Strength Steel Research Articles

Fatigue strength of an ultra-high strength low alloy steel fabricated via laser powder bed fusion

High strength low alloy (HSLA) steels are important metallic materials especially in the defense and military sectors due to their superior mechanical properties and cost-effectiveness compared to superalloys. This study investigates monotonic tension/compression strength and fatigue strength, including both the low cyclic fatigue (LCF) and high cyclic fatigue (HCF), of a HSLA steel produced using laser powder bed fusion (LPBF) additive manufacturing (AM). The LCF tests were strain-controlled (from 0.45% to 1.2% strain amplitudes) achieving a constant strain rate of 0.0005/s, while the HCF tests were stress-controlled (from 467 MPa to 1007 MPa stress range), all carried out at room temperature. Specimens of the alloy were manufactured in two build orientations to evaluate anisotropy. Moreover, a set of rolled plate and as-cast specimens were tested to facilitate comparisons. The alloy exhibited excellent performance confirming the potential of LPBF AM to manufacture HSLA steels with significant performance gains over as-cast counterparts. Nevertheless, the rolled material exhibited the highest performances. Details pertaining to the processing along with the testing results and characterization of microstructural origins of the high strength are presented and discussed in this paper.

Effect of nitrogen content on grain boundary engineering and corrosion resistance of 316LN stainless steel

Nitrogen solid solution can effectively enhance the strength of steels, but its effect on grain boundary engineering and the corrosion mechanism of steels is still unclear and systematic investigation is needed. In this study, 316LN steel specimens with five nitrogen concentrations were melted and subjected to compression and annealing. The grain boundary character distribution was investigated by electron backscatter diffraction. The corrosion behavior was investigated by the electrochemical polarization curves, electrical impedance spectroscopy, and X-ray photoelectron spectroscopy. The results showed that nitrogen promotes the formation of low-Σ boundaries when the pre-strain is not above 10%. The polarization resistance increases with increasing nitrogen content. The quantitative relationship between the nitrogen content and the pitting potential was obtained. Nitrogen favors the formation of oxides, hydroxides, and NH3/NH4+ on the passivated surface. The effect of planar slip on grain boundary engineering and the influence of NH4+ on the corrosion resistance are discussed.

Effect of hot rolling mode on crys-tallographic texture and mechanical properties of directly quenched bainitic steel

The paper considers the effect of hot rolling mode on the structure, mechanical properties and crystallographic texture, as determined by EBSD, for a directly quenched thick plate of high strength bainitic steel. According to the obtained results, stronger strain hardening of austenite before the phase transformation leads to appearance of granular bainite and hence diminishes the final steel strength.

Unexpected strengthening of the Mo-containing low-density steel achieved through a novel nano-ordered phase

Mo element has been considered as an effective element to regulate the nano-precipitation behavior in low-density steel. In this work, a novel nano-ordered phase was found in austenite of the Fe–28Mn–10Al-1.2C (wt.%) low-density steels with 3 wt% and 5 wt% Mo addition. HRTEM characterization indicated that this nano-ordered phase had a chemical composition similar to austenite matrix and a L′12 structure similar to κ-carbides. Although the Mo addition suppressed the precipitation and growth of κ-carbides in austenite, the nano-hardness testing showed that the formation of the novel nano-ordered phase caused by Mo addition resulted in a higher hardness of austenite. Based on the tensile property analysis of the Mo-containing steels, although the Mo addition decreased the precipitation strengthening of intragranular κ-carbides, the novel nano-ordered phase resulted in an additional strength contribution of 309 MPa. In addition, the formation of this novel nano-ordered phase had significant improvement on the strain hardening rate. Consequently, the Mo addition simultaneously increased the strength and plasticity of the low-density steels. The novel nano-ordered phase caused by Mo has rarely been reported in literatures, its contribution on the strength and strain hardening ability indicated its potential application in the regulation of mechanical properties.

