Strength Steel Research Articles

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.

The impact of corrosion on the fatigue strength of steel used in bridge structures

The impact of corrosion on the fatigue strength of steel used in bridge structures

Behavior and design of steel beam to multi-celled corrugated-plate CFST wall joints

In recent years, there has been a notable increase in the utilization of multi-celled corrugated-plate CFST (MC-CFST) walls. In this paper, a connection joint between a steel beam and an MC-CFST wall was proposed. The joint consisted of MC-CFST walls, end columns, horizontal diaphragms, vertical plates, and steel beams, with each steel component being interconnected through welding. To investigate its mechanical properties, a refined finite element model was developed, and the accuracy of the modeling method was validated through existing test results. The stiffness and moment capacity of the joints were analyzed, focusing on the effects of various factors, including the insertion depth of the horizontal diaphragm, beam-to-column linear stiffness ratio, width-to-thickness ratio of the end column plate, diameter of the horizontal diaphragm opening, axial compression ratio, and concrete and steel strengths. Based on numerical and theoretical analyses, a calculation method for determining the moment capacity of the joint and establishing the lower limit value for the steel tube thickness of the end columns was proposed by using the yield line analysis. The predicted value of the moment capacity of the joint was found to deviate from the finite element result by no more than 10%, suggesting that the formula exhibited excellent accuracy and applicability. The minimum steel tube thickness of the end columns was determined by considering the yield of the beam flange section. The design method presented in this paper has the potential to offer a valuable reference for practical designs of the proposed connection joint.

High temperature oxidation behaviors of steels at initial stages in air

The initial stage oxidation behaviors of commercial low carbon steel (LCS) and advanced high strength steel (AHSS) were comparatively studied at 1150 °C in air. Two reaction fronts were in situ observed during the oxidation of LCS, which was not found for AHSS. It was confirmed that the early detachment of LCS occurred after 30-s oxidation, however, the scale of AHSS maintained adherence due to the formation of a liquid phase. Besides, the scale thickness increased parabolically with oxidation time for both LCS and AHSS, with the estimated parabolic rate constants of 4.7 × 10−6 µm2/s and 4.9 × 10−6 µm2/s, respectively.

Trussed concrete-filled steel tubular hybrid structures subjected to axial compression: Performance and design calculation

This paper presents a study on the axial compression performance of trussed concrete-filled steel tubular (CFST) hybrid structures. A total of 26 specimens were tested, including 15 circular and 11 square cross-sectional specimens. The effects of cross-sectional types, yield strength of steel of CFST chord, web types, number of chords, and equivalent slenderness ratio were investigated to clarify the mechanical behavior of trussed structure. The analyses of the full-range response, the typical failure mode, and the distribution of the transverse deflection and strain of the structures were carried out. A finite element analysis (FEA) modelling of trussed CFST structure was established and validated against the experimental results. Through the numerical simulation, the analysis of the internal force and stress was conducted, and the hybrid effect between the web and CFST chord was revealed. Based on parameter analysis, the design calculation was proposed, and the trussed CFST hybrid structure was equivalent to a single CFST to calculate its load-bearing capacity by introducing equivalent slenderness ratio proposed.

Role of martensite morphology on mechanical response of dual-phase steel produced by partial reversion from martensite

This work focused on the effects of martensite morphology on the mechanical properties of the dual phase (DP) steels produced via partial reversion from martensite, which is different from the traditional DP steels. A commercial DP780 steel was used, and the needle and blocky DP steels were produced by controlled heat-treatments. Combinations of SEM, EBSD and tensile tests were performed for understanding the property and damage mechanisms. The needle DP steel has higher ductility while lower strength than that of blocky DP steel. Much higher density micro-voids were uniformly formed in the needle DP steel than that of blocky case, and the coalescence of micro-voids into microcracks is strongly suppressed, due to the high-density ferrite/martensite interfaces and uniform local plastic strain. On the other hand, the blocky martensite islands distribution leads to the non-uniform local plastic strain, which caused the lower density micro-voids formation and the generation of micro-cracks, thus the resultant lower ductility. In addition, the higher strength of the blocky DP steel is due to strengthening of the sub-micron sized needle martensite.

