Ultra-high Strength Steel Research Articles

Study on the formation of surface affected layer in grinding ultra-high strength steel

Study on the formation of surface affected layer in grinding ultra-high strength steel

Understanding the correlation of aging treatments and stress corrosion crack growth of a secondary hardening ultra-high strength steel

The correlation of aging treatments and stress corrosion crack growth of a secondary hardening ultra-high strength steel was investigated using microstructural characterization, electrochemical hydrogen penetration experiment, thermal desorption spectroscopy analysis, and stress corrosion crack growth test. The results showed that increase of aging duration led to slight increase in the fraction of reversed austenite but decrease in dislocation density while no notable variation in grain boundary misorientation distribution and the corrosion behavior was observed. The shrinkage and fracture toughness continued to increase with aging duration while the yield strength reaching a maximum value after 5 hours of aging. Longer aging durations led to the decrease in effective hydrogen diffusion coefficient but improvement of stress corrosion cracking resistance, mainly due to the coarsening and nucleation of M2C carbides, as well as the higher fraction of reversed austenite. This work provided the theoretical and technical basis for improving the stress corrosion cracking resistance of this steel.

Nonlinear mechanical response of UHSS rectangular plate under thermo-mechanical-metallurgical coupling

The mechanical properties of ultrahigh-strength steel (UHSS) produced through the gradient thermoforming process (GTP) are significantly influenced by residual stress and deformation. However, the precise distribution of these factors during GTP remains unclear. To address this issue, a detailed thermal model is developed, incorporating key heat transfer mechanisms, including coupled heat conduction and radiation, using the method of separation of variables. Furthermore, the displacement and stress fields of UHSS rectangular plates are derived using the differential quadrature method (DQM), accounting for phase transitions and external load. The combination of the separation of variables and DQM methods significantly improves computational efficiency. Furthermore, this study presents the first analysis of the combined effects of thermal load, phase transitions, and external forces on UHSS rectangular plates. Finally, through a combination of theoretical analysis, experimental validation, and case studies, the influence of material properties, geometric parameters, and external load on the temperature, stress, and displacement fields is examined. The findings reveal that heating rate, phase transitions, and external load play a critical role in the stress and displacement distribution in UHSS plates during GTP. This research provides a theoretical foundation for optimizing GTP process parameters and material design, ultimately enhancing the quality and performance of UHSS components.

Research on fracture prediction and control of UHSS thin-walled component with complex section in roll forming process

Aiming at the fracture of an ultra-high-strength steel (UHSS) thin-walled component with complex section in the roll forming process, the three-dimensional finite element model was established. It is found that the stress triaxiality and equivalent plastic strain are the main factors affecting the fracture behaviour of the UHSS thin-walled component with complex section, and the influence laws of forming strategies and process parameters on the fracture behaviour were discussed. The results show that the damage value, equivalent plastic strain and stress triaxiality at the fracture position are reduced by increasing the inter-distance and roll diameter, decreasing friction coefficient and forming velocity. The mathematical model of the UDP comprehensive measure with the forming strategies and process parameters was established by the response surface method. The numerical simulation and industrial test verify the effectiveness of the UDP comprehensive measure, which provides a theoretical and practical basis for the fracture prevention control.

The evolution and toughening mechanism of austenite in high Co–Ni ultrahigh strength steel

The austenite, as a “soft” phase, has a significant impact on the mechanical properties in ultrahigh strength steel. The morphology, size, and distribution of austenite play a critical role in toughness of steel. This study concentrates on morphology, size, and distribution of austenite subjected to solid solution, cryogenic, and aging treatments and explores its evolution characterized by transmission electron microscopy, scanning electron microscopy, electron backscatter diffraction and x-ray diffraction. The toughening mechanism of austenite with different characterization was investigated through charpy impact testing and fracture morphology analysis. The results indicate that filmy and blocky austenite existed at the lath boundary of martensite. Blocky austenite and some filmy austenite were eliminated after cryogenic treatment, while approximately less than 15 nm filmy reverted austenite was formed after aging treatment. Filmy reverted austenite at the boundary of martensite laths can enhance toughness. Retained austenite without cryogenic treatment can induce the formation of larger blocky austenite, which decreased toughness.

