fracture surfaces with crack initiation at the surface or, more often, at non-metallic inclusions. Morphology and crack initiation mechanisms were changed by the severe plastic deformation. The residual fracture surface of specimens with spheroidal initial microstructures showed well-defined dimple structures also after HPT at high fatigue limits and high hardness values. In contrast, the specimens with a tempered initial microstructure showed rather brittle and rough residual fracture surfaces after HPT.

Abstract High strength low alloy steels (HSLAs) containing copper, nickel and low amount of Carbon are a new generation of high strength low alloy steels that are widely used in shipbuilding. Strengthening elements such as Cu, Ni, and to some extend C, play a significant role in this family of steels. By increasing the amount of Cu, Ni, and by selection of proper final temperature of rolling, austenitization and aging, it is possible to enhance the yield strength of this steel up to 160 ksi (1102 MPa) while at the same time toughness remains at an acceptable level. In this investigation, the effect of increasing Cu and Ni on mechanical properties of these steels was elaborated. Four steels with varying amounts of Cu and Ni were cast, and hot rolled by a thermomechanical controlled processing. The steels were then austenitized at 920 °C for 45 min followed by quenching in water. Finally, the steels were aged at 650 °C for one hour. Optical microscope, and Filed Emission Scanning Electron Microscope (FE-SEM) were used to study microstructural evolution. Mechanical properties variation was evaluated by tensile and microhardness tests. Results showed that by increasing the amount of these two elements, hardness and tensile strength were increased in thermomechanically controlled processed, austenitized and aged specimens. Yield strength values of the rolled samples were somehow scattered which was due to a slight difference in final rolling temperature and the subsequent rapid cooling. Yield strength under austenitization and quenching conditions increased by increasing the amount of Cu and Ni which could be due to the presence of Nb (C, N) precipitates. Increasing the amount of Cu precipitates after quench could also be another reason for yield strength increment. Increasing the amount of Cu and Ni in samples containing 4.16 and 7.65 wt% for each of these alloys led to the reduction of A c1 (austenite formation temperature). Selecting aging temperature (650 °C) higher than the austenite formation temperature could probably lead to the growth of the carbides, softening of martensite layers and lower amount of dislocations density hence for the reduction of yield strength of the aged steels. As well, with the addition of the mentioned amounts of Cu and Ni, yield strength of 705 MPa (102.2 ksi), tensile strength of 1072 MPa (155.4 ksi), elongation of 14% and area reduction of 63% were obtained in the aged steels of this investigation. Highlights • High Cu/Ni steel contained mixed of phases together with Cu and Nb(C, N) precipitates. • Retained austenite increased by increasing the amount of Cu and Ni elements. • Increasing Cu & Ni elements in HSLA 100 steels increased hardness and tensile strength. • Appropriate aging at 650 °C was obtained for optimum mechanical properties. • Steel with Cu (3.75) and Ni (7.5) led to high strengths and acceptable toughness. [ABSTRACT FROM AUTHOR]

Highlights • In VHCF with subsurface crack initiation, Slip Marking occurrences are found. • The subsurface crack initiation mechanism seems similar to crack surface initiation. • An identical mechanism to SMAT process could lead to the FGA formation. Abstract The present paper aims to explain the subsurface crack formation occurring in the Very High Cycle Fatigue domain for bcc steels. For this purpose, two high strength steels (a low alloyed steel AISI 5120 and a martensitic stainless steel X40CrMo13) were studied under ultrasonic push-pull fatigue tests. For all tests, a fish eye developed before fracture and, in most cases, the subsurface crack initiation originated from inclusions. Also, during fatigue tests for both materials, the thermal field on specimen surface was recorded by infrared camera and fracture surface observations were performed by Scanning Electron Microscopy (SEM). The SEM pictures reveal the presence of a Fine Granular Area (FGA) surrounding the inclusion in the crack initiation stage. Moreover, earlier in the crack process, slips markings were found in the matrix inside the inclusion dimple indicating that some similarities exist between surface and subsurface crack initiation mechanisms. [ABSTRACT FROM AUTHOR]

