Intragranular Acicular Ferrite Research Articles

In-depth analysis of intragranular acicular ferrite three-dimensional morphology

Precipitation of intragranular acicular ferrite (IAF) is the principle means for increasing the strength of steel. This study investigated the three-dimensional morphology and growth behavior of IAF. Focused ion beam, confocal laser scanning microscopy and electron backscattered diffraction revealed that IAF had a plate-like morphology with numerous pits and defects when precipitated at 750 °C for 1 min and was needle-like with a smooth surface when precipitated at 700 °C for 1 min. Furthermore, sub-grains were observed within the IAF. This study elucidates more on the theoretical data of the IAF 3D morphology and provides novel insights into IAF.

An in-situ microscopy study on nucleation and growth of acicular ferrite in Ti-Ca-Zr deoxidized low-carbon steel

We describe here an in-situ microscopy study on the nucleation and growth of acicular ferrite in Ti-Ca-Zr deoxidized low-carbon steel as a function of cooling rate. The study indicated the starting transformation temperature of acicular ferrite decreased with increasing cooling rate. Moreover, the average growth rate of acicular ferrite was increased from ~2.2 μm/s to ~14.4 μm/s with the increase of cooling rate from 2 °C/s to 15 °C/s. In Ti-Ca-Zr deoxidized steel, (Ti, Ca, Zr, Al)-O-Mn-S oxide inclusions effectively promoted the formation of intragranular acicular ferrite. A significantly high proportion of acicular ferrite was characterized by high angle grain boundaries and reduced grain size. When the cooling rate was low at 5 °C/s, the proportion of acicular ferrite was high at ~85%, with ~48% high angle grain boundaries and small average grain size of ~2.7 μm. Additionally, MnS precipitated in the vicinity of (Ti, Ca, Zr, Al)-O-Mn-S oxide particles, which promoted the nucleation of acicular ferrite.

Mechanism of Improving Heat-Affected Zone Toughness of Steel Plate with Mg Deoxidation after High-Heat-Input Welding

In the present study, the mechanism of improving HAZ toughness of steel plate with Mg deoxidation after the simulated welding with the heat input of 400 kJ/cm was investigated through in situ observation, characterization with SEM-EDS and TEM-EDS, and thermodynamic calculation. It was found that intragranular acicular ferrite (IAF) and polygonal ferrite (PF) contributed to the improvements of HAZ toughness in steels with Mg deoxidation. With the increase of Mg content in steel, the oxide in micron size inclusion was firstly changed to MgO-Ti2O3, then to MgO with the further increase of Mg content in steel. The formation of nanoscale TiN particles was promoted more obviously with the higher Mg content in the steel. The growth rates of austenite grains at the high-temperature stage (1400~1250 °C) during the HAZ thermal cycle of steels with conventional Al deoxidation and Mg deoxidation containing 0.0027 and 0.0099 wt% Mg were 10.55, 0.89, 0.01 μm/s, respectively. It was indicated that nanoscale TiN particles formed in steel with Mg deoxidation were effective to inhibit the growth of austenite grain. The excellent HAZ toughness of steel plates after welding with a heat input of 400 kJ/cm could be obtained by control of the Mg content in steel to selectively promote the formation of IAF or retard the growth of austenite grain.

Constructing the Three-Dimensional Morphology of Intragranular Acicular Ferrite to Reveal the Formation Mechanism at the View of Growth Behavior

Precipitation of intragranular acicular ferrite (IAF) and interlocking microstructure is the chief principal, which can increase the strength and toughness of steel. Herein, the three-dimensional morphology and growth behavior of IAF is discussed. Focused Ion Beam, confocal laser scanning microscopy and electron backscattered diffraction were used. Results show that IAF was plate-like shape with much pits and defects, and needle-like with smooth surface which isolate in 750oC and 700oC for 1min respectively. The growth behavior of IAF and interlocking microstructure were studied. Additionally, we provide a theoretical basis for the three-dimensional morphology forming mechanism in a new perspective of IAF.

