S700MC Steel Research Articles

Effect of Corrosion and Post-Weld Treatment on the Fatigue Behavior of Multipass Robot GMAW Welds of S700MC Steel

High-strength steel is a candidate material for offshore structures, which are currently being constructed with regular-strength steel. These structures are constantly exposed to harsh environmental conditions and experience cyclic loadings, which can lead to premature failure due to the synergistic effects of corrosion and fatigue. In this regard, the current study aims to investigate the effects of corrosion and High-Frequency Mechanical Impact (HFMI) treatment on the fatigue behavior of welded joints made of S700MC steel. Multipass butt-welded joints were fabricated via the Robot GMAW method at an optimally selected heat input of 0.7405 kJ/mm. The microstructure of the weldments was studied using light optical microscopy. Tensile and Vickers microhardness tests were performed to evaluate the mechanical properties of the welded joints. To simulate marine environment corrosion in the laboratory, the as-welded samples were exposed to salt fog spray for 720 h. Subsequently, specimens were subjected to cyclic loading to evaluate their fatigue strength, while SEM and stereomicroscopy were used to analyze the fractured surfaces, providing a comprehensive understanding of the fracture mode. The findings suggest that although corrosion led to increased surface roughness and the formation of corrosion pits, its influence on the fatigue behavior of the weldments might be less significant compared to other geometrical factors, at least for the exposure time employed in the study.

Formation of Oxides and Sulfides during the Welding Process of S700MC Steel by Using New Electrodes Wires.

To receive a high-quality welding structure of high-strength S700MC steel for applications in the automotive industry, newly developed electrode wires with increased silicon and manganese content were used. The strength and structural tests of the obtained joints were performed. In the weld, we identified the beneficial oxides strengthening the joint structure and unfavorable MnS inclusions. The non-metallic inclusions were formed inside the weld. Their arrangement, morphology, and chemical composition is described. A view on the high-temperature mechanisms of the formations included during the welding process with new electrode wires is presented. It was found that the dominant mechanism of the inclusion formation and the temperature of the welding process impact the content and varied morphology of inclusions, thus determining the exploitation time of the welded joints obtained. The obtained MAG joints made S700MC steel, due to the formation mainly of oxide inclusions and a relatively small amount of MnS phase, were characterized by a high value of yield and tensile strength, which makes them a promising solution for the automotive industry, especially against the background of connections from the discussed steel grade presented in the literature.

The effect of thermomechanical welding on the microstructure and mechanical properties of S700MC steel welds

Grain refinement by plastic deformation during conventional TIG welding can help to compensate for the loss of mechanical properties of welded joints. The thermomechanical welding (TMW) tests were performed on S700MC steel with different combinations of TIG arc energy and high frequency hammering over three target cooling times (t8/5 = 5s, 15s, and 25s). Additionally, the effect of initial microstructures on the weld joint quality was analysed by testing three materials conditions: hot-rolled (as-received) and cold-rolled with 10% and 30% thickness reductions, respectively. The effects of plastic deformation and the mechanical vibration on the grain refinement were studied separately. Optical microscopy, electron backscattered diffraction, and Vickers hardness were used to characterise the weld microstructure heterogeneity. The weld width and depth and the mean grain size were correlated as the function of cooling time t8/5. The results show that the weld dimensions increase with increasing the t8/5. The weld microstructures transformed from the mixed martensite and bainite into mixed ferrite and bainite with increasing the t8/5 time, and the related mean grain size increased gradually. The TMW welds exhibit smaller grains compared to TIG welds due to the coupled effects of mechanical vibration and plastic deformation. The mechanical vibration contributes to weld metal homogenisation, accelerating TiN precipitation in the fusion zone. The proposed TMW process can refine the weld microstructure of S700MC steel, enhancing its mechanical properties.

Control of the Martensitic Transformation During Resistance Spot Welding of High Strength S700MC Steel

Control of the Martensitic Transformation During Resistance Spot Welding of High Strength S700MC Steel

Characterization of the heat-affected zones in the laser welded joints of quenched and thermo-mechanically processed high-strength steels

The objective of this research is to compare the heat-affected zone (HAZ) characteristics of quenched and thermo-mechanically processed high-strength steels. Laser-welded butt-welded joints of S690QL, S1100QL, and S700MC steels were used for the analysis. The aim was to describe the effect of microstructure, chemical composition, and manufacturing process on the HAZ´s properties. The considered parameters included the grain size, the width of individual HAZ subzones, and the distribution of hardness throughout the HAZ. The results showed that the content of C, Mo, and microalloying elements Nb and Ti has the greatest influence on the HAZ´s properties. This was observed mainly through the average grain size in the CGHAZ and the hardenability of the CGHAZ and FGHAZ. Based on measured data, the mean grain size of quenched steels is up to 15% larger and the width of CGHAZ is 20% wider than that of thermo-mechanically processed steels. At the same time, it is possible to observe a martensitic structure in the CGHAZ and FGHAZ of QL steels that is absent in S700MC steel.

