Segregation Of Nitrogen Research Articles

Bainite Formation in a Carbon-Free Fe–Cr–N System

In order to promote the understanding of bainitic transformation in nitrogen alloyed steels, the microstructure, transformation kinetics, and diffusion/precipitation characteristics of an isothermally heat-treated carbon-free system Fe–5Cr–N were investigated. Quenching dilatometry was used to perform heat treatments at temperatures between 400 °C and 500 °C and simultaneously receive information about the transformation kinetics. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) investigations indicate a plate- to lenticular-like, fully ferritic microstructure with nanometer scale CrN. In order to analyze the diffusion and precipitation behavior of nitrogen, atom probe tomography measurements were performed. On the one hand, nitrogen segregation was found, and on the other hand, the presence of CrN precipitates and preliminary stages of CrN nitrides were validated.

Research on gas pore formation and inhibition mechanism of high nitrogen steel during laser direct metal deposition

Nitrogen pore formation is an inherently tricky problem that limits the application of high nitrogen steel (HNS) in various fields. In the present study, a CA-LBM coupling model was proposed to understand the nitrogen pore formation mechanism during laser direct metal deposition (DMD) of HNS. Based on the proposed model and theoretical analysis, the effects of pulse frequency and shielding gas nitrogen content on the porosity were clarified systematically. Results showed that after the onset of solidification, the ferrite phase with extremely low nitrogen solubility precipitated first, inducing the continuous enrichment of nitrogen in the residual liquid phase, which in turn provided the prerequisite for nitrogen pore nucleation. The application of quasi-continuous-wave (QCW) laser mode accelerated the molten pool solidification, and shortened the nitrogen supersaturated region, allowing more nitrogen to be solid dissolved in the matrix. On this basis, the porosity was inhibited to a certain extent. Compared with the high-frequency pulse, the low-frequency pulse showed better porosity inhibition. In addition, following the classic nucleation theory, nitrogen segregation and nitrogen solubility synergistically dominated nitrogen pore formation. The nitrogen partial pressure, determined by the shielding gas nitrogen content, played a dual role in porosity defects. Elevated nitrogen partial pressure served to increase nitrogen solubility while promoting the nitrogen absorption process to intensify nitrogen segregation. However, due to the short duration of the molten pool, the nitrogen absorption process could not be fully processed. Nitrogen solubility enhancement was the key factor in inhibiting porosity during the DMD process.

Nitrogen-assisted laser-based powder bed fusion fabricated NiTi with high hardness and low modulus

The role of the protective gases in laser-based powder bed fusion (L-PBF) is mainly to avoid the oxidation of metals during the melting and solidification process. However, the gaseous environment can also interact with metals, and result in changes in microstructure and properties. In a NiTi alloy, we found the L-PBF processes in the N2 gaseous environment can induce TiN particles and facilitate the formation of a precipitate-dislocation network and therefore improve mechanical properties. Nitrogen segregation also led to grain refinement and the generation of high-angle grain boundaries. The as-printed NiTi in N2 loses its shape memory effect, however, an aging process at 500 °C resulted in unchanged grain size, parallel arrays of Ni4Ti3 precipitates around TiN, and a recovery of shape memory effect. The optimization of microstructure finally resulted in a high hardness of ∼4.0 GPa and a low reduced elastic modulus of ∼57.5 GPa. The manufacturing process presented in this study opens a new way for making biomedical devices with low elastic constant, high strength to weight, and high resistance to wear.

Effect of dislocation density on competitive segregation of solute atoms to dislocations

We report on both experimental and modeling of carbon and nitrogen segregation to dislocations in C–Mn ferritic steels. A model based on McLean theory is established to describe the competitive segregation of carbon and nitrogen in the Cottrell atmospheres around dislocations, considering the effect of dislocation density. It is expected from the model that a high dislocation density strongly depletes interstitial atoms from the matrix, resulting in a low excess quantity of atoms on dislocations. Atom probe tomography analyses performed on two different C–Mn weld steels are in accordance with the model: carbon excess is tenfold higher in comparison to nitrogen excess in the deposited metal. However, after tensile test up to failure, this trend is no longer valid and nitrogen segregates preferentially than carbon. This suggests that plastic strain can modify the material properties by changing the nature and quantity of segregated atoms around dislocations.

