Ferrite Fraction Research Articles

Enhancing strength and toughness in a pearlitic rail steel via multi-alloying and accelerated cooling

The demand for higher speeds and heavier loads has emphasized the necessity to tackle the inherent trade-off between strength and toughness in pearlitic rail steels. Herein, the co-additions of Cr, Ni and Cu and accelerated cooling rate on the microstructure and mechanical properties of a medium carbon pearlitic steel were systematically investigated. In comparison with base alloy, the new steel exhibits smaller prior austenite grain size (PAGS) due to the drag effect of Cu and Ni solutes. The finer PAGS and lower C content increased the volume fraction of proeutectoid ferrite. The finer pearlitic nodule size (PNS) and pearlitic colony size (PCS) were attributed to the small PAGS and greater extent of undercooling due to the combined effect of multi-alloying and accelerated cooling. Additionally, the synergistic effects of multi-alloying and accelerated cooling rate decrease the pearlitic phase transformation temperature, accountable for the finer interlamellar spacing (IS). Compared with the base steel, the yield strength and ultimate tensile strength of the new steel increased from 587 MPa and 1069 MPa to 740MPa and 1178MPa, respectively, with a simultaneous increase of ductility from 12.4% to 14.7%. The strength increments are mainly attributed to the finer IS in the new steel. Meanwhile, the new steel possesses a higher impact toughness compared to the base steel due to the increased volume fraction of proeutectoid ferrite, and finer PNS and IS.

Effect of Zr addition on MnS inclusion characteristics and mechanical properties in medium carbon ferrite-pearlite steel

In this paper, the influence of decreasing Zr content (0.0110wt%, 0.0044wt%) on the microstructure, MnS inclusion characteristics, and mechanical properties of medium carbon ferrite-pearlite steel was studied. The results show that the volume fraction of intragranular ferrite (IGF) in steel increases with a reduction in Zr content. The ductility of the steel is reduced by 1.83%, the impact toughness is increased by 2.5 times, and the yield strength is almost unchanged. Compound MnS inclusions are the main inclusions that induce IGF formation. The proportion of heterogeneous nucleation of MnS inclusions using oxides as nucleation sites is increased, which is the main reason for the increase in the volume fraction of IGF. Furthermore, MnS inclusions change from type Ⅲ to type Ⅰ and Ⅱ, and their distribution is gradually uniform. Thermodynamic analysis reveals that the underlying cause for the morphological transformation of MnS inclusions is the rapid surge in the supersaturation of S element in molten steel. The presence of liquid-phase low-melting inclusions (MnS-Al2O3) promotes the formation of type Ⅰ MnS inclusions. The volume fraction and the dimension perpendicular to the fracture surface of the MnS inclusions are increased by at least 33% and 111%, while their aspect ratio is reduced by a maximum of up to 19%. The change in the volume fraction of MnS inclusions is the primary factor contributing to the decrease of tensile plasticity. The toughness of steel is mainly affected by the aspect ratio and the dimension perpendicular to the fracture surface of MnS inclusions.

조선용 B Gr. 강의 용접부 미세조직과 기계적 성질에 미치는 용접기법의 영향

The microstructurale and mechanical properties of B Gr. steel were evaluated for shipbuilding according to a welding process. The following conclusions were drawn. In tandem submerged arc welding(TSAW), compared to single submerged arc welding(SAW), the grain boundary ferrite was coarsened, and the fraction was higher owing to the high heat input and low flux basicity ; conversely, the volume fraction of acicular ferrite was lower. Fine intragranular acicular ferrite and polygonal ferrite were observed in the weld metal of the electrogas welding (EGW), which had the highest heat input. In the microstructure near the fusion line, as the amount of heat input increased, the prior austenite grain boundary became coarse. Further, both the intergranular grainboundaryferrite and intragranular bainite became coarse, and the heat-affected zone became wider. The hardness of weld metal was independent of the amount of heat input and was demonstrated to be high in the order of EGW, single SAW, and TSAW owing to the microstructure difference caused by the chemical composition of the filler material and the dilution rate of the base metal. Furthermore, the tensile strength of all three welding processes resulted in stable satisfaction with the class society rules.

