Formation Of Intragranular Acicular Ferrite Research Articles

Improvement of impact toughness by microstructure refinement of simulated CGHAZ through enhancing welding heat input of low carbon Mo-V-Ti-N-B steel

Welding heat input greatly influences the microstructure and impacts the affected zone’s toughness. To interpret the relationships between the welding heat input, microstructure, and low-temperature toughness of the coarse-grained heat-affected zone (CGHAZ) of Mo-V-Ti-N-B steels, welding heat cycles with different heat inputs (25–75 kJ cm−1) were performed on a Gleeble 3500 simulator. Intragranular ferrite in the simulated samples subjected to different thermal cycles was characterized and quantified, and the impact energies of simulated samples were evaluated at −20 °C. Upon increasing the heat input, the intragranular ferrite content rose sharply from 4.3% to 76.0%. The V(C, N) enrichment on the precipitate surface increased the size of precipitates, providing favourable nucleation conditions for intragranular ferrite. The prior austenite grain (PAG) and martensite/austenite (M/A) constituents became rough, and the content of the M/A constituent increased while the impact energy of the CGHAZ increased. This behaviour occurred due to the formation of intragranular acicular ferrite (IGAF), which refined the microstructure of the CGHAZ. Grain refinement eliminated the negative influence of higher M/A content on the impact toughness of the CGHAZ.

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.

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.

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 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.

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.

Effect of Ti-Al-O inclusions on the formation of intragranular acicular ferrite

Abstract The role of Ti-Al-O inclusions on nucleation of intragranular acicular ferrite (IAF) was investigated by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). From SEM-EDS mapping images, two kinds of titanium containing oxides are observed. Type I is a Ti-Al-O phase, in which the elements of Al and Ti coexist. Type II is the titanium containing oxide, which precipitates on Al2O3. By statistical analysis, it can be seen that Type I is dominating in number, and its molar ratio of Ti/Al varied in a wide range, which reflects its composition complexity. After etching, microstructure of IAF is observed. It can be seen that sulfur accumulates on the periphery of nucleation Ti-Al-O inclusion, which is attributed to the precipitation of MnS. Moreover, this accumulation accompanies increasing in content ratio of Al/Ti, which could be explained by titanium vacancies. These vacancies can absorb Mn ions from the steel matrix around inclusions, which leads to a Mn depletion zone (MDZ). Its existence is confirmed by EDS line scanning, and considered to facilitate the formation of IAF since Mn is an austenite stabilizer.

Study on the Formation of Intragranular Acicular Ferrite in a Zr–Mg–Al Deoxidized Low Carbon Steel

The formation of intragranular acicular ferrite (IAF) in a Zr–Mg–Al deoxidized low carbon steel was investigated by means of dilatometry, thermo‐mechanical controlled processing, and combined these results with metallographical observations. The typical inclusions were characterized by scanning electron microscopy and the chemical composition of the inclusions was determined by energy dispersive spectroscopy. The role of these inclusions on the formation of IAF was discussed. The results indicate that the Al2O3–MgO–ZrO2 and Al2O3–MgO–ZrO2–MnS multiphase particles with 1.5–3.0 μm size can be the effective nuclei of IAF, whereas the Al2O3–MnS inclusions are inert. A Mn–depletion zone is observed adjacent to the Al2O3–MgO–ZrO2–MnS inclusion, which is believed to be one of the possible mechanisms to promote the IAF formation. Through dilatometric tests, for Zr–Mg steel, the IAF structure forms in the cooling rate range of 1–10 °C s−1, and the most appropriate cooling rate for IAF is in the range of 5–10 °C s−1. A fine grained microstructure and an improvement of toughness and tensile properties are obtained after subjecting the steel grade with Zr and Mg additions to a special thermo‐mechanical regime.

