Ni-free Austenitic Stainless Steel Research Articles

A novel surface grain boundary engineering approach to improving corrosion resistance of a high-N and Ni-free austenitic stainless steel

Grain boundary engineering (GBE) can effectively improve the corrosion resistance of materials, but the traditional thermal-mechanical process still exhibits some limitations. In the present work, a novel surface spinning strengthening (3 S) technology followed by heat treatment was applied to modify the microstructure near the surface (∼ 400 μm) of a high-N Ni-free austenitic stainless steel. Under an optimal condition of annealing at 1323 K for 60 min after 3 S, the fraction of special boundaries mainly involving Σ3 boundaries in GBE layer was significantly improved from 48.3 % to 69.1 %, thus achieving excellent resistances to intergranular corrosion and stress corrosion cracking.

Powder bed fusion of high-Mn-N Ni-free austenitic stainless steel: achieving low porosity and high mechanical strength through process parameter selection

The emergence of innovative high-performance stainless steels is essential at the forefront of material science. This research highlights the meticulous development of a novel high-Mn-N Ni-free austenitic stainless steel via the powder bed fusion laser–based (PBF-LB) technique. We strategically optimized the laser parameters, achieving ultralow porosity and a refined microstructure with defects under 2%. Comprehensive analysis revealed superior dendritic cellular formations at melt pool boundaries, underlining our method’s precision. Unveiling the strength of this novel steel, sample S11 (scanning speed of 800 mm/s and laser power of 147 W) showed a remarkable tensile strength of 1190 ± 20 MPa and an impressive elongation to fracture of 35 ± 3%. Interestingly, twin formations became evident under external loads, enhancing mechanical resistance while preserving ductility. Advanced quantification methods were employed to ensure accuracy, especially for low atomic number elements, overcoming previous measurement constraints. This pioneering study introduces a game-changing austenitic Ni-free stainless steel enriched by Mn and N. It sets a new benchmark in material development and application, synergizing exceptional mechanical attributes with robust ductility.

Influence of Nitrogen Content on the Corrosion Behavior of Powder Metallurgy Nickel-Free Austenitic Stainless Steel

Herein, powder metallurgy gas-solid nitriding was used to prepare a high-nitrogen Ni-free austenitic stainless steel with more than 1.0 wt.% N. Study of the microstructures and corrosion resistances of the as-prepared samples revealed that the corrosion weight loss, size, number of corrosion pits, and corrosion depth decreased as the N content increased from 1.19 to 1.37 wt.%. The self-corrosion current densities of the alloys were determined from their potentiodynamic polarization curves. The changes in the capacitive arc radius as demonstrated by the Nyquist curves were consistent with the conclusions of the immersion corrosion analysis; that is, a high N content leads to a high corrosion resistance.

Effect of recrystallization on degree of sensitization in nickel free austenitic stainless steel

PurposeEffect of grain size on degree of sensitization (DOS) was been evaluated in Nickel free steel. Manganese and nitrogen contained alloy is a Ni-free austenitic stainless steels (ASS) having type 202 grade. The main purpose of this investigation is to find the effect of recrystallization on the DOS of stainless steel after the thermo-mechanical processing (cold work and thermal aging).Design/methodology/approachIn the present investigation, the deformation of 202 grade analyzed using X-ray diffraction (XRD) and microstructural testing. Optical microstructure of Ni-free ASS has been done for cold worked samples with thermally aged at 900°C_6 h. Double loop electrochemical potentiodynamic reactivation test used for findings of degree of sensitization.FindingsNi-free ASS appears to be deformed more rapidly due to its higher stacking fault energy which gave results in rapid transformation from strain induced martensite to austenite in form of recrystallized grains, i.e. it concluded that as cold work percentage increases more rapidly recrystallization occurs. XRD results also indicate that more fraction of martensite formed as percentage of CW increases but as thermal aging reverted those all martensite to austenite. So investigation gives the conclusion which suggests that with high deformation at higher temperature and duration gives very less DOS.Originality/valueVarious literatures available for 300 series steel related to the effect of cold work on mechanical properties and sensitization mechanism. However, no one has investigated the effect of recrystallization through thermomechanical processing on the sensitization of nickel-free steel.

