Advanced Steels Research Articles

Microstructural inhomogeneity and associated hetero mechanical behavior in advanced bainitic weathering steel weldments

A new filler metal was developed to join the new generation weathering steel Q690qNH. Microstructural evolutions and the associated mechanical behavior of weldments were comparatively studied in hot rolled and tempered thick Q690qNH plates with initial microstructure of lath bainite and granular bainite respectively. Phase transformations in the HAZ promote the work hardening capacity of the weld jointed tempered Q690qNH (Q690T) plate and enable the joints to exhibit a different mechanical response upon the tensile loading in comparison to the weld jointed hot rolled Q690qNH (Q690R) plate. Prior austenite reconstruction was carried out to understand the crack nucleation behavior in HAZ. The plastic incompatibility between martensitic-austenitic (M-A) and bainitic ferrite leads to the nucleation of plastic micro-voids from the prior austenite grain boundary or M-A boundary. The fracture of Q690R joint is caused by coalescing micro-voids in HAZ with a strength reduction of 10%–14% that caused by the welding process. The continuous yielding behavior regarding the retained austinite in HAZ results in a competition of the plastic deformation in each subzone of the Q690T joint, of which the catastrophic fracture is dominated by the initial microstructure comprising the ferrite and the granular bainite.

Assessment of minimum allowable thickness of advanced steel (FeCrAl) cladding for accident tolerant fuel

The ferritic iron-chromium-aluminum (FeCrAl) alloy cladding is considered to be the most promising for near-term application in the ATF framework to replace existing zirconium alloy cladding. Although FeCrAl cladding presents several advantages, it is well known that there are at least two main drawbacks, one is the increased thermal neutron absorption cross-section compared to the current Zr-based cladding resulting in a neutronic penalty and another is tritium higher permeation. In the present study, the minimum allowable thickness of cladding is addressed considering neutronic penalty reduction and the mechanical-structural behavior under the LOCA accident condition. The neutronic penalty assessment was performed using the Monte Carlo code and mechanical-structural performance of the FeCrAl cladding using the TRANSURANUS fuel code, which was modified to consider properly the FeCrAl cladding.

Numerical and analytical studies for a novel steel brace retrofit system

To upgrade the existing RC frame structure with insufficient seismic design, a novel and advanced gapped eccentric steel brace (GESB) system was proposed. The finite element modeling methods based on fiber cross-section and macroscopic models were established and calibrated through the test results. The impact of three key design parameters, including axial load ratio (ALR) isolation thickness (IT) and Clamping Joint Spacing (CJS), was investigated. Based on the results of the numerical analysis, it is recommended that the drift demand controlled by IT should be between 0.6% and 1%, while the CJS should be 1/4 to 1/3 in response to the ductility and strength requirements. And the effect of ALR on the retrofitted frame can be ignored. An analytical method was also presented, which proved to be accurate for estimating the mechanical response of the retrofitted frame by comparison with the experimental and numerical response.

Development of advanced cementite-free bainitic steel using Ni, Al and Cu additions – Design concept and phase transformations study

A novel concept, consisting of the stabilization of retained austenite with Ni, Al, and Cu additions instead of using Si as the major alloying element to prevent carbide formation, is proposed to design nanocrystalline, cementite-free bainitic steels. In this work, the novel steel (wt.%) Fe-0.40C-0.50Mn-8.0Ni-2.65Al-0.8Cu was designed. Dilatometry investigations were performed to obtain the experimental CCT and TTT diagrams for this material. It was concluded that the bainitic reaction occurs between 400 and 225 °C during a transformation time ranging from 2 h to 66.5 h, respectively. An in-depth analysis performed on the resulting materials confirmed the presence of an ultrafine structure free of cementite consisting of carbon supersaturated laths of tetragonal bainitic ferrite and retained austenite with thin film and block morphologies. The use of Ni, Al and Cu as alloying elements leads to high carbon content in both populations of retained austenite, which prevents, the formation of fresh martensite during cooling to room temperature. The results of this research constitute a novel insight into the design of ultra-fine, cementite-free bainitic steels.

