High Nitrogen Stainless Steel Research Articles

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.

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.

Physical simulation of the heat-affected zone in nickel-free high-nitrogen stainless steel weld at various peak temperatures and cooling rates

Nickel-free high-nitrogen stainless steel (NFHNSS) stands out for its remarkable strength, ductility and toughness making it a preferred choice for applications demanding robust structural integrity. Physical simulations of NFHNSS heat-affected zone under different peak temperatures and cooling rates were studied. Based on the phase diagram simulated from Thermo Calc, physical simulations were carried out on 16 specimens with varying peak temperatures ( 800 °C, 900 °C, 1000 °C and 1250 °C) and various cooling rates (CR) as (25 °C/s, 15 °C/s, 5 °C/s and 2 °C/s). The specimens were grouped and characterised based on peak temperature under varying cooling rates. The microstructural changes, precipitates, hardness and tensile properties of the base metal and heat-affected zone (HAZ), were investigated. HAZ subzones are very sensitive to peak temperature and cooling rate lead to variation in grain size. The test results showed significant refinement in HAZ grain size and significant improvement in strength with a fast cooling rate. Furthermore, transmission electron microscopy analysis showed Cr2N formation with respect to the cooling rate. Yield and tensile strength increased with an increase in CR and ductility reduced slightly. Whereas, with an increase in peak temperature yield strength, tensile strength reduced and ductility increased due to grain coarsening

Hot Crack Susceptibility Studies on High-Nitrogen Stainless Steel

Hot Crack Susceptibility Studies on High-Nitrogen Stainless Steel

The Tribocorrosion Behavior of High-Nitrogen Bearing Stainless Steel in Acetic Acid at Various Applied Loads

High-nitrogen stainless steels, which are developed by replacing nickel with nitrogen, have been widely applied in manufacturing wear parts in mechanical engineering. In this study, the tribocorrosion performance of a ferritic high-nitrogen bearing stainless steel (40Cr15Mo2VN) under acetic acid solution with a pH of 3.0 was investigated under different loads ranging from 25 N to 125 N. Quantitative calculations indicated that pure mechanical wear was the predominant cause of material degradation, while the corrosion-accelerated wear component also played a crucial role. The material loss induced by both tribocorrosion and mechanical wear increased with increasing load, leading to severe delamination at sliding surfaces and larger wear debris.

Optimization of Mold Powders for High‐Nitrogen Stainless Steel Based on Mold Thermocouple Temperature Variation

High‐nitrogen stainless steel (HNSS) is prone to surface depression during continuous casting; however, its high‐temperature solidification characteristics and mold powder design have not been reported. Based on the analysis of mold thermocouple temperature variation, this study clarifies the high‐temperature solidification characteristics of HNSS and the cause of surface depression. It optimizes the mold powder for HNSS continuous casting according to factory and laboratory research. The factory trials indicate that the mold thermocouple temperature at the corresponding location decreases by 6.0–20.9 °C when the slab surface is depressed. The analysis shows that the high initial solidification strength of HNSS causes its slab surface depressions. Owing to this characteristic, the mold powder for HNSS should have a suitable melting rate, low basicity, and proper Al2O3. This low‐basicity Al2O3‐containing slag has a crystal slag film near the mold side and a thick liquid slag film near the slab. It can both provide good lubrication and control horizontal heat transfer.

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.

Nitriding Behaviour and Microstructure of High-Nitrogen Stainless Steel during Selective Laser Melting.

High-nitrogen stainless steels are widely used due to their excellent comprehensive performance. In this study, the effects of process parameters (laser power, scanning speed, and cavity pressure) on the formation of high-nitrogen stainless steels were studied by using conventional selective laser melting and high-pressure selective laser melting (HPSLM). The nitrogen content, nitrogen emission, phase composition, microstructure, and microhardness of the high-nitrogen stainless steel samples obtained through selective laser melting (SLM) were analysed by using an oxygen/nitrogen/hydrogen analyser, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and electron backscatter diffraction. The results showed that the maximum nitrogen emission in the SLM sample was 0.175 wt.%, the emission rate reached up to 54.7%, and the maximum nitrogen content in the HPSLM sample was 1.07 wt.%. There was no significant difference between the phase peak positions of the SLM samples with different laser powers and the original powder. The main phase of the HPSLM sample changed at 0.3 MPa (from α-Fe to γ-Fe phase); the microstructure of the SLM sample was mainly composed of columnar and cellular crystals, and columnar crystal bands formed along the direction of heat flow. The HPSLM sample was mainly composed of equiaxed crystals with a grain size of 10-15 μm. At an energy density of 136 J/mm3, the microhardness and relative density reached their peak values of 409 HV and 98.85%, respectively.