Surface fatigue characterization and its enhancement by the engineered ultrasonic rolling process for 300 M ultrahigh strength steel

The fatigue life of surface layer (FSL) is innovatively characterized to accurately capture the effect of surface integrity on the fatigue behavior of the components. The sensitivity of FSL of 300 M ultrahigh strength steel to the surface integrity indexes induced by representative manufacturing processes was analyzed by analytical modeling and fatigue tests from the perspective of engineering fracture mechanics. The ultrasonic surface rolling process (USRP) was introduced to optimize the fatigue limit of 300 M steel, and the improvement in fatigue life was experimentally quantified. Besides, the fracture analysis was conducted to provide the reason why USRP can enhance the fatigue behavior of 300 M steel. The results show that FSL occupies a majority with percentage of over 95 % in the entire fatigue life and a strong relation with the machined surface defects, especially the surface machining marks. The excellent surface finishing effect and high compressive residual stresses induced by engineered USRP could transfer the crack source from surface machining marks into subsurface inclusions, which greatly prolongs the fatigue crack initiation life and thereby the entire fatigue life. To be specific, the fatigue life was elevated by more than 40 times and the fatigue strength was increased by 34.7 % after USRP for the tested 300 M steel.

Designing a new ultra-high strength steel with multicomponent precipitates under material genetic design

Ultra-high strength steels with excellent strength, ductility and toughness have become one of indispensable high-performance structural materials. The development of ultra-high strength steels with higher performance has been widely concerned to break the strength-ductility trade-off. This study proposes a material inverse design strategy that combines material genetic design with thermodynamic calculation to search for new alloys with desirable microstructure and optimal strengthening contributions. Based on the advancement of the 'Pareto Front', a new steel with superior properties was successfully screened from 390,625 candidate individuals, which was verified by experiments. The experimental results show that a new steel Fe-0.25C-14.8Ni-17.0Co-2.54Mo-1.56Cr-0.53Al-1.06Cu-0.38V (wt. %) has been successfully developed. After quenching-cryogenic-aging process, the new steel achieved a superior combination of strength, ductility and toughness, with a yield strength of 2.2 GPa, an elongation-to-failure of 10% and a V-notch impact energy of 15 J. Multi-scale microstructure characterization indicates that such superior property synergy arises from the unique microstructure: lath martensite matrix with high-density dislocations and interstitial solid soluble carbon atoms, embedded fine M2C, NiAl and Cu-rich nanoprecipitates. We present an in-depth discussion on the origins of superior strength/ductility/toughness combinations of the new steel to validate the design strategy and provide an accessible pathway exploiting high-performance ultra-high strength steels.

Influence of pre-torsion strengthening on the fatigue properties of 45 CrNiMoVA ultra-high strength steel

Aiming at the demand for fatigue life improvement of 45CrNiMoVA ultra-high strength steel shaft parts, a pre-torsion strengthening process using a combination of two different angles before and after was proposed based on the mechanisms of strain hardening, fine grain strengthening and dislocation strengthening. Through surface integrity measurement, torsional fatigue test and fatigue fracture characterization of the specimens, the main surface integrity factors affecting the fatigue life of the pre-torsion strengthening were analyzed. The results indicated that the grain size was refined and the plastic deformation was increased by the two-angle pre-torsion strengthening compared with the single-angle pre-torsion strengthening. The obtained surface residual compressive stress σx was the main factor affecting the fatigue life of pre-torsion strengthening, and the change in pre-torsion angle results in a significant secondary strengthening layer in the subsurface layer of the material. In summary, the study of the pre-torsion strengthening process with different combinations of angles has a guiding significance for improving the fatigue life of ultra-high strength steel.

Design of tri-phase lamellar architectures for enhanced ductility in ultra-strong steel

Achieving exceptional ductility in ultra-high strength steels has long been a formidable challenge, particularly when the yield strength reaches 2.0 GPa. In this study, we have designed an innovative tri-phase lamellar microstructure in medium Mn steel that successfully achieves both an ultrahigh yield strength of approximately 2.0 GPa and an impressive uniform elongation ranging from 22.5 % to 24.5 %. The ultra-high yield strength of this steel is primarily due to the ultrafine lamellar grain with high-density dislocations and HDI strengthening resulting from meticulously designed tri-phase lamellae. The remarkable ductility is attributed to the synergistic action of multiple plasticity mechanisms. The inherently excellent plasticity of the lamellar ferrite, in coordination with the lamellar martensite, generates significant HDI hardening, thereby suppressing the onset of necking in the early stages of deformation. During the mid-stage of deformation, high strain activates the transformation-induced plasticity effect in the highly stable austenite, which remains stable due to substantial HDI hardening and effectively mitigates strain localization. In the later stages of deformation, delamination cracking relieves stress accumulation at the interfaces, delaying material failure. These mechanisms elevate yield strength and uniform elongation product to 47.3 GPa·%, showcasing the tri-phase lamellar structure’s potential for ultrahigh-strength, high-ductility steels.

