Nitride Precipitation Research Articles

Computational Thermodynamics and Kinetics of Aluminum Nitride Precipitation in Steel—An Overview with Emphasis on Austenitic Grain Size Control

Computational Thermodynamics and Kinetics of Aluminum Nitride Precipitation in Steel—An Overview with Emphasis on Austenitic Grain Size Control

Effects of rare earth on nitriding microstructure and properties of a La-Ce micro-alloyed Q235RE steel

Accelerating nitriding is an interesting topic in chemical heat treatment of ferrous parts. In order to have a deeper understanding of the catalysis principles of rare earth in nitriding, a Q235 carbon steel and a Q235RE steel micro-alloyed with rare earth elements La and Ce were nitrided respectively by plasma nitriding and gas nitriding. The microstructure, phase constitution, and composition of the nitrided specimens were analyzed. The hardness profile of nitriding layer and the brittleness of surface compound layer were measured. The results demonstrate that the Q235RE specimens obtained a higher hardness and deeper effective hardening layer than the Q235 specimens without rare earth microalloying. The rare earth catalysis effects were presented as the acceleration of nitrogen diffusion and the inhibition of nitride precipitation in the diffusion layer of the Q235RE specimens. In addition, the surface compound layer of the Q235RE specimens showed higher toughness and better adhesion to the matrix than that of the Q235 specimens, which is considered to be strongly related to the rare earth elements diffused into the compound and the enhanced residual compressive stress in the layer.

Regulation of mechanical properties in vanadium-containing high-nitrogen austenitic stainless steel via nano-Sized precipitated phase control

Nitride precipitation in high-nitrogen austenitic stainless steels significantly affects the mechanical properties of said steels. In this study, vanadium microalloying was employed to prepare high-nitrogen austenitic stainless steel with excellent comprehensive mechanical performance. The characteristics of the different precipitated phases, precipitation and dissolution behaviors, and their effects on mechanical properties were investigated. The impact mechanism of the precipitated phases on the mechanical properties of stainless steel was also discussed. The results indicate that the Me2X-type precipitated phase in the vanadium-containing high-nitrogen stainless steel exhibits a hexagonal close-packed (HCP) structure, forming a incoherent relationship with the matrix. This leads to a significant reduction in ductility owing to the larger size of particles enriched at the grain boundaries with minimal strengthening effects. Conversely, the MeX-type precipitated phase exhibits a face-centered cubic (FCC) structure, maintaining a semi-coherent relationship with the matrix. This results in smaller particle sizes and a more dispersed distribution, leading to a pronounced strengthening effect without compromising plasticity. Effective control of the precipitated phase can be achieved by solution treatment at different temperatures. Specifically, stainless steel subjected to a 1100 °C solution treatment exhibits only fine, dispersed MeX-type precipitated phases, resulting in optimal comprehensive mechanical properties: yield and ultimate tensile strengths of 587 and 924 MPa, respectively, with an elongation of 55% and a strength-ductility product of 49.6 GPa·%.

Morphological and Dimensional Evolution of Nanosized Amorphous Silicon Nitride in α-Fe: Diffusional and Elastic Effects

We present a detailed analysis based on both experimental and 3D modelling approaches of the unique silicon nitride precipitation sequence observed in ferritic Fe-Si alloys upon nitriding. At 570 °C, Si3N4 silicon nitride was shown to form as an amorphous phase into α-Fe ferrite matrix, which is morphologically unstable over time. Precipitates nucleated with a spheroidal shape, then developed a cuboidal shape for intermediate sizes and octapod-like morphology for a longer time. Using transmission electron microscopy, we show that the transition between spheroid and cuboid morphology depended on particle size and resulted from competition between interfacial energy and elastic strain energy. The resulting morphology was then shown to be a cuboid shape whose faces were always parallel to the {100} planes of the α-Fe; the <100> directions of the matrix corresponded to the elastically soft directions. There was a critical size of around 45 nm for which the transition between the cuboid shape and the octapod-like morphology took place. This was characterised by a transformation of quasi-flat facets into concave ones and the development of lobes in the <111> directions of the bcc crystal. To better assess the kinetic effects of diffusion fields and internal stresses on the morphological instability observed, an original 3D model that explicitly coupled phase transformations and mechanical fields was developed and applied. The latter, validated on the basis of model cases, was shown to be able to describe the time-evolution of both chemical and mechanical fields and their interactions in diffusive mass transport. Using a model system, it was shown that the concentration field around the precipitates and the internal stresses played opposing roles in the cuboid to octapod-like morphological instability. This work gives some clarification regarding the morphological evolution of amorphous Si3N4 precipitates, an important point for controlling the mechanical properties of nitrogen steels.

