High Nitrogen Austenitic Steel Research Articles

Weldability of corrosion-resistant high-nitrogen austenitic Kh22AG16N8M-type steels

The influence of thermal treatment on the structures and mechanical properties of welds of corrosion-resistant high-nitrogen austenitic 05Kh22AG16N8M-type steels is studied. In these steels, austenite is found to be highly resistant to discontinuous precipitation and the formation of σ phase and δ ferrite upon cooling regardless of the temperature of heating for quenching (from 900 to 1250°C) and the cooling conditions (water, air, furnace). Welding of these steels can produce high-strength welds with an enhanced impact toughness.

Transgranular fracture in low temperature brittle fracture of high nitrogen austenitic steel

Morphological features of transgranular fracture facets in low temperature brittle fracture of 18Cr–18Mn–0.7N high nitrogen austenitic steel have been studied by means of scanning electron microscopy and their formation mechanisms have been discussed. The transgranular fracture facets are fairly coarse compared with intergranular fracture facets and annealing twin boundary fracture facets. There are parallel steps and river patterns on the facets. Dual-surface observation indicated that these patterns are parallel to {111} planes and of strict crystallographic features. Microstructure observation shows that there are a lot of planar deformation structures formed prior to low temperature transgranular fracture. Transgranular fracture originates from microcracks formed at the intersections of the deformation structures on different {111} planes. These microcracks propagate toward adjacent microcracks on different {111} planes, forming transgranular fracture facets with steps and river patterns.

Progress in the study of high-nitrogen corrosion-resistant aging nonmagnetic vanadium steels

Studies of the structure and properties of high-nitrogen corrosion-resistant aging nonmagnetic vanadium steels have been analyzed. The hardening of these steels by fine vanadium nitride and carbonitride particles is considered. The mechanisms of the discontinuous decomposition of austenite supersaturated by nitrogen upon heat treatment are described. Methods for the removal of the negative effect of the discontinuous decomposition on the mechanical properties of high-nitrogen steels have been found. Steels with an overequilibrium nitrogen content are shown to exhibit superplasticity. The dependences of the microstructure and properties of high-nitrogen vanadium austenitic steels on their alloying and the conditions of thermal and thermoplastic treatments are considered.

Strain rate dependence of deformation behavior of high-nitrogen austenitic steels

Abstract Tensile deformation of high-nitrogen austenitic steel under various strain rate conditions was characterized to investigate the deformation mechanism of the steel at different strain rates. Room-temperature tensile tests of the steel with nitrogen contents of 0.51 wt.% and 0.88 wt.% were performed at different strain rates ranging from 5 × 10 −5 s −1 to 5 × 10 −1 s −1 . Tensile curves of the steel were analyzed and compared, to obtain strain hardening characteristics, yield and tensile strengths as well as the elongation. Microstructures of the tested specimens were examined to understand the deformation behavior of the steel. The high-nitrogen steel with a complete austenite microstructure did not exhibit noticeable flow–stress dependence on strain rates. However, the steel with secondary phases such as nitrides in an austenite matrix showed strong strain rate dependency. The flow stress level increased significantly and the elongation decreased at strain rates higher than 5 × 10 −2 s −1 . The change of the mechanical property became more significant as the nitrogen content in the steel increased. The deformation behavior of the nitrogen steel was elucidated by planar slip and twinning, which operated differently at different strain rates.

On the crystal structure of Cr2N precipitates in high-nitrogen austenitic stainless steel. III. Neutron diffraction study on the ordered Cr2N superstructure

The ordered structure of Cr(2)N precipitates in high-nitrogen austenitic steel was investigated utilizing high-resolution neutron powder diffractometry (HRPD). On the basis of the Rietveld refinement of neutron diffraction patterns, the ordered Cr2N superstructure was confirmed to be trigonal (space group P31m), with lattice parameters a=4.800 (4) and c=4.472 (5) A, as suggested in previous transmission electron microscopy studies [Lee, Oh, Han, Lee, Kim & Takaki (2005). Acta Cryst. B61, 137-144; Lee, Kim & Takaki (2006). Acta Cryst. B62, 190-196]. The occupancies of the N atoms in four crystallographic sites [1(a), 1(b), 2(d) and 2(c) Wyckoff sites] were determined to be 1.00 (5), 0.0, 0.74 (9) and 0.12 (3), respectively, reflecting a partial disordering of N atoms along the c axis. The position of the metal atom was specified to be x=0.346 (8) and z=0.244 (6), corresponding to a deviation from the ideal position (x=0.333 and z=0.250). This deviation caused the ((1/3 1/3)(0))-type superlattice reflection to appear. A comparison between the ideal and measured crystal structures of Cr2N was performed using a computer simulation of selected-area diffraction patterns.

