Bainitic Ferrite Research Articles

Pre-quenching induced lath structures and enhanced TRIP effect to optimize the strength-ductility of ultrahigh-strength hot-galvanized steel

Compared with the conventional hot-dip galvanized steel, an optimized strength-ductility combination was obtained by the combination of continuous galvanizing line (CGL) compatible quenching and austempering (QAT) treatment and pre-quenching, i.e., “pre-quenching + CGL-QAT”. The effects of the pre-quenching on the microstructure and mechanical performance were analyzed by using dilatometry, SEM, AES-EBSD, TEM and XRD technique. Compared with QAT samples, pre-quenching could eliminate the blocky M-A islands and promote the formation of lath reversed austenite and bainite ferrite in “pre-quenching + CGL-QAT” samples. Meanwhile, a large fraction (22.7–25.8%) of lath RA were obtained, such that brittle intergranular fracture can be prevented. The lath bainite/ferrite structures and sufficient TRIP effect enhanced the strength-ductility combination with an UTS reaching 1000 MPa and TEL around 30%. The fracture mainly presents ductile fracture and results in a ductile fracture surface with large dimples. The excellent strength-ductility combination with a UTS above 980 MPa, a YS of 625 MPa, a TEL of 29.1% and a higher PSE around 30 GPa·% can be obtained, which exceeds the current commercial hot-dip galvanized steel grades.

Microstructure and properties of magnetic field assisted laser wire-filled welded 22MnB5 steel joints

The magnetic field-assisted laser wire-filled welding test of 1.5 mm automotive 22MnB5 steel is performed to investigate the influence of magnetic field on the microstructure and properties of the welded joints. When no magnetic field is applied, and the laser heat input is 190 J mm−1, the welded joint width and the grain size of the coarse grain region are large. Also, there is an obvious hump defect at the bottom of the weld. Under the same heat input conditions, when a 5 mT and 15 mT steady magnetic field is applied, the thermoelectric magnetic force generated by the magnetic field promoted the flow of molten pool and concentrated laser energy. It is found that the hump defect is eliminated, the width of the welded joint is reduced, the grain size of the coarse grain region is significantly reduced, and the overall hardness of the welded joint is improved. However, different magnetic induction intensities have different effects on the solid phase transformation of the weld. When no magnetic field is added, the weld center is mainly composed of granular bainite and polygonal ferrite due to the slow cooling rate of the molten pool. When the applied magnetic field is 5 mT, the center of the weld is mainly composed of brittle and hard upper bainite because the thermoelectric magnetic force stirs the molten pool and accelerates the cooling rate of the molten pool but the overall mechanical properties of the welded joint were relatively poor. At 15 mT, lath martensite and lower bainite predominate in the weld center due to the increased cooling rate of the molten pool, thereby increasing the overall mechanical properties of the welded joint. Therefore, choosing the appropriate magnetic induction intensity is critical for improving the microstructure and properties of welded joints.

Effect of heat input on the microstructure and impact toughness of submerged arc weld metal for high-performance weathering steel

This study aims to elucidate the appropriate heat input (E j ) range for submerged arc welding (SAW) of high-performance weathering steel. Generally, by increasing E j , the welding efficiency can be improved, but the toughness of the weld metal may be deteriorated. Therefore, SAW was employed to produce the weld microstructure under varying E j from 20 to 50 kJ cm−1. The Charpy V-notch impact tests were conducted at −40 °C, and the weld microstructures were characterized by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). The results indicate that the weld microstructures consist of polygonal ferrite (PF), acicular ferrite (AF), granular bainitic ferrite (GBF), and martensite/austenite (M/A) constituents under each E j . With the increase in E j , the proportion of PF increases, while AF and GBF are coarsened, and the area fraction (f M/A) and mean size (dM/A) of M/A constituents increase monotonically. Further, the fraction (f MTA>15°) of high-angle grain boundaries (HAGBs) with the misorientation tolerance angles (MTAs) greater than 15° is reduced, while the mean equivalent diameter (MEDMTA≥15°) of ferrite grains with HAGBs increases. Accordingly, with the increase in E j , the impact toughness of weld degrades from 128.4 to 47.6 J. The higher degree of micro-strain concentration caused by the increase in M/A size and area leads to the formation of larger microcracks under small plastic deformation, while the reduced HAGBs have a lower inhibition effect on crack propagation. Finally, the impact toughness decreases with the increase of E j . Overall, the findings suggest that the E j of SAW should not exceed 40 kJ cm−1 in the construction of high-performance weathering steel.

