Plasticity Steel Research Articles

Numerical Study of the Nonlinear Soil–Pile–Structure Interaction Effects on the Lateral Response of Marine Jetties

This study presents three-dimensional finite element analyses of two marine structures subjected to lateral loading to approximate environmental forces (e.g., wind, waves, currents, earthquakes). The first structure is a marine jetty supported by twenty-four piles, representative of an existing structure in Cyprus, while the second is a simplified four-pile marine structure. Soil–pile interaction is modelled using nonlinear p-y, τ-z, and q-z springs that are distributed along the piles, while steel plasticity is also considered. This study examines the relationship between failure modes, deformation modes, and plastic hinge locations with soil behaviour and soil reaction forces. It also aims at investigating the behaviour of the above structures in lateral loading and quantifying the consequences of unrealistic assumptions such as soil and steel linearity or tension-resistant q-z springs. The results indicate that such assumptions can lead to the wrong prediction of failure modes, plastic hinges, and critical elements while emphasising the crucial role of soil nonlinearity and axial pile–soil behaviour on the structural response. It is demonstrated that the dominant nonlinear sources relevant to this study, whether soil nonlinearity, plastic hinge formation, or a combination of the two, are primarily influenced by the axial capacity of soil–pile foundation systems, particularly their tensile component.

Cryogenic rapid quenching simultaneously enhances the strength and ductility of steel–Aluminum composite plates

Roll-bonded steel–aluminum composite plates exhibit poor plasticity due to work hardening induced by severe plastic deformation. In this study, the mechanisms for improving the plasticity of steel–aluminum composite plates were investigated through liquid-nitrogen quenching from both material science and mechanical perspectives. From a materials science perspective, rapid cryogenic quenching forms numerous dislocation walls and cells within the Al matrix. These dislocation configurations effectively limit dislocation movement during tensile deformation. Additionally, the difference in thermal expansion properties between the steel and aluminum matrices generate significant microstrain near the interface, enhancing the back stress-induced strengthening effect. From a mechanical perspective, rapid cryogenic quenching increases the hardness and tensile strength of the aluminum matrix. A stronger aluminum matrix restricts the deformation of the steel matrix during tensile loading, delaying the onset of necking in the steel matrix. Additionally, the significant hardening of the aluminum matrix can cause the tensile specimen to bend, creating two potential necking points that divide the tensile strain, thereby significantly increasing the pre-necking deformation of the composite plate. Compared with untreated specimens, the tensile elongation and tensile strength of the specimens treated with rapid cryogenic quenching increased by 49.1% and 10.8%, respectively. This study provides a novel approach to simultaneously enhance the strength and plasticity of composite plates.

Effect of Ti–Mg–Ce Complex Deoxidation on the Inclusions and Mechanical Properties of S355 Steel

Oxide metallurgy technology not only refines the microstructure but also modifies the type, density, and size of inclusions. This study examines the impact of Ti–Mg–Ce composite deoxidation on the inclusions and mechanical properties of S355 steel used for heavy‐duty H‐beams. Compared to base alloy, Ti–Mg–Ce‐added alloy features a higher presence of inclusions such as MgO, TiN (containing Ce), TiOx, and TiS, except for SiO2, Al2O3, MnS, and MnO. Following Ti–Mg–Ce composite deoxidation, the number density of inclusions increases by 16.4%, and the equivalent diameter, length, and width of the inclusions decrease by 41.0%, 41.2%, and 37.9%, respectively. The length of large‐angle grain boundaries in the same area increases by 20%, and the average grain size reduces by 12.5%, resulting in an enhancement of yield strength by 23.6 MPa, tensile strength by 37 MPa, total elongation by 2.1%, and impact energy by 92 J. By employing Ti–Mg–Ce composite deoxidation to optimize inclusions and microstructure, the strength, plasticity, and toughness of the experimental steel are significantly improved.

