Bainitic Microstructure Research Articles

Comparison of Novel 1 GPa Low‐Carbon, Low‐Alloyed Steel Produced with Simulated and Laboratory‐Scale Thermomechanical Controlled Processes

A laboratory‐scale hot‐rolled Ti–Mo–V–Nb steel with 1 GPa tensile strength is produced, and its microstructure and tensile properties are characterized using advanced analysis techniques and uniaxial tensile testing. A Gleeble 3800 thermomechanical simulator is used to determine a process window for the thermomechanical controlled processing (TMCP) procedure. Although the simulated TMCP specimens are fully ferritic at coiling temperatures (CT) of 590 and 630 °C, the bainitic and mixed (bainitic + ferritic) microstructure is formed in the hot‐rolled steels. The variation in the microstructure causes variations in the dislocation density through the sheet thickness, which significantly reduces the steel's ductility properties, whereas a 16% elongation is achieved with the fully bainitic microstructure. Another significant difference between the simulated TMCP and hot‐rolled specimens is the precipitation behavior. No nanosized interphase‐precipitated (IP) carbides are formed in the hot‐rolled steel during the austenite‐to‐ferrite phase transformation, although the formation of the nanosized spherical IPs is observed within the polygonal ferrite grains of the simulated TMCP specimens at the CT of 630 °C. Relatively coarse (5–20 nm) spherical (V,Mo,Ti,Nb)C carbides do not strongly affect the tensile properties of the hot‐rolled Ti–Mo–V–Nb steel. The results show that the dislocation and grain boundary strengthening mainly contribute to the strength properties of this steel.

Unravelling the impact of Al-Ti content on the microstructure-toughness relationship in the coarse-grained heated-affected zone of super-high-strength pipeline steels

This study aims to investigate the role of Al-Ti addition on particle modification in super-high-strength pipeline steels, and its following effect on microstructural evolution and impact toughness in the coarse-grained heated-affected zone (CGHAZ) is also explored. The steel plates with low and high Al-Ti content (0.009 wt-% Al + 0.006 wt-% Ti vs. 0.033 wt-% Al + 0.025 wt-% Ti), respectively, are subjected to gas metal arc welding with 6–7 kJ/cm heat input. The results reveal that the second-phase particles including micron-scale inclusion of Al-Mg oxide covered with CaS and nano-scale precipitate of titanium nitride are obtained in low Al-Ti steel, which are modified to the Al-Mg-Ti oxide core surrounded by CaS and TiN precipitate in high Al-Ti steel. The nano-scale TiN precipitate and micron-scale inclusion can restrict the austenite grain growth at high temperatures and induce acicular ferrite nucleation at medium temperatures, respectively, leading to the fine-grained mixed microstructure of bainite and acicular ferrite in CGHAZ of both steels. Because of the variant selection during the decomposition of austenite to bainite and acicular ferrite, those fine-grain structures with amounts of high-angle grain boundaries exhibit excellent CGHAZ toughness in both specimens. Compared with low Al-Ti steel, more precipitates induce smaller austenite grains, and the inclusions containing titanium increase the ability of inclusion to promote acicular ferrite nucleation in CGHAZ of high Al-Ti steel. Both of them resulted in smaller crystallographic grain and toughness improvement in CGHAZ of high Al-Ti steel than that in low Al-Ti steel.

Effect of Filler Wire Composition on Weld Metal Microstructure and Mechanical Properties in X80 Steel Laser Welds.

Laser welding was performed using different filler wires, ER70S steel, commercially pure iron, and pure nickel filler, in the context of welding X80 pipeline steel to assess the microstructure and mechanical properties of the weld metal. Introducing an ER70S wire promoted acicular ferrite formation in the fusion zone, compared to a bainitic microstructure in an autogenous laser weld. The use of pure iron wire was considered as a potential strategy for reducing hardenability, as it led to the dilution of alloying elements in the fusion zone, increasing ferrite content and reducing weld metal hardness to a level compliant with API pipeline standards. The addition of pure nickel wire was used to reveal the degree of weld metal mixing imposed by the laser (thus providing an unambiguous tracer element) when it is combined with filler material dilution in the fusion zone, revealing that the upper region contained 38% wire material and the lower region only 12%. This accounts for the differences observed between the upper versus lower portions of the weld metal when other wires are used, and the use of hardness mapping and micro-indentation demonstrates the correlation between the variations in mechanical properties and microstructural differences introduced by incomplete mixing of the filler wire elements.

