Ferrite Microstructure Research Articles

Influence on Microstructure and Magnetic Performance of Co-doped Mg Ferrites by Low Temperature Sintering

Influence on Microstructure and Magnetic Performance of Co-doped Mg Ferrites by Low Temperature Sintering

Effect of Hydrogen on Tensile Properties and Fracture Behavior of Two High-Strength American Petroleum Institute Linepipe Steels

This study explored the influence of hydrogen on the tensile properties and fracture behavior of high-strength API X70 and X80 linepipe steels with bainitic microstructures under varying hydrogen charging conditions. The X70 steel exhibited a ferritic microstructure with some pearlite, while the X80 steel showed a bainitic microstructure and fine pearlite due to the addition of molybdenum. Slow strain rate tests (SSRTs) were conducted using both electrochemical ex situ and in situ hydrogen charging methods subjected to different current densities. The SSRT results showed that in situ hydrogen-charged SSRT, performed at current densities above 1 A/m2, led to more pronounced hydrogen embrittlement compared to ex situ hydrogen-charged SSRT. This occurred because hydrogen was continuously supplied during deformation, exceeding the critical concentration even in the center regions, leading to quasi-cleavage fractures marked by localized cleavage and tearing ridges. Thermal desorption analysis (TDA) confirmed that a greater amount of hydrogen was trapped at dislocations during in situ hydrogen-charged SSRT, intensifying hydrogen embrittlement, even with a shorter hydrogen charging duration. These findings highlight the importance of selecting appropriate hydrogen charging methods and understanding the hydrogen embrittlement behavior of linepipe steels.

Microstructural investigation of SAF 2507 stainless steel laser cladding developed on AISI 316L substrate

SAF 2507, a superduplex stainless steel, combines mechanical strength with high corrosion resistance. As a powder, it can be used in conjunction with the laser cladding deposition process (LCD), restoring damaged components by erosion and improving their properties, regarding surface hardness and resistance to corrosive environments. In the current study, a SAF 2507 powder was employed to fabricate various multi-pass clads on an austenitic 316 L substrate via laser cladding deposition (LCD). Specimens were later subjected to heat treatment procedures, in order to restore the phase balance ratio. The attained microstructures were observed through light optical and scanning electron microscopy (LOM and SEM), coupled with energy-dispersive spectroscopy (EDS) analysis. Vickers hardness tests were also conducted in order to evaluate the hardness of every clad layer. Finally, X-Ray Diffraction (XRD) was employed in order to ascertain the experimental results, as well as investigate the existence of undesirable phases. While a mostly ferritic microstructure was anticipated after the LCD process, the resulting clads were characterized by a relatively balanced phase ratio, with austenite being the dominant phase instead. After heat-treating, further austenite growth occurred, leading to an overall decrease in hardness, although nitrides that had precipitated during the deposition were no were no longer present.

The influence of laser-induced tempering on the microstructure and mechanical properties of 1500 MPa martensitic steel friction stir welded joints

A comprehensive comparison illustrates the role of laser technology in altering the macro morphology and microstructure of martensitic steel joints produced by friction stir welding (FSW). In this study, laser-induced tempering (LIT) was introduced to the FSW of 1500 MPa martensitic steel. The mechanism by which LIT enhanced the mechanical properties of the FSW joints was elucidated by comparing joints produced with and without LIT. Experimental findings revealed that LIT effectively eradicated tunnel defects under the same FSW parameters. Even with an increase in travelling speed from 60 mm/min to 120 mm/min, flawless welds were achieved under LIT conditions. At 500 rpm and 60 mm/min, the bottom of the stir zone (SZ) in conventional FSW (C-FSW) joint exhibited a ferrite and martensite dual-phase microstructure. In contrast, the SZ of the LIT-FSW joint had a fully martensitic structure. Moreover, LIT resulted in significant variations in the martensitic hierarchical structure within the SZ. LIT reduced ferrite distribution in the dual-phase zone, caused recrystallized ferrite to appear further from the SZ. The stress concentration in the dual-phase zone attributed to joint fracture. The improvement in the mechanical properties of martensitic steel FSW joints was explained by analyzing the correlation between the stress concentration factor and phase fraction. Additionally, the increase in traveling speed narrowed the dual-phase zone, triggered a constraint effect that led to a maximum ultimate tensile strength (UTS) increase from 986 MPa at 60 mm/min joint to 1142 MPa at 120 mm/min joint.

