Steel Components Research Articles

Modeling of Hydrogen-Charged Notched Tensile Tests of an X70 Pipeline Steel with a Hydrogen-Informed Gurson Model.

Hydrogen can degrade the mechanical properties of steel components, which is commonly referred to as "hydrogen embrittlement" (HE). Quantifying the effect of HE on the structural integrity of components and structures remains challenging. The authors investigated an X70 pipeline steel through uncharged and hydrogen-charged (notched) tensile tests. This paper presents a combination of experimental results and numerical simulations using a micro-mechanics-inspired damage model. Four specimen geometries and three hydrogen concentrations (including uncharged) were targeted, which allowed for the construction of a fracture locus that depended on the stress triaxiality and hydrogen concentration. The multi-physical finite element model includes hydrogen diffusion and damage on the basis of the complete Gurson model. Hydrogen-Assisted degradation was implemented through an acceleration of the void nucleation process, as supported by experimental observations. The damage parameters were determined through inverse analysis, and the numerical results were in good agreement with the experimental data. The presented model couples micro-mechanical with macro-mechanical results and makes it possible to evaluate the damage evolution during hydrogen-charged mechanical tests. In particular, the well-known ductility loss due to hydrogen was captured well in the form of embrittlement indices for the different geometries and hydrogen concentrations. The limitations of the damage model regarding the stress state are discussed in this paper.

Microstructure and dynamic mechanical behavior of wire-arc additive manufactured high-strength steel

The working environment of high-strength steel components produced by wire-arc additive manufacturing (WAAM) often faced challenges of explosion and impact, but the microstructure evolution and dynamic mechanical behavior of WAAM produced high-strength steel were still unclear. In this study, low alloy high-strength steel （HSS） components were produced by WAAM. Split Hopkinson Pressure Bar (SHPB) tests at different strain rates were carried out on WAAM-HSS, and the specimens before and after dynamic compression were characterized by techniques such as SEM, EDS, XRD, EBSD, etc. The results showed that： The as-built WAAM-HSS plate mainly consists of five typical structures: lath martensite (ML), acicular ferrite (AF), polygonal ferrite (PF), martensite-austenite (M-A), and precipitated phase δ. The central area of the precipitated phase is Si and Mn, and the C element is mainly distributed in the surrounding area of the precipitated phase. Under dynamic compression, the as-built WAAM-HSS exhibited obvious yield phenomenon, and the yield stress increased with the increase of strain rate. The yield-strengthening mechanism is mainly contributed by grain refinement strengthening mechanism and dislocation density strengthening mechanism. Under dynamic compression, the martensite transformation occurred in the WAAM-HSS. The deformation mechanism is dislocation slip and twinning, which compete with each other. Dislocation slip deformation is dominant at low strain rate and twinning deformation is more dominant at high strain rate.

Quantitative-regulated material removal rate in solid dielectric electrochemical polishing (QRR-SDEP) for smoothing high roughness surface of additively manufactured 316L stainless steel components

Quantitative-regulated material removal rate in solid dielectric electrochemical polishing (QRR-SDEP) for smoothing high roughness surface of additively manufactured 316L stainless steel components

Effect of Heat Treatments and Carbon Content on the Damping Properties of Structural Steel

Low- and medium-carbon structural steel components face random vibration and dynamic loads (like earthquakes) in many applications. Thus a modification to improve their mechanical properties, essentially damping properties, is required. The present study focuses on improving and developing these properties, significantly dampening properties, without losing the other mechanical properties. The specimens used in the present study are structural steel ribbed bar ISO 6935 subjected to heating temperatures of (850, 950, and 1050) ˚C, and cooling schemes of annealing, normalizing, sand, and quenching was selected. The damping properties of the specimens were measured experimentally with the area under the curve for the loading and unloading paths experienced from the tensile test. Considering the effect of different parameters on the damping properties, such as heat treatment temperatures, cooling rates, and carbon content, the results show that the damping properties in the annealing process at different temperatures have interesting damping properties, among other processes. Also, the highest damping energy for the annealing cooling scheme was attained at a heating temperature of 1050 ˚C, irrespective of the carbon content. Finally, better damping properties for the medium carbon content of (0.299%C) is achieved for all types of heat treatment process compared with a low carbon content of (0.188% C); and, in general, with increasing carbon content from medium to low, steel response to heat treatment increases and better damping properties are obtained.

