Ultra-high Strength Levels Research Articles

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.

Design of cooling route for carbide-free bainitic rail steels and resultant microstructures and properties

In the present work, two new low-alloy, carbide-free bainitic rail steels (steel A: 0.29C + 2.49MnCr & steel B: 0.19C + 3.18MnCr wt.%) were designed to match the controlled cooling production process that combines forced air-cooling and nature air-cooling. The effect of alloy design and continuous cooling rate on the bainite production processing window and resultant microstructure was investigated, followed by the establishment of controlled cooling routes. The microstructure, transformation kinetics, and crystallographic characteristics of the specimens under the process were studied, and cuboid specimens were employed to validate the processing feasibility and investigate the correlation between related microstructures and mechanical properties. The results showed that both steels exhibited ultrahigh strength levels, with a tensile strength approaching 1400 MPa and elongation exceeding 16 %. However, the impact toughness of Steel A deteriorated due to the formation of granular bainite. Due to the homogeneous lath bainite morphology and refined bainitic ferrite substructure, Steel B exhibits excellent strength and toughness, essentially meeting the Chinese technical specifications for the procurement of 43 kg/m ∼75 kg/m rail.

Edge-dislocation-induced ultrahigh elevated-temperature strength of HfMoNbTaW refractory high-entropy alloys

ABSTRACT Over 150 refractory high-entropy alloys (RHEAs) have been proposed in the last decade. Early alloys such as MoNbTaW and MoNbTaVW still show an unparalleled yield strength of approximately 400 MPa at 1600°C. However, RHEAs with even elevated high-temperature strength are necessary in aerospace vehicles and nuclear reactors to cope with advanced technology in the future. Here, solid-solution strengthening calculation and melting point prediction are combined to design single-phase RHEA for attaining ultrahigh strength at 1600°C. The results show that Hf0.5MoNbTaW and HfMoNbTaW alloys after fully homogeneous treatment at 2100°C for 2 h reveal a homogenous body-centered cubic phase. HfMoNbTaW alloy exhibits a yield strength of 571 MPa at 1600°C, much higher than that of MoNbTaVW (477 MPa). It is found that a plateau of strength occurs from 800°C to 1200°C, which is important for raising the strength level of RHEAs at high temperatures. This strengthening mechanism is explained with the change of deformation mode from screw to edge dislocations, which contributes an edge-dislocation-induced strength. A similar alloy design strategy could be applied to develop more RHEAs with an ultrahigh strength level.

Temperature Dependence of Fracture Characteristics of Variously Heat-Treated Grades of Ultra-High-Strength Steel: Experimental and Modelling.

The temperature dependence of tensile characteristics and fracture toughness of the standardly heat-treated low-alloyed steel OCHN3MFA along with three additionally heat-treated grades was experimentally studied. In the temperature range of 〈−196; 22〉 °C, all the additional heat treatments transferred the standard steel from a high- to ultra-high strength levels even with improved tensile ductility characteristics. This could be explained by a reduction of the inclusion content, refinement of the martensitic blocks, ductile retained austenite content, and homogenization of the shape ratio of martensitic laths as revealed by metallographic, X-ray, and EBSD techniques. On the other hand, the values of the fracture toughness of all grades were found to be comparable in the whole temperature range as the cause of a high stress triaxiality in the pre-cracked Charpy V-notch samples. The values of the fracture toughness of the standard steel grade could be predicted well using the fracture model proposed by Pokluda et al. based on the tensile characteristics. Such a prediction failed in the case of additionally heat-treated grades due to the different temperature dependence of the fracture mechanisms occurring in the tensile and fracture-toughness tests. While the tensile samples fractured in a ductile-dimple mode at all temperatures, the fracture-toughness specimens exhibited a transition from the ductile to quasi-brittle fracture mode with decreasing temperature. This transition could be interpreted in terms of a transfer from the model proposed by Rice and Johnson to the model of Tvergaard and Hutchinson.