Effect of Sintering Temperature and Solution Treatment on Phase Changes and Mechanical Properties of High-Nitrogen Stainless Steel Prepared by MIM

High-nitrogen stainless steel (HNSS) has been widely concerned and studied owing to its excellent mechanical, corrosion resistance, and biocompatibility properties. A series of HNSS was prepared by metal injection molding (MIM) using gas atomized Fe-Cr-Mn-Mo-0.3 N duplex stainless steel powders. Both sintering and solution treatments were carried out in an N2 atmosphere. The effects of nitrogen distribution and phase transformation on the mechanical properties of MIM HNSS during sintering and solution were studied. The results show that as the sintering temperature increased, the sample density increased, but the total nitrogen content decreased. Nitrogen and Cr2N concentration gradients along the cross-section of as-sintered samples were formed after cooling. The high nitrogen content promotes the decomposition of γ: γsaturated translated to γ and Cr2N. Meanwhile, the low Mn content in austenite also decomposes γ: γ translated to α and Cr2N. After solution treatment, a single γ phase was obtained for samples sintered at 1200 to 1320 °C. For solution treatment samples sintered at 1320 and 1350 °C, their tensile strength was 988.76 and 1036.12 MPa; yield strength was 615.61 and 636.14 MPa, and elongation was 42.58 and 40.08%, respectively. These values vastly exceeded the published MIM HNSS values.

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.

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.

Influence Mechanism of Crucible Materials on Cleanliness and Inclusion Characteristics of High-Nitrogen Stainless Bearing Steel During Vacuum Carbon Deoxidation

Influence Mechanism of Crucible Materials on Cleanliness and Inclusion Characteristics of High-Nitrogen Stainless Bearing Steel During Vacuum Carbon Deoxidation

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.

Thermomechanical processing model and abnormal microstructure evolution of high-nitrogen stainless steel X30CrMoN15-1

The high temperature deformation behavior of a X30CrMoN15-1 high-nitrogen stainless steel has been investigated using uniaxial compression test in the temperature range of 850–1250 ℃ and strain rate of 0.001–10 s−1. An Arrhenius-based hyperbolic sine equation was used to establish the flow stress constitutive model of the alloy at high temperatures, and the activation energy was 385.5 kJ/mol. Processing maps based on the dynamic material model were developed for true strains of 0.2, 0.4, 0.6 and 0.8. The domain of the safe region was in two parts: first, a strain rate range of 1–10 s−1 and temperature range of 1000–1100 ℃, and second, a strain rate range of 0.3–0.001 s−1 and temperature range of 1100–1250 ℃. The deformed microstructure at 1050 °C was characterized at different strain rates, strong< 100 >fiber textures parallel to the compression axis developed during high temperature deformation, and the strength gradually increased and became more concentrated with decreasing strain rate. The maximum dynamic recrystallization fraction and geometrically necessary dislocation densities were recorded at a medium strain rate (ε̇=0.1s−1), which was more conducive than a low strain rate to the continuous dynamic recrystallization process. Discontinuous and continuous dynamic recrystallization both contributed to the microstructural evolution of the columnar grains studied in this research, but discontinuous dynamic recrystallization was the dominant mechanism.

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.

Improving Cleanliness and Controlling Inclusion Characteristics in High-Nitrogen Stainless Bearing Steels by Optimizing Addition Order and Contents of Mg and Ce

Improving Cleanliness and Controlling Inclusion Characteristics in High-Nitrogen Stainless Bearing Steels by Optimizing Addition Order and Contents of Mg and Ce

Effect of aging treatment on microstructure and corrosion behavior of a Fe-18Cr-15Mn-0.66N stainless steel

• Effect of isothermal aging on pitting of high-nitrogen stainless steel is studied. • Aging time had minor influence on corrosion potential and corrosion current density, but a profound decrease of the pitting potential. • The fraction of Cr 2 O 3 in the passive film decreased with aging time. The effect of aging treatment on the microstructure and corrosion behavior of a Fe-18Cr-15Mn-0.66N high-nitrogen stainless steel (HNSS) in 3.5 wt.% NaCl solution was investigated using a series of electrochemical tests, scanning electronic microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results showed that the aging treatment led to the precipitation of Cr 2 N particles along the grain boundaries and their morphologies changed from dispersive particles to continuous network as the aging time increased up to 60 min. Aging time had minor effects on the corrosion potential and corrosion current density, but resulted in the sharp decrease in the pitting corrosion potential. The passive film behaved as a n-type semiconductor, and the donor density of the passive film increased with the aging time. Meanwhile, the fraction of stable oxide (Cr 2 O 3 ) in the passive film decreased with the aging time. It demonstrates that the aging treatment deteriorated the protectiveness of the passive film, hence weakened the corrosion resistance 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