High Nitrogen Austenitic Stainless Steel Research Articles

A comparison of wire-feed additive manufacturing and hybrid additive/subtractive manufacturing of high nitrogen steel

High nitrogen austenitic stainless steel, referred to as high nitrogen steel (HNS), has been widely studied and applied because of its excellent properties such as high strength, good plasticity and toughness. However, its high nitrogen content makes the deposition metal face problems such as nitrogen porosity, nitrogen escape and loss when it is processed by additive manufacturing (AM), which has a negative impact on the mechanical properties. Two thin-walled specimens without obvious defects were obtained by wire-feed AM and hybrid additive/subtractive manufacturing (HASM) using the self-made HNS wire with 0.7 wt.% N, respectively. The porosity, microstructures and tensile properties of thin-walled specimens were compared. The porosity of the two specimens was tested by an industrial computed tomography (CT) system. The defect volume ratio and the diameter of the largest pore of HASM specimen were reduced about 54% and by 46.6% than those of wire-feed AM, respectively. The morphology of ferrite at different height of both specimens was different including skeletal, lathy and granular. The tensile strength of HASM specimen was better than that of wire-feed AM, which is mainly attributed to the decrease of porosity and grain size.

Stacking fault energy during DRV-DRX competition in high-nitrogen austenitic stainless steel used in orthopedic implants

High-nitrogen (high-N) austenitic stainless steels (ASS) are used in orthopedic implants due to their mechanical properties, human biocompatibility, and affordable cost. However, the high content of (Ni–Nb–N)-rich precipitates can cause allergic reactions and significant challenges in the manufacturing of prostheses. This study investigates the competition between work hardening (WH), dynamic recovery (DRV), and dynamic recrystallization (DRX) during the physical simulation of an ASTM F-1586 steel by thermomechanical processing, using the Kocks-Mecking and Avrami constitutive models. The parameters were obtained through continuous isothermal torsion tests at the temperature range of 900–1200 °C, strain rates between 0.01 and 10 s−1 and a total deformation of 4.0. Determined by compositional-analytical methods, the stacking fault energy (γsfe) was correlated with the stress level (σi) according to the Uesugi and Dai models. Results indicated a high activation energy for hot deformation (Qdef = 587 kJ/mol), affecting the shape of the curves. The γsfe value varied along the curves, delaying the onset of the DRX (σc = 0.928σp). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses revealed a direct competition between work-hardened and recrystallized grains in the early part of the curves, due to WH-DRV synergy. After the peak stress (σp), the progress of DRX was slow, with the Avrami exponent (n) between 1.1 and 1.9, completing only after large deformations (σss = 0.714σp). The moderate γsfe value (69 mJ/m2) and fine precipitates of the Z-phase (CrNbN) (<20 nm) influence the grain boundary mobility during the DRV-DRX competition, delineating the stress-strain curve shape.

The hot deformation behavior and processing map analysis of a high-nitrogen austenitic stainless steel

Understanding the hot deformation behavior of steel and establishing corresponding hot processing maps are crucial methods for determining appropriate hot processing parameters. In this article, the hot deformation behavior of a high-nitrogen austenitic stainless steel was investigated in the temperature range of 900–1150 °C and the strain rate range of 0.01–10 s−1 on a Gleeble-3500 thermo-mechanical simulator. The results show that the flow stress of experimental steel increases with decreasing deformation temperature and increasing strain rate. Based on the Arrhenius model, a constitutive equation was developed for the experimental steel, with a hot deformation activation energy of 603.347 kJ/mol. The hot processing maps were created employing the dynamic material model to determine the optimum hot processing conditions for the experimental steel at different strain levels.

Strengthening high-nitrogen austenitic stainless steel via constructing multi-scaled heterostructure

High nitrogen austenitic stainless steels (HNASSs) have received widespread attention owing to their outstanding corrosion resistance, excellent weldability and workability. However, these materials usually exhibit insufficient yield strength to satisfy the requirement in service because of their face-centered cubic (fcc) structure. Here, we report a novel material design strategy of constructing both grain-scaled and atomic-scaled heterostructure through applying appropriate thermo-mechanical treatment to improve the comprehensive mechanical properties of HNASS. The optimal comprehensive mechanical properties with a yield strength of 1361 MPa and a uniform elongation of 10.6% are achieved. The results show that the ultrahigh yield strength mainly originates from the lath structure consisting of local chemical order domains (∼490 MPa), and the satisfactory ductility benefits from both deformation twinning and dislocation slip. This work proposes for the first time the strategy to improve mechanical properties of HNASS by introducing the multi-scaled heterostructure, and guides the development of high-performance steels and many other advanced fcc materials.

