Formation Of Acicular Ferrite Research Articles

Effect of Mg addition on formation of intragranular acicular ferrite in heat-affected zone of steel plate after high-heat-input welding

The effects of Mg content, inclusion size, and austenite grain size on the intragranular acicular ferrite (IAF) nucleation in heat-affected zone of steel plate after high-heat-input welding of 400 kJ/cm were investigated by welding simulation and observation using a scanning electron microscope equipped with an energy dispersive spectrometer and an optical microscope. The IAFs are observed in steel with Mg addition, and the volume fraction of IAF is as high as 55.4% in the steel containing 0.0027 mass% Mg. The MgO–Al2O3–Ti2O3–MnS inclusions with size around 2 μm are effective nucleation sites for IAF, whereas Al2O3–MnS inclusions are impotent to nucleate the acicular ferrite. The prior-austenite grain (PAG) size distribution in low Mg steel is similar to that in steel without Mg addition. The austenite grain with size about 200 μm is favorable for the IAF formation. In the steel with high Mg content of 0.0099%, the growth of PAG is greatly inhibited, and PAG sizes are smaller than 100 μm. Therefore, the nucleation of IAF can hardly be observed.

Enhancement of mechanical properties of low carbon steel joints via graphene addition

ABSTRACTSteel base metal laps or welding electrode surfaces were coated using graphene suspensions with various concentrations, and then the steel plates were welded using the shielded metal arc welding process. Microstructural observations showed that the addition of graphene to the weldment significantly refines the microstructure and promotes the formation of fine acicular ferrite. The results of mechanical testing indicated that with lower concentrations of graphene in the weldment, both the strength and ductility improve, but the hardness remains unchanged in comparison to the unreinforced weld metal. However, reinforcing with a higher concentration of graphene gives rise to the significant enhancement of the hardness and strength without deterioration of the ductility.

Effect of Ni content on the microstructure and mechanical properties of weld metal with both-side submerged arc welding technique

The effects of nickel (Ni) content on the mechanical properties and microstructure evolution in the weld metals of commercial K65 oil and gas pipeline steel were studied. Increase Ni content could significantly improve the strength and low temperature impact toughness (−40 °C and −60 °C) of the weld metal. The above influences of Ni were attributed to the formation of predominant acicular ferrite (AF), at the expense of grain boundary ferrite (GBF), ferrite side plates (FSP) and martensite/austenite (M/A) constituent in weld metal. EBSD results indicated that GBF keeps K-S or N-W relations only with the prior austensite grain that belongs to one side of the grain boundary and the variants belong to the same bain group which always forms low angle boundary. Moreover, the retained austenite as one part of M/A constituent was mainly found in the weld metal with lower Ni content. It indicated that the transformation process could be promoted more completely by the increase of Ni content. The mechanism of the toughness improvement brought by Ni increasing was ascribed to the relatively uniform transformation to final microstructure, which reduces the appearance of GBF and M/A constituents.

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.

Effect of double-pulsed gas metal arc welding (DP-GMAW) process variables on microstructural constituents and hardness of low carbon steel weld deposits

In this study, a bead on plate welding on a low carbon steel plate at different parameter combinations in double-pulsed gas metal arc welding (DP-GMAW) process has been carried out. The effect of heat input, pulse frequency and thermal pulse frequency on solidification variables and microstructure of the weld metal have been thoroughly studied. Formation of inclusion and acicular ferrite during solidification has been calculated to observe the effect of process parameters on the microstructural constituents. Furthermore, the microhardness analysis of the weld metals has been carried out to evaluate the effect of microstructural variations on the property. Several linear and non-linear equations have also been developed to predict the responses as a function of different process variables. The results indicate that decrease in heat input, pulse frequency and thermal pulse frequency increases the volume fraction of inclusion and acicular ferrite in the weld metals. The higher volume fraction of acicular ferrite ultimately results in fine grain microstructure with the higher hardness of weld metals. On the other hand, the grain size of coarse grain heat affected zone (CGHAZ) at lower heat input was finer which ultimately increases the heat affected zone hardness.

