Ferrite Nucleation Research Articles

Transformation behavior of a Ti–Zr deoxidized steel: Microstructure and toughness of simulated coarse grain heat affected zone

Continuous cooling and isothermal transformation behavior of a Ti–Zr deoxidized microalloyed steel was studied and compared with that of C–Mn steel, in conjunction with the evolution of microstructure and measurement of toughness of simulated high heat input welding coarse grain heat-affected zone (CGHAZ). The study indicates that the inclusions in Ti–Zr steel were primarily composite inclusions (ZrO2·MnS) with high density of inclusions of size less than 1μm. Polygonal ferrite, acicular ferrite, multi-directional bainite, and segmented martensite were the typical microstructures obtained with increase in cooling rate. Each of these phases was analyzed with decreasing isothermal temperature. Intragranular ferrite nucleation was significantly promoted by ZrO2·MnS inclusions. Inclusions of smaller size were more effective with decreasing temperature such that greater fraction of ferrite plates nucleated on one inclusion. Mn-depletion is the operating mechanism and precipitation of TiN was complementary. The simulated CGHAZ was characterized by interlock acicular ferrite microstructure and high impact toughness such that the dimple fracture and fine cleavage facets contributed to high impact energy.

The Effect of Inclusions on the Formation of Intragranular Ferrite

In order to make a more intuitive and easier analysis the influence of different inclusions on the formation of ferrite, a layer of high purity oxide powder was added in the two parts of the Q235 steel artificially. The effect of the inclusions Ti2O3MgOZrO2 and Al2O3 on the nucleation of intragranular ferrite were studied by means of the physical simulation method. The microstructure of the micro-zone adjacent to the inclusions was observed and the elements of the micro-zone adjacent were analyzed. The results showed that the inclusions Ti2O3 and Al2O3 can form the ferrite layers at the oxides-steels interface; the inclusions Ti2O3 has an ability to induce the nucleation of intragranular ferrite; the inclusions Ti2O3 can change the chemical composition of the metal micro-zone adjacent to the inclusions, where the Mn depletion area was observed.

Effect of titanium and nitrogen on the transformation characteristics of acicular ferrite in reheated C–Mn steel weld metals

The nucleation and growth processes of acicular ferrite in C–Mn steel weld metals containing various levels of titanium and nitrogen have been studied using dilatometry combined with transmission electron microscopy associated with energy-dispersive X-ray analysis. The as-deposited weld metals were thermally cycled under continuous cooling conditions, and fully developed and partially transformed microstructures were studied. Results showed that austenite transformed to ferrite in titanium-free C–Mn steel weld metals over a comparatively large range, and the resulting microstructure was dominated by Widmanstatten ferrite. The addition of titanium, typically 410 ppm, to C–Mn steel weld metals was found to accelerate the transformation kinetics, reducing the range in transformation temperature and modifying the weld microstructure with Widmanstatten ferrite being replaced by intragranularly nucleated acicular ferrite. Subsequent additions of nitrogen of around 240 ppm to C–Mn and C–Mn–Ti inhibited the transformation, but this soluble nitrogen had a little effect on weld microstructure. It is concluded that the development of acicular ferrite in C–Mn–Ti–N steel weld metals is associated with TiO- and TiN-type inclusions which act as effective sites for nucleation of acicular ferrite. This finding illustrates the possibility of using nitrogen as a deliberate alloying addition under some circumstances.

In Situ Observations of the Formation of Fine‐Grained Mixed Microstructures of Acicular Ferrite and Bainite in the Simulated Coarse‐Grained Heated‐Affected Zone

The formation of fine‐grained mixed microstructure of acicular ferrite and bainite in the simulated coarse‐grained heated‐affected zone was investigated by means of in situ observation and EBSD analysis. Single, multiple, and sympathetic nucleation of acicular ferrite were observed. Acicular ferrite laths or plates were formed prior to bainite and effectively partitioned austenite grains into many smaller and separated regions. The later formed bainite at lower temperature was confined in those smaller regions, thus resulting in the fine‐grained mixed microstructures of acicular ferrite and fine bainite plate.

