Nucleation Of Intragranular Acicular Ferrite Research Articles

Effect of Ca Addition on Inclusions of Welding Heat-Affected Zone in Pressure Vessel Steels

Pressure vessel steels are used in the manufacture of tanks for the storage of gases, chemical materials and oil. To meet the increasing production demands, high-wire-energy welding is widely used in the manufacture of pressure vessel steels. This means that the weldability of pressure vessel steels needs to be improved. Therefore, in order to reveal the microalloying effect of Ca in pressure vessel steel, this study took a commonly used pressure vessel steel as the research object, and three groups of experimental steels with different Ca mass fractions were prepared using vacuum metallurgy, controlled rolling and controlled cooling. Welding heat simulation technology was used to simulate the welding heat of experimental steel and the welding heat-affected zone (HAZ) was investigated. The inclusions of the welding HAZ in the experimental steels were observed by using a metallographic microscope and scanning electron microscope (SEM). The mechanism of intragranular acicular ferrite (IAF) nucleation induced by the inclusions containing Ca elements in the welding HAZ of pressure vessel steels was also discussed. The research results show that the addition of Ca increased the number density of effective inclusions in the welding HAZ of the experimental steel up to 535.60 pieces/mm2. The addition of the Ca element was beneficial for producing more pinning inclusions in the experimental steel welding HAZ under the experimental conditions, and the inclusions were mainly elliptical oxide complex inclusions of Ca-Si with a size of about 2 μm. Meanwhile, Al2O3 and MnS were precipitated. After the addition of Ca elements, Mn-poor regions appeared around the inclusions containing Ca in the welding HAZ. IAF nucleation was mainly induced by the local compositional change mechanism and supplemented by the stress–strain energy mechanism and inert interface energy mechanism. This study provides a valuable reference for optimizing the welding process of pressure vessel steels and is of great importance for understanding the IAF nucleation mechanism of Ca-containing inclusions in the welding HAZ of pressure vessel steels.

Review of Research on Titanium Oxides in Low‐Carbon Steel

Titanium oxides, which act as nuclei for intragranular ferrite (IGF), are common inclusions in low‐carbon steel. The interlocking structure of IGF greatly improves the mechanical properties of low‐carbon steel. To make the best use of this kind of oxide, the types, sizes, and distribution properties of effective titanium oxides are reviewed that can promote IGF nucleation. The control techniques of titanium oxides are summarized as follows: (1) Ti2O3 is one of the most effective of the common titanium oxides, followed by TiO and TiO2. The type of titanium oxide can be affected by controlling the Ti/O mass ratio in molten steel. (2) Titanium oxide with a size range of 0.4–4 μm can effectively promote the nucleation of IGF or intragranular acicular ferrite (IAF), among them, titanium oxide of about 1 μm is the most beneficial to IAF nucleation. (3) To obtain titanium oxides with effective size, the precipitation of titanium oxides should be controlled during the solidification process, and this process can be achieved with the existing rapid solidification technology. Under the same conditions, the higher solidification rate can obtain finer titanium oxide.

Extremely high temperature carbide precipitation induced intragranular acicular ferrite transformation of M2 steel during semi-solid cooling

Intragranular acicular ferrite (IAF) transformation of M2 steel occurred at extremely high temperature (1180 ∼ 960 °C) during semi-solid cooling. The uncommon IAF transformation was observed in situ via confocal laser scanning microscope. Small VC carbide particles precipitated explosively during semi-solid cooling and they acted as the preferential site for the nucleation of IAF due to the low lattice misfit with ferrite. The length of ferrite lath showed a bimodal behavior that long primary ferrite laths and short secondary ferrite laths both existed. The acicular laths intersected with each other and terminated at the solid boundary or interface. The prior solid austenite grain was filled with acicular ferrite. The IAF might help refine the effective grain size and improve the strength of semi-solid M2 steel.

