Nitrogen Stainless Steel Research Articles

Unveiling the effect of bovine serum albumin on the corrosion resistance of high nitrogen stainless steel for cardiac stents

Herein, the effect of bovine serum albumin (BSA) on the corrosion resistance of high nitrogen stainless steel (HNS) is systematically investigated based on experimental characterizations and first-principle calculations. Electrochemical measurements firstly prove that the anodic dissolution rate of HNS has significantly increased due to BSA adsorption and the potential deterioration of the passive film. By subsequent morphology and composition characterizations, it is demonstrated that BSA forms a 1.9 μm porous adsorption layer on HNS surface via the peptide bond mostly in -helix and -turn structure. Furthermore, Mott-Schottky tests confirm that the carrier (cation vacancy) density in the passive film has dramatically risen from 6.29×1021 cm-3 to 1.97×1022 cm-3 after BSA adsorption, which could be ascribed to the detachment of Cr atom from its original lattice due to the preference interaction between BSA molecule and Cr atom. Most importantly, such an increased defects will further promote the dissolution process of both the passive film and HNS matrix according to point defect model (PDM), which will definitely undermine the protective ability of the passive film and eventually result in the declined corrosion resistance of HNS in BSA-contained solution.

Effect of Solution Treatment Temperature on Microstructure, Mechanical Properties and Corrosion Resistance of Ultra-High Nitrogen Stainless Steel

Effect of Solution Treatment Temperature on Microstructure, Mechanical Properties and Corrosion Resistance of Ultra-High Nitrogen Stainless Steel

Mechanical properties of a novel high nitrogen and low nickel stainless steel S35657

Stainless steel exhibits excellent ductility and corrosion resistance, making it highly promising for applications in civil engineering. In order to investigate the mechanical properties of a novel stainless steel S35657, a total of 150 coupons including 125 plates and 25 rods were tested. The failure modes, stress-strain curves, and mechanical parameters of coupons were obtained. Fractographic analysis using scanning electron microscopy was also performed to investigate the failure mechanisms. It was found that all coupons exhibited ductile failure mode with elongation up to 52.08%. The nominal yield strength standard value reached 370.50 MPa, while the ultimate tensile strength standard value reached 736.07 MPa. The various existing typical constitutive models were assessed, indicating that these models provide a good fit in the elastic stage but are not applicable in the hardening stage. Based on the Ramberg-Osgood model and test results, a new constitutive model applicable to stainless steel S35657 was proposed. Moreover, a design reliability analysis was carried out using the Equivalent Normalization Method (JC method), and partial factors for resistance of stainless steel S35657 were calibrated, which were 1.05 under windless load combinations and 1.15 under windy combinations.

New insights of the nucleation and subsequent phase transformation in duplex stainless steel

The solidification behavior, especially involving multi-phase, of duplex stainless steel (DSS) directly determines its microstructures. However, thermodynamic calculations and other reports, which indicates that δ-ferrite is always thermodynamically stable as the primary phase, cannot precisely describe the nucleation and subsequent phase transformations in DSS. Both high temperature confocal microscopy (HTCM) and differential thermal analysis (DTA) are well-established tools for studying solidification behaviors and phase transformation. Here, an in-house build apparatus was employed to investigate the solidification process of DSS. It enables contemporaneous in-situ observation of the surface and thermal analysis of the bulk thermal events. Meanwhile, N2 and Ar are used as protective gas. Dual-peak DTA signals of solidification were recorded under an N2 atmosphere. Furthermore, through the real-time comparison between the DTA signals and the in-situ observations, a solid-state phase transition was accurately located. We propose the following new insight into the solidification mechanism for DSS, by combining experimental results, thermodynamic arguments (Gibbs free energy), and microstructural characterization using electron back scattered diffraction (EBSD): The γ nucleates and grows via a competitive nucleation process, and completely transforms into δ-ferrite in a solid-state phase transformation. After that, part of the δ-ferrite transforms into γ, and the duplex structure remains (L → γ → δ → δ + γ). Heterogeneous nucleation of γ on the liquid surface was also observed, which is quite different from the nucleation and growth pattern of δ-ferrite. These new insights contribute to the improvement of solidification mechanisms of high nitrogen stainless steels and, in turn, microstructures and physical properties of steels in the course of materials production processes.

