Intragranular Ferrite Nucleation Research Articles

Effect of Initial Intergranular Ferrite Size on Induction Hardening Microstructure of Microalloyed Steel 38MnVS6

In this study, we investigated the effect of grain size of an initial microstructure (pearlite + ferrite) on a resulting microstructure of induction-hardened microalloyed steel 38MnVS6, which is one topical medium carbon vanadium microalloyed non-quenched and tempered steel used in manufacturing crankshafts for high-power engines. The results show that a coarse initial microstructure could contribute to the incomplete transformation of pearlite + ferrite into austenite in reaustenitization transformation by rapid heating, and the undissolved ferrite remains and locates between the neighboring prior austenite grains after the induction-hardening process. As the coarseness level of the initial microstructure increases from 102 μm to 156 μm, the morphology of undissolved ferrite varies as granule, film, semi-network, and network, in sequence. The undissolved ferrite structures have a thickness of 250–500 nm and appear dark under an optical metallographic view field. To achieve better engineering applications, it is not recommended to eliminate the undissolved ferrite by increasing much heating time for samples with coarser initial microstructures. It is better to achieve a fine original microstructure before the induction-hardening process. For example, microalloying addition of vanadium and titanium plays a role of metallurgical grain refinement via intragranular ferrite nucleation on more sites, and the heating temperature and time of the forging process should be strictly controlled to ensure the existence of fine prior austenite grains before subsequent isothermal phase transformation to pearlite + ferrite.

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)).

The comparative study of Ti-bearing oxides introduced by different methods

Abstract The Ti-bearing oxides are introduced into HRB400 steel melt through external addition method (EAM, TiO2) and internal precipitation method (IPM, Ti alloy) using a vacuum induction furnace, respectively. For each experiment, the instantaneous evolution of oxide properties was investigated through the analysis of intermediate samples. For the EAM method, it was found that the titanium content gradually increased with increase in the treatment time. Similar increasing trend is also observed for Ti-bearing oxide properties, such as the number density and normalized titanium content. These phenomena are totally different from those in the IPM intermediate samples, in which both Ti content and Ti-bearing oxide features show rapid increase, reaching saturation at 1 min after the addition of Ti alloy, and remain relatively stable thereafter. This comparison hints that after TiO2 addition, they first dissolve in molten steel, then decompose, and finally again form Ti-bearing oxides. Furthermore, in the case of similar titanium content, EAM intermediate samples exhibit higher Ti-bearing oxide number density and higher normalized titanium content compared to IPM samples. After heat treatment, the intragranular ferrite nucleation was observed in cast sample of EAM, which was induced by the introduced Ti-bearing oxides.

Refinement mechanism of large heat-input welding CGHAZ microstructure by N addition and its effect on toughness of a V-Ti-N microalloying weathering steel

The large heat input welding performance of uncoated weathering steel remains a great challenge since their coarse grain heat-affected zones (CGHAZs) are usually occupied by coarse and brittle microstructure consisting of granular bainitic ferrite (GBF) and martensite/austenite (M/A) constituents. In this article, a V-Ti-N microalloying weathering steel suitable for large heat input welding was achieved through the N addition, and the mechanism dominating the modified microstructure and the enhanced impact toughness of CGHAZ was deeply elucidated mainly by HRTEM. The results indicated that in the simulated CGHAZ with large heat input of 80 kJ/cm, the increase of N content from 0.0030 wt% to 0.0075 wt%, and 0.0115 wt% elevated the density of nano- and submicron-sized particles of (Ti, V)(C, N) containing VN, which in turn resulted in the refinement of prior austenite grains (PAGs) by the improved grain boundary pinning effect and the enhancement in heterogeneous nucleation of intragranular ferrite (IGF) by the decreased lattice disregistry (δ) between (Ti, V)(C, N) and IGF. The fraction of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) higher than 15° was thereby increased while the mean equivalent diameter (MED) of ferritic grains with HAGBs was decreased. Simultaneously, the N addition scattered the M/A constituents, promoted the internal substructure of dislocation-type M/A at the expense of twin-type M/A, and accordingly reduced the hardness difference between the M/A constituents and the matrix. All these factors retarded brittle crack initiation and propagation, and low-temperature impact toughness of simulated CGHAZ increased from 62.8 J to 141.2 J.