A novel low-cost ultra-strong maraging steel by additive manufacturing

A novel Co/Mo free maraging steel, hardened by nano-sized β-NiAl precipitates, has been developed for additive manufacturing (AM). The composition was designed to achieve ultra-high strength with low cost, simultaneously maintaining the formability. The steel was successfully fabricated using laser powder bed fuse (L-PBF) additive manufacturing (AM) technology, and the effect of heat-treatment on the microstructure and mechanical properties of fabricated samples was investigated. Due to the refined microstructure and high-density dislocations, the as-built samples displayed in excess of 1400 MPa ultimate tensile strength with about 16% total elongation. This is the highest reported strength to date for any as-built AM materials. Upon direct aging treatment, nanoscale ordered β-NiAl precipitated in the martensite matrix, resulting in a remarkable combination of mechanical properties, i.e. the strength levels exceeding 2000 MPa and elongation up to ∼8% at room temperature. This work contributes to the development of advanced ultrahigh-strength steel for AM.

A multi-scale model combining martensitic transformations with multi-phase crystallographic slip

Mechanically induced martensitic transformations are an essential mechanism influencing the constitutive behaviour of many advanced steel grades, such as austenitic stainless steels and Transformation Induced Plasticity (TRIP) alloys. In this work, a single-crystal constitutive model is developed to predict their complex microstructural behaviour during mechanical loading. The model is based on the work of Vieira de Carvalho et al. [16] and crucially proposes crystallographic slip in the martensitic phase after transformation from retained austenite. Austenite and martensite plasticity are incorporated following a standard finite strain single crystal plasticity approach and are fully coupled with the martensite formation kinematics, enabling the simultaneous evolution of the three mechanisms. The numerical treatment of the coupled constitutive equations is described in detail. In particular, the solution of the monolithic system of equations with the Newton-Raphson methodology, including the exact linearisation of both the local and global problems. An algorithmic strategy for handling the martensite volume fraction constraints is proposed for return-mappings in the restarted step. The model is validated using macroscopic experimental results for an austenitic stainless steel where an efficient calibration procedure for the micro-constitutive parameters is devised using derivative-free optimisation methods.

Laboratory and numerical investigation on the longitudinal resistance of ballasted railway tracks with steel sleepers

In this research, we experimentally investigated ballasted railway tracks' longitudinal resistance and stiffness with standard and advanced Y-shaped steel sleepers. We examined the shares of various track components in providing longitudinal resistance by measuring the displacement of the track panel. Moreover, we validated the results via modeling in finite element software. According to the results, the shares of the rail fasteners and the sleepers in the longitudinal resistance can vary based on the flexibility or rigidity of the track-to-sleeper connection. In addition, a comparison between the longitudinal resistance of these panels with wooden and concrete ones indicated that steel sleepers could provide acceptable levels of longitudinal resistance by requiring less ballast due to their unique geometry. Steel sleepers can therefore provide the necessary stability in track sections where we could not utilize other sleepers due to height restrictions. This resistance also depends on the prestressed torque force in the fastener springs. Precisely, higher prestressed-force results in higher resistance and stiffness. Based on the results, we obtained the longitudinal resistance per sleeper in the track panels with standard steel sleepers and Y-shaped sleepers at spring prestressed torque forces of 80 N.m, 60 N.m, and 100 N.m to be about 5.05, 4.55, 7.5, and 11.4 kN/sleeper, respectively. The results of FE models showed a difference of 5.23, 3.55, 2.61, and 2.06 times when applying a vertical load of 300 kN for the longitudinal resistance compared to the condition of no loading.

3D-printed strong and ductile high-entropy alloys with orientation arranged nanostructure complex

