High Nitrogen Austenitic Steel Research Articles

Effect of Cold Deformation on Microstructure and Mechanical Properties of a Fe–20Mn–19Cr–0.5C–0.6N High Nitrogen Austenitic Steel

The effect of cold deformation on microstructure and mechanical properties of a Fe–20Mn–19Cr–0.5C–0.6N high nitrogen austenitic steel has been studied. Microstructural observations show that the low cold deformation is fully dominated by the dislocation structures, namely, pile‐ups, Taylor lattice, and dislocation cells. With further cold deformation, the effect of twinning induced plasticity and grains fragmentation play an important role during cold deformation process. At critical cold deformation reduction range of 26–36%, the microstructural transitions by dislocation planar slip and multiplication gradually transits to twinning. For mechanical properties, with the increase of cold deformation reduction, the yield strength increases from 0.52 to 1.60 GPa, being increased by three times. The flow stress–strain model of the solution‐treated specimen can be described as . In addition, the strength‐elongation product and the cold deformation reduction present a linear relationship with a correlation coefficient of −0.27 to estimate the variation rate of the strength‐elongation product during cold deformation.

Microstructural Evolution and Hardness Profile of High Nitrogen Austenitic Steel After Arc-laser Hybrid Welding

An arc-laser hybrid welding was performed on high nitrogen austenitic steel which was used in the industries. Micro-vickers hardness tester, Optical microscopy, secondary and backscattered electron beam image analysis, image analyzer and Energy Dispersive X-ray Spectroscopy (EDS) in Scanning Electron Microscopy (SEM) were applied to reveal the microstructural details. The results indicate that, the microstructures of the weld bead are consisted of the dendrite-like austenitic phase and the small rod-like delta ferritic phase located between the dendrites. The solidification mode of the bead is AF mode. The grain size in heat affected zone (HAZ) is larger than the parent metal, and the near the bead area, the larger the size of the grain.

Structural and phase transformations and micromechanical properties of the high-nitrogen austenitic steel deformed by shear under pressure

Using transmission electron microscopy, X-ray diffraction analysis, Mossbauer spectroscopy, microdurometry, and microindentation, the effect of large plastic deformations (through shear under pressure in Bridgman anvils) on the structure, phase composition, and micromechanical properties of high-nitrogen (1.24 wt % N) 08Kh22GA1.24 steel has been investigated. The steel was obtained by the casting method with counterpressure of nitrogen and was subjected to different heat treatments (quenching from1180°С, aging at 450 and 550°С) that form an austenitic (FCC) structure of the metallic matrix with chromium nitrides. It has been established that deformation by shear under pressure at room temperature results in the dispersion and deformation-induced partial dissolution of primary nitrides Cr2N in quenched and aged steel and in the complete (after aging at 450°С) and partial (after aging at 550°С) dissolution of secondary nitrides CrN. It has been noted that, for aged steel that contains finely dispersed secondary chromium nitrides upon shear deformation, as compared to the quenched state, the dispersion of the austenitic structure (down to nano- and submicrocrystalline states) is more intense and the enhancement in the microhardness and resistance to elastic–plastic deformations upon contact loading is more effective.

Effect of hydrogenation on mechanical properties and tensile fracture mechanism of a high-nitrogen austenitic steel

Austenitic stainless steels are frequently used for hydrogen applications due to their high ductility at low temperatures and lower hydrogen environment embrittlement compared to ferritic steels. We study the effect of electrochemical hydrogen saturation up to 40 h on tensile behavior and fracture mechanisms in high-nitrogen austenitic 17Cr–24Mn–1.3V–0.2C–1.3N steel. Hydrogen saturation weakly influences the characteristic of stress–strain curves, but decreases steel ductility, yield stress, and ultimate tensile stress. Hydrogenation provides a change in steel fracture peculiarities—a hydrogen-assisted thin brittle surface layer of ≈5 μm and ductile subsurface layer of 50–150 μm in width in hydrogen-saturated specimens. The subsurface layer shows ductile transgranular fracture with elongated dimples and flat facets. The central parts of fracture surfaces for hydrogenated specimens show ductile fracture mode similar to hydrogen-free state, but they include numerous secondary cracks both for central part and for transition zone between ductile central part and subsurface layer associated with highest hydrogen saturation. The possible reasons of decrease in hydrogen-associated ductility and change in fracture character are discussed.

