Nitrogen-alloyed Stainless Steel Research Articles

Nitrogen Control in Production of N-Alloyed Stainless Steels in AOD Converter: Application of Sieverts’ Law

In numerous stainless steels, nitrogen is used as an alloying element to improve mechanical and corrosion properties. The typical nitrogen content is up to 0.5 pct. A common method for alloying is to blow nitrogen gas into an Argon Oxygen Decarburization (AOD) converter. Depending on the steel composition, operating practice, and process conditions, the hitting grade of the N content can be a problem. According to Sieverts’ law, nitrogen solubility depends on the nitrogen partial pressure. In this study, the applicability of Sieverts’ law to control the nitrogen content in stainless steels was investigated by test trials in an industrial AOD converter using N2–Ar mixtures. The validity was established in a wide range of nitrogen content (0.1–0.4 pct) and large variety of alloying (Cr, Ni, Mo, Mn) with considerable influence on N solubility. The validity was well established by approaching the equilibrium N content both from above and below. As a conclusion, the nitrogen content can be controlled by regulating the partial pressure of nitrogen in the blowing gas mixture. For practical purposes, the final nitrogen content can be predicted in different situations and guide graphs were drawn to determine the required N2–Ar ratio. These tools can be utilized when designing blowing practices for the AOD converter for nitrogen-alloyed stainless steels.

Microstructure and properties of high-strength C + N austenitic stainless steel processed by laser powder bed fusion

In the developing field of laser powder bed fusion (L-PBF), austenitic stainless steels, such as AISI 316L, have gained great importance owing to their excellent processability. However, the moderate strength of these steels limits their applicability. This can be counteracted by the use of nitrogen as an alloying element to improve both strength and corrosion resistance.In this work, nitrogen-alloyed high-strength austenitic stainless steel X40MnCrMoN19-18-1 was processed by L-PBF, and the resulting microstructural and mechanical properties were investigated. The same material was also processed by hot isostatic pressing (HIP), which was used as a reference state. In the L-PBF process, argon and nitrogen were used as process gases to investigate the influence of process atmosphere on the microstructure and on changes in the chemical composition during processing. The results show a minor decrease in the nitrogen content of the steel after L-PBF, independently of the process gas, whereby argon resulted in a slightly higher specimen density. The microstructure after L-PBF processing contained small precipitates that could be removed by a short solution-annealing treatment. The tensile properties of the L-PBF-built steel are comparable to those of the steel produced by hot isostatic pressing in terms of ultimate tensile strength, but had lower elongation to fracture values. The ductility of the material was enhanced by solution annealing without significant impairment of the ultimate tensile strength. This work demonstrates that nitrogen-alloyed stainless steels can be processed by means of L-PBF and can extend the variety of appropriate steels towards applications with high requirements for the material strength and chemical resistance.

Phases and defects upon the solidification of nitrogen-alloyed stainless steels

One of the technological problems in alloying molten steels with nitrogen is the precipitation of nitrogen into the gas phase upon the solidification of steels and the formation of nitrogen bubbles and porosity in steel ingots with the result that potentials of nitrogen as an alloying element are used incompletely. The formation of bubbles and pores in nitrogen-alloyed stainless steels occurs heterogeneously and is limited by the desorption rate of nitrogen from the molten-metal surface into bubbles. For this reason, it is convenient to determine the critical nitrogen concentration in steels as the concentration at which in the process of equilibrium solidification the content of nitrogen in the residual liquid phase at all temperatures of semisolid state does not exceed its solubility in the residual liquid at a given pressure. Hereafter, this concentration should be refined experimentally for concrete conditions of solidification. To calculate the nitrogen concentration in the arising phases, a Thermo-Calc software can be employed.

Shielding Gas Oxygen Additions as a Means of Curbing Nitrogen Degassing During the Autogenous Arc Welding of Nitrogen-Alloyed Stainless Steel

This study examined the influence of oxygen additions (0.5% to 2.0%) to argon-rich shielding gas on nitrogen degassing during the autogenous arc welding of a high-manganese nitrogen-alloyed austenitic stainless steel, previously commercially available under the trade name of Cromanite. Autogenous arc welding of this steel in inert shielding gas results in considerable nitrogen losses from the weld pool, characterised by an unstable arc, spattering and violent metal expulsion from the weld pool. Oxygen additions to the shielding gas stabilise the arc and curb nitrogen-induced porosity, but at least 2.0% oxygen (by volume) is required to maintain the weld metal nitrogen content at the level of the parent material prior to welding. The beneficial effect of oxygen additions to the shielding gas is attributed to the formation of a solid MnCr2O4 spinel phase on the weld pool surface during welding, which retards nitrogen degassing by reducing the area available for the adsorption of nitrogen atoms prior to their recombination to form N2. This layer has a granular, irregular appearance and presents a less effective barrier to nitrogen degassing than the continuous, uniform liquid slag layer that forms when the Cr-Ni 300-series austenitic stainless steels are welded in oxygen-containing shielding gas mixtures.