The rapid development of 3D printing technologies demands new materials with novel properties, as many high-strength metallic materials (up to over 1000 MPa) suffer from a significant weakness in ductility despite their enhanced mechanical properties, particularly in tension strength. To address this issue, we fabricated precipitation-hardened high-entropy alloy (Fe28Co29.5Ni27.5Al8.5Ti6.5) and employed aging post-treatment to obtain a duplex multicomponent intermetallic nanoprecipitate arrangement while retaining the in-situ dislocation network features. This approach resulted in the formation of a novel orientation arranged nanostructure complex (NSC) in the alloy. The NSC structure is composed of duplex multicomponent intermetallic nanoprecipitates (L12 and L21 phases), disordered multicomponent FCC matrix, and in-situ dislocation network. This resulted in ultra-high tensile strengths of 1430 MPa and an excellent tensile elongation close to 16%, surpassing the performance of other advanced commercial steels and alloys. These findings offer an effective approach to facilitate the design of high-strength and high-ductility structural materials for industrial applications. This is achieved by tailoring the microstructure through a combination of in-situ generation of high-density dislocations structure and multicomponent intermetallic nanoprecipitates in additive manufacturing. As such, this study represents an important step forward in the development of advanced materials for 3D printing technologies.

Distinct Mechanisms for Channel Segregation in Ultra-Heavy Advanced Steel Ingots via Numerical Simulation and Experimental Characterization

Distinct Mechanisms for Channel Segregation in Ultra-Heavy Advanced Steel Ingots via Numerical Simulation and Experimental Characterization

Preface

The 2023 10th International Conference on Advanced Manufacturing Technology and Materials Engineering (AMTME 2023), sponsored by Guangdong Dr. Council for the Promotion of Innovation and Development and Guangzhou Matals Association, is motivated to bridge the scientists, experts, practitioners, and students in the scientific forum through sharing ideas and issues about theoretical and practical knowledge in advanced manufacturing technology and materials engineering.This Seminar took place via hybrid form on March 24th to 26th, 2023 in Zhuhai, China, bringing together about 200 attendees worldwide. The previous conferences of AMTME have been successfully held in Beijing, China (2013, 2021), Guangzhou, China (2014, 2015, 2016, 2017), Hohhot, China (2018), Chongqing, China (2019, 2020). As an annual conference, it’s held in China every year, attracting more than 600 attendees from different countries all around the world. With strong organization team, dependable reputation and wide sponsors globally, AMTME 2023 is a leading annual conference of advanced manufacturing technology and materials engineering for all researchers at home and abroad.This Conference is considered to accommodate discussion among researchers in the field of advanced manufacturing technology and materials engineering as a scientific forum. Therefore, the invited speakers in this Conference are experts in the field of advanced manufacturing technology and materials engineering.The keynote presentations were provided particularly to show the contribution of scholars and experts in related research fields towards research and knowledge sharing. We had six keynote speakers coming mainly from Baosteel Central Research Institute, China, South China University of Technology, China, Shandong Jianzhu University, China, and Tianjin University, China. Professor Xinping Mao from Baosteel Central Research Institute has long been engaged in the research of advanced steel manufacturing processes and high-performance steel materials, and shared with us his personal experience and insights in material engineering.The AMTME 2023 involved more than 150 papers from various topics including Advanced Manufacturing Technology, Advanced Design Technology, New Materials and Advanced Materials, Manufacturing Systems and Automation, etc.. The selected papers were checked through a rigorous peer review process, and will be published in the present conference proceedings. Reviews were conducted by expert referees to the professional and scientific standards expected of the proceedings journal.We would like to thank all the members of the Local Organizing Committee for their efficient preparation for organizing this event. We would also extend our best gratitude to keynote speakers for their invaluable contribution and worthwhile ideas shared in the Conference. Last but not the least is our appreciation to Journal of Physics: Conference Series for every process its members conducted to make this volume published.The Committee of AMTME 2023List of Committee Member is available in this pdf.

Fuzhou–Xiamen high-speed railway: a new paradigm for China’s cross-sea railways

The new 277 km Fuzhou–Xiamen high-speed railway in China is the country’s first cross-sea railway designed for operational speeds of 350 km/h. Completed in 2023, it involved various advanced construction technologies, including the world’s first 1000 t bridge-building machine to be used offshore, advanced corrosion-resistant steel, pioneering rotation of a main bridge pylon, and aerodynamic steel–concrete composite bridge decks with ballastless tracks. The innovative project represents a milestone for China’s high-speed railway, heralding a new era of cross-sea transportation.