Flexural performance of round-ended concrete-filled steel tubular specimens

Round-ended concrete-filled steel tubular (CFST) columns have numerous benefits, including easy construction, aesthetic appearance, and variable section arrangement. This study experimentally and numerically investigated the flexural behavior of round-ended CFST columns. The six testing results showed that as the aspect ratio increased from 1.38 to 1.86, the flexural resistance and stiffness increased by 82.0% and 159.9%, respectively. Every round-ended CFST specimen had excellent ductility. Concrete can be adequately confined by the round-ended steel tube. Mechanistic and parametric analyses were carried out using a finite element model. Mechanistic analysis demonstrated that the round segment of the steel tube can effectively confine the concrete, resulting in a considerable strength gain near the edge of the compression zone. The parametric analyses showed that as the concrete strength increased from 24 to 110 MPa, the flexural resistance increased by average 20.4%; as the steel strength improved from 235 to 700 MPa, the flexural resistance increased by average 155.4%; as the steel ratio enhanced from 0.05 to 0.2, the flexural resistance increased by average 179.7%; and as the aspect ratio increased from 1 to 3, the flexural resistance of the major (minor) axis increased (decreased) by about 59.9% (36.5%). The analysis showed that the flexural stiffness of this composite specimen can be accurately predicted by AISC 360–2016. Finally, a simplified model was proposed for estimating the flexural resistance of the round-ended CFST specimens.

Powder bed fusion of high-Mn-N Ni-free austenitic stainless steel: achieving low porosity and high mechanical strength through process parameter selection

The emergence of innovative high-performance stainless steels is essential at the forefront of material science. This research highlights the meticulous development of a novel high-Mn-N Ni-free austenitic stainless steel via the powder bed fusion laser–based (PBF-LB) technique. We strategically optimized the laser parameters, achieving ultralow porosity and a refined microstructure with defects under 2%. Comprehensive analysis revealed superior dendritic cellular formations at melt pool boundaries, underlining our method’s precision. Unveiling the strength of this novel steel, sample S11 (scanning speed of 800 mm/s and laser power of 147 W) showed a remarkable tensile strength of 1190 ± 20 MPa and an impressive elongation to fracture of 35 ± 3%. Interestingly, twin formations became evident under external loads, enhancing mechanical resistance while preserving ductility. Advanced quantification methods were employed to ensure accuracy, especially for low atomic number elements, overcoming previous measurement constraints. This pioneering study introduces a game-changing austenitic Ni-free stainless steel enriched by Mn and N. It sets a new benchmark in material development and application, synergizing exceptional mechanical attributes with robust ductility.

The Influence of Tempering Parameters on the Microstructure, Mechanical Property, and the Corresponding Strengthening Mechanism of Metastable 301 Austenitic Stainless Steel

Metastable austenitic stainless steels (MASS) are widely used in various industrial applications due to their exceptional mechanical properties. The microstructure of MASS can be regulated through severe plastic deformation, heat treatments, and surface treatments to achieve further improvement of mechanical properties. Herein, nanograin and quasiheterostructure are prepared by cold rolling and annealing process in Fe–17Cr–6Ni MASS. The mechanical response, deformation mechanism, and strengthening contribution of the two steels with representative microstructure are studied. The results indicate that nanograin austenitic steel (803 MPa, 22%) has superior yield strength and similar ductility compared with that of quasiheterostructure steel (600 MPa, 25%). In the nanograin steel, the deformation mechanism is mainly dislocation slip and deformation‐induced martensite transformation, while in quasiheterostructure steel, the deformation twinning is also included for the relative lower stacking fault energy in the coarse grain. The yield strength of nanograin structural steel is about 200 MPa higher than that of quasiheterostructure steel, which can be attributed to the differences in grain refinement strengthening, phase‐transformation strengthening, and precipitation strengthening of Cu particles. The findings presented in this study are informative for modulating MASS properties and ultimately improving the lifespan in a variety of industrial settings.