Characterization of the stress-state dependent ductile fracture behavior for Q960 ultra-high-strength structural steel

Ultra-high-strength structural steels are gaining popularity in civil engineering due to their exceptional strength-to-weight ratio. However, their inherent lower ductility compared to normal strength structural steel warrants particular attention and further investigation for safety assessment. In this study, a total of 24 specimens made of Q960 ultra-high-strength steel (UHSS), including 21 tensile specimens with various stress states and 3 three-point bending specimens, were tested to assess its mechanical behavior undergoing large deformation. Subsequently, the impact of stress state on plasticity, damage evolution and fracture initiation was analyzed and characterized. The Bai-Wierzbicki yield surface with a deviatoric associated flow rule was identified to characterize the complex plasticity behavior of Q960 UHSS. With respect to damage evolution, the stress-state dependent fracture energy Gf was proposed in this study to describe the damage softening behavior. The uncoupled Hosford-Coulomb locus with a non-linear weight function was utilized to predict the fracture initiation behavior. Finally, the utilized stress-state dependent constitutive models for Q960 UHSS, with the calibrated parameters and user-defined material subroutine VUMAT, were successfully verified through the three-point bending test with good agreement. Moreover, based on a parametric analysis of radius-to-thickness ratio, the bendability design regarding the minimum radius-to-thickness ratio of Q960 UHSS was proposed.

High temperature electropalsticity in Aermet100 steel by decoupling electron wind effect

Aermet100 ultra-high strength steel (UHSS) is one of the highest strength-plasticity-matched metallic materials. It is widely used in critical load-bearing components in aerospace and weaponry. Compared to traditional forging processes, electropulsing assisted forming technology significantly enhances manufacturing efficiency and forming limits. Particularly in the forming of hard-to-deform materials into complex structures, it demonstrates significant application potential. However, quantitative studies on the decoupling of electron wind effects during high-temperature deformation and their impact on microstructural evolution, deformation mechanisms, and mechanical properties remain few reported, resulting in limiting the ability to select optimal process parameters in electropulsing assisted forming technology to precisely control material structure and properties. In this paper, the impact of varying degrees of electron wind effects on flow stress through high-temperature compression tests was investigated. The evolution of the microstructure was analyzed in conjunction with the thermodynamic and kinetic mechanisms of recrystallization. Moreover, the hitherto unexplained relationship between the activation of slip systems and texture components in Aermet100 UHSS is examined. Results indicate that with the enhancement of the electron wind effect, the flow stress of Aermet100 UHSS is reduced by 59.4 %. The electron wind effect not only reduces the activation energy for recrystallization but also enhances vacancy concentration, dislocation climb diffusion flux, and driving force, thus promoting continuous dynamic recrystallization. Furthermore, as the intensity of the electron wind effect increases to 0.85 Ew and above, S and Brass textures are observed for the first time. It is notable that the Dillamore texture, Brass texture, and S texture respectively activate specific slip systems (1‾ 10)[11 1‾], (0 1‾1‾)[11 1‾] and (110)[1‾ 11‾] during the hot deformation process of Aermet100 UHSS. This investigation sheds new insight into tailoring high temperature electroplasticity by regulating electron wind effects.