Assessment of the Effect of Residual Stresses Arising in the HAZ of Welds on the Fatigue Life of S700MC Steel

Fine-grained steels, which belong to the HSLA (High-Strength Low-Alloy) group of steels, are increasingly used for parts of statically and dynamically loaded constructions. Due to the thermal effect of welding, combined with the inherent stiffness and clamping stiffness of the part, residual stresses are generated in the HAZ (heat-affected zone) which affect the fatigue life of the sub-weld and the entire construction. In this article, a specific temperature cycle measured during welding is used, which, together with a defined clamping stiffness, produces residual stresses of a defined shape and value in the sample. Subsequently, the effect of these stresses on the fatigue life on the change of the S–N curve compared to the annealed material, is assessed. Temperature cycles were applied using a Gleeble 3500 and the residual stresses were analyzed by X-ray diffraction (XRD). It was found that the effect of residual stresses decreased the fatigue strength by 33% compared to the annealed material. It was further found that by using annealing to reduce the residual stresses, it is possible to restore the fatigue life of S700MC steel to the original value of the base material.

Beneficial use of hyperbaric process conditions on the welding of high-strength low alloy steels

Hyperbaric welding is used for different steels in many underwater applications as a repair welding process. A difference between wet and dry welding processes can be made. Due to the increased ambient pressure, these processes have special features inherent in the process that influence the cooling and penetration behaviour. The positive use of these effects outside underwater applications is currently rarely addressed in science and application. The presented work establishes these advantages on the basis of a higher strength structural steel and characterizes the effects on the microstructure of a joined S700MC steel and on the mechanical properties of the joint. It will be shown that a hyperbaric environment can be used to orient the weld more towards the depth of the sheet. Furthermore, it will be shown that this change leads to modified cooling, which in itself influences the mechanical properties of the weld seam.

Thermo mechanical and microstructural constituents of dissimilar S700MC-S960QC high-strength steel welded joints using overmatched filler wire

In this study, a mechanical and microstructural constituent of welded joints of dissimilar high-strength and ultra-high-strength steels (S700MC/S960QC) using overmatched filler wire was evaluated. Three different heat inputs (18 kJ/cm, 8 kJ/cm, and 10 kJ/cm) and overmatched filler wire were applied using the GMAW process. Micro-hardness measurement was conducted using Vickers hardness test, tensile test, and microstructural constituents by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed in this analysis. A dissimilar welded sample with a cooling time of 51s (18 kJ/cm) showed a substantial hardness reduction in the fine grain heat-affected zone (HAZ) of the S700MC steel side, which was between 220-240 HV5. The cause could be the increase of heat input but also the use of overmatched filler wire, which can be the result of an increase of ferrite to austenite formation in the HAZ. SEM/EDS results confirmed the increase of carbide clusters, and tempered Martensite in the CGHAZ of the S960QC side, and ferrite-bainite on the S700MC side. An increase in the presence of high carbide content and the formation of Ni, Mo, and Mn were observed on the S960QC side. The increase of carbide formation in the CGHAZ of both sides reduced the hardness and strength of the welded joints. The tensile test confirms the softening observed in the FGHAZ of the S700MC side, which caused the joint fracture on this side during the test. Â

Effect of coiling temperature on the structure and properties of thermo-mechanically rolled S700MC steel

The boron-free S700MC steel is usually produced by exploiting the properties of a ferrite-bainite mixed microstructure formed by coiling the strips at a temperature of about 450?C, i.e.below the bainite starting temperature. With the aim of further enhancing the mechanical properties of 6 to 10 mm thick strips, industrial tests were carried out at a coiling temperature of 600?C to promote the formation of a structure of ferrite and carbides, which is also acceptable for this type of steel. Unexpectedly, a microstructure composed of ferrite and martensite was obtained. Compared to the ferritic-bainitic grade, the new structure is characterized by a slight decrease of the yield point but by an increase of the ultimate tensile strength by no less than 80 MPa, with a transition from a quasi-discontinuous to a clearly continuous yielding behaviour. Accordingly, the ratio of yield strength to tensile strength decreases from 0.90 to 0.75 and the impact energy decreases by 35 J and 60 J for the two gauge levels, respectively. The mechanical behaviour of the strips coiled at high temperature is explained as a direct consequence of the dual phase structure with a hard phase interspersed in a soft ferrite matrix. The presence of martensite is explained by the so-called incomplete bainite reaction. The partial transformation into ferrite after coiling and the long time required for the coil to cool down stabilize the untransformed austenite due to the carbon enrichment making bainite formation at lower temperatures impossible.