Effect of Mo on the Phase Transformation, Microstructure, and Hardness of Q500qENH Weathering Steel of Bridge Construction during Continuous Cooling

This article investigates the precipitation behavior of Mo element in the continuous cooling phase transformation process of weathering steel of bridge construction and its influence on microstructure and hardness. The results demonstrate that the addition of Mo leads to the narrowing of the phase transformation interval of proeutectoid ferrite and pearlite, the expansion of the bainite transformation zone, a delayed bainite phase transformation process, and the refinement of bainite microstructure. Moreover, theoretical calculations of bainite phase transformation kinetics reveal that the average activation energy for bainite transformation in Mo‐free and Mo‐containing steels is 94.8 and 134.0 kJ mol−1, respectively. The new grains of Mo‐free steel mainly grow in a 2D mode, while Mo containing steel mainly grows in a 1D mode, which confirms the retarding effect of Mo on bainite grain growth. Additionally, Mo significantly increases the number density and refines the size of precipitated phases in weathering steel of bridge construction. Within the cooling rate range of 0.1–30 °C s−1, the hardness of weathering steel of bridge construction is enhanced by Mo, with the magnitude of hardness increment decreasing as the cooling rate increases. This behavior is primarily governed by the combined influence of phase transformation strengthening and precipitation strengthening.

An Investigation of the Effects of Flame Straightening Heating on the High-Strength Structural Steels Microstructure

AbstractIn this research, the effects of heating cycles that are performed correctly and excessively—well above the limitations given by the steel manufacturer—on flame straightening are assessed and analyzed. The paper focuses on the microstructural changes caused by the overheating during the flame straightening process. To create geometrical changes in a metal construction, a tiny section of the element or structure is heated to the straightening temperature during the flame straightening process. Strenx® 960 is typically produced through a combination of thermomechanical rolling and quenching and tempering, resulting in a fine-grained martensitic or bainitic microstructure. These processes are designed to achieve a balance of high strength and good toughness. The microstructural changes associated with different working conditions in the case of welding include HAZ softening and grain growth. The study aimed to highlight the influence of flame hardening on the microstructure of the material.

Influencing TRIP threshold and variant pairing through minor cold and cryo-rolling in bainitic steel

An attempt has been made to compare the effects of small reduction cold and cryo-rolling on the microstructural evolution and stability of retained austenite in a 3rd generation advanced high-strength bainitic steel. The steel was heat treated and isothermally held at 300 °C for 2 h above Ms temperature to obtain bainitic microstructure. The heat-treated steel was subsequently subjected to cold-rolling and cryo-rolling (10 % reduction in thickness). The substructure of bainite consists of packets, blocks, and sub-blocks. Both the crystallite size and micro-strain of bainitic ferrite shows a decreasing trend after rolling compared to the undeformed condition. However, cryo-rolling induces higher micro-strain in retained austenite, which enhances its mechanical stability requiring higher stress to trigger the transformation induced plasticity (TRIP) effect, thus contributing to increased overall strength. Crystallographic variant analysis showed an increase in the boundary density and frequency of specific V1-V3(V5) (high angle block boundaries) and V1-V4 (sub-block boundaries) variant pairs after cryo-rolling following the K-S orientation relationship. While the heat-treated, undeformed state exhibited all the variants without preference for any particular variant pair. The results suggest that the mechanical properties of the steel after minor cryo-rolling are significantly influenced by crystallographic variant pairing and microstrain. Additionally, minor cryo-rolling proved to be superior to cold rolling in bainitic steel as it increased the threshold strain required for the TRIP effect during rolling.

Hydrogen Embrittlement Behavior of API X70 Linepipe Steel under Ex Situ and In Situ Hydrogen Charging.

This study investigates the hydrogen embrittlement behavior of API X70 linepipe steel. The microstructure was primarily composed of a dislocation-rich bainitic microstructure and polygonal ferrite. Slow strain-rate tests (SSRTs) were performed under both ex situ and in situ electrochemical hydrogen charging conditions to examine the difference between hydrogen diffusion and trapping behaviors. The ex situ SSRTs showed almost the same tensile properties as air and a limited brittle fracture confined to near the surface. In contrast, the in situ SSRTs showed an abrupt failure after the maximum tensile load, leading to a brittle fracture across the entire fracture surface with stress-oriented hydrogen-induced cracking (SOHIC). The crack trace analysis results indicated that SOHIC propagation paths were influenced by localized hydrogen accumulation due to high-stress fields. As a result, the dominant hydrogen embrittlement mechanisms, such as hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE), changed. These findings provide critical insights into the microstructural factors affecting hydrogen embrittlement, which are essential for the design of hydrogen-resistant steels in hydrogen infrastructure applications.

