Conventional High Strength Steel Research Articles

Evaluation of Fatigue Limit Improvement and Harmless Crack Size of Maraging Steel Using Shot Peening

The fatigue strength of maraging steel, which is an ultra-high-strength steel, is relatively low, compared to that of conventional high-strength steel. The fatigue life of a structure is highly dependent on the surface conditions, because fatigue cracks generally start at the surface of the material. In particular, surface cracks considerably degrade the fatigue limit. To expand the application range of maraging steel, it is necessary to improve the fatigue limit, and render the surface cracks harmless. This study aims to investigate the effect of shot peening (SP) on the fatigue strength of maraging steel with surface cracks. The SP application introduced a compressive residual stress from the specimen surface to a depth of 170 μm, and increased the fatigue limit by 77 %. The estimated crack size that can be rendered harmless, based on fracture mechanics, is (0.170 − 0.202) μm in the range As = (1.0 − 0.1). The intersections of the harmless crack sizes were determined at depth. A semicircular surface crack below this value is harmless in terms of fatigue limit. The usefulness of non-destructive inspection (NDI) and non-damaging technology was evaluated in relation to ahml, aNDI, a25,50, and As. Thus, the SP process can improve the reliability of the maraging steel. Compressive residual stress is the dominant factor to improve fatigue strength and render the surface crack harmless.

Structural Synergy of NanoAl2O3/NanoAl Composites with High Thermomechanical Properties and Ductility

Achieving a combination of high strength and ductility in metal-based composites is still a difficult task, and it is especially challenging in a wide temperature range. Here, nanoAl2O3/nanoAl composites with high tensile and compressive strength and excellent ductility at 25 and 500 °C were obtained using Al and Al2O3 nanopowders via a combination of high-energy ball milling (HEBM) and spark plasma sintering (SPS). Being about three times lighter than conventional high-strength steel (with a density of 2.7 g/cm3 vs. that of 7.8 g/cm3 for steel), the nanoAl2O3/nanoAl materials demonstrated tensile strength and elongation before failure comparable with those of steel. The nanoAl2O3/nanoAl composites were strengthened with two types of Al2O3 NPs, in situ formed, and introduced into the powder mixture. The resulting materials had a bimodal microstructure consisting of Al with micron and submicron grains surrounded by an Al/Al2O3 framework whose structural components were all in the size range of 20–50 nm. Among the studied compositions (0, 1, 2, 3, 4, 5, 10, and 20 wt.% of Al2O3), the Al-3%Al2O3 material showed the best thermomechanical properties, such as a tensile strength of 512 MPa and 280 MPa and a compressive strength of 489 MPa and 344 MPa at 25 and 500 °C, respectively, with an elongation to failure of 15–18%. These results show the promise of nanoAl2O3/nanoAl composites for use as small items in the automotive and aviation industries.

Material characterisation for strength and formability limits of DP 1180 sheet

ABSTRACT Dual phase steels were specifically developed to address the weight and safety-related problems of the automotive industry. Nevertheless, these steels behave quite differently from conventional high strength steels (single phase) during the stamping processes. The unusual material behaviour limits the application of these steels. This study focuses on the material characterisation of cold rolled and hot dip galvanised dual phase 1180 sheet material. Microstructure and elemental analyses were performed using an optical microscope and a scanning electron microscope with energy dispersive X-ray spectroscopy. Mechanical properties were determined through Rockwell hardness, Vickers hardness, tensile, Nakazima, and hole flanging tests. A digital image correlation system was utilised for Nakazima and hole flanging tests. The average micro hardnesses of the steel and the coating were 341 HV0.5 and 51 HV0.01 respectively. Uniform elongation was ∼6.6%. The lowest point on the forming limit diagram was ∼0.1. The average hole expansion ratio was ∼22%.

Opportunities and partial problems of HFMI implementation in design of welded structures of rail vehicles

This paper highlights the main impacts of HFMI (High Frequency Mechanical Impact) technology on increasing the fatigue resistance and extending the fatigue life of welded joints. In the case study there are SN curves of T-welded joints made of two different steels (conventional steel S235JR and high strength steel S460MC). For both materials there is a comparison of SN curves with and without HFMI treatment. Both samples are loaded by bending for cycle asymmetry R = 0. The failure criterion is a decrease in stiffness caused by the formation of a macrocrack. The results of fatigue testing are then applied in a parametric study, which assumes a random loading of a selected structural detail in a railway application. Opportunities and barriers of the implementation of HFMI in practice are mentioned and documented according to results of laboratory experiments, which take place in cooperation with the university research center and the manufacturer of railway vehicles.