STUDY OF THE INFLUENCE OF THE CHEMICAL COMPOSITION OF RAILWAY AXLE STEEL ON THE UNIFORMITY OF THE FINAL MICROSTRUCTURE

The relevance of the work. For modern railway transport, increasing the strength and operational reliability of rolling stock parts, in particular railway axles, is an urgent task. It is known that the inhomogeneity of the distribution of chemical elements in the structure of structural grade carbon steels is formed mainly during their crystallization and may subsequently affect the final structural state and set of properties. Purpose. Study of the influence of the chemical composition of steels for railway axles on the uniformity of the structure. Methodology. Experimental ingots of different chemical compositions with different amounts and ratios of the main chemical elements within the range of OS and EA1N were produced in laboratory conditions. The grain structure was studied on microsections after etching with nital. Chemical inhomogeneity (“traces” of the dendritic structure) − the distribution of chemical elements in the microstructure of cast samples was detected by etching in a hot solution of sodium picrate. Metallographic analysis was performed on a light (optical) microscope “Axiovert 200 M MAT” manufactured by “Carl Zeiss”. The results. The effect of changing the chemical composition of carbon steel, which is intended for the manufacture of railway axles, on the peculiarities of the formation of ferrite-pearlite heterogeneity and grain size is determined. It is shown that in samples of steels corresponding to the EA1N brand with a lower carbon content, the distribution of phases is uneven. The mechanism of formation of such a structure is explained from the point of view of two theories. In samples of steels with an average carbon content, which correspond to steel of the OS grade, the distribution of the pearlite and ferrite phases is more uniform, the relationship between the size and the degree of darkening of the pearlite areas with the order of the former dendritic branches has been established. Based on the results of the quantitative analysis of the final grain structure, it was established that OS steel with an average carbon content at different Mn/Si ratios has a more uniform grain structure. EA1N steel with a lower carbon content has a pronounced grain size, with an increase in the Mn/Si ratio, the largest maximum of the dependence of the distribution shifts to a larger grain size (smaller number). Practical results. Increasing the homogeneity of the final microstructure contributes to increasing the plasticity and viscosity of steel, which directly affects the reliability and durability of railway axles.

Electron Concept of Hydrogen Embrittlement and Hydrogen-Increased Plasticity of Metals

Based on theoretical and experimental studies of hydrogen effect on the electron structure of iron, nickel and titanium, an electron concept is proposed for hydrogen embrittlement as well as for hydrogen-improved plasticity of engineering metallic materials. This concept implies a hydrogen-caused redistribution of valence electrons across their energy levels and an increase in the density of electron states at the Fermi level, causing a softening of the crystal lattice and, thereby, leading to a decrease in the specific energy of dislocations with consequent increase in their mobility. Innate phenomena in metallic solid solutions, namely, short-range atomic order in its two versions, short-range ordering and decomposition, are shown to be a precondition for the localization of plastic deformation. Hydrogen enhances merely this effect resulting in pseudo-brittle fracture. The role of hydrogen-induced superabundant vacancies in hydrogen-caused localization of plastic deformation and grain-boundary fracture in pure metals is discussed. Using the temperature- and strain-dependent internal friction, the enthalpies of hydrogen diffusion and hydrogen–dislocation binding are studied, and their controlling effect on the temperature- and strain-rate-dependent hydrogen embrittlement is demonstrated. Finally, a physical rationale is proposed for using hydrogen as a temporary alloying element in the technological processing of titanium alloys, and for a positive hydrogen effect on the fatigue life and plasticity of austenitic steels.

Enhancing mechanical properties of medium Mn steel by warm rolling based on laminated elemental segregation

The hot-rolled microstructure of medium Mn steel has coarse grains and severe elemental segregation, resulting in low strength and plasticity. Constructing a multiphase structure, refining the microstructure, and regulating elemental segregation enhance the mechanical properties. In this study, liquid nitrogen treatment created a layered distribution of austenite and martensite. Warm rolling was then used to reduce layer thickness and refine grain structure. After liquid nitrogen and warm rolling treatments, the strength and plasticity of medium Mn steel increased to 1270 MPa and 23.3%, respectively, far exceeding the hot-rolled state (724 MPa, 12.8%). Warm rolling also triggers austenite reverted transformation (ART) and introduces high-density dislocations, further improving austenite stability. This strengthening effect is higher than that from intercritical annealing alone. Improved austenite stability delays the transformation induced plasticity (TRIP) effect, preventing brittle fracture and enhancing deformation coordination between layers, significantly increasing the plastic deformation capacity of medium Mn steel.