Designing gradient nanograined dual-phase structure in duplex stainless steel for superior strength-ductility synergy

Similar to other metallic materials, duplex stainless steel dramatically loses its advantage of high ductility as they are strengthened. Here, we produce a gradient nanograined dual-phase structure in the 2101 duplex stainless steel, thus facilitating a superior strength-ductility synergy: a yield strength of 1009.5 MPa being two times higher than that of the as-received sample, a total elongation of 23.4% and a uniform elongation of 5.9%. This novel structure is produced through a processing route of ultrasonic severe surface rolling and annealing, which realizes a superposition of gradient nanostructure and lamellar dual-phase structure with austenite and ferrite. During the tension deformation of gradient nanograined dual-phase structured duplex stainless steel, a significant accumulation of geometrically necessary dislocations occurs. These dislocations are formed to accommodate the deformation incompatibility caused by the layer-by-layer difference in strength and hardness of individual phase domains, as well as the inherent difference in properties between the austenite and ferrite domains. This results in a stronger hetero-deformation induced strengthening and hardening significantly contributing to superior mechanical properties. Our study provides a new avenue to develop advanced steels with high strength and ductility.

Dynamic strain aging behavior of an advanced alumina-forming austenite stainless steel based on Fe–30Ni–15Cr–3.5Al

Alumina-forming austenite (AFA) steels have emerged as promising candidate materials for ultra-supercritical units because of their excellent high-temperature performance. Dynamic strain aging (DSA) can increase the strength, but at the expense of plasticity, which will inevitably reduce the service life of materials. The aim of this study was to determine the serration behavior of a new AFA steel from room temperature (RT) to 700 °C at different strain rates from 1.33 × 10−5 s−1 to 1.33 × 10−2 s−1. Serrations were found at 300–650 °C under the tested conditions. Regardless of the strain rate, three types of serration arose: type A, A + B, and C. Plateaus of yield strength, ductility minima, and sensitivity to negative strain rates were observed. Ludwik's relationship was more accurate than Holloman's relationship for expressing the relationship between the true stress and the true strain during plastic deformation. Below 600 °C, the critical plastic strain showed a positive DSA, but it showed an inverse DSA above 600 °C. The value of (m+β) at the normal DSA was 2.27, and the activation energy was 81.624 kJ/mol. Depending on the value of the activation energy, the DSA was attributed to pinning dislocations by substitutional aluminum atoms. SEM examination of the fracture surfaces revealed mixed mode with dimples and facets at tested temperature, but facts fracture became more intense with increasing temperature and ductility minima arose at 650 °C. The results of transmission electron microscopy showed increase interaction of dislocations with solute atoms with increase in the temperature up to 700 °C.

Low-cycle fatigue behavior of differently matched welded joints of quenched and tempered steel

The appropriate matching of base and filler material is a complex task, where yield strength matching is the most general aspect. As the strength properties of structural steels have significantly improved in the past decades, the matching problem has become more relevant today. The mismatch phenomenon significantly affects the behavior of welded joints under dynamic and cyclic loading. Among cyclic loading, low-cycle fatigue (LCF) often occurs in welded steel constructions; furthermore, the LCF resistance of these advanced steels and their welded joints is limitedly known. In this paper, welding experiments are presented for the analysis of the LCF behavior of differently matched butt-welded joints made from two grades of quenched and tempered (Q + T) high-strength steels. For S690QL steel, matched and overmatched consumables were applied, while for S960QL steel, matched and undermatched filler materials were used. Material tests were performed to determine the mechanical properties of the different welded joints. In both examined steel grades, the welded joints tolerated a smaller number of cycles until failure than the base materials. In the case of S690QL, the LCF resistance of the matched welded joints was higher than the overmatched filler material. At S960QL in the higher strain amplitude range, the number of cycles to failure was higher at the same total and plastic strain amplitudes when undermatched filler material was used; however, an opposite ratio can be observed at lower strain values, compared to the matching filler material.