Fractographic investigation and microstructural characterization of a diffusion-bonded sinter-hardened steel: A case study

This study analyzes the fracture on a sinter-hardened steel component during bench-testing. The component was made from a Fe-base powder with pre-alloyed Mo (1.5 %), diffusion-bonded Cu (2.0 %) and mixed C (0.7 %), cold-compacted and sinter-hardened in an endogas atmosphere. The fractured surface was analyzed by scanning electron microscopy to investigate the failure mechanism. The same fractured component and a new one, taken as a reference, were observed by optical microscopy to identify the metallographic features, which were compared through microhardness measures. The mean porosity and some morphological characteristics of pores were determined by image analysis. Density was measured using Archimedes' principle, carbon content was quantified by combustion analysis. Based on the results, the failure mode was related to microstructure, microhardness and porosity morphology. Critical aspects in the compaction phase and in the sintering process were identified: carburization due to endogas sintering atmosphere was related to the occurred failure.

Hybrid manufacturing of steel construction parts via arc welding of LPBF-produced and hot-rolled stainless steels

AbstractThe demand for free-form steel structures having improved performances, reducing labour and resource usage is increasing in the construction sector. Structural nodes are some of the most critical regions for steel structures characterised often by large dimensions. These nodes can exploit the geometrical freedom of metal additive manufacturing (MAM) processes. Laser powder bed fusion (LPBF) is arguably the most developed MAM process, which has limitations regarding the size of the parts to be produced. A way to overcome the size limits of LPBF for producing structural nodes while still exploiting its geometrical capacity is producing hybrid components by welding them to traditionally manufactured beams. Such hybrid joints would constitute a complex system from a mechanical design perspective requiring a systematic analysis in order to be certified for structural use. Accordingly, this work studies the mechanical behaviour of hybrid steel components generated by welding LPBF plates and quarto plates made of AISI 316L stainless steel. The work was guided by a case study based on a large steel node, which helped defining the requirements to fill the gap of the international standards. The mechanical characterisation of LPBF-produced plates and quarto plates, as well as the welded hybrid components revealed a maximum of 10% difference between the properties of the differently manufactured plates. Through the digital image correlation (DIC) analyses, the anisotropic deformation behaviour along the LPBF, weld seam, and quarto plate regions have been identified, and the properties after welding did not show relevant modifications. The tests allowed to define that the failure behaviour is mainly governed by interlayer bounds, and a 0.9 safety reduction parameter for considering the reduction of ductility induced by arc welding to LPBF. Finally, design and production suggestions have been provided for a correct evaluation of gross and effective sections of the designed nodes.

Effect of powder-pack aluminizing on microstructure and oxidation resistance of wire arc additively manufactured stainless steels

This study investigated the effect of powder-pack aluminizing treatment on the high-temperature oxidation of ER307 stainless steel components fabricated by wire arc additive manufacturing (WAAM) during isothermal oxidation at 1000 °C for 5 h, 25 h, and 50 h. Scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), X-ray diffraction (XRD), X-Ray fluorescence (XRF), nanoindentation testing, and oxidation testing were used to characterize the aluminized and non-aluminized samples produced by WAAM. The results showed that the powder-pack aluminizing increased the surface nano-hardness up to 13.95 GPa and the modulus of elasticity up to 159 GPa, as well as improving the microstructure of WAAM ER307 stainless steel. Indeed, aluminide coatings remained stable up to temperatures exceeding 1000 °C, and the growth of hematite, the main oxide phase, was inhibited by a preferential alumina growth (Al2O3), resulting in an improvement in oxidation resistance in the range of 46–70 %. In addition, owing to the advantages of low-temperature aluminizing, the microstructure, mechanical properties, and oxidation resistance of these alloys have been improved without causing sigma phase formations, which constitute a significant problem in high-temperature heat treatment of stainless steels.

Experimental research on shear performance of frontal fillet weld with beading defects

Weld beading is one of the typical welding defects commonly found in steel connections. Severe defects of weld beadings may significantly decrease the welding quality between the steel components; therefore, improving the safety risk in steel connection joints. To obtain a prediction model for shear resistance of frontal fillet weld (FFW) with weld beading defects, the influence laws of the weld beading on the shear properties of (FFW) was investigated in terms of the uniaxial shear loading test on 90 FFW specimens. The test results show that FFW specimens with a higher number of weld beads as well as greater toe height tend to suffer shear failure on one side of the weld. The contribution of the extended weld length to the strengthening of the ultimate shear load capacity of FFW specimens with weld defects is limited, while the amplification of the ductility ratio and initial shear stiffness is more pronounced. When the weld length exceeds 200 mm, the difference in weld toe height should be fully considered for the influence on the shear properties of the frontal fillet weld. Moreover, if the ratio of weld diameter to weld toe height is greater than 2, the impact of weld defects on FFW shear properties rises sharply as the weld diameter increases. A prediction model for the shear load capacity of FFWs with weld beading defects is proposed, which matches well with the test results and can serve as guidance for assessing the shear performance of FFWs with weld beading defects.

