Ferrite Microstructure Research Articles

Effect of intercritical heat treatment process on the microstructure and mechanical properties of low alloy structural steels with residual element tin

ABSTRACT The intercritical heat treatment process is considered a novel technology to solve the mismatch of strength and toughness of low alloy steel containing residual elements. In the present study, the effects of initial microstructure and intercritical quenching temperature on microstructural morphology, mechanical properties and fracture appearance of the experimental tin-bearing steel have been investigated. The results show that, under the same intercritical quenching and tempering (IQ-T) heat treatment parameters, the initial microstructures of bainite and ferrite (B-F) and complete martensite (M) yield different ferrite morphologies, block and strip-like ferrite, respectively. It is found that after IQ-T process, the tensile strength of the steel containing 0.09% tin with B-F as initial microstructure is 1061 MPa, but the impact energy value is only 26 J; while the steel with M has achieved the combined properties of high strength and excellent toughness, and the optimal intercritical quenching temperature is 770°C.

Evaluation of the corrosion behavior of different regions of the deposited weld metal from the Ni-modified E8018-G electrode

In this study, the corrosion behavior of different regions of the weld metal produced from the E8018-G electrode containing 7.0 wt% Ni in a corrosive solution has been investigated. X-ray diffraction (XRD) method has been used to compare the phase intensity in different welding regions. Electrochemical evaluations show that in the weld joint, by moving from the root of the weld metal to the upper layers of the center and cap, the corrosion resistance of the weld metal increases. In addition, the results demonstrates that the corrosion of the St-52 welded joint mainly occurs in the heat affected zone (HAZ). According to the microstructures observed from the spectra obtained by the field emission scanning electron microscopy (FE-SEM), the corrosion behavior of the weld metal has been evaluated. The examination of the local weld metal layers indicates that not only the corrosion resistance of the cap layer has improved compared to the central and root layers, but also the corrosion resistance has increased compared to the base metal. This improvement is attributed to the existence of a homogeneous acicular ferrite microstructure in the cap layer and higher nickel content.

Effect of Cooling Rate on Phase Transformation Kinetics and Microstructure of Nb–Ti Microalloyed Low Carbon HSLA Steel

Nb–Ti microalloyed low carbon high strength low alloy (HSLA) steels are used to fabricate components, particularly for the automobile and piping applications requiring optimum combination of mechanical properties along with good weldability. These properties depend on the transformed ferrite microstructure and grain size obtained after cooling from hot rolling temperatures which are always well above the austenitizing temperature. Hence, an attempt was made to study the austenite decomposition (phase transformation) kinetics and the accompanying microstructural evolution in Nb–Ti microalloyed steel by subjecting it to austenitization at 1100 °C for 3 min followed by cooling at different rates ranging from 1 to 100 °C/sec in a B\(\ddot{a}\)hr DIL 805 A/D dilatometer. The first derivative method was employed to identify critical transformation temperatures from the dilation curves. A modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis (used for non-isothermal conditions) was carried out to find the time exponent (n’) values controlling the rate of transformation at different cooling rates. The nature of transformed ferrite was observed to change from polygonal to acicular type, and its grain size was found to decrease with an increase in cooling rate. EBSD analysis also revealed the cooling rate to have a profound effect on the microtexture evolution of the concerned alloy. The “\(\gamma\)” fibre and rotated cube components are replaced by the transformed copper (“\(\alpha\)” fibre) components owing to faster transformation and lack of recrystallization of transformed \(\alpha \) with increase in cooling rate. Finally, a power law and logarithmic relationship of grain size and microhardness with the applied cooling rate were established.

Hydrogen gaseous embrittlement effect over mechanical properties of an experimental X-120 microalloyed steel subjected to heat treatments and different cooling rates

This research work aimed to determine the hydrogen gas pressure effect on the mechanical properties of an experimental X-120 microalloyed steel, subjected to heat treatments and quenched in different mediums. The steel in its as-received condition was reheated at 900 °C and quenched in spray water (900QSW), pressurized air (900QPA), and emulsion of water-oil medium (900QWO) which produces complex microstructures formed by martensite–bainite–acicular ferrite; otherwise, reheated at 820 °C and quenched in oil media (820QO) which produces a banded martensite-polygonal ferrite microstructure. To determine the hydrogen embrittlement susceptibility, in-situ tension tests were developed at 1, 4, and 7 MPa of hydrogen gas pressure. The results showed that as hydrogen gas pressure increases, the mechanical properties reduce in all quenched conditions, being the most susceptible condition the 820QO sample which presented the higher embrittlement index; on the contrary, the less susceptible condition was the 900QPA sample.

