High-nitrogen Steel Research Articles

High-nitrogen steel laser-arc hybrid welding in vibration condition

ABSTRACT Welding porosity and nitrogen content are considered significant factors affecting the mechanical properties of fusion-welding joints of high-nitrogen steel. In this study, a method of applying mechanical vibration in the welding process to reduce weld porosity and increase weld nitrogen content was investigated. The effects of mechanical vibration on porosity, tensile, and impact properties were analysed. The results indicated that the bubble floating speed in the vibrating weld pool is faster than that in the general welding mode. With the increase of mechanical vibration frequency, the porosity of the weld decreased at first and then rose. The tensile strength and impact energy increased first and then decreased, and the fracture surface indicated a ductile fracture.

Process design for the forming of semi-tubular self-piercing rivets made of high nitrogen steel

The aim to reduce pollutant emission has led to a trend towards lightweight construction in car body development during the last years. As a consequence of the resulting need for multi-material design, mechanical joining technologies become increasingly important. Mechanical joining allows for the combination of dissimilar materials, while thermic joining techniques reach their limits. Self-piercing riveting enables the joining of dissimilar materials by using semi-tubular rivets as mechanical fasteners. The rivet production, however, is costly and time-consuming, as the rivets generally have to be hardened, tempered and coated after forming, in order to achieve an adequate strength and corrosion resistance. A promising approach to improve the efficiency of the rivet manufacturing is the use of high-strength high nitrogen steel as rivet material because these additional process steps would not be necessary anymore. As a result of the comparatively high nitrogen content, such steels have various beneficial properties like higher strength, good ductility and improved corrosion resistance. By cold bulk forming of high nitrogen steels high-strength parts can be manufactured due to the strengthening which is caused by the high strain hardening. However, high tool loads thereby have to be expected and are a major challenge during the production process. Consequently, there is a need for appropriate forming strategies. This paper presents key aspects concerning the process design for the manufacturing of semi-tubular self-piercing rivets made of high-strength steel. The aim is to produce the rivets in several forming stages without intermediate heat treatment between the single stages. Due to the high strain hardening of the material, a two stage forming concept will be investigated. Cup-backward extrusion is chosen as the first process step in order to form the rivet shank without forming the rivet foot. Thus, the strain hardening effects in the area of the rivet foot are minimized and the tool loads during the following process step can be reduced. During the second and final forming stage the detailed geometry of the rivet foot and the rivet head is formed. In this context, the effect of different variations, for example concerning the final geometry of the rivet foot, on the tool load is investigated using multistage numerical analysis. Furthermore, the influence of the process temperature on occurring stresses is analysed. Based on the results of the investigations, an adequate forming strategy and a tool concept for the manufacturing of semi-tubular self-piercing rivets made of high-strength steel are presented.

On the Superplastic Deformation in Vanadium-Alloyed High-Nitrogen Steel

The experimental evidence for the realization of a superplastic behavior with 900% elongation in V-alloyed high-nitrogen austenitic Fe-19Cr-22Mn-1.5V-0.3C-0.6N steel was proposed. Using thermomechanical processing, a misoriented grain/subgrain austenitic microstructure with a high density of deformation-assisted defects and precipitates was developed in the steel. During high-temperature tensile deformation in a temperature interval from 850 to 1000 °C and strain-rate range from 4 × 10−4 s−1 to 6 × 10−3 s−1, this microstructure demonstrated the characteristics of superplastic flow: elongation in the interval 400–900%, strain-rate sensitivity exponent m = 0.40–0.49, grain boundary sliding mechanism. The maximum elongation to failure (900%) was reached at deformation temperature 950 °C and strain rate 4 × 10−4 s−1.

3D magnetic patterning in additive manufacturing via site-specific in-situ alloy modification

Controlling the process-microstructure-properties relationship is at the heart of materials science. Thus in-situ compositional control in additive manufacturing has the potential to innovate present materials AM processing. We demonstrate in-situ alloy modification during laser powder bed fusion (LPBF) processing with the site-specific control of the para- and ferromagnetic properties in a high-nitrogen steel part fabricated by LPBF. Nitrogen can be selectively evaporated during the LPBF process by varying the volumetric energy density. The controlled evaporation of nitrogen stabilizes the ferromagnetic bcc phase at the expense of the paramagnetic fcc phase as shown by experiments and thermodynamic simulations. We demonstrate the feasibility to print complex magnetic structures by a 3D magnetic chessboard pattern comprised of non-ferromagnetic and partially ferromagnetic areas. The presented approach is applicable to all alloys where volatile elements affect the microstructure.