Effect of grain refinement on sensitization of high manganese austenitic stainless steel

Effect of grain size on degree of sensitization (DOS) was been evaluated in Nickel free steel. Manganese and nitrogen contained alloy is a Ni free austenitic stainless steel (ASS) having type 202 grade. In the present investigation, the deformation of 202 grade analyzed using XRD and microstructural testing. Optical Microstructure of Ni free austenitic stainless steel (ASS) has been done for cold worked samples with thermally aged at 900 °C_6 Hrs. Ni free austenitic stainless steel (ASS) appears to be deformed more rapidly due to its higher stacking fault energy which gave results in rapid transformation from strain induced martensite to austenite in form of recrystallized grains, i.e., it concluded that as cold work percentage increases more rapidly recrystallization occurs. XRD results also indicate that more fraction of martensite formed as percentage of CW increases but as thermal ageing reverted those all martensite to austenite. Double Loop Electro-potentio Reactivation Method (DLEPR) test used for findings of degree of sensitization. So investigation gives the conclusion which suggest that with high deformation at higher temperature and duration gives very less degree of sensitization (DOS).

Effect of sintering temperature and solution treatment on microstructure and mechanical properties of high-N Ni-free austenitic stainless steel prepared by metal injection molding

High-N austenitic stainless steel was prepared by injection molding and sintering under nitrogen atmosphere by using gas atomized Cr-Mn-Mo-N duplex stainless steel powders. The powders and binder were mixed, injected, debinded and sintered. Effect of sintering temperature and solution treatment on microstructure and mechanical properties were studied. It shows that sintering temperature has significant effect on N content and density. With the increase of sintering temperature, the sample density was increased, but the N content was decreased. Samples sintered at 1200 °C was composed of austenite and Cr2N, while only austenite can be detected by XRD for samples sintered at 1250 and 1300 °C. The fracture mode of the samples sintered at 1200 and 1250 °C is brittle fracture, along the 45° direction and stress axis, but samples sintered at 1300 °C did not fractured at the maximum load (21 kN) and showed ductility during compression. After solution treatment, the Cr2N was disappeared and samples sintered at 1250 °C did not fractured during compression.

Gaseous hydrogen embrittlement of a Ni-free austenitic stainless steel containing 1 mass% nitrogen: Effects of nitrogen-enhanced dislocation planarity

We investigated the effect of hydrogen on degradation of tensile properties in a Fe–25Cr–1N austenitic stainless steel. Hydrogen was introduced by exposure to a hydrogen gas atmosphere at 100 MPa and 270 °C. Hydrogen charging caused significant ductility loss associated with nitrogen-enhanced dislocation planarity. Specifically, even without hydrogen, the nitrogen-enhanced planar dislocation glide induced micro-stress concentration, which assisted the occurrence of hydrogen-induced intergranular and quasi-cleavage fractures. The hydrogen-assisted intergranular cracking occurred along boundaries of grains where primary slip was predominantly activated. On the other hand, the hydrogen-assisted quasi-cleavage fracture took place when multiple slip systems were activated. The hydrogen-related cracks emerged, but their growth was arrested via crack blunting associated with a significant plastic deformation. Instead, new cracks formed near the crack tips. Therefore, hydrogen-assisted crack propagation occurred through repetition of crack blunting, initiation, and coalescence.

Microstructure, Mechanical, and Corrosion Properties of Ni-Free Austenitic Stainless Steel Prepared by Mechanical Alloying and HIPping.