Effect of carbon and manganese contents on intra-granular acicular ferrite nucleation in steel containing nanoparticles

In the present study, the experimental steels containing nanoparticles were manufactured under different mass fractions of C and Mn. A systematic analysis of inclusion characteristics, Intra-granular acicular ferrites (IAF) inducing potency of active inclusions, and microstructure morphologies revealed that Al2O3-MgO had the largest number fraction with higher inducement ratios than other inclusions. Steel samples containing 0.21wt% C contained the largest share of fine acicular ferrite (AF) than other samples due to relatively stronger inducing potency of active inclusions on IAF. On the other hand, the underlying microstructures in steel samples with Mn content less than 0.6wt% were due to weaker inducing potencies of active inclusions on IAF.

Inclusion and microstructure characteristics in steel with TiO2 nanoparticle additions

Abstract TiO2 nanoparticles were directly added to molten steel. The corresponding analyses of inclusion and microstructure were carried out through scanning electron microscopy (SEM) and an energy-dispersive X-ray spectrometer (EDS). On the one hand, a large number of micrometer scale Ti-bearing inclusions were observed. Besides Ti, other elements, such as Al, Mn, Si and S, were present in these inclusions. These phenomena not only confirmed the validity of adding TiO2 nanoparticles but likewise their growth and reactivity. This can be explained by the interaction between TiO2 and a Fe matrix, the decomposition reaction of TiO2 and a higher Gibbs free energy for the nanoparticles. On the other hand, these Ti-bearing inclusions can induce the nucleation of intragranular acicular ferrite. This is similar in appearance to a Ti-bearing inclusion originating through internal precipitation and may be explained by the theory of a Mn-depletion zone.

Microstructural characteristics and impact fracture behaviors of low-carbon vanadium-microalloyed steel with different nitrogen contents

The influence of nitrogen content between 0.0032 and 0.0081 wt % on the microstructures and the impact toughness of low-C V-microalloyed steel was investigated. The isothermal transformation experiments in the medium temperature were performed at Gleeble-3800. The microstructures and impact fracture behaviours were obtained through a series of characterization means and the mechanism were investigated. Results indicated that increasing the nitrogen content created a larger number of finer precipitated (Ti,V) (C,N) particles as well as micro-sized particles. The former decreased the average diameter of prior austenite grains (PAGs) and the latter promoted the nucleation of intragranular acicular ferrite (AF), which led to an increased amount of AF and a reduced mean equivalent diameter (MED) of ferrite grains. Moreover, the increased nitrogen content increased the amount of fine martensite-austenite (M/A) constituents and tended to change the internal substructure of M/A constituent from twin-type to lath-type. Thereby these variations in microstructure resulted in the decrease in 50% fracture appearance transition temperature (50% FATT), and alterations in the fracture behavior in the upper shelf region and lower shelf region. Moreover, the increasing N content led to changes in the mechanism of crack initiation from micro-cleavage to microvoid, and the mechanism of crack propagation from predominantly transgranular cleavage to predominantly intragranular microvoid coalescence at the test temperature of -20 °C. As a result, the initiation and propagation energy increased significantly with the increased nitrogen content.

The effect of MgTiO 3 Adding on Inclusion Characteristics

Abstract MgTiO3 powder was directly added into molten steel at 1873K, and its effect on inclusion characteristics was studied by a scanning electron microscope and an energy dispersive spectrometer. Thermodynamic calculation indicates that MgTiO3 is unstable in molten steel and may decompose into [Ti], [O] and MgO. However, only Ti-bearing inclusions were observed in the treated sample, and Mg-bearing inclusions were absent. This can be explained by the features of wettability and stability for MgO. Compared with a Non-treated sample, both oxide and coarse MnS in treated sample were refined. For the oxide, this originates from the formation of Ti-bearing inclusion. For coarse MnS, this may be due to the fact that Ti can aggravate S segregation. Besides, this aggravation makes coarse MnS less globular. After etching, it was found that in the treated sample, Ti-bearing inclusion can induce the nucleation of intragranular acicular ferrites. This appearance was totally different from that of Non-treated sample, and indicates the effectiveness of external adding method in oxide metallurgy.

