Proeutectoid Ferrite Research Articles

Dissimilar joining of AISI 304/Q345 steels in keyhole tungsten inert gas welding process

Keyhole tungsten inert gas welding (K-TIG) was used to join AISI 304 stainless steel with Q345 low-alloy steel of 8 mm thickness in a single pass. Welding formation and process windows in different welding speeds were discussed. Microstructure and welding properties of dissimilar joints were investigated. The results show that in a relatively high welding speed (more than 300 mm/min), high-quality weld of dissimilar steels can be obtained in the welding process window with a current of 50A. Weld microstructure is lath martensite and retained austenite. Proeutectoid ferrite and Widmanstatten structure are observed in Q345 heat-affected zone (HAZ) while no obvious grain growth is found in AISI 304 HAZ. Microhardness of the weld metal is much higher than the base metal because of the martensite structure in the weld. Tensile test was conducted with digital image correlation (DIC). The result shows that the tensile strength of the weld is almost the same as the Q345 base metal and the elongation is slightly reduced, indicating good qualities both in strength and plasticity.

Promoting of bainitic transformation in pre-alloyed Fe-Cr-Mo steels with different compositions by nickel additions

Nickel is one of the alloying elements promoting the formation of acicular ferrite at the expense of proeutectoid ferrite. The Ni addition reduces the steady-state nucleation rates of allotriomorphic ferrite in Fe-C-Ni alloys. Due to such reasons, Ni was added to modify the microstructure of two sintered steels with different compositions-with the aim of improvement in mechanical properties. Two pre-alloyed Fe-3.0Cr-0.5Mo and Fe-1.5Cr-0.2Mo powders were employed as starting powders for experimental works. The sintered steels were produced under the same cooling rate of 0.1 °C/s. The pre-alloyed powder compositions, differentiated by alloying element contents of Cr and Mo, showed clear influence on the final microstructures of the sintered steels. For the sintered Fe-3.0Cr-0.5Mo-0.3C-xNi steels, additions of ≥ 1 wt.% Ni could change the microstructure from dual-phase type to bainitic structure. Ni additions in the sintered Fe-1.5Cr-0.2Mo-0.3C-xNi steels could also promote bainitic transformation. However, bainitic grain portion in the sintered Fe-1.5Cr-0.2Mo -0.3C-xNi steels was lower than that of the sintered Fe-3.0Cr-0.5Mo-0.3C-xNi steels. The microstructures with dual-phase nature still existed in the sintered Fe-1.5Cr-0.2Mo-0.3C-xNi steels, even Ni additions of up to 3.0 wt.% were conducted. The Ni addition not only altered the sintered steel microstructure but also improved the hardness of sintered steels.

Effects of oxygen content on microstructure and impact toughness of high heat input flux-cored wire weld metals

Three weld metals with different oxygen contents were developed. The influence of oxygen contents on the microstructure and impact toughness of weld metal was investigated through high heat input welding tests. The results showed that a large number of fine inclusions were formed and distributed randomly in the weld metal with oxygen content of 500 ppm under the heat input condition of 341 kJ/cm. Substantial cross interlocked acicular ferritic grains were induced to generate in the vicinity of the inclusions, primarily leading to the high impact toughness at low temperature for the weld metal. With the increase of oxygen content, the number of fine inclusions distributed in the weld metal increased and the grain size of intragranular acicular ferrites decreased, which enhanced the impact toughness of the weld metal. Nevertheless, a further increase of oxygen content would contribute to a great diminution of the austenitic grain size. Following that the fraction of grain boundary and the start temperature of transformation increased, which facilitated the abundant formation of pro-eutectoid ferrites and resulted in a deteriorative impact toughness of the weld metal.