Characteristics of Low Temperature Brittle Fracture Surface of High Nitrogen Austenitic Steel

Fracture surface and crack propagation in low temperature brittle fracture (LTBF) of an 18Cr-18Mn-0.7N high nitrogen austenitic steel (HNAS) were examined by means of scanning electronic microscopy, and compared with behaviours of LTBF of low carbon steel. Similar to BCC low carbon steel, the HNAS experienced a typical ductile-to-brittle transition (DBT) with decreasing temperature, and the appearance of the fracture surface transited from fibrous to granular. Dual-surface observation revealed that there were three types of fracture modes in LTBF of the HNAS: annealing twin boundary fracture, intergranular fracture, and transgranular fracture. The annealing twin boundary fracture facets were parallel to {111} planes, and were fairly flat and smooth, with a pattern of three sets of parallel straight-lines intersecting at 60. There were also bent steps that originated and terminated at grain boundaries. The transgranular fracture facets were coarse and uneven, with uniformly distributed small pits and partially river pattern on them. The intergranular fracture facets were smoothly curved ones on which more than three sets of parallel deformation structure trace lines were observed. Careful observation on crack propagation demonstrated that during LTBF of the HNAS, microcracks formed firstly at grain boundary and annealing twin boundary, and then these microcracks came together and coalesced to induce crack propagation through grains, resulting in a fracture appearance with shiny facets distributing in dull facets.

Dissolution and precipitation of excess phases and the nitrogen distribution between a solid solution and nitrides in a corrosion-resistant steel

The dissolution and precipitation of nitrides are experimentally studied during the solidification and heat treatment of high-nitrogen austenitic steels. The majority of nitrogen is found to be in nitrides even upon rapid solidification, and the dissolution of these nitrides requires high temperatures and long-term holding. The nitrogen content has to be optimized according to the required steel structure, namely, austenite or austenite + excess phases. A high nitrogen content in a steel is not always effective to provide its high strength.

固相窒素吸収法の特徴と得られる高窒素ステンレス鋼の特性

Solution nitriding (nitrogen absorption treatment) is one of the techniques to produce high nitrogen stainless steels. In this article, the merits and problems of solution nitriding, the general theory and phenomenon of nitrogen absorption, and the properties of solution-nitrided high nitrogen austenitic steels were reviewed on the basis of the results obtained by authors in the recent research. Especially, the thermodynamics and kinetics of nitrogen absorption were emphasized because of their importance in the production of solution-nitrided materials. In addition, the potential of high nitrogen steels produced by solution nitriding was also briefly discussed in terms of the prospective application of nickel-free stainless steels to fine or micro-sized structural materials for biomedical uses.

Effect of nitrogen on crevice corrosion in austenitic stainless steel

Corrosion properties of high nitrogen austenitic steels in chloride solutions have been investigated. Nitrogen behavior was evaluated at various electrode potentials, and analysis of the surface film was carried out with XPS. The alloy used for the experiments had a composition of 23%Cr–4%Ni–0–1%Mo–0.7–1%N and was obtained through electro-slag remelting (ESR) under high nitrogen pressure. High nitrogen austenitic steel produced NH 4 + in the solution by crevice corrosion under a constant potential of 0.2 V (SCE). In the transpassive region and at 0.7 V (SCE), the products in the solution were NH 4 + , NO 3 - and NO 2 - . The amount of dissolved NO 3 - and NO 2 - increased with the electrode potential. NH 4 + in the solution suppressed decreases of pH, having a re-passivation effect. For crevice corrosion under a higher electrode potential than 0.4 V (SCE), the number of crevice corrosion points and the corrosion loss decreased as the electrode potential increased. This behavior can be attributed to the corrosion suppressing effect of NO 3 - dissolved in the solution as a product of crevice corrosion. The presence of chromium and iron oxides in the passivation film and crevice corrosion surface film were identified from XPS analysis. N 1s spectra indicated the presence of a nitride (CrN) or NH 3.