Segregation of Mn, Si, Al, and Oxygen During the Friction Stir Welding of DH36 Steel

This work investigates the role of welding speed in elemental segregation of Mn, Si, Al, and oxygen during friction stir welding (FSW) in DH36 steel. The experimental work undertaken showed that when the speed of the FSW process exceeds 500 RPM with a traverse speed of 400 mm/min, then elemental segregation of Mn, Si, Al, and O occurred. The mechanism of this segregation is not fully understood; additionally, the presence of oxygen within these segregated elements needs investigation. This work examines the elemental segregation within DH36 steel by conducting heat treatment experiments on unwelded samples incrementally in the range of 1200–1500 °C and at cooling rates similar to that in FSW process. The results of heat treatments were compared with samples welded under two extremes of weld tool speeds, namely W1 low tool speeds (200 RPM with traverse speed of 100 mm/min) and W2 high tool speeds (550 RPM with traverse speed of 400 mm/min). The results from the heat treatment trials showed that segregation commences when the temperature exceeds 1400 °C and Mn, Si, Al, and oxygen segregation progress occurs at 1450 °C and at a cooling rate associated with acicular ferrite formation. It was also found that high rotational speeds exceeding 500 RPM caused localized melting at the advancing-trailing side of the friction stir-welded samples. The study aims to estimate peak temperature limits at which elemental segregation does not occur and hence prevent their occurrence in practice by applying the findings to the tool’s rotational and traverse speed that correspond to the defined temperature.

First-principles study on influence of molybdenum on acicular ferrite formation on TiC particles in microallyed steels

In this study, influences of molybdenum on acicular ferrite formation on precipitated TiC particles are investigated from thermodynamic and kinetic respects. In thermodynamics, Segregation of Mo towards austenite/TiC interface releases the interfacial energy and induces phase transformation from austenite to acicular ferrite on the precipitated TiC particles. The Phase transformation can be achieved by displacive deformation along uniaxial Bain path. In addition, the segregation of Mo atom will also lead to the enhanced stability of ferrite in comparison with austenite no matter at low temperature or at high temperature. In kinetics, the Mo solute in acicular ferrite can effectively suppress the diffusion of carbon atoms, which ensures that orientation relationship between acicular ferrite and austenitized matrix can be satisfied during the diffusionless phase transformation. In contrast to ineffectiveness of TiC particles, the alloying Mo element can facilitate the formation of acicular ferrite on precipitated TiC particles, which is attributed to the above thermodynamic and kinetic reasons. Furthermore, Interfacial toughness and ductility of as-formed acicular ferrite/TiC interface can be improved simultaneously by segregation of Mo atom.

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.

Toughness improvement by Zr addition in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

ABSTRACTThe effect of Zr addition on the microstructure and impact toughness in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels subjected to 100 kJ cm-1 heat input was investigated. The second- phase particles were mainly Al–Ti complex oxides and (Ti,Nb)N precipitates in Zr-free steel, whereas lots of finer Zr–Al–Ti complex oxides and (Al,Ti,Nb)N precipitates were formed in Zr-bearing steel because of Zr addition. These finer oxides and precipitates effectively restricted the austenite grain growth by pinning effect during welding thermal cycle, and smaller and more uniform prior austenite grains were obtained in CGHAZ of Zr-bearing steel. Furthermore, more acicular ferrite grains nucleated on Zr–Al–Ti complex oxides, inducing formation of fine-grained microstructure in CGHAZ of Zr-bearing steel. The toughness improvement in CGHAZ of Zr-bearing steel with dimple fracture surface was attributed to the grain refinement by pinning effect and acicular ferrite formation.