The effects of severe plastic deformation on the mechanical and corrosion characteristics of a bioresorbable Mg-ZKQX6000 alloy.

In this work, a bioresorbable Mg-ZKQX6000 (Mg-6Zn-0.6Zr-0.4Ag-0.2Ca (wt%)) alloy was severely plastically deformed via equal channel angular pressing (ECAP) according to three unique hybrid routes at low temperatures (200°C to 125°C). The roles of ECAP processing on microstructure, and ensuing mechanical properties and corrosion rates, are assessed. Microstructurally, ECAP induces a complex plethora of features, especially variations in grain sizes and precipitates' sizes, distributions, and morphologies for individual cases. Mechanically, ECAP generally refined grain size, resulting in ultra-high strength levels of about 400MPa in ultimate tensile strength for several cases; however, deformation via ECAP of precipitates induced embrittlement and low elongation to failure levels. Corrosion testing, conducted in simulated bodily fluid at bodily pH levels to mimic conditions in the human body, revealed consistent corrosion rates across several techniques (mass loss, hydrogen evolution, and electrochemical impedance spectroscopy (EIS)), showing that severe plastic deformation deteriorates corrosion resistance for this material. In-situ corrosion monitoring explained that corrosion accelerated after ECAP due to the creation of heterogeneous, anodic shear zones, which exhibited dense regions of refined grains and fine precipitates. Suggestions for future design and thermomechanical processing of Mg alloys for bioresorbable orthopedic implants are provided.

Improvement of strength – ductility balance of B2-strengthened lightweight steel

The microstructure evolution and tensile properties of a newly designed Fe-21Mn-10Al-1C-5Ni (wt.%) lightweight steel subjected to two annealing conditions (inducing partial recrystallization and full recrystallization) and subsequent aging treatment have been investigated. In the as-annealed condition, the microstructure of the steel consists of polygonal B2 particles along grain boundaries of recrystallized austenite grains and plate-shape B2 particles within unrecrystallized austenite grains, with no B2 bands. In addition, there is a formation of nano-sized κ-carbide and D03 particles within austenite grains and B2 particles, respectively. Subsequent aging promotes the formation of intragranular κ-carbide and D03 nanoparticles within austenite grains and B2 particles, respectively. As a result, the present steel shows an ultrahigh yield strength of 1.6 GPa after aging, which is mainly due to the presence of fine B2 particles within austenite grains and along grain boundaries as well as the additional strengthening effect of nano-sized κ-carbide and D03 particles present in austenite grains and B2 particles, respectively. The steel possesses good ductility (total elongation of 20%) even at such an ultrahigh strength level in the as-aged condition, surpassing the tensile properties of other B2 and κ-carbide strengthened lightweight steels.

Development of continuously cooled low-carbon, low-alloy, high strength carbide-free bainitic rail steels

In the present work, attempts have been made to design and develop low-carbon, low-alloy, high strength carbide-free bainitic steels for application in heavy-haul rail tracks. The study starts with an analytical approach for understanding the effects of different alloying elements on the thermodynamic and kinetic aspects of bainite transformation and the resultant microstructure and mechanical properties. Based on the analysis, two steel compositions were designed for further investigation. The alloys were prepared by melting and casting and processed by hot-rolling and air cooling, which is the usual route for the industrial production of rail sections. Microstructures of the developed steels primarily consisted of plates of carbide-free bainitic ferrite, interspersed with fine films of retained austenite. These continuously cooled steels offer ultra-high strength levels with very good ductility which are far superior to existing pearlitic grade rail steels. Detailed characterization and quantification of microstructural constituents have been carried out and they are correlated with the tensile proprieties using semi-empirical formulations available in literature.