Microstructural Characteristics of Multiphase Low Carbon Steel Welds with Different Ti and B Element Contents

In this work, a high strength, high toughness low carbon steel weld was developed by controlling the contents of B and Ti which are the minor but critical elements to the weld microstructure. The weld with the low B - Ti content exhibited the high strength close to 1 GPa and the excellent toughness over 70 J at 40 °C. The weld microstructure consisted of the multiphase which composed acicular ferrite, bainite, martensite, under welding condition. A weld with the high B - Ti content was fully martensite and exhibited the high strength over 1 GPa but the poor toughness below 15 J at 40 °C. The present results inform that, when the low carbon steel weld contains the substantial amount of various alloying elements to obtain the high strength, the B content should be low as possible so that a substantial amount of acicular ferrite is ensured by screening the hardenability improving effects from other elements. The Ti content was found to affect the size of the multi-component oxides such that the less the Ti content, the smaller the oxide size which is critical to the acicular ferrite nucleation.

Effect of Aluminum on the Nucleation of Intragranular Ferrite in Ti-Added Carbon Structural Steel

AbstractThe features of inclusion and microstructure for Ti-added carbon structural steel were studied through the scanning electron microscope (SEM) and Energy Dispersive Spectrometer (EDS). From the SEM-EDS mapping images, two kinds of titanium-contained oxides are observed. The type I is the Al-Ti-O phase, in which the elements of Al and Ti coexist. The type II is the titanium-contained oxides, which precipitate on the Al2O3. Through the statistical analysis, it can be seen that the former type is dominating in number and smaller in size. After etching, the microstructure of intragranular acicular ferrites (IAF) is observed, which is totally different from that in the comparison sample without Ti addition. Furthermore, it’s found that the type I inclusion can act as nucleation sites for the IAF. However, the similar nucleation phenomenon is complex for the type II inclusion. This complexity is attributed to the diversity of precipitation of Ti and Mn.

Quantitative analysis of austempering kinetics of ductile iron with Cu addition

The kinetics of austempering of two ductile iron alloys, Fe–3·5C–2·5Si and Fe–3·6C–2·7Si–0·7Cu, was investigated. The alloys were austempered for various times at 623 K (350°C) and 673 K (400°C), followed by water quenching. The first ferrite needles nucleate mainly at the graphite/austenite interface. Copper addition decreases the ferrite needles’ dimensions, increases the autocatalysis factor and increases the austempering rate. Decreasing the austempering temperature from 673 K (400°C) to 623 K (350°C) results in a decrease in the ferrite needles’ dimensions, an increase in the autocatalysis factor and a decrease in the austempering rate. A modified quantitative kinetic model for austempering of ductile iron, which takes into account the graphite nodules as ferrite nucleation sites, was developed. The change in the ferrite needles’ dimensions is explained by the interplay of the γ→α transformation driving force and the dislocation density that formed near the ferrite needles during their growth.

Effect of welding heat input on microstructures and toughness in simulated CGHAZ of V–N high strength steel

For the purpose of obtaining the appropriate heat input in the simulated weld CGHAZ of the hot-rolled V–N microalloyed high strength S-lean steel, the microstructural evolution, hardness, and toughness subjected to four different heat inputs were investigated. The results indicate that the hardness decreases with increase in the heat input, while the toughness first increases and then decreases. Moderate heat input is optimum, and the microstructure is fine polygonal ferrite, granular bainite, and acicular ferrite with dispersive nano-scale V(C,N) precipitates. The hardness is well-matched with that of the base metal. Moreover, the occurrence of energy dissipating micromechanisms (ductile dimples, tear ridges) contributes to the maximum total impact energy. The detrimental effect of the free N atoms on the toughness can be partly remedied by optimizing the microstructural type, fraction, morphologies, and crystallographic characteristics. The potency of V(C,N) precipitates on intragranular ferrite nucleation without MnS assistance under different heat inputs was discussed.

