Nitride Precipitation Research Articles

The as-cast precipitation-strengthened N-doped Fe40Mn40Co10Cr10 high-entropy alloys

The as-cast Fe40Mn40Co10Cr10-based (in at.%) high-entropy alloys were doped with different amounts of V and N. It was shown that a certain percentage of V and N cause significant strengthening due to the V2N-type nitrides precipitation. The precipitation-strengthened alloy with 2 at.% V and 1 at.% N had yield strength of 440/940 MPa and uniform elongation of 36/18 % at 293/77 K. Experimental data on the phase composition of the alloys agree reasonably with the CALPHAD predictions.

Investigation of the oxidation behavior of Zircaloy-4 cladding in a mixture of air and steam

Previous studies had shown that in certain conditions, the rate of oxidation of zirconium (Zr) based alloy fuel cladding is higher in air–steam mixtures than in dry air. In severe accidents in the spent fuel pool and in other air ingress accidents in nuclear power plants, the cladding is likely to be oxidized in an air–steam mixture, which makes it crucial to have an in-depth understanding of the nature of oxidation and its kinetics in that environment. Oxidation tests were conducted at 800 °C on Zircaloy-4 specimens in a mix of (air + steam) with various component ratios. Oxidation kinetics, details of the oxide layer, and hydrogen pick-up in the specimen were studied to investigate the mechanism of oxidation in each of these sets of conditions. Zirconium nitride precipitation in the oxide layer during the initial stages of the pre-breakaway oxidation stage and the widespread porous oxide growth on the cladding surface in the latter post-BA oxidation stage are related to the oxidation mechanism in the air–steam mixture. The differences in the mechanism of oxidation of the cladding in dry air and air–steam mixtures are discussed based on the experimental results.

Influence of Hydrogen Reduction on the Properties of Porous High-Nitrogen Austenitic Stainless Steel.

This work explores the impact of hydrogen reduction on sintering and nitriding of porous high-nitrogen austenitic stainless steel (HNASS) processed via powder metallurgy. A temperature-resolved hydrogen reduction (temperature range of 700–1250 °C) was performed to evaluate the phase composition of porous HNASS. The systematic microstructure was characterized by a scanning electron microscope (SEM) with energy disperse spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The compressive mechanical properties and electrochemical corrosion behavior of the unreduced and reduced samples were discussed. Samples reduced in hydrogen at 1100 °C and 1250 °C show better compressive properties while still retaining good corrosion resistance. Reduction of oxide facilitates sintering thus improves the compressive properties. Increasing the content of solute nitrogen and reducing the precipitation of nitride can effectively improve the corrosion resistance of porous HNASS.

Modelling of Nitride Precipitation during Isothermal Heating after Rapidly Cooled from High Temperature in Duplex Stainless Steel

Modelling of Nitride Precipitation during Isothermal Heating after Rapidly Cooled from High Temperature in Duplex Stainless Steel

Microstructural Study of CrNiCoFeMn High Entropy Alloy Obtained by Selective Laser Melting.

The high entropy alloy (HEA) of equiatomic composition CrNiFeCoMn and with FCC crystal structure was additively manufactured in a selective laser melting (SLM) process starting from mechanically alloyed powders. The as-produced alloy shows fine nitride and σ phase precipitates, which are Cr-rich and stable up to about 900 K. The precipitates increase in number and dimensions after long-period annealing at 900–1300 K, with a change in the HEA mechanical properties. Higher aging temperatures in the furnace, above 1300 K, turn the alloy into a single FCC structure, with the disappearance of the nitride and σ phase precipitates inside the grains and at the grain boundaries, but still with the presence of a finer Cr-rich nitride precipitation phase. These results suggest that the as-produced HEA is a supersaturated solid solution at low and intermediate temperature with nitrides and σ nanostructures.