Basic Physicochemical Concepts for Controlling the δ-Ferrite Content When Welding with Austenite-Ferrite Materials

Abstract—By the example of the metal of a seam obtained using welding materials based on 10Kh19N11M4F steel which are currently used for the welding of high-strength low-alloy steels, the influence of its chemical composition on the behavior of δ-ferrite throughout the entire temperature range of its existence is shown. On the basis of these studies, the prospects of the use of 10Kh19N11M4F steel for the welding of high-nitrogen corrosion-resistant steels with the retention of their nonmagnetization, including in the area of the welded joint, are shown. The critical parameters determining the behavior of δ-ferrite upon crystallization and subsequent cooling of solid steel are found using thermodynamic modeling. It is shown that the depth of the σ-ferritic transformation and the maximum equilibrium temperature of austenitization used for the interpretation of the experimental data obtained during hot physical modeling of welding are the most important among them. The areas of promising compositions of the metal of the seam in the case of welding of low-alloy high-strength and high-nitrogen corrosion-resistant steels without hot cracks and providing, if necessary, nonmagnetization of the seam are found and depicted on the fragment of the improved Scheffler–Speidel diagram.

Effect of Preliminary Cold Working on High-Temperature Tensile Behavior of Nickel-Free High-Nitrogen Austenitic Steel

The effect of solution treatment and preliminary cold working on the high-temperature tensile behavior of a high-nitrogen nickel-free corrosion-resistant austenitic steel at 875 – 950°C and different deformation rates (from 10 – 2 to 10 – 4 sec – 1 ) is studied. It is shown that the steel has high mechanical properties after the solution treatment. After the cold rolling the strength of the specimens increases and the ductility decreases due to strong strain hardening. At a high temperature, the stress-strain curves exhibit serrations at the lower deformation rates. The strength of such specimens is lower than after the solution treatment.

Melting of High-Nitrogen Austenitic Steels at Atmospheric Pressure

The article considers conditions for nitrogen bubble formation at the crystallization front for iron and stainless steels. Using the results obtained during plasma-arc remelting, a semi-empirical formula is proposed for calculating maximum permissible nitrogen concentrations. It is shown that during steel crystallization with austenite formation at atmospheric pressure it is possible to obtain dense steel ingots with a nitrogen content exceeding its solubility at the liquidus temperature by a factor of 1.4–1.5.

Effect of boron on intragranular ferrite nucleation mechanism in coarse grain heat-affected zone of high-nitrogen steel

Abstract The effect of boron on intragranular ferrite (IGF) nucleation mechanism in the simulated coarse grain heated-affected zone (CGHAZ) of a high-nitrogen steel was investigated. When boron (B) was added to the steel, the ferrite transformation temperature increased from 775 °C to 800 °C and the amount of polygonal ferrite increased from 63.2% to 78.6%. Consequently, the ductile-to-brittle transition temperature decreased from −30 °C to −50 °C. (Ti,V)(C,N)-MnS-BN, MnS-BN, Al2O3-MnS-BN, and BN served as IGF nucleation sites. The core of the complex precipitates was formed by undissolved (Ti,V)(C,N)-MnS, Al2O3-MnS, and MnS, with a BN cap formed on the undissolved precipitates, which increased the IGF nucleation potency.

Dislocation motion at a fatigue crack tip in a high-nitrogen steel clarified through in situ electron channeling contrast imaging

The dislocation motion was observed at the tip of a fatigue crack in the high nitrogen steel (HNS) by in situ test through electron channeling contrast imaging. No dislocation motion was observed near the crack tip even at a stress amplitude higher than the fatigue limit. In contrast, dislocation pileup was observed a bit from the crack tip with very narrow dislocation spacing. In addition, with increasing number of cycles, the dislocations were extended to stacking fault. The stacking fault showed a reversible behavior of shrink and expansion, which may influence in enhancing the crack growth resistance of the HNS.

Технологическая свариваемость высокоазотистых сталей применительно к судовым корпусным конструкциям

Технологическая свариваемость высокоазотистых сталей применительно к судовым корпусным конструкциям

Structural Features and Application Prospects for High-Nitrogen Austenitic Steels

The particular features of structure formation in high-nitrogen austenitic steels characterized by intermittent decomposition of solid solution and ductile-brittle transition are considered. The effects of heat treatment, thermoplastic treatment, and alloying elements on the physical and mechanical properties of high-nitrogen austenitic steels are considered. Possibilities for using such steels as structural materials are analyzed.