Martensitic transformation induced planar deformation of AlN nanoprecipitates in high nitrogen stainless steels

Boosting the applications of high nitrogen steels requires delicate control of detrimental brittle AlN precipitates which often initiate cracking and thus degrade overall mechanical performance. Here we report that stacking faults in nano-sized AlN precipitates can be activated by martensitic transformation during quenching in a high nitrogen martensitic stainless bearing steel. Atomic-scale structure and strain analyses illuminate that the planar deformation mode, which is rarely seen in inorganic AlN ceramic precipitates, is stimulated by the martensitic transformation of the steel matrix to accommodate the transformation stresses and strains at the precipitate/matrix interfaces. The critical resolved shear stresses derived from the density functional theory study are consistent with the stress arising from the martensitic transformation obtained from the phenomenological theory of martensitic crystallography combined with phase field modeling, further validating the martensitic transformation induced plastic deformation in AlN nanoprecipitates. Our finding paves a new way to mitigate the harmful effect of AlN precipitates in high nitrogen steels and enables the production of high-quality high nitrogen steels with an upper limit of Al content raised to the normal steelmaking level, which offers new guidelines for effectively reducing the production costs of high nitrogen steels.

Sintering optimization of high nitrogen nickel free austenitic stainless steel prepared by Metal Injection Molding

High nitrogen and nickel free austenitic (HNNFA) stainless steel prepared by powder metallurgy has become a research hotspot because of solving nickel precipitation and saving cost. In this paper, metal injection molding (MIM)method was used to prepare such kinds of steel. The effect of sintering parameters such as sintering temperature, holding time, and nitriding pressure on the density, shrinkage rate, and nitrogen content of austenitic stainless steel was systematically evaluated by an orthogonal experiment to obtain the optimum sintering parameters. When the sintering temperature is 1200°C, the holding time is 6 h, and the nitriding pressure is 1100 mbar, the sample possesses a higher density (7.52g/cm3) and nitrogen content (0.76%). The matrix of stainless steel is austenite after nitriding sintering.

Enhancement of microbiologically influenced corrosion resistance of copper-containing nickel-free high nitrogen stainless steel against marine corrosive Pseudomonas aeruginosa

The antibacterial performance and marine microbiologically influenced corrosion (MIC) resistance of a novel copper-containing nickel-free high‑nitrogen stainless steel (HNS) against marine Pseudomonas aeruginosa were studied. The electrochemical tests indicated that the copper content (0, 2 and 4 wt%) would hardly affect the corrosion resistance of HNS in abiotic simulated seawater. In the presence of bacteria, HNS-4Cu showed the best MIC resistance, while MIC rates of HNS-0Cu and HNS-2Cu were comparatively faster. Addition of 4 wt% copper significantly improved the antibacterial effect and the quality of the passive film of HNS, achieving a better MIC resistance.

Thermomechanical Simulation of Heat-Affected Zones in Nickel-Free High Nitrogen Stainless Steel: Microstructural Evolution and Mechanical Property Studies

Three different Heat Affected Zones (HAZ) in hot rolled Nickel Free High Nitrogen Stainless Steels (NFHNSS) based on three different peak temperatures were physically simulated using Gleeble Simulator to investigate microstructural evolution and structure-property correlation. Optical microscopy revealed that the austenite grains are recrystallized in the simulated heat affected zone in the peak temperature range of 750 oC to 1050 oC. Extent of recrystallization of grains and nucleation of precipitates varied with peak temperatures. TEM characterization showed the presence of Cr2N precipitate having an average particle size in the range of 300 nm to 395 nm in the simulated HAZ were confirmed by Selected Area Electron Diffraction (SAED) analysis. Precipitation kinetics of Cr2N were simulated using Thermo-Calc were found to correlate well with experimental values. Mechanical properties of specimens taken from three different HAZ were evaluated for tensile strength and hardness. Variation in strength of the different specimens has been discussed using various strengthening models. Fractography analysis was also carried out to understand the effect of peak temperature on fracture behaviour. Transition in fracture patterns in NFHNSS from ductile to mixed mode was observed for different specimens.