First-principles study of the effect of N on the ∑5 (210) [001] grain boundary of γ-Fe

High-nitrogen austenitic steels possess excellent properties owing to the solid solution strengthening of nitrogen, which is widely used in many fields. So it’s significant to investigate the effect of nitrogen on the properties of high austenitic nitrogen steels. In order to get atomic scale understanding of the influence of nitrogen, the first principles method was used to investigate the energetics, site preference, and segregation of nitrogen at γ-Fe ∑5 (210) [001] grain boundary (GB) and different atomic layers near GB plane. It is found that the nitrogen atom prefers to occupy the octahedral site. As nitrogen locates at different atomic layers away from the GB plane, the segregation energy presents increase first and then decrease. The mechanical properties were analyzed by Rice-Wang theory and theoretical tensile stress test, which shows that nitrogen at ∑5 (210) [001] GB has an embrittling effect on GB cohesion, while nitrogen located away from the GB plane presents a strengthening effect. The underlying mechanism was analyzed by bond length, charge density, and density of states (DOS). The segregation of nitrogen at ∑5(210) [001] grain boundary has negative effect on grain boundary cohesion, while nitrogen locating away from grain boundary plane presents strengthening effect on grain boundary.

Effect of Cooling Rate and High N Content on the Precipitation and Growth of AlN Inclusions in Fe–23Mn–2Al–0.08 V High‐Manganese Nonmagnetic Steel

Industrial experiments are carried out to investigate the precipitation and growth of AlN in Fe–23Mn–2Al–0.08 V steel with nitrogen content up to 80–92 ppm. AlN inclusions can be precipitated before solidification. The morphologies of AlN are mainly divided into hexagonal and needle‐like shapes. With the cooling rate decreasing from 36.66 to 0.71 K s−1, the equivalent diameters of typical AlN increase from 7.56 to 24.20 μm, and the total amount decreases from 203.01 to 60.00 mm−2. The relationship between the cooling rate and the number density is obtained: . The element segregation analyzed by electronic probe microanalyzer shows that aluminum concentration is low in interdendritic regions but high in dendrites. Aluminum segregation is very weak, and there is almost no nitrogen segregation. Thermodynamic calculation shows that AlN can precipitate before solidification with the nitrogen content higher than 58 ppm. The predicted results of AlN growth by kinetic analysis method can well reveal the growth trend. The nitrogen content (≥58 ppm) has a negligible effect on the size of AlN when the cooling rates are 3.02 and 0.71 K s−1, while the cooling rate has a significant effect on the size of AlN.

In situ nitriding of Fe2VAl during laser surface remelting to manipulate microstructure and crystalline defects

Tailoring the physical properties of complex materials for targeted applications requires optimizing the microstructure and crystalline defects that influence electrical and thermal transport and mechanical properties. Laser surface remelting can be used to modify the subsurface microstructure of bulk materials and hence manipulate their properties locally. Here, we introduce an approach to perform remelting in a reactive nitrogen atmosphere to form nitrides and induce segregation of nitrogen to structural defects. These defects arise from the fast solidification of the full-Heusler ${\mathrm{Fe}}_{2}\mathrm{VAl}$ compound that is a promising thermoelectric material. Advanced scanning electron microscopy, including electron channeling contrast imaging and three-dimensional electron backscatter diffraction, is complemented by atom probe tomography to study the distribution of crystalline defects and their local chemical composition. We reveal a high density of dislocations, which are stable due to their character as geometrically necessary dislocations. At these dislocations and low-angle grain boundaries, we observe segregation of nitrogen and vanadium, which can be enhanced by repeated remelting in nitrogen atmosphere. We propose that this approach can be generalized to other additive manufacturing processes to promote local segregation and precipitation states, thereby manipulating physical properties.

A Promising Pressurized Duplex Manufacturing Route of High Nitrogen Stainless Steel

By comparing the advantages and disadvantages of the existing single‐step pressurized metallurgical manufacturing technologies of high nitrogen stainless steel (HNS), a promising pressurized duplex manufacturing route, i.e., pressurized induction melting (PIM) and pressurized electroslag remelting (PESR), is proposed. The PIM stage mainly carries on the nitrogen alloying, deoxidation and desulfurization, and the PESR process mainly takes responsibility for further desulfurization, removal of large‐size inclusion, and elevating solidification quality. During PIM, the deoxidation is achieved by vacuum carbon deoxidation combined with magnesium and rare‐earth compound treatment. Subsequently, the PESR is conducted under high nitrogen pressure without adding nitrided ferroalloys. To precisely control the nitrogen content and avoid segregation and escape of nitrogen, the three‐stage nitrogen pressure collocation is developed. The high nitrogen stainless bearing steel 30Cr15Mo1N (DIN 1.4108) is manufactured as an example by the duplex route, and a PIM–PESR ingot with high nitrogen content, high cleanliness, fine and dispersive inclusions, and compact solidification structure is obtained. Moreover, the steel possesses a homogeneous microstructure, slightly better mechanical properties, and corrosion resistance compared with that manufactured by the single‐step PESR process. Finally, a pressurized ladle refining and PESR duplex route are proposed for large‐scale industrial production of HNSs.