Enhanced stretch flangeability and crack propagation behavior of an 1100 MPa grade TRIP-aided bainitic ferrite steel

In order to face the challenges of lightweight, safety, and emission reduction in the automotive industry, transformation-induced plasticity aided bainitic ferrite (TBF) steel was fabricated by controlling the size and shape of bainitic ferrite and compared with quenching and partitioning (Q&P) steels. The effects of bainite transformation on the mechanical properties, microvoid formation, and crack propagation were analyzed by dilatometry measurement and microstructure characterization. At lower isothermal bainite transformation (IBT) temperature, the lathy bainitic matrix can prevent crack formation and limit the crack propagation inside its laths, in favor of improving the strength and stretch flangeability simultaneously. In addition, granular bainite, coarse martensite, and austenite (M-A) islands would increase the risk of void formation at higher IBT temperatures. These voids provided the initial sites for crack formation, which was also responsible for the reduction of the hole expansion ratio (HER). In conclusion, the outstanding properties were achieved in the T-375 samples with the HER of 33.5%, the total elongation of 16.9%, the tensile strength of 1191 MPa, and the yield strength of 934 MPa.

Automated classification of granular bainite and polygonal ferrite by electron backscatter diffraction verified through local structural and mechanical analyses

Differentiation of granular bainite and polygonal ferrite in high-strength low-alloy (HSLA) steels possesses a significant challenge, where both nanoindentation and chemical analyses do not achieve an adequate phase classification due to the similar mechanical and chemical properties of both constituents. Here, the kernel average misorientation from electron backscatter diffraction (EBSD) was implemented into a Matlab code to differentiate and quantify the microstructural constituents. Correlative electron channeling contrast imaging (ECCI) validated the automated phase classification results and was further employed to investigate the effect of the grain tolerance angle on classification. Moreover, ECCI investigations highlighted that the grain structure of HSLA steels can be subdivided into four grain categories. Each category contained a different nanohardness or substructure size that precluded a nanoindentation-based phase classification. Consequently, the automated EBSD classification approach based on local misorientation achieved a reliable result using a grain tolerance angle of 5°.Graphical abstract

Dilatometric study of high silicon bainitic steels: Solid-state transformations

High silicon bainitic steels have gained significant recognition in various applications due to their exceptional properties, such as high strength, favourable corrosion resistance, and excellent high-temperature stability. This study investigates an alloy capable of generating a finer bainitic structure through a transformation at 260 °C, leading to the desired microstructure. Interestingly, other phenomena were discovered while pursuing optimal heat treatment conditions. At temperatures exceeding 850 °C, the alloy exhibits a tendency for graphite formation, which has intriguing implications for its mechanical properties. The high silicon concentration in the alloy significantly retards cementite growth, resulting in a microstructure composed solely of bainitic ferrite and residual austenite through the transformation of austenite below the bainite start temperature. Furthermore, it is observed that pearlite formed during rapid transformation at 650 °C does not exhibit the predicted equilibrium chemical composition. This discrepancy challenges the existing models of pearlite growth, which assume local equilibrium at the shared interface with austenite. This research aims to investigate the influence of silicon content on solid-state transformations in high-silicon steels using dilatometry, optical microscopy, scanning electron microscopy, and X–ray diffraction techniques. These analytical methods will provide insights into the intricate processes occurring during isothermal transformation temperatures, contributing to a deeper understanding of the material's behaviour and its potential applications.

Comparison of microstructural characteristics and mechanical properties of the high-strength low-alloy steels fabricated by wire arc additive manufacturing versus conventional casting

A comparison study on the microstructure and mechanical property between the wire arc additive manufacture (WAAM) high-strength low-alloy (HSLA) steel and the as-cast one was carried out. A more optimized CMT-based WAAM parameter was selected to fabricate a nearly defect-free single-passed thin-wall part. It was found that the main body of the deposit at the middle zone exhibited a typical granular bainite (GB) duplex microstructure consisting of bainite ferrite (BF) and martensite/austenite (M/A) island. Also, the middle zone exhibited a band segregation between the two adjacent columnar prior-γ grains. The band segregation distance is apparently shorter compared with the cellular segregation distance in the as-cast steel. In addition, the as-cast steel has the bainitic microstructure consisting of larger grain-sized BF and nano-twinned martensite island. On a basis of the microstructure characteristics, the nonequilibrium solidification and solid-solid phase transition accompanying with the segregation behavior during the WAAM deposition of the molten pools were studied. Such the above structured as-deposited steel, in comparison to the as-cast one, was decreased in σy from 594.9 MPa to 426.2 MPa, but increased in σuts from 778.6 MPa to 858.4 MPa, and also greatly increased in εtotal from 10.9% to 20.7%. The apparently enhanced εtotal of the as-deposited steel was interpreted in connection with the fabrication processing-induced microstructural variation of the two steels.