Achieving excellent properties of laser tailor-welded thin Al-Si coated press-hardened steel joint via controlling dilution rate

In laser tailor welded traditional 25 μm thick Al-Si coated press-hardened steel (PHS), a lot of ferrite was easily formed in the weld, so filling with expensive high-alloys wire is usually necessary. In this work, a new thin Al-Si coated PHS with 18 μm thickness was laser tailor welded, and a cheap low alloy wire was attempted to fill into the weld to via control the dilution rate. At a high dilution rate of 94%, the weld contained a ferrite fraction of 38%, resulting in the fracture occurring in the weld during tension and bending. At a medium dilution rate of 85%, the ferrite fraction in the weld decreased to 18%, obtaining excellent tensile and bending properties. The tensile strength and elongation of the joint reached 1528 MPa and 5.6%, respectively, with fracture being in the base metal (BM). The maximum bending load and displacement were 1291 N and 7.0 mm, respectively, and cracks initiated and propagated in the BM. This is attributed to the small amount of ferrite and stress concentration in the narrow weld. At a low dilution rate of 76%, the ferrite reduced to 6%, and the bending cracks initiated and propagated in the BM. However, the tensile specimen fractured in the weld, which should be related to the relatively low C concentration and the stress concentration at the weld toe due to the weld reinforcements.

Anisotropic sintering behavior of stainless steel 316L printed by binder jetting additive manufacturing

The influence of sintering temperature and printing orientation on the anisotropic sintering shrinkage of binder jetted (BJ) stainless steel 316L (SS316L) was investigated. Near quasi-isotropic shrinkage behavior with very little anisotropy (≤4 %) in the powder bed plane was observed. The out-of-plane anisotropic shrinkage of up to 16 % was attributed to the primitive lines formed perpendicular to the build direction. This follows the power law, and the activation energy is not affected by the anisotropy. For all printing orientations, grain boundary diffusion (25–162 kJ/mol) was the dominant sintering mechanism between 1100 °C and 1300 °C. Above 1300 °C, volume diffusion (490–623 kJ/mol) takes over as the dominating sintering mechanism, leading to significantly higher shrinkage up to 15 %. The anisotropic shrinkage behavior does not affect the microstructure, as similar evolution of grain sizes, ferrite fractions, and bulk densities (up to 96 %) is observed upon sintering.

An Overview on Additive Manufacturing of Duplex Stainless Steels: Microstructure, Mechanical Properties, Corrosion Resistance, Postheat Treatment, and Future Perspectives

Additive manufacturing (AM) is a cutting‐edge technique for constructing intricate components with unique microstructural features and strength comparable to wrought alloys. Due to their exceptional corrosion resistance and mechanical properties, duplex stainless steels (DSS) are used in a wide range of critical applications. Over the past several years, a substantial body of research has been conducted on the AM of DSS. In‐depth knowledge is required to understand the complete benefits of the AM process. This review overviews the AM‐processed DSS parts based on process‐specific microstructural changes, mechanical behavior, electrochemical performance, and postheat treatment processes based on the classifications of directed energy deposition and powder bed fusion AM techniques along with future perspectives. Major challenges in AM of DSS are optimizing the austenite–ferrite fractions and controlling the formations of deleterious phases. This review will be extensively useful to researchers and industries working in the AM of DSS.

Effects of Autogenous Gas Tungsten Arc Welding (GTAW) on Corrosion Resistance of Stainless Steel 316L

The autogenous manual gas tungsten arc welding (GTAW) process was used for cladding austenitic stainless steel 316L using a single pass with various contact tip-to-work distances (CTWDs). Immersion and electrochemical tests were used to evaluate the corrosion resistance of the welded specimens, and a microstructural analysis was conducted to investigate the chemical composition of the molten pool and the heat-affected zone of welding. The key findings of this study indicate that the corrosion resistance improved under a CTWD of 5 mm due to the optimal distribution of ferrite and a refined microstructure. Additionally, the highest hardness was observed in specimens with a CTWD of 3 mm, attributed to the increased ferrite content in the weld metal. As the CTWD increased, the ferrite fraction decreased, and the hardness also diminished. However, in the CTWD 7 mm case, the higher heat input influenced the microstructure and molten pool shape significantly through the Marangoni effect, resulting in a lower corrosion resistance. These results suggest that optimizing the CTWD can enhance the corrosion resistance of welded 316L stainless steel.

