Ultra-high Strength Steel Research Articles

Thermal behavior analysis of UHSS rectangular plates via gradient thermoforming process under coupled heat conduction and radiation

The mechanical properties of ultrahigh-strength steel (UHSS) fabricated by the gradient thermoforming process (GTP) are directly governed by conduction-radiation coupled thermal behaviors. However, the thermal behavior of UHSS during gradient thermoforming remains unclear. This study examined the specific effects of coupled thermal conduction, radiation, and convection on heat transfer in UHSS via GTP. An effective thermal model was proposed by considering heat transfer mechanisms, including coupled heat conduction and radiation. An analytical model for predicting the 3D transient temperature within rectangular UHSS plates during GTP was developed with the separated variable method. The model incorporated thermal conduction-radiation coupling and was validated through comparisons with existing references. Furthermore, the effects of different physical parameters on the temperature distribution of the UHSS rectangular plates in the GTP were explored. The results demonstrate that the heating rate, geometry, and model parameters significantly affect the temperature distribution of GTP-built UHSS rectangular plates. This research provides a theoretical foundation for optimizing the process parameters and material design and improving the formation quality of GTP-built UHSS rectangular plates.

Hydrogen Embrittlement in as-Quenched Martensitic Steels - Application of Incremental Step Loading Technique with Novel Tuning-Fork Test

Hydrogen embrittlement (HE) is a well-known issue, especially with ultrahigh-strength steels (UHSS). Various testing methods are utilised to study HE, but they typically require tensile test equipment, or are impractical due to limited stress control with standard geometries. We have developed a novel Tuning-fork test (TFT) to study HE susceptibility of steels with a new specimen geometry, which can be stressed accurately without tensile test equipment. The test method utilises in situ electrochemical hydrogen charging and constant displacement for stressing of the notched specimens by bending. Crack initiation and propagation are controlled with an isolated tensile stress region, and the failure process is monitored with a loadcell. TFT is a simple and fast testing method, which allows ranking of UHSSs, and to investigate, e.g., microstructural effects on susceptibility to HE and H-induced fracture processes. Here in this study, we present the state-of-the-art with the improved more precise second-generation TFT setup, which benefits from a more sensitive loadcell and a more stable fine-tuneable differential screw adjustment. We extend TFT to testing of martensitic steels with nominal hardness from 400 HBW to 600 HBW with the Incremental step loading technique (ISLT). The results show that TFT with ISLT is well applicable for ranking ultrahigh-strength steels based on their susceptibility to HE. Force-time data from ISLT can also be used for the determination of a material-specific threshold stress level, and the last step for the calculation of a crack initiation-time and time-to-fracture. However, the current manual operation of the loading screw can still limit maximum duration of a test.

Ultra-high strength concrete filled steel tube members: Classification, experimental database, and design

Current design codes do not endorse the use of ultra-high strength steel and concrete materials (Fy ≥ 690 MPa and f’c ≥ 120 MPa) for concrete filled steel tube (CFST) members. This paper makes efforts to address this gap using a four-step method. In the first step, the classification of CFST members based on material strengths and tube slenderness ratio is introduced. In the second step, an experimental database comprised of 805 ultra-high strength concrete filled steel tube (UCFST) specimens under various loading conditions is compiled, and the research gaps in the database are identified. In the third step, the possibility of extending Eurocode 4 and AISC 360–22 to UCFST members are evaluated using the experimental database. Finally, a research program including experimental tests, finite element (FE) parametric studies and new design equations for UCFST rectangular members is introduced.

Effect of hot rolling on microstructure and mechanical properties of hot isostatically pressed 30CrMnSiNi2A ultrahigh strength steel

