High Nitrogen Austenitic Stainless Steel Research Articles

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.

Microstructural Evolution, Ferrite Content, and Grain Size Study in the Coalescence of High Nitrogen Austenitic Stainless Steel Thick Plates Using Cold Metal Transfer Welding Process

Microstructural Evolution, Ferrite Content, and Grain Size Study in the Coalescence of High Nitrogen Austenitic Stainless Steel Thick Plates Using Cold Metal Transfer Welding Process

Molecular dynamics simulation of Cr-N clusters formation in high nitrogen austenitic stainless steel

The Cr2N precipitation has a prominent effect to high nitrogen austenitic stainless steel (HNASS) performance. The formation of Cr-N clusters as precursor of Cr2N phase based on a model of Cr and N along grain boundaries under aging at 1023 K were simulated using LAMMPS molecular dynamics (MD) approach. The Cr and N atoms at austenite grain boundary can aggregate into tightly structured Cr-N clusters with atomic number from 19 to 169. The variation of calculated diffusion coefficients with time suggests Cr-N clusters formation is controlled by the diffusion of Cr, N and Cr-N cluster. Furthermore, the calculated radius of pair gyration of Cr-N clusters indicates that the structural features of Cr-N clusters at grain boundaries are shifted from a chaotic distribution to more orderly structure. The mechanism on Cr-N clusters revealed by this study may be of guiding significance to the optimization design of controlling Cr2N precipitation.

Low-Temperature Deformation and Fracture of Cr-Mn-N Stainless Steel: Tensile and Impact Bending Tests

The paper presents the results of tensile and impact bending tests of 17%Cr-19%Mn-0.53%N high-nitrogen austenitic stainless steel in temperatures ranging from −196 to 20 °C. The steel microstructure and fracture surfaces were investigated using transmission and scanning electron microscopes, as well as X-ray diffraction analysis. The steel experiences a ductile-to-brittle transition (DBT); however, it possessed high tensile and impact strength characteristics, as well as the ductile fracture behavior at temperatures down to −114 °C. The correspondence between γ–ε microstructure and fracture surface morphologies was revealed after the tensile test at the temperature of −196 °C. In this case, the transgranular brittle and layered fracture surface was induced by ε-martensite formation. Under the impact bending test at −196 °C, the brittle intergranular fracture occurred at the elastic deflection stage without significant plastic strains, which preceded a failure due to the high internal stresses localized at the boundaries of the austenite grains. The stresses were induced by: (i) segregation of nitrogen atoms at the grain boundaries and in the near-boundary regions, (ii) quenching stresses, and (iii) reducing fcc lattice volume with the test temperature decrease and incorporation of nitrogen atoms into fcc austenite lattice. Anisotropy of residual stresses was revealed. This was manifested in the localization of elastic deformations of the fcc lattice and, consequently, the stress localization in <100>-oriented grains; this is suggested to be the reason of brittle cleavage fracture.

Effect of Aging Temperature on Plastic Flow Behaviour and Toughness of Nickel Free High Nitrogen Austenitic Stainless Steels

In the present study, tensile properties, plastic flow behaviour and impact toughness of Nickel free High Nitrogen Austenitic Stainless Steels (NFHNSS) were evaluated under solution annealed and aged condition (at 700 oC, 800 oC and 900 oC for 14 hours). Plastic flow behaviour was analyzed using Holloman and Ludwigson flow equations. Samples in solution annealed and aged conditions exhibited a flow transition behaviour. Ludwigson flow equation produces best fit for flow transition behaviour. Transmission Electron Microscope (TEM) investigations of NFHNSS samples after tensile test revealed dislocation network and planar arrangement of dislocations adjancent to Grain Boundaries (GB) in the solution annealing and aged conditions respectively. Plastic deformation of NFHNSS occurs by a combination of planar glide and twinning. Impact energy was significantly higher in solution annealed condition than aged condition. Impact energy values decreased with increasing aging temperature. Precipitation and growth as well as morphology of Cr2N at the GBs reduce the impact energy values significantly. Presence of larger precipitates readily pull out from GB easily than smaller precipitates at low temperatures. Large precipitate, at the GB’s readily pull out at low temperature, compared to slower ones.