An influence of the powder metallurgy route on the phase structure, mechanical properties, and corrosion resistance of Fe–18%Cr–12%Mn–N nickel-free austenitic stainless steel as a potential material for medical applications were studied. The powder was mechanically alloyed in a high purity nitrogen atmosphere for 90 h followed by Hot Isostatic Pressing at 1150 °C (1423 K) and heat treatment at 1175 °C (1423 K) for 1 h in a vacuum with furnace cooling and water quenching. More than 96% of theoretical density was obtained for the samples after Hot Isostatic Pressing that had a direct influence on the tensile strength of the tested samples (Ultimate Tensile Strength is 935 MPa) with the total elongation of 0.5%. Heat treatment did not affect the tensile strength of the tested material, however, an elongation was improved by up to 3.5%. Corrosion properties of the tested austenitic stainless steel in various stages of the manufacturing process were evaluated applying the anodic polarization measurements and compared with the austenitic 316LV stainless steel. In general, the heat treatment applied after Hot Isostatic Pressing improved the corrosion resistance. The Hot Isostatic Pressing sample shows dissolution, while heat treatment causes a passivity range, the noblest corrosion potential, and lower current density of this sample.

On the Reversion and Recrystallization of Austenite in the Interstitially Alloyed Ni-Free Nano/Ultrafine Grained Austenitic Stainless Steels

The martensite reversion treatment was conducted on two grades of Ni-free austenitic stainless steels interstitially alloyed with C and N. The hot rolled sheets of steels were cold rolled up to 80% thickness reduction to acquire strain-induced α′-martensite and subsequently reversion annealed at temperatures from 700 to 850 °C for 1 to 1000 s to revert the α′-martensite to austenite. Microstructural evolution was investigated using optical microscopy, X-ray diffraction, electron backscatter diffraction, and magnetic measurement techniques. Mechanical properties were measured using tensile tests at room temperature. The resultant microstructures contained both reverted and recrystallized austenite when reverted at 700 and 750 °C with the annealing time less than 100 s. A nonuniform grain structure was characterized under these conditions consisting of nano/ultrafine grains formed via α′-martensite reversion and coarser grains by recrystallization of the retained austenite. However, a more uniform austenite grain size with average size of 1 μm was obtained at 850 °C for 1000 s. The specimens having nonuniform grain structure exhibited excellent combinations of strength and ductility. A variety of mechanical properties was achieved depending on the annealing condition. The work hardening behavior affected UTS and ductility of the studied steels. The shift of the work hardening peaks to the higher strains was found suitable for ductility. Addition of C to N-containing Ni-free steels deteriorated mechanical properties. Best combination of strength and elongation was obtained in the test material with lower C/N ratio.

On the Hot Deformation Behavior of a Ni-Free Austenitic Stainless Steel Interstitially Alloyed with Low Nitrogen Content

Hot deformation behavior of a Ni-free, Fe-17Cr-12Mn-0.28N-0.06C (wt.%) austenitic stainless steel, hereinafter coded as FeCrMnN, was investigated using hot compression tests conducted under different deformation conditions comprising temperature and strain rate ranges of 800-1200 °C and 0.01-10 s−1, respectively. While the hot deformation at high strain rate and low temperatures (e.g., 10 s−1 and 800 or 900 °C) showed essentially dynamic recovery, resulting in a pancake-shaped microstructure, most of the other conditions exhibited occurrence of dynamic recrystallization (DRX). Increasing deformation temperature and decreasing strain rate showed a decrease in the critical stress and strain for initiating DRX. In general, DRX resulted in extensive microstructural reconstitution and grain refinement. For instance, hot deformation at 1000 °C/0.01 s−1 resulted in a fully recrystallized fine-grained microstructure with an average grain size of about 15 μm in comparison with the initial grain size of 60 µm. Increasing the temperature enhanced grain growth, but an increase in strain rate resulted in a finer grain structure. The amount of delta ferrite in the present steel varied under different conditions of deformation such that the lowest amount of delta ferrite (about 4.5%) was observed at 1000 °C. The activation energy of deformation (Qdef) for the present FeCrMnN steel with the initial grain size of 60 µm was estimated to be about 502 kJ/mol, which is higher than that of the conventional austenitic stainless steels.