Evolutions of Microstructure and Impact Toughness of Submerged Arc Weld Metal via Introducing Varied Si for Weathering Bridge Steel

In this paper, the influence of the silicon (Si) content on microstructure and impact property of submerged arc weld metals (WMs) for weathering bridge steel was clarified. Actual submerged arc welding (SAW) was carried out to produce WMs with 0.18 wt.%, 0.36 wt.%, 0.51 wt.%, and 0.60 wt.% of Si. The low temperature impact property of weld metal was detected, and the weld microstructures were characterized by optical microscopy (OM), scanning and transmission electron microscope (SEM and TEM), and electron backscatter diffraction (EBSD). The results indicate that WMs consist of polygon ferrite (PF), acicular ferrite (AF), granular bainitic ferrite (GBF), and martensite/austenite (M/A) constituents in each Si content. With increasing Si, the proportion of PF increased, while AF and GBF coarsened, the area fraction, fM/A, and the mean size, dM/A, of M/A constituents increased, the number of inclusions decreased, but the size increased. Further, the fraction of high-angle grain boundaries (HAGBs) with the misorientation tolerance angles (MTAs) greater than 15° reduced, while the mean equivalent diameter, MEDMTA≥15°, of ferrite grains with HAGBs increased. Accordingly, the impact toughness of WM was degraded from 108.1 J to 39.4 J with the increase in Si. The increase in M/A constituents and inclusions size reduced the critical fracture stress, thereby formation of larger microcracks and cleavage planes occurred. The reduced HAGBs exhibited a low hindering effect on crack propagation, and, consequently, the impact toughness decreased with increasing Si content.

Tempering Behavior of a Si-Rich Low-Alloy Medium-Carbon Steel

Owing to the addition of Si, 0.33C-1.8Si-1.44Mn-0.58Cr steel exhibits a unique tempering behavior. The tempering takes place in two distinct sequential stages that are significantly different from those in steels containing 0.2–0.5 wt.% of Si. Stage I is associated with the precipitation of transition carbides in a paraequilibrium manner, can take place in temperatures ranging from ~200 to ~474 °C, and concurrently increases strength, ductility, and toughness. Stage II is associated with the decomposition of retained austenite to bainitic ferrite and transition carbides. As a result, no significant effect of overlapping of Stage I with Stage II takes place. Stage III does not occur at temperatures below ~474 °C, since the precipitation of cementite in a orthoequilibrium manner is suppressed by the addition of 1.8 wt.% of Si. It was shown that a major portion of carbon atoms redistributes to Cottrell atmospheres under quenching. During low-temperature tempering at 200–400 °C, the precipitation of transition carbides consumes a large portion of carbon atoms, thereby increasing the number of ductile fractures and improving the impact toughness without strength degradation. The formation of chains of cementite particles on boundaries takes place in Stage IV at a tempering temperature of 500 °C. This process results in the full depletion of excess carbon from a ferritic matrix that provides increased ductility and toughness but decreased strength.

Promoting ductility and formability in a carbide free bainitic steel via pre-annealing treatment

This study aimed to overcome the existing trade-off in strength–ductility in carbide free bainitic steel and improve ductility without compromising strength. Herein, we proposed a novel and simple method to obtain Mn-rich cementite via a pre-annealing (PA) treatment. PA promoted the partitioning of Mn into cementite particles. The Mn-rich cementite increased the Mn content in the retained austenite (RA) and enhanced the RA stability. Moreover, the initial number of Mn-rich cementite microstructures is positively correlated with RA. The effect of PA on the bainitic ferrite (BF) content was evaluated via comparison with the non-PA sample, revealing that the PA treatment promoted BF nucleation and increased the BF content. The PA-treated samples exhibited the optimal transformation induced plasticity (TRIP) effect and mechanical properties; in particular, the PA-3 sample (with 3 h treatment period) improved ductility and formability, with the total elongation increased from 11.9% in the non-PA sample to 18.0% in the PA-3 sample, corresponding to an increase of ∼51.2%. Additionally, the Erichsen cupping value increased from 7.52 mm in the non-PA sample to 9.78 mm in the PA-3 sample, corresponding to an increase of 30.1%. The overall applicability of PA for the improvement of formability of the carbide free bainitic steel is evident in all PA-treated samples.

