Ultra-high Strength Steel Research Articles

Microstructure and mechanical properties of an ultra-high strength steel fabricated by laser hybrid additive manufacturing

35CrMnSiA is a type of ultra-high-strength steel that finds extensive application in key components characterized by geometrically regular structures and complex extensions, such as structural shells. Laser hybrid additive manufacturing (LHAM) is an advanced manufacturing technology that combines the benefits of traditional manufacturing and additive manufacturing. LHAM has the potential for rapid and cost-effective production of these components. However, the formation mechanism of the microstructure in different zones of 35CrMnSiA steel fabricated by LHAM and its impact on the overall tensile performance remain unclear, thereby tremendously restricting its application. To address this issue, this study employed wrought 35CrMnSiA steel as the base material and utilized directed energy deposition (DED) to fabricate defect-free LHAM parts. The microstructure of LHAM-fabricated 35CrMnSiA steel is location-related. The laser deposition zone comprised ferrite with internal and boundary-distributed carbides, whereas the substrate zone comprised martensite. The heat-affected zone exhibited a gradient microstructure between the two. The microstructure in the deposition zone was primarily governed by solidification behavior and thermal history during the deposition process, while the heat-affected zone was mainly influenced by recrystallization. The non-uniform characteristics of the microstructure significantly influenced its mechanical properties, resulting in a “cask effect,” namely, the tensile performance of LHAM-fabricated 35CrMnSiA steel depended on the weakest laser deposition zone. The ultimate tensile strength and elongation of LHAM-fabricated 35CrMnSiA steel are 959 MP and 15.2%, respectively. This study establishes a basis for controlling the microstructure and optimizing the performance of hybrid-manufactured ultra-high-strength steel.

Study on grinding performance during ultrasonic vibration-assisted grinding ultra-high strength steels

Study on grinding performance during ultrasonic vibration-assisted grinding ultra-high strength steels

Investigation of Hardfacing on Ultra-High Strength Steel Base Material

The application of high-strength steels is increasing rapidly nowadays, and steels with more than 1000 MPa yield strength are usually used in welded structures. The welding of these materials has many difficulties, so very important the precise technology planning, and disciplined work during welding. The weldability of these materials is commonly investigated field in case of joining. The application of ultra-high strength steels expands rapidly, and in the last years, it started to use them as a base material for hardfacing. Besides the wearing, there is a claim about higher strength of base materials in case of relatively extremely loaded machines. Because this ultra-high strength steel appears as a base material for hardfacing and it brings new challenges for welding technologists. In case of joining, the welding technology is complicated, usually need preheating before welding, is important to calculate and to use the right t8/5 cooling time, and basically necessary to decrease the heat input as much as possible. The bad effect of welding heat input can be compensated by the filler material too in some cases. In contrast in case of hardfacing the base material itself usually has a big thickness, and no joint preparation, additionally important to reach deep fusion on the surface. It basically determines the heat input which has a different heat cycle as in case of joining. Therefore, the heat affected zone (HAZ) differs from the HAZ in case of joining application. In this investigation, four different hardfacing were made with four different technological parameters by robotic gas metal arc welding on S1100QL steel. During the welding parameter determination, we try to find a series of heat inputs from the lowest to the practically usable highest heat input. For the experiments, two filler materials used, one for the buffer zone, and for the hardfacing itself. Microstructural evaluation and hardness tests were made on the specimens which can show the differences between the heat affected zones.

High Cycle Fatigue Testing of Lap Shear RSW Joints from Martensitic MS1400 Steel Sheets