Interactions between irradiation defects and nitrogen in [formula omitted]‐Fe: an integrated experimental and theoretical study

This paper aims at clarifying, by means of an integrated experimental and theoretical approach, the properties of the interaction between interstitial nitrogen (N) and irradiation generated lattice defects in α-Fe. For this purpose, N-doped and pure Fe specimens were irradiated at low temperature by high energy protons. The evolution of radiation defects and their interaction with N was monitored by electrical resistivity measurements during post-irradiation annealing. In parallel, density functional theory (DFT) was employed to study the properties of N solutes, vacancies and their mutual interaction in the Fe matrix. The DFT results were confronted to the experiment via kinetic rate theory modelling, employed to quantitatively simulate the measured resistivity evolution. One of the most important results is the experimental validation of the theoretically predicted strong binding energy of vacancy-N complexes, which reconciles previous discrepancies. Furthermore, a quantitative interpretation is provided of how irradiation competes with nitride precipitation.

Unlocking the influence of microstructural evolution on hardness and pitting corrosion in duplex stainless welded joints

In this study, the evolution of microstructure and its correlation with hardness and pitting corrosion resistance in duplex stainless steel (DSS) welded joints were investigated. The results showed that the precipitation of chromium nitride, refined grain size and large residual deformations contributed to the increase of microhardness in the heat affected zone (HAZ) and weld metal (WM). Poor pitting corrosion resistance was found in the HAZ, especially in the high temperature heat affected zone (HT-HAZ). The chromium nitride precipitates and secondary austenite were the main pitting nucleation sites.

Stable nitride precipitation in additively manufactured nickel superalloys

With negligible quantities of nitrogen, wrought and welded solid solution strengthened nickel superalloys usually contain carbides and topologically close-packed phases, such as Laves phase. However, appreciable nitrogen levels on the order of 0.1 % mass fraction drove the precipitation of a range of unanticipated nitride phases in additively manufactured Inconel 625 in the as-deposited and post process hot isostatically pressed conditions. Different nitride phases were observed with small changes in alloy chemistry. Cubic metal nitrides (MN), tetragonal Z-phase (CrNbN), and diamond-cubic metal η-nitrides (M 6 N) were found within the γ matrix of Inconel 625 containing relatively low Fe (1 %), low Ti (0.02 %), and high Si (0.39 %) mass fractions. Conversely, these phases were replaced by only MN nitrides in a similar Inconel 625 alloy with elevated mass fractions of Ti (0.21 %) and Fe (4 %). These various phases, however, were not fully predicted using state-of-the-art computational thermodynamic tools and databases, indicating a sparsity of data for nickel superalloys. Even after hot isostatic pressing, many nitrides persisted and only experienced slight changes in composition and lattice parameters in both materials. The stability of these nitride phases presents a potential pathway for achieving enhanced high temperature and creep properties within this and similar alloy systems. • Significant nitrogen contents are measured in Inconel 625 powder. • Only nitrides (MN, Z-phase, η-nitride) are observed in two as-deposited Inconel 625 alloys. • Minor alloying elements influence the type of nitride that forms. • Most nitrides persisted after post-process hot isostatic pressing.

Influence of N on precipitation behavior and transformation kinetics of super austenitic stainless steels after isothermal ageing at 900 °C

The precipitation behavior and transformation kinetics of secondary phases of super austenitic stainless steels 654SMO containing 0.4wt.% N and 0.5wt.% N were investigated following ageing at 900 °C for varied time. It was revealed that the increase of N did not lead to the refinement of sigma precipitates. Super austenitic stainless steel with 0.4wt.% N presented more uniform and homogeneous precipitation. This unexpected event was caused by a higher sigma nucleation rate than the growth rate of the 0.4wt.% N alloy. The area fraction of precipitates of 0.5wt.% N alloy was lower than that of 0.4wt.% N alloy, which was most likely due to the lesser number of particles nucleated overcompensating the fast growth of sigma particles caused by rising N concentration. Furthermore, increasing the N content did not speed up but slowed down the precipitation of the Cr2N phase. The Cr2N phases were found nucleated at the σ/γ interface, implying that the higher sigma transformation fraction of the 0.4wt.% N alloy promoted the precipitation of nitrides. The modified Johnson-Mehl-Avrami model was used to describe the transformation kinetics of precipitates in both alloys. There was a good agreement between experimental data and model.