Elucidating the heat input on CGHAZ microstructure and its irregular effect on impact toughness for a novel V–N microalloying weathering steel

This study investigated the effect of heat inputs on coarse-grained heat affected zone (CGHAZ) microstructure and impact properties of a novel vanadium (V) and nitrogen (N) microalloyed weathering steel via a Gleeble™ system, revealing the metallurgical essence of the steel suitable for large heat input (Ej) welding. The results indicate that in CGHAZ, the dominant microstructure was composed of lath bainitic ferrite (LBF) at Ej = 15 kJ/cm, granular bainitic ferrite (GBF) at Ej = 35 kJ/cm, and a mixture of intragranular acicular ferrite (IGAF), intragranular polygon ferrite (IGPF) and grain boundary polygonal ferrite (GBPF) at Ej = 55 and 75 kJ/cm, respectively, apart from martensite/austenite (M/A) constituents in each one. The area fraction, fM/A, and mean size, dM/A, of M/A constituents increased monotonously with the Ej. In addition, with increasing Ej from 15 to 35 kJ/cm, the fraction, fMTAs＞15°, of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) greater than 15° reduced, while the mean equivalent diameter, MEDMTA≥15°, of grains with HAGBs increased. However, as Ej further increased to 55 and 75 kJ/cm as the two large heat inputs, the precipitation of (Ti, V) (C, N) and thereby the heterogeneous nucleation of polygonal and acicular ferrite were promoted by a lowered degree of undercooling, resulting in the decreased MEDMTA≥15° and the increased fraction fMTAs＞15°. Accordingly, the impact toughness of CGHAZ was degraded with the increase of Ej from 15 to 35 kJ/cm, but enhanced with the increase of Ej from 35 to 75 kJ/cm.

One-step quenching and partitioning treatment of directly hot rolled low carbon steel

ABSTRACT Making cost-effective quenching and partitioning (Q&P) steel with good mechanical performance is attractive to the automotive industry. A low carbon steel was hot rolled, directly quenched, and isothermally partitioned using salt bath treatment. Effects of partitioning temperature and time on the microstructures and properties of the steel were studied. The microstructure consisted of carbide-free bainitic ferrite, polygonal ferrite, and retained austenite. The hot rolling introduced strain energy to the austenite, promoting the formation of ferrite, and enhancing the diffusion of carbon in the subsequent partitioning process with an accompaniment of the growth of ferrite. The ferrite was inferred to not only improve the plasticity of the steel but also make the retained austenite more stable. Steel exhibited excellent mechanical properties with high strength and elongation (>23 GPa·%). This study presents a preliminary approach for economical online Q&P steel production.

Microstructural evolutions and impact toughness in simulated welding heat affected zones for a high-strength carbide-free bainitic rail steel

Systematic experimental investigations were conducted to study the microstructures and impact toughness of each heat affected zone (HAZ) formed during rail flash-butt welding. A high-strength carbide-free bainitic rail steel was subjected to different thermal simulation cycles to separately reproduce each HAZ subzone by tailoring the peak temperature (PT) with respect to 700, 850, 920, 1000 and 1350 °C, and hence to generate the corresponding microstructures by using Gleeble-3500 simulator. Results show that the HAZ subzones exhibit complicated microstructures depending on the PTs, and with increasing PT the dominant bainitic microstructure type evolves from polygonal bainitic ferrite (700 °C) to a mixture of fine bainitic ferrite and granular bainite (850–1000 °C), and finally to coarse bainitic ferrite and granular bainite (1350 °C). Impact tests demonstrate that the impact toughness initially increases significantly as the PT reaches 920 °C (i.e., fine-grained HAZ), beyond which the impact toughness starts to decrease. The fine-grained HAZ displays optimal impact toughness in HAZs, yet which is lower than the base metal. Moreover, the morphology and distribution of martensite-austenite (M-A) constituents is strongly dependent on the welding PT, and the high fraction blocky and coarse slender M-A constituents is considered to be detrimental for the impact toughness.