Low and elevated temperature mechanical properties of cold-formed Q1200 ultra-high strength steel sections: Experiment and constitutive model

Ultra-high strength steel (UHSS) has extensive application prospects in building structures due to its excellent mechanical properties. Because of the cold-forming process, the mechanical properties of the corner portion differed from those of the flat portion. However, research on the mechanical properties of the corner and flat portions of cold-formed UHSS sections at low and elevated temperatures was very limited. Therefore, the steady-state test was employed to investigate the mechanical properties and constitutive models of the Q1200 UHSS corner and flat couple specimens at low and elevated temperatures. The target temperatures set in this study were −80 °C, −60 °C, −40 °C, −20 °C, 20 °C, 200 °C, 300 °C, 400 °C, and 500 °C. After the steady-state test, the failure modes, fracture surfaces, stress-strain curves, and key mechanical parameters of the Q1200 UHSS corner and flat couple specimens at different temperatures were obtained. In addition to increasing the elastic modulus of the corner specimen by more than 10 %, the effect of low temperature on the mechanical properties of the flat and corner specimens was within 10 %. The strength properties of Q1200 UHSS decreased with increasing elevated temperature, but the ductility properties increased with increasing elevated temperature. Remarkably, the ultimate strength of the flat and corner specimens at 200 °C increased by 5 % and 2 %, respectively. At 500 °C, the strength properties decreased by more than 60 %. The low- and elevated-temperature reduction factors of yield strength, ultimate strength, and elastic modulus of different structural steels obtained from recent studies and current codes were compared with those of the Q1200 UHSS. Based on the experimental results, predictive formulae for the key mechanical parameters of Q1200 UHSS corner and flat specimens at low and elevated temperatures were proposed. Furthermore, constitutive models of Q1200 UHSS corner and flat specimens at different temperatures with strains less than the ultimate strain were developed. The proposed predictive formulae and constitutive models provided an important basis for the structural analysis and design of cold-formed Q1200 UHSS structures in low- and elevated-temperature environments.

Nanotwinning grain refinement induced by micro-needle peening in arc-welded ultra-high strength steel sheet

Generally, the fatigue strength of ultra-high strength steel (UHSS) and high strength steel (HSS) arc-welded joints are comparable regardless of base metal's strength. Still, the micro-needle peening (MNP) method can improve the fatigue strength to the level of those of base metals. To understand the mechanism of this improvement, this paper investigates the microstructure of UHSS (tensile stress grade of 980 MPa) arc-welded joints treated with MNP and compares it to HSS (tensile stress grade of 440 MPa) joints. We focus on the presence of nanotwins, which exhibited a minimum thickness of 4.7 nm, observed in the UHSS joints following the MNP treatment. Importantly, these nanotwins demonstrated remarkable stability even under cyclic loading conditions (nominal stress σn = 600 MPa, N = 3 × 106 cycles). This indicates that the nanotwins contribute to the significant improvement in fatigue strength demonstrated by MNP. However, the nanotwins were not observed in the HSS joints, suggesting sufficient driving stress is necessary for their occurrence. The dislocation pileup stress at the grain boundary during twinning was estimated by the thickness of the twin, which was 8.1 GPa. This value is of the same order of magnitude as the 3.7 GPa estimated by the Hall-Petch coefficient for ferritic steel. The lower levels of C, Si, and Mn can contribute to the lower pileup stress, resulting in absence of the nanotwins in the 440 MPa joint. Overall, this study provides insights into the microstructural changes induced by MNP treatment and their impact on the fatigue strength.

Very high cycle fatigue properties of ultrahigh strength steels in the presence of hydrogen

The very high cycle fatigue (VHCF) properties of 2000 MPa ultrahigh strength martensitic steels after hydrogen pre-charging were investigated by means of ultrasonic fatigue tests. Results showed that the VHCF lives of the experimental steels were drastically decreased by about two orders after pre-charged with a hydrogen concentration of CH = 1.23 wppm. The experimental steel tempered at a higher temperature of 300 °C with some epsilon-carbides and a lower density of dislocations showed higher VHCF lives in the presence of hydrogen. It could be attributed to the lower hydrogen diffusion coefficient since hydrogen might accelerate fatigue crack propagation in the granular bright facet (GBF) area.