Grain growth behavior of niobium microalloyed 20MnCr5 steel and the effect of boron

AbstractGrain growth behaviors of niobium free, 0.021 wt. % niobium microalloyed and 0.024 wt. % niobium‐0.0022 wt. % boron microalloyed 20MnCr5 steels are investigated in this paper. Optical microscopy and field‐emission scanning electron microscopy were used to analyze the prior austenite grain. Transmission electron microscopy and energy dispersive spectroscopy were used to investigate the morphologies and compositions of the precipitates, respectively. The results show that adding niobium reduces the size of prior austenite grain. The addition of boron to niobium microalloyed steel, however, causes grain coarsening. Thermodynamic and kinetic calculations were performed to explain the mechanism of grain growth behavior. The results indicate that the addition of niobium and boron causes the difference in solution temperatures of aluminum nitride and niobium carbonitride precipitates. The addition of boron inhibits the precipitation of niobium carbonitride and aluminum nitride and causes precipitates coarsening. The precipitation observation backs up the calculation results.

Initiation mechanism of pitting corrosion in weld heat affected zone of duplex stainless steel

The pitting corrosion resistance of duplex stainless steel welds was investigated. Pitting corrosion mainly occurs in high-temperature heat-affected zone (HT-HAZ), where the peak temperature reaches ferrite single-phase region. Most pitting corrosion in the HT-HAZ initiates from (Ti,Cr)N in the ferrite. Even the titanium content is less than 50 ppm, titanium-containing nitrides precipitate in the HT-HAZ. While no clear Cr-depleted zone develops around (Ti,Cr)N, almost no passive film forms on (Ti,Cr)N surface. Therefore, pitting corrosion is initiated not only by Cr-depleted zone around Cr nitride but also by the lack or vulnerability of the passive film on the (Ti,Cr)N surface.

Influence of Nickel on Niobium Nitride Formation in As-Cast Stainless Steels

The influence of the nickel content and cooling rate on niobium nitride precipitation in as-cast stainless steels were analysed. Niobium microalloying is important for mechanical properties and the prevention of intergranular corrosion in stainless steels. However, coarse precipitates can negatively affect steel properties. The precipitation of NbN depends on thermodynamic conditions, which are dictated by the chemical composition and temperature. The thermodynamic computations were used to estimate niobium nitride precipitation. Additionally, segregation models were used to predict precipitation. Three steel batches with different nickel contents (9 wt.%, 4.7 wt.%, and 0.16 wt.%) were prepared in an induction furnace and cast into sand moulds. The polished and etched samples were examined with an optical microscope, followed by a more detailed examination using a scanning electron microscope. An automatic scanning electron microscope analysis of the niobium particles was performed to obtain particle number and size distribution. Primary niobium carbonitrides, eutectic phases, and heterogenous nucleations on MnS inclusions were observed. As the proportion of nickel in the solution decreased, the solubility of nitrogen in the melt increased, which is manifested by a lower formation of primary and eutectic niobium carbonitrides, while MnS non-metallic inclusions played an important role in the heterogeneous nucleation.

Analyzing the Effects of Heat Treatment on SMAW Duplex Stainless Steel Weld Overlays

Duplex stainless steel (DSS) has a reasonably high resistance to chloride stress corrosion cracking for offshore and marine applications. However, DSS weld overlay has not been successfully demonstrated due to some inherent problems in achieving pitting and crevice corrosion resistance. In this research work, isothermal heat treatments (350, 650 and 1050 °C) with and different cooling rates have been performed DMR249 Grade A by using shield metal arc welding (SMAW) with an E2209 electrode. Micrographs have shown two phase microstructures of the DSS weld metal, the amounts of austenite phase increased with increment of post-weld heat treatment (PWHT) temperatures. The dilution has maintained consistent values except solution annealing that has shown the disappearance of the heat affected zone in micrographs. The weld metal hardness values increased with PWHT temperatures and remained low at solid solution annealing temperatures. The major alloying elements (C, Mo, Cr, Ni, N, and Fe) were analyzed, as these elements can contribute to intermetallic phases. The results showed that C and Cr content slightly increased with PWHT except for solid solution annealing, Mo showed consistently low content due to dilution effects. Ni maintained higher content, although the heat-treated samples showed slight fluctuations. Nitrogen produced consistent values, as recommended to prevent critical involvement in nitride precipitation.