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.

Revisiting the Concept of Property Optimization of Low Temperature Phase Transformed Microstructures of GIGASTEEL Welds

Although the need for automobile lightening continues to increase in preparation for the era of electrical vehicles, the application of steel, which has strengths in price and recycling in the automotive industry, is expected to continue along with the developments of various driving energy sources. Recently, when lightening and stiffness reinforcement are required for the electric vehicles or medium and large SUVs, newly-designed parts using GIGASTEEL with a tensile strength of 980MPa or higher are being developed. However, it is difficult to secure sufficient weld strength and fatigue performances when applying welding consumables that have been applied normally and ultra-high strength welding materials that have been commercialized so far. It has also limitations that are not easy to apply due to excessive manufacturing costs in the automotive industry. In this study, the application of commercial welding consumables by strength grade to GIGASTEEL was reviewed and the properties of the weldments were compared to address all of the above issues. Here, we consider the effects of C, Mn, Si, Cr, Ni, Mo and microalloying to implement interlocked nature with dense and complex microstructures of lath-like bainitic and acicular ferrite that can effectively improve both strength and toughness of GIGASTEEL welds in a more economical way.

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.

Microstructure Refinement via Nucleation of Intragranular Acicular Ferrite Stimulated by Ti-Containing Core-Shell Structured Particles in Low-Carbon Steel.

This study investigated the microstructure, mechanical properties, and nucleation mechanism of acicular ferrite (AF) present in hot-rolled Ti deoxidized steel. In our experiments, the impact toughness of Ti deoxidized steel is significantly increased to 144 J at -20 °C, while those Mn and Al deoxidized steels are only 9 J and 18 J, respectively. Interlocked AF is the primary microstructure of Ti deoxidized steel. The second-phase particles of the core-shell-type structure, in which Ti2O3 is the nucleus and TiO is the outermost shell, act as effective nucleating agents to stimulate AF nucleation. The low lattice disregistry between TiO and AF is the main factor contributing to the production of AF. It is also revealed that Ti2O3 and MnS fulfill the particular orientation relationship, contributing to the formation of an Mn-depleted zone (MDZ) adjacent to MnS, proposed to be one of the possible mechanisms for promoting AF nucleation.

Microstructure, hardness and creep properties for P91 steel after long-term service in a ultra-supercritical power plant

3 P91 pipes enduring 5∼6 × 104 h service had been found in an ultra-supercritical power plant. Hardness tests, microstructure observation(OM, SEM/EBSD, TEM/EDS) and creep rupture tests were performed, and the relationship between hardness, microstructure and creep rupture strength were discussed. Results showed that 1 of the 3 pipes still featured lath martensite, and its mechanical properties complied with relevant criteria. Other 2 pipes were characteristic of a mixture of ferrite and highly recovered martensite. Their hardness and creep rupture strength experienced severe degradation. Laves phase was observed in all 3 pipes. The most distinctive feature of the 2 low hardness pipes was their coarse and intragranularly distributed M23C6. This might be caused by abnormal heat treatments which could lead to the mixed microstructure of martensite and ferrite, and this kind of microstructure would degrade fast during service. Thus the low hardness had nothing to do with the precipitation of Laves phase.