Relationship between MnS precipitation and respective effects of Ti-Mg bearing inclusions on the induction of intergranular acicular ferrite

Abstract The role of MnS in the nucleation of intragranular acicular ferrites (IAF) has been explored in Ti-Mg treated steel sample through scanning electron microscope (SEM) and energy dspersive spectrometer (EDS). When compared with a non-treated sample, Ti-Mg treatment evolves from Al-Mn-Si-O inclusion to Ti-Mg bearing inclusion and causes a reduction in pure MnS. In addition to this reduction in pure MnS, a precipitation of MnS around the Ti-Mg bearing inclusion can be observed. Moreover, after etching, it can be found that Ti-Mg bearing inclusions can induce IAF nucleation. Based on EDS line analysis and a comparison with Al-Mn-Si-O inclusion in non-treated sample, this effect can be explained by the presence of an Mn-depletion zone (MDZ) caused by the vacancy property and crystal structure of the inclusion. It should be added, however, that in the same sample, a similarly induced nucleation effect as well as an MDZ have not been observed around pure MnS. This comparison implies that the formation of an MDZ may be independent of MnS precipitation.

Inclusion and Microstructure Characteristics in a Steel Sample with TiO2 Nanoparticle Addition and Mg Treatment

TiO2 nanoparticles and Mg alloy were added to molten steel in sequence to investigate the inclusion and microstructure characteristics. Compared with a non-treated sample, these additives resulted in the formation of Ti–Mg-bearing inclusions, which proves that the additives were valid. The size evolution from nanometer-scale TiO2 to micrometer-scale oxides hints at the agglomeration and growth of the TiO2 nanoparticles, which is due to the possible formation of a liquid-capillary force, the decomposition reaction of TiO2, and the higher Gibbs free energy of the nanoparticle. Furthermore, the statistical analysis of the oxides indicated that with the addition of the TiO2 nanoparticles and Mg alloy, the oxides were refined and their density was higher. Few pure MnS were observed in the treated sample. This is due to the fact that most oxides separated out in the liquid region at 1873 K based on the oxide composition and the calculated Al2O3–Ti3O5–MgO phase diagram. Thus, MnS preferred to segregate on them during solidification. After etching, it was found that the Ti–Mg-bearing oxide can induce the nucleation of intragranular acicular ferrites. The appearance of these acicular ferrites was not observed in the non-treated sample. This comparison indicates the effectiveness of the external adding method in oxide metallurgy.

Influence of Al Content on the Inclusion-Microstructure Relationship in the Heat-Affected Zone of a Steel Plate with Mg Deoxidation after High-Heat-Input Welding

The effects of Al content on inclusions, microstructures, and heat-affected zone (HAZ) toughness in a steel plate with Mg deoxidation have been investigated by using simulated high-heat-input welding and an automated feature system. The studies indicated that the main kind of oxysulfide complex inclusions in two steels without and with Al addition were both MgO-MnS. The number densities and mean sizes of inclusions were 96.65 mm−2 and 3.47 μm, 95.03 mm−2 and 2.03 μm, respectively. The morphologies of MgO-MnS complex inclusions in steel were changed obviously with the addition of Al. When containing 0.001 wt.% Al, they consisted of a central single MgO particle and outside, the MnS phase. When containing 0.020 wt.% Al, they comprised several small MgO particles entrapped by the MnS phase. Because the former could nucleate intragranular acicular ferrites (IAFs) and the latter was non-nucleant, the main intragranular microstructures in HAZs were ductile IAFs and brittle ferrite side plates (FSPs), respectively. Therefore, HAZ toughness of the steel plate without Al addition after high-heat-input welding of 400 kJ/cm was significantly better than that of the steel plate with Al addition.