INJECTION MOLDED GLASS FIBER REINFORCED POLYPHENYLENE SULFIDE COMPOSITE WEAR IN SIMULATED FUEL CONDITIONS

The required lowering of CO2 emission results in increased loading of a fuel system components and thus materials, which require adaptation to the changed working conditions. The hypothesis was that glass-fibre reinforced polyphenylene sulphide is reliable in high loaded movable couplings with steel components under a hot gasoline environment. Plate samples were formed in injection moulding process where mould temperature was 140-150°C and injection pressure 165 MPa, while the temperature at last barrel distance was increased from 285°C to 325°C. Plates were annealed in 207°C/4h and cooled down -180°C/4h. Plates went through 4300h operational tests and 1.2E6 start/stop load intervals in gasoline mixture. The plates after 30-40% of the simulated operating time range showed signs of wear that were within the acceptable range for some applications. After 40-50% of the working time, there was damage on the edges of the tiles, followed by fatigue damage. Tests revealed that injection moulded 40% GF-PPS is suitable for high loaded parts only to a limited extent.

Effect of ultrasonic impact peening on stress corrosion cracking resistance of austenitic stainless-steel welds for nuclear canister applications

In nuclear applications, the utilization of austenitic-based stainless steel (ASS) components is an ideal material choice for storing spent nuclear fuels in dry storage canisters (DSC). However, due to the chloride rich environments in which they operate, failure in the form of stress corrosion cracking (SCC) frequently occurs. The following study assesses the efficacy of ultrasonic impact peening (UIP) on SCC susceptibility of gas tungsten arc welded (GTAW) stainless steel (SS) components, with an aim to control the SCC failure. The UIP induces residual compressive stresses (RCS) and a severely plastically deformed (SPD) layer containing nanograins on the surface. The superior SCC resistance of UIP'ed specimens is due to the synergistic effect of grain refinement and RCS. The underlying mechanisms for these findings were elucidated based on the modification of microstructural features and mechanical properties. These findings demonstrate that UIP is a potential technique for DSC application to mitigate the SCC susceptibility of ASS weld joints.

An experimental investigation into the influence of incorrect root gaps in welded-in transverse stiffeners on fatigue performance

The paper presents a comprehensive set of special experimental fatigue tests designed to determine the fatigue strength sensitivity of steel components, under circumstances where fabrication quality levels deviate from regulatory standards. Precise monitoring of the manufacturing process of test specimens containing internal imperfections facilitates accurate documentation of the phenomena that contribute to the eventual shape of the imperfection. Welded-in stiffeners with incorrect root gaps of varying sizes were subjected to fatigue testing, as such studies have never been carried out in experimental form before, as the background data to EN 1993-1-9 disclosed. The experimental investigation confirms numerical estimations, which predicted that even large root gaps do not result in lower fatigue strength values, when compared to stiffeners that were properly fitted. Moreover, details in welded beams that feature an intersection of three welds at the stiffener exhibit no discernible decrease in fatigue performance compared to rolled sections.

Influence of initial residual stresses on welding residual stresses in ultra-high strength steel S960

In general, the as-received steel components before welding have usually suffered from numerous manufacturing processes. There is no doubt that initial residual stresses (IRS) remain in the steels with different magnitudes and even signs if no any stress-relieving process is applied before welding. Nevertheless, the relationship between IRS and welding residual stresses (WRS) is still not unclear yet, especially in ultra-high strength steels (UHSS). The current work aims to elucidate the influence of IRS on WRS in ultra-high strength steel S960. The X-ray diffraction (XRD) was applied to measure both IRS and WRS here. The experimental results exhibit that IRS in the weld area are completely eliminated because of the metallurgical and mechanical melting effect, which have no influence on WRS. In the heated zone out of the weld area, initial longitudinal residual stress (σRSL) can significantly affect welding σRSLonly if it is in tension. In the cold area, WRS are the combination of IRS and welding-induced residual stresses (WIRS). Furthermore, IRS play the dominant role here in general. No matter how much and what sign IRS are in the cold area, IRS here almost have no impact on WRS in the weld area.

