High-nitrogen Steel Research Articles

Investigation of additive manufactured micro-lattice structures for defence applications

Abstract This study investigates the blast mitigation capabilities of A286 steel micro-lattice structures produced through additive manufacturing. The research explores the effects of different manufacturing conditions, such as stress relief and heat treatment, on the mechanical properties and blast resistance of honeycomb and gyroid lattice structures in correlation with armour steel structures. Comprehensive evaluations, including surface morphology, corrosion resistance, and compressive residual stress analysis, reveal notable findings for micro-lattice structures. Micro-lattice structures demonstrated 57.23% higher corrosion resistance compared to conventional materials, presently available in the form of rolled homogeneous armour, medium hardness armour, and high-nitrogen steel. Additionally, honeycomb lattice structures exhibit compressive residual stresses of up to 581.90 MPa, providing significant advantages in blast mitigation potential. These results underscore the significance of lattice geometry, material microstructure, and residual stress in enhancing blast resistance. The research offers valuable insights into optimizing additive manufactured structures as an alternative modular solution for defence applications.

Effect of mixed powder proportion on microstructure and defects of high-nitrogen steel welded joints

Effect of mixed powder proportion on microstructure and defects of high-nitrogen steel welded joints

Weldability of high nitrogen steels: A review

Abstract High nitrogen steel (HNS) have been widely used in many industrial fields in place of stainless steels. As we know, the welding is the main fabricating method of the HNS structural components. In this article, the recent investigations of joining methods of HNS, such as tungsten inert gas welding, melt inert-gas welding, laser welding, laser-arc hybrid welding, friction stir welding (FSW), and brazing method are summarized. First, the effect of nitrogen content in shielding gas, welding wire, and base metal on the evolution of microstructure, mechanical properties, and corrosion susceptibility of fusion welded joints are discussed systemically. Then the existing problem during FSW and brazing process of HNS are analyzed. Additionally, the key issues and future trends in the joining of HNS are proposed. The main purpose of this review is to provide a technical reference and theoretical basis for research and technological development during the welding of HNS.

Study on the Microstructure and Properties of Flash-Butt Welding Joints of High Nitrogen Steel

A thermomechanical coupling model for the solid-state flashing process of high nitrogen steel was established, based on finite element simulations and experiments. The effect of flash current on the microstructure and mechanical properties of the welded joint was investigated, and the temperature field of the flash-butt welding (FBW) process was simulated. The phase composition of the joint was determined according to the phase diagram and cooling curve. In addition, the joint with optimal parameters was subjected to full immersion corrosion tests. The results demonstrated that the interface structure was composed of austenite and δ-ferrite with a thyristor angle (flash current) of 45°. The microstructure of the overheated zone (OZ) was composed of austenite, ferrite and a small amount of the M2 phase, in which the heat-affected zone exhibited a single-phase austenite microstructure. The joint hardness displayed a “V” shaped distribution with the lowest interface hardness. As the flash current increased, the hardness and tensile strength of the interface area of the joint first increased and then decreased, with a maximum tensile strength of 902 MPa at 45°. During the full immersion corrosion tests, the joint exhibited the most serious corrosion in the interface center and gradually reduced corrosion on both sides.

Effect of Powder Formulation and Energy Density on the Nitrogen Content, Microstructure, and Mechanical Properties of SLMed High-Nitrogen Steel

The effects of powder formulation, including elemental mixed powder (EMP) and alloy mixed powder (AMP), and energy density on the nitrogen content and microstructural characteristics of high-nitrogen steel prepared by selective laser melting were investigated. The results reveal that the samples prepared with EMP had more nonfusion flaws and a relatively low density, with a maximum of only 92.36%, while samples prepared with AMP had fewer defects and a relative density of up to 97.21%. The nitrogen content and microstructural characteristics were significantly influenced by the laser energy density. The relative density of the EMP samples increased from 88.29% to 92.36% as the laser energy density increased from 83.3 J/mm3 to 125 J/mm3, while the relative density of the AMP samples rose from 93.31% to 97.21%, and the number of defects and the nitrogen content decreased. The mechanical properties of the AMP samples were superior to those of the EMP samples when the energy density rose, and the strength of the high-nitrogen steel first rose and then fell. The AMP samples showed the best mechanical properties when the energy density was 104.2 J/mm3, which corresponds to a laser power of 250 W, a scanning speed of 1000 mm/s, and a layer thickness of 30 μm. The corresponding values of yield strength, ultimate tensile strength, and elongation were 958.8 MPa, 1189.2 MPa, and 30.66%, respectively.

