Nucleation Of Acicular Ferrite Research Articles

Inclusions Evolution of Ca‐Treated Steel During High‐Heat Input Welding

This article has investigated a 60 mm Ca‐treated Q550 steel used for high‐heat input welding. The results have shown that the Ca‐type inclusions account for 70% of the inclusions in the steel which are mainly CaAlMg type, Ca–Al type, and a small amount of CaS type while the non‐Ca‐type inclusions are mainly FeCO. After undergoing the welding thermal cycle, the number of Ca‐containing inclusions above 2 μm in size is decreased while the number of non‐Ca‐containing inclusions below 1 μm (mainly FeO) is increased significantly. However, the total area of inclusions is decreased. It has been found out that the CaAlSO‐type inclusions undergo “diffusion” of O element to the surroundings during the thermal cycle, forming a ring‐shaped oxygen‐rich zone and carbon‐rich zone which promotes the nucleation of acicular ferrite. In contrast, the CaAlMgSO‐type inclusions undergo “decomposition” during the thermal cycle which forms a group of inclusions with different sizes and types, promoting nucleation of multioriented acicular ferrite.

An improved toughness process for high-temperature hot-rolled HSLA steel via inclusion-induced acicular ferrite nucleation

A feasible method of enhancing the impact toughness (−20 °C) of high-temperature hot-rolled HSLA steels was proposed. The results show that the toughness of the improved hot-rolled steel containing TiOx-CaO-ZrO2 oxide particles was significantly enhanced by ∼132 J compared with conventional steel containing Al-Ca oxide particles. The fine (Ti, Ca, Zr) Ox particles with high-density of ∼937 counts/mm2 significantly promotes anti-cracking acicular ferrite (AF) nucleation. Moreover, the fractional volume of AF increased from ∼79 % to ∼85 % when the rolling temperature was increased from ∼1055 °C to ∼1085 °C.

Evolution of Complex Oxide Inclusions During the Smelting Process of Oxide Metallurgical Steel and Their Effect on Acicular Ferrite Nucleation

Evolution of Complex Oxide Inclusions During the Smelting Process of Oxide Metallurgical Steel and Their Effect on Acicular Ferrite Nucleation

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.

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

With the development of the pressure vessel industry, high-energy wire welding has a great future. However, this means higher demands on the weldability of pressure vessel steels. Controlling inclusions via oxidative metallurgy is a reliable method of improving the weldability of pressure vessel steels. Hence, in this paper, experimental steels with different Mg element mass fractions were prepared using vacuum metallurgy. Simulated welding for high-heat input welding was carried out using the Gleeble-2000 welding thermal simulation test machine. The inclusions in the welding heat-affected zone (HAZ) in the experimental steels were observed using an optical microscope (OM) and scanning electron microscope (SEM). The compositions of the inclusions were analyzed using an energy-dispersive spectrometer (EDS). The research results indicated that the addition of Mg could increase the number density of the inclusions in the welding HAZ. With the addition of Mg from 0 to 5 wt.%, the total number density of the inclusions increased from 133 to 687 pieces/mm2, and the number density of the inclusions with a size of 0–5 μm2 increased from 122 to 579 pieces/mm2. The inclusions in the experimental steel welding HAZ with Mg elements were mainly elliptical composite inclusions composed of (Mg-Zr-O) + MnS. Moreover, MnS precipitated on the surface of the Mg-containing inclusions in the welding HAZ. Intragranular acicular ferrite (IAF) nucleation was primarily induced via the minimum lattice mismatch mechanism, supplemented with stress-strain energy and inert interface energy mechanisms.

A novel dual-strengthening technology combining external second phase nanoparticle method and hot compression deformation process in micro-alloyed medium carbon steel

Herein, a novel dual-strengthening technology that combines the external second-phase nanoparticle addition method with the hot compression deformation tests was carried out to investigate the effects of surface-modified nanoparticles (NPs) and deformation conditions on the characteristics of inclusions and the evolution of microstructure in the micro-alloyed (MA) medium carbon steels. The results revealed that the inclusions in the steels were divided into single phase inclusions and composite phase inclusions. With the addition of NPs, Al2O3 inclusions were replaced by MgAl2O4 spinel, and the composite inclusions mainly consisted of TiN–VN, MgAl2O4–TiN and MgAl2O4–TiN–MnS inclusions, and the same types of inclusions became significantly smaller in size. According to the EDS results, some of the inclusions acted to induce ferrite nucleation, and the MgAl2O4–TiN–MnS composite inclusion was more favorable for the nucleation of acicular ferrite (AF). The deformation induced ferrite (DIF) started to form at the deformation temperature of 750 °C and the maximum stress peak in the NPs-bearing steel was 516 MPa, which was 28.4% higher than that in the original steel. The proportion of ferrite increased as the deformation temperature decreased. The dispersed NPs contributed to the ferrite nucleation and the grain refinement as the inclusion-modifying agent, which improved the material properties, and the relevant strengthening mechanisms were explained.