Dissimilar MIG Welding Optimization of C20 and SUS201 by Taguchi Method

This study looks at how welding intensity, speed, voltage, and stick-out affect the structural and mechanical characteristics of metal inert gas (MIG) welding on SUS 201 stainless steel and C20 steel. The Taguchi method is used to optimize the study’s experiment findings. The results show that the welding current has a more significant effect on the tensile test than the welding voltage, stick-out, and welding speed. Welding voltage has the lowest influence. In addition to the base metals’ ferrite, pearlite, and austenite phases, the weld bead area contains martensite and bainite microstructures. The optimal parameters for the ultimate tensile strength (UTS), yield strength, and elongation values are a 110 amp welding current, 15 V of voltage, a 500 mm.min−1 welding speed, and a 10 mm stick-out. The confirmed UTS, yield strength, and elongation values are 452.78 MPa, 374.65 MPa, and 38.55%, respectively, comparable with the expected value derived using the Taguchi method. In the flexural test, the welding current is the most critical element affecting flexural strength. A welding current of 110 amp, an arc voltage of 15 V, a welding speed of 500 mm.min−1, and a stick-out of 12 mm are the ideal values for flexural strength. The flexural strength, confirmed at 1756.78 MPa, is more than that of the other samples. The study’s conclusions can offer more details regarding the dissimilar welding industry.

Formation of Bainite in a Low‐Carbon Steel at Slow Cooling Rate – Experimental Observations and Thermodynamic Validation

Bainitic microstructures in high‐strength steels are obtained either by continuous cooling or isothermal holding. Both scenarios necessitate faster cooling to keep the parent austenite phase untransformed till the bainite‐start temperature. The present study reports the development of bainitic microstructure in a low‐carbon steel with minimal alloying additions, under continuous cooling at very slow rates, similar to furnace cooling. For understanding the related transformation pathways, samples from the forged‐steel ingot are austenitized and cooled at different rates, viz. water quenching, air cooling, and furnace cooling. Microstructural characterization reveals development of acicular microstructures in all samples including the forged one, with gross absence of carbides. X‐ray diffraction confirms the ferritic nature of acicular plates and also indicated retained austenite present in some samples, the content of which could be correlated to the extent of bainitic transformation. Thermodynamic calculations together with microstructural observations (e.g., ferrite plate size) and hardness data established the development of fully martensitic microstructure on water quenching, while that of a mixed microstructure comprising predominantly of bainite in the forged, air cooled, and furnace‐cooled condition. The aforementioned findings could have wider implications in developing fully bainitic microstructures in large components, where uniform rapid cooling is not practically feasible.

A variational framework for Cahn–Hilliard-type diffusion coupled with Allen–Cahn-type multi-phase transformations in elastic and dissipative solids

This article presents a variational framework for coupled chemo-mechanical solids undergoing irreversible micro-structural changes at infinitesimal strains. The coupled problem is characterised by phenomena such as phase transitions, micro-structure coarsening and swelling. It is an extension of our previous work on variational inelasticity for a conserved chemo-mechanical setting to a unified conserved and non-conserved setting which include multi-phase transformations. The variational framework, again governed by continuous-time, discrete-time and discrete-space–time incremental variational principles, is outlined for coupled diffusion-phase transformation phenomena in elastic and dissipative solids. For the sake of simplicity, focus is restricted to isothermal conditions. It is shown that the governing macro- and micro-balance equations of the coupled problem appear as Euler equations of these minimisation and saddle point principles. In contrast to our previous work, extended variational principles (with the gradient of the chemical potential and phase fractions) are constructed that account for diffusion-phase transformation coupling. This is achieved by Legendre transformations. Note that the local–global solution strategy is still preserved and the resulting system of symmetric non-linear algebraic equations are solved by Newton–Raphson-type iterative methods. The applicability of the proposed framework is demonstrated by numerical simulations that qualitatively characterise lower bainitic micro-structure.