A high stability B2-containing refractory multi-principal element alloy

An equiatomic HfNbTaRuZr refractory multi-principal alloy was cast via arc melting and characterized in both as-cast and vacuum annealed conditions. (Nb, Ta)-rich body-centered cubic (BCC), (Hf, Ru)-rich B2, and (Hf, Zr)-rich hexagonal close-packed (HCP) phases in annealed materials were identified by both x-ray diffraction and transmission electron microscopy, with all phases stable from 1350 to 1575 °C. At 1575 °C the alloy contained 35.9 % of a morphologically stable B2 phase. Preferential segregation of oxygen and nitrogen to the HCP phase was observed. The volume fractions of the HCP and B2 phases increased and decreased, respectively, with increasing annealing temperature. Lamellar microstructures present along grain boundaries were determined to result from a discontinuous transformation from a disordered BCC phase to B2 and BCC. Refinement of the grain size and microstructure was achieved by splat quenching, resulting in exceptional hardness due to nano-scale precipitates.

Oscillatory Mechanism of α-Fe(N) ↔ γ'-Fe4N Phase Transformations during Nanocrystalline Iron Nitriding.

The kinetics of nanocrystalline α-iron nitriding to γ’-iron nitride in an ammonia atmosphere was studied at 598–648 K and at atmospheric pressure. Oscillatory changes in nitriding reaction rates depending on nitrogen concentration in a solid sample were observed. This phenomenon was explained by a gradual change in the iron active surface coverage degree, with nitrogen resulting from a gradual change in the free enthalpy of nitrogen segregation. The α-Fe(N) nanocrystallites’ transformation into γ’-Fe4N went through six metastable FeNx states. The continuous function proposed by Fowler and Guggenheim was modified to a stepwise variable function.

Verification of Thermodynamic Models for Predicting Grain Boundary Segregation of Carbon and Nitrogen in Ferritic Steels

Verification of Thermodynamic Models for Predicting Grain Boundary Segregation of Carbon and Nitrogen in Ferritic Steels

A High Stability B2-Containing Refractory Multi-Principal Element Alloy

An equiatomic HfNbTaRuZr refractory multi-principal alloy was cast via arc melting and characterized in both ascast and vacuum annealed conditions. (Nb, Ta)-rich body-centered cubic (BCC), (Hf, Ru)-rich B2, and (Hf, Zr)-rich hexagonal close-packed (HCP) phases in annealed materials were identified by both x-ray diffraction and transmission electron microscopy, with all phases stable from 1350 ◦ C to 1575 ◦ C. At 1575 ◦ C the alloy contained 35.9% of a morphologically stable B2 phase. Preferential segregation of oxygen and nitrogen to the HCP phase was observed. The volume fractions of the HCP and B2 phases increased and decreased, respectively, with increasing annealing temperature. Lamellar microstructures present along grain boundaries were determined to result from a discontinuous transformation from a disordered BCC phase to B2 and BCC. Refinement of the grain size and microstructure was achieved by splat quenching, resulting in exceptional hardness due to nano-scale precipitates.

XPS study of oxide nucleation and growth mechanisms on a model FeCrNiMo stainless steel surface

High resolution X-ray photoelectron spectroscopy analysis has been used to investigate, in-situ, the mechanisms for oxide nucleation and growth on a FeCrNiMo single crystal alloy. Ultra-high vacuum conditions provided the ideal environment in which an oxide-free surface could be oxidised in a controlled manner, using extremely low pressures and step-wise exposures of oxygen. In-situ snapshot measurements, which records an XPS spectrum every 10 s, provided insight into the mechanisms of chromium and nitrogen segregation during annealing of the sample. High resolution XPS spectra was used to follow the evolution of the key elements at step-wise intervals, allowing for detailed insights into the nucleation and growth mechanisms. Angular XPS analysis provided further information on the in-depth distribution and stratification of the oxide film.

