Formation Of Intragranular Ferrite Research Articles

Effect of Zr addition on MnS inclusion characteristics and mechanical properties in medium carbon ferrite-pearlite steel

In this paper, the influence of decreasing Zr content (0.0110wt%, 0.0044wt%) on the microstructure, MnS inclusion characteristics, and mechanical properties of medium carbon ferrite-pearlite steel was studied. The results show that the volume fraction of intragranular ferrite (IGF) in steel increases with a reduction in Zr content. The ductility of the steel is reduced by 1.83%, the impact toughness is increased by 2.5 times, and the yield strength is almost unchanged. Compound MnS inclusions are the main inclusions that induce IGF formation. The proportion of heterogeneous nucleation of MnS inclusions using oxides as nucleation sites is increased, which is the main reason for the increase in the volume fraction of IGF. Furthermore, MnS inclusions change from type Ⅲ to type Ⅰ and Ⅱ, and their distribution is gradually uniform. Thermodynamic analysis reveals that the underlying cause for the morphological transformation of MnS inclusions is the rapid surge in the supersaturation of S element in molten steel. The presence of liquid-phase low-melting inclusions (MnS-Al2O3) promotes the formation of type Ⅰ MnS inclusions. The volume fraction and the dimension perpendicular to the fracture surface of the MnS inclusions are increased by at least 33% and 111%, while their aspect ratio is reduced by a maximum of up to 19%. The change in the volume fraction of MnS inclusions is the primary factor contributing to the decrease of tensile plasticity. The toughness of steel is mainly affected by the aspect ratio and the dimension perpendicular to the fracture surface of MnS inclusions.

Interfacial structures and properties of sulfide-Fe matrix in steel: A first principles study

Fine sulfides can promote the refinement of microstructure and improve the strength and toughness of materials. The interface properties between sulfides and steel matrix are associated with this phenomenon. The first principles calculation combined with two-dimensional lattice mismatch and two-step nucleation theory is adopted to calculate the interface properties of TiS/bcc-Fe, TiS/fcc-Fe and TiS/MnS at the atomic level. The results show that TiS has a weak ability to promote the transition of austenite to intragranular ferrite (IGF) due to the low adhesion work value. And TiS needs to overcome energy barriers to induce the formation of IGF because the higher interface energy of TiS/bcc-Fe than TiS/fcc-Fe. The heterogeneous nucleation ability of TiS(001)/MnS(111) is stronger than that of TiS(001)/MnS(110), indicating that TiS is beneficial for improving the deformation performance of MnS without affecting the ability of MnS (110) surface to induce IGF nucleation for grain refinement. Based on the two-step nucleation theory, the structures and properties of TiS clusters are analyzed. The formation pathway of TiS/Fe interface can be described as: Tiatom + Satom + Featom → core (TiSn)–shell (fcc-Fe(111)) → core (TiS(001))–shell (fcc-Fe(110) + bcc-Fe(100)).

Understanding and Controlling the Weld Microstructure of Steels

In this article, selected studies are reviewed with a focus on the analysis of microstructure development in steel weld metals. In the study of austenitic stainless steel weld metals, microstructure development in the primary ferrite solidification mode (FA mode) was clarified and related to why FAmode welds are resistant to hot cracking. In studies of duplex stainless steel weld metals and high-Cr ferritic stainless steel weld metals, nitrogen-driven microstructure development and TiN-assisted grain refinement, respectively, were described, and discussions about the mechanism of equiaxed grain formation in the weld metals were added in the latter. In the study of low-alloy steel weld metals, the roles of titanium oxide and titanium nitride (TiN) inclusions on intragranular ferrite formation and the refinement of weld microstructure were described based on crystallographic analysis and the first principles calculation. At the end, the potential importance of the application of different multiscale, multiphysics simulations to welding research was pointed out.