Study on microstructure characterization of steel using on multi-frequency electromagnetic detection and FE simulation calculation

Microstructures dominate mechanical properties of steel materials. Monitoring and controlling microstructure variations are important for high-quality steel productions. In this study, the multi-frequency electromagnetic technology is used to effectively characterize changes of steel microstructures. Steel samples with individual micro- components are obtained using different heat treatments and are measured using a self-designed multi-frequency electromagnetic system. Micro- and Macro- FE models are developed to discuss electromagnetic responses on steel microstructures with different phases, grain size and grain boundaries. The relative permeability of different steel microstructures are calculated and discussed. Results show that the low frequency inductance will increase with the increase of ferrite grain size and ferrite fraction. The relative permeability of single ferrite grain boundary is determined to be 165 and the orientation of grain boundary can influence the relative permeability of steel microstructures.

Achieving an excellent balance of strength, plasticity, and stretch flangeability in a 1000 MPa grade TRIP-assisted bainitic ferrite steel

To address the requirements for safety enhancement, weight reduction, and emission reduction, a novel transformation-induced plasticity-assisted bainitic ferrite (TBF) steel was processed by involving a pre-quenching (PQ) process. This steel was designed with varying PQ temperatures to achieve different fractions of intercritical ferrite and bainite, and its influence on mechanical properties of TBF steels was thoroughly investigated through tensile tests and hole expansion tests. The results demonstrated that the introduction of high-temperature PQ treatment before heat treatment can dramatically accelerate the formation of austenite during the secondary annealing process, thereby facilitating a more sufficient bainitic transformation in the subsequent isothermal process. With the increase of PQ temperature from 750 to 900 °C, abundant intercritical ferrite was replaced by lathy bainite. In comparison with conventional specimens, the high-temperature PQ treatment could reduce the fraction of intercritical ferrite and martensite-austenite (M-A) islands, in favor of delaying the initiation of microvoid and microcrack and improving local formability. In summary, the P-900 specimen exhibited optimal comprehensive mechanical properties, with the highest yield strength of 657 MPa, ultimate tensile strength of 1081 MPa, elongation of 28.7%, and hole expansion ratio of 35.1%.

Evolution of inclusions in Ti-Ce-Mg deoxidized steel and its effect on acicular ferrite transformation in simulated welding HAZ

The characteristics of non-metallic inclusions in Ti-Ce-Mg deoxidized steel and their influence on the microstructure and toughness of welding heat-affected zone (HAZ) were studied. Results showed that Ti-Ce-Mg deoxidation led to a refined inclusion size and an increase in the number of inclusions below 1 μm. The highest acicular ferrite fraction was obtained in the HAZ of Ti-Ce-Mg steel, and the impact toughness was significantly improved. Ti-Ce deoxidation enhanced the stability of Ti-containing oxide, while Mg treatment promoted its fine dispersion. The final formation of composite inclusions of Ti-Ce-Mg oxide with TiN and MnS enhanced the ability to induce acicular ferrite transformation.