Friction Stir Processing of Beta C and Ti-185: A Unique Pathway to Engineer Microstructures for Exceptional Properties in β Titanium Alloys

Metastable β titanium alloys offer a wide range of attractive property combinations. Conventional processing options have practical limitations in achieving maximum attainable properties and utilizing them to their fullest potential. In the current study, friction stir processing (FSP) of β alloys is explored as a unique path for microstructural engineering. Two metastable β titanium alloys, Ti-1Al-8V-5Fe (Ti-185) and Ti-3Al-8V-6Cr-4Mo-4Zr (Beta C), subjected to FSP with two different tool rotation rates at a constant traverse speed were analyzed in different microstructural conditions. Fully retained β grain structures with grain sizes in the range 4 to 7 µm in Ti-185 and 10 to 15 µm in Beta C were obtained in the as-FSP condition. Post-FSP duplex aging treatment of the low heat input condition resulted in better tensile properties compared to those of high heat input condition, attributable to high number density of nucleation sites generated during FSP. Transmission electron microscopy observations of high-strength Ti-185 sample revealed fine α platelets of length in the range 50 to 130 nm and an aspect ratio in the range 3 to 10, providing significant strengthening contribution. Approximate nose temperatures and nose time calculations of the continuous cooling transformation of β to α were made for various commercial Ti alloys to assess the potential of achieving ultra-high strength levels via FSP.

Biaxial deformation behavior of friction stir processed TRIP steel sheets

In this study, effects of Friction Stir Processing (FSP) on the biaxial deformation behavior of 1.95 mm thick TRIP 780 steel sheets were investigated. FSP induced large plastic shear strains imposed at elevated temperature of about 945°C have drastically changed both microstructure and flow behavior of the steel. For these reason, after the FSP, significant changes in the microstructural and mechanical properties were obtained. After FSP, initial microstructure of the TRIP-steel transformed into a microstructure that mainly dominated by martensite grains. This transformation resulted with nearly two-fold hardness increase in stir zone. Similarly, lath martensite dominated microstructure elevated the FSPed condition into an ultra-high strength level with expense of room temperature ductility. After FSP, yield strength and UTS increased from 415MPa and 829 MPa to about 1280 MPa and 1475 MPa. Uniform elongation and elongation to failure decreased from 23% and 11% to 34% and 22% respectively. In accordance to decreased ductility, Erichsen index (EI) of the steel decreased from 9.16 mm to 4.90 mm under biaxial stretching conditions In contrast to strength enhancement punch force at EI of TRIP-780 also decreased from 80.6 kN to 45.4 kN respectively. This simultaneous decrease in both Ei and FEi attributed to increase in cracking tendency of the FSP induced microstructure.

Improving the ductility of ultrahigh-strength medium Mn steels via introducing pre-existed austenite acting as a “reservoir” for Mn atoms

We propose a novel “Mn preservation-cold deformation-intercritical annealing” treatment for medium Mn steels. The process produces a large fraction of retained austenite with an enhanced stability, which increases the ductility by ~40% compared to the conventional single-step intercritical annealing in a model 10Mn steel, without sacrificing the ultrahigh strength level.

Correlation Between Structure and Properties of Low-Carbon Cu-Ni-Mo-Ti-Nb Ultrahigh-Strength Steel

Low-carbon Cu-Ni-Mo-Ti-Nb steel having more than 1700 MPa tensile strength and good low-temperature impact toughness was successfully designed and processed for automobile, defence and structural applications. The steel was thermomechanical controlled processed at a different finish rolling temperatures (750-850 °C) and cooled at two different conditions: air cooling and water quenching. Evolutions of microstructure and precipitates were thoroughly characterised to understand the effect of different factors, such as steel composition, finish rolling temperature and cooling rate on the mechanical properties. The results indicate that the steel processed at 750 °C finish rolling temperature (FRT) followed by water quenching has exhibited superior tensile properties with a yield strength of 1034-1449 MPa, the tensile strength of 1598-1726 MPa and total elongation of 10-13%. Such an ultrahigh-strength level is primarily attributed to the combined effect of lower bainite dominated microstructure along with precipitation hardening. The satisfactory low-temperature toughness is promoted by low finish rolling temperature and higher amount of misorientation offered by the boundaries within the lower bainite.