Effect of Deformation Parameters on Microstructure and Properties During DIFT of X70HD Pipeline Steel

Grain refinement is a critical approach to improve the strength of materials without damaging the toughness. The grains of deformation-induced ferrite are considerably smaller than those of proeutectoid ferrite. Grain refinement is crucial to the application of deformation-induced ferrite. The composition of ferrite and bainite or martensite is important in controlling the performance of X70HD pipeline steel, and cooling significantly influences the control of their ratio and grain size. By analyzing the static and dynamic phase-transition points using Gleeble-3800 thermal simulator, thermal simulations were performed through two-stage deformations in the austenite zone. Ferrite transformation rules were studied with thermal simulation tests under different deformation and cooling parameters based on the actual production of cumulative deformation. The influence of deformation parameters on the microstructure transformation was analyzed. Numerous fine-grain deformation-induced ferrites were obtained by regulating various parameters, including deformation temperature, strain rate, cooling rate, final cooling temperature and other parameters. Results of metallographic observation and microtensile testing revealed that the selection of appropriate parameters can refine the grains and improve the performance of the X70HD pipeline steel.

Control of Carbon Content in Steel by Introducing Proeutectoid Ferrite Transformation into Hot-Rolled Q&P Process

A processing strategy involving primary and secondary carbon partitioning is proposed for the hot-rolled quenching and partitioning process through the introduction of proeutectoid ferrite transformation after rolling. The microstructures of the steels were characterized using scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, and x-ray diffraction, and the mechanical properties were evaluated using a universal tensile machine. The blocky retained austenite that distributed along the ferrite grain boundaries was promoted based on the coupling action of the primary and secondary carbon partitioning, which enhanced the transformation-induced plasticity effect during deformation despite the high carbon concentration. A ferrite formation temperature range of ~ 760 to 800 °C was proposed. In addition, from the perspective of industrialization, the observed ‘plateau trends’ for the retained austenite fraction and product of strength and elongation suggest the availability of a wide processing window of 215-362 °C for controlling the coiling temperature.

Preparation and printability of 24CrNiMo alloy steel powder for selective laser melting fabricating brake disc

ABSTRACTSpherical 24CrNiMo alloy steel powder used for selective laser melting (SLM) fabricating high-speed train brake disc was prepared by the vacuum induction melting gas atomisation (VIGA) method. Powder morphology, particle size, flowability and microstructure were measured. Part properties fabricated by SLM were investigated via some modern analysis method. The experimental results showed that powder mean particle size D50 was 75 μm, flowability was 16.69 s/50 g and apparent density was 4.71 g cm−3. 24CrNiMo alloy steel specimen microstructures prepared by SLM consisted of proeutectoid ferrite and granular bainite. Average microhardness was 346 HV, tensile strength was 1223 MPa, extensibility was 13.1% and the product of strength and elongation was 16.1 GPa%. 24CrNiMo alloy steel powder prepared by the VIGA method had good laser printability and huge potential application value for SLM-fabricated brake disc.

Assessment of nickel alloy 625 weld overlays deposited by the electroslag process

Nickel-base alloy weld overlays are commonly used in the oil and gas industry to extend the life of equipment under aggressive corrosion environments, since the overlays improve the corrosion resistance without a significant increase in the cost of manufacture when compared to massive equipment. Usually, the joints are welded by SMAW, GMAW or GTAW processes. In this respect, the electro slag welding process (ESW), which promotes high heat inputs and low dilution welds, can be an interesting option for this application as it provides high productivity as only one layer is necessary. The present work evaluates mechanical, microstructural and corrosion properties of an Alloy 625 weld overlay deposited on ASTM A516 Grade 70 carbon steel by the ESW process. The deposition was done with one and two layers in plates of dimension 50 mm × 400 mm × 400 mm, in the flat position and an average energy welding 11.7 kJ/mm. After welding, a post-weld heat treatment at 620 °C for 10 h was performed, this condition being compared with the as-welded condition. Bending tests showed no evidences of cracks. Microstructural evaluation performed using both optical (OM), scanning (SEM) and transmission electron microscopy techniques showed an austenitic microstructure of the weld deposit with a low proportion of secondary phases for all conditions, and the post-welding heat treatment did not promote significant changes in the mechanical properties. At the coarse grain heat-affected zone (CGHAZ), the occurrence of proeutectoid ferrite, pearlite and bainite were found for one layer deposit and refined pearlite and ferrite for two layer deposits where the microstructural constituents were observed due to the low cooling rate. In addition, no evidence of partially diluted zones (PDZ) were verified. Corrosion tests conducted on samples removed from the top layer of the weld overlay cladding deposit according to ASTM G 48 Method A Standard were considered satisfactory once no evidence of pittings was verified and the loss of mass was very much reduced.