Partial austenitisation and TBF steel composed of ferrite, bainitic ferrite, and austenite

A TRIP-aided bainitic-ferritic (TBF) steel with a chemical composition of Fe–0.19C–1.7Mn–1.09Si–0.51Al–0.05Nb (wt-%) was partially austenitised from a hot-rolled martensitic initial microstructure. After the hot rolling, the martensitic specimens were reheated to different intercritical temperatures and then austempered at 350°C. Thus, the effect of the initial microstructure of TBF steel on intercritical austenite formation during partial austenitisation was studied. The microstructures were investigated by scanning electron microscopy and electron backscatter diffraction (EBSD), and the tensile properties were tested. Microstructural observations revealed that a final microstructure of fine ferrite, bainitic ferrite, and retained austenite can be obtained. The steel partial austenitised at 770°C showed a good combination of ultimate tensile strength and total elongation.

Effects of aluminum substitution on the microstructure and magnetic properties of cobalt ferrites prepared by the co-precipitation precursor

Effects of aluminum substitution on the microstructure and magnetic properties of cobalt ferrites prepared by the co-precipitation precursor

Performance and Characteristics of Stationary Arc and Rotating Arc-Gas Metal Arc Welded DMR 249 Naval Grade Steel Joints

Abstract In this investigation, 12-mm thickness naval grade high-strength low-alloy steel plates were welded by rotating arc-gas metal arc welding (RA-GMAW) and stationary arc-gas metal arc welding (SA-GMAW) processes. The main objective of this work was to carry out the comparative analysis of mechanical properties and microstructural characteristics of the joints fabricated by the outlined welding processes. From this experimental work, it is found that the ultimate tensile strength, yield strength, and impact toughness of the RA-GMAW joint are 15 % higher than those of the SA-GMAW joint. This is because of the formation of a higher volume percentage of acicular ferrite microstructure and island martensite/austenite constituent in the weld metal region of the RA-GMAW joint. Moreover, from the productivity point of view, the number of passes in the RA-GMAW process was minimized to three (from six passes), and the width of the heat-affected zone was also reduced by 45 % to SA-GMAW processes. Overall, the RA-GMAW joints are found to have superior mechanical properties and microstructural characteristics compared to SA-GMAW joints.

Welding heat input for synergistic improvement in toughness and stress corrosion resistance of X65 pipeline steel with pre-strain

The microstructure, hydrogen embrittlement sensitivity, fracture toughness, and influence of pre-strain on the sulfide stress corrosion cracking (SSCC) sensitivity of the coarse-grained heat-affected zone of X65 pipeline steel with large deformation were studied to determine the optimal welding heat input in an acidic H2S environment. After pre-straining, the microstructures of the specimens changed insignificantly, but their ductility decreased. The acicular ferrite microstructure obtained under a low heat input demonstrated the synergistic improvement in the SSCC resistance and fracture toughness under pre-strain, especially at t8/5 = 20 s. The M/A constituents interface potentially causes microcrack initiation and interconnection under pre-strain.

Development of Desirable Fine Ferrite Grain Size and Random Second Phase Dual-Phase Steel Microstructures Using Composition and/or Processing Modifications

Microstructural morphology is known to have a significant impact on the mechanical properties of dual-phase steels. A fine ferrite grain size and random distribution of small second phase islands are desirable to provide superior isotropic properties compared to the banded second phase distribution that is typical for this type of steel. A rapid alloy prototyping (RAP) facility has been used to investigate three different DP 800 variants by systematically varying the compositions and/or process parameters compared to the ‘standard’ DP800 composition and processing that gives a banded microstructure. For Variant 1, the heating rate during the annealing cycle after cold rolling varied between 0.65 and 30 °C/s for the 45%, 60% and 75% cold reduction samples. It was found that a cold reduction of 75% and heating rate of 15 °C/s resulted in the microstructure that can give the best combination of strength and ductility because of the fine grain size and high martensite volume fraction. For Variant 2, the effect of changing the hot rolled (HR) microstructure (ferrite–pearlite, ferrite–bainite or martensite) on the final microstructure was investigated. Both the ferrite–50% bainite and fully martensite/bainite HR materials for all cold reductions resulted in annealed microstructures with necklace martensite morphology and finer ferrite grains compared to the ferrite–pearlite HR material, which gave a typical banded ferrite–martensite microstructure with a coarser ferrite grain size. For Variant 3, the Mn content was reduced, and increased Nb was used to achieve higher pancaking during the hot rolling stage, which refined ferrite grains in the HR condition with the same hardness. After annealing with the standard parameters only the 45% cold-reduced material produced a finer ferrite grain size than the standard material, whereas the 60% and 75% cold-reduced samples required a higher heating rate to achieve finer ferrite grain sizes due to rapid recrystallisation and growth kinetics.