Relationship between crystallographic structure of the Ti2O3/MnS complex inclusion and microstructure in the heat-affected zone (HAZ) in steel processed by oxide metallurgy route and impact toughness

A new method based on electron back scattered diffraction (EBSD) is proposed to determine the structure of titanium oxide/MnS complex inclusion which induced the formation of intragranular acicular ferrite (IAF) in heat-affected zone (HAZ) in steel processed by oxide metallurgy route. It was found that the complex inclusion was Ti2O3/MnS, the orientation relationship between Ti2O3 and MnS was also examined, and the crystallographic orientation relationship among IAF, Ti2O3/MnS complex inclusion, austenite, bainite formed at lower temperature is researched systematically. It was observed that MnS precipitated on Ti2O3 at specific habit plane and direction and MnS had a specific orientation relationship ({0001} Ti2O3//{111} MnS), <10–10> Ti2O3//<110> MnS) with respect to Ti2O3. Intragranular acicular ferrite (IAF) nucleated on MnS part of the Ti2O3/MnS complex inclusion had no specific orientation relationship with MnS. IAF and the surrounding bainite had different Bain groups, so that there was an increase in high angle boundaries, which was beneficial for the toughness of HAZ.

Crystallographic analysis for acicular ferrite formation in low carbon steel weld metals

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view in low carbon low alloy steelweld metals. The samples from electro slag welding (ESW) and submerged arc welding (SAW) deposits with various cooling rates were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from inclusions. The inclusions contributing to acicular ferrite formation were of multi-phase type consisting of amorphous phase, spinel type and MnS. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction patterns and energy-dispersive X-ray spectrometer analyses that the Ti-enriched layer was TiO. The acicular ferrite had a Baker–Nutting orientation relationship with the TiO layer on the inclusion surface. The misfit was 3.0% at the interface between the acicular ferrite and TiO. Therefore, it is considered that TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit. However, themorphologies of ferrite growth which nucleated from inclusions were different in both samples. Whereas the growth of ferrites nucleated from TiO was enough in ESW, the size of nucleated ferrite in SAW was a few hundred millimetres in size. In the early stage of nucleation from TiO, ferrite had small deviation from Kurdjumov–Sachs orientation relationship (K–S relationship) in both ESW and SAW. However, there was a difference in the growth stage of ferrite. The ferrite orientations were gradually changed to fit to the K–S relationship in ESW. On the other hand, the nucleated ferrite in SAW stopped growing and the newly nucleated ferrite which had K–S relationship prior to austenite was formed adjacently because of large super cooling due to small heat input.

In Situ Observation of the Evolution of Intragranular Acicular Ferrite at Mg-containing Inclusions in 16Mn Steel

The effects of chemical compositions and austenitizing temperature on the formation of intragranular acicular ferrite (IAF) of 16Mn steel, a grade of low alloying steel with carbon content of 0.13∼0.19% and manganese content of 1.20∼1.60%, were systematically investigated. The in situ observation of the evolution of IAF at Mg-containing inclusions was carried out using a high-temperature laser scanning confocal microscopy (LSCM). The nucleation and growth of ferrites, the formation of IAF induced by inclusions, and the size of austenite grain at 1200 °C was directly observed using the LSCM. The results show that the inclusions can be transformed into MgAl2O4, Mg2SiO4 and MgO after treated by Mg. Trace amount of Mg can induce the IAF nucleation. With the increasing of Mg addition, the percentage of IAF in steel greatly increases, while the optimum content of Mg is around 0.0022 wt%. The appropriate austenitizing temperature is around 1200 °C and the optimal prior austenite grain size is about 130 µm. The temperature range for IAF transformation is around between 571 °C and 627 °C, and the time for which is about 12 s. The disregistries between MgAl2O4, MgO and α-Fe are as small as 0.6% and 4.03%, they are so small that may act as highly effective nuclei of IAF during γ → α transformation.