Correlation between the granular structure and the mechanical properties of high-nitrogen austenitic 02Kh20AG10N4MFB steel after annealing

The effect of the annealing temperature and time on the formation of a granular structure in high-nitrogen austenitic 02Kh20AG10N4MFB steel has been studied. The hardness and the strength properties of the steel are shown to be related to the mean grain size by an inverse dependence, according to the Hall–Petch relation, and the impact toughness is proportional to the mean grain size. At annealing temperatures to 1100°C, structure formation is determined by the precipitation of secondary phases; at higher annealing temperatures, it is determined by the recrystallization of austenite grains.

Modelling approach to support design of new high nitrogen austenitic steel for wind turbines components

The alloy design of a new high nitrogen austenitic steel for bearing used in wind turbine gearboxes has been approached. The alloy design was based on a modelling approach to the different aspects of manufacturing routes. The mechanical strength was predicted by means of empirical models. Nitrogen solubility in liquid steel was estimated on the basis of Sievert’s law and Thermo-Calc calculations. Scheil calculations and models developed by Centro Sviluppo Materiali SpA (CSM) were used for estimating nitrogen solubility. The stability of austenite has been studied by thermodynamic calculations. The results show that amounts of at least N = 0.5% are required to fall in austenite domain approaching the required mechanical strength. Vacuum induction melting + pressure electroslag remelting is required to manufacture alloys with N ⩾ 0.6%, but the addition of elements as Mn can potentially allow to increase N amounts achievable with only vacuum induction melting route processing.

Effect of heat treatment on the mechanical properties and the structure of a high-nitrogen austenitic 02Kh20AG10N4MFB steel

The effect of heat treatment on the mechanical properties of a high-nitrogen austenitic 02Kh20AG10N4MFB steel has been studied in the temperature region 550—1200°C. The yield strength and the ultimate tensile strength are shown to change nonmonotonically as a function of the heat treatment temperature. They sharply decrease in the annealing temperature range 850—900°C, which can demonstrate a change in the character of the structure–phase state of this steel. After annealing at 850—900°C, aging occurs with the precipitation of embrittling phases; at higher annealing temperatures, these particles dissolve and austenite recrystallizes. The study of the stress–strain diagrams makes it possible to find the laws of strain hardening of the 02Kh20AG10N4MFB steel as a function of the heat treatment temperature.

The effect of hydrogen on strain hardening and fracture mechanism of high-nitrogen austenitic steel

High-nitrogen austenitic steels are perspective materials for an electron-beam welding and for producing of wear-resistant coatings, which can be used for application in aggressive atmospheres. The tensile behavior and fracture mechanism of high-nitrogen austenitic steel Fe-20Cr-22Mn-1.5V-0.2C-0.6N (in wt.%) after electrochemical hydrogen charging for 2, 10 and 40 hours have been investigated. Hydrogenation of steel provides a loss of yield strength, uniform elongation and tensile strength. The degradation of tensile properties becomes stronger with increase in charging duration - it occurs more intensive in specimens hydrogenated for 40 hours as compared to ones charged for 2-10 hours. Fracture analysis reveals a hydrogen-induced formation of brittle surface layers up to 6 μm thick after 40 hours of saturation. Hydrogenation changes fracture mode of steel from mixed intergranular-transgranular to mainly transgranular one.

Effect of the rolling temperature on the structure and the mechanical properties of high-nitrogen austenitic steels 05Kh21G9N7AMF and 04Kh22G12N4AMF

The structure and the mechanical properties of high-nitrogen austenitic 05Kh21G9N7AMF (0.56% N) and 04Kh22G12N4AMF (0.49% N) steels have been studied after hot rolling. It is found that the temperatures of the onset and end of hot deformation influence the structure and the mechanical properties of these steels. The higher set of mechanical properties of steel 05Kh21G9N7AMF after rolling in the temperature range 1100–900°C is due to the formation of a lamellar and equiaxed fragmented structure.

Twip-Effect in Nickel-Free High-Nitrogen Austenitic Cr – Mn Steels

The mechanism of plastic straining and the structure of high-nitrogen austenitic steel 06Kh18AG19M2 (0.81% N) after a hardening treatment involving quenching, aging, and plastic deformation by equichannel angular pressing (ECAP) are studied. Electron microscope investigation is used to determine the mechanism of straining and fragmentation of the structure of the steel under the ECAP, i.e., twinning. The method of x-ray diffraction analysis is used to evaluate the contribution of the parameters of structural flaws into the broadening of austenite lines under the ECAP.