The detection and classification method of sub-macroscopic defects inside steel with an ultrasonic testing and CatBoost-based stacking model

With the increasing demand for advanced steel, the internal cleanness of steel becomes an important evaluation indicator of material quality. Sub-macroscopic defects are randomly distributed inside steel materials, which have seriously affected material stability and fatigue life because they are not covered by existing testing standards. Besides, the existing detection methods generally have problems such as low efficiency and complexity. In this paper, we propose a non-destructive inclusion testing and classification framework based on ultrasonic testing experiments, signal feature extraction and machine-learning methods. Under the optimal experimental detection conditions we found through experiments, a large-scale sub-macroscopic inclusion signal data set is established to realize the classification of defects. Moreover, empirical mode decomposition (EMD) and other feature extraction algorithms are applied to further boost the model performance. We propose a CatBoost-based stacking fused model named Stacked-CBT, which obtains state-of-the-art experimental results with an accuracy rate of 86.65% and demonstrates that the proposed framework is feasible to classify the sub-macroscopic inclusion signals. To the best of our knowledge, there is no previous study in this field that has acquired such a large amount of experimental sub-macroscopic signal data while taking into consideration classification-specific designs.

Nanoparticle-strengthened Ni2CoCrNb0.2 medium-entropy alloy with an ultrastrong cryogenic yield strength fabricated by additive manufacturing

To improve the yield strength of metallic materials at low temperatures, a strategy of combining the calculation of phase diagrams (CALPHAD) technique with the overall valence electron concentration (OVEC) principle is applied, and a Ni2CoCrNb0.2 medium-entropy alloy (MEA) with D022 superlattice (noted as the γ′′ phase) is designed. Bulk MEA samples without defects were successfully fabricated using laser additive manufacturing (AM), followed by solution treatment at 1200 °C for 1 h and then aging at 650 °C for 120 h. The nanoscale γ′′ phase precipitated. The tensile results indicated that the MEA had superior yield strengths of ∼1180 MPa and ∼1320 MPa and tensile strengths of ∼1335 MPa and ∼1552 MPa at 293 K and 77 K, respectively. The yield strength obtained was superior to that of currently reported medium/high-entropy alloys and typical advanced cryogenic steel. The mechanical properties of the Ni2CoCrNb0.2 MEA demonstrated a strong temperature dependence, and the increased yield strength was mainly attributed to the increase in lattice friction stress at low temperatures. This research provides a new strategy for producing materials with ultrastrong cryogenic yield strengths by AM.

Low-cycle fatigue behaviour of hot-rolled titanium-clad bimetallic steel

Titanium-clad (TC) bimetallic steel is able to integrate the superior corrosion resistance of titanium alloy into traditional corrosion-susceptible structural steels, producing prominent life-cycle safety, serviceability and durability. Nevertheless, the relevant seismic performance and fatigue behaviour reported in existing literature are still limited and insufficient to support the codification for structural design of such advanced steel. In order to develop proper plastic/seismic design methodology for TC bimetallic steel structure, the cyclic behaviours and low-cycle fatigue life prediction of such metal must be thoroughly investigated. This paper aims to study the low-cycle fatigue behaviour of hot-rolled bonded TC bimetallic steel manufactured from TA2 titanium alloy and Q355B structural steel. The strain amplitude of low-cycle fatigue (LCF) test ranges from 0.5% to 2.5%, and the strain ratio applied herein includes 0 and -1. The experimental phenomenon, LCF failure mechanism, cyclic stress-strain relationship of different tests are thoroughly compared and discussed. Besides, the influence of strain rate and bond strength are simultaneously demonstrated. The LCF life are fitted and validated through Basquin-Coffin-Manson model, Kuroda model and energy method, respectively. The research outcomes demonstrate that the LCF cracks of TC bimetallic steel tend to be initiated at the cladding layer and propagated into the substrate layer due to a coupled buckling-fatigue failure mechanism. Several representative fatigue behaviour of conventional mild (CM) steels including the three-stage cyclic softening and mean stress relaxation have not been observed in this experimental study. The strain rate exhibits distinct influence on the LCF life and failure mechanism. Both Basquin-Coffin-Manson model and energy method can provide more favourable prediction accuracy for LCF life of TC bimetallic steel than the Kuroda model. Generally the LCF life of TC bimetallic steel is shorter than that of structural steels, stainless steels, titanium alloys and stainless-clad (SC) bimetallic steels. The test records and fatigue-life model parameters obtained can lay a solid foundation for seismic analysis and damage evaluation of TC bimetallic steel structure.