The high-strength steel and its weld under impact

The paper presents the results of Charpy impact tests conducted on the S700MC steel and its weld under the range of impact energy amounts from 50 J to 300 J. The material responses in the form of accumulated energy, force, projectile velocity, and deflection versus time are determined and discussed in detail. Fracture zones were analyzed by means of the macro-photography technique giving their features such as a shape of cracks and type of damage (brittle or ductile). In case of the parent material and weld an influence of the impact energy amount on their properties was well visible. A significant change of the fracture zone features with the increase of the energy amount was observed during an examination of the steel. The weld fracture was less sensitive on the energy amount. Physical magnitudes for the steel and the weld expressed a 30% reduction of the accumulated energy in comparison of that for the joint observed. In the case of weld the values of projectile velocity exceeded those for the parent material obtained. Deflection courses of the parent material and weld tested were the same at the initial phase of the experiment, however, finally they were higher for the weld material. The data captured during this research enabled a technical quality examination of the welding joint tested.

Plasma Welding of Thermo-Mechanically Processed Steel

The article presents the course and results of tests involving the technological plasma welding of 3 mm thick butt joints made of steel S700MC using the melt-in welding technique and the key-hole welding technique. The technological tests made it possible to determine parameters enabling the obtainment of joints representing quality level B in accordance with the PN-EN ISO 5817:2014 standard. The tests discussed in the article included tensile tests, bend tests, hardness tests as well as macro and microscopic metallographic tests involving the use of light microscopy. The test results were subjected to detailed analysis performed to identify the effect of high arc energy used in plasma welding on the structure and properties of the test joints. The test results revealed that the melt-in welding technique (characteristic of the TIG method) did not ensure the obtainment of minimum tensile strength.

Analysis of precipitates in heat treated thermo-mechanically processed steel with a high yield strength

The aim of this study was to analyse the precipitates and determine their impact on the structure of thermomechanically processed high strength S700MC steel. The tested samples were subjected to a heating process at temperatures from 100°C up to 1300°C with a 100°C step. The chemical composition analysis in the micro-areas of the samples, thin-film tests and quantitative measurements of carbonitride precipitates were performed. In order to describe the grain size, a method of calculating the number of grains cut through the measuring line was used. In order to determine the effect of heat treatment on strength and plastic properties, Charpy V-notch, Vickers hardness and static tensile tests were carried out. The study showed that S700MC steel is characterized by a non-equivalent, fine-grained bainitic-ferritic structure. The test showed a significant impact of heat treatment on the strength and plastic properties of S700MC steel.

Numerical Modeling for the Submerging Arc Welding of S700MC Steel

This paper presents a finite element model of submerged arc welding. The modeling of the filler material has been performed by applying the technique for deactivation and subsequent activation of the finite elements involved in the welding seams. The nonlinear material properties are taken into account. The description of the peaks in the specific heat capacity in the phase changes has been done by introducing latent heat. The welding arc is modeled as a heat source with a constant heat flux density. Experiments have been conducted to validate and verify the numerical model. The influence of the ratio between surface and volume of the set heat sources has been studied. Tack welds have been taken into account. Data on the relationship between the accuracy of the obtained results and the number of passages has been presented.

Effect of the Root Gap on the Structure and Properties of High Strength Steel S700MC Welds

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.

MAG Welding Process with Micro-Jet Cooling as the Effective Method for Manufacturing Joints for S700MC Steel

Advanced high-strength steel (AHSS) steels are relatively not very well weldable because of the dominant martensitic structure with coarse ferrite and bainite. The utmost difficulty in welding these steels is their tendency to crack both in the heat affected zone (HAZ) and in weld. The significant disadvantage is that the strength of the welded joint is much lower in comparison to base material. Adopting the new technology regarding micro-jet cooling (MJC) after welding with micro-jet cooling could be the way to steer the microstructure of weld metal deposit. Welding with micro-jet cooling might be treated as a very promising welding S700MC steel process. Tensile and fatigue tests were mainly carried out as the main destructive experiments for examining the weld. Also bending probes, metallographic structure analysis, and some non-destructive measurements were performed. The welds were created using innovative technology by MAG welding with micro-jet cooling. The paper aims to verify the fatigue and tensile properties of the thin-walled S700MC steel structure after welding with various parameters of micro-cooling. For the first time, micro-jet cooling was used to weld S700MC steel in order to check the proper mechanical properties of the joint. The main results are processed in the form of the Wöhler’s S–N curves (alternating stress versus number cycles to failure).

