Search Results (28)

In this work, we aimed to study the austenite grain growth behavior of an ultra-high-strength stainless steel within the temperature range of 900–1150 °C and holding time range of 0–120 min, using a metallographic microscope and metallographic image analysis software to perform a statistical analysis of grain size variation. The undissolved phases of the steel were investigated using a field emission scanning electron microscope (SEM) and transmission electron microscope (TEM). Within the temperature range of 900–950 °C, the grain growth rate of the steel was slow, while within the range of 1000–1150 °C, the grain growth rate was relatively fast. This is attributed to the precipitation of a large number of M₆C-type carbides during the forging and annealing processes. In the temperature range of 900–950 °C, the solid solubility of the M₆C phase was low and the pinning effect was significant, which hindered the growth of austenite grains. Above 950 °C, the carbides were dissolved extensively, weakening the pinning effect on the grain boundaries and accelerating the grain growth rate. A predictive mathematical model for the growth of the original austenite grains was established based on the Arrhenius equation, elucidating the effects of heating temperature, holding time, initial grain size, and number of carbides on the growth of austenite grains, providing a theoretical basis for heat treatment process design in actual production. Full article

With the increasing use of concrete-filled steel tubular (CFST) structures, exposed steel tubes are highly susceptible to corrosion, posing potential safety hazards. This study innovatively proposes the use of stainless-steel tubes instead of traditional carbon-steel ones and introduces coarse aggregates into ultra-high-performance concrete (UHPC), forming a coarse aggregate-incorporated ultra-high-performance concrete-filled stainless-steel tube (CA-UFSST). The inclusion of coarse aggregates not only compensates for the shortcomings of UHPC but also enhances the overall mechanical performance of the composite structure. Twenty sets of specimens were designed to analyze the influence of four parameters, including the coarse aggregate content, compressive strength, stainless-steel-tube thickness, and stainless-steel type on the axial compression performance of UHPC. The experimental results indicate that the failure mode of UHPC is related to the confinement ratio. As the confinement ratio increases, the failure mode transitions from shear failure to bulging failure. The addition of coarse aggregates enhances the stiffness of the specimens. Furthermore, this paper discusses the applicability of six current codes in predicting the bearing capacity of CA-UFSST and finds that the European code exhibits the best prediction performance. However, as the confinement ratio increases, the prediction accuracy becomes notably insufficient. Therefore, it is necessary to establish a more accurate calculation model for the axial compression bearing capacity. Full article

The strength of ultra-low carbon maraging stainless steels can be significantly enhanced by precipitating nanoscale intermetallic secondary phases. Retained or reversed austenite in the steel can improve its toughness, which is key to achieving an ideal combination of strength and toughness. Ti and Al are often used as cost-effective strengthening elements in maraging stainless steels but the synergistic toughening and strengthening mechanisms of Ti and Al have not been studied. To investigate the synergistic toughening and strengthening mechanisms of Ti and Al in Co-free maraging stainless steels, this paper focuses on the microstructure and mechanical properties of three alloys: Fe-12Cr-11Ni-1.7Al-0.5Ti (Steel A), Fe-12Cr-11Ni-0.5Ti (Steel B), and Fe-12Cr-11Ni-1.7Al (Steel C). The impact of Ti and Al on the microstructure and mechanical properties was investigated using X-ray diffraction (XRD), high-resolution transmission electron microscopy (TEM), and thermodynamic simulations. The relationship between microstructure, strength, and toughness is also discussed. The results indicated that Steel A, containing both Al and Ti, exhibited the highest strength level after solution treatment at 900 °C, with an ultimate tensile strength reaching 1571 MPa after aging at 540 °C. This is attributed to the simultaneous precipitation of spherical β-NiAl and rod-shaped η-Ni₃Ti phases. Steel B, with only Ti, formed a significant amount of Ni-rich reversed austenite during aging, reducing its ultimate tensile strength to 1096 MPa. Steel C, with only Al, showed a high strength–toughness combination, which was achieved by forming dispersive nano-sized intermetallic precipitates of β-NiAl in the martensitic matrix with a slight amount of austenite. It is highlighted that Al has superior toughening and strengthening effects compared to Ti in the alloy system. Full article