Quantification of the carbon content of single grains in martensite-ferrite dual phase steel by UHV-EDXS

The detection and quantification of carbon by conventional energy dispersive X-ray spectroscopy (EDXS) performed under standard conditions is not feasible due to occurring contaminations in common electron microscopes. In contrast, novel ultra high vacuum EDXS (UHV-EDXS) was used to acquire elemental mappings of carbon on dual phase (DP) steel, which exhibits a microstructure consisting of ferrite and martensite. These phases differ in hardness, local dislocation density and, most importantly, in their carbon content. Since the UHV conditions in combination with a customized windowless EDXS detector ensured a minimization of hydrocarbon contamination during the measurements and a maximization of the sensitivity for the detection of light elements, UHV-EDXS carbon mappings could successfully be obtained, which clearly reflect the ferrite and martensite microstructure of the investigated DP steels, as confirmed by electron back scatter diffraction (EBSD). Most importantly, it was possible to quantify the carbon content of individual grains with concentrations as low as 0.25 wt%. Furthermore, nano-hardness tests were performed on the very same grains which were characterized by UHV-EDXS and EBSD. It is shown that the hardness of the martensite grains is correlated to their carbon content, as directly determined by UHV-EDXS. With this new method an additional tool for advanced material characterization of multiphase materials on the level of individual grains is now available. • Quantification of low carbon concentrations is obtainable by EDXS under UHV conditions. • The carbon content of single martensite grains was determined by UHV-EDXS in DP steels. • Nanoindentation was equally performed on individual martensite and ferrite grains of the DP steels. • Correlation between the hardness of the grains, the martensite volume fraction and the carbon content in the martensite was proven.

Effect of welding mode and filler metal on microstructure and mechanical properties of dissimilar joints of S900MC thermomechanical steel to St52 carbon steel welded by gas tungsten arc welding

In this research, S900MC thermomechanical steel was jointed to St52 carbon steel through a gas tungsten arc welding process in the continuous and pulsed modes. The welding was performed in two different weld designs of V-groove and close square, with and without using an ER70S filler metal, respectively. Furthermore, the microstructure, hardness, and tensile strength of the joints were investigated. A bainitic microstructure was observed in the weld metal of the samples jointed in the continuous mode. While the weld metal microstructure of the pulsed joints was changed from a ferritic microstructure to a tempered martensitic microstructure by implementing filler metal in the V-groove weld design. In all joints, the heat-affected zone was narrower at the S900M side and had a fine bainitic microstructure. Among all joints, the maximum hardness of 389 HV was achieved for the weld metal of V-groove joints welded in the pulsed mode. At a normal strain rate, there was no considerable contrast between the yield and tensile strength of all joints, however, the elongation of the continuous joints was better than that of the pulsed joints. Increasing the strain rate led to the drastic improvement of yield and tensile strength of the joint welded in the continuous mode without using filler metal.

Effect of weld microstructure on the tensile properties and impact toughness of the naval, marine-grade steel weld joints

Pulsed direct current (PDC) gas tungsten arc welding (GTAW) of 12 mm thick plates of naval, marine-grade high strength low alloy steel (HSLA) using ER80S–Ni3 filler metal was investigated. The microstructural characteristics were examined by both optical microscopy and field-emission scanning electron microscopy (FE-SEM) techniques. The fusion zone is comprised of mixed microstructures of acicular ferrite (AF), polygonal ferrite (PF), grain boundary ferrite (GBF), and bainite ferrite (BF) laths in the various welding passes. The yield and tensile strengths of the PDCGTA weld joints were found to be superior to that of the base metal. A joint efficiency of 122% was observed for the weld seams while conducting the notch tensile studies (NTS). The impact toughness of the weld joints was higher than that of the base metal. The tensile and impact properties in the room temperature (RT) conditions were superior due to the formation of acicular ferrite (AF) and bainite ferrite (BF) laths in the weld seam microstructure. A higher toughness value of 173 J and ductile fracture mode was observed when the joints were impact loaded at −40 °C. On the other hand, the brittle cleavage fracture was observed for the joints subjected to impact loading at −196 °C. The impact toughness data indicated that the joints experienced a transition from a ductile to a brittle mode of fracture on lowering the temperature.