Improvement of Heat-Affected Zone Toughness of Steel Plates for High Heat Input Welding by Inclusion Control with Ca Deoxidation

The characteristics of inclusions and microstructure in heat-affected zones (HAZs) of steel plates with Ca deoxidation after high heat input welding of 400 kJ·cm−1 were investigated through simulated welding experiments and inclusions automatic analyzer systems. Typical inclusions in HAZs of steels containing 11 ppm and 27 ppm Ca were recognized as complex inclusions with the size in the range of 1~3 μm. They consisted of central Al2O3 and peripheral CaS + MnS with TiN distributing at the edge (Al2O3 + CaS + MnS + TiN). With increasing Ca content in steel, the average size of inclusions decreased from 2.23 to 1.46 μm, and the number density increased steadily from 33.7 to 45.0 mm−2. Al2O3 + CaS + MnS + TiN complex inclusions were potent to induce the formation of intragranular acicular ferrite (IAF). Therefore, the HAZ toughness of steel plates after high heat input welding was improved significantly by utilizing oxide metallurgy technology with Ca deoxidation.

Effects of Heat-Treatment Temperature on the Microstructure and Mechanical Properties of Steel by MgO Nanoparticle Additions.

The characteristics and formation mechanisms of intragranular acicular ferrite (IAF) in steel with MgO nanoparticle additions were systematically investigated for different isothermal heat-treatment temperatures, and its influence on mechanical properties was also clarified. The results indicate that the inclusions were finely dispersed and refined after adding MgO nanoparticles. In addition, with decreasing heat-treatment temperature, the microstructure changed from grain boundary ferrite (GBF) and polygonal ferrite (PF) to intragranular acicular ferrite. Moreover, the steel with MgO additions had excellent mechanical properties in the temperature range of 973 to 823 K and an average Charpy absorbed energies value of around 174 J at 873 K due to the significant refinement of the microstructure and nucleation of intragranular acicular ferrite.

Effect of Mg Treatment on Refining the Microstructure and Improving the Toughness of the Heat-Affected Zone in Shipbuilding Steel

The effect of Mg treatment on the microstructure and toughness of the heat-affected zone (HAZ) of shipbuilding steel after high-heat-input welding was investigated via laboratory and industrial testing. The welding process and Charpy impact tests were also carried out to evaluate the HAZ toughness of steel plates. First, typical inclusion characteristics were characterised with an ASPEX PSEM Explorer. Then, confocal laser scanning microscopy (CLSM) was used to observe the diameters of austenite grains under different holding times. The results showed that when the addition of microalloy elements were in the order of Al–Mg–Ti, this had an effect on dispersing inclusions, the largest proportion of which were micro-inclusions that had a particle size range of 1.0–1.5 μm. This accounted for 25.4% of the total inclusions, which was the highest amount. The micro inclusion particle size that was mainly distributed in the range of 0.5–3.5 μm accounted for 82.8% of all the micro-inclusions. The inclusion structure induced intragranular acicular ferrite (IAF) in austenite as follows: MgO and Al2O3 formed the core and Ti2O3 adhered to the Al–Mg complex inclusions to produce smaller particle sizes and dispersions of Al, Mg, and Ti complex inclusions. The 40-mm-thick plate obtained in the industrial test after welding had an average impact absorbed energy 2 mm from the weld joint in the heat-affected zone of 198.9 J at −20 °C, while the welding heat input was 150 kJ/cm, compared with the parent material’s low-temperature performance, which exceeded 88%.