Characterization of structural and mechanical properties of HfNbTaTiZr refractory high entropy alloy after gas nitriding

This study was initiated to improve surface hardness and wear resistance of a HfNbTaTiZr refractory high entropy alloy (RHEA) by gas nitriding at a medium temperature (600 °C) for 3 h. Structural characterizations conducted by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) equipped scanning electron microscope (SEM) revealed that nitriding led to formation of a 1.5 μm thick surface layer containing precipitates of oxides and nitrides of the alloying elements. Detection of oxides within the surface layer was attributed to the presence residual oxygen in the nitriding atmosphere. Nevertheless, the employed gas nitriding provided remarkably higher scratch resistance compared to the untreated state, as the results of increment in the surface hardness and development of larger compressive residual stress.

Microstructural characteristics of different heat-affected zones in welded joints of UNS S32304 duplex stainless steel using the GMAW process: analysis of the pitting corrosion resistance

Abstract The influence of specific microstructural characteristics on the properties of single-pass welding joints was assessed by optical processed images, transmission electron microscopy, microhardness measurements and corrosion tests conducted in various regions of the heat-affected zone (HAZ) in a lean duplex stainless steel. The welded joints were obtained with heat inputs of 1.5 and 2.5 kJ/mm using a gas metal arc welding (GMAW) process with a shielding gas enriched in Ar. Three selected regions in the HAZ showed different ferrite grain sizes and austenite fractions. The place in the welded joint where the HAZ was narrowest, and therefore experiences the highest cooling rate, is most prone to the formation of cubic CrN metastable nitrides. Conversely, the place where the HAZ was wider promotes the precipitation of stable Cr2N nitrides with more coalesced intragranular austenite (IGA) particles, where presumably random interfaces predominate. The HAZ region where the cooling rate was the highest presented more pitting corrosion resistance.

RESEARCH ON INCREASING THE HARDENING OF A GEAR REDUCER USING PLASMA TECHNOLOGY

Different case hardening methods are used to increase the hardening of machine-building products and their resistance to abrasion, temperature and corrosion. The most modern methods include plasma nitriding technology. This method can be applied for the following parts: crankshaft, gears, piston parts, etc. A common characteristic feature of workpiece that undergo case hardening is that their mechanical processing after hardening is difficult or impossible due to high hardness and complex geometric shape. Plasma nitriding is the process of low-temperature common case hardening for highly loaded gears. When nitriding, a diffusion layer with very hard complex nitride precipitation with a thickness of several microns is formed on the surface of the gear wheel, which greatly increases the technical performance of the part. [1] This article summarizes the state of knowledge in the field of plasma nitriding technology equipments and surface nitriding of gears, provides a overeview of the current state of research in the field of nitrided gears. Keywords: hardening, plasma nitriding, nitriding, gears, mechanical processing, reducer.

Effects of nitride precipitation on delta phase formation in additively manufactured nickel superalloys

Additively manufactured Inconel 625 is particularly susceptible to the formation of niobium-rich δ-phase precipitates in the interdendritic regions during high temperature exposure. With specific minor alloying element combinations and nitrogen mass fractions on the order of 0.1 %, the formation of δ-phase can be suppressed through the precipitation of nitrides. For example, mass fractions of 0.39 % silicon and 0.03 % titanium led to the precipitation of Z-phase and η-nitrides, which consumed the excess niobium in the interdendritic regions and limited δ-phase formation. Even when holding the material at a temperature of 870 °C for 1000 h, the δ-phase volume fraction was approximately 2 %, which is far below the 6 % level observed in the wrought condition. When the titanium mass fraction was increased to 0.21 % and the silicon mass fraction decreased to 0.05 %, titanium-rich MN nitrides formed within the interdendritic regions instead. Since much of the Nb in these regions was not consumed by the nitrides, δ-phase formation was promoted and reached volume fractions above 10 %. The addition of a hot isostatic pressing step prior to high temperature exposure in both alloys produced a more uniform niobium distribution and minimized δ-phase formation at shorter times. At extended times, trends in the δ-phase volume fractions were similar to those observed in the as-deposited condition, with the initial alloy compositions driving differences in the distribution of excess niobium available for δ-phase formation.