N-induced microstructure refinement and toughness improvement in the coarse grain heat-affected zone of a low carbon Mo–V–Ti–B steel subjected to a high heat input welding thermal cycle

In the coarse grain heat-affected zone (CGHAZ) of a low carbon Mo–V–Ti–B steel subjected to a simulative welding thermal cycle with a heat input as high as 75 kJ/cm, the influence of an increase in nitrogen content from 0.0085 to 0.0144 wt% on the microstructures and impact properties of the steel was comparatively estimated using Gleeble-3800 simulation, microstructure characterization and impact testing. With a rise in the nitrogen content, the number of fine and coarse particles also increased. An increase in the number of fine precipitates can effectively pin the prior austenite grain (PAG) boundary and inhibit the grain growth, resulting in the refinement of PAG in the CGHAZ. The intragranular and intergranular V-containing coarse particles effectively promoted the nucleation of intragranular acicular ferrite (IGAF) and grain boundary polygonal ferrite (GBPF). Accordingly, the contents of heterogeneously-nucleated IGAF and polygonal ferrite (PF) increased; however, the bainite content decreased with a rise in N content, leading to a prominent microstructure refinement in this CGHAZ with a high heat input. Furthermore, the nitrogen addition led to the formation of finer and more dispersed martensite-austenite (M/A) constituents in the CGHAZ. Therefore, the impact toughness of this CGHAZ was enhanced. This can be attributed to the microstructure refinement of the CGHAZ, which in turn was ascribed to an increasing nitrogen.

Investigation of Inclusion Characteristics and Intragranular Acicular Ferrite Nucleation in Mg-Containing Low-Carbon Steel

Investigation of Inclusion Characteristics and Intragranular Acicular Ferrite Nucleation in Mg-Containing Low-Carbon Steel

Microstructural evolution of 2.25Cr-1Mo-0.25V submerged-arc weld metal

Heat-resistant V-modified 2.25Cr-1Mo-0.25V-weld metal is commonly used in petrochemical industry for heavy wall pressure vessels in high-temperature hydrogen service. In order to improve the reactor efficiency, the weldments have to endure even higher temperatures and pressures. Acicular ferrite (AF) is often regarded as the optimum microstructure due to its good combination of strength and toughness. As few literature about the evolution of microstructure and the final microstructure constituents of 2.25Cr-1Mo-0.25V weld metal is available, the current paper intends to provide comprehensive information by means of microscopy, crystallographic examination via electron backscatter diffraction and in situ observation of the austenite to ferrite phase transformation via high-temperature laser scanning confocal microscopy (HT-LSCM). The investigated weld metal exhibits a high density of complex aluminium-silicon-manganese oxides with a spherical shape and large prior austenite grains, which in combination is beneficial for intragranular nucleation of AF. Nonetheless, the examination of the transformed final microstructure was not sufficient to make an unambiguous statement about the presence of AF within the 2.25Cr-1Mo-0.25V weld metal. Via in-situ HT-LSCM of the phase transformation, intragranular nucleation of AF at non-metallic inclusions within the austenite grains was detected, which confirms that even though the microstructure of 2.25Cr-1Mo-0.25V weld metal is mainly bainitic, small amounts of AF are present.

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.

Microstructure and mechanical properties of hot-rolled low-carbon steel containing Ti–Ca oxide particles: a comparison between base metal and HAZ

The effect of inclusion-induced nucleation on hot-rolled steel base metal was evaluated in comparison with welding heat-affected zone (HAZ). Microstructure and mechanical properties of hot-rolled low-carbon steel containing Ti–Ca oxide particles were studied. The results showed that inclusions in Ti–Ca deoxidized steel distributed dispersely and were effective for intragranular acicular ferrite nucleation. Under hot rolling and controlled cooling conditions, microstructure in steel base metal was significantly refined and mainly consisted of acicular ferrite and intragranular bainite, which exhibited higher strength and excellent toughness. The microstructural evolution behavior followed the process that acicular ferrite plates divided the austenite grain, intragranular bainite packets formed between interlocking acicular ferrite plates, and the remaining austenite decomposed into fine polygonal ferrite grains. The resultant complex microstructure improved the impact toughness significantly. By comparison, in HAZ microstructure, laminar grain boundary ferrite having similar crystallography orientation showed adverse effect on toughness.