On the effect of oxide scale stability on the internal nitridation process in high-temperature alloys

AbstractInternal high-temperature corrosion of metallic materials is an important process often determining the service live of components used for high-temperature processes. As soon as a material looses the ability to form a dense and adherent oxide scale, non-metallic elements (e. g., O, N, C, and S) can diffuse from the atmosphere into the bulk material and react with alloying elements, forming different kinds of internal precipitates. The corrosion behaviour of two commercial alloys, Nicrofer 7520Ti and CMSX-4, and of the model alloy Ni – 20Cr – 6Ti was investigated in air and nitriding atmosphere at 1000 °C and 1100 °C under isothermal and thermal-cycling conditions. The study was performed using a continuous thermogravimetric method, which permits mass changes to be measured in different atmospheres and under thermal-cycling conditions. It was shown that damage in the oxide scale, caused by cracking or spallation, leads to an enhanced internal corrosion attack manifesting itself in the precipitation of oxides and nitrides. The experimental results were described in a mechanismbased approach by means of a simulation model that combines a numerical finite-difference treatment of the diffusion differential equations and the commercial thermodynamic program ChemApp. The model is able to predict the complex material behaviour during high-temperature exposure.

Effect of electric potentials on high‐temperature plasma nitriding of 2Cr13 stainless steel with active screen assisted

AbstractThe influence of electric potentials on the characteristics of modified layers with plasma nitriding of 2Cr13 stainless steel at a high temperature was investigated. Three active screens were placed on the cathodic plate. Three groups of 2Cr13 stainless steel samples were set with cathodic, anodic, and floating potentials. Samples were nitrided at 500 °C for 5 h in an ammonia atmosphere. The treated samples were analyzed using x‐ray diffraction, scanning electron microscopy with energy dispersive x‐ray spectroscopy, surface roughness tester, and atomic force microscopy. It is proved that nitriding treatments can be suitably carried out at three electric potentials, but the corresponding modified layers take on different microstructure, morphology, and hardness behaviors. It found that the three nitrided samples are mainly composed of the ϵ−Fe2‐3N and γ’−Fe4N phases. The order of thicknesses of the nitrided layers is: floating < anodic < cathodic potential. The results also demonstrated that electric potentials play essential roles in the corrosion resistance and tribological properties in plasma nitriding treatment. Because there is no visible precipitation of chromium nitride in the modified layer, the corrosion resistance of the floating nitrided sample is better than that of the other two samples.

Structure and Properties of Gas-Nitrided, Precipitation-Hardened Martensitic Stainless Steel.

Nanoflex stainless steel is a promising material for medical applications. However, improvement of its mechanical properties without compromising its corrosion resistance is still a challenge. In order to investigate the effect of the nitriding process on the corrosion and wear resistance of Sandvik NanoflexTM steel, a number of processes were carried out in a gas atmosphere with differing ammonia contents in the temperature range of 425–475 °C for 4 h. The mechanical properties and wear resistance of the layers were tested using the nanoindentation and pin-on-disc methods, respectively. In order to assess corrosion resistance, potentiodynamic tests were carried out in Ringer’s artificial body fluid and in a 3% aqueous solution of sodium chloride. The results are discussed herein with respect to the microstructural characteristics of the layers studied using light and scanning electron microscopy, X-ray diffraction phase analysis and wavelength dispersive X-ray microanalysis. The structure of nitrided layers included three zones: the subsurface zone composed of nitrides and the zones composed of metastable phases, i.e., the S phase (γN) and expanded martensite (αN) with possible precipitates of nitrides. The third zone adjacent to the steel core was enriched with carbon. The nitrided samples showed significant improvement in the wear rate while maintaining good corrosion resistance in comparison to the non-treated steel. We concluded that nitriding should be carried out at a temperature below 450 °C and in an atmosphere containing no more than approximately 50% ammonia in order to avoid nitrides precipitation.