Influence of temperature- and phase-dependent yield strength on residual stresses in ultra-high strength steel S960 weldments

The temperature- and phase-dependent yield strength plays a decisive role in the precise prediction of residual stresses in structural steel weldments. Since it is very expensive and time-consuming to directly measure the yield strength of a given steel by experimental method, numerical simulation has commonly been performed using the available yield strength of the same steel grades in scientific literature and material database or using the evaluated data based on the empirical formulations in standards/codes for the given steel. Nevertheless, the yield strengths of the same steel grades are usually different mainly due to the disparity in chemical composition and grain size. Furthermore, there are considerable differences in the yield strength at elevated temperatures obtained from the predictive functions for the given steel. Therefore, it is necessary to clarify the influence of the variation in temperature- and phase-dependent yield strength obtained from different methods on the predicted welding residual stresses in numerical simulation. In this study, numerical sensitivity analyses have been conducted to clarify the effect of yield strength on the predicted residual stresses in ultra-high strength steel S960 weldments. Furthermore, tensile tests and welding experiments were carried out for comparison and validation. Based on the current work, the effect of temperature- and phase-dependent yield strength on the calculated welding residual stresses is clarified. Besides, the reliability of the proposed functions to evaluate yield strength based on the tensile test results mainly from mild steels and conventional high strength steels is assessed for the recently developed ultra-high strength steel S960. Finally, guidance on how to economically determine the reliable temperature- and phase-dependent yield strength for S960 steel in numerical welding simulation is provided.

On the effect of carbon content and tempering on mechanical properties and stiffness of martensitic Fe–18.8Cr–1.8B–xC high modulus steels

We studied the effects of C concentration (0.1, 0.3 and 0.5 wt%) and tempering temperature (100–700 °C) on the dynamic Young's modulus, mass density, hardness and impact toughness of quenched and tempered martensitic Fe–18.8Cr–1.8B (wt.%) based high modulus steels. Increasing C concentration lead to consistent hardness levels above 600 HV5 for tempering temperatures below 500 °C. While the tensile strength could be adjusted tailored from about 1000 MPa to more than 2000 MPa depending on the tempering temperature and respective C concentration, dynamic Young's modulus and mass density were left virtually unaffected. The E/ρ ratio shows little variability between approximately 31.2–33 GPa g−1 cm3, which is superior to conventional high strength steels and comparable to ferritic Fe–Cr–B steels without carbon additions. Particles identified within the martensitic matrices are mainly (Fe,Cr)2B type borides stemming from solidification, as well as an increasing fraction of (Fe,Cr)23(C,B)6 carbo-borides with increasing C content and tempering temperatures.

Dissimilar welding between conventional and high strength low alloy naval steels with the use of robotic metal cored arc welding

The increasing trend of high strength low alloy steel application in the shipbuilding industry dictates the necessity for an investigation of their welds with conventional high strength steels. In this study, conventional and high strength low alloy naval steel plates 12 mm thick, were joined together in a butt joint configuration, using the metal cored arc welding technique and low heat input (1.0 kJ/mm). The weld was examined in terms of microhardness, microstructure, and tensile performance. The effect of multiple thermal cycles on the formation of primary and secondary microstructures was also discussed. The results revealed a smooth distribution of the microhardness across the different weld regions. Inside the heat affected zones (HAZs) and the weld metal various constituents were revealed, with ferrite in various forms (lath-like, acicular, semi-equiaxed, polygonal) appearing as the predominant phase, alongside martensite and austenite grains, and pearlite (lamellar and degenerated). The tensile testing indicated the base metal of the conventional high strength steel to be the weakest link of the weld, which was mainly attributed to its ferritic-pearlitic microstructure and the coarse grain size. The overall tensile performance of the weld remained at high levels, with a simultaneous reduction in the elongation after fracture in comparison to the base metals.

Investigating the behaviour of steel end-plate connections with shape memory alloy bolts

Conventional extended end-plate connections with normal high-strength steel (HSS) bolts show considerable residual deformation following an earthquake. The application of shape memory alloys (SMAs) has recently been considered in seismic regions because of their capabilities, such as the ability to tolerate cyclic deformations and dissipate energy. In this paper, extended end-plate connections equipped with SMA bolts are investigated. A hybrid extended end-plate connection utilising SMA bolts only for the outer rows and conventional HSS bolts for the remaining rows is a new and viable alternative to existing connections. Finite-element models of connections with different bolt lengths and pre-stressing levels have been developed using Ansys software. The cyclic performance of the models has been assessed and compared. Results indicated that conventional and hybrid connections have better capabilities in energy absorption compared to SMA connections. The hybrid connections and also connections with 69 mm long SMA bolts are classified as high-ductility connections according to the American Institute of Steel Construction (AISC). Increasing pre-stressing over 50% fy has no significant effect on the cyclic performance of connections.