Effect of N Content on the Microstructure and Impact Properties of Normalized Vanadium Micro-Alloyed P460NL1 Steel

In this work, the effect of nitrogen doping on vanadium micro-alloyed P460NL1 steel is studied in terms of microstructures and impact toughness. As the nitrogen content increased from 0.0036% to 0.0165%, the number of V (C,N) particles increased. The fine precipitates of V (C,N) effectively pin the prior austenite grain boundary, resulting in the refinement of the austenite grain. The intragranular and intergranular V-containing coarse particles enhanced the nucleation of intragranular ferrite and the grain boundaries of polygonal ferrite during air cooling. Accordingly, the proportion of heterogeneously nucleated ferrite increased, and the grain size of ferrite decreased. Notably, the size of the pearlite microstructure decreased, and the bainite microstructure appeared with a high doping of N. With the increase in N content, the impact toughness of vanadium micro-alloyed P460NL1 steel was enhanced. This can be attributed to the refinement of ferrite and the reduction in pearlite, which, in turn, was ascribed to the increase in nitrogen.

Intragranular ferrite nucleation on MX carbonitrides and dislocations

The competing mechanisms of ferrite nucleation on (Ti,V)(C,N) MX carbonitrides and dislocations, as well as their dependence on deformation, are investigated experimentally by thermo-mechanical treatments and via simulation. The impact of recrystallization and the resulting austenite grain size on intragranular ferrite nucleation is evaluated. The austenite-to-ferrite transformation temperatures, affected by different microstructures, are examined by isothermal uniaxial single-hit compression tests on a dilatometer DIL 805 and compared to thermal treatments without deformation. Different resulting microstructures are analyzed by using optical light microscopy. The experimental data is used for the validation of thermo-kinetic simulations with the mean-field modeling software MatCalc using the implemented models for the austenite grain evolution, the on-particle nucleation of ferrite on the surface of MX particles, the dislocation density evolution, and recrystallization.

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.

Effect of Ti3O5 addition on oxide evolution for HRB400

Abstract The transient evolution of oxide was studied after directly adding Ti3O5 powder into HRB400 steel. This experiment was carried out in magnesia crucible with vacuum induction furnace and the intermediate samples were taken at 1, 5, 10, and 15 min after Ti3O5 addition. At 20th min, the furnace was powered off to get furnace-cooled cast sample. For intermediate samples, it is found that with increasing treatment time, the titanium content increased though the acid–soluble aluminum content remained stable. Besides, the Ti-bearing oxides were observed by scanning electron microscope (SEM). Moreover, statistical analysis indicated that for Ti-bearing oxide, both number density and titanium content after further normalization increased with increasing processing time. For cast sample, the characteristic of Ti-bearing oxide at different heights are similar. These results confirmed the adding valid of Ti3O5, which may be due to its decomposition. Finally, after heat treatment, these introduced Ti-bearing oxides can induce the intragranular ferrite nucleation. This indicates the effectiveness of external adding method in oxide metallurgy.

Effect of Mg-Ce Treatment on Inclusion Characteristics and Pitting Corrosion Behavior in EH420 Marine Steel

Reasonable regulation of nonmetallic inclusions in steel can significantly improve its strength, toughness, and corrosion resistance. In this paper, EH420 marine steel was treated with Mg, Ce, and Mg-Ce to modify the inclusions. The effects of different treatments on the morphology, composition, size distribution, induced intragranular ferrite (IGF) nucleation, and pitting resistance of inclusions were systematically analyzed using various methods. The results show that the Mg-Ce composite treatment can modify irregular MgAl2O4 inclusions into spherical Mg-Ce-O composite inclusions and MgO-dominated inclusions. The density of inclusions is increased from 74.8/mm2 to 186.0/mm2, and the average size of inclusions is decreased from 2.60 μm to 1.07 μm. The Mg-Ce-O composite inclusions are effective inclusions for inducing IGF. Furthermore, the pitting potential is increased from −503 mV to −487 mV, and the corrosion rate is decreased. The order of average electronic work function is ΦMgO < ΦCe2O3 < Φα-Fe < ΦAl2O3. Ce2O3 is hard to induce pitting corrosion due to its similar electronic work function to the steel matrix. Thus, the Mg-Ce composite treatment is better than that for Mg and Ce treatment alone, and has better application prospects.

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 Welding Heat Input on Microstructure and Impact Toughness in the Simulated CGHAZ of Low Carbon Mo-V-Ti-N-B Steel

Welding thermal cycles with heat inputs ranging from 25 to 75 kJ/cm were performed on a Gleeble 3500. The impact energy improved significantly (from 10 to 112 J), whereas the simulated coarse-grain heat-affected zone (CGHAZ) microstructure changed from lath bainite ferrite (LBF) and granular bainite ferrite (GBF) + martensite/austenite (M/A) to acicular ferrite (AF) + polygonal ferrite (PF) + M/A as the heat input increased. Simultaneously, the mean coarse precipitate sizes and the degree of V(C,N) enrichment on the precipitate surface increased, which provided favorable conditions for intragranular ferrite nucleation. The Ar3 of CGHAZ increased from 593 °C to 793 °C with increasing heat inputs; the longer high-temperature residence time inhibited the bainite transformation and promoted the ferrite transformation. As a result, acicular ferrite increased and bainite decreased in the CGHAZ. The CGHAZ microstructure was refined for the acicular ferrite segmentation of the prior austenite, and the microstructure mean equivalent diameter (MED) in the CGHAZ decreased from 7.6 µm to 4.2 µm; the densities of grain boundaries higher than 15° increased from 20.3% to 45.5% and significantly increased the impact toughness. The correlation of heat input, microstructure, and impact toughness was investigated in detail. These results may provide new ideas for the development of high welding heat input multiphase steels.