Understanding and exploring anisotropy mechanism of mechanical properties for ferrous alloy under different cooling paths

To understand and illuminate the anisotropy mechanism of mechanical properties with respect to typical ferrous alloy, two different kinds of microstructures were fabricated by adopting thermo-mechanical controlled processing (TMCP) with identical rolling schedule and different cooling paths, named high coiling temperature process (HC) and low coiling temperature process (LC). The influences of microstructure together with distribution of crystallographic orientation on the anisotropy discipline were investigated, and the anisotropy mechanism were further explored on the basis of plastic deformation theory. Results indicated that the microstructure processed by HC was primarily composed of quasi-polygonal ferrite (QPF) and small fraction of bainitic ferrite (BF). While the LC-processed microstructure predominantly consisted of acicular ferrite (AF), BF as well as martensite-austenite island (M/A). The LC-processed alloy exhibited more obvious anisotropy performance indicated by tensile properties along different tensile directions compared to the HC-processed alloy. There was a phenomenon of uniform distribution of macro-texture and uneven distribution of micro-texture with respect to the both achieved microstructures, and the uneven distribution of micro-texture together with high density of dislocation for BF were responsible for the obvious anisotropy performance of the studied ferrous alloy.

Cold Metal Transfer Welding of Ferritic and Austenitic Stainless Steel: Microstructural, Mechanical, and Electrochemical Studies

In the present study, cold metal transfer arc welding was employed to weld the 304L austenitic stainless steel (ASS) and Ti-stabilized 439 ferritic stainless steel (FSS) using a 309L filler electrode. Dissimilar joints were prepared using low heat input (HI; W1 ~ 247 J/mm) and high HI (W2 ~ 282 J/mm). The solidification mode for both weldments were the ferritic-austenitic mode and the weld zone (WZ) regions of both the weldments consists of columnar austenites, lathy and skeletal ferrite phases. The interfaces between WZ and ASS base metal showed the unmixed zone, whereas a conventional heat-affected zone (HAZ) was formed between the WZ and FSS base metals. The formation of ferrite stringers were observed in the unmixed zone, whereas peppery features of chromium-rich carbides were observed in HAZ. Moreover, electron backscattered diffraction technique was used to distinguish the microstructural differences between W1 and W2 weldments. Increase in the HIs resulted in decreased ferrite fraction in WZ as well as decrease in the mechanical strength of the joints. The W1 weldment depicted higher values of average micro-hardness (WZ ≈ 334.32 HV) than W2 (WZ ≈ 310.92 HV)) weldment. The electrochemical behaviour of the weldments was analysed for both the base metals and WZ of weldments. The higher degree of sensitization (DOS ~ 9.24%) of W1-WZ showed lower intergranular corrosion resistance than W2-WZ (DOS ~ 7.77%), however, the opposite trend was observed for impedance and pitting resistance.

The Effects of Heat Inputs on LDSS 2101 GTAW and SMAW Weld Microstructure and Mechanical Behavior

The welding has been utilized to stabilize the phase fractions in the microstructure of lean duplex stainless steel (LDSS) to build massive mechanical structures. The influence of heat input on the microstructure, mechanical properties, and corrosion behavior of LDSS 2101 during the gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW) processes is investigated in the present work. Specifically, we compared the outcomes between low heat input (LHI) at 0.85 kJ/mm and high heat input (HHI) at 1.3 kJ/mm for both welding techniques. Throughout the welding process, ER2209 filler wire was utilized. To assess the microstructural changes in the weldments, we employed an optical microscope, a scanning electron microscope, and X-ray diffraction. The results revealed that the volume phase fraction of ferrite was significantly higher in the LHI sample of GTAW compared to HHI GTAW and all SMAW welds. LHI GTAW welds have 18.2% greater Charpy impact toughness than LHI SMAW, whereas HHI GTAW has 35.7% higher than HHI SMAW specimens. The microhardness of the LHI GTAW weldments increased (from 230 ± 3.2 to 252 ± 4.8 HV10), whereas the microhardness of the LHI SMAW weldments increased (from 227 ± 2.8 to 246 ± 5.2 HV10). GTAW exhibited a fine grain structure, showcasing favorable tensile properties and higher hardness compared to SMAW. Conversely, the SMAW welds and their heat-affected regions exhibited coarse grain structures. These findings highlight the superior performance of GTAW in terms of microstructural characteristics, and mechanical properties when working with LD SS 2101 in comparison to SMAW.