Resistance spot welding (RSW) is one of the most common welding methods for steel sheets, as it is mainly used to join the automotive body structure parts. Different types of ultra-high strength steels (UHSS) have become widely used in the automotive body to obtain the required demands such as lower car weight, improving crashworthiness behavior, and enhancing strength–ductility combination. Martensitic UHSS belong to the highest grades width their tensile strength above 1000 MPa. During the lifetime of the vehicle cyclic loading generally occurs, therefore the optimization of welding technology should be performed considering the fatigue resistance of the welded joints. In our research 1 mm thick standardized lap shear sheets of martensitic MS1400 steel were welded by a TECNA 8007 RSW equipment with two different welding parameter combinations. The idea was to analyze the effect of welding and pulsation parameters on joint properties under static and cyclic loading. The welding parameters have been calibrated to produce the same weld nugget size for both technological combinations. Macroscopic, hardness, and tensile-shear tests were carried out to determine the fundamental mechanical characteristics of the RSW joints. The relation between the weld nugget microstructure and mechanical properties was explored. The high cycle fatigue (HCF) tests were performed on an MTS 810.23 universal electro-hydraulic materials testing system. A statistical approach was applied during the preparation and evaluation of the investigations, which increased their reliability. Measured and analyzed data of the lap shear welded joints, prepared by different technological parameters, were compared and discussed. The parameters of the HCF experiments were calculated considering the Japanese testing method (JSME S 002-1981). In most of the samples it was observed from both welding parameter combinations that the fatigue cracks initiate and grow in curvature shape in the softened part of the heat-affected zone towards the base metals in both directions symmetrically. A slight difference was observed in the HCF resistance of the welded joints prepared by different welding parameters.

Experimental study on the post-fire cyclic behavior of Q960E ultra-high-strength structural steel

This paper presents an experimental study on the cyclic behavior of Q960E ultra-high-strength steel (UHSS) after exposure to elevated temperatures and subsequent cooling down by water. The test program comprised six temperature levels ranging from room temperature to 900 °C, along with four cyclic loading protocols. A total of 46 tests were conducted in this study, and the failure mode, hysteretic curve, springback behavior, Young’s modulus, and fracture strain were discussed in detail. Subsequently, a method was proposed to calibrate the equivalent strain–stress data and true strain–stress data based on the reversion method with axisymmetric model. Based on the calibrated data, the stress–strain relationship and strain hardening behavior were examined, and the corresponding parameters were also calibrated. Finally, comparisons of the numerical simulation results and experimental data demonstrated that the suggested constitutive model accurately described the cyclic behavior of Q960E UHSS after elevated temperatures. The findings of this study have potential applications in evaluating the seismic performance of UHSS structures following exposure to fire.

Oxidation behavior and corrosion properties of 2-GPa grade Co-reduced ultrahigh-strength stainless steel

This study investigated the oxidation behavior and corrosion resistance of 2-GPa grade ultrahigh-strength stainless steel. Results revealed that the addition of V to Co-reduced FerriumS53 (7Co1V) accelerated the precipitation of MC carbide during aging, while inhibiting the precipitation of Cr-rich M23C6 carbide. A significant amount of Cr was predicted in the microstructure of the alloys, and this microstructure increased the partitioning of Cr to the alloy surface and provided enhanced resistance against corrosion pitting. These findings suggest that the addition of V into Co-reduced stainless secondary hardening steels can enhance their oxidation and corrosion resistance without compromising their mechanical properties.

Optimization of GMAW Process Parameters in Ultra-High-Strength Steel Based on Prediction

Ultra-high-strength steel (UHSS) is a complex and sophisticated material that allows the development of products with reduced weight but increased strength and can assist, for example, in the automotive industry, saving fuel in vehicles and decreasing greenhouse gas emissions. Welding UHSS has a certain complexity, mainly due to the higher alloys and heat treatments involved, which can result in a microstructure with higher sensitivity to welding. The primary purpose of the current work was to select the best parameters of the gas metal arc welding (GMAW) for welding the S960 material based on prediction methods. To achieve the expected results, a finite element analysis (FEA) was used to simulate and evaluate the results. It was found that the welding parameters and, consequently, the heat input derived from the process greatly affected the UHSS microstructure. Using FEA and estimating the extension of the heat-affected zone (HAZ), the peak temperature, and even the effect of distortion and shrinkage was possible. With an increase in the heat input of 8.4 kJ/cm, the estimated cooling rate was around 70 °C/s. The presence of a softening area in the coarse grain heat-affected zone (CGHAZ) of welded joints was identified. These results led to an increase in the carbon content (3.4%) compared to the base metal. These results could help predict behaviors or microstructures based on a few changes in the welding parameters.