Processing routes, resulting microstructures, and strain rate dependent deformation behaviour of advanced high strength steels for automotive applications

Automobile industry is continuously striving to obtain light body-in-white structures to meet tightened regulations on flue-gas emissions/crash-testing parameters. ‘Advanced high strength steels (AHSS)’ find increased applications in the automotive industry because of improved crashworthiness/formability at reasonably low costs. AHSS category mainly includes transformation induced plasticity (TRIP) steels, twinning induced plasticity (TWIP) steels, dual phase (DP) steels, complex-phase (CP) steels, and quenching-partitioning (Q&P) steels. AHSSs provide superior strength-ductility combination than conventional high-strength steels by virtue of their multi-phase microstructures. Mechanical properties of AHSSs are greatly influenced by processing routes/derived microstructures. Furthermore, mechanical properties/tensile deformation behavior are also strain rate dependent. During an automobile crash, deformation occurs at strain rates which are exceedingly higher than quasi-static conditions. So, investigation of AHSS properties under both quasi-static as well as high strain rates conditions is important to check applicability for superior crash-resistance. The present work critically reviews details of processing routes, room temperature microstructures, mechanical properties, and finally strain rate dependence of tensile deformation behaviour of AHSSs. Finally, main gaps in existing literature/scope for future research with regards to high strain rate deformation dependent properties of this steel category are presented.

High-Performance Bamboo Steel Derived from Natural Bamboo.

It is highly desirable to develop green and renewable structural materials from biomaterials to replace synthetic materials involved from civil engineering to aerospace industries. Herein, we put forward a facile but effective top-down strategy to convert natural bamboo into bamboo steel. The fabrication process of bamboo steel involves the removal of lignin and hemicellulose, freeze-drying followed by epoxy infiltration, and densification combined with in situ solidification. The prepared bamboo steel is a super-strong composite material with a high specific tensile strength (302 MPa g-1 cm3), which is higher than that (227 MPa g-1 cm3) of conventional high specific strength steel. The bamboo steel demonstrates a high tensile strength of 407.6 MPa, a record flexural strength of 513.8 MPa, and a high toughness of 14.08 MJ/m3, which is improved by 360, 290, and 380% over those of natural bamboo, respectively. Particularly, the mechanical properties of the bamboo steel are the highest among the biofiber-reinforced polymer composites reported previously. The well-preserved bamboo scaffolds assure the integrity of bamboo fibers, while the densification under high pressure results in a high-fiber volume fraction with an improved hydrogen bonding among the adjacent bamboo fibers, and the epoxy resin impregnated enhances the stress transfer because of its chemical crosslinking with cellulose molecules. These endow the bamboo steel with superior mechanical performance. Furthermore, the bamboo steel demonstrates an excellent thermal insulating capability with a low thermal conductivity (about 0.29 W/mK). In addition, the bamboo steel shows a low coefficient of thermal expansion (about 6.3 × 10-6 K-1) and a very high-dimensional stability to moisture attack. The strategy of fabricating high-performance bamboo steel with green and abundant natural bamboo as raw materials is highly attractive for the sustainable development of structural engineering materials.

Low and Medium Carbon Advanced High-Strength Forging Steels for Automotive Applications

This paper presents the microstructural and mechanical properties of low and medium carbon advanced high-strength forging steels developed based on the third generation advanced high-strength sheet steels, in conjunction with those of conventional high-strength forging steels. Hot-forging followed by an isothermal transformation process considerably improved the mechanical properties of the forging steels. The improvement mechanisms of the mechanical properties were summarized by relating to the matrix structure, the strain-induced transformation of metastable retained austenite, and/or a mixture of martensite and austenite.