Dissimilar welding of high nitrogen stainless steel and low alloy high strength steel under different shielding gas composition: Process, microstructure and mechanical properties

Ar–N2-O2 ternary shielding gas is employed in dissimilar welding between high nitrogen steel and low alloy steel. The effect of O2 and N2 is investigated based on the systematical analysis of the metal transfer, nitrogen escape phenomenon, weld appearance, nondestructive detection, nitrogen content distribution, microstructure and mechanical properties. There are two nitrogen sources of the nitrogen in the weld: high nitrogen base material and shielding gas. The effect of shielding gas is mainly reflected in these two aspects. The change of the droplet transfer mode affects the fusion ratio, N2 in the shielding gas can increase nitrogen content and promote the nitrogen uniform distribution. The addition of 2% O2 to Ar matrix can change the metal transfer from globular transfer to spray transfer, high nitrogen base material is thereby dissolved more to the molten pool, making nitrogen content increase, ferrite decrease and the mechanical properties improve. When applying N2-containing shielding gas, arc stability becomes poor and short-circuiting transfer frequency increases due to the nitrogen escape from droplets and the molten pool. Performance of the joints is improved with N2 increasing, but internal gas pores are easier to appear because of the poor capacity of low alloy steel to dissolve nitrogen, The generation of pores will greatly reduce the impact resistance. 4–8% N2 content in shielding gas is recommended in this study considering the integrated properties of the dissimilar welded joint.

A Promising Pressurized Duplex Manufacturing Route of High Nitrogen Stainless Steel

By comparing the advantages and disadvantages of the existing single‐step pressurized metallurgical manufacturing technologies of high nitrogen stainless steel (HNS), a promising pressurized duplex manufacturing route, i.e., pressurized induction melting (PIM) and pressurized electroslag remelting (PESR), is proposed. The PIM stage mainly carries on the nitrogen alloying, deoxidation and desulfurization, and the PESR process mainly takes responsibility for further desulfurization, removal of large‐size inclusion, and elevating solidification quality. During PIM, the deoxidation is achieved by vacuum carbon deoxidation combined with magnesium and rare‐earth compound treatment. Subsequently, the PESR is conducted under high nitrogen pressure without adding nitrided ferroalloys. To precisely control the nitrogen content and avoid segregation and escape of nitrogen, the three‐stage nitrogen pressure collocation is developed. The high nitrogen stainless bearing steel 30Cr15Mo1N (DIN 1.4108) is manufactured as an example by the duplex route, and a PIM–PESR ingot with high nitrogen content, high cleanliness, fine and dispersive inclusions, and compact solidification structure is obtained. Moreover, the steel possesses a homogeneous microstructure, slightly better mechanical properties, and corrosion resistance compared with that manufactured by the single‐step PESR process. Finally, a pressurized ladle refining and PESR duplex route are proposed for large‐scale industrial production of HNSs.

Reaction Mechanism and Control Strategy of Aluminum Increase in High Nitrogen Stainless Bearing Steel During Pressurized Electroslag Remelting

Reaction Mechanism and Control Strategy of Aluminum Increase in High Nitrogen Stainless Bearing Steel During Pressurized Electroslag Remelting

A review of progress on high nitrogen austenitic stainless-steel research

High nitrogen austenitic stainless steels are commonly used in wide range of applications because of their excellent properties, attracting super attention over the past decades. Compared with other metal materials, high nitrogen austenitic stainless steel increases the nitrogen content under the premise that the structure is austenite, giving it excellent mechanical properties and corrosion resistance. Based on relevant documents from the past ten years, this article summarizes and compares three preparation methods for high nitrogen austenitic stainless steels, namely: powder nitriding, melt nitriding and bulk nitriding. They can be divided into six categories according to other differences as explained by the latest research progress on strengthening and toughening mechanism for high nitrogen austenitic stainless steels: composite structure strengthening, fine grain strengthening, precipitation strengthening and strain strengthening. This article also reviews the research progress on excellent properties of high nitrogen stainless steel, including strength, hardness and corrosion resistance. It further describes the emerging nickel-free high nitrogen austenitic stainless steels and its biocompatibility. Welding applications of high nitrogen austenitic stainless steels are also described from three aspects: friction stir welding, arc welding and brass solder. Finally, this article puts forward the development direction of high nitrogen austenitic stainless steels in the future.