Microstructural Evolution and Hot Deformation Behavior of Cu‐Containing Twinning‐Induced Plasticity Steel

High manganese twinning‐induced plasticity (TWIP) steels with excellent strength and plasticity have promising applications in automotive manufacturing, but limited corrosion resistance affects their further development. Alloying with Cu is a viable solution to improve corrosion resistance. However, there remains a paucity of research concerning the effect of Cu on the hot deformation behavior of TWIP steels. So, the investigation of the recrystallization behavior and microstructural evolution in Fe‐23Mn‐6Cr‐3Al‐0.2C‐xCu (x = 0, 2.5) TWIP steels has been conducted using uniaxial hot compression test. The results reveal that Cu alloying exerts minimal influence on the high‐temperatur flow behavior, with dynamic recrystallization emerging as the predominant softening mechanism in Cu‐containing TWIP steels. However, owing to the drag effect of solute atoms, Cu alloying increases the activation energy for hot deformation and marginally reduces hot workability. Fine recrystallized grains in Cu‐containing TWIP steels can be achieved by hot deformation at lower temperatures and strain rate regions. The recrystallization behavior of Cu‐containing TWIP steels hot deformed at low temperatures and high strain rates is obviously inhibited by the increase of the activation energy and stacking fault energy, coupled with the hindering effect of Cu solute atoms clustered at grain boundaries on grain boundary migration.

Nonlinear Soil–Pile–Structure Interaction Behaviour of Marine Jetty Structures

Nonlinear soil–pile–structure interaction (SPSI) phenomena are known to play a vital role in the response of bottom-fixed marine structures. For such structures, these phenomena are commonly considered by the imposition of p-y, τ-z, and q-z springs, representing the lateral and axial shaft and axial base soil resistances, respectively. The importance of each resistance mechanism depends on the type of foundation system, with only very limited studies investigating their roles in the response of piled marine structures, such as jetties. Within this context, this study presents numerical three-dimensional pushover analysis results for two marine jetties, a smaller model with four piles and a larger model supported by twenty-four piles. SPSI effects are considered through p-y, τ-z, and q-z springs, the behaviours of which are determined by following commonly employed procedures. The structures’ responses are investigated under the influence of various assumptions regarding the behaviours of springs, as well as steel plasticity. The current investigation underscores the substantial influence of the axial soil–pile interaction on the response of the jetty, particularly in terms of its failure mode. Moreover, it demonstrates the importance of incorporating p-y springs, even though the choice between their linear or nonlinear constitutive behaviour is found to be less critical. Finally, the study concludes that the behaviours of the springs significantly affect the system’s ductility and the degree of steel yielding in the piles, while also highlighting the unconservative influence of neglecting SPSI phenomena.

Modelling of hot deformation of cast austenitic corrosion-resistant steel type 20Mn–18Cr–3Ni–2Mo–0.4N

The deformation behaviour of cast austenitic corrosion-resistant steel type 20Mn–18Cr–3Ni–2Mo–0.4N in the temperature range 1000–1250 °C and strain rates in the range 0.1–10 s–1 has been studied. It is shown that the flow stresses drop with increasing temperature and decreasing strain rate in accordance with the change of the Ziner–Hollomon parameter of the temperature-velocity deformation regime. From the analysis of peak flow stresses, the value of the effective activation energy of hot deformation (Q = 490.3 kJ/mol) required for the calculation of the Ziner–Hollomon parameter is determined. An expression for the peak flow stress in the form of a hyperbolic function of the Ziner–Hollomon parameter was obtained. A comparative evaluation of hot plasticity of austenitic non-magnetic steel by finding the strain value corresponding to the appearance of the first macrocracks on the surface of the specimen is carried out.