Usage of Ti-surface-modified filler material to increase the joint strength of High-Strength Low Alloyed (HSLA) steels under different load types

Welding-related loss of strength, especially in the case of fatigue, significantly reduces the range of applications for high-strength fine-grained structural steels. In order to counteract this situation, the aim of the work is to increase the strength of welded joints made of high-strength fine-grained structural steels by using coated welding consumables. This is described using the example of a titanium coating for quasi-static and abrupt dynamic load and fatigue. The thermomechanical rolled fine-grained structural steel S700MC is used as the base material, using a welding filler of the same type. MAG welding was used to produce the fillet welds on a T-joint. In addition to tensile tests at four different load speeds up to 2 m/s, the results of fatigue tests are presented. In addition, the microstructure of the weld seams is examined by metallographic methods and the scanning electron microscope. A comparison with two joints from an unmodified variant and another steel grade with comparable properties (S690QL) serves to classify the results. It is shown that the use of modified filler metals has a significant influence on the overall strength of the welded joint due to the rounding of the weld toe. Thus, the fatigue strength can be increased by around 50%. In addition, the strength under sudden dynamic load can be increased by 10%.

Experiments and Numerical Simulations of the Annealing Temperature Influence on the Residual Stresses Level in S700MC Steel Welded Elements.

The article presents the results of research on the influence of temperature and time changes of the annealing process on the values and distribution of stresses in the simulated heat-affected zone of S700MC steel welded joints. For this purpose, tests were carried out on a thermal cycle simulator, as well as heating the prepared samples in accordance with the recorded welding thermal cycles, and then annealing at temperatures from 200 to 550 °C. The stresses values in the tested samples before and after the annealing process were measured by using X-ray diffraction (XRD). The performed tests were verified with the results of numerical analyses using the finite element method (FEM) performed in the VisualWeld (SYSWELD) environment as, on the one hand, the verification of the obtained results, and, on the other hand, the source of data for the development of a methodology for conducting analyses of heat treatment processes of S700MC steel welded structures. Also presented are three examples of numerical analyses for Gas Metal Arc (GMAW), laser and hybrid welding and then the annealing process of the obtained joints at selected temperatures. The main purpose of the work was to broaden the knowledge on the influence of annealing parameters on the values and distribution of stresses in welded joints, but also to signal the possibility of using modern software in engineering practice.

Influence of the Welding Process on the Mechanical Characteristics and Fracture of the S700MC High Strength Steel under Various Types of Loading.

This paper focuses on the mechanical properties analysis of the high strength S700MC steel applied in welding joints. The research comprised mechanical tests for checking what the changes of tensile characteristics, mechanical parameters, resistance to impact, and fracture toughness look like in selected regions of the welding joint. Stress-strain curves have shown significant differences in the tensile characteristic shape and the values of Young’s modulus, yield stress, ultimate tensile strength, and ductility due to the welding process applied. In the case of Charpy tests, the courses of the accumulated energy, force, deflection, and project velocity are presented, indicating the maximum value of absorbed energy, the same level of force during the first contact of the projectile with the specimens, and the significant variation of the velocity for the impact energy ranging from 50 J up to 300 J. On the basis of the fracture toughness tests, the distributions of the CTOD (Crack Tip Opening Displacement) values are presented for the parent material, HAZ (Heat Affected Zone) and weld. Moreover, the characteristic features of the fatigue pre-crack, transient and crack propagation zones are identified and discussed.

Deep-Penetration Welding as a Method for Saving Materials and Increasing the Load-Carrying Capacity of Welded Structures

The study presents possible savings resulting from the use of deep-penetration welding processes in the fabrication of welded structures. The study-related tests revealed 80% savings of filler metals and the 50% reduction of welding distortions without compromising the maximum load-carrying capacity of welded joints. The tests involved steel grades S355J2, S460NL, S700MC, S690QL and 450HBW (Hardox) having thicknesses restricted within the range of 8 mm to 20 mm as well as filler metal grades G4Si1 and G69.

Electron Beam Welding of TMCP steel S700MC

The article discusses tests aimed to determine the effect of electron beam welding on the properties of butt welded joints made in 10 mm thick TMCP steel S700MC. The welding process was performed in the flat position (PA) using an XW150:30/756 welding and surface processing machine (Cambridge Vacuum Engineering). The joints obtained in the tests were subjected to non-destructive tests including visual tests and magnetic particle tests. The joints were also subjected to destructive tests including static tensile tests, bend tests, toughness tests (performed at a temperature of -30°C), hardness tests as well as macro and microscopic metallographic tests. The destructive tests revealed that the joint represented quality level B in accordance with the PN EN ISO 13919-1 standard. The analysis of the destructive test results related to the electron beam butt welded joint (made in steel S700MC) revealed its high mechanical and plastic properties. The toughness tests revealed a decrease in toughness in the HAZ (27 J/cm2) in comparison with that of the base material (50 J/cm2). In addition, the hardness of the HAZ and of the weld increased up to approximately 330 HV; the hardness of the base material amounted to 280 HV.