High-nitrogen Steel Research Articles

A Perspective on Powder Metallurgy and Additive Manufacturing of High‐Nitrogen Alloyed Stainless Steels

Incorporating nitrogen as an alloying element in stainless steels can significantly enhance their mechanical and chemical properties. However, the limited solubility of nitrogen, particularly in the liquid phase, presents challenges. This perspective article explores an innovative powder metallurgical approach to producing high‐nitrogen steels (HNS) by utilizing a mixture of stainless steel and Si3N4. This mixture undergoes hot isostatic pressing (HIP) followed by direct quenching, facilitating diffusion alloying, and solution annealing in a single step. The article also examines adapting this method to powder bed fusion of metals using a laser beam (PBF‐LB/M) to overcome nitrogen solubility limits, which currently constrain the achievable nitrogen content in PBF‐LB/M‐manufactured stainless steels. The approach aims to retain Si3N4 particles within the matrix during PBF‐LB/M to enrich the steel with nitrogen during subsequent HIP. However, laser interaction with Si3N4 can lead to nitrogen loss, prompting an alternative strategy: a shell–core structure based on a gas‐tight shell enclosing loose Si3N4 particles. These particles dissolve during HIP, enriching the matrix with nitrogen and enabling the production of HNS. This article highlights the potential for extending this approach to other stainless steel groups, broadening the possibilities for HNS production through both conventional HIP and PBF‐LB/M manufacturing routes.

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

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

High nitrogen steels produced by laser powder bed fusion – Processability of an additivated austenitic steel powder

The additivation of conventional metal powders allows the expansion of the range of materials and alloys suitable for powder bed fusion additive manufacturing of metals. The combination of material properties, the improvement of currently used powder properties, and their processability are the focus of this technique. This work investigates the influence of the additivation of Si3N4 particles to X2CrNi18–9 austenitic stainless-steel powder on the resulting powder properties and processability by PBF-LB/M. The aim is to process a composite material consisting of a steel matrix in which retained Si3N4 particles are embedded. A subsequent heat treatment by hot isostatic pressing should allow dissociation of Si3N4 and the associated nitrogen diffusion into the austenitic matrix to produce a high nitrogen steel (HNS). Despite some degradation in powder properties, as indicated by a decrease in the Hausner ratio from 1.24 to 1.15, PBF-LB/M successfully produced samples in a process window of 30–36 J/mm3.

Microstructures and wear properties of CoCrFexNi (x = 0, 1) medium-entropy alloy coatings prepared by laser cladding

PurposeHigh-nitrogen steel is a common material for the manufacture of drilling equipment such as drill collars. However, its poor surface properties often limit its applications. The purpose of this paper is to find a way to enhance the surface performance of high-nitrogen steel, which is expected to improve the wear resistance of high-nitrogen steel.Design/methodology/approachThe CoCrNi and CoCrFeNi medium-entropy alloy (MEA) coatings were prepared on high-nitrogen steel substrate by laser cladding technology. The microstructure, phase composition and element distribution of the fabricated coatings were investigated using scanning electronic microscopy, electron backscatter diffraction and X-ray diffraction. The phase structure, phase stability and structural stability of the CoCrNi and CoCrFeNi MEAs were investigated in combination with phase diagram calculation and molecular dynamics. Then the wear tests were carried out for coatings.FindingsThe results show that both prepared MEA coatings have good quality and contain a single face-centered cubic phase. The wear performance of MEA coatings is improved by the refinement of grain size and the increase of dislocation density. Due to the addition of Fe atoms, the lattice distortion of the CoCrFeNi system increased, resulting in a higher dislocation density of the coating. Cr atoms in the CoCrFeNi system are the largest, and the local lattice distortions induced by them are greater. Through this study, MEA coatings with high hardness can be expanded to drilling field applications.Originality/valueCoCrFeNi and CoCrNi MEA coatings were successfully prepared on the surface of high-nitrogen steel for the first time without obvious defects. The micromorphology and grain orientation of the different kinds of coatings were discussed in detail. The hardness-strengthening mechanism and structure stability of the coatings were illustrated by experiments and molecular dynamics simulations.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-04-2024-0116/

Revealing the impact of inclusion types on pitting corrosion behavior of high-nitrogen stainless bearing steels with different Ce contents: Induction effect of oxides/sulfides and inhibition effect of nitrides

The impact of inclusion types on pitting corrosion behavior of high-nitrogen stainless bearing steels (HNSBSs) with different Ce contents was investigated combining electrochemical tests, microstructure characterization and thermodynamic analysis. The oxide (Al2O3/Ce2O3) and sulfide (MnS/Ce2O2S) inclusions exhibited induction effect on pitting corrosion, while nitride (CeN) inclusion exhibited inhibition effect. Interestingly, CeN inclusion also acted as a protective armor wrapping around Ce2O3 inclusion, alleviating its adverse effect. Moreover, the mechanism of inclusion evolution with Ce content was clarified, and the optimum Ce content in HNSBS was obtained (0.020 wt%). This work provides theoretical guidance for improving corrosion resistance of high-nitrogen steels.