Effects of Mn and Al on the Intragranular Acicular Ferrite Formation in Rare Earth Treated C–Mn Steel

AbstractThe influence of Al, Mn and rare earth (RE) on microstructure of C–Mn steel was investigated. The capacities of different RE inclusions to induce intragranular acicular ferrite (AF) formation were compared. Result shows that RE treatment could make C–Mn steel from large amounts of intragranular AF. Al killed is detrimental to the formation of intragranular AF in RE-treated C–Mn steel. An upper bainite structure would replace the AF when Al content increased to 0.027 mass %. The optimal Mn content to form AF is about 0.75–1.31 mass %. The effective RE inclusion which could induce AF nucleation is La2O2S. When patches of MnS are attached on the surface of La2O2S inclusion, AF nucleation capacity of RE-containing inclusion could enlarge obviously. The existence of manganese-depleted zone and low lattice misfit would be the main reason of La-containing inclusion inducing AF nucleation in C–Mn steel.

Effect of vanadium on dynamic continuous cooling transformation behavior of medium-carbon forging steels

Dynamic continuous cooling transformation (CCT) behavior of medium-carbon forging steels microalloyed with different V contents up to 0.29% was investigated by means of dilatometric measurement, microstructural observation and hardness measurement. The results showed that the dynamic CCT diagrams were similar and the main difference was that the fields of the transformation products were shifted to the right side of the diagrams with the increase of V content, and this effect was more noticeable with an addition of 0.29% V. The Ac1 and Ac3 temperatures showed increasing trends with increasing V content, while the critical cooling rates decreased with increasing V content. The increase of V content resulted in significant increase of hardness and this tendency was enhanced with increasing cooling rate until the formation of acicular ferrite (AF). A promising approach of remarkably improving the toughness of ferritic-pearlitic medium-carbon forging steels with suitable V addition and the introduction of AF without notable penalty on its strength level was suggested.

On the Capability of Nonmetallic Inclusions to Act as Nuclei for Acicular Ferrite in Different Steel Grades

Acicular ferrite nucleates intragranularly on nonmetallic inclusions, forming a microstructure with excellent fracture toughness. The formation of acicular ferrite is strongly affected by the size, content, and composition of nonmetallic inclusions, but also by the composition of the steel matrix. The potential of inclusions in medium carbon HSLA (high-strength low-alloyed) steels has been the main focus in the literature so far. The current study evaluates the acicular ferrite capability of various inclusions types in four different steel grades with carbon contents varying between 0.04 and 0.65 wt pct. The investigated steels are produced by melting experiments on a laboratory scale and subsequent heat treatment in a High-Temperature Laser Scanning Confocal Microscope. Inclusions are exclusively formed by deoxidation and desulfurization reactions. No synthetic particles are added to the melt. The inclusion landscape is analyzed by Scanning Electron Microscopy. Final ductility of the samples is evaluated based on performed tensile tests. Inclusion types in every steel grade are assessed regarding their nucleation potential always considering the interaction with the steel composition, especially focusing on the role of manganese. The effects of (Ti,Al)Ox-, MnS-, and MgO-containing inclusions are discussed in detail.

In-situ microscopy study of grain refinement in the simulated heat-affected zone of high-strength low-alloy steel by TiN particle

ABSTRACTHigh-strength low-alloy steels subjected to high heat input welding are susceptible to failure due to low toughness caused by grain coarsening. The effect of TiN on grain refinement in the simulated heat-affected zone (HAZ) was investigated. Because of small amount of Ti addition, abundant dispersed nanoscale TiN precipitates were formed. The TiN precipitates tended to be stable at high temperature and effectively retarded the austenite grain growth by refining the grain size during thermal cycle. Furthermore, the TiN also covered on the surface of Al–Ti complex oxide with MnS and caused low interface energy with ferrite. The acicular ferrite grains nucleated on complex inclusion in austenite grains at intermediate temperature and induced the austenite grain transform to the fine-grained mixed microstructure of acicular ferrite and bainite. The crystallographic grain size became small in the simulated HAZ due to the effective pinning effect and acicular ferrite formation.