Enhanced strength and plasticity of 316L austenitic stainless steel through combined ECAP and annealing treatment

The mechanical properties of 316L austenitic stainless steel (ASS) were enhanced by equal channel angular pressing (ECAP) combined with annealing treatment. High-density dislocations, twins, and slip bands are generated during 4 passes ECAP. An appropriate annealing treatment is beneficial for overcoming ductility loss and maintaining strength of ECAP samples. In this study, we found that annealing at 700 °C for 1 h and 600 °C for 12 h can achieve this goal, mainly due to the recovery of stacked dislocations at boundaries and the retention of nanoscale twins and slip bands.

Modelling of hot deformation of cast austenitic corrosion-resistant steel type 20Mn–18Cr–3Ni–2Mo–0.4N

The deformation behaviour of cast austenitic corrosion-resistant steel type 20Mn–18Cr–3Ni–2Mo–0.4N in the temperature range 1000–1250 °C and strain rates in the range 0.1–10 s–1 has been studied. It is shown that the flow stresses drop with increasing temperature and decreasing strain rate in accordance with the change of the Ziner–Hollomon parameter of the temperature-velocity deformation regime. From the analysis of peak flow stresses, the value of the effective activation energy of hot deformation (Q = 490.3 kJ/mol) required for the calculation of the Ziner–Hollomon parameter is determined. An expression for the peak flow stress in the form of a hyperbolic function of the Ziner–Hollomon parameter was obtained. A comparative evaluation of hot plasticity of austenitic non-magnetic steel by finding the strain value corresponding to the appearance of the first macrocracks on the surface of the specimen is carried out.

Ultra-Fine Bainite in Medium-Carbon High-Silicon Bainitic Steel.

The effects of austenitizing and austempering temperatures on the bainite transformation kinetics and the microstructural and mechanical properties of a medium-carbon high-silicon ultra-fine bainitic steel were investigated via dilatometric measurements, microstructural characterization and mechanical tests. It is demonstrated that the optimum austenitizing temperature exists for 0.3 wt.%C ultra-fine bainitic steel. Although the finer austenite grain at 950 °C provides more bainite nuclei site and form finer bainitic ferrite plates, the lower dislocation density in plates and the higher volume fraction of the retained austenite reduces the strength and impact toughness of ultra-fine steel. When the austenitizing temperature exceeds 1000 °C, the true thickness of bainitic ferrite plates and the volume fraction of blocky retained austenite in the bainite microstructure increase significantly with the increases in austenitizing temperature, which do harm to the plasticity and impact toughness. The effect of austempering temperature on the transformation behavior and microstructural morphology of ultra-fine bainite is greater than that of austenitizing temperature. The prior martensite, formed when the austempering temperature below Ms, can refine the bainitic ferrite plates and improve the strength and impact toughness. However, the presence of prior martensite divides the untransformed austenite and inhibits the growth of bainite sheaves, thus prolonging the finishing time of bainite transformation. In addition, prior martensite also strengthens the stability of untransformed austenite though carbon partition and enhances the volume fraction of blocky retained austenite, which reduces the plasticity of ultra-fine bainitic steel. According to the experimental results, the optimum austempering process for 0.3 wt. %C ultra-fine bainitic steel is through austenitization at 1000 °C and austempering at 340 °C.

Microstructure and Mechanical Properties of a New TWIP Steel under Different Heat Treatments.

The effects of solution treatment and annealing temperature on the microstructure and mechanical properties of a new TWIP steel that was alloyed from aluminum (Al), silicon (Si), vanadium (V), and molybdenum (Mo) elements were investigated by a variety of techniques such as microstructural characterization and room tensile testing. The austenite grain size grew slowly with the increase in annealing temperature. The relatively weak effect of the solution treatment and annealing temperature on the austenite grain size was attributed to the precipitation of MC and M2C, which hindered the growth of the austenite grain. The plasticity of the TWIP steel in cold rolling and annealing after solution treatment was obviously higher than that in cold rolling and annealing without solution treatment. This was because the large-size precipitates redissolved in the matrix after solution treatment, which were not retained in the subsequently annealed structure. Through cold rolling and annealing at 800 °C after solution treatment, the prepared steel exhibited excellent strength and plasticity simultaneously, with a yield strength of 877 MPa, a tensile strength of 1457 MPa, and an elongation of 46.1%. The strength improvement of the designed TWIP steel was mainly attributed to the grain refinement and precipitation strengthening.