Effect of the nanostructuring by high-pressure torsion process on the secondary phase precipitation in UNS S32750 Superduplex stainless steel

In this work, the precipitation and the morphology of secondary phases after severe plastic deformation (SPD) processing followed by an isothermal treatment was investigated. High-pressure torsion (HPT) was the SPD process carried out on superduplex 2507 (UNS S32750) stainless steel material under P = 6 GPa at room temperature. At this high strain levels (ε up to 170) samples have shown grain size decrease and strained microstructure with high dislocation density and nanostructure features. After a short isothermal treatment at 830 °C, the sigma phase and chromium nitrides were revealed as the main secondary phases identified by scanning and transmission electron microscopy and element analysis by energy dispersive spectroscopy. Scanning precession electron diffraction and automated crystal orientation mapping have been carried out in order to confirm the precipitation of the secondary phases. In fact, the results provide evidence that the precipitation of chromium nitrides seems to be the preferred nucleation site for sigma phase at higher deformation strain, in addition to the intergranular precipitation of sigma. Both the sigma phases nucleated integranularly and besides chromium nitrides are randomly orientated.

On the mechanical alloying of novel austenitic dual-precipitation strengthened steels

Austenitic oxide dispersion strengthened steels were developed to serve as a structural material in power plants. Mechanical alloying is used with the aim to homogeneously distribute rare earth elements like yttrium inside a steel matrix to form strengthening nano-scaled particles. Due to the ductile nature of a face centered cubic austenite matrix, mechanical alloying results in a large amount of cold-welding of powder and the milling equipment, which significantly reduces the production yield and the reproducibility of the process. Here, a novel 2-step mechanical alloying process using nitrogen as a process control agent was developed, resulting in a production yield close to 100 %. Vanadium was added to the steel prealloy to sequester the introduced nitrogen and to promote precipitation of nitride precipitates. Optimizations of process parameters were suggested after characterizing chemical compositions, particle sizes, dislocation densities, crystallite sizes, phase compositions and harnesses. A comparison of X-ray diffraction and scanning electron microscopy results revealed a new approach to define an optimized milling duration. After hot-rolling, a successful dual-precipitate formation of nano-scaled oxides and nitrides, with mean diameters of 3 and 37 nm, respectively, was proven using transmission electron microscopy methods.

Effect of N on corrosion resistance of Fe-Cr-Ni-Mo-Mn alloy

In order to develop the Fe-Cr-Ni-Mo-Mn-N corrosion resistant alloys, analyze the influence of nitrogen on the corrosion resistance of Fe-Cr-Ni-Mo-Mn alloy, adjust the N content in the alloy system, and Flux cored wires with different N content were prepared. They were surfaced on low carbon steel by MIG welding. The phase composition, microstructure and corrosion resistance of the cladding metal were analyzed to study the effect of N content on the structure and performance of the surfacing metal. The results show that the addition of nitrogen does not change the matrix structure (which is Fe-Ni-Cr austenite), but with the increase of nitrogen content, the precipitation of nitrides (Cr2(C,N) and BN)is accompanied; when the amount of nitrogen added reaches 1.0%, the corrosion resistance of the surfacing metal is the best. At this time, the self-corrosion current density is at least 1.569 × 10–9 mA cm−2 and the self-corrosion potential is at most −0.318 V. The addition of nitrogen can promote the formation of austenite and nitride, inhibit the appearance of carbides, avoid the phenomenon of poor Cr, and improve the corrosion resistance of the material.

Nitride Precipitation during Isothermal Heating after Rapidly Cooled from High Temperature in Duplex Stainless Steel

Nitride Precipitation during Isothermal Heating after Rapidly Cooled from High Temperature in Duplex Stainless Steel