Influences of partial substitution of C by N on the microstructure and mechanical properties of 9Cr18Mo martensitic stainless steel

Large eutectic carbides presenting in the matrix deteriorate the mechanical properties of high carbon martensitic stainless steel. In the present work, nitrogen-alloyed martensitic stainless steel with different C/N ratios were designed by partial substitution of C by N of 9Cr18Mo, and their microstructure and mechanical properties were investigated. The results show that, with increasing the content of N up to 0.15 wt% in the experimental steel, the quantity and the size of eutectic carbides decrease； and the net-work eutectic carbides are gradually eliminated. The microstructure of all the steels is composed of lath martensite, twin martensite and M23C6 carbides. With partial substitution of C by N, the morphology of martensite blocks evolves from a lath shape to a blocky one, the quantity of twin martensite increases, and the nano-scaled Cr2N nitrides precipitate in the martensite matrix. The steel containing 0.15 wt% N exhibits the hardness of 60.5 HRC, a fracture strength of 1900 MPa and an elongation rate of 1.6 % due to hot rolling and heat treatment, which is attributed to the refinement of carbides and grains, solid solution of C and N, as well as the dense dislocation.

Heat-Affected Zone Characterization of X2CrNiN22-2 Lean Duplex Stainless Steel by Metallographic and Electrochemical Techniques

This research aimed to investigate the heat-affected zone of lean duplex stainless steel grade X2CrNiN22-2. Different heat-affected zone microstructures and grain morphologies were developed by Gleeble simulations. The governing microstructures were evaluated by metallographic techniques and electrochemical corrosion measurements. It was found that the 1200-800 °C cooling time significantly affects the microstructure, austenite content, and corrosion properties. The average austenite content in the case of 1 s cooling time is 30.7 ± 1%, which increased with the longer cooling times up to 38.6 ± 0.9%. The rapid cooling times resulted in a more ferritic microstructure, which promoted nitride precipitation in the ferrite grains. The nitride precipitations acted as nucleation sites for pitting initiation in 3.5 wt.% NaCl solution. The lowest pitting potential was measured in the case of the most rapidly cooled sample: 573 ± 31 mV, while the balanced, annealed microstructure had much better pitting corrosion resistance, showing a pitting potential of 1308 ± 62 mV vs. the Ag/AgCl (KCl sat.) reference electrode. The results of this research can be used in designing welding parameters for the welding of the X2CrNiN22-2 lean duplex stainless steel.Graphical

Structure–Phase Transitions in the Friction Contact Zone of High-Nitrogen Chromium–Manganese Austenitic Steel

The influence of contact stresses on the phase and concentration composition of thin surface layers and wear products in the tribological contact zone of high-nitrogen FeMn22Cr18N0.83 steel was studied using Mössbauer spectroscopy, X-ray structural analysis, and electron microscopy. It was shown that contact compressive stresses developing under the conditions of dry sliding friction in the surface layers (20–25 microns) resulted in the strain-induced dissolution of cellular precipitation products (nitrides Cr2N) and increased the average content of nitrogen in austenite. Antiferromagnetic ordering in austenite caused by the precipitation of secondary nitrides with low chromium and nitrogen content was observed in tiny external layers (~0.1 microns) of the friction surface and products of steel adhesive wear. The effect of tension stresses in the friction contact zone on the formation of strain-induced martensite and nitrides with α″-Fe16N2 structures was established in the wear products.

Optimization of Grain Boundary Character Distribution of Fe-19Cr-10Mn-1Ni-0.53N High Nitrogen Austenitic Stainless Steel

High nitrogen austenitic stainless steel (HNASS) has outstanding mechanical properties, but in its hot working, welding, and high-temperature use, there will be a precipitation phase, especially nitride precipitation that reduces its intergranular corrosion, stress corrosion, corrosion fatigue, and other grain boundary-related properties. At present, the traditional methods of suppressing precipitation phase precipitation, such as solution treatment and alloying, have drawbacks. Grain boundary character distribution (GBCD) optimization of Fe-19Cr-10Mn-1Ni-0.53N HNASS was characterized by electron backscatter diffraction (EBSD) in this work. This study reveals that after grain boundary engineering (GBE) treatment, the percentage of low Σ coincidence site lattice (CSL) boundaries of the solid solution treated sample rises from 53.94% to 82.41%, because the sample was cold-rolled (CR) by 5%, followed by annealing at 1423 K for 10 min, and the twin related domains (TRD) size increases from 23.99 μm to 169.82 μm and ν (the ratio of TRD size to grain size) raises from 1.74 to 7.52. The connectivity of the random high-angle grain boundary (RHAGB) web is interrupted and the GBCD of the experimental steel is remarkably optimized.