The prior particle boundaries (PPBs) are a typical powder metallurgy (PM) defect, which usually leads to the deterioration of mechanical properties in PM materials. In this study, two different hot rolling modes (unidirectional rolling and cross rolling) are designed to improve PPBs in hot isostatic pressed (HIPed) 30CrMnSiNi2A ultrahigh strength steel. The microstructure, mechanical properties and fracture features of the HIPed compact, the hot unidirectional rolled (HURed) and hot cross rolled (HCRed) sheets are systematically compared. The results manifests that hot rolling has a significant influence on size and distribution of PPBs. PPBs in HIPed compact are comprised of Al-rich oxides which is distributed along particle boundaries. After hot rolling, large PPBs inclusions are crushed and the size of PPBs are mostly concentrated in 0–0.20 μm. Unidirectional rolling makes continuous PPBs structure broken into short-chain fragments and dispersed oxides completely, while cross rolling makes most of continuous PPBs structure maintained and only a few oxides are dispersed. Excellent mechanical properties are obtained by hot rolling, reflecting on an ultimate tensile strength (UTS) of about 2.0 GPa, a total elongation (TE) of 17.9 %, a Charpy impact energy of 28.6J along rolling direction and a UTS of over 2.1 GPa, a TE of 16.1 %, a Charpy impact energy of 18.1J along transverse direction, respectively. High performances can be attributed to formation of martensite, grain refinement, improvement of PPBs and work hardening. Furthermore, hot-rolled samples manifests an obvious anisotropy in mechanical properties. The HCRed sample reveals a lower anisotropy compared to the HURed sample, mainly owing to differences of PPBs distribution and rolling textures.

Effects of laser welding speed on the microstructure and microhardness of ultra-high strength steel S1100

Ultra-high strength steels strengthened via thermomechanical processing and low alloying constituents are currently quite popular for industrial applications. The popularity is primarily due to these steels being relatively economical compared with their high-alloy rivals, making them feasible to provide significant strength-to-weight ratios with reasonable costs in steel structures. However, welded joints in most steel structures cannot be avoided, and ultra-high strength steels, due to their thermomechanical strengthening mechanisms, are prone to loss of strength, toughness, or ductility after welding. Also, due to the ultra-high strength levels of the base metals, providing a suitable filler material with mechanical properties comparable to those of the base metals for filling the joint gap seems challenging. Accordingly, laser welding can be a relatively more suitable approach than arc welding to weld ultra-high strength steels, considering its easier control over the weld heat input, joint shape, and the possibility of welding without the requirement of filler metals. Therefore, this study investigates the influence of travel speed and its subsequent change in heat input of laser welding on the joint shape, microstructure, and hardness of the S1100 low-alloy carbon steel, one of the most commonly used ultra-high strength steels in modernized steel structures.

Evaluation on Joint Performances of Advanced Multi-Material Dissimilar Lap Joints

As a result of R&D efforts about the advanced materials, an ultra-high strength steel (UHHS) and a carbon fiber reinforced thermos-plastic (CFRTP) have been developed. In this study, various advanced multi-material dissimilar lap joints using UHHS, CFRTP and an aluminum alloy of A5083P-O were fabricated by the resistance spot welding (RSW), refill friction stir spot welding (RFSSW), friction stir spot welding (FSSW) and laser irradiation assistance plastic welding (LIAPW). Where, the total heat input for each joining method was varied in order to examine the influence of welding condition on joint performances, which were the tensile shear strength, shear fatigue property and corrosion shear fatigue property. The tensile shear strength of UHHS and A5083P-O joined by RFSSW is almost two times of that produced by RSW, and this difference seems to be mainly caused by the difference of joint area. The difference of joint area also affects the tensile shear strength of UHHS & CFRTP or A5083P-O &CFRTP lap joints made by FSSW and LIAPW, and the joint strength of LIAPW is larger than that of FSSW. As for the shear fatigue tests, it is found that the fatigue strength seems to decrease almost monotonically with increasing the applied load regardless of the types of dissimilar materials. Moreover, through the corrosion shear fatigue tests for A5083P-O and CFRTP joined by FSSW or LIAPW, it is found that the corrosion fatigue property seems to be controlled by the crack propagation behavior.

Effect of Rapid Induction Tempering on the Mechanical Properties and Microstructure of Ultra-High Strength Steel

Rapid induction can be utilized to decrease the time and energy used for heat-treatment of steels. In the present study, a commercial 500 HB grade wear-resistant steel was subjected to rapid induction tempering and compared to conventionally furnace tempered samples. The martensitic ultra-high strength steel was cut to narrow thin sheets, which were tempered at 200, 300, 400, and 500 °C with both methods. The rapid tempering was applied with an in-house built induction line, in which the samples were moved through an induction coil. The velocity of the samples was adjusted to ensure constant temperature control. The applied heating rate was 1000–1100 °C/s resulting in extremely rapid tempering times. The conventionally tempered samples were heated in a pre-heated furnace for 45 min and cooled in still air. The samples were tested for tensile and hardness properties and microstructural characterization was conducted. Results revealed only minor differences between the differently treated steels. Elongation was slightly improved with the induction treatment. Therefore, the induction tempering appeared to result in similar or even slightly better tensile properties and can be considered a promising alternative for tempering processes in future steelmaking.