Research on Influential Mechanism of HAZ Impact Toughness for Shipbuilding Steel with Mg Addition

The welding performance of shipbuilding steel under large heat input could be improved greatly by the addition of Mg to the steel, but the impact toughness of the heat affect zone (HAZ) is not stable. According to the three different thickness steel plates obtained in the industrial experiment, the large heat input welding was carried out by different heat input, and the impact toughness analysis, impact fracture analysis, metallographic microstructure analysis and inclusions analysis were carried out. The results showed that, the HAZ of three kinds of thickness plates induced much intragranular acicular ferrite (IAF); with Mg addition, the inclusion dimension had been reduced effectively, and the IAF-induced ability of the inclusions had also been improved. The difference of HAZ impact toughness with different welding heat input and different impact temperature is significant; considering the influence of welding heat input and metallographic microstructure on the impact toughness of HAZ, the welding heat load had a far greater effect than the metallographic microstructure on ductile–brittle transition temperature. At the same time, if the original metallographic microstructure of steel was coarse, the pinning effect of the inclusions would be reduced significantly, and the microstructure of HAZ would be coarsened and the impact toughness of HAZ would be decreased, so there is a certain matching relationship between the metallographic microstructure and the inclusion dimension.

Influence of Cooling Process on Microstructure and Property of Low-Carbon V–N Micro-alloyed Medium and Heavy High-Strength Steel Plate

Low-carbon V–N micro-alloyed steel plates (30 mm thick) were produced via various cooling processes. The differences between the microstructure and the properties of the experimental steels were analyzed and compared. The results indicated that a large amount of intragranular acicular ferrite with high density of dislocation and high misorientation boundaries formed in the experimental steel, and the quantities of finer V-containing precipitates increased at a finish cooling temperature of 538 °C. The strength and the impact toughness improved. The crystallographic characteristics of the experimental steels were analyzed via using electron back-scattered diffraction. The microstructures of experimental steels were observed on an optical microscope, a scanning electron microscope, and transmission electron microscopy. The strength enhancement was attributed to the refinement strengthening, the dislocation strengthening, the precipitation strengthening, and the transformation strengthening. The improvement of toughness was attributed to the microstructure refinement and the formation of high fraction acicular ferrite.

The Relationship between MnS Precipitation and Induced Nucleation Effect of Mg-Bearing Inclusion

Abstract The relationship between MnS precipitation and induced nucleation effect of Mg-bearing inclusion has been explored through scanning electron microscope and energy dispersive spectrometer (EDS). Results indicate that MnS prefers to precipitate on Mg-bearing inclusions. Statistical analysis suggests that MgAl2O4 and MgO may coexist in inclusion. After etching, it is found that Mg-bearing inclusions can induce the nucleation of intragranular acicular ferrites. Based on EDS line analysis and comparison with Al-Mn-Si-O inclusion in non-Mg-treated sample, this effect can be explained by Mn-depletion zone (MDZ), which is due to the vacancy property and crystal structure of MgAl2O4. In the same sample, similar induced nucleation effect and MDZ are not observed around pure MnS. This comparison implies that the formation of MDZ may be independent of MnS precipitation.

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.

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.

Research of microalloy elements to induce intragranular acicular ferrite in shipbuilding steel

ABSTRACTBased on the minimum degree of disregistry mechanism in oxide metallurgy, laboratory and industrial research have been conducted on intragranular acicular ferrite (IAF) induced by microalloying elements in austenite. Based on the chemical compositions of DH36 steel and Mg, Al, Ti, V, Nb microalloyed steel, experimental results show that in ingots' organization, both V and Nb can induce IAF, but when the adding sequence was Al-Mg-Ti, smaller and dispersion inclusions were formed in austenite. When the Mg content was 0.005 wt%, the inclusion structure induced IAF in austenite is as follows: MgO and Al2O3 forms the core and TixOy adheres to the Al-Mg complex inclusions to produce smaller particle size and dispersions of Al, Mg, Ti complex inclusions. Finally, upon lowering the temperature, carbonitrides of Ti, V, and Nb were precipitated on the outermost layer of the inclusions. These carbonitrides with small disregistry contribute to induce intragranular acicular ferrite.