Effect of V on intragranular ferrite nucleation of high Ti bearing steel

The effect of 0.1V on the intragranular ferrite nucleation of 0.2Ti steel was studied. By adding 0.1V, the grain size was refined to 1–3 μm. The (Ti,V)(C,N) complex precipitates were the nucleation sites of the intragranular ferrite. The core of the precipitate was undissolved Ti–C-rich (Ti,V)(C,N), and the cap was V–C-rich (V,Ti)(C,N) generated by stress induction. The V-rich cap forming on the Ti-rich core increased the nucleation potency. The precipitates reached critical nucleation size with the assistance of the core.

Effect of Niobium on the Ferrite Continuous-Cooling-Transformation (CCT) Curve of Ultrahigh-Thickness Cr-Mo Steel

Pressure vessels made for petrochemical and power plants using Cr-Mo steel must be thick (≥400 mm) and have high tensile strength (≥600 MPa). However, the tensile strength in the middle portion of the vessel is very low as a result of ferrite formation. Therefore, research was performed to study the ferrite transformation that occurs in the middle portion of high-thickness Cr-Mo steel when Nb is added to it. The ferrite-formation start time of the continuous-cooling-transformation (CCT) curve decreased with an increase in Nb content until the latter reached 0.05 pct. On cooling from the austenitizing temperature, some of the NbC present at the austenitizing temperature of 1203 K (930 °C) goes into austenite solution in the temperature range of 1173 K to 1073 K (900 °C to 800 °C). However, the ferrite curve shifted to the left for the alloy containing 0.075 pct Nb. It is postulated that the surplus NbC could act as ferrite nucleation sites despite the lower cooling rate. As a result, the hardenability improved in the order of the following Nb content: 0.05 pct, 0.025 pct, 0 pct, and 0.075 pct.

Effect of aluminium content on acicular ferrite formation in low carbon steel weld metals

This paper describes the effect of Al content on the formation of acicular ferrite in low carbon steel welds. Inclusions and the microstructures of welds with three levels of Al content were analysed. Acicular ferrite was formed in the low and medium Al content samples, but not in the high Al content sample. In the low and medium Al samples, a TiO layer, which encourages acicular ferrite nucleation, was formed around the inclusions. Grain boundary ferrite was formed only in the low Al content sample. Inclusions in the low Al sample were predominantly composed of the amorphous phase, whereas those in the medium Al sample were composed of spinel structures. It was confirmed by electron energy loss spectroscopy analysis that boron existed in the amorphous phase. The formation of the amorphous phase enhances grain boundary transformation by absorbing boron, which stabilises austenite grain boundary, whereas the formation of spinel structures enhances intragranular transformation by suppressing the boron absorption.

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.

The Influence of Oxide Inclusion on the Refinement of Bainite and Toughness in HAZ for Low Carbon Steels

The influence of Ti oxide on the toughness of heat affected zone for low carbon bainitic steels has been investigated. The optical microscope, SEM and TEM were used to analyze the composition, size and distribution of the inclusions, and the microstructure and mechanical properties after welding thermal simulation were also investigated. The effect of Ti oxide inclusion on the transformation of acicular ferrite has also been studied. The results show that after the melting with Ti dioxide technique the inclusion is complex, in the core is Ti oxides about 1-3 micron and around it is MnS. It has been found the acicular ferrite can nucleate at the inclusions and the Ti oxide inclusion will promote the nucleation of acicular ferrite, and the acicular ferrite will block the growth of bainite. Therefore by introducing the Ti oxide in the steels the microstructure of HAZ could be refined markedly therefore the toughness of HAZ can be improved evidently.