Origin of Serrated Markings on the Hydrogen Related Quasi-cleavage Fracture in Low-carbon Steel with Ferrite Microstructure

Origin of Serrated Markings on the Hydrogen Related Quasi-cleavage Fracture in Low-carbon Steel with Ferrite Microstructure

Ball milling and annealing effect in structural and magnetic properties of copper ferrite by ceramic synthesis

This study aims to better understand the relationship between the microstructure and magnetic properties of copper ferrite, which is an inverse spinel that present a body-centered tetragonal structure associated with a remarkable Jahn–Teller (JT) effect. For this goal, a sample has been synthesized by the ceramic route from a stoichiometric mixture of high-purity CuO and Fe2O3 powders activated mechanically in a high-energy planetary ball mill. This sample was calcined in air furnace at 1000 ºC for 6 h. As synthetized sample with a cubic structure was divided into two parts and one of them was annealed at 650 ºC for 3 h and slowly cooled to room temperature to obtain a pure tetragonal spinel sample. Furthermore, these two samples were subjected to the same processes of severe plastic deformation by milling for up to 70 h and subsequent annealing at temperatures ranging between 300 and 600 ºC to obtain samples with a mixture of phases in different proportions. For the cubic one, there is no change in its structure due to milling, always remaining 100 % cubic, and the evolution in the saturation magnetization was related to the changes in the density of defects present. On the other hand, copper cations are always found in the octahedral sites of spinel with a tetragonal structure. The small decrease in the degree of inversion to 0.95 induced by milling in this sample is capable of breaking the JT distortion, giving rise to the appearance of a cubic phase. This work demonstrates how the structure, microstructure, defects like stacking faults and deformation twins, and degree of inversion of the different phases are related to changes in magnetic properties.

253MA heat-resistant stainless steel by wire arc additive manufacturing: microstructure and mechanical properties

ABSTRACT The 253MA stainless steel fabricated via wire arc additive manufacturing displays a mixed austenite and ferrite microstructure. It features many dislocations, minimal inclusions, and carbides, observed through TEM. Notably, CeO2 spherical inclusions enhance its high–temperature and oxidation resistance. Microhardness test 2 shows average values of 187.9 HV0.2 and 183.9 HV0.2 in transverse and longitudinal directions, respectively. Mechanical properties at room temperature include a yield strength of 388 MPa, a tensile strength of 681 MPa, 18.89% elongation, and 34% area reduction post–fracture. Impact toughness measured 89.7J and 133.7J in transverse and longitudinal directions, respectively. At 600°C, the steel's yield strength and tensile strength drop to 208 MPa and 412 MPa, with elongation increasing to 32.06% and area reduction to 53%. The stress–strain curve at 600°C shows serrated flow, suggesting dynamic strain ageing. SEM analysis indicates ductile fracture with larger dimples at elevated temperatures.

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.

Laser powder bed fusion of 2507 duplex stainless steel: Microstructure, mechanical properties, and corrosion performance

Duplex stainless steels offer a unique combination of advantages derived from both ferritic and austenitic structures, providing excellent mechanical properties and corrosion resistance. Nevertheless, conventional metallurgical techniques such as casting and rolling encounter challenges when it comes to the rapid fabrication of intricate components. In this study, we explore the microstructure, mechanical properties, and corrosion performance of 2507 duplex stainless steel manufactured through laser powder bed fusion (L-PBF). Through meticulous refinement of L-PBF parameters, we successfully achieved a dense sample featuring a hardness exceeding 400 HV, a relative density surpassing 98 %, and a surface roughness below 15 μm. This was obtained using a scanning speed of 700 mm/s and a laser power of 210 W. Due to the high cooling rate of L-PBF process, L-PBFed 2507 exhibits a ferritic microstructure distinct from that of rolled 2507 duplex stainless steel. Additionally, a considerable presence of internal dislocations can be observed in the L-PBFed 2507 material. The 2507 stainless steel exhibited an outstanding yield strength of 1198 MPa and an ultimate tensile strength of 1269 MPa, surpassing the performance of rolled 2507 duplex stainless steel. The corrosion current density was nearly negligible (approximately 10−6 A cm−2), with a passivation window expanding 1.2 V relative to the corrosion potential and a passivation platform below 10−4 A cm−2. These corrosion characteristics were comparable to those observed in rolled 2507 duplex stainless steel. This research demonstrates the suitability of L-PBFed 2507 stainless steel in environments requiring high strength and corrosion resistance, thereby opening up avenues for high-performance components in advanced engineering applications.