Automated Workflow for Phase‐Field Simulations: Unveiling the Impact of Heat‐Treatment Parameters on Bainitic Microstructure in Steel

Bainitic steels are extensively utilized across various sectors, such as the automotive and railway industries, owing to their impressive mechanical properties, including strength, hardness, and fatigue resistance. However, the pursuit of achieving the desired optimal mechanical properties presents considerable challenges due to the intricate bainitic microstructures consisting of multiple phases. To tackle these challenges, an automated workflow is used for extracting 2D and 3D microstructural features. The proposed method allows for a detailed examination of the correlations between microstructure characteristics and the processing parameters, specifically the holding temperature during transformation. In these findings, it is revealed that as the holding temperature decreases, there is a notable reduction in microstructural element size and carbon partitioning. Some of the observations are microstructural features such as area, perimeter, and thickness of the bainitic ferrite grains under two different holding temperatures. Phase‐field simulations results show that the microstructures at lower holding temperatures have finer grains. The distributions of grain areas and perimeters are uniform, with smaller grains dominating at low and high isothermal holding temperatures. While the grain thickness measurements from simulations and experiments at high temperature are qualitatively aligned, data from low temperatures show discrepancies.

Effect of initial microstructure on microstructure and properties of bainitic steels achieved by rapid heat treatment

Rapid heat treatment is essential in manufacturing advanced high-strength steels. The initial microstructure significantly influences the steel's microstructure and properties after rapid heat treatment. The present study investigated how the initial microstructure affected phase transformation, microstructure, and properties of bainite in high-carbon low-alloy steel during rapid heat treatment. Under rapid heating, ultrafine bainite (UB) sample with homogeneous composition exhibits the lowest austenitization temperature, while globular pearlite (GP) sample with poor composition homogeneity has a higher austenitization temperature. After rapid heat treatment, the compositional distribution of the initial microstructure was partially retained in the final sample. The increase of alloying elements inhibits the nucleation of bainitic ferrite, making it easier for bainitic ferrite to grow at low-alloy positions. Finally, a large amount of retained austenite is obtained in the alloying element-rich area. The difference in microstructure results in samples with different initial microstructures having similar strengths but differing plasticities. In samples with an initial lamellar pearlite (LP) microstructure, there was a substantial amount of thicker, filmy retained austenite, with an average thickness of 96.8 nm. This led to a small increase in Kernel Average Misorientation (KAM) with strain during the tensile process. This improvement in coordinated deformation during straining indicates that the sample has achieved excellent performance, with a tensile strength of 1483 MPa and a uniform elongation of 23 %. When the initial microstructure was GP, the sample contained a large amount of crumbly retained austenite with a low aspect ratio. This structure poorly coordinated deformation with higher strain, resulting in numerous microcracks.

Effects of Silicon and Aluminum Alloying on Phase Transformation and Microstructure Evolution in Fe–0.2C–2.5Mn Steel: Insights from Continuous–Cooling–Transformation and Time–Temperature–Transformation Diagrams

The impact of silicon and aluminum on phase transformation behavior, particularly bainite, and microstructure evolution in Fe–0.2C–2.5Mn steel are presented. Continuous–cooling–transformation (CCT) and time–temperature–transformation (TTT) diagrams are determined experimentally. An aluminum extended empirical formula is introduced to estimate the martensite start temperature (Ms) with a thorough assessment of existing formulae. Results show that aluminum significantly increases Ms and has a stronger influence on promoting ferritic microstructures than silicon. During continuous cooling, alongside bainite, formation of Widmanstätten structures is induced in aluminum‐alloyed steel at higher cooling rates due to increased prior austenite grain size. Silicon decelerates bainite transformation kinetics by enhancing austenite's chemical stability through carbon enrichment via preventing carbide precipitation and by strengthening austenite against displacive phase transformation via solid solution hardening. Although aluminum has similar effects, incubation time is shortened during isothermal treatment because of the increased driving force, which overcompensates for the retardation effects. A finer carbide‐free bainitic microstructure is achieved in aluminum‐alloyed steel with more pronounced film‐like retained austenite (RA) formation and superior carbon enrichment, improving RA stability and suppressing martensite–austenite island formation. Finally, with the proposed formula, an accurate approximation to experimental Ms is accomplished.