Evolution of ultrafine precipitates and its influence on wear mechanism in cryoprocessed high nitrogen martensitic steel

ABSTRACTDevelopment of the space engines with liquid propellants necessitates the use of bearings of high quality. During recent years, a new family of high strength corrosion resistant steels, with partial replacement of carbon by nitrogen, known as high nitrogen steels were developed having higher impact strength and fracture toughness coupled with high hardness for antifriction bearing of cryopumps in rocket. In this research work, wear characteristics of cryogenically treated high nitrogen martensitic stainless steels were investigated by Pin-on-disc wear testing machine. After conventional hardening treatment, specimens were subjected to cryogenic treatment at −196°C with cryosoaking period varying from 8 to 32 h in the interval of 8 h followed by soft tempering. These specimens were analysed for hardness, wear resistance, subsurface and worn surface features by SEM. It is established that there is a profound influence of nucleation and growth of precipitates on the wear behaviour and the underlying wear mechanisms.

High nitrogen steels

The article provides a brief overview of the properties and production technology of high-nitrogen steels (HNS), which have several advantages over traditional ones. The main advantages are: up to four times higher yield strength with unique preservation of the remaining characteristics; reduction in consumption or a 100 % elimination of the use of some expensive alloying elements, such as Ni, Mo, Co, W, and others; effective alloying with unconventional elements (Ca, Zn, Pb, etc.). The basics of HNS technology, dependence of the properties on nitrogen content in steels, producing technologies for ferritic-pearlitic, martensitic and austenitic steel, their properties and applicability are discussed. Alloying with nitrogen for ferritic-pearlitic steel requires more precise adherence to the chemical composition in order to prevent the formation of insoluble nitrides during heat treatment (due to its greater solubility compared to carbon). Features of martensitic steels are associated with the possibility of formation of nitrides and carbonitrides during tempering. The possible effect of nitrogen in these steels may be as a decrease in the size of nitride particles as compared with carbide ones. Increased stability temperature of nitrides and carbonitrides provides increased mechanical and physical properties. In austenitic steels, nitrogen, due to the strong γ-forming equivalence to nickel, replaces it in a ratio of 1 kg of nitrogen ≈ 6 – 39 kg Ni. In austenitic-martensitic steels, the main role is played by thermal martensite. Stable austenite is obtained in the process of its aging at operating temperatures. Examples of effective use of HNS in important details are described.

Study on corrosion behavior of high-nitrogen steel laser-arc hybrid welding joints in different heat source action area

This paper investigates the effects on corrosion mechanism and pitting resistance in high-nitrogen austenite steel laser-arc hybrid weld. Electrochemical corrosion technology was used to study the corrosion behavior in 3.5% NaCl solution. The conclusions as follow: Both from microcosmic and macroscopical analysis, the δ-ferrite precipitated from dendrites is preferentially be corroded and exfoliated. The growth pattern and dense degree of dendrites is found to be the most significant factors that affect the corrosion tendency. Compared to the arc acting area, the laser acting area at the weld bottom prefer better corrosion mechanism because of the enhancement of Laser keyhole action. Pitting corrosion mainly concentrated in the center of the small fir-tree crystals and leave regular special secondary pitting hole. This phenomenon only exists in the austenite welds with dendritic distribution. The coarse dendrites with large structure is the starting layer of exfoliation corrosion which promote the further erosion of Cl−. Through the study of the weld hardness, it is further proved that the laser acting area does have more uniform structure distribution and thus has excellent corrosion resistance. Research reveals that the hybrid weld has unique corrosion behavior which is different from the conventional Austenite stainless steel.