Atomic-scale understanding of solute interaction effects on grain boundary segregation, precipitation, and fracture of ultrahigh-strength maraging steels

Understanding the fundamental mechanisms of embrittlement and de-embrittlement is crucial for the development of strategies toward advanced steels with improved performance. In this study, the solute interaction effects on grain boundary (GB) segregation, precipitation, and fracture of Fe–Ni–Ti–(Mo) maraging steels were systematically investigated through a combination of experimental and theoretical techniques, including scanning transmission electron microscopy, atom probe tomography, and first-principles calculations. Our results reveal that the Fe–Ni–Ti maraging steel suffers from serious intergranular embrittlement and the mechanism is related to the formation of coarse Ni3Ti precipitates and associated precipitate-free zones (PFZs) at GBs, which facilitate the crack initiation and growth along the GBs. Interestingly, adding Mo to the maraging steel effectively suppresses the intergranular embrittlement, thereby substantially improving the ductility. Atomistic analyses reveal that Mo de-embrittles the GBs by reducing the segregation of Ni and Ti, which substantially inhibits the formation of coarse Ni3Ti precipitates and PFZs at the GBs, thereby alleviating their harmful impact on the GB cracking. In addition, the Mo segregation enhances the GB cohesion, which may play a minor role in suppressing the GB fracture.

Dwell-fatigue crack growth behaviour of Alloy 709

Dwell-fatigue crack growth behaviour of an advanced austenitic stainless steel Alloy 709 has been investigated and compared with that of a conventional Type 316H stainless steel. The test procedure employed alternation of 1 h dwell-fatigue loading and 0.25 Hz fast cycling so that crack growth rates (da/dN) obtained from dwell-fatigue loading can be compared to those purely result from fatigue mechanism on the same test-piece. Tests were conducted at 550, 650 and 750 °C in air using 0.5 T compact tension test-pieces under a fixed maximum load of 8 kN and a stress ratio, R, of 0.1. For the investigated temperature and ΔK ranges, crack growth mechanisms of fatigue alone, creep alone and mixed fatigue-creep have all been observed. Detailed fractographic and metallographic observations were conducted to interpret failure mechanisms and regimes of different failure modes. Compared to crack growth rates obtained under 0.25 Hz fatigue loading, dwell-fatigue produces: no obvious increases in crack growth rates at a test temperature of 550 °C; a moderate increase (∼ 2–5 times) at a test temperature of 650 °C; and, over a tenfold increase at a test temperature of 750 °C. Compared to 316H, Alloy 709 has much improved resistance against creep-fatigue crack growth, here confirmed at the single test temperature of 650 °C.

Effects of Mn content on austenite stability and mechanical properties of low Ni alumina-forming austenitic heat-resistant steel: a first-principles study

Low Ni alumina-forming austenitic (AFA) heat-resistant steel is an advanced high-temperature stainless steel with reduced cost, good machinability, high-temperature creep strength, and high-temperature corrosion resistance. Using the First-principles approach, this study examined the effect of Mn content on austenite stability and mechanical properties at the atomic level. Adding Mn to low Ni-AFA steel increases the unit cell volume with an accompanying increase in the absolute value of formation energy; the austenite formed more easily. The austenitic matrix binding energy decreases and remains negative, indicating austenite stability. As the Mn content increases from 3.2 to 12.8 wt%, the system's bulk modulus (B) rises significantly, and the shear modulus (G) falls. In addition, the system's strength and hardness decrease, and the Poisson ratio of the austenite matrix increases with improved elasticity; the system has excellent plasticity with an increase in the B/G. For the Fe22–Cr5–Ni3–Al2 system, with the increase of Mn content, the electron density distribution between the atoms is relatively uniform, and the electrons around the Mn atoms are slightly sparse, which will slightly reduce the structural stability of the matrix. The experiment demonstrated the matrix maintains the austenitic structure when adding 3.2–12.8 wt% Mn elements to low Ni-AFA steel. At an Mn content of 8 wt%, the overall mechanical properties of the high-Mn AFA steel are optimal, with a tensile strength of 581.64 MPa, a hardness of 186.17 HV, and an elongation of 39%.