Effects of Ti-bearing inclusions on intragranular ferrite nucleation

Abstract The effect of Ti-bearing inclusion characteristics, such as size and marginal composition, on intragranular ferrite (IGF) nucleation has been explored in single sample through SEM and EDS. It can be seen that Ti-bearing inclusions can induce IGF nucleation, and this nucleation ability was scaled by the number of its induced lathes. Statistical analysis suggested that this ability may be correlated with inclusion size, and independent of Ti2O3 content in inclusion. The former can be attributed to the classical theory of heterogeneous nucleation. The latter can be explained by the relationship between the diffusion quantity of Mn and its solubility in Ti2O3 based on the theory of an Mn-depletion zone. Moreover, a probabilistic feature was observed in the nucleation phenomenon which may be due to a small difference in formation energy between the ferrite side plate and the IGF. These results may be helpful to clarify the nature of oxide metallurgy of a Ti-bearing inclusion.

Effect of boron on intragranular ferrite nucleation mechanism in coarse grain heat-affected zone of high-nitrogen steel

Abstract The effect of boron on intragranular ferrite (IGF) nucleation mechanism in the simulated coarse grain heated-affected zone (CGHAZ) of a high-nitrogen steel was investigated. When boron (B) was added to the steel, the ferrite transformation temperature increased from 775 °C to 800 °C and the amount of polygonal ferrite increased from 63.2% to 78.6%. Consequently, the ductile-to-brittle transition temperature decreased from −30 °C to −50 °C. (Ti,V)(C,N)-MnS-BN, MnS-BN, Al2O3-MnS-BN, and BN served as IGF nucleation sites. The core of the complex precipitates was formed by undissolved (Ti,V)(C,N)-MnS, Al2O3-MnS, and MnS, with a BN cap formed on the undissolved precipitates, which increased the IGF nucleation potency.

Relationship between MnS precipitation and respective effects of Ti-Mg bearing inclusions on the induction of intergranular acicular ferrite

Abstract The role of MnS in the nucleation of intragranular acicular ferrites (IAF) has been explored in Ti-Mg treated steel sample through scanning electron microscope (SEM) and energy dspersive spectrometer (EDS). When compared with a non-treated sample, Ti-Mg treatment evolves from Al-Mn-Si-O inclusion to Ti-Mg bearing inclusion and causes a reduction in pure MnS. In addition to this reduction in pure MnS, a precipitation of MnS around the Ti-Mg bearing inclusion can be observed. Moreover, after etching, it can be found that Ti-Mg bearing inclusions can induce IAF nucleation. Based on EDS line analysis and a comparison with Al-Mn-Si-O inclusion in non-treated sample, this effect can be explained by the presence of an Mn-depletion zone (MDZ) caused by the vacancy property and crystal structure of the inclusion. It should be added, however, that in the same sample, a similarly induced nucleation effect as well as an MDZ have not been observed around pure MnS. This comparison implies that the formation of an MDZ may be independent of MnS precipitation.

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.

Study of Mn absorption by complex oxide inclusions in Al[sbnd]Ti[sbnd]Mg killed steels

In AlTiMg killed steels, the use of complex deoxidation products, which act as heterogeneous nucleation sites for fine intragranular ferrite, has been acknowledged as an effective way for grain refinement. In this study, we investigated the interaction between Mn solute atoms and different oxide inclusions (such as MgO, MgTi2O4, MgTiO3, Ti2O3, Ti3O5, Al2O3, MgAl2O4) experimentally and using first-principles calculations, to identify the oxides that can effectively lead to the formation of local Mn-depleted zones promoting the nucleation of intragranular ferrite. The results show that MgTi2O4, MgTiO3 are effective for partition of Mn atoms into oxides, while MgO, Al2O3, MgAl2O4 are ineffective for the formation of local Mn-depleted zones. Furthermore, the calculations show that Mn atoms exist as a simple solute by replacing Mg atoms in MgO, MgTi2O4 and MgTiO3. Except by forming complex Ti-Mn oxides, such as MnTi2O4 and MnTiO3, Mn atoms can also exist as a simple solute in Ti2O3 by occupying vacancy positions in their crystals. Ti3O5 oxides cannot absorb Mn atoms by replacing Ti atoms in it, meanwhile, the occupying vacancy pattern is not the main reason for Ti3O5 oxides absorb Mn atoms.

