Papers by Keyword: High Strength Steel

Abstract: Recently, light-weight and energy-saving requirements for the automobile industry are extremely important in order to protect the environment by a reduction of the emission of CO₂. Hence, high-strength steel (AHSS), even ultrahigh strength steels with tensile strength larger than 1GPa is used. Among AHSS, cold-rolled martensitic steels have attracted much attention due to their superior strength to improve crashworthiness. In this research, the influence of different annealing treatments, especially the auto-tempering, on the phase transformation further affecting the mechanical properties and microstructure was investigated. The result shows that the level of auto-tempering and strength in martensitic steel is dominated by the quenching/auto-tempering temperature. Furthermore, the auto-tempering carbides should be cementite which is fine enough to improve yield strength. The suitable chemical composition combined with auto-tempering method has been implemented to develop cold-rolled martensitic steels with a tensile strength of exceeding 1300MPa. These developed martensitic steels can meet the requirements of bumper reinforcement which has applied in a variety of automobiles.

Abstract: The application of high strength structural steels in welded structures is growing steadily and intensively. Quenched and tempered (Q+T) as well as thermomechanically treated (TM) steel base materials are developing faster than the filler metals for fusion welding processes, and therefore the selection of filler metal deserves special attention. Welded structures made of high strength steel are often subjected to cyclic loading, which can cause initiation and propagation of fatigue cracks and can lead to fatigue fracture failure of the structural element or the structure. This characteristic must also be taken into account when selecting the filler metal. In order to study this issue, welded joints were made from base materials in the 700-1300 MPa strength category using gas metal arc welding (GMAW) process. The applied filler metals were of the undermatching, matching or overmatching type, depending on the strength of the base material. Fatigue crack propagation (FCP) tests were performed on specimens machined from the welded joints, in which notch locations and crack propagation directions were statistical in nature. Therefore, the fatigue crack propagation directions were parallel and perpendicular to the longitudinal axis of the welded joints and located in different zones of the heat affected zone (HAZ). From these investigations, the two parameters (C and n) of the Paris-Erdogan equation were determined for each specimen and statistical samples were formed from the base material-filler metal matching pairs. During the evaluation of the results, it was found that the matching phenomenon has significant effect on the fatigue crack propagation behavior of the welded joints and that this effect depends on the strength category of the base material. Based on these results, recommendations for the applicable base material-filler metal pairings were proposed.

Abstract: When investigating the heterogeneous properties of welded joints, mechanical testing of certain Heat Affected Zone (HAZ) regions, using standard approach test specimens, is very difficult or sometimes even impossible. Inability to precisely position and extract mechanical test specimens, even ones of the subsize dimensions, from the narrow HAZ regions is a limiting factor in the mechanical testing implementation. Detailed investigation of the HAZ is made possible by the use of thermo-mechanical simulations on the Gleeble welding simulator. In scope of this paper several characteristic HAZ microstructures of S690QL grade High Strength Steel (HSS) are being simulated. Multi-pass welding simulations are done on special 10x10 mm square section bar specimens in order to reproduce thermal gradients and characteristic microstructures at any location in a weld. Such simulated HAZ microstructures are of a sufficiently large volume, with homogeneous and repeatable properties, that standard specimen methods for mechanical testing can be readily implemented. Metallographic optical examinations, as well as hardness measurements were done initially. Mechanical properties are focused on determining stress-strain curves for each characteristic weld region. The paper investigates whether the mechanical properties of Gleeble simulated hard-soft combined HAZ regions are better in comparison to exclusively hard or soft HAZ regions. The obtained results can subsequently be used for the material model development.

Abstract: Implementation of high strength steels in welded structural designs in the automotive, defence and construction industries is constantly increasing. Prolonged usage of such structures requires deep understanding of welded joint fatigue as well as a reliable and feasible life estimation methods development. Conventional fatigue testing methods often require costly, expensive in maintenance, high loading capacity equipment. They are also commonly restricted to specific specimen geometry and are time consuming due to the limit of a single specimen per test setup. This work presents high cycle fatigue (HCF) testing of high strength MARS 600 welded steel using a quick, simple and efficient resonance fatigue testing (RFT) method. The specimen is a simple cantilever fillet welded to a base plate using austenitic stainless steel 307L consumable electrode. Electrodynamic shaker is used for harmonic base excitation at a constant operating frequency. Several specimens welded to the common base are tested simultaneously, allowing completion of a high number of cycles and statistics in a relatively short time period. A hybrid, practical research approach combining experimental, finite element analysis (FEA), numerical and analytical calculations is presented. Fracture mechanics approach for fatigue life assessment is implemented. Crack growth calculation is based on the Paris - Erdogan law. Reduction in structural integrity due to crack propagation causes a reduction in natural frequency and transmissibility. The change in gain is evaluated via the open crack FEA model and integrated into the crack propagation algorithm. Resonance search, track and dwell module (RSTD) for maintaining constant gain throughout the test is not required. Fatigue life Wohler (SN) curve is constructed. Standard weld fatigue data is often provided for direct loading (tensile stress) and for different stress ratio (R) values. Corrections for mean stress and loading application are required. Current fully reversed (R = -1), indirect loading (bending stress) test results may be readily applied for random vibration fatigue analyses post processing. As expected, actual fatigue life results are higher compared to standard design curves, implying correctness of the manufacturing welding process of examined specimens. The presented procedure is of interest for research as well as for industrial welding processes testing, optimization and qualification.