Effect of quenching conditions on the formation of the grain structure and the mechanical properties of high-nitrogen austenitic 02Kh20AG14N8MF and 02Kh20AG12N4 steels

The formation of the grain structure of high-nitrogen 02Kh20AG14N8MF and 02Kh20AG12N4 steels in forging and quenching and their mechanical properties in this state have been studied. It is found that both steels have close mechanical properties under the same quenching conditions. In 02Kh20AG14N8MF steel, a homogeneous structure of primarily recrystallized austenite grains forms under the quenching conditions under study. In 02Kh20AG12N4 steel, the processes of secondary recrystallization and normal grain growth take place.

High-strength austenitic welded steel for shipbuilding

The paper depicts long-term research resulting in creation of high-nitrogen austenitic steels for shipbuilding. It shows the influence of the chemical composition and technology on the crystal lattice and steel structure, mechanical and corrosion properties, weldability, and other characteristics. The paper studies the fine structure of nitrogen austenitic steels and shows the important role of nitrogen, which impacts the strength and corrosion resistance more effectively than carbon while maintaining a high level of plastic properties and resistance to cyclic and dynamic loading. Magnetic and vibrodamping characteristics are evaluated. High-strength nitrogen austenitic steel with the yield strength of 650 MPa has been successfully adopted by the domestic industry. Its implementation for heavy duty structures is in the final stage. A prognostic evaluation for further development of nitrogen austenitic steels for various industries is given.

Austenitic high interstitial steels vs. CoCrMo – Comparison of fatigue behavior

The favourable combination of strength and ductility as well as excellent corrosion resistance of CrNiCN-austenitic steels is the reason for their use in biomedical engineering. However, Ni was attributed to cause allergies within a certain number of patients. Thus Ni-free alternatives like CoCrMo- and Ti-base alloys are often preferred. Other possible alternatives might be austenitic high nitrogen steels (AHNS) and austenitic high interstitial steels (AHIS). The aim of this study is to compare the fatigue properties of Ni-free austenitic CrMn(Mo)CN-steels with 0.85–1.07wt% C+N, CoCrMo-alloy with up to 0.25wt% C and conventional CrNiCN-steels with 0.01–0.42wt% C+N.The comparison was conducted using total strain controlled axial LCF and HCF fatigue tests at room temperature on solution-annealed samples. After mechanical testing the samples were investigated by means of SEM including EBSD and TEM in order to analyse the distribution of plastic strains.In contrast to the CrNiCN-steels, which show cyclic hardening before the generation of the first crack, the CrMn(Mo)CN as well as the CoCrMo-alloys only showed cyclic softening. Still within the HCF regime the Co-base alloys showed the best fatigue behavior in terms of numbers of cycles to fracture Nf for a given total strain amplitude, while the Ni-free CrMn(Mo)CN-steels were either as good or slightly worse. The results indicate a distinct correlation between slip behavior, stacking fault energy and the amount of interstitials in solid solution.

Determination of the temperature range of recrystallization based on structural studies of an austenitic high-nitrogen Cr–Mn–Mo steel

The effect of thermoplastic treatment, which includes cold plastic deformation by rolling, on the microstructure of a high-nitrogen 07Kh16AG13M3 steel has been studied. The temperature range of recrystallization is determined by three methods. An equation is proposed for calculating the melting temperature of the high-nitrogen austenitic steel

오스테나이트계 Fe-18Mn-18Cr-0.61N 고질소강의 인장 및 충격 특성에 미치는 냉간 가공의 영향

High-nitrogen Fe-18Mn-18Cr-N austenitic steels with higher yield strength have been recently developed and used for generator retaining rings because they have non-magnetic, high strength, high ductility, and good corrosion resistance. In the present study, a high-nitrogen Fe-18Mn-18Cr-0.61N austenitic steel was fabricated and then tensile and Charpy impact tests were conducted on them in order to investigate the effect of cold working on the mechanical properties. Although the yield and tensile strengths usually increased with cold working, the ductility and impact toughness significantly decreased after cold working. On the other hand, the high-nitrogen austenitic steel exhibited a ductile-brittle transition due to unusual brittle fracture at low temperatures despite having a face-centered cubic structure. The ductile-brittle transition temperature obtained from Charpy impact tests could be remarkably increased by <TEX>$60^{\circ}C$</TEX> after 20% cold working because of the enhanced cleavage-like brittle fracture.