Brazing of high-nitrogen steel to Al0·3CoCrFeNi using a BNi-2 filler: Microstructure, mechanical properties, and corrosion resistance

Studies that focus on brazing high-nitrogen steel (HNS) to high-entropy alloys (HEAs) are currently limited. In this study, the HEA Al0·3CoCrFeNi was successfully brazed to HNS using a commercial BNi-2 filler at 1080–1140 °C. The microstructure and shear strength of the Al0·3CoCrFeNi/HNS brazed joints were observed and evaluated. The typical microstructure of the brazed joint was HNS/infiltration layer/reaction layer/Ni(s,s) (containing NiAl3)/reaction layer/infiltration layer/Al0·3CoCrFeNi. The higher brazing temperature and longer holding time promoted the diffusion of B as a brazing seam with homogeneous Ni(s,s), and more intergranular Cr-rich borides formed in the base materials. The optimized shear strength attained 549.2 MPa after the joint brazed at 1120 °C for 15 min, and ductile fracture occurred in the reaction zone. The insufficient and excessive diffusion of B resulted in residual Cr–B compounds in the brazing seam and excess borides in the HNS, respectively, which negatively affected shear strength. All the joints showed poor corrosion resistance following immersion in a 3.5 wt% NaCl solution because the excess Cr-rich compounds resulted in decreased Cr content in the surrounding matrix. Therefore, Ni-based fillers without B were more suitable for obtaining Al0·3CoCrFeNi/HNS joints with a favorable corrosion resistance.

Calculations of phase composition of austenitic high-nitrogen welding wire and study of a welded joint made from it

The processability of a material is directly related to the possibility of its production, operation and maintainability. One of the most important indicators of the processability of any metal is weldability. Austenitic steels with a high nitrogen content proved themselves as high-strength, corrosion- and cold-resistant materials, but the issue of their weldability is still not fully understood. The lack of welding filler materials on the market specifically designed for welding high-nitrogen steels is the primary obstacle to solving this problem. Thus, the goal of the work was to develop and obtain a laboratory sample of high-nitrogen welding wire. Based on calculations of nitrogen solubility and the phase composition of the weld metal, the chemical composition of Cr – Mn – Ni – Mo – V, N steel was selected for this wire. A defect-free ingot with 0.57 % N was obtained, and wire with a nitrogen content of 0.57 wt. % was produced using hot plastic deformation and drawing methods. Testing of this wire to obtain a welded joint of austenitic cast steel, close to it in chemical composition, with the welding process carried out according to the developed technological recommendations, made it possible to obtain a defect-free welded joint without loss of nitrogen in the weld metal. With a microhardness of the base metal of 252 HV50 , due to the alloying of the welding wire steel with nitrogen and vanadium, the metal of the weld and fusion line had a high microhardness (278 and 273 HV50 , respectively), significantly exceeding the microhardness of Cr – Ni cast austenite. The metal of the welded joint has high strength (0.9 of the base metal strength) and high impact toughness. The fracture of impact samples is characterized by a dimple structure characteristic of viscous materials. According to the obtained results, the new welding wire showed itself to be a promising material for welding austenitic high-nitrogen steels.

Isothermal aging treatment and its effect on the mechanical properties of Fe-Cr-Mn high-nitrogen austenitic steel

The high-temperature solution nitriding process is a suitable treatment for producing high-nitrogen steels by the diffusion of nitrogen from the surface to the center of steels under high nitrogen pressure at high temperatures. On the other hand, long-term solution nitriding at high temperatures can cause the formation of coarse-grained austenite. This study focused on the grain size, strength, and elongation properties of Fe-Cr-Mn high-nitrogen austenitic steels by isothermal aging. For this purpose, we produced high-nitrogen steels by three-step phase transformations: (i) high-temperature solution nitriding, (ii) isothermal aging, and (iii) reaustenitization. After solution nitriding, high-nitrogen austenitic steel was treated with isothermal aging to induce austenite decomposition. Supersaturated austenite (ϒ) transformed to less nitrogen-saturated austenite (ϒ′) and discontinuous cellular precipitation (DCP) during the isothermal aging treatment. Subsequently, the decomposed structure was reversed into austenite through reaustenitization. The results suggested that austenite grain refinement and increasing hardness were achieved by nucleating reversed austenite grains after reaustenitization treatment. On the other hand, a noticeable increase in elongation appeared after reaustenitization treatment for 30 min.