Experimental testing on timber connections considering the influence of gap size and intumescent sealants

AbstractThis paper investigates the influence of gap size (openings/spaces between two structural members) and the presence of intumescent sealants on the development of temperature when timber connections are exposed to fire conditions. The experimental results of 21 samples representing a concealed steel‐to‐timber connection configuration that was exposed with a 0, 3, 6, and 10 mm gap to the ISO834 standard fire for 120 min are presented. Half of the samples in each sample group were protected with an intumescent fire protection sealant. The temperatures in the timber were measured at various locations around the gap and directly next to the steel component. The experimental results show that the presence of a gap increases the temperature of the timber more than predicted by the thermal penetration models available, with current code guidelines (e.g., Eurocode 5) possibly being non‐conservative. The use of an intumescent fire protection sealant in the gap is shown to be an effective protection method, but that the application configuration is important to ensure effective protection, especially as the gap becomes smaller. The results also show that the use of an intumescent fire protection sealant in a gap increases the predictability of the temperatures in the timber in longer duration fires. Unprotected samples and 0 mm gap samples exhibited large variability in the thermal development outcomes over time.

A decoupled T-stub component model for the cyclic modelling of steel joints

The reliable estimation of the cyclic behaviour of steel beam-to-column joints is essential for the safe design of steel structures under seismic loading. The T-stub is an essential component in steel beam-to-column joints. This paper presents the framework and implementation of a decoupled component model for the T-stub under cyclic loading. This decoupled model splits the single hysteretic spring representation of the T-stub into two separate hysteretic springs, one representing the quadrant where tensile forces are present and the other the quadrant with compressive forces. The results indicate that because of the degradation of the response of the T-stub with multiple loading cycles, the equivalence between the single spring model, and the decoupled double spring model is not trivial. Exact equivalence rules are proposed for the definition of the properties of the decoupled model. The accuracy of the model is validated by comparison with experimental test results. The decoupled model overcomes the problem of using a single spring to represent the T-stub in a steel joint that is not able to lead to correct results because it is unable to mobilize compressive forces in the T-stub as it occurs.

Anisotropy Evaluation and Defect Detection on Laser Power Bed Fusion 316L Stainless Steel.

Because of rapid heating, cooling, and solidification during metal additive manufacturing (AM), the resulting products exhibit strong anisotropy and are at risk of quality problems from metallurgical defects. The defects and anisotropy affect the fatigue resistance and material properties, including mechanical, electrical, and magnetic properties, which limit the applications of the additively manufactured components in the field of engineering. In this study, the anisotropy of laser power bed fusion 316L stainless steel components was first measured by conventional destructive approaches using metallographic methods, X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). Then, anisotropy was also evaluated by ultrasonic nondestructive characterization using the wave speed, attenuation, and diffuse backscatter results. The results from the destructive and nondestructive methods were compared. The wave speed fluctuated in a small range, while the attenuation and diffuse backscatter results were varied depending on the build direction. Furthermore, a laser power bed fusion 316L stainless steel sample with a series of artificial defects along the build direction was investigated via laser ultrasonic testing, which is more commonly used for AM defect detection. The corresponding ultrasonic imaging was improved with the synthetic aperture focusing technique (SAFT), which was found to be in good agreement with the results from the digital radiograph (DR). The outcomes of this study provide additional information for anisotropy evaluation and defect detection for improving the quality of additively manufactured products.

Process chain for forming and consolidating fiber-reinforced thermoplastics and metallic sheets in a two-stage isothermal tool

Due to their good mechanical properties and low structural weight, multi-material structures are a promising approach in the automotive industry to lightweight design, body construction and functionalization. Especially metal and plastic are mainly combined to achieve improved properties of the final component compared to mono-material structures. This paper describes the development of a manufacturing cell for the joint forming and heat-assisted press joining of steels and continuous fiber-reinforced thermoplastics in the form of unidirectional carbon-fiber tapes. In order to achieve shorter cycle times and to ensure economical production, a manufacturing cell, supplemented with automated handling by means of two robots and an isothermal, two forming stages tool concept was developed and tested. The composite components produced were tested with regard to their mechanical performance. The feasibility of the production was demonstrated. All composite components had a higher specific load capacity than a pure steel component. Cycle times of well under 60 seconds were achieved. An enormous reduction in process time compared to variothermal tool concepts could be achieved with the new manufacturing cell.