Acicular ferrite formation and Mn depletion behavior around the inclusions in Ti-Mg oxide metallurgy steel with different holding times

In this study, the effect of different holding times at 1250 °C on acicular ferrite formation, the inclusion characteristics, and Mn depletion behavior at the inclusion-matrix interface in Ti-Mg oxide metallurgy steel was systematically investigated. The results suggested that the volume fraction of grain boundary products decreased with increased holding time from 60 s to 3600 s, with the volume fraction of acicular ferrite initially increasing and then decreasing. Regarding the inclusion characteristics, different holding times showed no obvious effect on the inclusion type, but they affected the size and area number density of the inclusions. The domain inclusions at different holding times all consisted of MgTi2O4 +MnS, and these inclusions were found to be effective for the nucleation of acicular ferrite. The average diameter of the inclusions increased, and the area number density of the effective inclusions significantly decreased when the holding time was increased from 900 s to 1800 s and 3600 s. Mn-depleted zones (MDZs) with larger widths and higher Mn concentration gradients formed at the inclusion-matrix interfaces at holding times of 60 s and 900 s, and MDZ formation was weakened by the continuous diffusion of Mn when the holding time was increased to 1800 s and 3600 s. At a holding time of 900 s for the Ti-Mg oxide metallurgy steel, the volume fraction and relative nucleation potential of acicular ferrite reached their maximum values, and MDZ formation with the lowest Mn concentration (∼0.8 wt%) was observed around the MgTi2O4 +MnS inclusions. Therefore, 900 s was the optimal holding time for acicular ferrite formation.

Influence of Ti/N Ratio on Inclusions, Microstructures, and Toughness in Heat‐Affected Zone of Shipbuilding Steel Plates with Mg Deoxidation after High Heat Input Welding

The inclusions, microstructures, and toughness in heat‐affected zone (HAZ) of the shipbuilding steel plates with Mg deoxidation and the different Ti/N ratios are studied after the high heat input welding of 400 kJ cm−1. With increasing the Ti/N ratio from 3.00 to 5.67, the average size of inclusions increases, the mass fraction of total TiC inclusions increase from 2.8% to 5.0%, and the average size of MgO–TiN–MnS increases from 2.82 to 5.49 μm, while the number density that can induce intragranular acicular ferrite (IAF) nucleation decreases from 87 to 62 mm−2. These changes in inclusions are detrimental to induce the IAF nucleation. The HAZ microstructures are composed of the IAFs, polygonal ferrites, and upper bainites (UBs). With increasing the Ti/N ratio, the area fraction of the IAFs decreases from 44% to 14%, and the area fraction of UBs increases from 1.8% to 9.5%. The HAZ fracture mode changes from ductile fracture to brittle fracture after the high heat input welding of 400 kJ cm−1, and the HAZ toughness decreases from 183.3 to 48.7 J at −20 °C.

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.

Study on the mechanism of AF nucleation induced by complex oxide inclusions after LF refining in oxide metallurgical steel

The ladle furnace refining process (before LF, add Ti, add Al, add Mg, add Ca) of oxide metallurgical steel was sampled, and the effect of complex oxide inclusions generated after LF refining on acicular ferrite (AF) nucleation were studied by SEM, TEM, TKD and EBSD. The results show that the complex oxide inclusions after LF refining are composed of MgAl2O4, CaTiO3, TiO2, TiN, and MnS. Among them, MgAl2O4, CaTiO3, TiO2 and TiN have NW orientation relationship with adjacent AF laths. There are Mn depletion zone(MDZ) with a width of about 200 nm outside MgAl2O4, CaTiO3, TiO2 and MnS. Some AF laths can be nucleated by different inclusions under the combined action of MDZ mechanism and lattice mismatch mechanism. The first principle calculations show that MgAl2O4, CaTiO3 and TiO2 all have cation vacancies, and the diffusion of Mn ions into cation vacancies has thermodynamic advantages. The formation of MDZs is the combined effect of MnS precipitation on oxidized inclusions and Mn ions occupying cation vacancies in inclusions.