Microstructure and Properties of Porous High-N Ni-Free Austenitic Stainless Steel Fabricated by Powder Metallurgical Route.

Porous high-N Ni-free austenitic stainless steel was fabricated by a powder metallurgical route. The microstructure and properties of the prepared porous austenitic stainless steel were studied. Results reveal that the duplex stainless steel transforms into austenitic stainless steel after nitridation sintering for 2 h. The prepared high-N stainless steel consists of γ-Fe matrix and FCC structured CrN. Worm-shaped and granular-shaped CrN precipitates were observed in the prepared materials. The orientation relationship between CrN and austenite matrix is [011]CrN//[011]γ and (-1-11)CrN//(1-11)γ. Results show that the as-fabricated porous high-nitrogen austenitic stainless steel features a higher mechanical property than common stainless steel foam. Both compressive strength and Young’s modulus decrease with an increase in porosity. The 3D morphology of the prepared porous materials presents good pore connectivity. The prepared porous high-N Ni-free austenitic stainless steel has superior pore connectivity, a good combination of compressive strength and ductility, and low elastic modulus, which makes this porous high-N Ni-free austenitic stainless steel very attractive for metal foam applications.

Influence of the PM-Processing Route and Nitrogen Content on the Properties of Ni-Free Austenitic Stainless Steel

Abstract Ni-free austenitic steels alloyed with Cr and Mn are an alternative to conventional Ni-containing steels. Nitrogen alloying of these steel grades is beneficial for several reasons such as increased strength and corrosion resistance. Low solubility in liquid and δ-ferrite restricts the maximal N-content that can be achieved via conventional metallurgy. Higher contents can be alloyed by powder-metallurgical (PM) production via gas–solid interaction. The performance of sintered parts is determined by appropriate sintering parameters. Three major PM-processing routes, hot isostatic pressing, supersolidus liquid phase sintering (SLPS), and solid-state sintering, were performed to study the influence of PM-processing route and N-content on densification, fracture, and mechanical properties. Sintering routes are designed with the assistance of thermodynamic calculations, differential thermal analysis, and residual gas analysis. Fracture surfaces were studied by X-ray photoelectron spectroscopy, secondary electron microscopy, and energy dispersive X-ray spectroscopy. Tensile tests and X-ray diffraction were performed to study mechanical properties and austenite stability. This study demonstrates that SLPS process reaches high densification of the high-Mn-containing powder material while the desired N-contents were successfully alloyed via gas–solid interaction. Produced specimens show tensile strengths >1000 MPa combined with strain to fracture of 60 pct and thus overcome the other tested production routes as well as conventional stainless austenitic or martensitic grades.

Design of a new Ni-free austenitic stainless steel with unique ultrahigh strength-high ductility synergy

A conceptual approach was used to design a new Ni-free austenitic stainless steel with a unique combination of ultrahigh strength and ductility. The concept was based on the alloying of the 0.05C–18Cr–12Mn (wt.%) steel by 0.39%N and heavy warm rolling (84% reduction) at 1173K (900°C) to achieve the yield strength of minimum 1GPa and high tensile strength and elongation due to a proper stability of the austenite as a result of the optimized stacking fault energy (SFE). The yield strength of 1010MPa, tensile strength of 1150MPa and high fracture strain of 70% were measured for the steel designed. Dislocation and solid solution hardening mechanisms are introduced as the main contributors for the ultrahigh yield strength of the steel. The strain hardening is gradual and the hardening rate reaches a high level of ∼2400MPa at a high true strain of 40% due to slow α′-martensitic transformation and mechanical twinning. Consequently, the ductility of the designed steel is excellent.