The conventional joining methods like resistance spot welding and arc welding have several challenges during joining of thin sheets of high strength steel materials. One of the main challenges is that application of these joining methods may result in a severe distortion of welded structure. Therefore, laser welding process has emerged as an alternative joining process which can help mitigate some of these challenges. Lower heat input from laser during the welding process results in a smaller size weld heat affected zone and also in lower overall distortion of the structure. The laser welding process presents an exciting opportunity in designing lighter weight structures. However, the major roadblock to application of laser welding method for large structural parts is that fatigue behavior of laser welded joints is not yet well understood. In order to study the fatigue performance of laser welded joints, detailed experimental and numerical investigations have been carried out and the results are presented in this work. The scope of experimental studies included a large set of coupons with different thicknesses and material combinations. Experimental fatigue test data has been generated for the laser welded joints produced using thin sheets of three grades of high strength steel materials (HSLA and UHSS grades) of several thicknesses (1 mm, 1.6 mm, 2 mm and 3 mm). The fatigue test data sets were obtained at R-ratios of R  = 0.1, R  = 0.2 and R  = 0.3. Another variable introduced into experimental studies was an orientation of laser weld joint with respect to applied loading direction. After fatigue tests were completed, detailed metallurgical investigations have been carried out to understand the failure mechanism and the crack growth behavior in laser welded joints. Based on the observed experimental and numerical studies it was concluded that the strain life based fatigue analysis method which has been successfully applied to study weld toe failures for the arc weld joints is not sufficient for the evaluation of laser welded joints. This is due to the reason that laser welded joints have unique challenges due to weld root crack failures and extremely high stress concentration at the location of crack initiation in the root of laser welded joints between the plates. The fracture mechanics based method has been developed for the fatigue life assessment of laser welded joints. In order to apply this method comprehensive three-dimensional finite element studies were performed. Numerical studies show good correlation of the estimated fatigue lives obtained using proposed fracture mechanics method with the experimental data. [ABSTRACT FROM AUTHOR]

Abstract As weld pool geometry and thermomechanical strains are known to affect formation of solidification cracks, the influence of a superimposed beam oscillation on these characteristics is investigated for this paper. In this context the effects of the oscillation frequency and focal diameter on the weld pool and temperature field are determined by means of infrared thermography. As a result an increase of the weld pool size for larger focal diameters and a more even shape of the seam edges for higher frequencies can be identified. [ABSTRACT FROM AUTHOR]

Production volumes of advanced high strength steels (AHSS) are growing rapidly due to material and energy savings they provide in a number of application areas. In order to use their potential fully, it is necessary to minimize any danger of unexpected failures caused by hydrogen embrittlement. It is possible only if deeper understanding of underlying mechanisms is obtained through further research. Besides description of main grades of AHSS and mechanisms of HE, this paper reviews available tools for determination of hydrogen content and susceptibility to HE focusing on atmospheric conditions. Techniques such as slow strain rate testing, constant load testing, electrochemical permeation technique, scanning Kelvin probe and scanning Kelvin probe force microscopy have already been used to study the effect of hydrogen entered under atmospheric exposure conditions. Nanoindentation, hydrogen microprint technique, thermal desorption spectroscopy, Ag decoration or secondary ion mass spectrometry can be also conducted after atmospheric exposure. [ABSTRACT FROM AUTHOR]