Flow stress curves for 980MPa- and 1.5GPa-class ultra-high-strength steel sheets weakened under high-stress triaxiality

The weakening behavior of ultra-high-strength steel(UHSS) remains unknown, even though it is a requirement for the accurate prediction of strain localization and ductile fracture in automobile applications. In this study, we, therefore, revealed the stress–strain curves of UHSSs up to their ductile fracture under different stress triaxialities for the first time. Smooth and notched tiny round-bar specimens cut from UHSSs with 1.6 mm thickness were subjected to tensile tests, during which the forces and neck diameters of the specimens were measured. Notably, the evaluated flow stresses in the 1.5 GPa-class notched UHSS specimens exhibited weakening of up to 3%. This was in contrast to the 980 MPa-class UHSS specimens, which did not exhibit notch-induced weakening. The absence of weakening during stress measurements, as confirmed by synchrotron X-ray diffraction (XRD) analysis, suggested that weakening was caused by material damage. However, the weakening behavior did not follow the previously developed damage models based on microvoid formation. The volume fraction of the microvoids, as observed by X-ray computer tomography, was extremely small (at 0.2%) and cannot account for the 3% material weakening indicated by the flow stress measurement results. A new damage mechanism, associated with unobservable small lattice vacancies, was implied in UHSSs deformation under high stress triaxialities.

Review of Technological Advances in High Temperature Overhead Line Composite Conductors

ACSR (Aluminium Conductor Steel Reinforced) conductors consisting of pure aluminum wires helically stranded around a core of high strength galvanized steel core wires are widely used in overhead transmission line applications. While the steel wires provides the required strength, the outer aluminium serves as electricity conducting member. The thermal rating or ampacity of conductors is the maximum current that a circuit can carry within the temperature limits as dictated by the allowable conductor sag or by the annealing onset temperature of the conductor, whichever is lower. On a continuous basis, ACSR may be operated at temperatures up to 100°C without any significant change in the conductor’s physical properties. Above 100°C, aluminum wires loses tensile strength over time and becomes "fully annealed" giving rise to increased thermal expansion of the conductor; this causes excessive sagging reducing the clearance between the ground and the energized conductors.
 While, the electric utilities are posed with increased challenges of production of additional capacities and building new transmission circuits to meet the ever growing demand and the high cost and environmental restrictions of constructing new lines, methods are being explored to increase their capacity with minimum changes in the existing towers. Replacement of the existing conductor with a advanced high temperature composite (HTC) conductors operating upto temperatures of 200°C with reduced sagging characteristics have been attempted and an increase in capacity of 30-80%, can be achieved through application of these composite conductors.
 The advanced HTC conductors with varied combinations of core material and outer conducting wires are being tried so as to achieve wide ranging properties. In general, HTC conductors features the combinations of either soft aluminum wires with ultra high strength steel or temperature resistant aluminium with or fiber reinforced metal/polymer matrix composites as the core. Both the alumina fiber reinforced temperature resistant aluminium metal matrix composite and the carbon fiber reinforced high temperature polymer resin composite wires as the core material appears to have favorable applications. The comparative performance of the HTC depends on the degree to which both outer aluminum strand and reinforcing core’s physical properties are stable at high temperature and on the elastic, plastic, and thermal elongation of the combined HTC.
 The summary of various developments realized in the area of composite conductors and their relative merits and demerits have been discussed in detail.

Study on the effect of tempering on the impact abrasive wear performance of 45Si2MnCr2Mo ultra-high strength steel

In this paper, three-body impact abrasive wear tests of 45Si2MnCr2Mo ultra-high strength steel (UHSS) at different tempering temperatures were carried out by using the MLD-10 impact abrasive wear machine. The effects of different tempering temperatures on the wear weight loss and the depth of the hardened layer of UHSS were investigated. The results show that the weight reduction rate of the steels tempered at 150 and 220 °C decreases at first and then increases with time, but the steels tempered at 290 and 360 °C gradually increase with time. The surface and subsurface areas of UHSS are strengthened by work hardening after wear. The wear performance of UHSS should consider the change of weight reduction rate with time rather than just considering short-term weight loss. UHSS tempered at 150 °C has the best comprehensive surface hardness, hardening percentage, and effective hardened layer depth, and the wear process is more uniform.