Multifunctional graphitic carbon nitride/manganese dioxide/epoxy nanocomposite coating on steel for enhanced anticorrosion, flame retardant, mechanical, and hydrophobic properties

Benzidine (Bz) was used to modify the nanofiller, manganese dioxide (MnO2), and the resulting Bz/MnO2 was then encased in pure epoxy resin (EP) with graphitic carbon nitride (GCN). The effectiveness of epoxy-coated mild steel in protecting against varying concentrations of GCN/Bz-MnO2 was assessed through the use of electrochemical techniques in natural seawater. The flame retardancy capabilities of the epoxy coating were significantly improved by the incorporation of GCN/Bz-MnO2. The EP-GCN-Bz/MnO2 nanocomposites display peak heat release rate (PHRR) and total heat release (THR) values which are 79 % and 62 % lesser than that of pure EP, respectively. When compared to a pure epoxy coating, the results of salt spray tests indicated that the addition of GCN/Bz-MnO2 improved corrosion protection and decreased water absorption. Electrochemical impedance spectroscopy (EIS) studies showed that the epoxy-GCN/Bz-MnO2 coating still had an improved resistance of 6.40E10 Ω.cm2 even after 30 days of exposure to seawater. Scanning electrochemical microscopy (SECM) displayed lowest ferrous ion dissipation (1.0 I/nA) in the EP-GCN/Bz-MnO2 nanocomposite coated steel. Potentiodynamic polarization testing showed that EP-GCN-Bz/MnO2 had a lower current density (icorr: 0.03 µA/cm2) in comparison with other coatings. This is mainly because the Bz alteration of the MnO2 nanoparticles and their successful coordination with GCN sheets result in a noticeably more compact nanostructure that lowers aggregation and increases binding capacity. Surface-modified GCN was found to be improved in its breakdown products, leading to the production of a strong, inert nanolayered covering, according to Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) analysis. Water contact angle (WCA) of 160° demonstrates the exceptional water resistance of the recently developed EP-GCN/Bz-MnO2 coating. In terms of hardness and adhesion strength, Bz/MnO2 wrapped in GCN had good mechanical characteristics on the epoxy substrate. The improved adhesive strength (18.3 MPa) for mild steel coated with EP-GCN/Bz-MnO2 was attained prior to being immersed in seawater. The EP-GCN/Bz-MnO2 crystallizes form a robust, inert coating that prevents ions from accessing the specimen. The coating has increased adhesive strength as a result, and it can withstand deep immersions without losing its integrity. Hence, the EP-GCN/Bz-MnO2 nanocomposite might work as a useful coating component in the automotive industry.

Fracture prediction and damage evolution of Q690 HSS under various stress states

The high strength and light weight of the high strength steel (HSS) make it extensively used. However, the reduction in ductility often leads to prominent fracture issues. The Q690 steel, with a yield strength greater than 690 MPa, is one type of Chinese HSS, and its fracture behavior, especially under complex stress states, is a growing concern. Therefore, this study tests a series of monotonic tensile specimens of Q690 HSS with various stress states to obtain the fracture results. Subsequently, a new Lode-dependent enhanced Lemaitre (NLEL) model is proposed. This model integrates the damage effect of the Lode parameter into the original Lemaitre model, enhancing the accuracy of fracture prediction specifically under shear-dominated stress scenarios. Comparative analysis among the Lemaitre model, NLEL model, and another Lode-dependent enhanced Lemaitre (LEL) model reveals that the proposed NLEL model delivers more comprehensive fracture prediction across all tested specimens, including smooth round bars, notched round bar, flat grooved plates, pure-shear plates as well as tensile-shear plates. Furthermore, this study delves into the analysis of damage evolution under representative stress states and initiates discussions regarding the coupled damage model.