Column Watch: Methods for the Passivation of HPLC Instruments and Columns

Metal-analyte interaction is a type of nonspecific adsorption (NSA) that occurs mainly between acidic analytes and active adsorptive sites present in the metal surfaces of the fluidic path of any chromatographic system. NSA represents a chromatographic challenge since it may lead to poor peak shape, low recovery, and poor performance from the high performance liquid chromatography (HPLC) system. As a result, many solutions that circumvent and mitigate this type of secondary interaction have been published or commercialized in the last few years. Such solutions encompass swapping stainless steel components for non-reactive polymers, “biocompatible” metals or alloys, the addition of additives into the mobile phase, pretreating the system with large amounts of sample or strong acids, and the use of coated stainless steel materials. This article explores these solutions in more detail, particularly the recent adaptation and benefits of coated metal surfaces in LC.

Maraging steel powder recycling effect on the tensile and fatigue behavior of parts produced through the laser powder bed fusion (L-PBF) process

Additive manufacturing (AM) has advanced the manufacturing industry and has been employed in a wide range of industrial applications, including aerospace, automotive, medical, and die-casting equipment. To ensure the cost-effectiveness of the AM process, unfused powder must be recycled even if its characteristics may change after each cycle, making essential the validation of powder quality and component mechanical performances. Despite the research published to date, predicting the mechanical performance of printed parts issued from reused powder remains challenging since it is dependent on many AM process variables. Until now, no research has looked at the impact of powder recycling on the fatigue behavior of maraging steel components. This study investigates the impact of maraging steel powder reuse on powder characteristics, as well as on the tensile and fatigue properties of printed components. Our results indicate that the powder particle size distribution increased after eight powder reuses, particle morphology showed the presence of aggregates, broken particles, and shattered and deformed particles, while powder apparent density remained constant. Powder reusing had no significant impact on the surface roughness of as-built specimens. Although there was a slight decrease in mechanical properties over reuse cycles, tensile and fatigue performance remained globally stable, while the standard deviation of fatigue stress became narrower after eight cycles. Finally, fractography revealed that the fatigue fracture surfaces of components manufactured from an eight-time recycled powder have more fusion defects and carbon inclusions than the parts made from virgin powder.

Numerical modelling of shear cutting using particle methods

The use of Advanced High Strength Steel (AHSS) allows for lightweighting of sheet steel components, with maintained structural integrity of the part. However, AHSS grades show limitations in edge crack resistance, primarily influenced by sheared edge damage introduced by the shear cutting process. Numerical modelling of the shear cutting process can aid the understanding of the sheared edge damage, thus avoiding unforeseen edge cracking in the subsequent cold forming. However, the extreme deformations of the blank during the shear cutting process are likely to cause numerical instabilities and divergence using conventional Finite Element modelling. To overcome these challenges, this work presents the use of a particle-based numerical modelling method called the Particle Finite Element Method (PFEM). PFEM accurately solves some of the challenges encountered in shear cutting with the standard Finite Element method, such as large deformation, angular distortions, generation of new boundaries and presents an efficient way of transfer historical information from the old to the new mesh, minimising the results diffusion. The present work shows prediction of cut edge morphology of AHSS using a PFEM modelling scheme, where the numerical results are verified against experiments. With these results, the authors show new possibilities to obtain accurate numerical prediction of the shear cutting process, which promotes further advances in prediction of edge damaged related to shear cutting of AHSS.

Investigation of Plasma-Electrolytic Processing on EDMed Austenitic Steels.

This study investigates the effect of electrolytic plasma processing on the degree of defective layer removal from a damaged layer obtained after manufacturing operations. Electrical discharge machining (EDM) is widely accepted in modern industries for product development. However, these products may have undesirable surface defects that may require secondary operations. This work aims to study the die-sinking EDM of steel components followed by the application of plasma electrolytic polishing (PeP) to enhance the surface properties. The results showed that the decrease in the roughness of the EDMed part after PeP was 80.97%. The combined process of EDM and subsequent PeP makes it possible to obtain the desired surface finish and mechanical properties. In the case of finishing EDM processing and turning, followed by PeP processing, the fatigue life is enhanced without failure up to 109 cycles. However, the application of this combined method (EDM + PeP) requires further research to ensure consistent removal of the unwanted defective layer.