Production methods for reliable construction of ultra-high-performance concrete (UHPC) structures

Mix design procedures were developed around the core strategy of maximizing the packing and ensuring distributed size distribution of the particulate matter in UHPC for achieving ultra-high strength levels and desired fresh mix workability using locally available materials and concrete-making facilities. The linear density packing model and the continuously graded particle packing model provided the theoretical basis for proportioning the UHPC mixtures. Criteria were devised for selection of local materials to be used in UHPC structures. Experimental investigations were conducted in order to refine and optimize the UHPC mix proportions, yielding the targeted compressive strength of 200 MPa (30 ksi). The final UHPC mix design developed in the study was used for pilot-scale production of a large 1 m × 1 m × 1 m (3.3 ft × 3.3 ft × 3.3 ft) reinforced concrete block, with UHPC batched in a ready-mixed concrete plant and mixed/transported using a conventional concrete truck (transit mixer).

Effect of Carbon Content on Ti Inclusion Precipitated in Tire Cord Steel

The precipitation of TiN inclusion during solidification of different carbon content of 0.72%, 0.82% and 0.95% in tire cord steel is thermodynamically studied respectively. The results show that the carbon content has obvious effect on TiN inclusion precipitated in tire cord steel of different strength levels. With the carbon content of tire cord steel increasing, the temperature before solidifying reduced gradually and the required activity product of titanium and nitrogen for TiN inclusion precipitation also declined gradually. With the same condition of initial Ti and N content in liquid steel, the size of TiN inclusion precipitated in tire cord steel of higher carbon content is bigger than that of lower carbon content. In order to control the harmful effects on processability of TiN inclusion precipitated in hypereutectoid tire cord steel of the ultra high strength level, the measures of smelting process must be taken to further reduce the titanium and nitrogen content in liquid steel.

Bulk nanoscale materials in steel products

Although a number of nanoscale metallic materials exhibit interesting mechanical properties the fabrication paths are often complex and difficult to apply to bulk structural materials. However a number of steels which exhibit combinations of plasticity and phase transitions can be deformed to produce ultra high strength levels in the range 1 to 3 GPa. The resultant high stored energy and complex microstructures allow new nanoscale structures to be produced by combinations of recovery and recrystallisation.The resultant structures exhibit totally new combinations of strength and ductility to be achieved. In specific cases this also enables both the nature of the grain boundary structure and the spatial variation in structure to be controlled. In this presentation both the detailed microstructural features and their relation to the strength, work-hardening capacity and ductility will be discussed for a number of martensitic and austenitic steels.

The effect of severe marforming on shape memory characteristics of a Ti-rich NiTi alloy processed using equal channel angular extrusion

A Ti-49.8 at. pct Ni alloy was severely deformed at three different temperatures using equal-channel angular extrusion (ECAE). Three deformation temperatures—room temperature (below the martensite finish temperature), 50 °C (below the austenite start temperature), and 150 °C (above the austenite finish temperature)—were selected such that the initial deforming phase (B2 austenite or B19’ martensite) and the initial governing deformation mechanism (martensite reorientation, stress-induced martensitic transformation, or dislocation slip in martensite) would be different. The X-ray analysis results revealed that all processed samples mostly contained a deformed martensitic phase, regardless of the initial deforming phase and the deformation mechanism. Although the martensite start temperature did not change, the austenite start temperature decreased significantly in all deformation conditions, probably because of the effect of the internal stress field caused by the deformed microstructure. All deformation conditions led to an increase in the strength levels and some deterioration of shape-memory characteristics. However, a subsequent low-temperature annealing treatment significantly improved pseudoelastic strain levels while preserving the ultrahigh strength levels. The sample deformed at room temperature followed by the low-temperature annealing resulted in the most promising strength and shape-memory characteristics under compression, such that a 5.3 pct shape-memory strain at a 2200 MPa strength level and a 3.3 pct pseudoelastic strain at a 1900 MPa strength level were achieved. The differences between the strength levels and the shape-memory characteristics after severe deformation at different temperatures were attributed to the different amounts of plastic deformation and the resulting deformation textures, since at each deformation temperature the deformation mechanism was different. It is concluded that the severe marforming using ECAE could easily improve strength levels of NiTi alloys while preserving the shape-memory and pseudoelasticity (PE) characteristics and, thus, improve the thermomechanical fatigue behavior. However, lower deformation temperatures are necessary to hinder formation of macroshear bands, and ECAE angles larger than 90 deg should be used to reduce the amount of strain applied in one pass.