Effect of boron addition on the microstructure and cryogenic mechanical properties of N50 stainless steel after aging treatment

The effect of boron on the microstructure and cryogenic mechanical properties in N50 austenitic stainless steel after aging treatment was revealed by a comparative investigation of 0B and 25B (25 ppm) N50 steels. The results showed there is a large amount of irregular block and rod-like Cr2N precipitated along grain boundaries with intra-granular precipitation of lamellar Cr2N in 0B steel; nevertheless, the nucleation and growth of Cr2N were significantly suppressed in 25B steel. It is related to the retarded diffusion of Cr, Mo, and V atoms caused by the pre-occupied B atoms during the precipitation of nitride. Theoretical calculations verify that the high B segregation at grain boundaries is caused by both equilibrium and non-equilibrium segregation. The weakened precipitation of nitride and the enhanced cohesion of grain boundaries by segregated B synergistically suppress the initiation and extension of intergranular cracks. The impact toughness of 25B–N50 steel at both 77 K and 4.2 K can reach 200 J, which is about 2.5 times higher than that of 0B–N50 steel.

Investigation of Chromium Nitride Precipitation in UNS S39274 Stainless Steel

Investigation of Chromium Nitride Precipitation in UNS S39274 Stainless Steel

Design and performance of nitrogen-alloyed iron-based coating for enhancing galling resistance by plasma transferred arc welding

Iron-based alloys have been widely used as high-temperature galling resistant coatings. In this study, we designed and prepared a galling-resistant coating using plasma transferred arc welding with cored wires of varying nitrogen content. Our approach was based on the concept that nitrogen alloying can lead to the formation of hard precipitates and promote strain-induced phase transformations. The coating was primarily composed of austenite, and as the nitrogen content increased, the grain size decreased and the amount of nitride precipitation increased. The threshold galling stress of this cobalt-free coating was found to reach 80% of that of a laser-clad Stellite 21 coating at 300 °C. Following the galling test, the hardness of the worn area increased and a phase transformation from austenite to martensite occurred beneath the worn surface. The improvement in galling resistance under dry heavy load sliding conditions can be attributed to the synergistic effect resulting from multiple strengthening mechanisms, including fine-grain strengthening, second phase strengthening and phase transformation strengthening.

Abnormal improvement of high-temperature stress rupture life in a new low cost Ni-based single crystal superalloy by adding minor nitrogen

Generally, elevated nitrogen (N) content promotes the precipitation of nitrides, which increase the size and quantity of porosity and thus degrade the mechanical performance of superalloys. However, in this study, N content was found to have a beneficial impact on the stress rupture property of single-crystal (SX) superalloys. The results indicated that as the N content increased from 5 ppm to 35 ppm, the porosity first decreased and then increased. The porosity of the sample with 14 ppm N was the lowest, 0.072% ± 0.011%, which was mainly attributed to the TiN phases in interdendritic pools that compensated for volume shrinkage by lattice expansion. The addition of N reduced the volume fraction and antiphase boundary energy of the γ′ phase because TiN nitride formation consumed Ti in large quantities. Additionally, the rupture life at 1100 °C/130 MPa was abnormally improved to 251.67 h ± 19 h as the N content was increased to 14 ppm, while it was markedly decreased as the N content was continuously increased to 35 ppm. The improved stress fracture life of the alloy was primarily attributed to the formation of TiN with a small size, which retarded microporosity formation and effectively impeded dislocation movement in γ channels. Moreover, during the stress rupture test, the size of the tertiary γ′ phase first reduced to a minimum in the alloy containing 14 ppm N, which increased the resistance to dislocation movement. However, excessive addition of N drastically reduced the stress fracture life as the precipitation and solid solution strengthening in the γ′ phase weakened. In addition, the agglomeration of nitride inclusions formed defects that aggravated crack nucleation and propagation. The overall N impact was studied, and the optimal N content was ascertained to provide further instructions for the commercial development of Re-free second-generation SX alloys.