The effect of heat input in multi-pass GMAW of S960QL UHSS based on weaving and stringer bead procedure on microstructure and mechanical properties of HAZ

Quenched and tempered S960QL (yield strength ≥ 960 MPa) ultra-high strength steel (UHSS) thick plates were joined by multi-pass robotic gas metal arc welding (GMAW) using weaving and stringer bead techniques. The effects of microstructural changes in heat-affected zone (HAZ) of the joint on toughness and hardness were examined. Weaving and stringer bead techniques applied for the multi-pass welding procedure altered average peak temperatures and exposure time to those temperatures. Mechanical properties of HAZs were evaluated by utilizing notch impact and hardness tests, and these results were correlated with microstructural characterizations using optical (OM) and scanning electron microscopes (SEM). Prior austenite grain (PAG) coarsening occurred because of increased exposure time to peak temperature in coarse-grained HAZ (CGHAZ) of the W-5 (weaving pass) joint. CGHAZs at the face pass, which have not been subjected to a second thermal cycle, have the highest hardness in both joints. Hardness of SCHAZ and CGHAZ of S-12 joint was 7% and 1% higher compared with W-5 joint, respectively. Weld metal hardness of W-5 joint was 15% lower than that of S-12 joint. Both joints not only fulfilled the requirements of minimum 50 J per EN ISO 10025-6 at −20 °C but exceeded this limit by 50% (W-5) and 200% (S-12). Lateral expansions for impact toughness specimens were around 17.5% for S-12 joint, whereas it was 4% for W-5 joint. Since HAZ in the S-12 (stringer bead) joint is narrow compared with the one in the W-5 joint, impact toughness values were higher with the S-12 joint due to the locations of the notches of the impact specimens.

Microstructural Control and Alloy Design for Improving the Resistance to Delayed Fracture of Ultrahigh-Strength Automotive Steel Sheets

The demand for higher-strength automotive steel sheets has increased significantly for lightweight and safe body concepts. However, the increment of the steel strength is often limited by the potential occurrence of delayed fracture. This paper discusses proper microstructure control and alloy design to improve the resistance against the delayed fracture of ultrahigh-strength automotive steel sheets in order to increase the usable upper limit of their strength and provides basic data serving as a practical guide for solving the problem of delayed fracture in ultrahigh-strength automotive steel sheets. It is confirmed that grain refinement, the appropriate dual-phase structure of martensite with ferrite or retained austenite, and surface decarburization, increase the resistance to delayed fracture. In terms of alloy design, the effects of Nb, Mo, and B on the delayed fracture resistance of hot-stamped steels have been investigated. The results suggest that there are other reasons for Nb to improve delayed fracture resistance in addition to grain refinement and the ability to trap hydrogen by its precipitates, as has been conventionally believed. Regarding Mo, it was clearly demonstrated that the segregation of this element at the grain boundary plays a main role in improving the delayed fracture resistance.

Structural performance and design of S960 ultra-high strength steel non-slender welded I-sections subjected to combined loading

The structural performance of S960 ultra-high strength steel non-slender welded I-sections subjected to combined loading is studied in the present paper. Nonlinear finite element (FE) models were first created and validated with reference to test results collected from the literature, and then used for conducting parametric studies to derive numerical data over an extensive spectrum of cross-sectional geometries, aspect ratios, as well as loading combinations. Given the lack of existing design rules for S960 ultra-high strength steel structures, the suitability of the current provisions for lower strength steels given in the European, Australian and American specifications to S960 ultra-high strength steel was assessed based on the generated numerical data. It was found that (i) the European code and Australian standard can accurately predict the strengths of Class 1 and 2 (i.e. compact) S960 ultra-high strength steel welded I-sections subjected to both strong- and weak-axis combined loading, but results generally in conservative strength predictions for Class3 I-sections, and (ii) the American specification yields accurate strength predictions for S960 ultra-high strength steel welded I-sections subjected to strong-axis combined loading, but leads to conservative strength predictions for weak-axis combined loading. Finally, the continuous strength method (CSM) was applied to the studied S960 ultra-high strength steel non-slender welded I-sections subjected to combined loading and found to offer significantly improved design consistency and accuracy relative to the current codified provisions. The reliability of the latter design method was also confirmed by means of statistical analyses.