極短パス間時間多パス圧延プロセスにおけるフェライト結晶粒微細化への内部組織変化の影響

The optimum conditions for manufacturing ultrafine-grained hot strips in super short interval multi-pass rolling (SSMR) process were discussed. As the first means of evaluation, the changes in austenite grain size and shape during multi-pass rolling were observed using experimental equipment. Deformed austenite grains could be observed with immediate quenching after each rolling pass. As the second means, a new numerical simulation method was used, which could predict the microstructure change of austenite, ferrite nucleation inside austenite grains, and ferrite grain growth. The effects of austenite grain size before finish rolling and also of temperature control between rolling passes on ferrite grain refinement were investigated. It was found that the temperature control between rolling passes is effective for grain refinement, whereas austenite grain refinement before finish rolling is not so effective. Therefore, deformation strain accumulation in austenite is important for ferrite grain refinement in SSMR process.

Effects of Normalizing Processes on Microstructure and Impact Toughness in Ti-bearing Weld Metal of Multilayer MAG Welded HSLA Steel

The influences of different normalizing heat treatments on microstructure, non-metallic inclusions and impact toughness in MAG Ti-bearing weld metal of HSLA steel has been studied. It has been shown that for the Ti-bearing weld metal the impact toughness after normalizing treatment decreases significantly against prolonging holding time and increasing normalizing temperature. The Mn-depleted zone forms around the Ti-bearing phase (MnTiO3) precipitated on Mn-Si oxide. Proeutectoid ferrite preferentially nucleates at the Mn-depleted zone and the interface between austenite and proeutectoid ferrite becomes the nucleation sites for pearlite thereafter. Mn-depleted zone formation increases the pearlite nucleation sites, and makes the pearlite fine. The dissolve of Ti-bearing precipitate causes disappear of Mn-depleted zone at strong normalizing processes (longer normalizing time and higher normalizing temperature), and the number of ferrite nucleation sites decreases, then the pearlite become coarser, which causes the deterioration of impact toughness.

First-principles study on ferrite/TiC heterogeneous nucleation interface

Interface atomic structure, bonding character, cohesive energy and interfacial energy of ferrite (100)/TiC (100) were studied using a first-principles density functional plane-wave ultrasoft pseudopotential method. Meanwhile, the effectiveness of TiC as the heterogeneous nuclei of ferrite was analyzed. The results indicated that, TiC bonding is dominated by the C-2p, C-2s and Ti-3d electrons, which exhibits high covalency. With increase of the atomic layers, the interfacial energies of ferrite and TiC are both declined rapidly and stabilized gradually. There are two binding modes for TiC as the heterogeneous nuclei of ferrite, which are Fe atoms above the Ti atoms (Ti-termination) and Fe atoms above the C atoms (C-termination). Interfacial energy of the Ti-termination is larger than that of the C-termination, which means that for Fe atoms above the C atoms, the ability of TiC promotes ferrite heterogeneous nucleation on its surface is larger than that for Fe atoms above the Ti atoms.

Effect of Ti-containing inclusions on the nucleation of acicular ferrite and mechanical properties of multipass weld metals

In the present study, the influence of Ti-containing inclusions on the development of acicular ferrite microstructure and mechanical properties in the multipass weld metals has been studied. Shielded metal arc weld deposits were prepared by varying titanium content in the range of 0.003–0.021%. The variation in the titanium content was obtained by the addition of different amounts of titanium oxide nanoparticles to the electrode coating. The dispersion of titanium oxide nanoparticles, composition of inclusions, microstructural analysis, tensile properties and Charpy impact toughness were evaluated. As the amount of Ti-containing inclusions in the weld metal was increased, the microstructure of the weld metal was changed from the grain boundary allotriomorphic ferrite structure to acicular ferrite with the intragranular nucleation of ferrite on the Ti-containing inclusions, and the mechanical properties were improved. This improvement is attributable to the increased percentage of acicular ferrite due to the uniform dispersion of Ti-containing inclusions and the pinning force of oxide nanoparticles against the growth of allotriomorphic ferrite and Widmanstätten ferrite from the austenite grain boundaries.