Isothermal constant-strain-rate compression experiments of S280 ultrahigh-strength stainless steel were conducted at 800–1150 °C, 0.001–10 s⁻¹, and 70% height reduction. The flow stress behaviors were analyzed based on the compression data. The strain compensation Arrhenius constitutive relation, multiple linear regression constitutive relation, and back-propagation (BP) neural network constitutive relation of this alloy were established for the first time. The S280 ultrahigh-strength stainless steel is characterized by a positive strain rate and negative temperature sensitivity. Its flow stress at high temperature (1000–1150 °C) and low temperature (800–950 °C) is generally at the steady state and the softening state, respectively. The three new flow stress constitutive relations all meet the requirements for engineering applications in terms of predictive precision. The BP neural network constitutive relation shows the highest predictive precision, with correlation coefficient R of 0.999 and average absolute relative error AARE of 1.04%. The strain compensation Arrhenius constitutive relation shows the lowest predictive precision, with R of 0.994 and AARE of 14.748%. The multiple linear regression constitutive relation shows the modest predictive precision, with R of 0.994 and AARE of 6.24%. Full article

Refractory materials containing calcium aluminate cement (CAC) are commonly used in the metallurgical and petrochemical industries due to their exceptional mechanical resistance, even at temperatures exceeding 1000 °C, and do not require additional reinforcement. This study seeks to advance this practice by developing ultra-high-performance structures that offer building protection against fire and explosions. Such structures require bar reinforcement to withstand accidental tension stresses, and the bond performance becomes crucial. However, the compressive strength of these materials may not correlate with their bond resistance under high-temperature conditions. This study investigates the bond behavior of ribbed stainless austenitic steel bars in refractory materials typical for structural projects. The analysis considers three chamotte-based compositions, i.e., a conventional castable (CC) with 25 wt% CAC, a medium-cement castable (MCC) with 12 wt% CAC, a low-cement castable (LCC), and a low-cement bauxite-based castable (LCB); the LCC and LCB castables contain 7 wt% CAC. The first three refractory compositions were designed to achieve a cold compressive strength (CCS) of 100 MPa, while the LCB mix proportions were set to reach a CCS of 150 MPa. Mechanical and pull-out tests were conducted after treatment at 400 °C, 600 °C, 800 °C, and 1000 °C; reference specimens were not subjected to additional temperature treatment. This study used X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscopy (SEM) methods to capture the material alterations. The test results indicated that the bonding resistance, expressed in terms of the pull-out deformation energy, did not directly correlate with the compressive strength, supporting the research hypothesis. Full article

Sub-micro dislocation cellular structures formed during rapid solidification break the strength–ductility trade-off in laser powder bed fusion (LPBF)-processed 316L stainless steel through high-density dislocations and segregated elements or precipitates at the cellular boundaries. The high-density dislocation entangled at the cellular boundary accommodates solidification strains among the cellular structures and cooling stresses through elastoplastic deformation. Columnar grains with cellular structures typically form along the direction of thermal flux. However, the ultra-low misorientations between the adjacent cellular structures and their interactions with the cellular boundary formation remain unclear. In this study, we revealed the ultra-low misorientations between the cellular structures in LPBF-processed 316L stainless steel using conventional electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), and transmission electron microscopy (TEM). The conventional EBSD and TKD analysis results could provide misorientation angles smaller than 2°, while the resolution mainly depends on the specimen quality and scanning step size, and so on. A TEM technique with higher spatial resolution provides accurate information between adjacent dislocation cells with misorientation angles smaller than 1°. This study presents evidence that the TEM method is the better and more precise analytical method for the misorientation measurement of the cellular structures and provides insights into measuring the small misorientation angles between adjacent dislocation cells and nanograins in nanostructured metals and alloys with ultrafine-grained microstructures. Full article