Effect of dissolved niobium on eutectoid transformation behavior

The effect of dissolved niobium on the eutectoid transformation behavior in near-eutectoid high-carbon steels has been studied. Dissolved niobium is important in the eutectoid transformation behavior. It increases the eutectoid carbon content significantly (by ˜0.0477% per 0.00001% dissolved niobium), increases the hardenability of steel markedly, yields finer, more uniform, polygonal proeutectoid ferrite, induces a transition in morphology of eutectoid cementite from lamellar to somewhat spheroidal, and increases the misorientation angle of pearlite colonies from being focused near 0° to near 60°.

Modification of Microstructure and Tensile Property of Sintered Fe-Cr-Mo-C Steel by Nickel Addition

Nickel is one of the alloying elements promoting the formation of acicular ferrite at the expense of proeutectoid ferrite. The Ni addition reduces the steady-state nucleation rates of grain boundary ferrite allotriomorph in Fe-C-Ni alloys. Due to such reasons, Ni was added to modify the microstructure of the sintered steel, investigated in this study, with the aim of improved mechanical properties. The sintered steel, produced from pre-alloyed Fe-Cr-Mo powder mixed with 0.3 wt.% C, was modified by Ni addition and cooling rate. The alloy compositional change was performed additions of varied Ni contents of 1.0, 1.5, 2.0, 2.5 and 3.0 wt.%. The material processing variables were investigated by using two different cooling rates of 0.1 and 5.4 °Cs-1. Under the cooling rate of 0.1 °Cs-1, the sintered Fe-Cr-Mo-3C steel without Ni addition showed dual-phase microstructure consisting of ferrite (soft phase) and bainite (hard structure). With Ni additions, the dual-phase microstructure was replaced by bainitic structure. Microstructural heterogeneity was observed due to the presence of Ni-rich areas, which increased with increasing added Ni contents. Under the cooling rate of 5.4 °Cs-1, the sintered Fe-Cr-Mo-3C steels with and without Ni additions showed bainitic structure. Microstructural heterogeneity was similar to that of slowly cooled steel. Ni additions not only modified the sintered steel microstructure but increased tensile strength and elongation. In general, Ni pushes the C-curve of pearlite transformation to the right hand side and lowers the martensite start temperature. The absence of ferrite from the sintered steel with only 1 wt.% Ni addition and slowly cooled at 0.1 °Cs-1 suggests that Ni strongly suppresses the austenite → ferrite transformation. In another word, Ni promotes bainite formation in the sintered Fe-Cr-Mo-Ni-C steels.

Investigations on multi-run metal made of HSLA steel – Heterogeneous microstructure and mechanical properties

Multi-run high strength low alloy (HSLA) steel plates welded by manual metal arc (MMA) process were investigated. Heterogeneous weld metal microstructure was studied accompanied with its effect on the mechanical properties of the joint. Microstructural studies revealed: (1) in the last run, there were intergranular acicular ferrite and pro-eutectoid ferrite, (2) in previous weld runs, there were refined grains with precipitated particles and (3) in the interlayer region of previous runs, there were some heterogeneous ferrite grains, grain boundary discontinuities and precipitated microalloying elements. In the last run, in fusion zone weld and heat affected zone interface, the solidification is a phenomenon of an epitaxial growth type structure. However, in previous weld runs, there was neither epitaxial growth nor typical refined grain structure. The mechanical properties varied in different parts of the welded plate due to the metallurgical characteristics of weld metal in comparison with the production procedure of HSLA steels and their metallurgical parameters.