In this work tensile strength, fatigue resistance and fracture toughness of two electroslag remelted (ESR) tool steels and of 33CrMoV12 ESR steel (both in quenched and tempered condition, as well as nitrided condition) were evaluated. The role of hardness, residual stresses and inclusion sizes on the fatigue behavior was investigated. Tool steels have a tensile strength between about 1900 and 2300 MPa, fracture toughness between 35 and 33 MPa√m, while fatigue strength ranges between 725 and 992 MPa. The tensile strength and fracture toughness of the 33CrMoV12 ESR are, respectively, 1365 MPa and about 150 MPa√m. Nitriding induces a significant increase in fatigue strength from 560 to 980 MPa. These results highlight that appropriate ESR tool steels could replace nitrided steels. [ABSTRACT FROM AUTHOR]

The present study was attempted to evaluate the hydrogen embrittlement (HE) of commercial 20SiMn2CrNiMo, PSB1080 and PH13-8Mo high strength steels using slow strain rate tensile tests with smooth cylindrical tensile specimens. The results reveal that three steels exhibit the superior strength and plasticity in the absence of hydrogen. However, the mechanical properties are deteriorated after hydrogen charging and the resistance to HE is 20SiMn2CrNiMo steel, PH13-8Mo steel and PSB1080 steel, which is ranked in a decreasing order. Fine lath boundaries in 20SiMn2CrNiMo steel disperse hydrogen atoms, reduce local hydrogen accumulation and then result in the lowest HE. Coherent NiAl precipitates in PH13-8Mo steel and O-Mg-Al-Si-Ca type inclusions in PSB1080 steel are responsible for the high HE susceptibility due to the concurrent effect of the stress concentration and hydrogen-enhanced decohesion mechanism. It is thus suggested that the inclusions in steels should be eliminated and the efforts concerning to obtain low HE susceptibility in steels strengthened by the coherent precipitates should be conducted to guarantee the safe service of engineering components fabricated from high strength steels. [ABSTRACT FROM AUTHOR]

Well-defined uniaxial micro-tensile tests are performed on single-crystal ferrite specimens with three different orientations. All specimens reveal a highly reproducible plastic behavior. The {110}<111> and {112}<111> slip systems equally contribute to the deformation, while all other (complex) slip traces can be identified as cross-slip and ‘pencil glide’. No {123}<111> slip system traces were observed. The critical resolved shear stresses of the two active slip systems are close to each other, i.e. CRSS { 110 } = ( 1.0 ± 0.1 ) × CRSS { 112 } . In all the tested specimens, the activation of the primary slip systems (e.g. systems that activate first) follows the Schmid's law. At first glance, the activation of secondary slip systems does not seem to comply with the highest Schmid factor. However, detailed investigation supported by crystal plasticity simulations reveals that the boundary constraints acting on the primary slip direction triggers an increase of the Schmid factors of the activated secondary slip systems, i.e. Schmid's law correctly justified all observed slip traces. Application of the found ferrite slip parameters in crystal plasticity simulations of ferrite-containing steels shows that correct input values are crucial for obtaining meaningful macroscopic predictions. [ABSTRACT FROM AUTHOR]

To investigate the low cycle fatigue (LCF) performance and damage behaviours of marine high strength steels (HSSs), experimental analysis and constitutive modelling are carried out in this study. Basic mechanical properties are obtained by the monotonic tensile tests. Cyclic loading tests with different loading protocols are conducted to examine the cyclic stress-strain response. Based on the experimental data, the evolutions of peak stress, yield surface radius, backstress and LCF damage are discussed. The Ramberg-Osgood model and Manson-Coffin equation are fitted to characterize the cyclic stress-strain relation and LCF damage behaviours, respectively. Based on the continuum damage mechanics, a damage coupled cyclic elasto-plastic constitutive model is proposed to describe the hysteresis behaviours of marine HSSs. The model parameters are calibrated, and the model accuracy for stress-strain response and fatigue damage prediction is then verified by experimental data. • Constitutive model of marine high strength steels coupled with low cycle fatigue damage is proposed. • Hardening and fatigue performances of marine high strength steels are experimental investigated. • Parameters of Manson-Coffin equation and Hardening model are calibrated. [ABSTRACT FROM AUTHOR]

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.