Quantifying Cyanothece growth under DIC limitation

The photoautotrophic, unicellular N2-fixer, Cyanothece, is a model organism that has been widely used to study photosynthesis regulation, the structure of photosystems, and the temporal segregation of carbon (C) and nitrogen (N) fixation in light and dark phases of the diel cycle. Here, we present a simple quantitative model and experimental data that together, suggest external dissolved inorganic carbon (DIC) concentration as a major limiting factor for Cyanothece growth, due to its high C-storage requirement. Using experimental data from a parallel laboratory study as a basis, we show that after the onset of the light period, DIC was rapidly consumed by photosynthesis, leading to a sharp drop in the rate of photosynthesis and C accumulation. In N2-fixing cultures, high rates of photosynthesis in the morning enabled rapid conversion of DIC to intracellular C storage, hastening DIC consumption to levels that limited further uptake. The N2-fixing condition allows only a small fraction of fixed C for cellular growth since a large fraction was reserved in storage to fuel night-time N2 fixation. Our model provides a framework for resolving DIC limitation in aquatic ecosystem simulations, where DIC as a growth-limiting factor has rarely been considered, and importantly emphasizes the effect of intracellular C allocation on growth rate that varies depending on the growth environment.

Chemical composition influence on element segregation and properties of steel strip manufactured by strip casting route

The article presents the results of the chemical composition influence on element segregation and properties of steel strip manufactured by strip casting route. It is shown that in the transition from the slab production with a thickness of 220 mm to slabs with a thickness of 1.2 mm, the thickness of the segregated diffusion layer in the case of the continuous strip casting process is 4.1 - 12.4 times less than in the case of traditional continuous casting. The carbon, nitrogen, copper, tin, phosphorus and sulfur segregation percentage in the continuous strip casting process is 1.7 - 5.1 times less than in traditional continuous casting. A method is proposed for calculating the element segregation based on the equality of the segregation level in traditional and strip continuous casting. It has been established that the content of elements in steels during two-roll continuous casting can be increased for impurities such as S, O, N, P, H from a minimum of 3 for P to a maximum of 497% for S. For residual elements such as Pb, Bi, Sn, Cu, Sb, Zn, As minimum increase from 1.1 for Zn to maximum 401% for Pb. The content of such alloying elements as B, Se, Al, Te, Ca, Mg, Ce, C, La, Nb, Ti, Mn, Ni, Si can be increased from a minimum of 1.1 for Si and Mn to a maximum of 675% for B. The time and rate of cooling of a 20-ton coil of steel strip are calculated, which are, respectively, 13.7 hours and 0.0051 oC/s. Such cooling conditions create the prerequisites for the precipitation of chromium carbides and an increase in the tendency of steel to intergranular corrosion. It was found that for eliminate this problem, it is necessary to increase the cooling time with water after rolling to a strip temperature from 300 to 400 oC.

Effect of Alloying Elements on Grain Boundary Segregation of Nitrogen in α-Iron

Effect of Alloying Elements on Grain Boundary Segregation of Nitrogen in α-Iron

Properties and influence of microstructure and crystal defects in Fe2VAl modified by laser surface remelting

Laser surface remelting can be used to manipulate the microstructure of cast materials. Here, we present a detailed analysis of Fe2VAl following laser surface remelting. Within the melt pool, elongated grains grow nearly epitaxially from the heat-affected zone. These grains are separated by low-angle grain boundaries with 1°–5° misorientations. Segregation of vanadium, carbon, and nitrogen at grain boundaries and dislocations is observed using atom probe tomography. The local electrical resistivity was measured by an in-situ four-point-probe technique. A smaller increase in electrical resistivity is observed at these low-angle grain boundaries compared to high-angle grain boundaries in a cast sample. This indicates that grain boundary engineering could potentially be used to manipulate thermoelectric properties.