Characterization of Wear and Surface Reaction Layer Formation on Aerospace Bearing Steel M50 and a Nitrogen-alloyed Stainless Steel

Abstract The formation of reaction layers on surfaces in mechanical contact is strongly affected by the tribological loading conditions, the materials used, the lubricant and the service temperature. An appropriate balance between reactivity of material and lubricant in tribological systems decreases wear and friction and increases the durability. The goal of this paper is to compare the reaction layer formation on a standard aerospace bearing steel AMS 6491 (M50) with that on a high strength stainless steel grade AMS 5898 exhibiting a nominal chemical compositions of 0.82C-4.1Cr-1V-4.2Mo (wt. %) and 0.3C-0.4N-15.2Cr-1Mo (wt. %), respectively. As lubricant jet engine oil Mobil Jet II has been used. Rolling contact fatigue (contact pressure: 6 GPa) and ball-on-disk tests (contact pressure: 1.6 GPa, sliding speed 10 cm/s) at room temperature and at 150°C were employed to study the effect of extreme loading conditions and temperature dependence of the reaction layer formation. The contact areas were inspected by means of optical profilometry, scanning electron microscopy (SEM), and secondary ion mass spectrometry (SIMS) in order to determine type, thickness, homogeneity, and distribution of the reaction layers in the contact zone. The main result of the study is that the reaction layer formation is significantly less on the stainless steel grade compared with M50. SIMS depth profiles were determined in order to explain the differences in the wear characteristics of the two materials. The reaction layers are mainly built up of PXOY molecules, which might be phosphates resulting from the tricresyl phosphate (TCP) lubricant additive. The role of the chemical reaction of the steel and TCP regarding the layer formation will be discussed.

On the Nature, Properties and Wear Response of S-phase (nitrogen-alloyed Stainless Steel) Coatings on Aisi 316l

On the Nature, Properties and Wear Response of S-phase (nitrogen-alloyed Stainless Steel) Coatings on Aisi 316l

On the nature, properties and wear response of s-phase (nitrogen-alloyed stainless steel) coatings on AISI 316L

Nitrogen-alloyed AISI 316L coatings with nitrogen contents, [N], between 1.2 and 80 nitrogen atoms per 100 metal atoms (N/100 Me) were obtained by reactive sputter deposition. Coatings comprising only a nitrogen supersaturated metastable face-centred cubic (fcc) phase (s-phase) were obtained for 8.6 ≤ [N] ≤ 48 N/100 Me. For [N] ≤ 5.8 N/100 Me body-centred cubic (bcc) phases were present in the coatings while at [N] ≤ 65 N/100 Me the coatings comprised unidentified high nitrogen phases in addition to s-phase. The lattice parameter of the fully s-phase coatings increased linearly with increasing [N] according to a = 0.3585 + 0.00075 [N] nm The hardness of the s-phase coatings attained a maximum value of ≈20 GPa at [N] ≈ 30-40 N/100 Me. The nitrogen in the s-phase coatings reduced the wear of the coatings by 1-2 orders of magnitude compared to the wrought material by promoting an oxidational wear mechanism.

Thermodynamic Calculation of Stacking Fault Energy and its Effect on Fcc→Hcp Phase Transformation in Nitrogen Alloyed Stainless Steels

The observed fcc→hcp phase transformation in some iron-based alloys, such as Fe-Mn-based shape memory alloys, is due to the formation of stacking faults, expansion of the faults by the movement of Shockley partial dislocations followed by ordered overlapping of the faults. Therefore, alloy systems with low stacking fault energy (SFE) are naturally more prone to the fcc→hcp martensitic phase transformation. Nitrogen (N) is known as an austenite stabilizing element; however, some existing experimental data in the literature have shown that it decreases the stacking fault energy of austenitic steels. Therefore, one would expect that N alloying should promote the fcc(γ)→hcp(e) martensitic transformation and thus have a destabilizing effect on austenite. To resolve the above discrepancies we have considered the effect of N on the stacking fault energy of fcc iron-based alloys. Our calculations have shown that by increasing N in solid solution, the stacking fault energy of the system initially increases to a maximum and then decreases. Using a phenomenological description for fcc(γ)→hcp(e) heterogeneous martensitic nucleation kinetics, we have demonstrated that increasing nitrogen concentration decreases the driving force for the fcc(γ)→hcp(e) martensitic transformation and increases the friction work due to both nitrogen solid solution strengthening and segregation of nitrogen to stacking faults for fcc iron-based alloys.