Investigation of Microstructure and Properties of Ultra-High Strength Steel in Aero-Engine Components following Heat Treatment and Deformation Processes

C250 steel, renowned for its remarkable strength and toughness, is extensively utilized in the aerospace industry for manufacturing critical components. This research investigates the microstructure and properties of forgings produced through different heat treatment temperatures, aging durations, and thermal cycling intervals. The results demonstrate that the samples were compressed at 1050 °C followed by air-cooling using the conventional maraging treatment. For the cycle heat treatment, temperature was maintained at 1050 °C and cycled 1–2 times, with a heat preservation period of 1 h and subsequent water-cooling. Solution heat treatment at a temperature of 1050 °C, aging for 5 h, and then air hardening were performed to achieve the best forging hardness. Interestingly, the solution time under age hardening conditions had no significant effect on the grain size but had a significant effect on the hardness of martensitic aged steel.

Study of fracture damage criteria and influence of process parameters on punching of hot stamped ultra-high strength steel

Study of fracture damage criteria and influence of process parameters on punching of hot stamped ultra-high strength steel

The effects of geometric and metallurgical constraints on ultra-high strength steel weldments

The effect of geometric and metallurgical constraints on the ultimate capacity of weldments manufactured from direct-quenched (DQ) ultra-high strength steel (UHSS) is studied using numerical calculations. In addition, experimental tests are conducted to prove the phenomena in practice. DQ steels behave differently under static loading, especially their weldments, compared to the same-strength class steels, which are manufactured using different alloying and fabrication techniques. In these steels welding generates a distinct softened region at the heat-affected zone (HAZ), which, in certain conditions, can influence the ultimate load-carrying and elongation capacity of the weldment. Metallurgical constraints can form in welds where the soft zone is adjacent to zones with higher strength. Moreover, the current study shows that not only metallurgical constraints, but also geometric ones, can strengthen weldments depending on the joint type. Removing the geometrical constraints from weldments slightly reduces the ultimate load-carrying capacity, while a remarkable reduction is seen in the elongation capacity of the joint. Numerical analysis conducted for non-load carrying X-joints indicates that as the width of the soft zone increases, the effect of the geometric constraint diminishes. Thus, it appears that geometric constraints can, in certain cases, prevent the detrimental effect of strain localization from occurring in the soft regions in at the HAZ. This mechanism seems to increase both the ultimate load-carrying capacity and ultimate elongation of weldments.

Raman Spectroscopic Analysis of the Reaction between Al-Si Coatings and Steel.

Hot-stamped ultrahigh strength steel components are pivotal to automotive light-weighting. Steel blanks, often coated with an aluminum-silicon (Al-Si) layer to protect them from oxidation and decarburization, are austenitized within a furnace and then simultaneously quenched and formed into shape. The Al-Si coating melts within the furnace and reacts with iron from the steel to yield an intermetallic phase that provides some long-term corrosion protection. During the intermediate liquid phase, some of the coating may transfer to the furnace components, leading to maintenance costs and operational downtime. A detailed understanding of the coating transformation mechanism is needed to avoid such production issues while ensuring that final intermetallic coatings conform to specifications. We introduce cross-sectional Raman microscopic mapping as a method to rapidly elucidate the coating transformation mechanism. Raman spectroscopic fingerprints for relevant intermetallic compounds were determined using synthesized Al-Fe-Si ternary and Al-Fe binary compounds. These fingerprints were used to map the spatial distribution of intermetallic compounds through cross sections of Al-Si-coated 22MnB5 specimens that were heated at temperatures between 570 and 900 °C. These chemical maps show that the intermetallic fraction of the coating does not grow significantly until formation of η (Al5Fe2) at the steel interface, suggesting that η facilitates extraction of iron from the steel and subsequent diffusion through the coating. Under the heating conditions used here, a series of reactions ultimately lead to a silicon-rich τ2 (Al3FeSi) phase on top of the binary η phase. The technique presented here simplifies structural analysis of intermetallic compounds, which will facilitate prototyping of strategies to optimize hot stamping.