Performance of Austenitic High-Nitrogen Steels under Gross Slip Fretting Corrosion in Bovine Serum.

Modular artificial hip joints are a clinical standard today. However, the release of wear products from the head-taper interface, which includes wear particles in the nm size range, as well as metal ions, have raised concerns. Depending on the loading of such taper joints, a wide variety of different mechanisms have been found by retrieval analyses. From these, this paper concentrates on analyzing the contribution of gross slip fretting corrosion at ultra-mild wear rates using a bovine calf serum solution (BCS) as the lubricant. The parameters were chosen based on biomechanical considerations, producing wear rates of some ng/m wear path. In parallel, the evolution of tribomaterial (third bodies) was analyzed as to its constituents and generation rates. It has already been shown earlier that, by an advantageous combination of wear mechanisms and submechanisms, certain constituents of the tribomaterial remain inside the contact area and act like extreme-pressure lubricant additives. For the known wear and corrosion resistance of austenitic high-nitrogen steels (AHNSs), which outperform CoCrMo alloys even under inflammatory conditions, we hypothesized that such steels will generate ultra-mild wear rates under gross slip fretting. While testing AHNSs against commercially available biomedical-grade materials of CoCrMo and TiAlV alloys, as well as zirconia-toughened alumina (ZTA) and against itself, it was found that AHNSs in combination with a Ti6Al4V alloy generated the smallest wear rate under gross slip fretting corrosion. This paper then discusses the wear behavior on the basis of ex situ analyses of the worn surfaces as to the acting wear mechanisms and submechanisms, as well as to the tribological reaction products.

Dynamic recrystallization behaviors and cracking mechanism for the hot deformation of new-generation high nitrogen bearing steel

In this work, the hot deformation behaviors of the high-nitrogen steel X30CrMoN15-1 from 900 °C to 1200 °C have been investigated. The microstructure observation indicates that the maximum DRXed grain size reaches 59.05 μm under the deformation temperature of 1200 °C with strain rate of 0.005 s−1, while the minimum grain size is 2.12 μm under the condition of 900 °C with strain rate of 1 s−1. The fully dynamic recrystallization (DRX) occurs at the conditions of high temperature and high strain rate (1000 °C − 1200 °C, 1 s−1 –5 s−1). There are two main mechanisms of crack formation. Under lower temperatures (900 °C-1000 °C), the significant stress concentration produced by M2(C, N) should be responsible for crack propagation along the DRXed grain boundaries. As the temperature increases to 1100 °C, the formation of cracks is mainly attributed to the uncoordinated deformation and huge pinning of dislocations by Lomer-Cottrell (L-C) locks. A new hot processing map has been established to reveals that the optimal processing domains are located at 1020 °C − 1090 °C with strain rate of 0.37 s−1 –4 s−1 and 1140 °C − 1200 °C with strain rate of 2 s−1 –5 s−1.

Additive manufacturing of high nitrogen austenitic steel using shell-core strategy

Laser Powder Bed Fusion/Metal (PBF-LB/M) is a promising technology for industrial applications, but challenges such as long process times remain. Innovations such as the shell-core approach aim to address this by creating a dense shell around a minimally exposed powder core, significantly reducing processing times, with full densification and property adjustments achieved by subsequent hot isostatic pressing (HIP). This study focuses on the fabrication of shell-core samples using a powder mixture of austenitic steel and Si3N4 to produce high nitrogen steel PBF-LB/M components, which are otherwise difficult to produce due to the limited nitrogen solubility in the melt. PBF-LB/M induces Si3N4 decomposition, resulting in Si and N loss through laser-powder interaction. Si3N4 particles in the still powdered regions serve as a source of N enrichment during HIP, circumventing the limitations of nitrogen solubility in the melt and exploiting the higher solubility in the solid. After HIP, energy dispersive spectrometry and electron backscatter diffraction reveal a fully austenitic matrix with Si diffusion seams mainly in non-laser-exposed areas. The Si3N4 dissolution during HIP contributes to an interstitial dissolved N content of about 0.189 mass%, which, together with the higher Si content, increases hardness. Wavelength dispersive spectrometry (WDS) and nanoindentation line scans show decreasing Si and N concentrations from core to shell, resulting in reduced (nano)hardness in the shell. This innovative approach demonstrates the potential to produce AM components with enhanced properties by overcoming the limitations of nitrogen solubility in the steel melt during PBF-LB/M.