Influence of Thermal and Mechanical Processing on Room Temperature Mechanical Properties of Nickel Free High Nitrogen Austenitic Stainless Steels.

Nickel free high nitrogen austenitic stainless steels with a base composition around 18%Cr–18%Mn and varying in C and N contents and processing conditions were evaluated for their mechanical behaviour. A range of mechanical properties from high strengths to ultra high strength levels could be obtained. The steels develop ultra high strength levels in the as-hot worked condition, hot work1cold work condition and solution treated1cold worked conditions. They show high strength levels with very good ductility and impact toughness levels in the solution annealed condition. The Hall–Petch parameters and DBTT obtained in the solution annealed condition were found to be influenced by the carbon content of the steel. The work hardening behaviour of the steels have been examined at various processing conditions. Nitrogen steels with high carbon content show good mechanical properties in the solution treated and subsequently cold rolled conditions. In the as-formed condition, the high carbon steels show inferior ductility due to grain boundary lamellar nitride precipitation.

150kgf/mm<SUP>2</SUP>級中炭素鋼の低温焼もどし後の靱性におよぼす粒界りん偏析およびボロン添加の効果

Effects of phosphorus content and boron addition on impact toughness of medium carbon steels with an ultra-high strength level over 150 kgf/mm2 have been investigated. The toughness is deteriorated by two independent fracture modes, that is, intergranular fracture and Tempered Martensite Embrittlement (TME).Intergranular fracture is greatly enhanced with the increase in phosphorus content of steel, whereas the boron addition is effective in reducing it and increases toughness over the whole range of tempering temperatures below 400°C. However, phosphorus content and boron addition have no significant effect on TME.Auger Electron Spectroscopic (AES) observation reveals that boron addition reduces the intergranular fracture by reducing the phosphorus segregation at the grain boundaries. The estimation by diffusion calculation and McLean's grain boundary equilibrium segregation model shows that boron segregates to grain boundaries much faster than phosphorus does at austenitizing temperatures and reduces the boundary energy for phosphorus segregation.

The effect of loading mode on hydrogen embrittlement

Hydrogen embrittlement is shown to occur very easily in notched-round bars under opening modeI (tension) but not under antiplane shear modeIII (torsion). The stress tensor invariants under modeI,II, andIII loadings and how these affect interstitial diffusion are discussed. It is suggested that long range diffusion of hydrogen down orthogonal trajectories to the vicinity of the crack tip, which can occur under modeI but not modeIII, is a key part of any hydrogen embrittlement mechanism. This premise was evaluated with AISI 4340 steel heat treated to ultrahigh strength levels. It was found that an initial modeI stress intensity level of 17,000 psi-in.1/2 produced failure in several minutes. ModeIII stress intensity levels three times this produced no crack initiation in 300 min. Further analysis of the time-dependent hydrogen concentrating effect utilized a stress wave emission technique. This produced plausible critical hydrogen concentrations even though the present elastic analysis is a first order approximation of the stress field.