The current review work studies the adiabatic shear banding (ASB) mechanism in metals and alloys, focusing on its microstructural characteristics, dominant evolution mechanisms and final fracture. An ASB reflects a thermomechanical deformation instability developed under high strain and strain rates, finally leading to dynamic fracture. An ASB initially occurs under severe shear localization, followed by a significant rise in temperature due to high strain rate adiabatic conditions. That temperature increase activates thermal softening and mechanical degradation mechanisms, reacting to strain instability and facilitating micro-voiding, which, through its coalescence, results in cracking failure. This work aims to summarize and review the critical characteristics of an ASB’s microstructure and morphology, evolution mechanisms, the propensity of materials against an ASB and fracture mechanisms in order to highlight their stage-by-stage evolution and attribute them a more consecutive behavior rather than an uncontrollable one. In that way, this study focuses on underlining some ASB aspects that remain fuzzy, allowing for further research, such as research on the interaction between thermal and damage softening regarding their contribution to ASB evolution, the conversion of strain energy to internal heat, which proved to be material-dependent instead of constant, and the strain rate sensitivity effect, which also concerns whether the temperature rise reflects a precursor or a result of ASB. Except for conventional metals and alloys like steels (low carbon, stainless, maraging, armox, ultra-high-strength steels, etc.), titanium alloys, aluminum alloys, magnesium alloys, nickel superalloys, uranium alloys, zirconium alloys and pure copper, the ASB propensity of nanocrystalline and ultrafine-grained materials, metallic-laminated composites, bulk metallic glasses and high-entropy alloys is also evaluated. Finally, the need to develop a micro-/macroscopic coupling during the thermomechanical approach to the ASB phenomenon is pointed out, highlighting the interaction between microstructural softening mechanisms and macroscopic mechanical behavior during ASB evolution and fracture. Full article

Refractory castables, i.e., refractory aggregates and ultra-fine particle mixtures with calcium aluminate cement (CAC) and deflocculants, were created 40 years ago for the metallurgy and petrochemical industries. These materials demonstrate outstanding performance even over 1000 °C. Typically, they have no structural reinforcement, resisting compression stresses because of the combination of temperature and mechanical loads. This study is a part of the research project that develops high-temperature resistance composite material suitable for fire and explosion protection of building structures. However, this application is impossible without structural reinforcement, and the bond performance problem becomes essential under high temperatures. This experimental work conducts pull-out tests of austenitic stainless 304 steel bars and typical structural S500 steel bars embedded in refractory castables after high-temperature treatments. This study includes plain and ribbed bars and considers two castable materials designed with 25 wt% CAC content for 50 MPa compressive strength after drying (typical design) and 100 MPa strength (modified with 2.5 wt% microsilica). This test program includes 115 samples for pull-out tests and 88 specimens for compression. As expected, the tests demonstrated the plain bars’ inability to resist the bond stresses already at 400 °C; on the contrary, ribbed bars, even made of structural steel, could ensure a mechanical bond with cement matrix up to 1000 °C. However, only stainless steel bars formed a reliable bond with the high-performance castable, determining a promising object for high-temperature applications. Still, the scatter of the test results did not ensure a reliable bonding model. In addition, the castable strength might not be optimal to ensure maximum bond performance. Thus, the test results clarified the research objectives for further developing the reinforced composite. Full article

The use of ultra-short pulse lasers in the kW range, combined with an appropriate beam engineering approach, enables the achievement of high-throughput production of laser-functionalised surfaces. However, the manufacturing of complex parts still faces various challenges, such as difficulties in accessing regions with high aspect ratio shapes or intricate profiles, which often leads to the necessity of adapting the laser processing workstation to specific geometries. The forming process is a well-established technique for producing parts of any shape from metallic foils by imposing specific constraints. In this study, we aimed to assess the feasibility of producing laser-functionalised 3D complex products by the forming of laser-treated flat thin metallic sheets. Two-hundred micrometre-thick stainless-steel foils were textured with laser-induced periodic surface structures (LIPSS) through a roll-to-roll pilot line. First, we optimized the morphology of LIPSS. Subsequently, we conducted three types of mechanical tests on both laser-treated and untreated foils: standard tensile tests, fatigue tests, and cruciform specimen tests. We measured and compared parameters such as ultimate tensile strength, breaking strength, maximum elongation, and area reduction between specimens with and without LIPSS, all obtained from the same foil. Additionally, we utilized scanning electron microscopy (SEM) to compare the LIPSS morphology of laser-treated samples before and after mechanical tests. Full article