Nitride precipitation and formation in IN617 superalloy during creep

Precipitation behaviors of four types of nitrides (including π phase (Cr3.25Ni1.75N), AlN, Cr2N and TiN) were investigated in IN617 after creep rupture at 1000 °C/16 MPa. Nitride formation mechanisms were discussed based on their relative stabilities, crystallographic characteristics, and orientation relationships with the matrix. The influences of nitrides on creep rupture behaviors were discussed. The results indicated that the π phase, which was predicted unstable at 1000 °C by thermodynamical calculations, only precipitated at high-energy grain boundaries and phase boundaries. Thermodynamically stable AlN, Cr2N and TiN could be formed at relatively low-energy sites, such as inside the matrix, at dislocations and twining boundaries. Particularly, AlN and Cr2N are polar crystals and have hcp structures, much different from the fcc matrix. They precipitated from the habit planes with low atomic misfits and grew into a needle shape to reduce the strain energy. Fcc TiN was easy to nucleate and had good coherence with the matrix, so it could either precipitate from the regions with high-density dislocations in a needle shape, or directly from the matrix in spherical or dendritic shapes to minimize the interfacial energy and strain energy. Among those nitrides, the π phase and AlN formed coarse intergranular networks and were difficult to deform and release stress, dominantly responsible for the brittle rupture mode.

Influence of Inclusions in the Corrosion Behavior of Plasma Nitrided Stainless Steel

Plasma nitriding is a well‐established technique to improve hardness and tribological properties of austenitic stainless steel. It is proved that it is also possible to preserve the corrosion resistance after nitriding by controlling the process parameters such as time and temperature, obtaining the so‐called S phase. Herein, the corrosion behavior is evaluated for nitrided layers produced by three different plasma treatments: DC plasma nitriding, plasma immersion ion implantation (PIII), and low‐energy ion implantation (LEII), using different parameters in order to obtain the S‐phase in thickness from 1.2 to about 6 μm without nitride precipitation. The microstructure and chemical composition of the nitrided layers is characterized by means of X‐ray diffraction, secondary‐ion mass spectroscopy, and scanning electron microscopy–focused ion beam. The corrosion behavior is evaluated by means of the cyclic polarization tests in NaCl solution. The morphology of the corrosion attack is studied by optical microscopy and SEM‐FIB, revealing a change from crevice to pitting after the nitriding process. The inclusions are observed to be corrosion initiation sites. Due to this, the thickest nitrided layers (with high N concentration) show better corrosion behavior than the thinner ones.

Effects of Plasma Nitriding Process on AISI 304 Stainless Steel

AISI 304 stainless steel is a type of austenitic stainless steel that contains a high percentage of chromium and nickel. It is one of the most widely used grades of stainless steel and is commonly used in a variety of applications, including kitchen equipment, food processing equipment, and chemical processing equipment. AISI 304 stainless steel is a versatile and widely used material due to its excellent corrosion resistance, durability, and non-magnetic properties. Low-temperature processes like ion implantation, plasma nitriding can prevent the corrosion resistance of stainless steels by diffusion of plasma into the surface of the material, forming precipitation of Chromium nitride. For this research work, plasma nitriding is carried out on AISI 304 at low-temperatures 550°C for the time duration of 8 hrs, 16 hrs and 32 hrs. The formation of nitrogen-enriched layers with high nitrogen content promoted to increase in surface hardness. Wear test were carried out with pin on disc machine and the samples were undergone with hardness tests. The microstructures of plasma treated samples were compared with untreated microstructures. It was noted that phase change occurred from austenite to expanded austenite forming a hard layer from the surface level improving the wear resistance of the material.

Optimize the corrosion behavior of AISI 204Cu stainless steel in different environments under previous cold working and welding

Enhancing corrosion resistance in stainless-ssteel alloys is a paramount objective in the petroleum industry. This study investigated the effects of the previous cold working and welding processes on the mechanical properties and corrosion rates of 204 Cu stainless steel in different aggressive environments (crude oil, freshwater, and seawater). The experimental sets were supported by microstructure analysis. The mean weight loss method was employed to determine the corrosion rates, which were optimized using the Taguchi method. The ferrite and austenite phase bands, as well as the deformed portions of austenite, are pushed to flatten out during cold working, which increases the material’s hardness. Cold-worked steels were welded, creating an annealed area around the HAZ in addition to the usual weld zones, which demonstrated partial microstructure recovery and hardness reduction. HAZ showed signs of iron overload and chromium nitride precipitation. Cold-worked specimens only showed reduced corrosion resistance to 30% of the initial rate and reduced thickness. Moreover, the Taguchi optimization technique indicated that the corrosion environment has the most effect on the corrosion rate compared to the cold work ratio for welded and non-welded stainless-steel specimens.