Experimental and Modelling Research on the Effect of Prior Ferrite on Bainitic Transformation in Medium-Carbon Bainitic Steel

In this study, we investigate the impact of prior ferrite on the bainite transformation kinetics and microstructure of medium-carbon steel interrupted by an intercritical annealing (IAA) process. It was found that the incubation time and completion time decreased from 687 s and 6018 s to 20 s and 4680 s, with the volume fraction of ferrite increasing from 9.5% to 28.6%, while the maximum transformation rate increased from 00271 μm/s to 0.0436 μm/s. The ferrite/austenite interface is introduced, and the nucleation sites are increased to accelerate the subsequent bainite transformation due to the formation of prior ferrite. However, there is a competitive relationship between the number and activation energy of bainite nucleation. According to the experimental results and theoretical calculations, the activation energy of the bainite transformation in the medium-carbon bainite steel decreases gradually with an increase in the volume fraction of prior ferrite.

Effect of prior martensite on bainitic transformation of high Si hypereutectoid steel

The paper discusses the effect of prior martensite on the bainitic transformation of a high Si hypereutectoid steel. The research uses a combination of experimental techniques, high-resolution dilatometry and X-ray diffraction analysis, and theoretical calculations to investigate the impact of martensite on the transformation kinetics and microstructure evolution. The study shows that the presence of martensite, regardless of the amount, accelerates the bainitic transformation, particularly in the early and fastest stages, and at higher isothermal transformation temperatures. At lower temperatures, the preconditioning of the austenite prior to the bainitic transformation is such that its mechanical stability increases to the point where the whole transformation is inhibited. It is also suggested that the formation of cementite due to the tempering of martensite at the isothermal stage controls the final fraction of bainitic ferrite. The findings provide valuable insights into the role of prior martensite in bainitic transformation kinetics and microstructure development.

Microstructure, mechanical properties, and corrosion behaviour of wire arc additive manufactured martensitic stainless steel 410 for pressure vessel applications

High-strength martensitic stainless steel 410 (MSS 410) was successfully fabricated without defects via wire arc additive manufacturing (WAAM). The microstructural features, mechanical properties, and corrosion performance of WAAM-processed MSS 410 were examined. Microstructural analysis revealed the existence of equiaxed and coarse columnar dendrites and the formation of residual delta-ferrite and retained-austenite (RA) was confirmed within the martensitic matrix. The fraction of residual ferrite and RA varied along the building direction due to complex thermal cycles. Uni-axial tensile results exhibited anisotropic behavior and are related to layered microstructure. Microhardness values varied from bottom to top (384–460 HV). Ductile mode of failure was noticed, and the existence of RA influences the ductility. The corrosion behaviour of the WAAM 410 specimens in 3.5% NaCl solution was considerably acceptable, and the corrosion rate ranged between 0.05 and 1.37 mpy. This shall be attributed to the absence of defects, microstructural features, and ferrite fraction in the WAAM 410 samples. The mechanical properties and corrosion behaviour of the WAAM 410 were comparable to the wrought grade and are suitable for pressure vessel applications with adequate corrosion resistance.

A Hot‐Stamped Medium‐Mn Steel with Superior Strength–Ductility–Bendability Combination and Thin Oxidation Layer

A hot‐stamped medium‐Mn steel containing 3.84 wt% Mn is designed for the manufacture of hot‐stamped automotive components. The effects of intercritical temperatures employed for hot forming on the microstructure, tensile properties, and three‐point bending behaviors are investigated. The result shows that the microstructure can be controlled by intercritical temperatures, leading to different phase fractions of martensite, retained austenite, and ferrite in the steels. Besides, the tensile strength increases with increasing intercritical temperature, while both the total elongation and the bending angle show reverse change trend, which are related to the various phase fractions and the different transformation‐induced plasticity effects during deformation. Importantly, the medium‐Mn steel subjected to heat treatment at 800 °C possesses tensile strength of 1535 MPa, total elongation of 13.8%, and bending angle of ≈56°. Therefore, its total elongation is significantly higher than hot‐stamped 22MnB5 steel under the same tensile strength grade, and the bending angle of the former is close to the latter. Moreover, the thickness of oxidation layer on the medium‐Mn steel is only ≈3.5 μm after heat treatment at 800 °C, much thinner than the ≈57 μm‐thick oxidation layer on the 22MnB5 steel after austenitizing at 930 °C.