Decrease in the Brittle-to-ductile Transition Temperature in Cu Added Nickel-free Austenitic Stainless Steels

Brittle-to-ductile transition (BDT) in Cu added Ni-free austenitic stainless steel was investigated. Temperature dependence of apparent fracture toughness was measured using four-point bending tests, indicating that the BDT temperature was decreased with the increase in the Cu content. The activation energy was obtained from the deformation rate dependence of BDT temperatures. It was found that the values of the activation energy was decreased with the increase in the Cu content, suggesting that the dislocation mobility in austenitic stainless steels was increased by Cu addition. The increase in the dislocation mobility induces the decrease in the BDT temperature. The values of the activation energy are deviated from the regression line drawn on the data which obtained from the materials with high Peierls potentials. Temperature dependence of 0.2% proof stress indicated that the effective stress was nearly independent from the Cu content while the values of activation volume were decreased with the increase in the Cu content. A model for dislocation glide was proposed to explain the both decrease in the activation energy and the activation volume with the increase in the Cu content.

Effect of annealing temperature on nano/ultrafine grain of Ni-free austenitic stainless steel

In the present work, martensite thermomechanical treatment was employed to refine the microstructure of a metastable Ni-free austenitic stainless steel down to nano/ultrafine size. For this purpose, the studied alloy was cold rolled to about 80% reduction followed by reversion annealing at 973K (700°C)–1273K (1000°C) for 1–104s. It was observed that the finest austenitic microstructure is achieved in the sample reversion annealed at 1173K (900°C) for 100s. It had a bimodal microstructure composed of nano and ultrafine grains with an average grain size of about 240±117nm. Also, formation of nano-size (Cr,Fe,Mn)-rich carbonitrides was observed in the microstructure of this sample. The reversion mechanism was determined to be shear-type. The nano/ultrafine grained sample exhibited an ultrahigh strength (yield strength of 1150MPa and tensile strength of 1293MPa) and a good elongation of 21%. It was shown that grain refining down to nano/ultrafine range promotes deformation-induced martensitic transformation, which can improve the ductility of the alloy.

Comparative Evaluation of 316L and N-Bearing Ni-Free Austenitic Stainless Steel as a Potential Cost-Effective Replacement for Body Implants

Currently austenitic stainless steels, cobalt-chromium alloys and titanium alloys are used in body implants. As per ISO 5832-1, Cr-Ni-Mo alloy 316L with minimum 13% nickel is widely used for body implants. ASTM standard F 2229-07 also permits nitrogen strengthened essentially Ni-free Cr-Mn-Mo alloy (UNS S 29108) for this purpose. Nitrogen as austenite stabilizer is able to substitute nickel. It serves the dual purpose of increasing the strength as well as pitting corrosion resistance. This paper compares the corrosion behaviour of these two grades. Cyclic potentiodynamic tests were carried as per ASTM F2129 in Simulated Body Fluids (SBFs) like Ringers, Hanks and Phosphaste Buffer Saline solution at 37 °C, which corresponds to the human body temperature. The pitting potential was significantly higher for Ni free grade S29108 as compared to 316L. In addition, re-passivation potential of the S 29108 was also far superior than 316L. The reverse scan indicated that the breakdown of the passive film was not reached in S 29108, whereas a hysteresis loop was observed in 316L. The strength of annealed S 29108 is far superior and meets the property requirement of ISO 5832-1 for 316L under cold rolled conditions. Thus this alloy could replace annealed as well as cold rolled 316L as per ISO 5832-1. This promising alloy has an added advantage of being significantly cheaper as compared to 316L and other Ti, Co based alloys to enable cost effective medical care to common man. Keywords: High nitrogen stainless steel, 316L, Bio-Implants, Potentio-dynamic tests.