Heterogeneous Microstructure Provides a Good Combination of Strength and Ductility in Duplex Stainless Steel

SAF2507 duplex stainless steel (DSS) is often used as a structural component in ocean-going vessels and marine petroleum exploitation equipment, which require superior mechanical properties. In this study, we used cold rolling (CR) at room temperature with 55% or 80% deformation amounts and subsequent annealing at 1273 K in 1 min to prepare SAF2507 samples with a heterogeneous structure (HS) that was composed of ferrite and austenite phases with different grain sizes. Compared with the homogeneous structure samples, the yield strength of the HS samples increased, while the ductility did not decrease. The 55%-1273 and 80%-1273 samples exhibited the hetero-zone boundary-affected regions on both sides of the grain boundary, phase boundary, and twin boundary. This resulted in hetero-deformation-induced (HDI) strengthening and strain hardening of samples during tensile deformation, which improved the ultimate tensile strength of the HS samples while maintaining a good uniform elongation. In addition, the heterogeneous structure of DSS had better corrosion resistance than the initial sample of coarse grain (CG) structure; mainly because the HS samples had finer grains and more grain boundaries on the DSS surface than the CG structure, which is conducive to the formation of high-density passivation film on the surface of stainless steel. The current study provides a new method of material selection of some structural components with the demands of high strength and good ductility.

Artificial Neural Network Models for Determining the Load-Bearing Capacity of Eccentrically Compressed Short Concrete-Filled Steel Tubular Columns

Artificial neural networks (ANN) have a great promise in predicting the load-bearing capacity of building structures. The purpose of this work was to develop ANN models to determine the ultimate load of eccentrically compressed concrete-filled steel tubular (CFST) columns of circular cross-sections, which operated on the widest possible range of input parameters. Short columns were considered for which the amount of deflection does not affect the bending moment. A feedforward network was selected as the neural network type. The input parameters of the neural networks were the outer diameter of the columns, the thickness of the pipe wall, the yield strength of steel, the compressive strength of concrete and the relative eccentricity. Artificial neural networks were trained on synthetic data generated based on a theoretical model of the limit equilibrium of CFST columns. Two ANN models were created. When training the first model, the ultimate loads were determined at a given eccentricity of the axial force without taking into account additional random eccentricity. When training the second model, additional random eccentricity was taken into account. The total volume of the training dataset was 179,025 samples. Such a large training dataset size has never been used before. The training dataset covers a wide range of changes in the characteristics of the pipe metal and concrete of the core, pipe diameters and wall thicknesses, as well as eccentricities of the axial force. The trained models are characterized by high mean square error (MSE) scores. The correlation coefficients between the predicted and target values are very close to 1. The ANN models were tested on experimental data for 81 eccentrically compressed samples presented in five different works and 265 centrally compressed samples presented in twenty-six papers.

Meso–Macro Energy Exchange in Shock-Wave Processes and Dynamic Strength of AB2 Steel

Meso–Macro Energy Exchange in Shock-Wave Processes and Dynamic Strength of AB2 Steel

Effects of plasma surface quenching on strength and crack resistance property of PCrNi3 Mo steel

Plasma surface quenching technology uses a high-energy beam to rapidly heat the PCrNi3Mo steel material. Simultaneously, the treated workpiece is rapidly cooled by its own heat transfer to realize surface quenching. The metallographic structure showed that the grains of the PCrNi3Mo steel treated by plasma quenching were significantly refined. The DEFORM software was utilized to simulate the heat treatment process, and the results showed that the martensitic volume fraction of the plasma quenching treatment of PCrNi3Mo steel was as high as 83.6%~95.9%, and the simulated hardness value was HRC55~57. The actual hardness of the specimen after plasma surface quenching was HRC54~56, which was in good agreement with the simulation value. The plasma was employed to treat the PCrNi3Mo steel tensile specimen, the processing depth was only 1/10 of the thickness of the specimen. Through the MTS tensile experiment, it was observed that the tensile strength of specimens treated with plasma increased by 9.2% when compared to the untreated specimens. Despite the high hardness of the treated specimen, fracture strain exceeded 7%. Furthermore, before the fracture of the tensile specimen occurred, the plasma quenching part effectively prevented crack propagation and delayed fracture, and the fracture necking phenomenon of the specimen was not obvious.