Mechanisms of inclusion evolution and intra-granular acicular ferrite formation in steels containing rare earth elements

Inclusion characteristic and microstructure of rare earth (RE) elements containing steel were evaluated with scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), element-mapping, optical microscopy (OM), and automated feature analysis (AFA) option equipped with ASPEX PSEM. Factsage was used to calculate the equilibrium inclusion composition. Based on the calculation, an inclusion evolution mechanism was proposed. Furthermore, line scanning analysis was used to elucidate the intra-granular acicular ferrite (IAF) nucleation mechanism. The result showed that two difierent inclusions exist in sample steel: (Mn-Al-Si-Ti-La-Ce-O) +MnS complex inclusion and isolated MnS inclusion. Almost all nucleation sites for IAF are complex inclusions, while single MnS inclusion cannot induce IAF. A possible formation mechanism of complex inclusion is proposed based on calculated results using Factsage, which agrees well with experimental results. A Mn-depletion zone (MDZ) which exists adjacent to the (Mn-Al-Si-Ti-La-Ce-O) +MnS complex inclusion can account for the IAF formation. However, the low volume fraction (1.49£10 i7 )

Effect of tungsten addition on the mechanical properties and corrosion resistance of S355NL forging steel

A study was made to investigate the effect of tungsten (W) addition on the microstructure, tensile properties, Vickers hardness, and corrosion resistance of S355NL forging steel. Mechanical properties were evaluated and considered in the context of the interlamellar spacing of pearlite. Microstructural analysis revealed that the addition of W favors the formation of intragranular acicular ferrite and leads to a decrease in the interlamellar spacing of pearlite. After W addition, the corrosion rust film was intact. The steel containing W showed fewer microcracks distributed in the corroded surface compared with that without W. It was concluded that the addition of W is beneficial for improvement of both the mechanical properties and corrosion resistance of S355NL forging steel.

In Situ Observation of the Evolution of Intragranular Acicular Ferrite at Ce‐Containing Inclusions in 16Mn Steel

AbstractThe effects of chemical compositions and austenitizing temperature on the formation of intragranular acicular ferrite (IAF) of 16Mn steel were systematically investigated, and the in situ observation of the evolution of IAF at Ce‐containing inclusions was carried out using a high‐temperature laser scanning confocal microscopy. The results show that trace amount of Ce can induce the IAF nucleation, while the optimum content of Ce is around 0.02 wt%. The appropriate austenitizing temperature is around 1200°C and the optimal prior austenite grain size is about 120 µm. The temperature range for IAF transformation is around between 557 and 623°C, and the time for which is about 13 s. The disregistries between Ce‐containing inclusions and α‐Fe are so small that may act as highly effective nuclei of IAF.

In-situ observation of phase transformation during rapid cooling processes

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view to understand the formation mechanism for acicular ferrite microstructure in low alloy steel laser weld metals. The sample was low carbon Ti-B weld metals with an oxygen content of 480 ppm. In this sample, intragranular acicular ferrite formation was observed from some inclusions and acicular ferrite had Kurdjumov-Sachs orientation relationship with austenite matrix. The multi-phase inclusions contributed to nucleation of acicular ferrite. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction and EDS analysis that the Ti-enriched layer was TiO. The acicular ferrite had Baker-Nutting orientation relationship with TiO layer on the inclusion surface. The lattice misfit was 3.0 % at the interface between the acicular ferrite and TiO.Therefore, it is considered that the TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit.

Thermodynamic Analysis of Mn-depleted Zone Near Ti Oxide Inclusions for Intragranular Nucleation of Ferrite in Steel

A series of thermodynamic analyses have been performed to elucidate possible mechanism of Mn-depleted zone (MDZ) development near Ti oxide inclusions as nucleation sites of Intragranular Acicular Ferrite (IGF) transformation in steels, using a computational thermodynamic approach (CALPHAD). It is demonstrated that thermodynamic calculations are able to reproduce experimentally known inclusions evolution in the steels. The MDZ development near the Ti2O3 inclusions is shown to be a result of Mn absorption into the Ti2O3. Moreover, from thermodynamic analysis of phase equilibria in the Mn–Ti complex oxide system, it is proposed that a phase change of inclusion from (Ti3O5) s.s. (pseudobrookite structure solid solution dissolving Mn) to (Ti2O3) s.s. (ilmenite structure solid solution dissolving Mn) accelerate the Mn absorption into the Ti2O3 inclusion in steel. From a series of thermodynamic calculations, optimum thermal heating condition to enhance Mn absorption into Ti2O3 inclusions as well as IGF formation is discussed.