Study on the Low Plastic Behavior of Expansion Deformation of Austenitic Stainless Steel.

The plastic deformation of TWIP steel is greatly inhibited during the expansion process. The stress-strain curves obtained through expansion experiments and observations of fracture morphology confirmed the low plastic behavior of TWIP steel during expansion deformation. Through an analysis of the mechanical expansion model, it was found that the expansion process has a lower stress coefficient and a faster strain rate than stretching, which inhibits the plasticity of TWIP steel during expansion deformation. Using metallographic microscopy, transmission electron microscopy, and EBSD to observe the twin morphology during expansion deformation and tensile deformation, it was found that expansion deformation has a higher twin density, which is manifested in a denser twin arrangement and a large number of twin deliveries in the microscopic morphology. During the expansion deformation process, dislocation slips are hindered by twins, the free path of the slips is reduced, and dislocations accumulate significantly. The accumulation area is the initial point of crack expansion. The results show that the significant dislocation accumulation caused by the delivery of a large number of twins under expansion deformation is the main reason for the decrease in the plasticity of TWIP steel.

Microstructure Evolution and Mechanical Properties of Laser Welded Dissimilar/Similar Joints Between Medium‐Mn Transformation‐Induced Plasticity Steel and DP980/22MnB5 Steels

In this article, the microstructure and mechanical properties of the Fe–0.31C–6.86Mn–3.53Al steel and DP980 and 22MnB5 steel dissimilar/similar joints by fiber laser welding are studied. Based on experimental results, fusion zone (FZ) of transformation‐induced plasticity (TRIP)‐welded joint mainly consists of 77 vol% martensite and 23 vol% ferrite. The FZ of TRIP‐DP980 and TRIP‐22MnB5 welds mainly consist of martensite. The hardness profile of weld cross‐section exhibits significant asymmetry, and softening zones hardness of TRIP‐DP980 and TRIP‐22MnB5 accounts for 82% and 64% of DP980 and 22MnB5 base materials (BM), respectively. However, no softening zone is observed on TRIP steel side, primarily due to the lack of martensite in TRIP BM. The tensile results showed that welding efficiency of TRIP‐TRIP reached 99%, but elongation of welded specimen is only 50% of TRIP BM due to the high hardness and brittleness of FZ region. TRIP‐DP980 and TRIP‐22MnB5 joints did not fail in softening zone, because the constraint stress is generated on both sides of softening zone to inhibit the plastic deformation in softening zone during tensile process. The tensile stress is distributed on the surface of tensile specimens and concentrated in BM on the side with low yield strength (YS), resulting in the final fracture.

Discovery of microsegregation-aided transformation and twinning-induced plasticity in low Mn steel through directed energy deposition of functionally graded materials

Functionally graded materials (FGMs) combining two dissimilar steels, stainless steel 316 L and high-strength low-alloy steel, were additively manufactured using directed energy deposition. High-throughput characterization of the dissimilar steel FGM led to the discovery of a low Manganese (Mn<1 wt.%) TRIP (transformation-induced plasticity) and TWIP (twinning-induced plasticity) steel with a metastable microstructure enabled by additive manufacturing. Microsegregation from non-equilibrium solidification caused phase stability and stacking fault energy heterogeneity, leading to TRIP and TWIP effects in the as-built condition without heat treatment. Tensile testing of the new as-built TRIP and TWIP steel resulted in ultimate tensile strength of 960 MPa, yield strength of 415 MPa, total elongation of 26 %, and a unique strain hardening rate that increases after yielding. We compare experimental measurements of microsegregation with thermodynamic modeling to discuss the impact of microsegregation on phase stability, stacking fault energy, and solidification cracking susceptibility in additively manufactured FGMs. The highlights of this work include the discovery of a novel pathway for achieving TRIP and TWIP effects in additively manufactured steels without heat treatment. This work also shows that the TWIP effect can be introduced without high Manganese content playing a critical role in adjusting stacking fault energy.