Untypical Electrochemical Corrosion Behavior of High-Nitrogen Austenitic Stainless Steel: Non-Pitting in Transpassivation

Untypical Electrochemical Corrosion Behavior of High-Nitrogen Austenitic Stainless Steel: Non-Pitting in Transpassivation

Effect of Isothermal Ageing on Microstructure and Corrosion Behavior of Nickel and Molybdenum-Free High Nitrogen Austenitic Stainless Steel

Effect of Isothermal Ageing on Microstructure and Corrosion Behavior of Nickel and Molybdenum-Free High Nitrogen Austenitic Stainless Steel

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·%.

Static and dynamic mechanical behavior and constitutive parameters determination of high nitrogen austenitic stainless steel

Static and dynamic mechanical behavior and constitutive parameters determination of high nitrogen austenitic stainless steel

Heterogeneous nucleation interface between NbN(111) and TiN(111) in high-nitrogen austenitic stainless steels: First-principles calculation

Heterogeneous nucleation of NbN induced by TiN inclusions is important to play the role of precipitation strengthening in high-nitrogen austenitic stainless steels. In this work, the atomic structure, interfacial stability, and interfacial fracture toughness of TiN(111)/NbN(111) are investigated based on first-principles calculation. Furthermore, the interfacial bond structure and energy of TiN(111)/NbN(111) are also analyzed with and without Cr doping on the basis of Thermo-Calc phase diagrams. The results indicate that the lattice mismatch between the crystal face of TiN(111) and NbN(111) is lower than 5 %, which reveals that TiN can act as the heterogeneous nucleus for NbN to grow. Besides, the maximum adhesion work at 4.58 J/m2 and the minimum interfacial energy at 1.28 J/m2 are presented on the stacking site of N-Ti TL, which suggests that (Ti, Nb) N phases can precipitate through this interfacial bonding mode. Moreover, from the view of bonding properties on the interface with and without Cr doping, it can be found Cr doping in two layers of the interface can enhance interface stability except for N-Ti TL (2Cr). This work will give promising guidance and theoretical basis for the controllable preparation of high-nitrogen austenitic stainless steels with in-situ precipitated nitride particles.

Novel high nitrogen austenitic stainless steels: From high-throughput screening to experimental validation and properties relationship

The quest for affordable and price-stable alternatives for nickel as the main austenitizing alloying element is pushing the stainless steel industries to develop new types of Ni-lean austenitic stainless steels. This work focuses on the challenges of designing austenitic stainless steels with appreciable nitrogen content using computational methods, which can still be produced at 1 atm N2 pressure and thermomechanically processed without excessive cracking or N2 degassing. CALPHAD-based computational tools enabled the identification of promising alloys by high-throughput screening from more than 2000 candidates based on predefined criteria considering composition (Ni-poor, N-rich), phase stability (austenite), degassing (far from N2 loss from liquid or solid phase), cost (Ni-lean), and pitting resistance equivalent number (PREN > AISI 201 stainless steel). The resulting alloys exhibited an austenitic microstructure with dispersed micrometric and nanometric aluminum nitrides (AlN) precipitates. Even with the occurrence of appreciable AIN precipitates, hot rolling was performed without cracking, resulting in equiaxed austenitic grains. The mechanical properties of the developed alloys exceeded those of AISI 201 stainless steel in terms of hardness, yield strength, and ultimate tensile strength, with values of 304 HV, 534 MPa, and 953 MPa, respectively, yet with considerable ductility (∼40% elongation). Even in the presence of AlN that pick up some of the nitrogen from solid solution, the corrosion resistance of the developed alloy was superior to the AISI 201 stainless steel. This research provides valuable insights for developing and processing high-nitrogen austenitic stainless steels to compete with 201 and 304 stainless steels in terms of mechanical properties and corrosion resistance.

Effect of Cl− on Passivation Properties of Fe-20Cr-20Mn-0.75N High Nitrogen Austenitic Stainless Steel

In this work, Fe-20Cr-20Mn-0.75N (wt.%) high-nitrogen stainless steel (HNSS) was studied using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electrochemical testing. The corrosion behaviors of Fe-20Cr-20Mn-0.75N HNSS with different concentrations of NaCl were studied. The composition of a passive film on Fe-20Cr-20Mn-0.75N HNSS was analyzed using X-ray photoelectron spectroscopy (XPS) at an applied potential of 0.2VSCE. The results showed that, with the increase in Cl− concentration, the corrosion tendency and corrosion rate of Fe-20Cr-20Mn-0.75N HNSS get higher. In the solution of a low Cl− concentration, the fraction of Fe and Cr oxides in the passive film is higher, and the passive film is thicker and more stable. By increasing the stability of the passive film and preventing its rupture, the elevated NH4+ concentration can enhance the corrosion resistance of Fe-20Cr-20Mn-0.75N HNSS in a NaCl solution.