Effect of Nb Addition and Heat Input on Heat-Affected Zone Softening in High-Strength Low-Alloy Steel.

The effect of both Nb content and heat input on the softening phenomenon of the heat-affected zone (HAZ) of low-alloy high-strength steel was studied through welding thermal simulation experiments. The microstructure evolution, density variation of geometrically necessary dislocation, microhardness distribution and the second phase precipitation behavior in HAZ was characterized and analyzed by combining the optical microscope, scanning electron microscope, high-resolution transmission electron microscope with microhardness tests. The results showed that the softening appeared in the fine-grain HAZ (FGHAZ) of the low-alloy high-strength steel with the polygonal ferrite and bainite microstructure. With an increase in Nb content, the FGHAZ softening was inhibited even with high heat input; however, the hardness shows little variation. On the one hand, the increase in the Nb content increased the volume fraction of high-strength bainite in the FGHAZ. On the other hand, the remarkable strengthening was produced by the equally distributed precipitation nanoparticles. As a result, the two factors were the main reason for the solution of the FGHAZ softening problem in the low-alloyed high-strength steel with the mixed microstructure of ferrite and bainite.

Critical assessment 42: acicular ferrite formation and its influence on weld metal and heat-affected zone properties of steels

Toughness and fatigue resistance of welds and associated heat-affected zones (HAZs) in high-strength steels are important design parameters in structural applications. Acicular ferrite microstructures, which can be achieved through control of weld process and nucleant (inclusions or precipitates) characteristics, are often regarded as key to enhancing weld metal and HAZ properties of welded steels. This paper addresses the transformation characteristics of acicular ferrite related to alloying and weld processing as well as the specific role of acicular ferrite in weld performance, along with future research needs to identify weld processes suitable to specific alloying strategies.

Effect of sintering temperature on microstructure and magnetic and dielectric properties of M-type barium ferrites

In this study, the effect of sintering temperature on microstructure, magnetic properties and dielectric behavior of M-type barium ferrites (BaM) is systematically analyzed. BaBi0.45(CoTi)1.2Fe9.15O19 samples with a dense microstructure and typical hexagonal shape are obtained at the sintering temperature of 920 °C. Moreover, the permittivity (ε′) of BaM increases to 21.41 at 0.01 GHz and the permeability (μ′) increases to 10.82 at the optimized temperature of 920 °C. Based on mathematical fitting, the saturation magnetization (4πMs) of samples is calculated to be 3272.27 Gauss and the magnetocrystalline anisotropy constant K1 is found to be 4.18 × 106 erg cm−3. More interestingly, the magnetic loss angle tangent (tanδμ) is reduced to 2.7 × 10−1 and the dielectric loss angle tangent (tanδε) is reduced to 5.8 × 10−3. The optimized magnetic and dielectric properties, with low losses, enhance the suitability of BaM ferrites for miniaturized microstrip devices.

Microstructure Characteristics and Wear Performance of a Carburizing Bainitic Ferrite + Martensite Si/Al-Rich Gear Steel

In this paper, a new low-carbon alloy gear steel is designed via Si/Al alloying. The carburizing and austempering, at a temperature slightly higher than the martensitic transformation point (Ms) of the surface and much lower than the Ms of the core, for different times, were carried out on the newly designed gear steel. After heat treatment, a series of different microstructures (superfine bainitic ferrite + retained austenite, superfine bainitic ferrite + martensite + retained austenite, and martensite + retained austenite) were obtained on the surface, whilst the low-carbon lath martensitic microstructure was obtained in the core. The microstructure of the surface was examined using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The phase composition was analyzed using X-ray diffraction (XRD). The hardness and wear resistance of the surface as well as the hardness distribution of carburizing layer of the samples with different microstructures were studied. The results show that the Si/Al-rich gear steel, after carburizing and austempering at 200 °C for 8 h, not only has excellent mechanical properties but also has high wear resistance, which meets the technical requirements of heavy-duty gear steel. The research work in this paper can provide a data reference for the application of carburized steel with mixed microstructures of bainitic ferrite and martensite in the design of heavy-duty gear.