Structure-property relationship in vanadium micro-alloyed TRIP steel subjected to the isothermal bainite transformation process

Structure-property relationship in vanadium micro-alloyed TRIP steel is investigated focusing on austenite transformation, precipitation behavior and mechanical performance. Microstructural evolution of the steel during processing was characterized mainly using EBSD, TEM and APT techniques; and its influence on mechanical properties was discussed. The TRIP steel exhibits complex microstructure including intercritical ferrite (IF), bainitic ferrite (BF), retained austenite (RA) and martensite. After identical intercritical annealing (IA), IF fraction keeps constant, BF and RA fractions increase while martensite fraction decreases with decreasing isothermal bainite transformation (IBT) temperature from 470 to 380 °C. The initially precipitated vanadium carbides (VC) in the hot-rolled specimen undergo coarsening during IA process, accompanied with some newly precipitated VC in either ferrite or in austenite, confirmed by the co-existence of small-sized and coarsened VC after IA process, as well as occurrence of K–S orientation relationship between VC and the α-Fe matrix. Ultra-fine average grain size of less than 1 μm in the TRIP heat-treated steel is achieved by the refining effect of these nano-scale VC precipitates, which can inhibit phase boundary (PB) migration during IA process and promote bainite transformation during subsequent IBT process. The higher volume fraction of RA accompanied with higher carbon enrichment intensifies sustainable strain hardening and finally increased ductility of the specimens treated under lower IBT temperature. In the IBT-380 specimen with the highest RA and BF fractions, the occurrence of micro-yielding phenomenon is a result of the Cottrell atmosphere in soft domain BF and stress relaxation due to RA to martensite transformation, during which the soft domain IF/RA/BF and hard domain martensite yields step by step. An excellent mechanical performance is thus achieved in the IBT-380 specimen, meeting the requirements for the 3rd generation advanced auto-steel.

Influence of partitioning treatment on microstructure and mechanical properties of an alloyed ductile iron austempered at different temperatures

AbstractThe present study was conducted to uncover effects of partitioning treatment on Cu–Ni–Mo alloyed ductile iron (DI) austempered at different temperatures. For this purpose, the DI samples, produced via sand casting, were austenitized at 900 °C for 60 min, followed by austempering at the temperatures of 275–325–375 °C for 120 min and afterwards a partitioning treatment was applied at 200 °C for 15 min. In the characterization studies, dilatometer, image analysis, JMat-Pro, mechanical tests, XRD, optical microscope, and scanning electron microscope (SEM) equipped with EBSD detector were utilized. Characterization studies showed that the effects of partitioning treatment were directly correlated with austempering temperature and high carbon austenite volume fraction changed in the range of 19.48–35.45%. That redistribution of carbon (C) between bainitic ferrite and high carbon austenite occurred, in turn, the carbon content of high carbon austenite increased with the partitioning treatment irrespective of austempering temperature were uncovered. Furthermore, the partitioning treatment considerably changed the grain morphologies of both high carbon austenite and banitic ferrite. As a consequence of these microstructural differences, the highest tensile strength of 1489.2 MPa was established in the sample austempered at 275 °C and partitioned at 200 °C, whereas the highest ductility of 5.61% acquired at the austempering temperature of 375 °C.

Strength-Toughness of a Low-Alloy 0.25C Steel Treated by Q&P Processing.

Quenching and partitioning (Q&P) treatments were applied to 0.25C steel to produce the microstructures that exhibit an improved balance of mechanical properties. The simultaneous bainitic transformation and carbon enrichment of retained austenite (RA) during the partitioning stage at 350 °C result in the coexistence of RA islands with irregular shapes embedded in bainitic ferrite and film-like RA in the martensitic matrix. The decomposition of coarse RA islands and the tempering of primary martensite during partitioning is accompanied by a decrease in the dislocation density and the precipitation/growth of η-carbide in the lath interiors of primary martensite. The best combinations of a yield strength above 1200 MPa and an impact toughness of about 100 J were obtained in the steel samples quenched to 210-230 °C and subjected to partitioning at 350 °C for 100-600 s. A detailed analysis of the microstructures and the mechanical properties of the steel subjected to Q&P, water quenching, and isothermal treatment revealed that the ideal strength-toughness combinations could be attributed to the mixture of the tempered lath martensite with finely dispersed and stabilized RA and the particles of η-carbide located in the lath interiors.