Weld morphology and microstructure during simulated local dry underwater FCTIG

The metal transfer modes, the weld morphology and microstructure during simulated local dry underwater tungsten inert gas welding with a flux-cored wire (FCTIG) are investigated by analyzing high-speed photographs of the metal transfer process, the arc temperature based on a spectral diagnosis, and scanning electron microscopy images of the welds. When the wire-electrode gap (WEG) is 3–9mm, the metal transfer mode varies from the no-contact mode/slag column mode to the continuous liquid transfer mode with an increase in WEG. The appearance of the weld pool varies greatly with the change in the metal transfer mode. Compared to the welds produced using underwater local cavity TIG welding (LC-TIG), the welds formed using underwater local cavity FCTIG (LC-FCTIG) contain more acicular ferrite and less pro-eutectoid ferrite, due to the metallurgical effect of the flux-cored wire. The weld microstructures corresponding to the different metal transfer modes are distinct, given the effects of the metal transfer modes on the arc, which result in changes in the arc temperature field.

Research on microstructure evolution of austenitization in grinding hardening by cellular automata simulation and experiment

Heat generated within the grinding zone is used to generate a hardened layer in grinding hardening. The phase fraction and carbon content of the hardened layer have significant effects on its mechanical properties. Specially, the heating rate is extremely high and the duration time beyond A c1 (austenitization starting temperature) is short in grinding hardening, so the austenitization is sometimes incompleted and the carbon content is always non-uniform. In the present paper, three-dimensional finite element method (FE) incorporating a regular triangle heat source is used for computing temperature histories of the specimen. Microstructure evolution model of austenitization of AISI 1045 steel considering the heating rate in grinding hardening is developed by using the cellular automata method (CA). In the CA model, austenite nucleation, pearlite dissolution, austenite growth in proeutectoid ferrite, and carbon diffusion are considered and simulated based on their respective theories. A new variable t d (duration time beyond A c1) is defined to describe the temperature history characteristic. The effects of grinding parameters on the temperature characteristic variables are studied. Meanwhile, austenitization are simulated under different grinding parameters. The microstructure of the hardened layer is observed in grinding hardening experiment to validate the CA model. The results show that, firstly, the CA model can effectively simulate the microstructure evolution. Secondly, both peak temperature and t d have significant effects on the austenitization in grinding hardening. Thirdly, better microstructure and mechanical properties can be obtained by optimizing grinding parameters.

Investigations of Temperatures of Phase Transformations of Low-Alloyed Reinforcing Steel within the Heat Treatment Temperature Range

AbstractThe paper presents the results of DSC analysis of steel B500SP produced in the process of continuous casting, which is intended for the production reinforcement rods with high ductility. Studies were carried out in the temperature range below 1000°C in a protective atmosphere of helium during samples heating program. The main objective of the study was to determine the temperature range of austenite structure formation during heating. As a result of performed experiments:Ac1s,Ac1f– temperatures of the beginning and finish of the eutectoid transformation,Ac2– Curie temperature of the ferrite magnetic transformation and the temperature Ac3of transformation of proeutectoid ferrite into austenite were elaborated. Experimental determination of phase transformations temperatures of steel B500SP has great importance for production technology of reinforcement rods, because good mechanical properties of rods are formed by the special thermal treatment in Tempcore process.

Precipitation enhanced ultragrain refinement of Ti-Mo microalloyed ferritic steel during warm rolling

Ultragrain refinement caused by continuous dynamic recrystallization (cDRX) was studied from microstructure, texture and precipitation. Compared with microstructures of deformation induced ferrite transition (DIFT), ferrite matrix refined through cDRX presented more substructures. Textures formed through the cDRX mainly consisted of (001)[11¯0] and (111)[1¯1¯2] and the intensity of (001)[11¯0] could increase with strain. Rolling in γ non-recrystallization region facilitated the strain induced precipitation, hence more subboundaries formed during the subsequent warm rolling process. With the consumption of alloying elements, stability of remaining austenite was reduced and this promoted the formation of high temperature proeutectoid ferrite. Additionally, stress fields induced by the nanoscale precipitates could suppress the formation of microshear bands and further promote the generation of high angle boundaries, especially in the γ-fiber textures. Grains were refined to an average size of ~1µm through cDRX and most refined grains transformed from γ-fiber textures presented an orientation of <110>//ND. Moreover, the refined matrix performed better mechanical properties along the rolling direction and the precipitation enhanced ultragrain refinement through cDRX showed greater potential on the refining of ferrite grains.