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.

Tailoring of the Microstructure and Mechanical Properties of High Nitrogen Steel Repaired by Underwater Laser Direct Metal Deposition

Tailoring of the Microstructure and Mechanical Properties of High Nitrogen Steel Repaired by Underwater Laser Direct Metal Deposition

Research on gas pore formation and inhibition mechanism of high nitrogen steel during laser direct metal deposition

Nitrogen pore formation is an inherently tricky problem that limits the application of high nitrogen steel (HNS) in various fields. In the present study, a CA-LBM coupling model was proposed to understand the nitrogen pore formation mechanism during laser direct metal deposition (DMD) of HNS. Based on the proposed model and theoretical analysis, the effects of pulse frequency and shielding gas nitrogen content on the porosity were clarified systematically. Results showed that after the onset of solidification, the ferrite phase with extremely low nitrogen solubility precipitated first, inducing the continuous enrichment of nitrogen in the residual liquid phase, which in turn provided the prerequisite for nitrogen pore nucleation. The application of quasi-continuous-wave (QCW) laser mode accelerated the molten pool solidification, and shortened the nitrogen supersaturated region, allowing more nitrogen to be solid dissolved in the matrix. On this basis, the porosity was inhibited to a certain extent. Compared with the high-frequency pulse, the low-frequency pulse showed better porosity inhibition. In addition, following the classic nucleation theory, nitrogen segregation and nitrogen solubility synergistically dominated nitrogen pore formation. The nitrogen partial pressure, determined by the shielding gas nitrogen content, played a dual role in porosity defects. Elevated nitrogen partial pressure served to increase nitrogen solubility while promoting the nitrogen absorption process to intensify nitrogen segregation. However, due to the short duration of the molten pool, the nitrogen absorption process could not be fully processed. Nitrogen solubility enhancement was the key factor in inhibiting porosity during the DMD process.

Mechanical properties of high-nitrogen steel produced via selective laser melting using mechanically alloyed and spheroidized powders

In recent years, the development of additive technologies has been one of the priority tasks in the sector. Primarily, additive technologies enable the effective implementation of various design and engineering ideas in high-tech industries, such as the aircraft industry, engine technology, and rocket engineering. The expanded range of standardized materials for additive technologies will facilitate their integration into large-scale production. Of significant interest is the potential use of nitrogen-containing heat-resistant powder alloys to produce complex-shaped aircraft parts using additive technologies. This paper describes the complete process of obtaining samples from powders of alloys with superequilibrium nitrogen content using the selective laser melting (SLM) method. Four different compositions of high-nitrogen steels were obtained through mechanical alloying. Subsequently, the powders of these steels underwent processing using the plasma spheroidization method to be utilized in the SLM process. The SLM method was also employed to produce samples for mechanical tests. Throughout each stage of the process, the powders were thoroughly analyzed. One of the most critical parameters was the nitrogen content in the resulting powders. At each subsequent production stage, its proportion decreased, yet it remained at the superequilibrium content level of 0.13–0.44 wt. %. The mechanical tests confirmed that the alloys fabricated by the SLM method are not inferior in terms of their properties compared to those obtained using classical metallurgical technologies.

Corrosion behaviour of self-piercing riveted joints with uncoated rivets in high nitrogen steel

AbstractSelf-piercing riveting is an established joining technique for lightweight materials. To increase the sustainability of the rivet manufacturing process, the authors of the present paper have developed an approach for shortening the process chain by omitting the heat treatment and rivet coating. To do this, use is made of high nitrogen steel as the rivet material. Successful joining with these rivets has already been proven, and it has also been shown that a competitive joint strength can be achieved with these rivets. Up until now, no studies have been conducted of the corrosion behaviour of uncoated rivets in high nitrogen steel compared to conventional rivets made of heat-treatable steel with a coating of Almac® or zinc-nickel with topcoat, and the corrosion behaviour of joints manufactured with these rivets has also not been investigated. Furthermore, the suitability of rivets in high nitrogen steel for structures undergoing cathodic dip painting has not been evaluated to date. These are therefore the aims of the research work presented in this paper. Corrosion behaviour is tested by exposing rivets and joints to a salt spray atmosphere. Cross-cut tests are conducted in order to classify the adhesion of cathodic dip paint to the different rivet surfaces and materials. The results of the experimental test show that the cathodic dip paint has sufficient adhesion to the uncoated rivets in high nitrogen steel and that these rivets can therefore be used in the manufacture of car bodies. Due to the stainless properties of the high nitrogen steel, better corrosion resistance is seen by comparison to the commonly used coatings of Almac® and zinc-nickel with topcoat. A study of the corrosion behaviour of the joints shows that the rivet head diameter and rivet head position, in particular, are decisive for preventing crevice corrosion under the rivet head and contact corrosion within the joint.