Design and optimization of cooling channels in hot stamping dies applying additive manufacturing design concept

The stamping process is a process that represents a key manufacturing process in the automotive industry. With the implementation of the stamping process, mechanical elements obtain mechanical resistance, in addition to a very low ratio in quantity/cost. Within the design premises that govern the automotive industry, it is worth highlighting the requirement for mechanical resistance, passenger safety, and the lightness of materials. All of this contributes to the current use of alloyed materials with high mechanical resistance. These materials in addition to keeping the mechanical requirements of the different components subjected to load, must satisfy the geometric requirements established by the designers. In the last few years, the use of the Ultra High Strength steels has grown in the automotive industry. This type of alloyed steel contributes to car safety and clean mobility. Given the mechanical nature of these ultra-high-strength alloys, they require the use of hot stamping. This makes the deformability of the different steels easier and allows that, using a conventional stamping method, would not be able to deform according to the geometric requirements of the different mechanical components.

Effect of Mo and Mo+Nb Additions on the Phase Transformation and Microstructure of a Developed Low-Carbon CrNiMnB Ultrahigh-Strength Steels with a Preceding Hot Deformation

The influence of molybdenum, and molybdenum with niobium addition on the phase transformation behaviour of a developed low-carbon CrNiMnB ultrahigh-strength steels, was investigated. Gleeble 3800 thermomechanical simulator was employed to simulate the hot-rolling process and to get the dilatation curves. After austenitization at 1250 °C for the complete dissolution of carbides, specimens received 0.6 total strain (i.e., 0.2 at 1100 °C and 2 x 0.2 at 900 °C) followed by cooling at various cooling rates (CRs) in the range of 2-60 °C/s. The final microstructures were investigated using laser scanning confocal microscopy, field emission scanning electron microscopy, and hardness measurements. Then the continuous cooling transformation diagrams were constructed based on the dilatation curves, microstructure, and hardness values. The electrolytic extraction method was used to assess the elements' distribution and the composition of the forming precipitates. The addition of Mo increased the hardenability, decreased the transformation temperatures, and promoted the formation of low-temperature transformation products i.e., martensite and bainite ferrite, at different CRs and inhibit the formation of polygonal ferrite. The formation of coarse precipitates neglected the effect of Mo+Nb addition, decreased the hardenability and expanded the region of BF formation to high CRs. The variation in the hardness with microstructural changes was discussed.

Numerical study and design of ultra-high-strength steel plate girders subjected to patch loading after elevated temperature

When a steel structure does not collapse after a fire, it is meaningful to study its residual resistance after exposure to elevated temperatures to evaluate its post-fire service capacity. The elevated temperatures of a fire change the residual mechanical properties of structural steel, which influences the patch-loading resistance performance of steel plate girders. However, the effects of temperature on the residual resistance performance of Q960 ultra-high-strength steel (UHSS) plate girders subjected to patch loading remain unclear. This study aims to determine the variation trends of the post-fire bearing performance of Q960 UHSS plate girders subjected to patch loading based on their mechanical properties after exposure to elevated temperature and natural cooling. The effects of the web width-to-height ratio, web height-to-thickness ratio, load length, and exposure temperature were considered in the parametric study; a total of 400 numerical models were included. Additionally, the influence of the exposure temperature on the serviceability limit state of Q960 UHSS plate girders was considered. The applicability of EN 1993-1-5 was verified based on the finite element analysis results, which showed that the design methods in EN 1993-1-5 significantly underestimated the ultimate resistance of Q960 UHSS plate girders exposed to elevated temperatures. Thus, the strength reduction function in EN 1993-1-5 was revised to improve the accuracy of the statistical analysis.