Microstructural and mechanical properties of electron beam welded super duplex stainless steel

The electron beam welding of super duplex stainless steels is associated with challenges due to the concentrated heat input and the nitrogen loss that result in a predominantly ferritic structure after the solidification. This study presents an approach to overcome this issue by feeding nickel-based filler wire into the melt pool in welding of 2507 super duplex stainless steel. Results showed that the high-frequency beam oscillation combined with a multi-beam technique led a good mixing between the base metal and the filler wire, even at a large depth-to-width ratio. Additionally, the weld geometry was characterized by near-parallel fusion lines and a narrow heat-affected zone. The nickel addition resulted in a balanced microstructure in the weld metal with ferrite fractions of 35–55 %, despite a significant nitrogen loss, consequently leading to impact energy values of 215 ± 15 J and hardness values of 285 ± 15 HV. The findings of this investigation allow fabricators to effectively design electron beam welding processes for producing thick-walled super duplex stainless steel components.

Correlation of microstructure and hardness distribution of high-speed train wheels under original and service statuses

High-speed train wheels have complex service environments during wheel–rail contact, and a comprehensive understanding of the microstructural inhomogeneity at the rim is critical for service safety assessment. The local quenching process in the vicinity of the rim will cause a difference in microstructure, directly affecting the service performance of the material. In this study, high-throughput characterization method based on material genome engineering was used, the methods for characterising and identifying the microstructure of wheel steel were developed using high-throughput scanning electron microscopy, and the accuracy and reproducibility of the methods were evaluated. A point-to-point research scheme for microstructure image and hardness data was designed based on the concept of statistical mapping characterization. The area fraction of the proeutectoid ferrite and micro-Vickers hardness distribution in a large size range of the key area of the rim and their internal correlation was obtained, these helped avoid statistical and representativeness limitations of traditional local selection analysis of microstructure and hardness. The results indicates that the area fraction of proeutectoid ferrite and micro-Vickers hardness showed a clear gradient distribution, and opposite distribution trend, with a high negative correlation. In addition, by observing the correlation between microstructure and hardness, and studying the evolution of microstructure, it was found that the microstructure and hardness of the rim near the tread demonstrated a significant synergistic evolution law under the influence of service, this was observed both statistically and microscopically. These findings enhanced the application prospects of the microstructure-performance statistical mapping method, and were beneficial in efficiently evaluating the service life and safety of key components.

Influence of Cr and Cr+Nb on the interphase precipitation and mechanical properties of V–Mo microalloyed steels

There has been much research on steels strengthened by interphase precipitation, but the role of Cr remains unclear. Isothermal transformations have been performed at different temperatures to investigate the effect of Cr and Cr + Nb additions on the interphase precipitation resulting from the austenite-to-ferrite transformations in V–Mo based dual phase steels. Optical microscopy (OM) plus transmission electron microscopy (TEM) and tensile testing were used to characterize and correlate microstructural evolution to the tensile properties and the interphase precipitation formed during the austenite to ferrite (γ→α) transformation. It was found that an addition of 0.5 wt% Cr to V–Mo microalloyed steel accelerates the transformation rate of ferrite, producing a higher volume fraction of ferrite. Importantly, the addition of Cr reduces the diameter of the interphase precipitates giving an important contribution to the yield strength. A further addition of 0.03 wt% Nb to the Cr–V–Mo reduces the contribution made by the interphase precipitation and increased the grain refined contribution. A new method was used to calculate the volume fraction of interphase precipitation with TEM images from extraction replicas is presented and included in a root-mean-square model which includes the precipitation hardening made by IP on microalloyed DP steels to effectively predict their yield strength.