Hydrogen in metallic alloys ─ embrittlement and enhanced plasticity: a review

Abstract The evolution of ideas concerning the nature of hydrogen embrittlement of engineering metallic materials is described based on a number of the proposed hypotheses and corresponding experiments. The main attention is paid to two of them, namely hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). Recent attempts to interconnect the both models as HELP + HEDE and HELP-mediated HEDE ones are also estimated. A conclusion is made that HELP model is preferential for understanding the entire array of experimental data with a caveat that it is necessary to consider the chemical nature of hydrogen atoms and view them not only as point defects. Based on the studies of hydrogen effect on the atomic interactions in iron, nickel, titanium, and its alloys, it is shown that the electron approach to HELP phenomenon adequately describes two competitive features of hydrogen behavior in metals: increased brittleness and enhanced plasticity. Due to the increase in the concentration of free electrons, hydrogen decreases the elasticity moduli, which causes the crystal lattice to soften. For this reason, the formation of hydrogen atmospheres around the dislocations decreases the start stress of dislocation sources, as well as line tension of emitted dislocations, enhancing thereby their mobility, and weakens repulsion between dislocations in their pile-ups. The range of temperatures and strain rates in which hydrogen embrittlement occurs is controlled by the enthalpies of hydrogen atoms’ diffusion and their binding to dislocations. The resulting consequences for mechanical properties depend on the short-range atomic order, SRO, which inherently occurs in the metallic solid solutions and localizes plastic deformation both in the cases of short-range atomic ordering and of short-range atomic decomposition. Hydrogen enhances slip localization because of its different solubility in the submicrovolumes of short-range decomposed solid solutions. If SRO is absent or not remarkably formed, the hydrogen-increased concentration of free electrons results in enhanced plasticity. Available positive hydrogen effects on the plasticity of titanium β-alloys and austenitic steels are presented and interpreted.

Excellent ductility of an austenitic stainless steel at a high strength level achieved by a simple process

In the pursuit of simultaneously improving the yield strength and plasticity of austenitic stainless steel, a new austenitic stainless steel was fabricated by induction smelting using a pure N2 atmosphere, hot forging, cryogenic rolling, and annealing. The material was characterized by microstructures with 3–4 μm uniform finer grains, fine precipitates, high thermal stability austenite, and extensive high-angle grain boundaries. The elongation after fracture, yield strength, and ultimate tensile strength of the samples reached 53.5 %, 707 MPa, and 1020 MPa, respectively, as well as 61 %, 600 MPa, and 977 MPa, respectively, at the same time. Moreover, a high strain hardening rate was achieved in the new stainless steel. The appropriate uniform finer grains not only played a role in grain-refined strengthening but also provided intragranular spaces and sufficient mean free available paths for dislocation accumulation and movement. Precipitates, which were coherent or semi-coherent with the matrix, provided interfaces for dislocation accumulation and obstructions for dislocation movement. Extensive high-angle grain boundaries with appropriate finer grains served as another important factor for excellent ductility due to the inhabitation and resulting deviation of crack propagation. In addition, strain-induced mechanical twinning in the current austenitic stainless steel contributed to excellent ductility and high strength.

A comparative study on the wear behavior of dual phase (DP) steel and quenching and partitioning (QP) steel

Present study aims to elucidate the wear resistance/morphology/mechanism at wear distances from 3 to 1000 m based on the microstructure and tensile properties in DP steel and QP steel with similar initial hardness by means of sliding wear test and electron microscopes. QP steel shows better wear resistance at higher wear distance over 150 m opposite to lower wear distance, compared to DP steel, due to continuous increase in hardened layer during wear test in QP steel. This is because of higher fraction of adhesive wearing (e.g., galling) without significant wear loss originated from higher capacity of strain−hardening caused by effective impacts of grain refinement of ferrite, transformation-induced plasticity, and multiphase hardening in QP steel.

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.