The Role of MX Carbonitrides for the Particle-Stimulated Nucleation of Ferrite in Microalloyed Steel

The role of titanium and vanadium carbonitrides (Ti,V)(C,N) of the MX series on the austenite-to-ferrite transformation in microalloyed steels and their potential of acting as pre-nuclei for intragranular ferrite nucleation is examined experimentally and by computational simulation. Thermal treatments and single-hit compression tests are performed on a dilatometer and a Gleeble® 3800 thermomechanical simulator to investigate the phase transition and precipitation sequences within microalloyed steel. The analysis of the microstructure and examination of formed precipitates is carried out by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In situ observations are realized via high-temperature laser scanning confocal microscopy (HT-LSCM). The experimental results are compared to kinetic precipitation simulations for MX precipitates’ particle-stimulated intragranular ferrite formation using an on-particle nucleation model for ferrite on the surface of the MX carbonitrides. A particular focus is the comparison of precipitation trends between non-deformed and deformed samples. The physically appropriate modeling of MX precipitation kinetics in combination with intragranular ferrite formation serves as a basis for future industrial process optimizations without extensive experimental work.

Effect of Permanent Magnet Stirring on Solidification of High‐Sulfur Micro‐Alloyed Steel under Different Magnetic Flux Densities

The effects of permanent magnet stirring (PMS) with different center magnetic flux densities (0, 850, 1450, and 1800 Gs) on the solidification process of high‐sulfur micro‐alloyed 49MnVS3 steel are investigated by analyzing the MnS precipitation, microstructure, and tensile properties. The results indicate that with an increase in the magnetic flux density, an enhanced turbulent flow is introduced to the melt, and the MnS precipitates are not limited by grain boundary and distributed randomly and uniformly in the steel. When the magnetic flux density of PMS increases from 0 to 1800 Gs, the mean length of MnS decreases from 6.8 to 2.9 μm, and the number density of MnS increases from 1562 to 2293 n mm−2. Meanwhile, the content of the intragranular ferrite (IGF) increases from 0.4% to 3.1%, and the ratio of IGF in the total ferrite increases significantly from 4.2% to 30.1%, due to the growing precipitation of finer MnS‐ (Nb, V, Ti) (C, N) complex inclusions after PMS that induces the formation of IGF. In addition, both the tensile strength and toughness of the steel are improved under PMS with enhanced magnetic flux densities, which is attributed to the increased small‐sized MnS precipitates and promotes the formation of IGF.

Effects of Ti and Al on the formation of intragranular ferrites in the ultra-low-oxygen Si-Mn weld metals of low-carbon steel

Studies on intragranular ferrite with ultra-low O content are scarce, and establishing a technology for the refinement of the microstructure of a weld metal with ultra-low O content through the formation of intragranular ferrite is challenging. In this report, the effect of addition of Ti and Al on the formation of intragranular ferrite in ultra-low-oxygen weld metals was studied. The formation process of the oxide was discussed based on the detailed observation of the oxides and thermodynamic calculations. In the weld metal of Ti added steel with the highest intragranular ferrite area fraction, it was observed that the significant Mn depleted zone (MDZ) formed around a complex oxide containing Ti, Mn and Si. The formation of the MDZ increases the transformation temperature around the oxide and promotes the formation of intragranular ferrite with the oxide as a nucleus. Since the increase in Mn content at equilibrium due to the change in the stable phase during cooling is larger for the inclusion in Ti added steel, it was considered that more Mn diffused from the matrix to the inclusion in Ti added steel and the MDZ formed at the interface between matrix and the inclusion. The promotion of intragranular ferrite formation in an ultra-low-oxygen weld metal is enhanced by oxides with a large increase in the equilibrium Mn content during cooling.

Effect of S and Si on the Formation of Intragranular Ferrite and Inclusions in Ultra-low Oxygen Weld Metal of Low Carbon Steel

Effect of S and Si on the Formation of Intragranular Ferrite and Inclusions in Ultra-low Oxygen Weld Metal of Low Carbon Steel

Significant Influence of Welding Heat Input on the Microstructural Characteristics and Mechanical Properties of the Simulated CGHAZ in High Nitrogen V-Alloyed Steel