Effect of prior microstructure on carbide precipitation in a Cr-Mo-V pressure vessel steel at 650 °C

Cr-Mo-V steels are key materials used for constructing thick reactor pressure vessels (RPV) owing to an impressive combination of their mechanical properties and weldability. Conventionally, these steels are subjected to a quality heat treatment that comprises austenitization, quenching and tempering. Owing to its large thickness, an RPV experiences widely different cooling rates across its thickness during quenching, leading to considerable variation in the microstructure from the surface to the centre of the vessel. In this study, a Cr-Mo-V steel was austenitized at 950 °C for 1 h and cooled at two rates (water quenching and 10 °C/min), representative of the cooling rates at the surface and centre of a typical RPV, to study the effect of prior microstructure on the carbide precipitation during tempering of the steel at 650 °C. Electron microscopy of the as-cooled samples revealed the presence of martensitic microstructure in water quenched sample and predominantly granular bainitic microstructure in slow cooled sample. Synchrotron studies revealed the presence of retained austenite containing ∼0.8% carbon in the slow-cooled sample. Tempering of the steel at 650 °C for 200 h precipitated M7C3 and MC (VC) carbides in the water-quenched sample, while the slow cooled samples precipitated M23C6 as an additional constituent. This difference in the carbide precipitation in the two samples has been attributed to the excess carbon in the martensite/austenite (M/A) regions in slow cooled samples and explained on the basis of carbide precipitation sequence predicted using Thermo-kinetic simulations. This analysis has shown that M23C6 and M7C3 carbides in the slow cooled sample precipitate simultaneously as critical radii of their nucleation as well as their incubation time become comparable when the matrix contains more than 0.25% C. The present study has demonstrated that the microstructure prior to tempering may significantly affect the precipitation sequence of carbides during tempering.

Study of bainite and martensite tempering in a medium C high Si steel. microstructural disparities and equilibrium convergence

This study investigates the tempering behavior of bainite and martensite in a medium carbon, high silicon steel, with a focus on the microstructural evolution and the attainment of equilibrium, over a temperature range of 200–650 °C. The dissimilarities between the characteristics of the two initial microstructures, both comprising a C-saturated tetragonal ferrite matrix and retained austenite, are reflected in the differences observed in their evolution towards equilibrium as the tempering temperature increases. Therefore, while retained austenite plays a pivotal role in the bainitic microstructure, in the martensitic microstructure it is the ferritic matrix, which is highly dislocated and enriched in carbon, that plays a determinant role. The findings demonstrate that while both bainite and martensite can converge towards the same equilibrium state upon high-temperature tempering (600–650 °C), the pathway to this convergence is markedly different, with bainite exhibiting a slower transformation rate. This path to equilibrium has been characterised by means of high-resolution dilatometry, scanning electron microscopy and detailed X-ray diffraction analysis. This has provided a dynamic and detailed picture of the most relevant microstructural changes and tempering mechanisms in high silicon steels. It has also provided a foundation for tailoring heat treatment processes to optimise the mechanical properties of advanced bainitic and martensitic steels.

Additive manufacturing of high-strength low-alloy AISI 4340 steel with an optimal strength-ductility-toughness trade-off

Additive manufacturing (AM) has revolutionised steel part fabrication, yet not all steels are amenable to its unique solidification features, characterised by cyclic and rapid heating/cooling, and directional solidification. These conditions often result in challenges such as columnar grain formation, microstructural heterogeneity, and consequently, inferior mechanical performance, brittleness and severe anisotropy in particular. Recent studies have adopted inoculation or post-fabrication treatments to address this issue, but often entailing extra cost and processing time. This study aims to verify that some steels such as AISI 4340 steel are inherently compatible to AM, producing components that are innately robust and ready for use in the as-built state. The medium carbon content and low alloying element concentration enable AM processing to produce a uniform and refined bainite microstructure with minimal elemental segregation, avoiding the formation of unstable retained austenite. The high AM-processability of this steel is demonstrated by achieving high densification (>99.9 %) across a broad processing window, which allows precise microstructural control via proper tunning the processing parameter modifications, inducing a transition from upper bainite to lower bainite dominance, to tailor mechanical properties for specific applications. The as AM-fabricated AISI 4340 steel exhibits a good combination of strength, ductility, and toughness, manifested by a yield strength range from 1240 to 1370 MPa, an ultimate tensile strength from 1360 to 1740 MPa, an elongation from 7 % to 14 %, and an impact toughness range of 11–44 J. The mechanical properties of the AM-fabricated 4340 steel are comparable to those of the wrought counterpart and superior to the majority of other AM-fabricated steels. This research reveals the high potential of AM to process high-strength low-alloy steels.