Tensile Properties and Fracture Analysis of Duplex (2205) and Super Duplex (2507) Stainless Steels, Produced via Laser Powder Bed Fusion Additive Manufacturing

Additive manufacturing of duplex (DSS) and super duplex stainless steel (SDSS) has been successfully demonstrated using laser powder bed fusion (LPBF) in recent years. Owing to the high cooling rates, as-built LPBF-processed DSS and SDSS exhibit close to 100% ferritic microstructures and require heat treatment at 1000–1300 °C to obtain the desired duplex microstructure. In this work, the mechanical properties of DSS and SDSS processed via LPBF were investigated in three building directions (vertical, horizontal, diagonal) and three processing conditions (as-built, stress-relieved, annealed, and quenched) using uniaxial tensile testing. As-built samples exhibited tensile and yield strength greater than 1000 MPa accompanied by less than 20% elongation at break. In comparison, the water-quenched samples and samples annealed at 1100 °C exhibited elongation at break greater than 34% with yield and tensile strength values less than 950 MPa. Stress relief annealing at 300 °C had a negligible impact on the mechanical properties. Austenite formation upon high-temperature annealing restored the reduced ductility of the as-built samples. The as-built and stress-relieved SDSS showed the highest yield and tensile strength values in the horizontal build direction, reaching up to ≈1400 and ≈1500 MPa (for SDSS), respectively, as compared to the vertical and diagonal directions. Fractographic investigation after tensile testing revealed predominantly a quasi-ductile failure mechanism, showing fine size dimple formation and cleavage facets in the as-built state and a fully ductile fracture in the annealed and quenched conditions. The findings in this study demonstrate the mechanical anisotropy of DSS and SDSS along three different build orientations, 0°, 45°, 90°, and three post-processing conditions.

Nucleation Stages Onset and Intra-granular Ferrite Morphologies in Titanium Free Micro-Alloyed Steel

The aim of this work is focused on nucleation stages with emphasis on the development of intra-granular ferrite morphologies during isothermal austenite transformation in titanium free micro-alloyed steel. Isothermal treatment was carried out in the temperature range 350 to 600οC. These treatments were interrupted at different times between 2 and 1800 s in order to analyze the evolution of the microstructure. Metallographic evaluation was done by using optical and scanning electron microscopy (SEM) enabled determination of the nucleation onset at all treatments and subsequent on the development of intra-granular ferrite of isothermally treated Ti free micro-alloyed steel. The results show that at high temperatures ( 500  C) polygonal intra-granularly nucleated ferrite idiomorphs, combined with grain boundary ferrite and pearlite were produced and followed by an incomplete transformation phenomenon, At intermediate temperatures (450 and 500  C) an interlocked acicular ferrite (AF) microstructure is produced, and at low temperatures (400 and 350  C) the sheave of parallel acicular ferrite plates, similar to bainitic sheaves but intra-granularly nucleated were observed. In addition to sheaf type acicular ferrite, the grain boundary nucleated bainitic sheaves are observed.

Cluster mediated high strength and large ductility in a strip casting micro-alloyed steel

Exhibiting exceptional mechanical properties and formability, high strength low alloy steels characterized by a single ferrite microstructure with finely dispersed nano-precipitates (ferritic HSLA steel) have garnered notable attention in the automotive industry. Nevertheless, to maximally utilize the precipitation hardening effect, these steels necessitate substantial additions of carbide-forming elements, unavoidably narrowing the process window and escalating the cost. Strip casting, featuring a streamlined process chain and high energy efficiency, has emerged as a promising technique for developing ferritic HSLA steels. In this work, leveraging the process characteristics of strip casting, we report that a novel single ferrite microstructure with multi-atomic layered clusters distributed in both interphase-precipitation and random fashions was engineered in a low Nb micro-alloyed ferritic HSLAs via raising the coiling temperature to 650 ℃. The multi-atomic layered clusters play a pivotal role in tailoring dislocation behaviors, facilitating local double cross-slips, contributing to dislocation multiplication and homogeneous distribution. These mechanisms collectively sustain mild work hardening to higher strains, leading to combined strength and ductility increments. In comparison to their cluster-free bainitic counterparts coiled at 480 ℃, the results demonstrate significant mechanical improvements with an increase in ultimate strength (630 MPa to 670 MPa) and a 90 % rise in plasticity (10.3 % to 19.1 %), signifying an alternative pathway for advancing the utilization of strip casting technology in designing and processing novel low-cost, high-performance HSLAs.