Cleanliness Control of High Nitrogen Stainless Bearing Steel by Vacuum Carbon Deoxidation in a PVIM Furnace

Cleanliness Control of High Nitrogen Stainless Bearing Steel by Vacuum Carbon Deoxidation in a PVIM Furnace

Inhibition of Nitride Precipitation in a Fe-Cr-Mn-Mo-N High-Nitrogen Austenitic Stainless Steel through Grain Boundary Character Distribution Optimization

The precipitation behavior of the second phase in Fe-18Cr-17Mn-2Mo-0.85N high nitrogen and nickel-free austenitic stainless steel after grain boundary character distribution (GBCD) optimization was investigated. The results show that the fraction of low Σ coincidence site lattice (CSL) boundaries increases from 47.9% for the solid solution treated specimen to 82.25% for the specimen cold-rolled by 5% and then annealed at 1423 K for 72 h (r5%-a1423 K/72 h). Compared with the solution treated sample, nitride precipitation is obviously restrained and the fraction of precipitates is the least in r5%-a1423 K/72 h sample with the highest proportion of SBs. The appearance of a high proportion of CSL boundaries with low energy inhibits the nitrides precipitation through GBCD optimization.

A Proposition to Standardize the Microstructural Grain Size Measurements of Hip Stems

Abstract Femoral stem fractures in total hip arthroplasty (THA) are a problem in clinical practice that results in great morbidity and high cost of revision hip surgery. Stem fractures are multifactorial events that are usually related to a combination of factors that increase the mechanical stress on the stem or decrease the mechanical strength of the implant. Failure analyses of hip prosthesis have identified that the presence of inadequate grain size may lead to implant failure. The aim of this article is to develop a rational to set specific sites to perform grain size measurements along stems used in THA as well as appropriate procedures to evaluate the heterogeneity of the microstructure related to the grain size distribution. In the present study, nonmodular femoral stems from three manufacturers with different wrought materials were chosen: stainless steel ISO 5832-1 (Manufacturer I), high nitrogen stainless steel ISO 5832-9 (Manufacturer II), and cobalt-chromium-molybdenum alloy ISO 5832-12 (Manufacturer III). The results of this study showed a great variability of grain size number depending on the cross section and fields evaluated. Therefore, the current technical standards for evaluating THA stems need to be modified. Analyses of grain sizes at different cross sections and inside each cross section of the stem is necessary to ensure the safety of hip stems.

Microstructure and mechanical properties of flash butt welding joint of high nitrogen austenitic stainless steel

The microstructure and properties of high nitrogen stainless steel welded joints under different flash currents were studied by flash butt welding. The results show that, the flash welding parameters have obvious influence on the joint structure and mechanical properties. The joint is organized into symmetrical distribution, and the grains obtained in the interface area are small, mostly pearlite and ferrite. With the increase of flash current, grain coarsening in overheated zone (OZ) and welding zone (WZ) of joint, which was mainly attributed to the increase of welding heat input. Martensite exists on both sides of the weld, in which the hardness reaches the peak. The tensile strength of welded parts can reach up to 97% of the base material, and the fracture is in the form of ductile fracture.