Laser-directed energy deposition of low-carbon, low-temperature ultra-fine bainitic multi-physical modeling, microstructure and performance studies

An innovative iron-based alloy powder characterized by reduced carbon content and elevated Mn content was developed, leading to the fabrication of iron-based ultrafine bainitic steel with exceptional mechanical properties utilizing laser-directed energy deposition (L-DED). The layers' morphology, microstructure, phase composition, and elemental distribution were analyzed, indicating that the melting depth of the layer was deep and ultra-fine dendrite formed in the top region. A multi-physical powder and melt pool simulation was conducted to analyze the melt flow and pool solidification characteristics, showing that the strong downward flow inside the molten pool caused by the powder impingement increased the melting depth. The cooling rate G × R, where G was the temperature gradient and R was the solidification rate, was higher at the top pool boundary, facilitating the ultra-fine dendrite formation at this location. Following subsequent low-temperature isothermal heat treatment, a refined bainitic microstructure emerged within the deposited layer. The bainitic plates in the 573 K isothermally transformed bainite were interspersed with film-retained austenite (RA). Conversely, the bainitic plates formed at a lower temperature (523 K) exhibited a coalesced morphology stemming from the diminished stability of super-cooled austenite. The bainite plates underwent growth and coalescence, ultimately giving rise to the bainitic microstructure. The 523 K transformed coatings demonstrated superior hardness (509.2 HV), strength (1435 MPa), and elongation (11.5 %) compared to the 573 K transformed coatings (456.5 HV, 1241 MPa, 10.1 %). This research holds significant implications for advancing low-carbon bainitic materials with remarkable comprehensive mechanical properties in additive manufacturing applications.

Role of M-A constituents in bainitic microstructure on crack propagation behavior in the ICCGHAZ of HSLA steels for offshore applications

The objective of the study is to understand the effect of martensite-austenite (MA) constituents along with the neighbouring bainitic matrix and its effect on low-temperature impact toughness in the thermo-mechanically simulated inter-critically reheated coarse-grained heat-affected zone (ICCGHAZ) of offshore High Strength Low Alloy (HSLA) steels. ICCGHAZ was simulated using a Gleeble 3500 thermomechanical simulator between the inter-critical temperature near Ac1 (lower ICT) and at a temperature 50 °C higher (upper ICT) for two different steels (steel A and steel B) with predominant microstructures composed of granular bainite (GB) and bainitic ferrite (BF), respectively. MA constituents were observed in the form of necklaces along prior austenitic grain boundaries (PAGB) of simulated ICCGHAZ for both steel A and steel B. Steel A combrised of discrete island MA constituents inside the GB grains and steel B consists of lath shaped MA constituents inside BF grains. Fractographical analysis showed that secondary cracks were initiated at the MA constituents along PAGB and propagated linearly into the GB/BF grains in the ICCGHAZ. The cracks initiated at the bulky MA at grain boundaries of steel A moved towards the pre-cracked or de-bonded discrete MA inside the grains and propagated through the interlinking of microcracks. In contrast, for steel B a notable percentage of cracks were initiated through the necklace-like MA at the grain boundaries and propagated through the BF lath. Pre-cracking of the slender MA present at the lath boundaries facilitated crack propagation in steel B. Furthermore, increasing the inter-critical temperature improved the impact toughness of steel A with a predominantly GB microstructure, whereas impact toughness improvement was limited in steel B with a BF microstructure.

Novel bainitic Ti alloys designed for additive manufacturing

Novel ternary titanium alloys with a bainitic microstructure have been designed specifically for additive manufacturing (AM). The Ti-xCu-yFe alloys take advantage of constitutional supercooling to suppress the growth of large columnar grains usually associated with AM Ti alloys. In the as-built condition the bainitic microstructure consists of a refined α-phase within a matrix of β-phase. The intermetallic phase, Ti2Cu, forms predominantly on the interface of the α- and β-phase and within the β matrix. The high strength of these materials is attributed to the refined α-phase and Ti2Cu particles as well as significant solid solution hardening. This work demonstrates a viable way to fabricate structural components with unique and excellent properties using low-cost elemental powders with AM.

Developing Ultra‐High Strength Bainitic Steel by Hot Stamping Process and Anisothermal Holding

Hot stamping is a well‐established process in the automotive industry for structural parts to decrease the weight of a vehicle without reducing the safety and stiffness of components. In this work, a bainitic approach during the hot stamping process is studied by anisothermal annealing. The effect of different strategies on the microstructure and mechanical properties of the hot‐stamped parts is investigated. The microstructure is analyzed using light microscopy (LM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) techniques. Tensile and hardness tests are applied to analyze mechanical properties. A microstructure consisting of a carbide‐free bainite matrix containing martensite, fine‐grained ferrite, and retained austenite is obtained. SEM analysis of fracture surfaces from tensile samples reveals that the bainitic microstructure showed mainly a ductile fracture by dimples and voids along with some quasi‐cleavage fracture.