Effects of Different Sterilization Methods on Orthodontic Wires: An In Vitro Study

ABSTRACT Objective: The purpose of this study was to determine the effect of common methods of sterilization on the tensile strength of Beta titanium, Stainless steel, Australian Stainless steel, Copper Nickel-Titanium, and Nickel-Titanium wires. It also aimed to evaluate the changes in tensile strength values caused by repeated cycles of sterilization. Materials and Methods: A sample of 225 orthodontic wires, i.e., beta-titanium, stainless steel, Australian stainless steel, copper nickel-titanium, and nickel-titanium wires, were collected from different manufacturers. These wires were divided into three groups, which consists of Groups 1, 2, and 3. Four methods of sterilization used in this study were as follows: (i) autoclave (250°F for 20 min), (ii) dry heat sterilization (375°F for 20 min), (iii) ethylene oxide sterilization (54°C for 4 hrs), and (iv) 2.45% acidic glutaraldehyde (10 hrs). Results: The results of this study showed that there was increase in tensile strength of beta-titanium and nickel-titanium wires using autoclave and dry heat sterilization. No statistically significant difference in tensile strength of stainless steel and Australian stainless steel archwires. The tensile strength of copper nickel-titanium wires decreased following 0, 1, and 5 cycles of sterilization. Conclusion: The lack of statistically significant differences established in the study of new and sterilized orthodontic archwires gives us reason to conclude that the orthodontic arch wires can be sterilized because the sterilizing processes do not affect their tensile strength and the orthodontists could thus ensure the maximum safety of their patients.

Variety of yield strength caused by precipitations of L12-Ni3Al and B2-NiAl phases in alumina-forming austenitic steel

Alumina-forming austenitic (AFA) steel exhibits excellent high-temperature properties and has attracted significant attention in thermal power generation industry. Microstructure evolutions and yield strengths of 20Ni-AFA steel during isothermal aging are investigated. The results show that five types of phases are found in 20Ni-AFA steel during isothermal ageing: NbC, L12-Ni3Al, M23C6, B2-NiAl and Laves phases. Addition of Cu causes precipitation of L12-Ni3Al phase in 20Ni-AFA steel at the early stage of isothermal aging. L12-Ni3Al phase grows and gradually transforms into B2-NiAl phase which possesses a more stable structure with the extension in isothermal aging time. The transformation leads to yield strengths of 20Ni-AFA steel to decrease anomalously during 24-120 h isothermal aging. Yield strengths of 20Ni-AFA steel increase again as isothermal aging time continues to increase.

Pull-out capacities of screw connections in thin steel battens exposed to bushfire conditions

External building envelopes made of cold-formed steel (CFS) claddings are commonly used in bushfire prone areas due to their non-combustible properties. However, their designs should ensure safety under the combined action of enhanced wind suction loading and elevated temperatures experienced during bushfires. Although numerous studies have investigated the critical pull-out failures in CFS batten screw connections under wind suction loading at ambient temperature, the pull-out failure behaviour of batten screw connections under bushfire conditions has not been investigated. The main objective of this study is to examine the pull-out failure mechanisms and capacities of screw connections in CFS battens under wind suction loading at elevated temperatures. Two hundred and sixteen small-scale tests were conducted on CFS battens made of three commonly used thicknesses and both high (G550) and low (G300) strength steels. Tests were conducted at six different temperatures using three types of screw fasteners to investigate the critical localised pull-out failures of batten screw connections. The results of these tests were utilised to develop applicable design equations and capacity reduction factors for the pull-out capacities of batten screw connections under the combined action of wind suction loading and elevated temperatures. The findings of this study will help design and build structures safely in bushfire-prone regions.