Precipitation Behavior of the Second Phase in High-Nitrogen Austenitic Stainless Steel Alloyed with V and Nb

The retaining ring is one of the components with the highest stress on the rotor of the turbine generator. Due to the high content of alloying elements in this type of steel, it is extremely easy to precipitate a large amount of the second phase during the forming process of a material, which deteriorates the processability, Therefore, it is very important to study the second phase precipitation of high-nitrogen steel. This work addresses the precipitation behavior of the second phase in a high-nitrogen austenitic stainless steel alloyed with V and Nb. The optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffractometry were applied to investigate the influence of heat treatment parameters on the morphology and composition of the precipitated particles. The addition of V and Nb results in the formation of NbVN and VN particles, of which the amount is slightly decreased after a subsequent solution treatment at 1200 °C for 1 h. The results demonstrate that an additional aging treatment at 800~1000 °C for three leads to the participation of lamellar Cr2N and the undissolved MX phase. The content of the precipitates increases with increasing both the aging temperature and the aging time. The addition of V/Nb elements effectively improves the stability of MX phase, and the existence of high energy zones (the MX phase interface and grain boundary) promotes the early formation of Cr2N. Cr2N is shown to nucleate in the high energy zones, which thereafter grows into austenite grains, demonstrating a normal discontinuous cellular growth.

Mechanical properties of pre-cracked high nitrogen steel under dynamic composite loading

In this paper, the mechanical properties of pre-cracked high nitrogen steel specimens with different loading angles of 30°, 45° and 60° were studied by using Hopkinson tension bar. The corresponding force-displacement curves under different loading angles were obtained. It was found that with the decrease of the pre-crack loading angle, the proportion of the shear stress on the specimens in the composite loading gradually increased, and the stress concentration appeared during the composite loading process, the fracture strength of the specimens decreased significantly. At the same time, however, the dispersion of the experimental results decreased, and the fracture morphology tended to be consistent with the elongation at break. The obtained mechanical properties of high nitrogen steel under combined loading can provide reference and basis for its technological improvement, safe production and reliable application

Corrosion Behavior of High-Nitrogen Steel Hybrid Welded Joints Fabricated by Hybrid Laser-Arc Welding.

To study the corrosion mechanism of high-nitrogen steel welds, this study investigated the effects of laser outputs on the corrosion behavior of high-nitrogen steel hybrid welded joints in hybrid laser-arc welding. The relationship between the ferrite content and laser output was characterized. The ferrite content increased with the increase in the laser power. The corrosion phenomenon first occurred at the two-phase interface, thereby forming corrosion pits. Ferritic dendrites were first corroded to form dendritic corrosion channels. Furthermore, first-principles calculations were performed to investigate the properties of the austenite and ferrite content. The work function and surface energy indicated that solid-solution nitrogen austenite exhibits a higher surface structural stability than austenite and ferrite. This study provides useful information for high-nitrogen steel weld corrosion.

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.

Elimination Mechanism of Shrinkage Porosity During Pressurized Solidification Process of 19Cr14Mn4Mo1N High-Nitrogen Steel Ingot

Elimination Mechanism of Shrinkage Porosity During Pressurized Solidification Process of 19Cr14Mn4Mo1N High-Nitrogen Steel Ingot

Influence mechanisms of underwater hyperbaric environment on nitrogen behavior, phase evolution, and mechanical properties of high nitrogen steel repaired by underwater laser direct metal deposition

Nitrogen outgassing and nitrogen pore formation are tricky problems inherent in the fabrication/repair of high nitrogen steel (HNS). In the current research, HNS powder with a nitrogen content of 0.42 wt% was used to repair damaged HNS plates by the underwater laser direct metal deposition (UDMD) technique at an ambient pressure of 0.3 MPa (sample U1). Under the same process parameters, the DMD repaired sample (sample A1) in the ground environment served as a comparison. The influence mechanisms of the underwater hyperbaric environment on the nitrogen behavior, interface crystallographic characteristics, and mechanical properties were clarified. Results showed that nitrogen outgassing and nitrogen pore formation occurred in sample A1. In sample U1, the elevated ambient pressure increased the nitrogen solubility in the molten pool from 0.628 wt% to 1.088 wt%, thus eliminating porosity defects. In addition, there was a higher content of Kurdjumov-Sachs (KS) and Nishiyama-Wassermann (NW) interfaces (95%) in sample A1. Conversely, the water cooling inhibited the austenite shear transformation period at the relatively low temperature of sample U1. Hence, lower KS/NW interfaces (56%) were obtained due to the changed austenite nucleation and growth mechanism. Benefitting from the denser dislocation density and finer grain size caused by the underwater environment, the comprehensive mechanical properties of defect-free sample U1 were superior to those of sample A1. The nitrogen pores in sample A1 did not weaken the tensile strength but did seriously impair the impact toughness and ductility.