Experimental and numerical investigations of S960 ultra-high strength steel welded I-section beams with in-plane flexural failure

A testing and numerical modelling programme was performed to investigate the in-plane flexural behaviour and cross-section bending capacities of S960 ultra-high strength steel welded I-section beams and presented in this paper. The testing programme included material testing, membrane residual stress measurements, initial local geometric imperfection measurements and eight in-plane four-point bending tests, with four performed about each of the cross-section minor and major principal axes. This was followed by the numerical modelling programme, where finite element models were developed and validated against the test results and then used to perform parametric studies to generate further numerical data over a wide variety of cross-section dimensions and span lengths. The obtained test and numerical data were used to evaluate the applicability of the design rules for cross-section classification and bending capacities, as prescribed in the European code, Australian standard and American specification, to S960 ultra-high strength steel welded I-section beams. The evaluation results revealed that all codified slenderness limits were safe (though conservative to different extents) when used for classifying the constituent plate elements of S960 ultra-high strength steel welded I-sections in bending. Moreover, all three considered design codes were shown to result in accurate and consistent bending capacity predictions for Class 1 and 2 (or compact) S960 ultra-high strength steel welded I-sections, but conservative bending capacity predictions, to different extents, for Class 3 (or non-compact) and Class 4 (or slender) S960 ultra-high strength steel welded I-sections.

Effect of Aging Treatment on the Microstructure and Properties of 2.2 GPa Tungsten-Containing Maraging Steel.

Maraging steel is a prominent category of ultrahigh-strength steel (UHSS) characterized by excellent comprehensive properties, and it finds wide applications in manufacturing load-bearing structural components. In this study, a novel tungsten-containing maraging steel, C-250W, was designed. The effects of aging treatments on the mechanical properties, microstructure, precipitations, and reverted austenite of C-250W steel were investigated. The results revealed that the optimal combination of strength and toughness could be achieved through an aging treatment of C-250W steel carried out for 5 h at 480 °C after solution treatment at 1000 °C for 1 h. As the aging temperature increased, the proportion of dimples in the impact fracture gradually decreased while that of quasi-cleavage increased, leading to a reduction in Charpy impact energy. The boundary of martensitic lath decomposed gradually as the aging temperature increased, and it disappeared entirely at temperatures higher than 550 °C. Moreover, the aging process resulted in the formation of phases, including spherical Fe2M (M represents Mo, W) and thin strip-shaped Ni3N (N represents Mo, Ti) precipitates. These precipitates coarsened from 5 nm to 50-200 nm with increasing aging temperature. Additionally, the content of reverted austenite increased with the aging temperature. Within the temperature range of 400 °C to 500 °C for aging treatment, the content of film-shaped reverted austenite was approximately 3%, primarily distributed at the boundary of martensite lath. When the aging temperature exceeded 550 °C, the content of reverted austenite reached 20.2%, and its morphology changed from film-shaped to block-shaped, resulting in a decline in strength and toughness.

Fatigue crack propagating behavior from inner surface crack on bent sheet of ultra-high strength steel

Fatigue fracture of structures made of bent sheets often occurs from cracks on the sheet inner surface. This study investigated fatigue crack growth and non-propagating behavior in bent sheets of ultra-high strength steel (UHSS) using artificial cracks introduced by a picosecond pulse laser. A newly-designed fatigue test showed that the internal compressive residual stress introduced by bending arrests crack propagation in the depth direction. A crack propagation analysis using the software SCANP revealed that this internal compressive residual stress suppressed not only the stress intensity factor in the depth direction, ΔKa, but also that in the width direction, ΔKc, even though the surface residual stress was tensile. A stress relief heat treatment test demonstrated that the crack arrest behavior disappeared and the propagation life was shortened in the absence of this compressive residual stress. Non-propagating cracks were observed in the specimens after 1×107 test cycles. The relationship between the fatigue limit and the artificial crack depth (Kitagawa-Takahashi diagram) was generally fitted by the Line Method with a critical distance of 46.7 μm. This study provides insight into the fatigue behavior of cracks on bent sheets of UHSS, which are widely used in many industries such as automobile manufacturing.