A novel versatile ultrahigh-strength stainless bearing steel was prepared by first solution treating the steel at temperatures between 1000 °C and 1100 °C for 1 h, followed by performing cryogenic treatment at −73 °C for 2 h, and tempering at 500 °C for 2 h, with the cryogenic and tempering treatments being repeated twice. The microstructures were characterized using multiscale techniques, and the mechanical properties were investigated using tensile testing, as well as via Rockwell hardness and impact toughness measurements. Tensile strength was found to be independent of solution temperature, with a value of about 1800 MPa. In contrast, yield strength decreased from 1530 MPa to 1033 MPa with increasing solution temperature, while tensile elongation increased from 15.3% to 20.5%. This resulted in an excellent combined product of tensile strength and elongation for steels initially treated at 1080 °C and 1100 °C, with values of 33.9 GPa·% and 37.0 GPa·%, respectively. Furthermore, the steels showed excellent impact toughness, increasing from 37.0 J to 86.2 J with increasing solution temperature. The microstructural and mechanical investigations reveal that the excellent mechanical properties and impact toughness are related to three factors, namely (i) a transformation-induced plasticity effect, mainly attributed to a high volume fraction of retained austenite, (ii) a high strengthening capacity arising from a high dislocation density, and (iii) a synergistic effect due to cobalt additions and the nanoprecipitation of M₂C and M₆C carbides. Full article

This study investigated the effect of hollow 304 stainless-steel fiber on the corrosion resistance and mechanical properties of ultra-high-performance concrete (UHPC), and prepared copper-coated-fiber-reinforced UHPC as the control group. The electrochemical performance of the prepared UHPC was compared with the results of X-ray computed tomography (X-CT). The results reveal that cavitation can improve the distribution of steel fibers in the UHPC. Compared with solid steel fibers, the compressive strength of UHPC with hollow stainless-steel fibers did not exhibit significant change, but the maximum flexural strength increased by 45.2% (2 vol% content, length–diameter ratio of 60). Hollow stainless-steel fiber could better improve the durability of UHPC compared with copper-plated steel fiber, and the gap between the two continued to increase as the durability test progressed. After the dry–wet cycle test, the flexural strength of the copper-coated-fiber-reinforced UHPC was 26 MPa, marking a decrease of 21.9%, while the flexural strength of the UHPC mixed with hollow stainless-steel fibers was 40.1 MPa, marking a decrease of only 5.6%. When the salt spray test had run for seven days, the difference in the flexural strength between the two was 18.4%, but when the test ended (180 days), the difference increased to 34%. The electrochemical performance of the hollow stainless-steel fiber improved, owing to the small carrying capacity of the hollow structure, and more uniform distribution in the UHPC and lower interconnection probability were achieved. According to the AC impedance test results, the charge transfer impedance of the UHPC doped with solid steel fiber is 5.8 KΩ, while that of the UHPC doped with hollow stainless-steel fiber is 8.8 KΩ. Full article

The present paper reports the analyses of results obtained from experiments carried out to explore the challenge of homogeneous, uniform, and deagglomerated dispersion of ultra-heavy nanoparticles (NPs) in the high-performance polyaryletherketone (PAEK) matrix. An equal and fixed amount of (0.5 vol. %) NPs of silicon carbide (SiC), zirconium carbide (ZrC), and tungsten carbide (WC) were dispersed in a PAEK matrix and compression molded to develop three different nanocomposites. Simultaneously, nano-adhesives of the same composition were also developed to join the stainless steel adherends. The composites and adhesives were characterized for their physical, thermal, thermo-mechanical, thermal conductivity (TC), and lap shear strength (LSS) behavior. It was observed that SiC NPs performed significantly better than ZrC and WC NCs in all performance properties (LSS: 154%, TC: 263%, tensile strength: 21%). Thermal conductivity (TC) and tensile properties were validated using various predictive models, such as the rule of mixture parallel model, the Chiew and Glandt model, and the Lewis model. Scanning electron micrographs were used for the morphological analysis of LSS samples to detect macro- and micro-failure. Micrographs showed evidence of micro-striation and plastic deformation as a micromodel, as well as mixed failure, i.e., adhesive–cohesive as a macro-failure mode. Full article