Structural Investigation and Control of Multi-Pass Gas-Shielded Flux-Cored Arc Weldments

Structure–property relationships for multi-pass gas-shielded flux-cored arc welds of X70 grade pipeline steel have been evaluated. It is known that the optimum structure for these kinds of welds would consist mainly of acicular ferrite with a minimum of allotriomorphic ferrite along prior-austenite grain boundaries. Non-metallic inclusions based on Ti2O3 play an important role for intragranular nucleation of acicular ferrite. Fine non-metallic inclusions of more complex compositions, Al2O3-CaO-SiO2-MnO-MgO-TixOy-MnS, were detected within the microstructure of the investigated welds. These inclusions were the substrates for formation of intragranular acicular ferrite in the weld metal. The effective combination of the number and thickness of different structural zones guarantees high toughness of the weld metal. The influence of constructional and technological parameters of multi-pass welding (welding gap, welding torch tilt during layer filling, welding direction for a hot pass, different heat inputs) on the primary as-welded and secondary reheated zones of the lower layers of the weld was investigated. Allotriomorphic ferrite volume fraction was estimated using panoramic images of the lower layers of welds along with the fracture surfaces of Charpy V–notch impact test bars. A method for quantitative estimation of allotriomorphic ferrite volume fraction (VAF) by image analysis was developed. This allows formulation of the requirements for critical values of VAF that guarantee high-impact toughness of the lower layers of welds. All results have been used to improve the welding technology.

Improvement of Utilization Ratio of Nanoparticles in Steel and Its Influence on Acicular Ferrite Formation

In the present study, 35MnVS experimental steels containing nanoparticles are manufactured using a vacuum induction furnace (VIF), as well as a vacuum induction levitation furnace (VILF). The differences on the utilization ratio of nanoparticles (URN), inclusion characteristics, and steel microstructure between the original steel and experimental steels are compared. The results reveal that the steel processed with a VILF has a higher URN that helps to form a finer inclusion size range. There is a critical size for each inclusion, and only when the inclusion size is ≤ the critical value, the inclusions can efficiently induce acicular ferrites (AF). Among the three steels, only the inclusion size range of the VILF steel is less than the critical size which is between ≈2.2 and ≈5.2 µ. Therefore, the inclusions in VILF steel have a relatively stronger ability on inducing AF, and that is revealed by its microstructure showing large proportions of AF.

Effect of Cu addition on microstructure and impact toughness in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The effects of Cu content on microstructure and impact toughness in the simulated coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels were investigated. It has been observed that the microstructure in the simulated CGHAZ of Cu-free steel is dominated by a small proportion of acicular ferrite and predominantly bainite with martensite–austenite constituent. Whereas, in the 0.45 and 1.01% Cu-containing steels, the acicular ferrite increased significantly due to the effective nucleation on intragranular inclusions with outer layer of MnS and CuS. The formation of acicular ferrite is attributed to superior high heat-affected zone impact toughness in the 0.45% Cu-containing steel. Furthermore, the increasing martensite–austenite constituent and ε-Cu precipitates in the simulated CGHAZ of 1.01% Cu-containing steel caused degradation in impact toughness.

Effect of Ti as a trace element on mechanical properties and microstructural evolution in direct austempered ductile cast iron

Abstract In this study, the effect of Ti as a trace element on acicular ferrite formation in direct austempered ductile cast irons was investigated. A step-block shape sample was used and the melt was poured into a sand mold at about 1450 °C. After cooling in the mold, the melt was hot knocked out at approx. 900 °C and then medium cooling to 350 °C was applied to the step-block shape sample. Afterwards, isothermal treatment was conducted in a commercial furnace at this temperature for 1 h, and then the sample was air-cooled down to room temperature. It was found that Ti as a trace element has an important effect on the nucleation of beneficial phases such as acicular ferrite in ductile irons which can result in increasing of mechanical properties. The microstructural examination confirmed that Ti actually acts as nucleation site for acicular ferrite. Moreover, the results show that in direct austempered ductile cast iron samples contain 0.25 wt.-% Ti, an increase from 820 to 950 MPa in tensile strength properties can be seen. Furthermore, the impact test results show that with an increase of tensile strength, the impact toughness does not decrease, but rather remains constant.