Advancement of strength and toughness in ultra-low carbon martensitic stainless steel by reversed austenite

It has been proven that martensite matrix accompanied by fine reversed austenite is beneficial to improve the strength and toughness in martensitic steels to a certain extent. In order to achieve this unique microstructure, the existing methods, like quenching at high temperatures, multiple heat treatment processes, increasing Ni content, raise the economic cost and operation difficulty. In this work, fine reversed austenite was formed in Fe13Cr4NiMo steel with relatively low nickel content (4 wt%) by one-time relatively low temperature quenching (860 °C) and tempering process. Then the formation mechanism and effect on mechanical properties of reversed austenite were discussed. The 0.55 vol% reversed austenite with the size of 260 nm can be formed in Fe13Cr4NiMo steel under quenching at 860 °C for 40 min and tempering at 650 °C for 2 h. The reversed austenite maintained a deviation of ∼2° from K-S relationship with the martensite matrix. In addition, the formation of reversed austenite was caused by short circuit diffusion of element-free redistribution, which caused the reversed austenite to be more inclined to nucleate at subgrain boundaries and grow up restrictedly. Furthermore, the Fe13Cr4NiMo steel that contained reversed austenite hindering dislocation movement and coordinating deformation possessed excellent plasticity of 18.4 % and Charpy impact energy of 289 J while maintaining high strength of 872 MPa at room temperature.

Microstructural Evolution and Mechanical Properties of V-Containing Medium-Mn Steel Adopting Simple Intercritical Annealing

The variations of the microstructure and mechanical properties of medium-Mn steel after vanadium (V) microalloying with different contents were investigated. After a one-step intercritical annealing (IA) at 730 °C, the steel containing 0.04 wt.% of V exhibited excellent comprehensive properties. The steel maintained an ultimate tensile strength (UTS) of 1000 MPa while also exhibiting a total elongation (TEL) of 37% and a product of strength and plasticity (PSE) of 37.7 GPa%. V-microalloying improved the yield strength (YS) and UTS of the experimental steel by refining ferrite grains and precipitation strengthening, however, it deteriorated its plasticity, which is difficult to compensate for through grain refinement and due to the TRIP effect of retained austenite (RA). The largest amount of RA and the appropriate stability also make a significant contribution to the outstanding UTS of the steel containing 0.04 wt.% of V through the TRIP effect. However, the further increase of V content led to decreased RA content and stability, weakening the TRIP effect and resulting in a weaker strength ductility balance.

Enhancing weld strength in high-strength steels: the role of regional preheating in RSW

Abstract In this study, automotive in sector use STRENX 700 CR and DP 800 steels were joined by resistance spot welding using medium frequency direct current. Some parts were joined under atmospheric conditions, while others were joined by applying regional preheating to their heat-affected zones before welding. The design, manufacturing, and adaptation of the preheating system to the welding machine were carried out as part of this study, considering it as a new system. After the joining process, nondestructive and destructive tests were performed on resistance spot welding connections between STRENX 700 CR and DP 800 steels. Hardness tests revealed that the preheating applied to the heat-affected zones before welding reduced the hardness of the weld metal by approximately 8 %, while the microstructure analysis showed that the regional preheating increased both the weld metal and the heat-affected zones width. The tensile-shear strength increased by about 7 %, and the cross-tension test strength increased by about 5 % with the preheating applied to the heat-affected zones before welding. In both the tensile-shear and cross-tension tests, all failures occurred in the form of button shearing from the heat-affected zones. The regional preheating treated specimens showed the highest fatigue life, with an average of 947,632 cycles.

Numerical FE Modeling and Design Methods of CCES Columns with Normal-Weight Crushed Dolomite Coarse Aggregate Fully Embedded IPE Steel-Section

The composite concrete-encased steel (CCES) column member is made by the steel section embedded and covered in concrete from all sides. Due to the ability of the composite sections to bear heavy loads while using smaller sections, CCES columns have been widely used. Analytical studies on the CCES columns’ behavior using crushed dolomite coarse aggregate (CDCA) with different shear connectors (SCs) types/shapes and sizes under axial loads are described here. This study also aims to evaluate the current design methods to determine the ultimate capacity of the CCES with CDCA concrete columns using nine available codes. The results show that the finite element (FE) analysis could accurately predict the ultimate capacity of the CCES columns; the column’s capacity improved by about 41.75% as fcu increased by 60%. Increasing the IPE-shaped steel strength (fss) strategy is not very effective and gives brittle behavior even though enhancing the fss improves the capacity. The column's capacity increased as the tie stirrups and steel bars ratios increased. The column’s capacity increased by about 17.63%, as steel bars ratios increased by 155.49%. The efficiency factors increased slightly as tie stirrups were raised but slightly decreased as steel bar ratios increased. Using the SCs system increases the columns’ capacity by an average value of about 4.9% of the specimen without SCs. The computed capacities using the nine available codes are conservative and safe. The closest estimates made by the YB9082-06 code are 26% less on average than the test results; in contrast, the safest predictions made by the ECP-LRFD code are 68% less, on average, than test results.Graphical