Strengthening of high nitrogen austenitic stainless steel by Nb addition

Nitrogen addition is known to be one of the most effective ways of strengthening austenitic stainless steels while maintaining good properties of the steels. This study investigates Nb addition as an additional strategy to strengthen a high nitrogen austenitic stainless steel. It was found that Nb-containing steels developed Z-phase precipitates during homogenization and hot-rolling. Their fraction and size hardly changed by subsequent aging at 1050 °C. They effectively delayed grain coarsening and fine grains were maintained throughout the aging. The yield strengths of Nb-containing steels were always higher than those of Nb-free steel by 30–83 MPa. Quantitative analyses revealed that the major strengthening mechanism was grain boundary strengthening. The contribution from precipitation hardening was small because the precipitates were not sufficiently fine and the negative contribution from solid solution strengthening due to N consumption by the precipitation was also insignificant.

Effect of nitrogen content on corrosion behavior of high-nitrogen austenitic stainless steel

A series of electrochemical tests combined with the techniques of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and X-ray photoelectron spectroscopy (XPS) were used to study the effect of nitrogen content on the composition, structure and protectiveness of passive films, which were formed on the surfaces of high-nitrogen austenitic stainless steels (HNSS) in 0.5 mol/L NaCl solution. The results showed that the HNSS with higher nitrogen content had a larger proportion of low-angle grain boundaries, and it also had a lower corrosion current density in 0.5 mol/L NaCl solution and thus a lower corrosion rate. The existence of a larger proportion of stable oxides (e.g., Cr2O3) in the passive films facilitates the passivation/repassivation process and contributes to the high corrosion resistance of HNSS.

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.

Microstructure and Tribological Behavior of Cr-Mn-N Steel with Age-Hardened Near-Surface Layer including CrN and Fe2N Particles Intended for Use in Orthopedic Implants

This paper presents the results of a study of 17%Cr-19%Mn-0.53%N high-nitrogen austenitic stainless steel with a 25 µm thick dispersion-hardened near-surface layer intended for orthopedic applications. It was modified using a mechanical–thermal treatment (MTT) that included both friction processing and subsequent electron beam processing. The friction processing enabled the formation of a microstructure with a high dislocation density and strain twins, and it also initiated strain aging in the near-surface layer. At this stage, the hardening was achieved via the formation of CrN particles coherent to the matrix with the face-centered cubic (FCC) lattice and via the relaxation of internal stresses. After electron beam processing, the volume fraction of the nanodispersed phases increased. In the near-surface layer, a highly dispersed microstructure with a grain size of 3 μm, reinforced with CrN and Fe2N nanoparticles, was observed using transmission electron microscopy. The MTT increased the microhardness of the surface layer, and this contributed to the enhancement in both the H/E and H3/E2 ratios. This indicated an improvement in the crack resistance of the steel under frictional loads. The MTT also enhanced both the yield point (up to 580 MPa) and the wear resistance (by 50% to 100%, depending on the applied load) compared with those of the same steel after it had undergone quenching. In addition, the wear resistance was many times greater than that of the Ti-6Al-4V alloy typically used for manufacturing orthopedic implants. After the MTT, the properties of the near-surface layer of the steel indicated its suitability for biomedical applications.

Precipitation Behavior of the Second Phase in High-Nitrogen Austenitic Stainless Steel Alloyed with V and Nb

The retaining ring is one of the components with the highest stress on the rotor of the turbine generator. Due to the high content of alloying elements in this type of steel, it is extremely easy to precipitate a large amount of the second phase during the forming process of a material, which deteriorates the processability, Therefore, it is very important to study the second phase precipitation of high-nitrogen steel. This work addresses the precipitation behavior of the second phase in a high-nitrogen austenitic stainless steel alloyed with V and Nb. The optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffractometry were applied to investigate the influence of heat treatment parameters on the morphology and composition of the precipitated particles. The addition of V and Nb results in the formation of NbVN and VN particles, of which the amount is slightly decreased after a subsequent solution treatment at 1200 °C for 1 h. The results demonstrate that an additional aging treatment at 800~1000 °C for three leads to the participation of lamellar Cr2N and the undissolved MX phase. The content of the precipitates increases with increasing both the aging temperature and the aging time. The addition of V/Nb elements effectively improves the stability of MX phase, and the existence of high energy zones (the MX phase interface and grain boundary) promotes the early formation of Cr2N. Cr2N is shown to nucleate in the high energy zones, which thereafter grows into austenite grains, demonstrating a normal discontinuous cellular growth.