Elucidating the heat input on CGHAZ microstructure and its irregular effect on impact toughness for a novel V–N microalloying weathering steel

This study investigated the effect of heat inputs on coarse-grained heat affected zone (CGHAZ) microstructure and impact properties of a novel vanadium (V) and nitrogen (N) microalloyed weathering steel via a Gleeble™ system, revealing the metallurgical essence of the steel suitable for large heat input (Ej) welding. The results indicate that in CGHAZ, the dominant microstructure was composed of lath bainitic ferrite (LBF) at Ej = 15 kJ/cm, granular bainitic ferrite (GBF) at Ej = 35 kJ/cm, and a mixture of intragranular acicular ferrite (IGAF), intragranular polygon ferrite (IGPF) and grain boundary polygonal ferrite (GBPF) at Ej = 55 and 75 kJ/cm, respectively, apart from martensite/austenite (M/A) constituents in each one. The area fraction, fM/A, and mean size, dM/A, of M/A constituents increased monotonously with the Ej. In addition, with increasing Ej from 15 to 35 kJ/cm, the fraction, fMTAs＞15°, of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) greater than 15° reduced, while the mean equivalent diameter, MEDMTA≥15°, of grains with HAGBs increased. However, as Ej further increased to 55 and 75 kJ/cm as the two large heat inputs, the precipitation of (Ti, V) (C, N) and thereby the heterogeneous nucleation of polygonal and acicular ferrite were promoted by a lowered degree of undercooling, resulting in the decreased MEDMTA≥15° and the increased fraction fMTAs＞15°. Accordingly, the impact toughness of CGHAZ was degraded with the increase of Ej from 15 to 35 kJ/cm, but enhanced with the increase of Ej from 35 to 75 kJ/cm.

Effect of Al or Ti addition on the ferrite formation in ultra-low oxygen weld metal of low carbon steel

Effect of Al or Ti addition on the ferrite formation in ultra-low oxygen (about a few tens ppm) weld metal of low carbon steels were investigated. A bead-on-plate welding experiments were performed with a Tungsten Inert Gas (TIG) welding system. The double nozzle was attached to the welding torch used, Ar was passed along the outer nozzle and He is added along the inner nozzle. Optical micrographs showed that primary ferrite appeared along prior austenite grain boundaries and that as the temperature is decreased ferrite side plates were grew into grain interior. Their formation positions were approached to the welding end as the decreasing Al and increasing Ti content. This change in ferrite formation position attributed on the ferrite formation temperature, which was assumed to increase as the decreasing Al and increasing Ti content. Energy dispersive X-ray spectroscopy analysis of inclusions showed that Al and Ti were mainly associated with 0, it was obvious that the inclusions in the steels were Al- and Ti- oxide inclusions. Al-oxide inclusions were not favourable acicular ferrite nucleation sites and the inclusions contributing to acicular ferrite formation were Ti-oxide ones. The number density of acicular ferrite was small. This indicated that acicular ferrite formation was suppressed because the number density of inclusions was low under the ultra-low oxygen welding condition.

Microstructure and mechanical properties of wire-fed arc-based directed energy deposition of high-strength steel after ultrasonic impact oxidation treatment

The ultrasonic impact oxidation treatment (UIOT) was applied to the local region of a high-strength steel specimen fabricated via a wire-arc directed energy deposition (DED) process. The microstructure and shear-punch strength of the specimen were investigated. The effects of temperature on the UIOT were also studied. Results show that UIOT produces an oxide layer on the treated surface and the elements in the pin are transferred to form the oxides. UIOT at room temperature results in the highest element transfer amount from the pin. The UIOT-induced oxides become tiny oxide inclusions dispersed in the DEDed specimen. Contents of Al, Si, Ti, Mn, and O of the inclusions are significantly higher than those in the deposited metal and the Mn-deplete zone is generated near the inclusion, which has the favorite conditions for the nucleation of acicular ferrite. The UIOT results in finer grains in the treated region than those in the untreated region, and it decreases the strength of the treated material while increasing the equivalent yielding-to-tensile ratio.