Sintering behaviours of iron ore with flue gas circulation

ABSTRACTFlue gas circulation is an important method for energy conservation and pollutant emission reduction in iron ore sintering. In this paper the effects of flue gas recirculation ratio on sintering of different iron ores including haematite, magnetite and limonite were studied by illustrating the variation of sinter bed temperature, atmosphere and mineralisation characteristics of different types of iron ores induced by the circulation. It shows that the proper flue gas circulation ratios for haematite, magnetite and limonite are 37, 30 and 25%, respectively. For magnetite ore, preheating and high consumption of oxygen in combustion zone caused more silicate minerals and less acicular calcium ferrite, thereby lowering sinter tumbler strength. As for haematite ore, the rapid change of temperatures of combustion, melting and solidification zones leads to elevated combustion efficiency and increased formation of acicular calcium ferrite, which enhances the sinter strength. When using limonite ore as the main raw material, high oxygen consumption, lower maximum temperature of sintering bed, higher cooling rate and larger porosity of sinter are observed.

Friction Stir Welding of DH36 Steel and Segregation of Mn, Si, Al, and Oxygen

This work investigates the role of welding speed in elemental segregation of Mn, Si, Al, and oxygen during Friction Stir Welding (FSW) in DH36 steel. The experimental work undertaken showed that when the speed of the FSW process exceeds 500 RPM with a traverse speed of 400 mm/min then elemental segregation of Mn, Si, Al and O occurred. The mechanism of this segregation is not fully understood; additionally the presence of oxygen within these segregated elements needs investigation. This work examines the elemental segregation within DH36 steel by conducting heat treatment experiments on un-welded samples incrementally in the range of 1200-1500°C and at cooling rates similar to that in FSW process. The results of heat treatments were compared with samples welded under two extremes of weld tool speeds namely W1 low tool speeds (200 RPM with traverse speed of 100 mm/min) and W2 high tool speeds (550 RPM with traverse speed of 400 mm/min). The results from the heat treatment trials showed that segregation commences when the temperature exceeds 1400°C and Mn, Si, Al and oxygen segregation progress occurs at 1450°C and at a cooling rate associated with acicular ferrite formation. It was also found that high rotational speeds exceeding 500 RPM caused localised melting at the advancing-trailing side of the friction stir welded samples. The study aims to estimate peak temperature limits at which elemental segregation does not occur and hence prevent their occurrence in practice by applying the findings to the tool’s rotational and traverse speed that correspond to the defined temperature.

Role of Lanthanum Addition on Acicular Ferrite Transformation in C–Mn Steel

Acicular ferrite formation in C–Mn steel after rare earth La addition and the potency of different La-containing inclusions inducing the formation of acicular ferrite have been investigated. It was found that there would be a large amount of acicular ferrite in quenched steel after 0.020 mass% La additions. The size of effective inclusion for acicular ferrite nucleation is mainly 1–4 µm. The best single La-containing effective inclusion is La2O2S. When MnS attached to the surface of La2O2S forming complex inclusion, the potency of inclusion on nucleating acicular ferrite is enhanced significantly. Low lattice mismatch between α–Fe and La2O2S plays an important role on acicular ferrite nucleation. When patches of MnS precipitated on the surface La2O2S forming the complex inclusion, the main mechanism for that inclusion is the combine of low degree of lattice mismatch and existing of Mn-depleted zone.