Abstract: In this study, elastic waves were detected when different bending stresses were applied to cracked specimens of high-strength steel (SKD11: HV550) immersed in a 0.057 M solution of acetic acid (CH₃COOH), and frequency characteristics were analyzed using time-frequency analysis. The dominant frequency obtained using the tensile test was approximately 103 kHz, and those in the acetic-acid solution without stress were approximately 32 and 101 kHz. The dominant frequencies of the crack specimens in which cracks propagated were approximately 30–40 (F1), 60–85 (F2), and 100–110 (F3) kHz. An elastic wave was obtained by corrosion, pitting, crack initiation, and propagation but not during the hydrogen aggregation time. The dominant frequencies of the crack specimens without crack propagation were approximately 28–33 (F1) and 94–109 (F3) kHz. These were the same as the dominant frequency in the acetic-acid solution under nonstress conditions. The fractured surface showed many traces of pitting and corrosion regardless of the applied stress, resulting in microcracks in the Cr carbide. -----------------------------------------------------------------------------------------------------------

Abstract: Owing to the progressive use of cold-formed high strength steel (HSS) for transportation applications, a characterisation of the fatigue behaviour of HSS has become a focal point for material scientists and design engineers. To mimic the behaviour of cold-formed components, a specimen was adopted from previous research that features multiple bent sections. The geometry was obtained by consecutive bending operations at room temperature. When subsequent tensile cyclic loading is applied to the specimen, the localised damage from forming and stress concentrations cause crack initiation on the inside of the bent area. To investigate the effect of cold-forming on the fatigue behaviour experimentally, the evolution of the strain, displacement or stiffness can be monitored during fatigue testing. The current paper presents an experimental framework for investigating the strain fields of a bent specimen during fatigue. The evolution of the strain fields is then linked with characteristic fatigue mechanisms, such as crack initiation and growth.

Abstract: The need for welded structures of high-strength steels requires detailed studies on the factors influencing the behaviour of these steels during welding. The present work introduces results on the influence of the welding gap on the structure and some mechanical and technological properties of welded joints of high-strength steel S960QL, joined by submerged arc welding.

Abstract: This paper demonstrates the results of the study of microstructure and physical-mechanical properties of the high-strength economically alloyed Fe-Cr-Mo steel, developed by RosNITI JSC for the production of the oil country tubular goods (OCTG) (casing and tubing). The main requirement for this steel is to provide simultaneous increased strength and resistance to sulfide stress cracking (SSC). It was shown that this problem could be solved by special heat treatment. As a result, the structure of this steel consists of a secondary sorbite with a lower dislocation density. Hardening is provided by dispersion-strengthened V, Nb carbides.

Abstract: Nowadays, the use of high strength (HS) and ultra-high strength steels (UHS) increases, notably in welded constructions. These steels are mainly exploited in heavy loaded welded constructions such as bridges, cranes and excavators, in pressure vessels, vehicles, ships, drilling rigs etc. working at room or lower temperatures. As the welded constructions have specific requirements, the development of high strength and ultra-high strength steels imposes the need for research on the factors influencing their weldability. Among the possible negative implications are: cold cracks formation, softening of the heat affected zone, brittleness in the coarse grained zone. When complying with the generally accepted rules for welding, HS and UHS are readily welded by all conventional welding methods. Recommendations for welding of steels after normalization, thermo-mechanical treatment and quenching and tempering are given in the standard EN 1011 -1, 2. The use of thermo-mechanically treated (hot-rolled) steels with low carbon equivalent, such as S700MC, allows reduction in time required for welding as the preheating temperature is lowered or even preheating is not necessary. A more pronounced negative effect on the weld quality has the presence of different defects. S700MC can be welded by all conventional methods, and a reduction in the softened zone can be achieved by using appropriate welding parameters. Joint preparation for welding of HS and UHS steels is described in the standards EN ISO 9692-1:2013 and EN ISO 9692-2:2001. Nevertheless, the root gap is often the closing part in constructions and does not comply with the standard recommendations. That is why the effect of the root gap on welds has to be researched. The present work introduces results of a research studying the effect of the root gap on the structure and some mechanical and technological properties of S700MS welds, welded by submerged arc welding.

Abstract: Recently the high strength steel has been applied in the automotive more and more widely. In this study, the effect of blank shape on the formability of an automotive part was analyzed. The three kinds of blank shapes were chosen, including a rectangular shaped blank, a blank with two corners cut straightly and a blank with two corners cut in curve. The effect of the variable blank holder force on the formability was studied. The four kinds of variable blank holder force were applied. The blank shape in this part is the blank with two corners cut curve. The results show that the blank with two corners curve is the most suitable. And the blank holder force from 1000 kN to 1500 kN is the most useful for the formability.