Plastic Properties of Prestressed High-Strength Steel Reinforced Concrete Continuous Beams

In this paper, the plastic properties of prestressed high-strength steel reinforced concrete (PHSSRC) beams are studied, by performing static load tests on eight built-in Q460 and Q690 prestressed high-strength steel reinforced concrete continuous beams and one built-in Q355 prestressed steel reinforced concrete (PSRC) continuous beam. The design parameters of the specimens were the steel strength grade, the steel ratio, the comprehensive reinforcement ratio, and the stirrup ratio. The failure mode, load–deflection curve, internal force redistribution ability, curvature ductility, plastic hinge performance, and moment modification coefficient of continuous beams under the influence of various parameters were analyzed to measure the plastic performance of the continuous beams. These results show that after the plastic hinge is formed in the middle support and mid-span of the prestressed high-strength steel reinforced concrete continuous beam, the test beam eventually becomes a rotating mechanism and is destroyed with increasing load. The built-in high-strength steel can significantly improve the bearing capacity of the specimen, and the maximum increase in the bearing capacity is 37.3%. The specimen still has a high bearing capacity after reaching the ultimate bearing capacity. With a decrease in the steel ratio, the degree of internal force redistribution is deepened, the curvature ductility is improved, and the plastic performance is enhanced. Increasing the comprehensive reinforcement ratio and the stirrup ratio can improve the plastic performance of the specimen. The calculation formula of the equivalent plastic hinge zone length is proposed. The calculation formula of the moment modification coefficient, with the relative plastic rotation angle and relative compression zone height as independent variables, is established. When the relative plastic rotation angle is not greater than 0.829 × 10−5, the moment modification coefficient increases with increasing plastic rotation angle and is not greater than 0.37. In the range of 0.3~0.4, the moment modification coefficient decreases with increasing height of the relative compression zone.

Achieving high strength and high ductility of dual-phase steel via alternating lamellar microstructure

Overcoming the strength-plasticity trade-off in GPa-grade dual-phase steel while simultaneously enhancing both properties poses a significant challenge. In this study, we propose an innovative strategy to break this inherent dilemma by designing an alternately arranged ferrite and martensite lamellar microstructure with appreciable deformation coordination in dual-phase steel. Compared to traditional dual-phase steel (DP steel) with a block microstructure and the same phase components, the newly developed heterogeneous lamellar dual-phase steel (L-DP steel) prepared by a cyclic intercritical quenching process demonstrates a remarkable 90 % increase in uniform elongation (from 9.1 ± 0.4 % in DP steel to 17.3 ± 0.3 % in L-DP steel). The ultimate tensile strength increases slightly from 1002 ± 10 MPa (DP steel) to 1034 ± 11 MPa (L-DP steel). The lamellar martensite and lamellar ferrite with a certain hardness difference lead to high back stress during plastic deformation, thereby ensuring the ultra-high tensile strength of the L-DP steel. The improved ductility of the L-DP steel is primarily attributed to the appreciable coordinated deformation ability of the lamellar microstructure and the considerable plastic deformation ability of lamellar martensite. These two outstanding capabilities allow this unique microstructure to elongate continuously along the tensile direction, facilitating persistent work hardening without premature cracking. Furthermore, the heterogeneous hardening triggered by the mutual constraints of the lamellar microstructure during the plastic deformation also contributes additional work hardening to enhance the ductility. The design of lamellar microstructure under a cyclic intercritical quenching process presents an efficient and feasible pathway for producing GPa-grade dual-phase steel with considerable ductility.