AbstractThe microstructural characteristics and mechanical properties of the simulated coarse grained heat affected zone (CGHAZ) in high N V-alloyed steel have been conducted under different welding heat input, characterized by the cooling time taken from 800°C to 500°C (t8/5). The experimental results show that the microstructure is dominantly composed of lath bainite (LB) and granular bainite (GB) at t8/5 30 s– 90 s. The content of LB decreases with t8/5 increasing, and that of GB increases. When t8/5 further increases to 120 s and 180 s, the microstructure mainly consists of intragranular polygonal ferrite (IPF) and acicular ferrite (IAF). The higher t8/5 leads to the increased content of intragranular ferrite (IGF). Meanwhile, the prior austenite grain size (PAGS) progressively increases from 56 ± 6.0 μm to 148 ± 9.9 μm as t8/5 increases from 30 s to 180 s. Besides, EBSD analysis indicates that the fraction of high angle grain boundaries (HAGBs) is 0.570, 0.427 and 0.624, respectively, corresponding to t8/5 30, 90 and 180 s. Moreover, the impact toughness decreases as t8/5 increases from 30 s to 90 s caused by the increased PAGS and GB content, and then sharply increases with t8/5 exceeding 90 s due to the increased formation of IGF, especially IAF. Furthermore, the high nitrogen content accelerates V(C,N) precipitation, which not only inhibits the coarsening of prior austenite grains, but promotes the formation of IGF, resulting in the increased number of HAGBs and raising impact toughness.

Effect of welding heat input on microstructural evolution, precipitation behavior and resultant properties of the simulated CGHAZ in high-N V-alloyed steel

The effect of welding heat input characterized by the cooling time taken from 800 °C to 500 °C (t8/5) on the microstructural evolution, V(C,N) precipitation and resultant mechanical properties of the simulated CGHAZ in high-N V-alloyed steel was comparatively investigated using a Gleeble-1500D thermomechanical simulator. Metallographic analysis indicated that the dominant microstructure transformed from lath bainite to granular bainite when t8/5 increased from 30 s to 90 s and then changed to intragranular ferrite at t8/5 of 180 s. The nanoscale V(C,N) precipitates were coarsened as t8/5 increased, coupled with a slightly increased number density. Furthermore, the orientation relationship [001]α//11¯0V(C,N) reduced the interfacial structural energy (MC/α) and enhanced the V(C,N) precipitation in the ferrite matrix. Moreover, the microhardness progressively decreased due to the combined effect of microconstituents, precipitates and grain boundaries. The impact toughness first decreased and then increased, and the optimal value was obtained at t8/5 of 180 s. Furthermore, the formation of intragranular ferrite, especially acicular ferrite, and the increased fraction of high angle grain boundaries could completely remedy the detrimental effect caused by the increased content of grain boundary ferrite and the coarsened effective grain size and precipitates. In addition, the high nitrogen content of 240 ppm accelerated V(C,N) precipitation in the austenite region, and the submicron V(C,N) precipitates could promote the formation of intragranular ferrite by providing heterogeneous nucleation sites, in combination with the larger prior austenite grain size caused by higher welding heat input.

Effect of surface-modified MgO nanoparticles on intragranular ferrite nucleated on inclusions in low-alloy steel

Despite extensive research into steel refining with nanoparticle addition, some aspects remain unclear, such as the low utilization ratio of the nanoparticles. In this study, application of surface-modified nanoparticles in the steel refining process was investigated. The effects of their addition amount on inclusion modifications and microstructure refinements were investigated and the mechanism of intragranular ferrite formation was clarified. Initial experiments were performed to determine the appropriate steelmaking holding time, and the utilization ratio of MgO@C nanoparticle was 65% from the calculated total oxygen content. After nanoparticle addition, the inclusions were mainly composed of MgAl2O4 spinel or MgAl2O4–TiN-based composite inclusions. The inclusion size sharply decreased with nanoparticle addition. The number of sub-micrometer inclusions accounted for 77.2% of all inclusions in S2 steel sample, which contributed to inhibit the migration of prior austenite grain boundaries, and effectively induced acicular ferrite nucleation. Based on the theory that inclusions act as heterogeneous nuclei to induce ferrite nucleation, the results from thermodynamic calculations showed that the critical nucleation diameter for the TiN inclusions is 0.346 μm, which is consistent with experimental data of the minimum inclusion size for ferrite nucleation in actual steel samples.