Wear resistance of hydrogenated high nitrogen steel at dry and solid state lubricants assistant friction

Purpose: This paper is devoted the investigation hydrogen influence on of wear resistance of high nitrogen steel (HNS) at dry and solid state lubricants assistant friction. It has been established that after hydrogenation at 250 N loading the wear rate increased by 2.9 ... 4.1 times. Microhardness of hydrogenated layer was 7.6 ... 8.2 GPa, that is increased after hydrogenation in two times. After adding the (GaSe)xIn1-x compounds to the tribo conjugates by X-ray diffraction analysis it has been established the appearance of new phases which formed during the friction process and detected on the friction surface. Design/methodology/approach: This work presents research results concerning the comparative tests of high nitrogen steels in the circumstances of dry rolling friction. It was conducted the experiments to determine the tribological properties of high-nitrogen steels under rolling friction. The test pieces were manufactured in the form of rollers, and rotated with a linear velocity 2.27 m/s (upper roller), 3.08 m/s (bottom roller). Upper roller is made from HNS was subjected for hydrogenation. Analysis of friction surfaces indicates the complex mechanism of fracture surfaces. The results of the local X-ray analysis and X-ray diffraction analysis has been established the appearance of new phases and elements on the friction surface. Findings: It has been found that the level of wear resistance of the investigated materials under hydrogenation. Compounds realize chemisorption, tribochemical mechanisms of the formation of thin protective (anti-wear, antifriction) layers on metal surfaces. Research limitations/implications: An essential problem is the verification of the results obtained using the standard mechanical tests, computer-based image analysis and other methods. Practical implications: The observed phenomena can be regarded as the basic explanation of reduces the plasticity characteristics after hydrogenation. Applying the (GaSe)xIn1-x compounds as a lubricant will allow the formation of films on friction surfaces that can minimize surface wear, which will contribute to the transition to a wear-free friction mode. The protective film is a barrier to high shear and normal loads, preserving the base metal of the part and providing reduced wear and friction. Originality/value: The value of this work is that conducted experiments permit to determine the tribological properties of high nitrogen steels under rolling friction after hydrogenation. After adding (GaSe)xIn1-x compounds to the tribo conjugates after due to X-ray diffraction analysis it has been established the appearance of new phases which formed during the friction process and detected on the friction surface.

Comparison of the Dislocation Structure of a CrMnN and a CrNi Austenite after Cyclic Deformation

In the literature, the effects of nitrogen on the strength of austenitic stainless steels as well as on cold deformation are well documented. However, the effect of N on fatigue behaviour is still an open issue, especially when comparing the two alloying concepts for austenitic stainless steels—CrNi and CrMnN—where the microstructures show a different evolution during cyclic deformation. In the present investigation, a representative sample of each alloying concept has been tested in a resonant testing machine at ambient temperature and under stress control single step tests with a stress ratio of 0.05. The following comparative analysis of the microstructures showed a preferred formation of cellular dislocation substructures in the case of the CrNi alloy and distinct planar dislocation glide in the CrMnN steel, also called high nitrogen steel (HNS). The discussion of these findings deals with potential explanations for the dislocation glide mechanism, the role of N on this phenomenon, and the consequences on fatigue behaviour.

High-Nitrogen Steel

The properties of high-nitrogen steels are briefly reviewed, along with their production technologies. High-nitrogen steels are preferable to traditional steels in many respects: specifically, their yield point is four times higher, with a distinctive combination of other properties; expensive alloying elements such as Ni, Mo, Co, and W are required in smaller quantities, if at all; and effective alloying with nontraditional elements (such as Ca, Zn, and Pb) is possible. The influence of the nitrogen content on the properties of such steels is considered. The production technologies for ferritic–pearlitic, martensitic, and austenitic steels are described, along with their applications. For ferritic–pearlitic steels, more careful maintenance of the chemical composition is required in alloying with nitrogen, which is more soluble than carbon, so as to prevent the formation of nitrides, which are insoluble on heat treatment. On tempering martensitic steels, nitrides and carbonitrides may be formed. The influence of nitrogen in those alloys may be associated with decrease in size of the nitride particles relative to the carbide particles. The increased thermal stability of nitrides and carbonitrides improves the mechanical and physical properties of the steel. Because nitrogen is closely equivalent to nickel in terms of the formation of γ phase, it may be used to replace nickel in austenitic steels, in the following proportions: 1 kg nitrogen to around 6–39 kg of nickel. In austenitic–martensitic steels, the main role is played by thermal martensite; stable austenite is formed on aging at the working temperatures. Examples of the effective use of high-nitrogen steels in machine parts are presented.