Microstructure evolution and crack formation mechanism of high-nitrogen bearing steel subjected to warm deformation

In this work, the microstructure evolution and crack formation mechanism of the X30CrMoN15-1 high-nitrogen steel under warm deformation (700 ℃ and 800 ℃) have been investigated. The microstructure observation indicates that the warm deformation (both at 700 °C and 800 ℃) facilitates the ferrite transition and the carbonitrides precipitation, which leads to the reduction of retained austenite by 40% in the specimen deformed 700 °C and 18% for the specimen deformed 800 °C. In addition, cracks are likely to generate under the low strain rates of 0.005 s−1. In the specimen deformed at 700 °C, the lamellar carbonitrides are precipitated along the interface between ferrite and M/A islands, while the spherical carbonitrides are precipitated inside the ferrite. The volume expansion generated by ferrite transformation and the growth of recrystallized grain are hindered by carbonitrides, resulting in the significant stress concentration and even crack initiation. For the specimen deformed at 800 ℃, the spherical carbonitrides are mostly precipitated from the recrystallized ferrite grain boundary. The significant dislocation aggregation around the austenite /ferrite interface and strength mismatch between ferrite and austenite together result in the interfacial debonding and even cracks propagation at phase boundary.

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.

Effect of interstitial nitrogen on bonding characteristics and electronic structure of high nitrogen steel

Interstitial nitrogen can significantly change the electronic structure and bonding characteristics of austenite, and has an important influence on the properties of austenite. The bonding properties and properties of high nitrogen steel with different nitrogen element ratios were calculated. The alloy model with different nitrogen content is established, and the properties of energy, density of states and elastic constant are calculated. Increasing the nitrogen content will reduce the density of states and energy, significantly improving the stability of the cell. The crystal cells with different nitrogen content meet the mechanical stability. With the increase of nitrogen content, the elastic modulus, Poisson’s ratio gradually decreases, strength, shear strain and deformation resistance increase, and plasticity and ductility decrease.

Influence of carbon on the structural-phase composition and hardness of steel ingots of the Fe-Cr-Mo-N-C system obtained by the SHS method under nitrogen pressure

The possibility of obtaining solid ingots (without gas porosity in the volume) from high-nitrogen steel of the Fe-Cr-Mo-N-C system by the method of self-propagating high-temperature synthesis under nitrogen pressure is shown in the present paper. It has been established that with an increase in the mass fraction of carbon from 0.09 % to 0.30 % in the as-cast metal, the hardness of the metal increases. At the same time, the structural-phase composition consists of a pseudo-pearlitic structure (ferrite-nitride mixture) and austenitic structure in different ratios since an increase in the carbon content leads to an increase in the amount of the pseudo-pearlite. The heat treatment of specimens with a mass fraction of carbon of 0.09 % and 0.30 % at 1250°C for two hours leads to their complete austenization. In this case, the hardness of steels of both compositions decreases to 22 – 24 HRC (0.09 wt.% of carbon) and to 28 – 30 HRC (0.30 wt.% of carbon).

Investigation of the weldability of dissimilar joint between high nitrogen steel and low alloy steel by comparing filler metals

Dissimilar welding of high nitrogen steel and low alloy martensitic steel has a large application prospect in power plants and offshore structures. In this paper, the weldability was studied by comparing and analyzing the metallurgical characteristics and mechanical properties of the dissimilar joints by three types of consumables. Among the welding materials of austenitic ER 307Mo, duplex ER 2209, and martensitic ER 120S-G, macroscopic gas pores appeared in the weld with ER 2209 and ER 120S-G, which were both caused by the mix of the nitrogen-containing molten metal and low alloy steel, where the elements that promote nitrogen solubility were rapidly diluted, resulting in a violent escape of nitrogen. In ER 307Mo weld, because of the high content of alloying elements and the minimal fusion ratio, nitrogen only escaped as the micropores and small precipitates, which have no obvious effect on the performance of the weld. It is a good solution to solve the problems of dissimilar welding by selecting a filler metal that does not contain nitrogen but can dissolve nitrogen, and using the dissolution of high nitrogen steel to strengthen the weld based on controlling the fusion ratio. Finally, nitrogen behavior during the welding process was discussed and analyzed combined with porosity distribution, nitrogen distribution, and microstructure.