Influence of intercritical austempering on the microstructure and mechanical properties of austempered ductile cast iron (ADI)

The focus of this investigation was to examine the influence of intercritical austempering process on the microstructure and mechanical properties of low-alloyed austempered ductile cast iron (ADI). The investigation also examined the influence of intercritical austempering process on the plane strain fracture toughness of the material. The effect of both austenitization and austempering temperature on the microstructure and mechanical properties was examined. The microstructural analysis was carried out using optical microscopy, scanning electron microscopy and X-ray diffraction. The test results indicate that by intercritical austempering it is possible to produce proeutectoid ferrite in the matrix microstructure. Lower austenitizing temperature produces more proeutectoid ferrite in the matrix. Furthermore, the yield, tensile strength and the fracture toughness of the ADI decreases with decrease in austenitizing temperature. A considerable increase in ductility was observed in the samples with higher proeutectoid ferrite content. The fracture surfaces of the ADI samples revealed that dimple ductile fracture produced higher fracture toughness of 60±5MPa√m in this intercritically austempered ADI.

Effect of ultrasonic vibration on microstructural evolution and mechanical properties of underwater wet welding joint

The propagation of high-intensity ultrasonic vibration into weld pool can influence on microstructure and mechanical properties of welded joints. This research introduced a hybrid method for improving weld quality by using superimposed ultrasonic vibration on workpiece directly in underwater flux cored arc welding (FCAW). With the purpose of revealing how macrostructure, microstructure and mechanical properties of welded joints are affected by ultrasonic vibration within the weld metal, the present study has been carried out. The results indicated that proeutectoid ferrite, ferrite side plate and acicular ferrite constitute the microstructures of weld metal. Ultrasonic vibration can affect the size and morphology of prior austenite grain during weld pool solidification and eventually accelerate the formation of fine ferrite microstructure of weld metal in high proportion in the subsequently solid-state phase transformation. Excellent balance of high tensile strength and impact toughness is realized as more fine columnar microstructures in weld metal with using ultrasonic vibration. These findings suggest that the addition of ultrasonic vibration is an effective way to improve the tensile strength and impact toughness of weld metal during underwater wet welding.

Effects of Mo, Ti and B on Microstructure and Mechanical Properties of Underwater Wet Welding Joints

Alloy components are designed and transferred into weld metal via electrode covering to address the deterioration of the microstructure and mechanical properties of underwater wet welds. Emphasis is placed on studying the effects that the Mo, Ti and B contents have on the microstructure, tensile strength and low-temperature toughness of the underwater wet welding joint. The as-welded metal obtained at a water depth of 3 m is analyzed. The results indicate that the addition of Mo depresses the formation of coarse pro-eutectoid ferrite. However, a higher Mo content (0.609 wt.%) results in the formation of lath-like bainite and martensite, which are harmful to the plasticity and toughness of the weld. Acicular ferrite nucleation increases with the combined addition of Ti and B. The deposited metal with the optimum alloy components ratio achieves good plasticity and toughness while maintaining its tensile strength of 592 MPa, with the impact toughness at 0 °C and elongation reaching 53.34 J and 16.2%, respectively.

Microestrutura e Propriedades Mecânicas de um Aço para Trilhos Ferroviários Soldado por Centelhamento