The effect of age-hardening mechanism on hydrogen embrittlement in high-nitrogen steels

The effect of age-hardening regime on peculiarities of hydrogen-assisted fracture and tensile properties in two high-nitrogen Fe-23Cr-17Mn-0.1C-0.6N and Fe-19Cr-22Mn-1.5V-0.3C-0.9N steels was studied. A large number of intergranular (austenite/austenite) and interphase boundaries (austenite/coarse particle) provides high fraction of trapping sites for hydrogen atoms in V-alloyed steel. This leads to a change in fracture regime from transgranular brittle mode in coarse-grained V-free steel to intergranular brittle fracture of hydrogen-assisted surface layers in fine-grained V-alloyed steel with coarse (V,Cr)(N,C) particles. The formation of cells (Cr2(N,C) particles and austenite) along the grain boundaries due to discontinuous precipitate-hardening reaction facilitates predominantly interphase hydrogen-assisted fracture for both steels. The complex reaction of the particle-strengthening mechanisms including discontinuous precipitation with formation of austenite/Cr2(N,C)-plates interfaces or homogeneous nucleation of coherent (V,Cr)(N,C) particles in austenite (age-hardening regime 700 °C, 10 h) promotes mainly transgranular cleavage-like fracture mode under hydrogen-charging. The structural scheme is proposed to describe a change in hydrogen-assisted fracture micromechanisms and tensile properties of the steels with different density and distribution of interphase and intergranular boundaries.

Effect of N2 shielding gas flow rate on microstructure and weld surface corrosion resistance of high nitrogen steel by laser-arc hybrid welding

In order to investigate the microstructure of welded joint and corrosion resistance of the weld surface of high nitrogen steel, a laser-arc hybrid welding process was used for welding high nitrogen steel in this paper. The Ar + CO2 mixed gas was used as the shielding gas above the weldment, N2 was used as the shielding gas under the weldment. The microstructure of welded joint and corrosion resistance of the weld surface of high nitrogen steel were studied by changing the flow rate of N2. The results showed that all of the welded joints have an excellent macroscopic appearance, weld nitrogen content increases with increasing N2 flow, the microstructure of welded joints are austenite and a small amount of delta-ferrite, the higher the nitrogen flow, the tinier the dendrite of ferrite is. Compared with the weld arc zone, the dendrite of ferrite of the weld laser zone is finer. As the nitrogen flow rate increases, the pitting potential of L1–L4 increase, and the corrosion resistance become better. Nitrogen pores appear in the weld of the L5 group, and a large number of pitting pits appears around the nitrogen pores, which significantly reduce the corrosion resistance. The corrosion resistance of the weld arc zone is not affected by nitrogen flow, the dendrites of ferrite in the weld arc zone are peeled from the crystal structure. Compared with the surface of the weld laser zone, the corrosion resistance of the weld arc zone is worse.

NITROGEN STEELS AND HIGH NITROGEN STEELS. INDUSTRIAL TECHNOLOGIES AND PROPERTIES

Nitrogen pressure can be a basis for the most general classification of steel, alloyed by nitrogen. Nitrogen steels are made under normal pressure, high nitrogen steels are made under pressure that is higher than atmospheric in special units. Nitrogen, as well as carbon, also strengthens austenite, but increases thermal stability of austenite, has the smaller sizes of ions and high solubility in γ- and α-phases. The result is smaller size of nitrides, smaller superficial energy, their higher strengthening effect and possibility of simultaneous increase in durability and corrosion resistance of austenite. The article considers the mechanisms of nitrogen influence on properties of steel, thermodynamics and kinetics of steels alloying with nitrogen, critical concentration of nitrogen and influence of nitrogen on properties of steel. There is no uniform balanced database and thermodynamic model now. Therefore any choice of values of equilibrium constant and parameters of interaction according to tabular data reduces the accuracy of calculations of nitrogen solubility in steel. In the circumstances it is better to use experimental data for concrete alloy from original works. At the choice of data it is necessary to be guided by the following control values: KN = 0,044, A ≥ 600. The nitrogen solubility in liquid metal, in α- and γ-phases is significantly various. Critical nitrogen concentration Nk, which excess leads to formation of bubbles and interstices at steel solidification, depends on composition of steel. Now the acceptable results when determining critical nitrogen concentration, can be received from the following condition: during the whole time of solidification the nitrogen content in residual liquid has to be less its equilibrium with the general pressure in the system of content in liquid metal at the same temperature T. Examples of nitrogen and high nitrogen steels, including steels with special properties, such as corrosion-resistant in bioactive environments, bactericidal steel, alloyed according on the scheme C + N steels, are given.