Effect of nitrogen on the plasma (ion)-carburized layer of high nitrogen austenitic stainless steel

Austenitic stainless steel or high nitrogen austenitic steel are used extensively as structural non-magnetic materials. The former, because of their low surface hardness, have low wear resistance. Recently developed plasma (ion)-carburizing improves the surface of austenitic stainless steel by removing the oxide scale. In contrast, high nitrogen austenitic stainless steel are expected as wear-resistance material because the hardness is higher than that of conventional austenitic stainless steel.In this study, two kinds of austenitic stainless steel were plasma (ion)-carburized in an atmosphere of CH4+H2 plasma at 1303 K for 14.4 ks. The metallurgical characteristics were then investigated using XRD (X-ray diffraction method), OM (optical microscopy), hardness testing, glow discharge optical emission spectroscopy (GDOES) and a scanning electron microscopy (SEM).The results show that the plasma (ion)-carburized low nitrogen austenitic stainless steel had the layer with structure similar to the common austenitic stainless steel. The plasma (ion)-carburized high nitrogen austenitic stainless steel had the carbide layer at the top surface, carbide dispersed layer in the matrix, and then lamellar structure of carbides toward the inner layer.

Influence of Modes of Thermoplastic Treatment on the Development of Microplastic Strain in Nitrogen-Bearing Steel Kh21G10N7MBF

The effect of the degree of reduction in cold rolling and subsequent heat holds on the development of microplastic strain in the bending of ribbon specimens fabricated from corrosion-resistant austenitic high-nitrogen steel Kh21G10N7MBF is studied.

Precipitation of σ-phase in high-nitrogen austenitic 18Cr–18Mn–2Mo–0.9N stainless steel during isothermal aging

Precipitation mechanism of σ-phase in austenitic high-nitrogen steel (HNS) upon aging was investigated using electron microscopy and thermodynamic calculation. The precipitation sequence was found to be grain boundary Cr 2N, cellular Cr 2N and σ-phase with aging time. It was concluded that the nitrogen-depleted zone near Cr 2N induces the nucleation of σ-phase.

Structural Stability of Austenitic CrMn(Mo)N Steels for High-Strength, Corrosion-Resistant Retaining Rings

The improved temperature resistance of insulating materials enables the service temperature of retaining rings on electric generator shafts to be raised from 90 to 130 °C. Therefore the structural stability of two high nitrogen austenitic steels in respect to cold expansion and ageing was investigated by XRD, STEM and internal friction. Cold working caused the formation of about 5 to 10 % E-martensite, which was promoted by the low stacking fault energy of about 30 mJ/m 2 . Ageing between 90 and 170 °C was accompanied by the precipitation of Fe 3 N which dissolved during subsequent aging at 400 °C because of the stronger bond of nitrogen to dislocations. Their pinning prevented long-term structural changes of a ring which was shrink fitted at 400 °C and more than 60.000 h in service.

Sliding wear behaviour of an electrochemically modified austenitic high-nitrogen steel surface

Wear debris from artificial metallic implant joints is known to provoke detrimental foreign-body reactions in the surrounding human tissue. Although commonly used biotolerant metals generate only a little amount of particles, wear is still a major cause for concern. It is the aim of this work to evaluate the sliding wear resistance of a topologically modified high-nitrogen austenitic stainless steel. A disc-on-pin test in self-mating contact is performed in distilled water. Submicron particles are trapped by the structured topography and form together with the plastically deformed metal a hybrid surface which has the potential to significantly improve the tribological behaviour of the tested high-nitrogen steel.

Ermüdungsverhalten des hochstickstofflegierten und nickelfreien Stahles X13CrMnMo18-14-3 an Luft und in Ringerlösung

The present work presents a new austenitic high-nitrogen and nickel-free steel X13CrMnMoN18-14-3, (1.4452). The cyclic fatigue behaviour of this materials was investigated under simulated body conditions. When compared to solution annealed CrNiMo-steels, fatigue endurance limit is higher. The pitting potential in Ringer's solution was measured being 1.1 V, which is in the range of CoCr29Mo6. Hence, the austenitic high-nitrogen steel X13CrMnMoN18-14-3 may be a promising alternative to CrNi-Mo-steels due to the absence of nickel and improved mechanical and chemical properties.

Fabrication of Fine-grained High Nitrogen Austenitic Steels through Mechanical Alloying Treatment.