Analysis of fracture toughness properties of wire + arc additive manufactured high strength low alloy structural steel components

The uncertainty surrounding the fracture behaviour of CMT-WAAM deposited steel, in terms of crack tip condition (J and CTOD) needed to cause crack tip extension, has made this manufacturing technique unpopular to date. Fracture toughness parameters are crucial in the structural integrity assessment of components and structures in various industries for assessing the suitability of a manufacturing process and material. In the offshore wind industry, the EN-GJS-400-18-LT ductile cast grade for the mainframe and hub has lower fracture toughness resistance for its high strength grade. Its high weight level affects the Eigen frequency of the tower and imposes high installation cost incurred from heavy lifting equipment usage. Poor fracture toughness is currently a challenge for wind turbine manufacturers in the quest for a cleaner and cheaper energy in the form of offshore wind. In this study, CMT-WAAM is used in depositing steel components with an oscillatory and single pass deposition strategy. The effects of microstructural variation, as a result of layer by layer deposition and the layer band spacing, on the fracture resistance in the build and welding direction was shown here. The fracture mechanics and failure mode of the WAAM deposited parts were investigated. The microstructural variation, again as a result of the layer by layer deposition and the layer band spacing, are the key parameters that control the fracture toughness of WAAM steel. Anisotropic behaviour in the Jq values was observed between both fracture orientations. The constructive transformation mechanism of the WAAM oscillatory process made way for intragranular nucleation of acicular ferrite on the Ti containing inclusion, thereby improving the toughness of the ER70S-6 deposit with a unique microstructure and Jq value of 640kJ/m2 .

The Effect of TiO2 Nanoparticle Adding on Inclusion and Microstructure

Abstract A mixture consisting of TiO2 nanoparticles and pure iron powder was directly added into molten steel, and its effects on inclusion and microstructure have been studied. Through a scanning electron microscope (SEM) and an Energy Dispersive Spectrometer (EDS), Ti-containing inclusions, in which the elements of Si, Mn and Al co-existed, were observed. Moreover, compared with the non-treated sample, statistical results indicated that the inclusion in the treated sample was refined and its number per unit area increased by 50 %. These phenomena meant the adding validity of TiO2 nanoparticle, and may be due to the decomposition reaction of TiO2 and the higher Gibbs free energy of the nanoparticles. After etching, it was found that the Ti-bearing inclusions can induce intragranular acicular ferrite nucleation. This appearance was consistent with the Ti-containing inclusion introduced through the internal precipitation method.

Effect of hot deformation parameters on flow stress and microstructure in a low carbon microalloyed steel

Hot deformation behavior of low carbon microalloyed steel was investigated by single-pass compression tests using a thermomechanical simulator. After the specimens were subjected to different hot deformation processes, their microstructures were obtained by constant cooling rate and then examined using optical microscopy and electron backscattering diffraction (EBSD) technique. The results showed that the absence of stress peak in the flow stress curves does not necessarily indicate the absence of dynamic recrystallization (DRX) because in some cases the partial DRX phenomenon does occur when the stress peaks are absent in the flow curves. The metallurgical events that occur during hot deformation directly influence the nature of phase transformation products. High dislocation density in the deformed austenite can promote the intragranular nucleation of acicular ferrite, while the DRX austenite grains prefer to form lath bainite/martensite. The EBSD results confirmed that the microstructures that were transformed from prior pancaked austenite grains always contain higher fraction of middle misorientation angle boundaries and larger average local strain as compared to those with prior DRX grain shape.

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 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.

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 yttrium on the microstructures and inclusions of EH36 shipbuilding steel

The effects of yttrium content and cooling speeds on the microstructures and inclusions of EH36 shipbuilding steels were investigated, the evolutionary processes of microstructures and inclusions were investigated using optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The results showed that adding trace amounts of yttrium could promote intragranular acicular ferrite (IAF) nucleation and refine the microstructures. The proportion of IAF first increased and then decreased with increasing yttrium content, and the optimal yttrium content to obtain microstructure of IAF was 0.01%. The appropriate cooling method was the steel mould cooling. The lattice disregistry between the α-ferrite phase and Y2 O2 S was 4.2%, which is very small. Y2 O2 S could effectively facilitate the nucleation of IAF. Moreover, the appropriate size of Y2 O2 S inducing IAF was approximately 1∼4 μm.