First principle study on the effect of Cr, Mn and Ni on electronic structure and mechanical properties of high nitrogen steel

First principle study on the effect of Cr, Mn and Ni on electronic structure and mechanical properties of high nitrogen steel

Effect of N–Cr compounds on the brazing of high nitrogen steel with Ag-based filler metal: first-principles calculation

The Cr element is present in the form of CrN and Cr2N compounds in the brazing of high nitrogen steel with Ag-based filler metals. The Cr compounds have a strong influence on the strength of the brazed joints. In this paper, the mechanical properties of CrN and Cr2N are evaluated based on first-principles. The results show that Cr2N has a stronger covalent bonds than CrN, resulting in higher Young's modulus and hardness of Cr2N than CrN. The 7 atomic layers Ag(111) surface, 9 atomic layers CrN(111) surface and 9 atomic layers Cr2N(0001) surface can represent the block structure. The Ag(111)/CrN(111) interface and Ag(111)/Cr2N(0001) interface were constructed. The Ag(111)/CrN(111) B-site interface was found to have the largest work of adhesion(Wad). The chemical bonding strength of Ag–N in the Ag(111)/CrN(111) interface is stronger than that in Ag(111)/Cr2N(0001) interface, indicating that the formation of Cr2N phase is more likely to produce microcracks in brazing joints. Hence, the formation of the Cr2N phase should be suppressed.

Вопросы сварки аустенитных сталей с высокой концентрацией азота (Обзор)

Alloying with nitrogen allows one to substantially increase the strength of steels, especially of the austenitic steels, in which the equilibrium nitrogen content can reach 0.4 – 0.7 wt % depending on their chemical composition. Nitrogen stabilizes austenite and enhances its corrosion and wear resistance. For this reason, nitrogen-containing austenitic steels are advantageous structural materials, in particular, for heavy loaded welded structures. However, wrong choice of the welding method, regime, or filler for high-nitrogen steels can cause critical defects such as pores and cracks in the welded joints and substantially decrease their mechanical properties and corrosion resistance compared to those of the base metal. More than hundred literature sources have been considered for the analysis of welding types and methods for austenitic steels containing ≥ 0.4 wt. % nitrogen, the criteria for their weldability, and the causes for potential negative phenomena occurring upon their welding. For such steels, information is provided on the welding filler materials as well as on the modes and parameters of the welding process (supplied heat, presence/absence of a protective atmosphere, its composition, etc.). The structure, phase composition, and properties of the resulting welded joints are considered. The examples are given on the effect of the subsequent processing of welded joints (heat treatment and thermal deformation processing for the elimination of the resulting distortion) on their basic properties.

Precipitation characteristics and tensile properties of high-nitrogen chromium-manganese steel fabricated by wire and arc additive manufacturing with isothermal post-heat treatment

Wire and arc additive manufacturing (WAAM) technology is a promising method for fabricating high-performance protective materials and structural parts from high-nitrogen steel (HNS). However, owing to the complicated thermal fields generated by the process, it may produce some undesirable precipitates, such as the intermetallic σ phase or chromate nitrides, especially when the fabricated part is large. This study focused on the precipitation features of WAAM-produced high-nitrogen Cr-Mn steel containing 0.7 wt% nitrogen. When the isothermal post-heat treatment was conducted at approximately 900 °C, the σ phase dominated the original inter-dendritic ferrite (Inter-F) region. And, there were some secondary austenite precipitates (γ2) along with a small amount of Cr2N. The oxide inclusions (especially the MnO particles) provided excellent nucleation sites for the σ phase in the Inter-F region or at the Inter-F and γ matrix interface due to the high interface energy and lattice matching index. Further, when aging above 1000 °C, no σ phase was found, and the decomposition of the Inter-F to γ matrix was dominated by element diffusion. With the precipitation of the σ phase, a typical brittle fracture was observed in the samples aged at low temperatures of 800 °C and 900 °C.