Mechanism of Acicular Ferrite Formation in Low Carbon Steel Ti–B Weld Metals with Different Oxygen Levels

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view to understand the formation mechanism for acicular ferrite microstructure in mild and low alloy steel weld metals.The samples with different oxygen content in Ti–B weld metal were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from some inclusions and acicular ferrite had Kurdjumov–Sachs orientation relationship with austenite matrix. The inclusions contributing to acicular ferrite nucleation were multi-phase type consisting of amorphous phase and MnS. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction patterns and analysis of EDS that the Ti-enriched layer was TiO. The acicular ferrite had Baker–Nutting orientation relationship with TiO layer on the inclusion surface. The misfit was 3.0% at the interface between the acicular ferrite and TiO.Therefore, it is considered that the TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit.

In situ observation of the formation of intragranular acicular ferrite at non-metallic inclusions in C–Mn steel

Intragranular acicular ferrite is regarded as a most desirable microstructure feature, in view of its strength and toughness, both in weld metals and in the heat-affected zone. This paper systematically investigated the effect of Ti addition on the evolution of intragranular acicular ferrite in the heat-affected zone of C–Mn steel. We also systematically studied the effects of austenite grain size, alloy content and the characteristic of inclusions on the formation of intragranular acicular ferrite. The nucleation and growth of intragranular acicular ferrite was directly observed by laser scanning confocal microscopy. Subsequently, microscopy analysis was used to quantitatively determine and distinguish the potent and inactive inclusions with respect to the nucleation of intragranular acicular ferrite. Finally, some possible reasons are given to explain the formation of intragranular acicular ferrite in the C–Mn steel.

FIB and TEM Studies on the Bainitic Microstructure in Low Carbon HSLA Steels

In the present study, focused ion beam (FIB) technique was applied to make site-specific TEM specimens and to identify the 3-dimensional grain morphologies of bainitic microstructure in low carbon HSLA steels such as granular bainite, acicular ferrite and bainitic ferrite. Granular bainite consisted of fine subgrains and 2nd phase constituents like M/A or pearlite located at grain and subgrain boundaries. Acicular ferrite was characterized by an aggregate of ramdomly orientated and irregular shaped grains. The high angle boundaries between adjacent acicular ferrite grains caused by intragranular nucleation during continuous cooling process. Bainitic ferrite revealed uniform and parallel lath structure within the prior austenite grain boundaries and its’ packet size could effectively decreased by the formation of intragranular acicular ferrite.

Nucleation of Intragranular Accicular Ferrite in a Ti-Containing Low Carbon Steel of High N Content

Formation of acicular ferrite grains by dispersing non-metallic inclusions which can act as intragranular nucleation sites within austenite grain is proposed as a method to obtain refined grain structure in steels. The microstructure of acicular ferrite steel consists of fine interlocking ferrite plates nucleated on dispersed non-metallic inclusions. This acicular ferrite structure provides a desirable combination of high strength and good toughness because of its small plate thickness and interlocking microstruture. in Ti-containing low carbon steels, researchers proposed that Ti2O3 or MnS, among inclusions, is the most effective nucleant for intragranular ferrite because of the formation of Mn depleted zone(MDZ) around Ti2O3 or MnS particles. TiN is also reported as a nucleation site owing to the coherency between ferrite and TiN crystals. But It has not been a simple task to identify which inclusions and what mechanisms are really effective in the formation of intragranular acicular ferrite.