Hot Workability of a Typical Ultrahigh Strength Steel During the Isothermal Forging Process

Hot Workability of a Typical Ultrahigh Strength Steel During the Isothermal Forging Process

Effect of high-frequency induction weld seam on the deformation of M1700 ultra-high strength steel shell structures considering residual tensile stress

The feasible cold forming procedure to massively manufacture ultra-high strength steel (UHSS) light-weighting profiles so far in industrial fields consists of roll forming, welding, cutting and bending. To reveal the role the high-frequency induction weld (HFIW) line plays in the subsequent plastic forming, a serials of deliberately designed collapse tests of the shell structures taken from M1700 roof rail pillar with a wall-thickness of 2 mm and triangle cross-section were conducted. Real geometries and inhomogeneous material properties of the weld seam and heat-affected zone (HAZ), together with the residual tensile stresses within the weld seam after HFIW, were fully considered in the numerical simulation. The Brozoo's modified CL damage model and Oyane damage model were employed and compared. The results show that the deformation resistance of the shell structures is modified by the material accumulation around the HFIW seams during the collapse tests. The cracks mainly occur within the critical HAZ outside the tube and the temper softening zone inside the tube under the action of softening effect in the HAZ of M1700 steel. The residual tensile stresses increase the crack probability of weld seams. The predicting accuracy of fracture location of the Oyane damage model is higher than that of the Brozoo's modified CL damage model.

Effect of different cooling and lubrication strategies on milling performance of ultra-high strength steel

Effect of different cooling and lubrication strategies on milling performance of ultra-high strength steel

An Experimental Investigation on the High Cycle Fatigue Behavior of Resistance Spot Welded Ultrahigh Strength Steel

An Experimental Investigation on the High Cycle Fatigue Behavior of Resistance Spot Welded Ultrahigh Strength Steel

Investigation of 300M ultra-high-strength steel deposited by wire-based gas metal arc additive manufacturing

300 M ultra-high-strength steel (UHSS) is widely used to produce landing gear components for aircraft. The conventional manufacturing route for these components involves extensive machining and significant material wastage. Here, the application of wire-based gas metal arc additive manufacturing to produce 300 M UHSS parts was investigated. In particular, the influence of torch shielding atmosphere on the process stability and material performance of 300 M UHSS was investigated. The shielding gases used for comparison are pure Ar, Ar with 2.5% CO2, Ar with 8% CO2, Ar with 20% CO2, and Ar with 2% CO2 and 38% He. It was found that the arc length decreased, the transfer mode changed from spray to droplet mode, and spattering became more severe as the CO2 proportion increased. Additionally, replacing Ar with He led to a broader arc core, and a slightly shorter arc length and maintained a spray transfer, which decreased spatter. The wall surface roughness followed the trend in spatter, becoming worse with the increasing CO2 proportion, and better with He addition. Adding CO2 and He in pure Ar significantly increased the bead and wall width. The microstructure and mechanical properties exhibited a strong location dependence in the as-built state, with fresh martensite and higher strength in the top region, and tempered martensite and better ductility in the reheated bulk. Generally, torch shielding gas composition appeared to have no significant effect on the microstructure evolution. This study provides a reference for the subsequent application of gas metal arc additive manufacturing to aircraft landing gear mass production to achieve a high deposition rate and process stability simultaneously.

3D free thermal bending of BR1500HS high-strength steel tube: Simulation and experimental investigation

Ultra-high strength steel (UHSS) pipes find widespread application in the automotive industry. To address the challenges associated with forming complex structural parts from UHSS pipes, the three-dimensional free thermal bending (3D-FTB) technology for UHSS pipes based on a robotic arm and a bending roller mold was firstly developed. A finite element simulation prediction model for accurately describing the forming process is established and discussed. In this paper, the thermal-mechanical coupling model of the material is established by Gleeble thermal tensile test and modified Arrhenius model. The effects of parameters such as current frequency, current density, axial feed rate, and quenching conditions on forming temperature and microstructure transformation are thoroughly investigated via finite element simulation analysis. The mechanical properties and potential defects of the pipe fittings after forming are analyzed and validated experimentally. The results demonstrate that the finite element simulation and analysis method based on the thermal-force coupling model of the BR1500HS UHSS pipe can accurately predict the forming process parameters. The optimal process conditions are identified as follows: current frequency of 55 kHz, current density of 650 A/mm2, and feed rate of 8 mm/s. Under these parameters, the austenite transformation rate of the pipe fittings reaches an impressive 98 % and the maximum tensile strength reaches 1540 MPa. Lastly, the overall wall thickness distortion rate of the formed pipe remains below 0.2 % and the bending rebound angle exhibits an average reduction of over 80 % compared to that under room temperature conditions.