Effect of MnS Inclusions Distribution on Intragranular Ferrite Formation in Medium Carbon Non-Quenched and Tempered Steel for Large-Sized Crankshaft

Effect of MnS Inclusions Distribution on Intragranular Ferrite Formation in Medium Carbon Non-Quenched and Tempered Steel for Large-Sized Crankshaft

The optimized microstructure and properties of a V-Ti microalloyed forging steel by boron addition

The microstructure and properties of a V-Ti microalloyed forging steel was optimized by boron addition. The uneven distribution of boron suppressed the formation of grain boundary ferrite (GBF) and promoted the formation of intragranular ferrite (IGF) idiomorphs. The volume fraction of GBF decreases from 21.2% to 14.3%, while that of IGF idiomorphs increases from 3.5% to 10.2%. After boron addition, the strength of the steel has little change, but the total elongation increases from 21.2% to 23.6%, and the impact energy increases from 38.2 J to 51.4 J, which is attributed to the higher volume fraction of IGF idiomorphs.

Effect of nitrogen contents on the microstructure and mechanical properties of V-Ti microalloyed steels for the forging of crankshafts

The microstructure and mechanical properties of vanadium-titanium microalloyed crankshaft steels with two different nitrogen concentrations were studied. The increasing nitrogen not content only refined the austenite grain size but also facilitated the formation of intragranular ferrite (IGF) idiomorphs. The V-Ti-N steel has a better combination of strength, plasticity and toughness than that of the V-Ti steel. Increasing the nitrogen content also facilitated the precipitation of V(C, N) particles in ferrite, which significantly strengthened it. This outcome remedied the strength reduction caused by the decrease in the pearlite volume fraction. The IGF idiomorphs can deform along with the outside pearlite compatibly and restrain the crack growth in pearlite while in tensile deformation. As a result, the plasticity is significantly improved from the increase of the IGF idiomorphs. Moreover, the effective grain size can be refined by the increase of the IGF idiomorphs, which improves the toughness.

Effect of Zr, Al Addition on Characteristics of MnS and Formation of Intragranular Ferrite in Non-Quenched and Tempered Steel

Effect of Zr, Al Addition on Characteristics of MnS and Formation of Intragranular Ferrite in Non-Quenched and Tempered Steel

The effect of carbon contents on intragranular ferrite formed in the V-Ti-N microalloyed steel with a carbon content gradient prepared by controlling the surface decarburization

The V-Ti-N microalloyed steel with a carbon content gradient was innovatively obtained through surface decarburization. Then, the effect of carbon contents on intragranular ferrite (IGF) formation was studied. With the carbon contents increasing from 0.39% to 0.47% (mass percent), the volume fraction of IGF decreases from 8.2% to 4.6%, but the number of IGF has no significant change. The critical size of IGF nuclei was calculated on the basis of classic heterogeneous nucleation theory. According to the results of calculation and statistics, with the carbon content increasing from 0.39% to 0.47%, the number of IGF nuclei does not decrease significantly. When the carbon content is relatively low, some (Ti, V)(C, N) particles precipitating in austenite matrix may also act as the nuclei of IGF to refine the pearlite-ferrite microstructure.

Effect of nitrogen on transformation behavior and intragranular ferrite formation in a V–Ti microalloyed steel for forging

The transformation behavior and intragranular ferrite formation in V–Ti microalloyed steel with a nitrogen content of 0.005 wt.% and 0.015 wt.% are studied by the DIL 805A dilatometer. The results show that increasing the nitrogen content has no significant effect on AC1, AC3and MStemperature. However, the continuous cooling transformation (CCT) diagrams are shifted to left side. The minimum cooling rates of bainitc and martensitic transformation are increased from 3 °C/s to 5 °C/s and from 5 °C/s to 10 °C/s, respectively. (Ti, V)(C, N) particles on MnS is the nuclei of intragranular ferrite, and (1 0 0)(Ti, V)(C, N)and (1̅ 0 1̅α) are just misoriented by 6.7°. With the increase of nitrogen content, the number of intragranular ferrite is increased from 73 to 170 per · mm2. The volume fraction of intragranular ferrite is increased from 0.23%∼0.79% to 0.79%∼4.6% at cooling rate of 1 °C/s∼0.1 °C/s. According to the industrial production experiment, the toughness of forging crankshaft is improved by increasing the nitrogen content. It is benefit for achieving fair matching of high strength and high toughness of crankshaft.