Fatigue of Austenitic High Interstitial Steels - The Role of N and C

In order to increase the strength and maintain the ductility of austenitic steels high Nitrogen austenitic steels (AHNS) emerged of which Ni was substituted by Mn so that up to 1 w% N could be alloyed and kept in solid solution. Cold working was added to gain strength values up to 3000 MPa. Still the endurance limit did not follow this trend. The low stacking fault energy was thought being the main reason for the solely planar slip but it became clear that other near-field effects might govern this behaviour as well. Thus the density of free electrons could be identified as being one for CrMn-steels being mainly influenced by the sum and the ratio of C and N. In order to investigate this strain-controlled fatigue tests are carried out. This contribution presents the results of strain-controlled fatigue tests and discusses them on the basis of SEM-EBSD and TEM investigations in relation to the microstructural characteristics.

Comparison of Energy Absorption Properties of High Nitrogen Austenitic Steel and Cast Alloy Determined Using Low Velocity Perforation Test

Abstract The results of energy absorbing analysis of VP159 austenitic steel and LH556 cast alloy were presented in this article. The assessment was carried out on the basis of drop-weight tower perforation test at impact energy equal to 500J and striker velocity equal to 12,5 m/s. Moreover, the basic mechanical properties of both tested materials were estimated in order to calibrate coefficients of the Johnson-Cook visco-plasticity model and Johnson-Cook damage initialization criterion as well. Subsequently, both models were applied for the finite element method simulation of perforation process. The reasonable agreement between measured and calculated shape of energy absorption curves were obtained for steel and cast alloy as well.

Deformation behavior of ferrite-austenite duplex high nitrogen steel

The tensile deformation behavior of ferrite-austenite duplex high nitrogen steel has been investigated by interrupted tensile tests and compared with that of fully austenitic high nitrogen steel. It shows that ferrite is softer than austenite and most of the strain in early and later stages of deformation is accommodated by ferrite, while austenite undergoes a deformation-induced martensitic transformation. Such accommodation of a large amount of strain in ferrite is responsible for rapidly increasing work hardening rate and the resultant higher ultimate tensile strength of duplex high nitrogen steel as compared to those of fully austenitic high nitrogen steel, although duplex steel contains a smaller amount of N than austenitic steel.

The effect of high-pressure torsion on microstructure and strength properties of high-nitrogen austenitic steel

In this paper, we investigate the microstructure and microhardness of high-nitrogen austenitic steel after high-pressure upset and high-pressure torsion (6 GPa) at room temperature. As the result of deformation, steel microhardness increases by 1.5 times after high-pressure torsion per one revolution while the distribution of microhardness is quasi-homogeneous across the disks. The level of solid solution hardening of steel remains high after deformation processing, and the main mechanisms determining fragmentation of the steel structure are slip, twinning, formation of localized deformation microbands, and precipitation hardening.

Metallurgical aspects on the fatigue of solution-annealed austenitic high interstitial steels

Austenitic stainless steels have been used for over 100years for their combination of strength and ductility. In order to further improve the mechanical and chemical properties of austenitic high nitrogen steels (AHNS) were developed. Ni reduces the solubility of N and, therefore, was substituted by Mn in order to allow for up to 1weight-% N to be alloyed. AHNS show an even higher strength for the solution annealed state, which can be increased further by cold working. Unfortunately the endurance limit did not follow this trend as it is known to for cold-worked Ni-containing steels. The solution annealed Ni-containing austenites allow for wavy slip and the generation of dislocation cells while the Mn-alloyed AHNS only show planar slip with twins and stacking faults. While the stacking fault energy was thought to be the main reason for planar slip, early results showed that there must be other near-field effects. The density of free electrons, which is mainly influenced by the sum and the ratio of C and N, might be responsible. Strain-controlled fatigue tests were carried out in CrMn-alloyed austenitic steels with different sums (C+N: 0.65–1.2) and ratios (C/N: 0.13–∞) of C and N. Manson–Coffin analyses revealed distinct differences in the fatigue behaviour to CrNi-alloyed C+N steels investigated earlier. This contribution presents these differences and discusses them in relation to microstructural characteristics as well their alterations under cyclic loading.