The mechanism of intragranular ferrite formed on Ti-rich (Ti,V)(C,N) precipitates in the coarse heat affected zone of a V–N–Ti microalloyed steel

The effect of Ti-rich (Ti,V)(C,N) precipitates on the evolution of intragranular ferrite in the coarse grain heated effected zone (CGHAZ) of V–N–Ti steel was systematically investigated via transmission electron microscopy. The coarse grain boundary ferrite, bainite, and ferritic side-plates in simulated CGHAZ were replaced by polygonal ferrite and acicular ferrite, when the nitrogen content was increased from 0.0044wt% to 0.0094wt%. In addition, the undissolved Ti-rich core acted as a nucleation site for the V-rich cap; the cap reached the critical size at which nucleation of intragranular ferrite occurs nucleation size of intragranular ferrite with the aid of the core. Owing to a cap-induced increase in the nucleation potency, the intragranular ferrite have formed on the Ti-rich (Ti,V)(C,N) hold the Baker-Nutting orientation relationship ((001)IGF//(001)(Ti,V)(C,N), [110]IGF//[100](Ti,V)(C,N)).

Kinetics and formation mechanisms of intragranular ferrite in V-N microalloyed 600 MPa high strength rebar steel

To systematically investigate the kinetics and formation mechanisms of intragranular ferrite (IGF), isothermal heat treatment in the temperature range of 450°C to 600°C with holding for 30 s to 300 s, analysis of the corresponding microstructures, and observation of the precipitated particles were conducted in V-N microalloyed 600 MPa high strength rebar steel. The potency of V(C,N) for IGF nucleation was also analyzed statistically. The results show that the dominant microstructure transforms from bainite (B) and acicular ferrite (AF) to grain boundary ferrite (GBF), intragranular polygonal ferrite (IPF), and pearlite (P) as the isothermal temperature increases from 450°C to 600°C. When the holding time at 600°C is extended from 30 s to 60 s, 120 s, and 300 s, the GBF content ranges from 6.0vol% to 6.5vol% and the IPF content increases from 0.5vol% to 2.8vol%, 13.1vol%, and 13.5vol%, respectively, because the ferrite transformation preferentially occurs at the grain boundaries and then occurs at the austenite grains. Notably, V(C,N) particles are the most effective nucleation site for the formation of IPF, accounting for 51% of the said formation.

Effect of Hot Deformation and Controlled Cooling Process on Microstructures of Ti–Zr Deoxidized Low Carbon Steel

It has been known that microstructures in steel welding heat affected zones can be improved by inclusion-induced intragranular ferrite nucleation. In this paper, the influence of inclusions on microstructure in steel base metal under thermo-mechanical conditions was further researched. Hot deformation and controlled cooling process were simulated with Ti–Zr deoxidized low carbon steels. Microstructural characteristics and transformation behaviors were analyzed. Results showed that inclusions were mainly (Ti, Zr)-rich oxide with MnS precipitation on the surface and they were effective for acicular ferrite nucleation in the coarse-grained heat affected zone. Acicular ferrite could still form in hot deformed and recrystallized austenite grains, but its volume fraction reduced with the decrease in deformation temperature. Accelerated cooling above 600°C and addition of boron significantly promoted acicular ferrite structure and the hardness value also increased. Hot rolling simulation indicated that for the experimental Ti–Zr–B steel, microstructure could not be further improved by applying conventional controlled rolling deformation. Instead, fine-grained acicular ferrite structure and high hardness value were obtained through high temperature deformation and controlled cooling process, during which (Ti, Zr)-oxide inclusions contributed to the intragranular acicular ferrite formation.

Prediction of intragranular ferrite nucleation from TiO, TiN, and VN inclusions

The current study presents a method to calculate the critical diameters of TiO, TiN, and VN inclusions for intragranular ferrite (IGF) nucleation in steels. Based on the calculation results, it was noted that the critical diameters of TiO, TiN, and VN inclusions for IGF nucleation were 0.192, 0.355, and 0.810 μm. The calculation results agreed with the experiment data of a minimum inclusion size for IGF nucleation in the actual steel samples. Moreover, the effects of Mn, C, and S contents on the critical diameters of inclusions were investigated. It was found that the critical diameters of TiO, TiN, and VN inclusions increased with the increasing Mn and C contents. In addition, it was found that S does not have a direct effect on the critical diameters of TiO, TiN, and VN inclusions. However, the increasing S content led to an increased amount of MnS precipitation in the actual steels. This is negative, since MnS is ineffective nucleation site for IGF nucleation. When the amount of MnS increases in steels, the area fraction of IGF slightly decreases. This fact has been investigated by in situ observation experiments.