Influence mechanism of underwater hyperbaric environment on the corrosion behavior of high nitrogen steel fabricated by underwater laser direct metal deposition

The underwater laser direct metal deposition (UDMD) technique presents a potential new approach for on-site repair in the field of underwater on-site repair. However, the influence mechanism of the underwater hyperbaric environment on the corrosion behavior of UDMD-fabricated samples remains unclear. In the current research, the corrosion behavior of high nitrogen steel (HNS) samples fabricated by UDMD (sample U1) at the ambient pressure of 0.3 MPa was evaluated. The sample fabricated by DMD in the ground environment with the same process parameters served as a comparison (sample A1). Results manifested that the elevated ambient pressure induced sample U1 to possess a higher nitrogen content than the raw HNS powder, unlike sample A1 where nitrogen outgassing occurred. The increased nitrogen content induced the formation of a denser passive film, enhanced re-passivation ability, and promoted synergistic effect between nitrogen and molybdenum. Furthermore, the water-quenching effect, on the one hand, inhibited elemental partitioning and resulted in a lower ∆|PRENγ-PRENα|, thereby leading to a slower galvanic corrosion rate. On the other hand, the water-quenching effect refined grain size and promoted the formation of LAGBs, further leading to the formation of a more stable and robust passive film. Based on the aforementioned factors, the corrosion resistance of sample U1 was substantially improved.

Microstructure and mechanical properties of high nitrogen steel – high strength steel bimetallic multi-layered steels fabricated by plasma-arc additive manufacturing

Three bimetallic multi-layered steel samples with different structures as well as a pure high nitrogen steel (HNS) sample and a pure high strength steel (HSS) sample were fabricated by plasma-arc additive manufacturing. The microstructure and mechanical properties of the samples were characterized by various methods. The results indicated that the microstructures of HNS and HSS are mainly austenite and martensite, respectively, and the grains in the interface area of HNS and HSS grew orderly compared with the inner area. The microhardness of the HSS sample (480.7 HV-499.2 HV) is higher than that of HNS (326.4–345.1 HV), and that of the bimetallic multilayered steels is consistent with the structure distribution. The bimetallic multi-layered steels combined the tensile property advantages of HNS and HSS well, and steels with both high strength and toughness were obtained.

High cycle fatigue properties of high-nitrogen steel hot-rolled with the yield strength level of 1000 MPa

The fatigue properties of an Fe-19Cr-20Mn-0.6N high-nitrogen steel (HNS), which was hot-rolled with the yield strength level of 1000 MPa, were investigated by means of tension–tension fatigue tests. The results showed that the fatigue life increased in a power law manner with the decrease of the maximum stress from 1040 MPa to 880 MPa. The fatigue limit at the 107 cycles was about 840 MPa, with the ratios of the fatigue limit to yield strength and tensile strength being 0.79 and 0.69, respectively. The high fatigue limit could be attributed to its high yield strength level, which was resulted from the mixed microstructure consisting of ultrafine grains and elongated grains with some slip bands.

A new approach to the powder metallurgical production of high nitrogen steels by integrated diffusion alloying in a hot isostatic press

Alloying with nitrogen in stainless steels, with nitrogen being dissolved interstitially, results in improved mechanical properties and higher resistance to local corrosion phenomena. However, the positive effect cannot be arbitrarily scaled to higher nitrogen contents, mainly due to the limited nitrogen solubility in the liquid phase. This paper presents a powder metallurgical approach to the production of a high nitrogen variant of the austenitic stainless steel X2CrNi18-9, in which a powder mixture of steel and Si3N4 powder is hot isostatically pressed and integrally cooled by uniform rapid quenching (URQ®). This procedure, known as diffusion alloying, allows nitrogen to dissociate from Si3N4 and dissolve into the solid austenitic phase, which provides a nitrogen solubility greater than 0.5 mass% under HIP conditions. With this approach, about 0.38 mass% N could be interstitially dissolved in the steel, which, however, can be further increased in the future by adjustments discussed here.