Mechanical alloying (MA) treatment was applied for the fabrication of fine-grained high nitrogen stainless steels. Chromium nitride (Cr 2 N) powder was mixed with Fe-Cr binary alloy powder to control its mean chemical composition to be Fe-23mass%Cr-1 mass%N which is enough to stabilize austenitic structure at room temperature. The powder mixture was mechanically alloyed up to 360 ks in an argon gas atmosphere (MA powder). The MA powder was packed in a stainless steel tube in a vacuum and consolidated by warm rolling at 1 073 K. The consolidated materials were finally heated to various temperatures (1 173-1 473 K) for austenitizing and then quenched without holding at the temperatures. Although the materials heat-treated below 1 323 K had bcc (martensitic) matrix, those heat-treated above 1 373 K had stable austenitic structure with a small amount of Cr 2 N. The grain size of matrix was maintained to be fine due to dispersed oxide particles within matrix in all steels. For example, the materials heat-treated at 1 473 K had fine austenitic structure in which the grain size was 2.2 μm and the solute nitrogen concentration was 0.86 mass%. The steel had very high yield strength of 1.1 GPa and moderate elongation of 30 %. Such a high strength of the steel was explained by the combined strengthening mechanism of nitrogen solid solution strengthening and grain refining strengthening.

Resistance of a high nitrogen austenitic steel to cavitation erosion

The resistance to cavitation erosion (CE) was measured using a rotating disk device for a high-nitrogen CrMn austenitic steel. The damaged morphology and microstructure characteristics in the surface layer exposed to CE were analyzed by the use of SEM and TEM. The results show that the resistance to CE of 18Mn18Cr0.5N (HNS) steel is 3.3 and 2.3 times higher than that of 0Cr13Ni6Mo (CrNi1) and 0Cr16Ni5Mo (CrNi2) steels, respectively, which are in commonly used for hydraulic turbine runners. The excellent resistance to CE of HNS steel is related to its good mechanical properties and the following microstructure changes: strengthening due to changes of dislocation configurations and the formation of substructures during CE, consumption of CE impact energy by the processes of CE-induced mechanical twins or stacking faults etc. as well as the strengthening due to the CE-induced products itself, the strengthening due to the block of strong N(C)Me covalent bond on dislocation motion and the reconfiguration of crystal lattices.

Development of high-strength and high-conductivity conductor materials for pulsed high-field magnets at Dresden

The work at the IFW Dresden is focused on the development of microcomposite Cu–Ag alloys and steel–copper macrocomposites with high-nitrogen steel and pearlitic steel jackets, respectively. In Cu–Ag alloys the investigation of continuously cast rods with different starting diameters suggests that the cooling rate during solidification determining the dendrite arm spacing has a minor influence on the development of the strength compared to the cooling velocity after solidification which determines the extent of the Ag-supersaturation in the Cu solid solution. Maximum strength at minimum drawing strain demands (i) a sufficient volume fraction of eutectic in order to suppress discontinuous precipitation (absence of grain boundaries) and (ii) a sufficiently rapid cooling after solidification in order to prevent pre-precipitation. With a continuously cast starting rod of 12 mm diameter a maximum tensile strength of 1.3 GPa was obtained after a drawing strain of only η=4.3. Steel–copper macrocomposites were fabricated by the ‘rod-in-tube’ technology. The experiments with austenitic high-nitrogen steels were performed with two alloys. With the commercial alloy Nicrofer 3033 a strength level of 1.2 GPa has been achieved with a 52 vol% Cu composite at a drawing strain of η=2.3. A composite with pearlitic C60-steel (0.6 wt% C) and 56 vol% Cu showed a tensile strength of 1.53 GPa after a final patenting at a diameter of 14.7 mm and a drawing strain of η=4.

Electrochemical Inhomogeneity and Corrosion of Welded Joints of High-Nitrogen Austenitic Steels

Electrochemical Inhomogeneity and Corrosion of Welded Joints of High-Nitrogen Austenitic Steels

Mössbauer Effect and Neutron Scattering in Austenitic High Nitrogen Steel Fe-19Cr-19Mn-0.9N

Austenitic high nitrogen Fe-19Cr-19Mn-0.9N steel was studied by means of Mossbauer analysis and neutron diffraction and small-angle neutron scattering (SANS) after solution treatment at 1125°C and ageing at 250-350°C. The redistribution of nitrogen atoms in austenite after ageing was detected in Mossbauer measurements. SANS was observed in annealed and egeined samples characterizing an existence of the inhomogeneities in steel of atomic and magnetic nature. The linear size and shape of inhomogeneities is estimated on the basis of analysis of the integral parameters of scattering curves.