Study on thermal deformation behavior and microstructure evolution of P550 high nitrogen austenitic stainless steel

The high nitrogen austenitic stainless steel is widely used as power generation and geological exploration equipment materials because of its excellent strength, corrosion resistance and non-magnetic. In this paper, the mechanical behavior and microstructure evolution of P550 steel in the range of 900 °C–1200 °C and 0.001–10 s−1 deformation conditions were studied by physical and heat treatment simulations, metallographic observations and thermal processing maps. The results showed that the flow curves quickly reach the peak and then soften to a steady state, which indicates dynamic recrystallization (DRX) behavior. DRX is easy to occur when the deformation temperature is above 1080 °C. The activation energy of the forged P550 stainless steel was calculated as 519 kJ mol−1. There is a positive correlation between the peak stress, DRX critical stress, strain and Z value of the tested steel. The instability of the tested steel is easy to produce in the high strain rate region and low temperature region during hot working. Crack germinates and expands preferentially at the ‘necklace structure’ of inadequate dynamic recrystallization. Under the deformation state of 0.001 s−1, coarse crystals and mixed crystals are easily emerged during subsequent heat treatment. Combining the hot working map, the maximum deformation resistance and the grain evolution behavior during hot working and heat treatment, the suggested working window is T = 1020 °C–1200 °C and έ = 0.01–1 s−1.

Effect of heat input on microstructure and corrosion resistance of high-nitrogen austenitic stainless steel welded joints

In this study, the microstructure and corrosion resistance of high-nitrogen austenitic stainless steel welded joints with different heat inputs were investigated. A high heat input leads to a large width and depth of the fusion pool and a low fraction of δ-ferrite in the weld sea. However, a high fraction of Cr-rich precipitates is formed under a high heat input and low heat promotes the formation of δ-ferrite. Therefore, following corrosion tests, the weld seam suffers severe intergranular corrosion under high heat input and low heat input, and the corrosion resistance of the weld is the highest at a heat input of 6.8 kJ/cm. A high heat input results in a high fraction of Cr-rich precipitates and a random boundary in the heat affected zone (HAZ), which decreases corrosion resistance. Furthermore, corrosion test results indicate that the Cr-depleted area and random boundary are initially corroded, and high heat input decreases corrosion resistance of the HAZ.

Microstructure, mechanical and magnetic properties of a nickel-free austenitic stainless steel fabricated by hot powder forging

Nickel-free high nitrogen austenitic stainless steel, with a composition according to ASTM F 2581, was produced by hot powder forging (HPF). To improve the sintering behavior and increase the density of this alloy, a biocompatible additive of Mn-11.5 wt% Si at 3, and 6 wt% was added to the alloying powder, and the effect of the additives on the physical and mechanical properties of the alloy was investigated. The structure of the specimens was examined by X-ray diffraction (XRD), metallography, optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy(TEM), selected area diffraction (SAD) and magnetometer. The densities of the samples were analyzed by the Archimedes and image analysis methods and the mechanical properties of the samples were determined by microhardness, hardness, compression and tensile tests. The XRD results demonstrate that the forged samples contain austenitic nanocrystalline and amorphous phases with no ferrite and martensitic phases in the structure. Densitometry proves that the sample with a 6 wt% additive consolidated to near 96% theoretical density. Hardness, microhardness, tensile and compression tests show that this alloy composition also has better mechanical properties than alloys containing 0 and 3 wt% additive. The results show that stainless steel 316L and forged nickel-free high nitrogen austenitic stainless steel behave similarly to soft magnetic materials, although the magnetic saturation of nickel-free steel is lower than that of 316L steel. This study proves that the alloy made by HPF has significantly better mechanical properties than conventional commercial 316L. The effect of the additive on the microstructure and other features is discussed in detail.

Cavitation erosion-corrosion behaviour of Fe-19Cr-15Mn-0.66 N high nitrogen austenitic stainless steel in sodium chloride solution

In this work, the cavitation erosion (CE)-corrosion behaviors of high nitrogen austenitic stainless steel (HNSS) Fe-19Cr-15Mn-0.66 N were investigated under both static and CE conditions in 0.5 mol l−1 NaCl solution. For the sample after 8 h CE, the cumulative mass loss increased by 15% in 0.5 mol l−1 NaCl solution compared to distilled water, indicating that the corrosive media promoted mass loss. The corrosion behavior of the samples under static and CE conditions was evaluated using electrochemical techniques. The results showed that compared with the samples under static conditions, the samples under CE had a higher corrosion rate, and the corrosion resistance of the samples gradually decreased with the prolongation of C E time.The open circuit potential and potentiodynamic polarization tests showed a higher corrosion rate under CE conditions. The electrochemical impedance spectroscopy results revealed a decrease in corrosion resistance with increasing CE time. The XPS results showed that CE has a significant effect on the composition of the passive film, promoting a notable increase in Cr2O3 and Fe2O3.