Evaluation of Shear-Punched Surface Layer Damage in Ultrahigh-Strength TRIP-Aided Steels with Bainitic Ferrite and/or Martensite Matrix Structure

Evaluation of Shear-Punched Surface Layer Damage in Ultrahigh-Strength TRIP-Aided Steels with Bainitic Ferrite and/or Martensite Matrix Structure

Coarsening of alloy carbides during a cyclic heat treatment for grain refinement in ultrahigh-strength steels

Strength–ductility trade-off dilemma is acute in materials science, especially for ultrahigh-strength steels. The improvement of toughness requires careful tuning of grains and precipitates. Thermal cyclic treatment and annealing before quenching are two effective ways of microstructural refinement for steels. However, a combination of the two methods has never been studied. Complex alloy carbides form in the process of annealing, but their effects on grain refinement in the subsequent quenching are unclear. Here, a systematic comparison of microstructure and mechanical properties is established between a thermal cyclic treatment with annealing (CHT) and traditional quench-temper treatment (QT). The results indicate that the CHT treatment leads to the coarsening of alloy carbides and subgrains, and the decrease in strength and toughness.

A polymer-like ultrahigh-strength metal alloy

Futuristic technologies such as morphing aircrafts and super-strong artificial muscles depend on metal alloys being as strong as ultrahigh-strength steel yet as flexible as a polymer1–3. However, achieving such ‘strong yet flexible’ alloys has proven challenging4–9 because of the inevitable trade-off between strength and flexibility5,8,10. Here we report a Ti–50.8 at.% Ni strain glass alloy showing a combination of ultrahigh yield strength of σy ≈ 1.8 GPa and polymer-like ultralow elastic modulus of E ≈ 10.5 GPa, together with super-large rubber-like elastic strain of approximately 8%. As a result, it possesses a high flexibility figure of merit of σy/E ≈ 0.17 compared with existing structural materials. In addition, it can maintain such properties over a wide temperature range of −80 °C to +80 °C and demonstrates excellent fatigue resistance at high strain. The alloy was fabricated by a simple three-step thermomechanical treatment that is scalable to industrial lines, which leads not only to ultrahigh strength because of deformation strengthening, but also to ultralow modulus by the formation of a unique ‘dual-seed strain glass’ microstructure, composed of a strain glass matrix embedded with a small number of aligned R and B19′ martensite ‘seeds’. In situ X-ray diffractometry shows that the polymer-like deformation behaviour of the alloy originates from a nucleation-free reversible transition between strain glass and R and B19′ martensite during loading and unloading. This exotic alloy with the potential for mass producibility may open a new horizon for many futuristic technologies, such as morphing aerospace vehicles, superman-type artificial muscles and artificial organs.

Microstructural and mechanical characterizations of weld metal of S960QL ultra high strength steel joints obtained with different multi-pass laying techniques using GMAW

Abstract 15 mm thick ultra-high strength steel plates with 960MPa yield strength were welded using different multi-pass laying techniques (i.e., stringer and weaving beads) with torch manipulation. Weld metals obtained were compared using different mechanical (i.e., micro tensile tests and Vickers hardness maps) and microstructural (i.e., optical microscope, scanning electron microscope, X-ray diffraction, electron backscatter diffraction) characterization techniques. Coarser grains and acicular ferrite were observed in weld metal obtained with weaving pass procedure. There were hardness differences in face and root passes of both weld metals. Yet, hardness values were 19% and 11% higher for face and root regions of the joint obtained by stringer pass procedure, respectively. Fractographs of micro tensile test specimens revealed dimples depicting ductile network structure for both joints.

Flexural behaviour of Q1100 ultra high strength steel welded I-sections

An experimental and numerical program investigating the in-plane flexural behaviour of Q1100 ultra high strength steel (UHSS) welded I-sections is presented in the current paper. The testing program comprised of material coupon tests, measurements of residual stresses, and in-plane 3-point bending tests. Subsequently, a numerical program was executed, involving the development of finite element (FE) models. The FE models validated against the experiments were subsequently employed for parametric studies, aiming at assessing the flexural behaviour of Q1100 welded I-sections across a broader spectrum of section geometries. The data obtained from the experiments and numerical simulations were used to assess the applicability of prevailing design provisions outlined in the European, Chinese and Australian codes for high-strength steel structures. These design provisions encompass classification limits for cross-sections and predicted methods for cross-sectional resistance in bending. Via reliability analyses, adjustments were made to the cross-section classification limits. Finally, a modified direct strength method was proposed to predict the in-plane bending resistance for Q1100 welded I-sections.

Mechanical properties and microstructure evolution of 1800 MPa grade low alloy ultrahigh strength steel during quenching and tempering process

Mechanical properties and microstructure evolution of 1800 MPa grade low alloy ultrahigh strength steel during quenching and tempering process