Effects of Ti–Mg Complex Inclusions on Acicular Ferrite Nucleation

Abstract The alloys of Ti and Mg are added in molten carbon structural steel in sequence to reveal their influence on features of inclusion and microstructure. Compared with non-treated steel, it is found that they leads to the formation of Ti-Mg-O inclusion and reduction in portion of pure MnS. Experimental results indicate that average inclusion size in treated steel significantly decreases and its number per unit volume increases by 48 %. Moreover, the margin mole ratio of Mg/Ti in all Ti-Mg-O inclusions is larger than 2, which hints they are more complex than single assumption of MgTi2O4. After etching, intragranular acicular ferrite (IAF) microstructure is observed. EDS line analysis suggests that Mn-depletion zone exists in steel matrix, which is proposed to explain the IAF nucleation effect induced by Ti-Mg-O inclusions. Through comparison with only Mg-treated sample, the refinement of inclusion and increase in portion of pure MnS due to Ti adding are revealed.

Effect of Mg Addition on Nucleation of Intra-granular Acicular Ferrite in Al-killed Low Carbon Steel

To verify the formation behaviors and mechanisms of intra-granular acicular ferrite (IAF) grains nucleated by Mg-Al-O in low carbon steel, the steels containing different Mg contents were refined in a vacuum induction furnace. The effect of Mg addition on the formation of IAF structure in Al-killed low carbon steel was investigated by optical microscope (OM) and scanning electron microscope with energy dispersive X-ray spectroscope (SEM-EDX). It reveals that the IAFs are only detected in Mg-added steels, and the volume fraction of IAF increases with the Mg concentration from 8×10−6 to 26×10−6. It shows that not only the MgO-Al2O3-MnS and MgO-Al2O3-P2O3 particles are the effective nucleation sites for IAF, but also the pure MgO · Al2O3 phase can promote the ferrite nucleation. A Mn-depletion zone (MDZ) is characterized adjacent to the MgO-Al2 O3-MnS, which is believed to be one of the possible mechanisms to explain the IAF nucleation. The MDZ around the MgO-Al2 O3-MnS inclusion would be induced by the MnS precipitation on the inclusion. It seems that the ability of Mg-containing inclusions to induce the nucleation of ferrite might be attributed to a new mechanism, i. e., the P-rich zone formed on a few Mg-Al-O inclusions might be another factor for promoting the IAF formation.

The Effect of Nb on the Continuous Cooling Transformation Curves of Ultra-Thin Strip CASTRIP© Steels

The effect of Nb on the hardenability of ultra-thin cast strip (UCS) steels produced via the unique regime of rapid solidification, large austenite grain size, and inclusion engineering of the CASTRIP© process was investigated. Continuous cooling transformation (CCT) diagrams were constructed for 0, 0.014, 0.024, 0.04, 0.06 and 0.08 wt% Nb containing UCS steels. Phase nomenclature for the identification of lower transformation product in low carbon steels was reviewed. Even a small addition of 0.014 wt% Nb showed a potent effect on hardenability, shifting the ferrite C-curve to the right and expanding the bainitic ferrite and acicular ferrite phase fields. Higher Nb additions increased hardenability further, suppressed the formation of ferrite to even lower cooling rates, progressively lowered the transformation start and finish temperatures and promoted the transformation of bainite instead of acicular ferrite. The latter was due to Nb suppressing the formation of allotriomorphic ferrite and allowing bainite to nucleate at prior austenite grain boundaries, a lower energy site than that for the intragranular nucleation of acicular ferrite at inclusions. Strength and hardness increased with increasing Nb additions, largely due to microstructural strengthening and solid solution hardening, but not from precipitation hardening.