To study the axial compression performance of a steel-reinforced ultra-high-performance concrete-filled square stainless-steel tube (SR-UHPCFSSST), eight specimens were designed with different length-to-diameter ratios and skeleton contents, and axial compression tests and numerical simulations were performed. Damage pattern, ultimate load capacity, and load–displacement curve data of the specimens were obtained. Finite element analysis was performed using ABAQUS software for parameter expansion. The damage mechanisms of the specimens and the influences of various parameters, such as the length-to-diameter ratio, skeleton content, diameter-to-thickness ratio, and concrete strength, on the damage processes and ultimate bearing capacities of the specimens were studied. The results showed that among the components of the test piece under the same axial load, the stress of the built-in steel skeleton was the first to approach its yield stress, and the steel pipe was the first to produce a bulging deformation. The ultimate bearing capacities of the specimens increased with the increase in the skeleton content and concrete strength and decreased with the increase in the length-to-diameter and diameter-to-thickness ratios. Based on the test and numerical simulation results, this paper puts forward a calculation formula of the axial compression bearing capacity of a square-stainless-steel-tube ultra-high-performance-concrete middle-length column with a steel skeleton, which provides a reference for engineering design and for compiling relevant codes. Full article

The current study applied thermomechanical processing (TMP) on 301 austenitic stainless steel to produce an ultrafine-grained austenitic structure, examining the dual effects of deformation at subzero temperature and TMP cycles on the strain-induced α′-martensitic transformation and austenite reversion occurring upon subsequent annealing. Three TMP schemes were adopted: (1) one cycle using a strain of 0.30, (2) two cycles using a strain of 0.20, and (3) three cycles using a strain of 0.15. Each cycle consisted of tensile deformation at −50 °C followed by annealing at 850 °C for 5 min. Compared to other schemes, the use of three cycles of the 0.15 strain scheme resulted in a significant formation of the martensitic phase to about 99 vol.%. Consequently, the austenite reversion occurred strongly, providing a mixture of the austenitic structure of reverted ultra-fine grains and retained coarse grains with an average grain size of 1.9 µm. The development of a mixed austenitic structure was found to lower the austenite stability and thus enhance the α′-martensitic transformation upon deformation in subsequent cycles. Moderate growth of high-angle grain boundaries occurred in the austenitic phase for all schemes, reaching a maximum of 64% in cycle 3 of the 0.15 strain scheme. The tensile behavior during subzero deformation was generally characterized by an initial strain hardening by slip (stage I), followed by a remarkable increase in strain hardening rate due to the strain-induced α′-martensitic transformation (stage II). Further straining promoted breakage of the α′-martensite banded lath structure for forming dislocation cell-type martensite, which was marked by a decline in strain hardening rate (stage III). Accordingly, the latter hardening stage had a lesser hardness enhancement of deformed samples with an increasing number of cycles. Nevertheless, the yield strength for samples processed by the 0.15 strain scheme improved from 450 MPa in cycle 1 to 515 MPa in cycle 3. Full article

Engineered cementitious composites (ECC) exhibit ultra-high ductility and post-cracking resistance, which makes it an attractive material in civil engineering. First, a monotonic uniaxial tensile test was performed, considering the effects of polyvinyl alcohol (PVA) fiber volume content and water-binder ratio. Then, the effects of the above variables on the tensile characteristics including the tensile stress–strain relationship, deformation capacity, and fracture energy were investigated based on test results; and when the water-binder ratio is 0.28 and the fiber volume content is 2%, the deformation performance of ECC is improved most significantly. Next, combined with damage mechanics theory, the damage evolution mechanism of ECC in monotonic uniaxial tension was revealed, based on which the damage factor and damage evolution equation of ECC were developed and the expressions of model parameters were proposed. Moreover, the comparison between the proposed model and test results demonstrated the accuracy of the proposed model. Finally, to further verify the feasibility of the proposed model, a finite element (FE) simulation analysis of the tensile performance of high-strength stainless steel wire rope (HSSWR) reinforced ECC by adopting the proposed model was compared with test results and the simulation analysis results by using anther existing model, the “trilinear model of ECC”. The comparison shows that the proposed model in this paper can predict more accurately. Full article