Influence of Pre-Stretching Levels on the Forming Limit Strain and Stress Curves of High Strength Steel Sheet

In this work, Forming Limit Curves (FLCs) of the conventional and pre-stretched High Strength Steel (HSS) sheet grade 440 (SCGA440-45) were investigated. The conventional forming limit curve was experimentally determined by using the Nakajima stretching test. Subsequently, the non-linear strain path FLCs were precisely developed through the Nakajima stretching test after the specimens were pre-stretched in biaxial direction up to several levels on the Marciniak In-plane stretching test. The gained non-linear strain path FLCs were compared with the conventional FLC.Additionally, the experimental Forming Limit Stress Curve (FLSCs) were calculated using the experimental FLC and non-linear strain path FLCs data from investigated steel sheet. The yield criterion Hill’48 was employed in combination with the Swift strain hardening law to describe anisotropic deformation and plastic flow behavior of the HSS sheet, respectively. Hereby, the influence of pre-stretching levels on the experimentally determined the FLCs and FLSCs were examined. The results prove a significant influence of the pre-stretching levels on the both FLCs and FLSCs of the investigated HSS sheet. For a low pre-stretching in biaxial loading the FLCs demonstrated a reduced formability and the FLSCs exhibited the limited stress levels depending on the experimental FLC data.

Possibilities for specific utilization of material properties for an optimal part design

High-strength, cold-formable steels offer great potential for meeting cost and safety requirements in the automotive industry. In view of strengths of up to 1200 MPa now attainable, certain aspects need to be analysed and evaluated in advance in the development process using these materials. In addition to early assessment of crash properties, it is also highly important to adapt the forming process to match the material potential. The steel making companies have widened their portfolios of cold-rolled dual-phase steels well beyond the conventional high-strength steels. There are added new grades which offer a customized selection of high energy absorption, deformation resistance or enhanced cold-forming properties. In this article the necessary components for material modelling for finite element simulation are discussed. Additionally the required tests for material model calibration are presented and the potentials of the thyssenkrupp Steel material data base are introduced. Besides classical tensile tests at different angles to rolling direction and the forming limit curve, the hydraulic bulge test is now available for a wide range of modern steel grades. Using the conventional DP-K®60/98 and the DP-K®700Y980T with higher yield strength the method for calibrating yield locus, hardening and formability is given. With reference to the examples of an A-pillar reinforcement and different crash tests the procedure is shown how the customer can evaluate an optimal steel grade for specific requirements. Although the investigated materials have different yield strengths, no large differences in the forming process between the two steel grades can be found. However some advantages of the high-yield grade can be detected in crash performance depending on the specific boundary and loading conditions.

05.03: Weld residual stresses effects in the design of welded plate girders: Simulation and Implementation

ABSTRACTEurocode 3 offers different possibilities on a numerically‐aided design of welded plate girders. In this, a geometrically and materially nonlinear analysis of the imperfect structure (GMNIA) provides the theoretically highest possible degree of realism. A correct implementation of “imperfections” is of critical importance in the practical application of this method. The current design rules are based on long term experience in terms of traditional (“old”) welding procedures and regular (“low strength”) steel grades. Thus, they provide very approximate (but safe) rules for these structures. With the introduction of modern shop welding operations and materials, many of these assumptions seem to be over‐conservative. A revision of these rules is being processed in different national and international research projects. This particular study focusses on the weld residual stresses and their effects in conventional steel S355 and high strength steel S690. Metal active gas (MAG) welding under shop welding conditions was used. The measurement of weld residual stresses on component‐like samples (carried out by the BAM Bundesanstalt für Materialforschung und ‐prüfung in Berlin) has been widely published. The hereupon based (simplified) welding simulation as part of a subsequent structural analysis is shown here. A novel numerical approach how to directly incorporate the results into large structural components models is presented. The capacity analysis is performed on a simple example for weak‐axis buckling. The results indicate significant improvements in the utilization of load‐bearing capacity and underline the superior buckling assignment for the S690 compared to conventional S355. The experimental validation of the findings is currently being prepared.

Stripping failure of punching pin in GPa-grade steels

GPa-grade steels which possess the ultimate tensile strength over 1000 MPa and higher elongation have been thoroughly researched to replace conventional high-strength steels in automobile industry to enhance fuel efficiency without sacrificing crashworthiness. However, high strength of GPa-grade steel does not only induce punching pin buckling due to high contact pressure, but also abrasive wear on the mold surface due to the unstable contact that results from the large amount of springback, which tend to impede widespread usage of GPa-grade steels. In this research, design criteria for an optimal aspect ratio of the punching pin preventing buckling failure has been proposed by linking with experimental and numerical analyses, which is validated with the analytic solutions. Furthermore, in order to prevent stripping failure, it is highly recommended that one of the main factors, a blank holder force (BHF), over 1.5 MPa has to be maintained during the withdrawal process to minimize the effect of springback in stamped panels applying GPa-grade steel, which is able to extend the tool life of the side punching pin about 100,000 cycles.