Effect of aging-deformation-treatment on the formation of intragranular ferrite in V-microalloyed steel

ABSTRACTThe influence of the aging-deformation-treatment on the formation of intragranular ferrite (IGF) in a vanadium microalloyed medium carbon steel (Fe–0.34C–1.53Mn) was studied. Effects of aging time on the volume fraction of IGF and austenite grain size were also investigated. The effect of the aging-deformation recrystallisation process on the amount of ferrite and the mechanism of IGF formation was discussed. The results show that aging-deformation-treatment makes the precipitation position of the carbonitride transfer from austenite grain boundary to intragranular, and the precipitated intragranular carbonitrides become the nucleation cores of the IGF during the recrystallisation. The content of precipitated carbonitrides, and the volume fraction of ferrites and the grain size of austenites, increase with the increasing aging time.

Intragranular Ferrite Formed in a V–Ti–N Medium‐Carbon Steel Containing MnS Inclusions

This paper studies the formation of intragranular ferrite in a V–Ti–N medium‐carbon steel containing MnS inclusions. The (Ti, V)(C, N) particles precipitating on MnS inclusions are confirmed to be the main nucleation sites of intragranular ferrite. Because of the small lattice mismatch and low‐energy ferrite/V(C, N) interphase boundary, the vanadium‐rich edge of (Ti, V)(C, N) particles increases the nucleation potency of intragranular ferrite. The interphase of MnS inclusions and austenite provides suitable sites for (Ti, V)(C, N) particles to grow big enough to reach the critical size for intragranular ferrite nucleation. Furthermore, the efficiency of the formation of intragranular ferrite starts to increase rapidly when the length of MnS inclusions decreases to less than 10 µm. The shape of intragranular ferrite is also significantly influenced by the length of MnS inclusions. In addition, the intragranular ferrite is confirmed to have a new type of orientation relationship with the particles precipitating in liquid phase, which indicates that the orientation relationship between the intragranular ferrite and (Ti, V)(C, N) particles is not sole, but diversified.

Recent Aspects on the Effect of Inclusion Characteristics on the Intragranular Ferrite Formation in Low Alloy Steels: A Review

AbstractIntragranular ferrite (IGF), which nucleates from specific inclusion surfaces in low alloy steels, is the desired microstructure to improve mechanical properties of steel such as the toughness. This microstructure is especially important in the coarse grain heat affected zone (CGHAZ) of weldments. The latest review paper focusing on the role of non-metallic inclusions in the IGF formation in steels has been reported by Sarma et al. in 2009 (ISIJ int., 49(2009), 1063–1074). In recent years, large amount of papers have been presented to investigate different issues of this topic. This paper mainly highlights the frontiers of experimental and theoretical investigations on the effects of inclusion characteristics, such as the composition, size distribution and number density, on the IGF formation in low carbon low-alloyed steels, undertaken by the group of Applied Process Metallurgy, KTH Royal Institute of Technology. Related results reported in previous studies are also introduced. Also, plausible future work regarding various items of IGF formation is mentioned in each section. This work aims to give a better control of improving the steel quality during casting and in the heat affected zone (HAZ) of weldment, according to the concept of oxide metallurgy.

Ferrite Formation Dynamics and Microstructure Due to Inclusion Engineering in Low-Alloy Steels by Ti2O3 and TiN Addition

The dynamics of intragranular ferrite (IGF) formation in inclusion engineered steels with either Ti2O3 or TiN addition were investigated using in situ high temperature confocal laser scanning microscopy. Furthermore, the chemical composition of the inclusions and the final microstructure after continuous cooling transformation was investigated using electron probe microanalysis and electron backscatter diffraction, respectively. It was found that there is a significant effect of the chemical composition of the inclusions, the cooling rate, and the prior austenite grain size on the phase fractions and the starting temperatures of IGF and grain boundary ferrite (GBF). The fraction of IGF is larger in the steel with Ti2O3 addition compared to the steel with TiN addition after the same thermal cycle has been imposed. The reason for this difference is the higher potency of the TiO x phase as nucleation sites for IGF formation compared to the TiN phase, which was supported by calculations using classical nucleation theory. The IGF fraction increases with increasing prior austenite grain size, while the fraction of IGF in both steels was the highest for the intermediate cooling rate of 70 °C/min, since competing phase transformations were avoided, the structure of the IGF was though refined with increasing cooling rate. Finally, regarding the starting temperatures of IGF and GBF, they decrease with increasing cooling rate and the starting temperature of GBF decreases with increasing grain size, while the starting temperature of IGF remains constant irrespective of grain size.