Ultra-high strength and high toughness 24CrNiMo alloy steel fabricated by laser powder bed fusion and subsequent quenching

A nearly fully dense grade 24CrNiMo alloy steel with superior comprehensive performance was fabricated by laser powder bed fusion (LPBF) under optimum laser parameters and subsequent quenching treatment was applied. The effect of quenching temperatures on microstructure evolution and mechanical properties of the LPBF 24CrNiMo parts were first systematically characterized and analyzed. The results demonstrated that the microstructure of LPBF 24CrNiMo steel (submicron grain size of 2.99 μm on average) was mainly constituted by block and stripe granular bainite containing twin substructure and θ-Fe3C carbides with no macroscopic segregation and anisotropy. After quenching, with the increase of quenching temperature from 800 to 1000 °C, the microstructure gradually changed from the acicular ferrite to martensite, finally to tempered martensite. The fine grains, the numerous twins were retained and the nanoscale carbides η-Fe2C were precipitated after quenching at 850 °C, simultaneously obtaining ultra-high mechanical properties including the hardness, yield strength, ultimate tensile strength and elongation were 446 HV and 1380 MPa, 1517 MPa, 13.8 %, respectively as well as the surface scratch resistance improved about 25 %. As quenching temperatures varied to 1000 °C, the mechanical properties lowered due to the coarsening and uneven distribution of the grain and θ-Fe3C carbides as well as the decrease in the number of twins. The improved mechanical properties via quenching treatment was attributed to phase transformation strengthening, grain refinement strengthening and precipitate strengthening.

Bond stress-slip behavior of ultrahigh-strength steel bars with spiral grooves embedded in plain and steel fiber high-strength recycled aggregate concrete

This study investigated the bond behavior of ultrahigh-strength steel bars with spiral grooves (UHSSBs) embedded in high-strength recycled aggregate concrete (RAC). Forty-five pullout specimens under monotonic loadings were designed to evaluate the effect of several parameters on the bond strength. The variables included concrete strength (50 MPa–60 MPa); recycled concrete aggregate (RCA) replacement ratio (0 %, 50 %, and 100 %); embedment length (5db, 10db, and 20db); concrete cover (25 mm, 45 mm, and 68.7 mm); surface treatment; volume fraction of steel fibers in RAC and stirrup ratio. The failure modes of these tests mainly concentrated on two types, pullout-type bond failure and splitting-type bond failure. According to the typical bond stress-slip curves and trendlines of plotted values, the bond strength increased with increasing of concrete compressive strength. For specimens with splitting-type bond failure, the higher normalized bond strength was observed in higher RCA replacement ratio and concrete cover thickness. The factors of embedment length and surface treatment were also proven to be effective for bond strength. For specimens with pullout-type bond failure, the presence of steel fibers and stirrups substantially restricted the splitting crack growth and improved the bond failure modes. Combined with previous experimental results, simple empirical equations for the average bond strength and bond stress-slip relationship between UHSSBs and RAC under different types of bond failure were developed by regression analysis, which can capture the experimental data with high accuracy.

Effect of temperature on the plastic flow and strain hardening of direct-quenched ultra-high strength steel S960MC

This study investigates the plastic deformation and hardening behavior of the direct-quenched ultra-high strength steel S960MC at various temperatures ranging from room temperature to 900 ℃. In this regard, the Hollomon and Voce equations are used to model the hardening behavior of the material at different temperatures. The suitability of each equation to predict the plastic flow of S960MC is evaluated based on the best resulted fit for the material. In addition, microstructural investigations are carried out to indicate the correlations between the microstructural changes, occurring in the range of room temperature to 900 ℃, and hardening behavior and governing parameters. The Hollomon approach showed deviations from the experimental results for room to intermediate temperatures; however, the Voce equation modeled the material's strain hardening and flow behavior more successfully for the entire temperature range of room temperature–900 ℃. Additionally, there was a significant consistency between the Kocks-Mecking and Voce parameters. Dislocation interactions, dynamic strain aging, dynamic recrystallization, dynamic recovery, tempering (martensite decomposition), and austenite formation were the most influential microstructural features on the hardening behavior at various temperatures. The correlations between these microstructural features and hardening parameters were established satisfactorily for both the Hollomon and Voce approaches.

Obtaining Excellent Mechanical Properties in an Ultrahigh-Strength Stainless Bearing Steel via Solution Treatment

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.