Influence of rare earth Ce on hot deformation behavior of as-cast Mn18Cr18N high nitrogen austenitic stainless steel

Influence of rare earth Ce on hot deformation behavior of as-cast Mn18Cr18N high nitrogen austenitic stainless steel

Effects of grain boundary character distribution on hydrogen-induced cracks initiation and propagation at different strain rates in a nickel-saving and high-nitrogen austenitic stainless steel

The effect of grain boundary character distribution (GBCD) on hydrogen-induced cracks (HICs) initiation and propagation in nickel-saving austenitic stainless steel was investigated at different strain rates with electrochemical hydrogen charging. The fracture strength and elongation of specimens decreased as the strain rates decreased due to more hydrogen atoms being transported by mobile dislocations. HICs are more likely to initiate at random grain boundaries (RGBs) because of poor deformation coordination of adjacent grains at RGBs after hydrogen charging. Special boundaries (SBs) deflected HICs and hindered the propagation of HICs owing to a more stable boundary structure and less strain localization during slow strain rate tensile (SSRT) deformation with hydrogen charging. Furthermore, highly efficient distributions of SBs contribute to great resistance to hydrogen embrittlement.

Corrosion Behaviors of Fe-22Cr-16Mn-0.55N High-Nitrogen Austenitic Stainless Steel in 3.5% NaCl Solution

In this study, the corrosion behavior of the high-nitrogen austenitic stainless steel (HNS) Fe-22Cr-16Mn-0.55N before and after solution treatment (ST) in 3.5% NaCl solution has been investigated. The effect of a solution temperature of 1100 °C and heat preservation for 30 min on the corrosion resistance and passive film protection of HNS steel was studied. Open-circuit potential, potentiodynamic polarization and electrochemical impedance tests were used to assess the corrosion resistance of treated and untreated steels. In addition, potentiostatic polarization and XPS techniques together with Mott–Schottky curves were applied to determine the composition and properties of the passive films. The results showed that after solution treatment, the grain size of HNS decreased and the grain became more uniform. Although corrosion occurred on HNSs both before and after treatment, solution treatment resulted in greater compactness in passive films, leading to lower carrier density, lower corrosion current density and better corrosion resistance.

On the strengthening mechanisms of high nitrogen austenitic stainless steels

The effect of microstructure and dispersion of secondary phases on tensile mechanical properties of two high nitrogen austenitic stainless steels subjected to annealing or rolling at 1000 °C was studied. The solid solution strengthening gave the main contribution to overall strength after annealing. Similar strength contribution was provided by the dislocation density after rolling that resulted in almost twofold increase in the yield strength.

Research on the pitting behavior of high nitrogen austenitic stainless steel 316LN in sodium chloride solution by using modified potentiostatic pulse test

• The modified potentiostatic pulse test is used to study the pitting behavior of 316LN. • The pitting size and number of 316LN are regulated. • The Si-enriched inclusions are initial sites of metastable pit on 316LN surface. • The pitting resistance of 316LN is enhanced after the potentiostatic pulse test. The pitting behavior of high nitrogen austenitic stainless steel 316LN was studied by using the modified potentiostatic pulse test (PPT). The results show that the duration of the high potential (E h ) and the low potential (E l ) significantly affect the pitting size and number of 316LN. However, the pitting behavior of 316LN only changes during the first five pulse cycles, because the metastable pitting initiates at the limited inclusions. The optimal PPT condition is regarded as E h = 0.5 V SCE for 1 s, E 1 = 0.2 V SCE for 2 s and ten pulse cycles. Metastable pits initiate at the susceptible sites of inclusions which are identified as SiO 2 and SiC. The metastable pits will further grow or restrain with the increasing time of E h or E l , respectively. After PPT modification, the pitting resistance of 316LN is enhanced due to the removal of the Si-enriched inclusions from the 316LN surface. Meanwhile, the fewer current transitions, the lower passivation current density and the higher impedance all suggest that the protectiveness of the oxidation film formed on 316LN surface is improved after PPT.