Applications of Nitrogen-Alloyed Stainless Steels

A selected number of applications for different types of nitrogen-alloyed stainless steels are described. The applications and grades are based on how nitrogen improves different properties. Conventional austenitie grades of type 304 and 316 can be alloyed with nitrogen to increase the strenght and to maintain the austenite stability after cold deformation when exposed to cryogenic temperatures. Such examples are presented. The addition of nitrogen to duplex grades of stainless steel such as 2205 improves the pitting resistance, among other things, and also enables faster information of the austenite in the heat affected zone. This means that heavy plate can be welded without pre-heating or post-weld heating. Such applications are covered. Modern highly alloyed austenitic stainless steels almost always contain nitrogen and all reasons for this are covered, i.e. to stabilise the austenite, to increase the strength, and to improve the pitting resistance. The increased strenght is the characteristic exemplified the least, since the higher strength of duplex grades is well known, but examples on austenite stability and improved pitting resistance are presented.

An Experimental and Theoretical Study of Nitrogen Flux in Stainless Steel TIG Welds

Welding of nitrogen-alloyed stainless steels is today performed both in production and on-site However achievement of the correct nitrogen balance during welding is delicate, and when the process is disturbed, it is not always clear which corrective measures should be made. In order to make rational decisions and allow for a reliable and robust process control, the mechanisms controlling the net nitrogen content of the weld metal must be identified, understood and mastered The paper illustrates the present situation and illuminates the factors governing the nitrogen exchange between the weld pool, plasma and shielding gas in TIG welding. A factor of major importance is the nitriding capacity of plasma which is strongly enhanced compared with a gas with corresponding nitrogen content Deviations from the ideal behaviour of a molecular diatomic gas are thus observed for the plasma and the Siewerts law is consequently not obeyed Techniques applicable to the study of the nitriding capacity of plasma and the rate controlling reactions of nitrogen absorption and desorption from liquid steel in contact with plasma have been examined. Experiments using stationary TIG are and real welds with travelling are are presented. both were performed using different materials and shielding gases. A model was derived that accounts for several of the most important welding parameters and the alloying levels of the material allowing for predictions of how the individual parameters influence the nitrogen content.

Progress in Bearing Performance of Advanced Nitrogen Alloyed Stainless Steel, Cronidur 30

The bearing rig tests performed in this study, demonstrate superior bearing performance of Cronidur 30 steel over conventional bearing steels. In these tests the L10 life of Cronidur 30 steel as calculated by the DIN/ISO 281 method was 80 times the unfactored L10 life under full lubrication conditions. In boundary lubrication conditions, the Cronidur 30 steel demonstrated the L10 life capability typical of EHD lubrication conditions, whereas the other steels showed a drastically reduced lives. In tests with predamaged races and boundary lubrication conditions, Cronidur 30 demonstrated 8 times the calculated L10 life, whereas the conventional steels exhibited further debit in lives as compared to the boundary lubrication testing whithout predamage. The improved performance of Cronidur 30 steel over conventional bearing steels is attributed to its unique compositional formulation and microstructure that results in provision of balanced properties in the alloy – hardness, toughness and corrosion resistance. Fortschritte bei der Lagerlebensdauer durch den Einsatz des stickstofflegierten nichtrostenden Stahles Cronidur 30 Die Prüfstandsversuche mit Wälzlagern, über die hier berichtet wird, ergaben mit dem Stahl Cronidur 30 ein erheblich besseres Laufverhalten als mit herkömmlichen Wälzlagerstählen. Unter elasto-hydrodynamischer Schmierung ergab sich im Vergleich zur L10-Lebensdauer, die ohne Berücksichtigung von Faktoren nach DIN/ISO 281 errechnet wurde, eine 80-fach längere Lebensdauer. Unter Mangelschmierung erreichten Lager aus Cronidur30 immer noch Lebensdauern wie andere Wälzlagerstähle unter Vollschmierung. Die Lebensdauern dieser Stähle sinkt unter Mangelschmierung drastisch ab. In Versuchen mit absichtlich vorgeschädigter Laufbahn und Mangelschmierung erreichten Lager aus Cronidur30 immer noch die 8-fache rechnerische L10-Lebensdauer, während die Lebensdauer der Lager aus anderen Stählen weitaus stärker absank. Die Überlegenheit des Cronidur30 über herkömmliche Wälzlagerstähle wird zurückgeführt auf die ungewöhnliche Stahlzusammensetzung und Mikrostruktur, die in einem ausgewogenen Verhältnis zwischen Härte, Zähigkeit und Korrosionswiderstand resultieren.