Effect of Boron on the Isothermal Bainite Transformation

The role of Boron on the isothermal bainitic transformation in low-C, lean-alloyed steel was investigated. B clearly affected both the transformation kinetics and the morphology of isothermally transformed bainite. The effect of B was more noticeable in the high-temperature range of the bainitic transformation. The microstructure of bainite formed at 773 K (500 °C) consisted of a bainitic ferrite matrix and the martensite/austenite constituent. While the martensite/austenite constituent had an elongated morphology in B-free steel, the martensite/austenite constituents in the B-added steel had a granular morphology. Two types of bainite unit nucleation were considered: the initial nuclei and the nuclei formed on previously formed units. Electron backscattered diffraction (EBSD) analysis showed that the initial bainitic ferrite nuclei were formed at austenite grain boundaries with a Kurdjumov-Sachs (K-S) crystallographic orientation relationship with respect to one of the neighboring austenite grains, revealing the importance of interfacial energy reduction in the nucleation stage. The nuclei of the bainite transformation in the B-added steel were confined to the austenite grain interior, and the bainitic ferrite nuclei had crystallographic orientations limited to K-S variants within the same Bain variant. The characteristic bainite microstructure in B-added steel is due to the inhibition of the bainitic ferrite nucleation at austenite grain boundaries.

Effect of Nitrogen and Boron on the Development of Acicular Ferrite iN Reheated C-Mn-Ti Steel Weld Metals

The transformation behaviour of C-Mn-Ti alloyed steel weld metals with different levels of nitrogen and boron contents has been investigated using a dilatometric technique in combination with transmission electron microscopy (TEM) associated with microanalysis techniques, i.e. energy dispersive X-ray analysis (EDXA) and electron energy loss spectroscopy (EELS) in order to clarify the role of nitrogen and boron on the nucleation and growth processes of weld metal microstructure in particular acicular ferrite which gives high strength and improved impact toughness of steel weld metals. Results of this investigation showed that the addition of a small amount of boron, typically 40 ppm to a C-Mn-Ti weld metal was sufficient to significantly reduce the transformation start temperature with a decrease in the amount of grain boundary ferrite and a concommittant rise in the amount of acicular ferrite. Further decrease in the transformation start temperature was observed as the level of boron content was increased up to approximately 160 ppm resulting in the formation of bainitic microstructures. However, a subsequent addition of nitrogen around 240 ppm to this type of weld metal increased the transformation temperature and modified the weld microstructure marked by the presence of intragranular acicular ferrite and polygonal ferrite which nucleated on multiphase inclusions principally of the ‘TiO’ type but on which has formed BN. This finding seems to suggest that BN is a potent substrate for nucleating ferrite and the amount of acicular ferrite in Ti-B-N system is controlled by the balance between BN as an energetically favourable site for acicular ferrite nucleation and soluble boron which acts as a hardenability element suppressing grain boundary ferrite formation.

Effect of Cooling Rates on the Second‐Phase Precipitation and Proeutectoid Phase Transformation of a Nb–Ti Microalloyed Steel Slab

AbstractDuring the continuous casting of low‐carbon Nb–Ti microalloyed steel, control of the slab surface microstructure and the behavior of the second‐phase precipitation are significantly influenced by the cooling rate. Through confocal laser scanning microscopy, the effect of the cooling rate on the behavior of ferrite precipitation both at the grain boundary and within the austenite was observed in situ and analyzed. The relationship between the cooling rate and precipitation of the microalloying elements on the slab surface microstructure was further analyzed by transmission electron microscopy. The results showed that the effect of microalloying element precipitation on proeutectoid ferrite phase transformation is mainly manifested in two aspects: (i) the carbonitrides of microalloying elements act as inoculant particles to promote nucleation of the proeutectoid ferrite and (ii) the carbon near the grain boundary is depleted when the microalloying elements precipitate into carbonitrides, inducing a decrease in the local carbon concentration and promoting ferrite precipitation.