Nanostructured Nickel-Free Austenitic Stainless Steel/Hydroxyapatite Composites

In this work Ni-free austenitic stainless steels with nanostructure and their nanocomposites with hydroxyapatite are presented and characterized by means of X-ray diffraction and optical profiling. The samples were synthesized by mechanical alloying, heat treatment and nitriding of elemental microcrystalline powders with addition of hydroxyapatite (HA). In our work we wanted to introduce into stainless steel hydroxyapatite ceramics that have been intensively studied for bone repair and replacement applications. Such applications were chosen because of their high biocompatibility and ability to bond to bone. Since nickel-free austenitic stainless steels seem to have better mechanical properties, corrosion resistance and biocompatibility compared to 316L stainless steels, it is possible that composite made of this steel and HA could improve properties, as well. Mechanical alloying and nitriding are very effective technologies to improve the corrosion resistance of stainless steel. Similar process in case of nanocomposites of stainless steel with hydroxyapatite helps achieve even better mechanical properties and corrosion resistance. Hence nanocrystalline nickel-free stainless steels and nickel-free stainless steel/hydroxyapatite nanocomposites could be promising bionanomaterials for use as a hard tissue replacement implants, e.g., orthopedic implants. In such application, the surface roughness and more specifically the surface topography influences the proliferation of cells (e.g., osteoblasts).

Nanostructured nickel-free austenitic stainless steel composites with different content of hydroxyapatite

The aim of this work is to show that Ni-free austenitic stainless steels with nanostructure and their nanocomposites with hydroxyapatite can be synthesized by mechanical alloying, heat treatment and nitriding of elemental microcrystalline powders with addition of hydroxyapatite (HA). Hydroxyapatite was introduced into stainless steel because it is intensively studied for bone repair and replacement applications. Nickel-free austenitic stainless steels seem to have better mechanical properties, corrosion resistance and biocompatibility compared to 316L stainless steels. Therefore it's combination with hydroxyapatite that has high biocompatibility and ability to bond to bone could have improved properties, as well.To confirm nanocrystalline structure of obtained material and reveal topographical features of the surface, small-angle X-ray analysis (SAXS) and atomic force microscopy (AFM) were used. Results are consistent and the mean grain size of the obtained materials do not exceed 100nm.

Corrosion behaviour of carbon S-phase created on Ni-free biomedical stainless steel

Following the success of forming a carbon S-phase (expanded austenite) surface layer on medical grade Ni-free austenitic stainless steel by DC plasma carburising, the established commercial carburising process Kolsterising® was performed on both Ni-containing (AISI 304) and Ni-free austenitic stainless steels. While the Ni-containing stainless steel responded very well to Kolsterising®, the Ni-free alloy did not. The carbon absorption and the hardness of the Kolsterised® Ni-free alloy are inferior to Kolsterised® AISI 304 Ni-containing stainless steel, however, the hardness of the untreated Ni-free alloy was doubled by Kolsterising®. The response of both Kolsterised® Ni-free and Ni-containing alloys to pitting, crevice corrosion and intergranular corrosion resistance was similar. From this work it can be concluded that the Kolsterised® austenitic stainless steels do not suffer from intergranular corrosion but are susceptible to intragranular pitting when tested in boiling sulphuric acid and copper sulphate solution. It was also observed that Kolsterising® improves significantly the pitting and crevice corrosion resistance of the alloys used in this study.

Corrosion Resistance of Nickel-Free Austenitic Stainless Steels and their Nanocomposites with Hydroxyapatite in Ringer's Solution

In this work Ni-free austenitic stainless steels with nanostructure and their nanocomposites were synthesized by mechanical alloying (MA), heat treatment and nitriding of elemental microcrystalline Fe, Cr, Mn and Mo powders with addition of hydroxyapatite (HA). Microhardness and corrosion tests' results of obtained materials are presented. Mechanical alloying and nitriding are very effective technologies to improve the corrosion resistance of stainless steel. Decreasing the corrosion current density is a distinct advantage for prevention of ion release and it leads to better cytocompatibility. Similar process in case of nanocomposites of stainless steel with hydroxyapatite helps achieve even better mechanical properties and corrosion resistance. Hence nanocrystalline nickel-free stainless steels and nickel-free stainless steel/hydroxyapatite nanocomposites could be promising bionanomaterials for use as a hard tissue replacement implants, e.g. orthopedic implants.