Resumo Trilhos com perfil TR-57 do tipo intermediário foram soldados por centelhamento em um estaleiro brasileiro e passaram por detalhada caracterização mecânica e metalográfica. Os dados quantitativos da caracterização metalográfica e dos ensaios mecânicos foram submetidos a análises de variância (ANOVA). Na metalografia confirmou-se a presença majoritária de perlita, porém há também ferrita proeutetóide na linha central da solda e globulização parcial da cementita em regiões da Zona Termicamente Afetada (ZTA), sobretudo na zona de transformação parcial. O tamanho das colônias perlíticas e o espaçamento interlamelar foram correlacionados à microdureza Vickers em toda a extensão da junta. Ensaios de tração revelaram que o metal base atendeu às especificações da Norma AREMA, no entanto, as juntas soldadas não atenderam e a fratura dos corpos de prova ocorreu na região de menor dureza da ZTA. Os resultados permitem compreender, em detalhes, as consequências do processo de soldagem sobre a estrutura do aço e as variações de propriedades em diversas regiões da junta, com destaque às alterações morfológicas da perlita. Relações com modos de falha anteriormente relatados na literatura científica também são apresentadas.

Phase-field modeling of ferrite morphology in austenite-to-ferrite transformation with considering anisotropic effects

The morphology of proeutectoid ferrite in steels has attracted much attention in view of its close correlation with the fundamentals about the phase transformation theory as well as its potential practice relating to the final microstructure and properties of the steel product. With the recent development of mesoscale microstructure-based transformation models, the approach to integrated microstructural simulation is ideally suited to provide a more in-depth insight into the mechanism and morphology complexity for this problem. Among the various mesoscopic models, the phase-field method can readily be used to simulate the complex morphological phenomena during the austenite-to-ferrite transformation in steels in view of its convenience to include the material properties, especially the grain boundary properties, in a phenomenological way, and thus to model the microstructural process in an anisotropic system. In this study, a modified multi-phase-field (MPF) model that takes into account various anisotropic interfacial conditions is developed to simulate the growth morphology of ferrite during the austenite-to-ferrite transformation in a Fe-C-Mn alloy. In this model, a quantitative relation between the MPF model parameters and the physical anisotropic interfacial properties, including the grain-boundary energy and the mobility, is carefully considered, which allows the identical width of the diffuse interface regarding arbitrary interfacial anisotropies in the MPF simulations. In this way, both the accuracy and the numerical stability of the model can be ensured. Using this model, the effects of the grain boundary anisotropy on the ferrite growth are studied. The simulation results indicate that apart from the interfacial energy of &lt;em&gt;σ&lt;/em&gt;&lt;sub&gt;&lt;em&gt;α&lt;/em&gt;, &lt;em&gt;γ&lt;/em&gt;&lt;/sub&gt;, &lt;sub&gt;&lt;/sub&gt;the grain boundary energy between the initial austenite grains, &lt;em&gt;σ&lt;/em&gt;&lt;sub&gt;&lt;em&gt;γ&lt;/em&gt;, &lt;em&gt;γ&lt;/em&gt;&lt;/sub&gt;; does also significantly influence the growing morphology of ferrite. The ferrite growth along the initial austenite grain boundaries is facilitated when increasing the ratio of &lt;em&gt;σ&lt;/em&gt;&lt;sub&gt;&lt;em&gt;γ&lt;/em&gt;, &lt;em&gt;γ&lt;/em&gt;&lt;/sub&gt;/&lt;em&gt;σ&lt;/em&gt;&lt;sub&gt;&lt;em&gt;α&lt;/em&gt;, &lt;em&gt;γ&lt;/em&gt;&lt;/sub&gt;, and hence leading to a smaller equilibrium angle at the triple junction. The results also indicate that misorientation-dependent grain boundary energy and mobility play important roles in determining the ferrite growth behavior. The growth of ferrite with a low misorientation &lt;em&gt;α&lt;/em&gt;/&lt;em&gt;γ&lt;/em&gt; interface is greatly inhibited. The ferrites nucleated at the triple junctions of the initial austenite grains present different growth scenarios while assigning different orientation relationships. Finally, the simulated ferrite morphologies in a polycrystalline structure are compared with the optical micrograph and are found that they are in good consistence with each other. This MPF model can replicate the morphology diversity of the ferrite grains in the austenite-to-ferrite transformation.

高精度FE-EPMAによる低炭素鋼の初析フェライト変態初期における炭素の分配

高精度FE-EPMAによる低炭素鋼の初析フェライト変態初期における炭素の分配