Study on Fatigue Behaviors of 0Cr21Mn17Mo2N0.83 High Nitrogen Stainless Steels

High cycle fatigue behaviors of 0Cr21Mn17Mo2N0.83 high nitrogen stainless steels at forged and solid solution state were investigated. High cycle fatigue tests were carried out up to 107cycles at a stress ratio R=0.1 and frequency of 70Hz on specimens using a high frequency fatigue machine. Fatigue fracture surfaces of specimens that in the high cycle fatigue tests were observed using a scanning electron microscope for revealing the micro-mechanisms of fatigue crack initiation and propagation. The results showed that the fatigue limit of test alloys at room temperature is 865.25 MPa (as-forged alloy) and 736.10MPa (solid solution alloy), respectively. The micro-fatigue fracture surface of the test alloys included three representative regions. These regions are fatigue initiation area, fatigue crack propagation area and fatigue fracture area. Fatigue cracks of the test alloys initiate principally at the precipitates, inclusion or uneven stress concentration sites of alloy surface, and propagate along the grain boundary. The fatigue striations of fatigue crack propagation area are very clear. The fatigue fracture of test specimens show the rupture characteristics of quasi cleavage and dimple fracture. The room temperature fatigue properties of as-forged alloy are generally higher than that of the solid solution high nitrogen stainless steel according to the S-N curves fitting results.

Design and evaluation of nitrogen-rich welding wires for high nitrogen stainless steel

Three nitrogen-rich welding wires with nitrogen content of 0.15 %, 0.6 % and 0.9 % for high nitrogen stainless steel welding were developed by a modified Schaeffler diagram and equilibrium phase diagram calculation. The effect of the chemical compositions especially the nitrogen on the weld defects, element distribution, microstructure, mechanical properties and droplet transfer were studied to evaluate the performance of the welding wires. Nitrogen element in the droplets of nitrogen-rich welding wires was partially lost due to the escape of nitrogen during the welding process, so nitrogen in the welding materials was not completely transported to the weld as designed, resulting in a deviation of actual weld from the design. The joint of the welding wire with 0.6 % nitrogen content had the highest mechanical properties, the tensile strength reached 912.5 MPa, and the impact energy of the weld reached 138.17 J. Full-austenite weld was only obtained in the weld of the wire with 0.15 % nitrogen content, but the tensile strength of the joint was lowest because of the low nitrogen content in the weld. Welding wire with 0.9 % nitrogen content had the most serious nitrogen loss, a few of pores were found in the weld. Moreover, the content of skeleton ferrite in the weld with the welding wire containing 0.9 % nitrogen content was high, resulting in the decrease of the weld impact energy.

Optimization of shielding gas composition in high nitrogen stainless steel gas metal arc welding

Abstract High nitrogen stainless steel has extensive applied foreground in industries. But the weldability limits the use of the steel. The weld without nitrogen can become the weakness of the joint in corrosion resistance and strength. In this study, response surface methodology was applied to optimize the Ar-N2-CO2 ternary shielding gas for a nitrogen-containing filler metal in high nitrogen stainless welding. The influence of the proportion of N2 and CO2 on the nitrogen content, the impact energy and the tensile strength were investigated by the statistical regression models. The results show that the tensile strength, nitrogen content and impact energy increase and then decrease with the increasing of CO2, which indicates that CO2 content should not be too high. N2 addition can increase the nitrogen content of the weld obviously. But the impact energy decreases when N2 content exceeds about 7%. Integrating the mathematic models of the three performances, the optimal shielding gas compositions were determined to be 87 %Ar-6.5 %N2-6.5 %CO2. With this optimal shielding gas, the tensile strength and impact energy reached 956.7 MPa and 166.8 J, respectively. The deviation between the experimental value and the predicted value was below 2%.

Effect of grain ultra-refinement on corrosion behavior of ultra-high strength high nitrogen stainless steel

High-performance high nitrogen stainless steel (HNS) with ultra-high strength up to ∼1.7 GPa and excellent corrosion resistance was successfully obtained by friction stir processing. The simultaneous enhancement in strength and corrosion resistance was mainly attributed to the grain ultra-refinement. Uniform ultrafine-grained microstructure (∼225 nm) significantly improved the repassivation behavior of HNS. With significantly increased element diffusion rate and element content, the passive film formed on the ultrafine-grained HNS showed high resistance to the nucleation and growth of pits. Grain ultra-refinement was confirmed to be an effectual method for improving corrosion resistance of HNS without sacrificing strength.