Fatigue and recrystallization evolution of carbide-free bainitic layer under high-stress rolling sliding contact

The rolling contact fatigue behavior of a newly developed bearing steel with carbide-free bainite, modified by a combination of tungsten inert gas surface melting and subsequent isothermal transformation was investigated. The rolling sliding tests were conducted to determine the vibration response and rolling contact fatigue life. The rolling contact fatigue life of the bainitic layer is 6.623 × 106 cycles, significantly higher than that of the commercial GCr15 steel sample (4.496 × 106 cycles). Focused ion beam technology and transmission Kikuchi diffraction studies on fatigue-tested samples showed that the toughness of the bainitic layer could be improved by the bainitic ferrite and retained austenite, which recrystallizes and forms nanocrystals during the rolling contact fatigue process. Furthermore, high-resolution transmission electron microscope study elucidated that the bainitic layer has better dislocation slip ability and is more prone to recrystallization on the rolling contact fatigue surface, preventing the rapid expansion of fatigue cracks. Moreover, the effect of dissolution of the Fe3C in GCr15 steel on the structure of distorted body-centered cubic crystals and pitting defects was also comparatively investigated.

Microstructure and properties of low alloy CrMo steel processed with varied isothermal temperatures

The microstructure and properties of low alloy CrMo steel after isothermal treatment at different temperatures were studied. The experiment process is to rapidly cool the samples that have been held at 940 ℃ for 2 h to a salt bath furnace at different temperatures (300–500 ℃) for isothermal treatment, and then air cooling to ambient temperature. The microstructure after isothermal treatment is granular bainite, which is a mixed microstructure composed of bainitic ferrite (BF) and martensite/austenite (M/A) island. The morphology, size and volume fraction of M/A islands change with the change of isothermal temperature. The mechanical properties are also closely related to isothermal temperature. With the increase of isothermal temperature, the impact toughness and yield strength (YS) decreased. By studying the effects of different strengthening mechanisms and multiphase on YS, the results show that YS mainly depends on BF. According to the SEM and EBSD characterization of the fracture, it is found that the stress in the sample is mainly concentrated near the M/A, which makes the crack easy to initiate on the M/A island or its interface. The brittle fracture tendency of materials is related to M/A island. The larger the size of the M/A island, the more likely brittle fracture will occur and the less ductile it will be.

Two design strategies for enhancing the thermal stability of bainitic structures

In response to the growing interest of the industry in advanced high-strength steels subjected to extreme operating conditions, two novel grades BainTS and BainNiAlCu bainitic steels have been developed. Since the in-use properties of bainitic steel are directly related to the thermal stability of the bainitic structure, two different alloy design concepts to improve tempering resistance considering various strengthening mechanisms were proposed. BainTS steel follows the standard assumptions of nanocrystalline bainitic steel design, where the retained austenite is stabilized with silicon. A potential secondary hardening was also considered using Cr, Mo, and V alloying additives. On the other hand, BainNiAlCu steel was designed taking into account the precipitation strengthening with intermetallic compounds (nickel aluminide and copper particles). It was revealed that both steels after heat treatment consist of bainitic ferrite laths and a significant content of retained austenite (above 25%) with film and blocky morphologies. After the tempering process (350 °C–650 °C), there was significant structural evolution following the gradual decomposition of the metastable bainitic structure. It was proved that BainTS steel is characterized by the higher thermal stability of both bainitic ferrite and retained austenite compared to BainNiAlCu. Despite this, the hardening effect after tempering BainNiAlCu steel was higher (by about 140 HV compared to heat treatment, approx. 30% increase). The presented results suggest that bainitic steels strengthened with intermetallic phases are extremely promising in the context of future industrialization in applications where operating conditions are exposed to elevated temperatures.

Weld morphology and grain growth characteristics of driven moving arc hollow stud welding

During the welding process of hollow studs, several issues can arise, including arc deflection, local non-fusion, welding pores, and cracks. In response to these challenges, a rotating magnetic field device has been developed to enhance the welding arc movement, which in turn improves the joint formation and weld grain growth behavior. According to the results, the arc moves in a periodic manner along the end face of the stud when exposed to a rotating magnetic field, due to the application of the Lorentz force. This leads to an enhanced uniformity of the melting process, a reduction in the occurrence of localized un-fusion and porosity defects, and an overall improvement in the quality of the weld morphology. Under the electromagnetic stirring effect, the undercooling of the molten pool changes and the microalloying elements in high-strength steel diffuse into the welded joint. The microstructure of the welded joint area is transformed into martensite, lower bainite, and needle-like ferrite. Under the action of a rotating magnetic field, the coarse grain in the welded joint is broken and the size is refined from 2.91 μm to 1.48 μm. The number of high-angle grain boundaries increases while the number of low-angle grain boundaries decreases. The shear strength of the joint is improved from 227 MPa to 315 MPa.