Strength and toughness properties of two nitrogen alloyed stainless steels and their submerged arc weldments at cryogenic temperatures

Submerged arc weldments of the two nitrogen-alloyed stainless steels X2 CrNiN 18 10 (similar to AISI 304 LN) and X4 CrNiMoN 18 14 (similar to AISI 316 LN) welded with the fully austenitic filler metal X2 CrNiMnMoN 20 16 were investigated. Tensile, impact toughness and single-specimen J-integral tests at room temperature, 77 K and 4 K were performed. The strength of the materials increased whereas the impact toughness and the fracture toughness decreased with decreasing temperature. The toughness of the steel X4 CrNiMoN 18 14 was superior to that of X2 CrNiN 18 10, and for the austenitic weld a good combination of strength and toughness was also found. On the fracture surface of the compact tension specimens, a stretch zone was found, the width of which was reduced with decreasing test temperature. For all three materials at all three temperatures the critical values, JIc, of the J integral determined according to ASTM E 813 are approximately twice the respective values for the J integral at physical crack initiation determined according to the German specification DVM 002 using the width of the stretch zone. © 1998 Chapman & Hall

Stickstofflegierte nichtrostende Dualphasenstähle

Nitrogen-alloyed stainless steels with a dual-phase microstructure have been developed to enhance the properties of conventional stainless steels. Several types of the basic alloy Fe-17 % Cr-1 % Mo with N and Ni added have been deformed and heat treated to produce dual-phase microstructures and to evaluate mechanical and chemical properties. The alloys N10CrNiMo17-1 (25 vol.-% martensite) and N5CrNiMoV17-1-1 (12 vol.- % martensite) with a microstructure consisting of homogeneously distributed martensite in fine-grained ferrite exhibit good properties under static, impact and cyclic load and in corrosive acids.

Laser surface alloying of carbon steel with FeCrSi3N4 powders

Laser surface alloying (LSA) is a relatively new technique for producing a highly alloyed layer on metal surfaces. Surface failures such as corrosion and wear can be improved by modifying surface chemistry with LSA. It reduces the amount of scarce and costly alloying elements, while retaining the bulk properties of the substrate. The beneficial effect of nitrogen on enhancing mechanical properties and corrosion resistance, especially local corrosion, as compared with conventional austenitic stainless steels (SSs), has been reported. During the last decade, there have been intensive studies on introducing large amounts of nitrogen into the SSs. Mass alloying of nitrogen into SSs can be achieved by using techniques such as pressurized induction melting, plasma remelting, hot isostatic pressure (HIP) melting, and pressurized electroslag remelting (PESR). The PESR process in which nitrogen is added directly into the melt via a metal nitride addition has become commercially available. Whether surface modification of carbon steel with high nitrogen stainless steel is possible is of interest. In the present investigation, a technique used to produced a nitrogen-alloyed SS surface layer by laser melting the Fe-Cr-Si[sub 3]N[sub 4] powders on carbon steel is attempted. The chemical states and concentration of the alloying elements in themore » LSA layers are analyzed and presented.« less

Precipitation in a nitrogen-alloyed stainless steel at 850°C

The highly-alloyed austenitic stainless steels, with molybdenum contents typically in excess of 4 wt %, are a group of materials which are undergoing continual development due to increasing harsh user demands. Nitrogen has proved particularly attractive as an alloying element in view of its strong austenite-stabilizing tendency, beneficial effect on localized corrosion resistance and ready availability. The influence of nitrogen alloying on sensitization behavior is complex in that it increases the risk of nitride precipitation but may retard the precipitation of intermetallic phases. This has been illustrated by Thier et al for 21 Cr 25Ni 6Mo. The same has also been demonstrated to be the case for 20Cr 20-30Ni 4.5Mo steels in that the addition of 0.21 wt % nitrogen retarded the first precipitating intermetallics (principally Laves phase) but caused the appearance of Cr[sub 2]N. The present work represents an elaboration of the latter study in which the effect of various exposure times at 850 C has been investigated for the alloy containing 0.21% nitrogen in order to elucidate the precipitation reactions and equilibrium state.

CRONIFER 2205 LCN

Abstract CRONIFER 2205 LCN is a low-carbon nitrogen-alloyed stainless steel with a ferritic-austenitic duplex structure. It has excellent resistance to stress-corrosion cracking in solutions containing chloride ions. Its resistance to pitting corrosion is good; both base metal and welded material successfully resist intergranual corrosion of the type that shows up in the Strauss test. For longtime use, the operating temperature should not exceed 300 C (572 F). The uses of CRONIFER 2205 LCN include chemical equipment and heat exchangers using seawater and/or brackish water. This datasheet provides information on composition and tensile properties. It also includes information on high temperature performance and corrosion resistance as well as forming, heat treating, machining, joining, and surface treatment. Filing Code: SS-415. Producer or source: Vereingte Deutsche Metallwerke AG.