Effects of Q&P Processing Conditions on Austenite Carbon Enrichment Studied by In Situ High-Energy X-ray Diffraction Experiments

We report the first ultra-fast time-resolved quantitative information on the quenching and partitioning process of conventional high-strength steel by an in situ high-energy X-ray diffraction (HEXRD) experiment. The time and temperature evolutions of phase fractions, their carbon content, and internal stresses were determined and discussed for different process parameters. It is shown that the austenite-to-martensite transformation below the martensite start temperature Ms is followed by a stage of fast carbon enrichment in austenite during isothermal holding at both 400 and 450 °C. The analysis proposed supports the concurrent bainite transformation and carbon diffusion from martensite to austenite as the main mechanisms of this enrichment. Furthermore, we give evidence that high hydrostatic tensile stresses in austenite are produced during the final quenching, and must be taken into account for the estimation of the carbon content in austenite. Finally, a large amount of carbon is shown to be trapped in the microstructure.

In-situ investigation of quenching and partitioning by High Energy X-Ray Diffraction experiments

We report the first ultra-fast time-resolved quantitative information on the quenching and partitioning process of conventional high strength steel by in-situ High Energy X-Ray Diffraction experiment. The time and temperature evolutions of phase fractions, their carbon content and internal stresses were determined. The austenite to martensite transformation below Ms is followed by a stagnant stage during which microstructural state remained unchanged. Afterwards, a fast kinetics of carbon enrichment of austenite during the partitioning step at 400°C is highlighted. The analysis proposed supports the carbon diffusion from martensite to austenite as the main mechanisms responsible for this enrichment.

The buckling resistance of welded plate girders taking into account the influence of post-welding imperfections – Part 1: Parameter study

Abstract Welding is the most important joining technique and offers the advantage of customizable plate thicknesses. On the other hand, welding causes residual stresses and deformations influencing the load carrying capacity. Their consideration in the design requires simple and fast models. Though welding simulation has contributed to accurately access to these values nowadays, their application to large components remains still in a less practicable range. Nevertheless, many studies emphasized the need to make corrections in recently available simplified models. Especially the influence of residual stresses seems somewhat overestimated in many cases if comparing conventional structural steel S355 and high-strength steel S690. In times of computer-aided design, an improved procedure to implement weld-inducted imperfections appears overdue. This will be presented in two parts. The first part illustrates the potential influence of post-welding imperfections exemplified for weak axis buckling in comparison with the general method in accordance with Eurocode 3. Residual stresses and initial crookedness were varied systematically in order to produce a scatter band of capacities. An approach to characterize the borders of these imperfections was untertaken before that. The excessive scattering of reduction factors for the load bearing capacity demonstrates the importance of these variables. Results were finally evaluated against advanced simulation models which will be further detailed in part two of this contribution.

Effect of Annealing Temperature and Time on Microstructure and Mechanical Properties of Multilayered Steel Composite Sheets

Multilayered composite steels consisting of alternating layers of martensitic phase and austenitic phase exhibit an excellent combination of strength and elongation compared with conventional advanced high-strength steels. The deformation processes underlying these properties are of considerable interest. In this article, microstructure, grain size, and phase are characterized by scanning electron microscopy (SEM) and electron backscattering diffraction. The hardness of each layer is analyzed by a microindentation hardness testing system. Finally, the deformation and failure processes in multilayered steel are investigated by in-situ SEM. The hardness results indicate that various hardening modes occur in the soft austenitic layer and the hard martensitic layer. In-situ SEM results combined with microstructure analysis and hardness results reveal that annealing temperature and annealing time have a significant impact on final microstructure, fracture behavior, strength, hardness, and ductility.

Third generation of advanced high-strength steels: Processing routes and properties

The automobile industry is presently focusing on processing of advanced steels with superior strength–ductility combination and lesser weight as compared to conventional high-strength steels. Advanced high-strength steels are a new class of materials to meet the need of high specific strength while maintaining the high formability required for processing, and that too at reasonably low cost. First and second generation of advanced high-strength steels suffered from some limitations. First generation had high strength but low formability while second generation possessed both strength and ductility but was not cost effective. Amongst the different types of advanced high-strength steels grades, dual-phase steels, transformation-induced plasticity steels, and complex phase steels are considered as very good options for being extended into third generation advanced high-strength steels. The present review presents the various processing routes for these grades developed and discussed by different authors. A novel processing route known as quenching and partitioning route is also discussed. The review also discusses the resulting microstructures and mechanical properties achieved under various processing conditions. Finally, the key findings with regards to further research required for the processing of advanced high-strength steels of third generation have been discussed.