A dual-precipitation strategy for simultaneously enhancing the strength and ductility of cost-effective medium Mn steels at low temperatures

A dual-precipitation strategy has been utilized to simultaneously improve the strength and ductility of the cost-effective medium Mn steels for cryogenic applications. When the temperature decreases from room temperature (RT) to liquid nitrogen temperature (LNT), the dual-precipitation state, including NiAl and Cu-rich intermetallics in ferrite and austenite respectively, exhibits a simultaneous increase in the yield strength (YS, to ∼1160 MPa) and tensile elongation (TEL, to ∼40 %), and a significantly higher secondary strain hardening. On the one hand, the NiAl precipitates can strengthen the ferrite matrix and provide dislocation storage capacity for strain accommodation. On the other hand, the Cu-rich precipitates can stabilize the austenite matrix, activate the formation of microbands (MBs) and delay the transformation-induced plasticity (TRIP) effect until large strains. Therefore, multistage cooperative precipitation hardening, microband-induced plasticity (MBIP) and TRIP give rise to comprehensive strength-ductility interactions, which are comparable to those of the best-performing expensive cryogenic materials.

Enhancing interfacial adhesion and electrical conductivity of steel/polymer composites via molecular adhesion for sustainable structural materials

Steel/Polymer Composites (SPCs) represent a promising class of materials that combine the strengths of steel and polymers, making them ideal candidates for various structural applications. Notably, they offer heat shielding properties, energy absorption, reduced noise, vibration, and harshness, high rigidity, as well as weight reduction benefits. Achieving robust interfacial adhesion between steel and polymer, while maintaining electrical conductivity for welding, presents a significant challenge in SPC development. Conventional SPCs relying on physical adhesion through polymer coating are susceptible to environmental factors, compromising durability. To address this, novel adhesion concepts are explored, leveraging molecular-level interactions. In this study, we investigate the adhesion between electro-galvanized iron steel treated with a phosphate conversion coating and four polymers: polyethylene (PE), acrylonitrile-butadiene-styrene (ABS), Nylon 6, and Polyketone. The primary amines are anchored on the steel surface using 3-aminopropyltriethoxysilane (APTES), while maleic anhydride (MA) is introduced onto the polymer chains. The adhesion strength of the SPCs dramatically increased from 0 to 11.5 MPa for PE, from 1.5 to 11.1 MPa for ABS, from 12.7 to 14.1 MPa for Nylon 6 and from 7.1 to 14.2 for Polyketone. In particular, the adhesion strength of Nylon 6 was maintained at 89.0% for 7 d aging despite moisture absorption being fatal to interfacial adhesion. The findings suggest two presumable interactions at the steel/polymer interface, including molecular adhesion. In order to enable spot welding of these SPCs, mechano-chemistry was employed to embed carbon-based graphitic materials within the plastic matrix, enhancing electrical conductivity. The selection of Polyketone as a core material facilitates the adsorption of conductive fillers, such as Carbon Nanotubes (CNT) and Graphite Platelets (GPs). Graphite Nanoplatelets-Polyketone, with smaller lateral dimensions (50 ∼ 100 nm), exhibits the highest electrical conductivity of 207.5 S/cm. Scanning Electron Microscopy (SEM) confirms effective filler adsorption without pre-treatment. This comprehensive study enhances our understanding of SPC development, shedding light on innovative approaches to achieve robust adhesion, electrical conductivity, and suitability for spot welding applications.