Tensile properties improvement by homogenized nitrogen solid solution strengthening of commercially pure titanium through powder metallurgy process

In this study, interstitial solid solution behavior of nitrogen atoms dissolved in commercially pure Ti fabricated by spark plasma sintering (SPS) followed by the extrusion process was investigated. Microstructural observations of the extruded Ti-(N) samples before homogenization, ExTi-(N), revealed the nitrogen element's segregated regions as a result of a short time of the SPS process. Because of the inadequate sintering time, nitrogen atoms were unable to dissolve in the α-Ti matrix homogeneously, which caused a material degradation through brittle failure initiating at high nitrogen absorbed regions. Applying homogenization heat treatment at 1123 K for 180 min followed by hot extrusion, improved microstructural homogeneity without the nitrogen segregation. Furthermore, grain morphology was transformed from coarse elongated grains with substructures into refined equiaxed α-Ti grains. The yield strength and ultimate tensile strength of the Ti-(N) sample with 0.69 wt%N reached maximum 957 MPa and 1117 MPa, respectively, which represented 148% and 214% increments over as-extruded pure Ti sample. In addition, a 50% enhancement of elongation of homogenized Ti-(N) alloys compared to the non-homogenizing condition was achieved. The improvement of strength and elongation was primarily attributed to grain refinement and solid solution strengthening. The quantitative evaluation of strengthening and mechanisms was discussed in detail by the Hall-Petch formula and Labusch model.

Low-temperature toughness properties of 500 MPa offshore steels and their simulated coarse-grained heat-affected zones

Offshore steels for cold climate conditions require not only relatively high strength to improve the cost-efficiency of the structures, but also excellent toughness at low temperatures to guarantee the safety of the structures in harsh environments. The most challenging locations to fulfil both requirements are in the welded joints of these steels because the weld thermal cycles cause irreversible changes to the steel microstructure. Titanium and niobium are often used in these steels to form nanoscale precipitates and reduce the austenite grain growth during the thermal cycle. However, there is a risk that in addition to the nanoscale precipitates formed from the solid state, microscale coarse inclusions are also formed from the melt. These inclusions are hard and brittle and thus likely to deteriorate mechanical properties such as fracture toughness and impact toughness. In this study, two experimental 500 MPa offshore steels with different carbon contents and with differing inclusion structures were studied to find out whether microscale titanium-based nitrides significantly deteriorate the impact and fracture toughness of the coarse-grained heat-affected zone (CGHAZ) of these steels at low temperatures. A Gleeble 3800 was used to simulate the CGHAZ with two different cooling rates, the fracture toughness was determined by three-point bend testing, and the impact toughness was determined by Charpy V-notch testing. The study was limited to single-pass welding scenarios. Inclusions were characterized using FESEM-EDS. It was found that both impact and fracture toughness of the steel with coarse titanium-based nitrides were lower and the scatter higher than in the steel with a higher quantity of calcium-based inclusions and without the coarse nitrides. Fractographic examination showed that the failure in the samples with the lowest toughness was initiated by the coarse titanium-niobium nitrides. Thus, to avoid CGHAZ brittleness under cold conditions, it is necessary to mitigate the formation of coarse nitrides by careful control of steelmaking process and continuous casting to avoid the segregation of titanium, niobium and nitrogen.

Effects of Nitrogen Segregation and Solubility on the Formation of Nitrogen Gas Pores in 21.5Cr-1.5Ni Duplex Stainless Steel

The nitrogen gas pore-formation mechanism was discussed with regard to the solidification of 21.5Cr-1.5Ni duplex stainless steels (DSSs) by considering nitrogen segregation and solubility. The segregation behavior of nitrogen was investigated with phase transformation using experimental detection methods and Thermo-Calc software calculations. The process associated with the formation of gas pores was illustrated clearly. The factors that influenced the formation of gas pores, including shrinkage, nitrogen content, solidification pressure, and alloying elements (Mn and Cr), were discussed in detail. The formation of nitrogen-rich phases [austenite phase (FCC), AlN, and hexagonal close packed] is beneficial to eliminate nitrogen segregation and suppressing gas pore formation. The nitrogen-depleted phase (ferrite phase (BCC)) exhibits an opposite effect. Regular gas pores are initially formed in locations consisting of the austenite phase. As the gas pores lengthen, ferrite and austenite phases alternately form around the gas pores. Solidification shrinkage can promote the formation of irregular gas pores at the centerline of the ingots. Increasing the nitrogen content is favorable to the formation of gas pores. Increasing solidification pressure is effective with regard to suppressing the formation of gas pore defects in DSSs. Increasing the Mn content can reduce the likelihood of gas pore formation; this can be attributed to the increased nitrogen solubility in the residual liquid surrounding the dendrites and the formation tendency of the nitrogen-rich phase. Increasing the Cr content exhibits a dual effect on gas pore formation, which is caused by the increased nitrogen solubility and segregation in the residual liquid.