Effect of Nb on inclusions and phase transformation in simulated high heat input coarse-grain HAZ of Nb/Ti low carbon microalloyed steel

In this paper, two groups of Nb/Ti low carbon microalloyed steels (Nb-bearing and Nb-free) were obtained through laboratorial methods. The evolution and effect of Nb/Ti-containing complex inclusions were characterised and analysed after being subjected to high heat input thermal simulation of 100 kJ/cm. The results showed that the prior austenite grain size in the coarse grain heat-affected zone (CGHAZ) was not affected by 0.045 wt% Nb addition. However, the acicular ferrite nucleation, induced by effective micron-level inclusions, was inhibited. Nb tended to converge towards the interfaces between the micron-level inclusions and matrix, which had a negative effect on the nucleation potential of acicular ferrite. The re-precipitation behaviour of nano-level inclusions satisfied the dynamic conditions during the high heat input thermal cycle, under which the dislocation movement was inhibited due to the pinning effect of re-precipitation particles. Besides, the size, morphology and composition of nano-level precipitates were changed by Nb (0.045 wt%) addition. In general, the evolution and effect of inclusions in simulated high heat input CGHAZ provide theoretical references for the study of welding performance in Nb/Ti low carbon microalloyed steels. • Nb segregation at the interface between micron inclusion and matrix affects the nucleation of acicular ferrite. • The acicular ferrite nucleation induced by effective inclusions is inhibited by a certain amount of Nb. • The size, morphology and even composition of the nano-level inclusions in CGHAZ are changed by 0.045wt%Nb addition.

New insights of heterogeneous nucleation and anisotropic growth of acicular ferrite on non-metallic inclusion

This work studies the acicular ferrite (AF) nucleation and growth from a classical defect, i.e. non-metallic inclusion surface. The heterogeneous AF-nucleation probability on NMI is predicted by a mathematical model following the classical nucleation theory. The calculation results are in consistent with the experimental data. The current model includes the exact form of the activation energy and Zeldovich factor for heterogeneous nucleation on pre-existing spherical surface. It generates an interaction map that for the first time quantitatively describes the superposition influences of the interfacial energies and NMI-size on the AF-nucleation potency. In the AF-growth aspect, we systematically reveal the single AF-plate evolution behavior through the synergy between the experimental observations and multiphase-field simulations. It is found that the AF-tip morphology is impacted by the misorientation angle between the favorable growth direction and normal vector. It also influences the AF-lengthening and widening kinetics. Additionally, the anisotropic stress field induced by the AF-NMI interface plays a significant role in controlling the complex curvature of the AF-plate. The current study opens new perspectives for understanding the AF-precipitation mechanisms.

Effect of in situ observation of cooling rates on acicular ferrite nucleation

Abstract The effect of cooling rates on acicular ferrite (AF) nucleation was observed in situ by laser scanning confocal microscopy. The precipitation characteristics of TiN–MnS inclusions, the austenite grain size, and the content and morphologies of AF were analyzed and compared under different cooling rates. The results indicated that with the increase of the cooling rate, the size of TiN–MnS inclusions precipitated in the test steel gradually decreased while the inclusion number increased. When the cooling rate increased from 0.2 to 10°C·s−1, the average inclusion size decreased from 2.7 to 1.3 µm. The austenite grain size decreased gradually with the increase of the cooling rate, and the larger the cooling rate was, the more uniform the grain size distribution. The AF proportion increased first and then decreased, it reached the maximum when the cooling rate was 5°C·s−1, which accounted for 82.6%. Meanwhile, the dendrite size decreased gradually, the average length decreased from 30 to 5 µm, and the average width decreased from 2.8 to 0.7 µm when the cooling rate increased from 0.2 to 10°C·s−1.

Laser Surface Melting and Consecutive Point-Mode Forging Hardening of DH36 Marine Steel: Mechanical and Precipitation Behavior