Structure formation during sheet steel production in a unit of continuous casting and deformation

The paper considers the main problem of improving quality of steel sheets for welded pipes obtained at plate rolling mills and the main reasons that reduce it. There is a possibility of improving quality of steel sheets using technological capabilities of the unit of combined continuous casting and deformation. Problem of determining stress state of metal in cyclic deformation zone is posed and solved. The results of obtaining 09G2S sheet steel in pilot unit of continuous casting and deformation are presented. Magnitude of stresses in the center of cyclic deformation during production of steel sheets was determined for welded pipes at the unit. The authors present results of experimental study of the process of steel sheets production at pilot unit of continuous casting and deformation. The article considers the results of microstructure analysis, microhardness determination and microanalysis of chemical composition of the studied steel in various zones of the strip cross section. Structural inhomogeneity development in ferritic-pearlitic steel is interpreted along the thickness of sheet under conditions of combined thermomechanical processing of metal solidified in the mold. Predominantly ferritic microstructure is generated in the central light interlayer of the strip compared to base metal, characterized by a sharp decrease in carbon concentration in it. There is a possibility of such structure genesis at the first stages of crystallization during forging of low-carbon steel, when the excess phase of high-temperature ferrite that solidifies first is located mainly in the strip center. The size of pearlite grains in the light interlayer covariates with the grain structure of pearlite in the strip base metal and does not form precipitates of lath morphology, which increase tendency of steel to brittle fracture.

The Influence of Microstructure on the Electromagnetic Behavior of Carbon Steel Wires

This paper describes the relations between microstructure, mechanical properties, and electromagnetic behavior of carbon steel wires submitted to different thermomechanical treatments. The electrical resistivity and bulk magnetic properties are determined through resistivity measurements down to 2 K and magnetic hysteresis loop measurements. In addition, magnetic domains are imaged by magnetic force microscopy despite the complex microstructures. The electromagnetic properties are mainly related to changes in the volume fraction, morphology, and distribution of the cementite phase within the α-ferrite matrix. Electrical conductivity and magnetic permeability increase in the order of martensite, tempered martensite, pearlite, proeutectoid ferrite-pearlite, spheroidite, and ferrite microstructures. The increase in carbon concentration enhances the electrons localization at atomic sites, assisting the covalent character of Fe–C interatomic bonds and thereby reducing conductivity. Moreover, the α-Fe3C interfaces that act as a physical barrier for dislocation slip in ferrite, affecting also the main free-paths for conductive electrons and magnetic domain walls displacements within the materials. As the electromagnetic behavior of steels results from individual contributions of microstructural elements that are often intrinsically related to one another, a careful interpretation of both electrical and magnetic responses is critical for a proper application of quality and process monitoring methods of carbon steel wires.

Effect of Al on the microstructure and corrosion resistance of high-Mn steel

Fe-16Mn-1.5C (wt%) steels with 0, 5 and 10 wt% Al content are prepared to investigate the effect of Al on the microstructure and corrosion resistance of high-Mn steel. When the Al content is 0 and 5 wt%, the samples exhibits a fully austenitic microstructure, whereas a ferritic microstructure appears in the sample with 10 wt% Al content. Electrochemical testing results show that the addition of Al improves the corrosion resistance of high-Mn steel in NaCl and NaHSO3 solutions. Al changes the composition of the oxide film on the surface of the high-Mn steel during electrochemical testing, and the Al2O3 film is beneficial for reducing the corrosion rate. Therefore, Al addition can improve the corrosion resistance of high-Mn steel.

Effect of Bainitic Isothermal Treatment on the Microstructure and Mechanical Properties of a CMnSiAl TRIP Steel

TRIP-assisted CMnSiAl steels with a fully martensitic initial microstructure have been studied in order to investigate the effect of partial substitution of Si by Al. The steel was fabricated by casting in a sand mold, hot forged, homogenizing, hot rolling, cold rolling, intercritical annealing, and finally, an isothermal bainitic treatment. During the intercritical annealing at 1023 K (750 °C) for 420 s, a matrix with a microstructure consisting of 50% proeutectoid ferrite was induced and after isothermal treatment at 663 K (390 °C) for 300 s, a microstructure of ferrite, bainite, and retained austenite was obtained. This austenite was carbon enriched and therefore stabilized, with an Ms lower than original austenite, before bainitic treatment. Microstructure and tensile properties have been analyzed, comparing these results with those corresponding to a commercial TRIP 780 steel. During tensile tests, retained austenite changes from 8.1 to 1.9%vol, with a total elongation of 23.2%, which demonstrates the TRIP effect. Texture analysis showed a weak γ-fiber in the steel, which deteriorates the drawing properties of the steel. The maximum elongation during the tensile test of this steel was obtained at 323 K (50 °C), correlating with the TRIP effect present in steel, which is favored when working within the critical zone of temperatures Msσ and Md. The results show that partial substitution of Si by Al decreases yield stress, ultimate tensile strength, increases the total elongation, and decreases the temperature for maximum elongation, related to TRIP 780.