Post-Processing Effect on the Corrosion Resistance of Super Duplex Stainless Steel Produced by Laser Powder Bed Fusion.

This study examines the microstructural characteristics and corrosion resistance of super duplex stainless steel (SDSS) produced through laser powder bed fusion (LPBF). The analysis shows that the as-printed samples mainly exhibit a ferritic microstructure, which is due to the fast-cooling rates of the LPBF technique. X-ray and microstructure analyses reveal the presence of minor austenite phases in the ferritic matrix. The process of solution annealing led to a more balanced microstructure. Analyses of corrosion resistance, such as potentiodynamic polarization tests and EIS, indicate that heat treatment has a significant impact on the corrosion behavior of SDSS. Solution annealing and stress relieving at 400 °C for 1 h can improve corrosion resistance by increasing polarization resistance and favorable EIS parameters. However, stress relieving at 550 °C for 5 h may reduce the material's corrosion resistance due to the formation of chromium nitride. Therefore, stress relieving at 400 °C for 1 h is a practical method to significantly enhance the corrosion resistance of LPBF-printed SDSS. This method offers a balance between microstructural integrity and material performance.

Micro‐alloying effects of lanthanum in thermo‐mechanical control process of manganese‐chromium‐molybdenum bainite rail steel

AbstractMicro‐alloying effects of lanthanum in thermo‐mechanical control process of manganese‐chromium‐molybdenum bainite rail steel are investigated through experimental simulation and microstructural characterization. The results show that the deformation strengthening effect is fully exerted by thermo‐mechanical control process in steel containing 0.015 % lanthanum. Finally a kind of multi‐layer bainite ferrite microstructure featured by 3.5 μm blocks, 0.41 μm plates and 108 nm sub‐plates with ultrafine sub‐subunits and 55 nm θ‐M3C inside is achieved, which enhances the strength and toughness synergistically. And the nanoscale refinement mechanism of bainite ferrite plates lies in the formation of massive ultrafine sub‐subunits with the average size of 20 nm ×32 nm. Besides, a large number of twinning martensite with the size of 2 nm to 20 nm and high‐density entangled dislocations can be found on the boundaries of ultrafine sub‐subunits. Further, the density of dislocation is increased by 2.92×1014 cm−2 and its contribution to the strength is calculated to be 18 MPa. Moreover, micro‐alloying effects of lanthanum in thermo‐mechanical control process are explored to be that lanthanum enhances the interaction between bainite ferrite plates and dislocations, strengthens the entanglement of carbon and θ‐M3C with dislocations, and promotes the pinning effect of θ‐M3C on ultrafine subunits.

Effect of Ti/N Ratio on TiN Particles, Microstructures, and Toughness of Heat‐Affected Zone After High Heat Input Welding of the Ca‐Treated Steel Plates

The high heat input welding (HHIW) can greatly enhance the welding efficiency of the steel plates. However, coarse grains and brittle microstructures will be formed in the heat‐affected zone (HAZ) during the HHIW process, which deteriorates the toughness of HAZ. In the present study, the effect of the Ti/N ratio on the size and distribution of titanium nitride (TiN) particles, microstructures, and toughness of HAZ after HHIW of 400 kJ cm−1 is investigated for the Ca‐treated steels with different Ti/N ratios of 1.61, 3.79, and 5.00 for TN16, TN38, and TN50 steels. The TN38 steel has the highest number density of the TiN particles with the sizes between 20 and 25 nm and the highest pinning force of the particles in three steels, resulting the smallest grain sizes in its HAZ. In addition, in three steels, the TN38 steel has the main microstructure of intragranular aciculate ferrites with the highest high‐angle grain boundaries density in its HAZ, thereby obtaining the highest value of low‐temperature impact toughness of the HAZ.