Environmental effect on fatigue strength of stainless steel in PWR primary water: Relation between fatigue life in air and environment

AbstractThe fatigue life of Type 316 stainless steel in the pressurized water reactor (PWR) water environment and its dependency on that in air were investigated. Specimens from two heats of the material were prepared for fatigue tests, and additionally, previous test findings obtained using another heat were applied. The fatigue test results obtained for various strain ranges and strain rates revealed that the fatigue life in the water environment Nf(env) did not depend on that in air Nf(air) when the strain rate was 0.4%/s. The variation in the ratio of Nf(env) to Nf(air) was larger when Nf(air) obtained for each heat of the material was used than that when a general Nf(air) was used. The changes in the penalty factor Fen with applied strain range and strain rate could be predicted by assuming Nf(env) depended not on strain rate but on rise time.

Fatigue strength of steel–aluminum alloy dissimilar lap joints fabricated by dimple spot welding for automotive application

AbstractRecently, a new dissimilar mechanical joining method, dimple spot welding (DSW), has been developed for joining steel sheets and aluminum alloys. In this study, tensile shear tests and fatigue tests were conducted on four types of dissimilar lap joints, with two types of steel sheets and two types of Al alloys joined by DSW. Both the tensile shear strength and fatigue strength of DSW joints were found equal to or greater than those of a self‐piercing riveting (SPR) joint. In all four types of DSW joints, fatigue cracks were initiated from the Al alloy and were associated with fretting induced by repeated contact and friction. A finite element analysis showed that the tangential stress was mainly governed by the friction coefficient and also affected by the steel sheet strength. Higher‐strength steel led to lower tangential stress, resulting in a longer fatigue life for the DSW joint.

Fatigue assessment of steel tube-to-flange welded joints with reinforcement ribs subjected to multiaxial loads according to the Peak Stress Method

In fatigue design of welded structures, the Peak Stress Method (PSM) is a FE-oriented tool to estimate the Notch Stress Intensity Factors (NSIFs). In compliance with proper fatigue design curves validated against more than 1300 experimental data, the PSM allows to estimate the fatigue lifetime of welded structures made of either aluminium alloys or structural steels and subjected to uniaxial and multiaxial loadings. Moreover, an interactive analysis tool has also been developed in Ansys® Mechanical to automate PSM application on welded structures and support the FE analyst. In this work, the fatigue strength of complex steel tube-to-flange welded joints with reinforcement ribs has been experimentally investigated under pure bending, pure torsion and in-phase as well as out-of-phase combined bending-torsion loadings. The tested joint consists of a steel SHS tube having 6.3 mm thickness, which is joined at both ends to a 15-mm-thick flange. In addition, tube and flanges are joined by fillet welding to 6-mm-thick steel reinforcement ribs. A test rig has been designed in order to perform the experimental fatigue tests, taking advantage of two hydraulic actuators. The experimental fatigue results have been compared with fatigue lifetime estimations performed using the automated PSM tool.

Effects of Boron Addition on Low Cycle Bending Fatigue Strength of Gas Carburized Low Alloy Steel

Effects of Boron Addition on Low Cycle Bending Fatigue Strength of Gas Carburized Low Alloy Steel

Fatigue life prediction of the nitrided steel by multiaxial high cycle fatigue criteria

The present study aims to predict the fatigue strength of ion-nitrided 42CrMo4 steel, using multiaxial high cycle fatigue (HCF) criteria and considering the effects of stabilized residual stresses and surface hardening. The predicted fatigue strength was compared to experimental data obtained after three-points bending fatigue tests at two stress ratios (0.1 and 0.5) and for notched (Kt = 1.6) nitrided and untreated specimens. A 3D finite element (FE) model of a three-point bending fatigue test was developed under ABAQUS software in order to determine the actual applied cyclic stress state at the notch. The results show that ion-nitriding treatment led to 32% improvement in the fatigue strength at 106 cycles compared to the untreated material. This improvement is explained by the advantageous effect of ion-nitriding treatment in terms of compressive residual stresses and surface layer hardening. Both stabilized residual stress state and hardening effects were successfully implemented into various multi-axial HCF criteria, including Sines, Crossland, Dang Van and Matake criteria. A sensitivity analysis has shown that Crossland criterion has permitted to predict the fatigue limits in accordance with the experimental results.