This study investigates the effect of the laser surface melting and consecutive point-mode forging process (LSM-CPF) on the mechanical properties and the microstructure of DH36 marine steel. The microstructual revolution during the LSM-CPF process are revealed by metallographic microscope (OM) and scanning electron microscope (SEM) technique, and the strengthening mechanisms for different samples are also elucidated. The results show that the best yield strength (σ0.2) and tensile strength (σb) for the sample treated with 4000 W laser power and 10% reduction ratio are 721.3 and 884.2 MPa, which are 49.55% and 41.54% higher than that of the DH36 matrix, respectively. The hardness of the coatings decreases along the normal direction with the maximum value of 586.4 HV in the CPF zone for the sample treated with 2000 W laser power and 20% reduction ratio. During the low power LSM-CPF treatment, the nanoscale cementite appear as intragranular due to the inhibited carbon diffusion. The coherent boundary of (110)NbC‖(110)Ferrite, [11¯0]NbC‖[001]Ferrite between NbC and ferrite reduces the nucleation barrier to promote the nucleation of acicular ferrite (AF). The strengthening mechanism for samples treated at 2000 W is found to be dislocation strengthening. During high power laser treatment, pearlite transformation is found to occur with a low cooling rate. In this case, the strengthening mechanism is the boundary strengthening of lamellar pearlite and dislocation strengthening.

Mechanism of BN-Promoting Acicular Ferrite Nucleation to Improve Heat-Affected Zone Toughness of V-N-Ti Microalloyed Offshore Steel.

This study examined the effect of boron (B) on the microstructure and toughness of the simulated coarse-grained heat-affected zone (CGHAZ) of normalized vanadium microalloyed offshore steel by using the welding thermal simulation method under different heat inputs for welding. The results showed that when t8/5 (the cooling time from 800 to 500 °C) increased from 14 s to 24 s, In the range of t8/5 of 24–44 s, the impact energy of the CGHAZ rose initially and then remained constant at around 125 J at −40 °C, and dropped to 79 J when t8/5 increased to 64 s. The particles (Ti,V)(C,N)-BN and BN contributed in the generation of acicular ferrite, which minimized the loss of CGHAZ toughness due to the presence of carbon. Furthermore, the microstructural parameters controlling CGHAZ toughness were the contents of the high misorientation grain boundaries and effective grain size at a tolerance angle of 15° at varied heat inputs.

Effect of Arc Length on Oxygen Content and Mechanical Properties of Weld Metal during Pulsed GMAW

Pulsed gas metal arc weld (GMAW) was widely used for the advantages of controllable heat input, all-position welding, and no spatter. In order to obtain an ideal welding process, the stability of the arc length was studied in many researches, but the influence of arc length on the properties of weld metal was ignored. In this paper, the effect of arc length on oxygen content and mechanical properties of weld metal during pulsed GMAW was studied. Q690 high strength steel was selected as the base metal, and ER69-G solid wire, with a diameter of 1.2 mm, was used as the electrode wire. Additionally, the shielding gas and the wire feed rate were 82% Ar + 18% CO2 and 4 m/min, respectively. The results showed that as the arc length raised from 2.9 mm to 9.2 mm, the oxidation reacted more completely in the droplet transfer zone, and the oxygen content of the weld metal increased significantly. The tensile strength of the weld metal reduced but the −40 °C impact energy heightened. Due to the longer arc, the proportion of acicular ferrite (AF) in the microstructure decreased, but the proportion of lath bainite (LB) and granular bainite (GB) decreased. The higher oxygen content of weld metal was useful for the formation of inclusions, which promoted the nucleation of acicular ferrite and dimples, contributing to the growth of plasticity and toughness of weld metal.

Effect of Al or Ti addition on the Ferrite Formation in Ultra-low Oxygen Weld Metal of Low Carbon Steel

Effect of Al or Ti addition on the ferrite formation in ultra-low oxygen (about a few tens ppm) weld metal of low carbon steels were investigated. A bead-on-plate welding experiments were performed with a Tungsten Inert Gas (TIG) welding system. The double nozzle was attached to the welding torch used, Ar was passed along the outer nozzle and He is added along the inner nozzle. Optical micrographs showed that primary ferrite appeared along prior austenite grain boundaries and that as the temperature is decreased ferrite side plates were grew into grain interior. Their formation positions were approached to the welding end as the decreasing Al and increasing Ti content. This change in ferrite formation position attributed on the ferrite formation temperature, which was assumed to increase as the decreasing Al and increasing Ti content. Energy dispersive X-ray spectroscopy analysis of inclusions showed that Al and Ti were mainly associated with O, it was obvious that the inclusions in the steels were Al- and Ti- oxide inclusions. Al-oxide inclusions were not favourable acicular ferrite nucleation sites and that the inclusions contributing to acicular ferrite formation were Ti-oxide ones. The number density of acicular ferrite was small. This indicated that acicular ferrite formation was suppressed because the number density of inclusions was low under the ultra-low oxygen welding condition.