Nitrogen Pick-up Research Articles

Effect of Refractory Element Additions on the Properties of Sintered Stainless Steels

This paper studies the change in properties of powder metallurgical 316L stainless steel when pure vanadium, molybdenum and tungsten (3% vol.) are added. The objective is to study the possibility of nitrogen pick-up by these refractory elements better than the chromium from plain steel. All materials have been manufactured mixing the powders with a 0.6% (wt.) of wax. Compaction was carried out at 700 MPa, and green density was evaluated. Finally, sintering was done (for 30 min) in 25%N 2 -75%H 2 at 1230 °C. Evaluated properties include sintering density, hardness, and corrosion (through salt spray test), and a broad microstructural study (optical and scanning electron microscopy) was also carried out.

State of the Art in Evaluation and Control of Steel Cleanliness.

This paper first reviews the current “state-of-the-art” in the evaluation of steel cleanliness, discussing over 20 different methods. The demand for cleaner steels requires lowering non-metallic oxide inclusions and also controlling their morphology, composition and size distribution. Because no single method can measure all of these aspects accurately, it is best to combine several methods together to quantify steel cleanliness in a given operation. Owing to the cost, time requirements, and sampling difficulties, steel cleanliness is widely inferred using total oxygen, nitrogen pick-up, and other indirect methods. Recent cleanliness values using these indicators are summarized for LCAK at many steel plants around the world. Secondly, this paper reviews operating practices to improve steel cleanliness at the ladle, tundish and continuous caster, emphasizing findings quantified with plant measurements. Inclusions come from many sources, including deoxidation, reoxidation, slag entrapment, refractory wear, and chemical reactions. They generate many defects such as cracks and slivers in the steel product. Steel cleanliness is controlled by attention to a wide range of important operating conditions throughout the steelmaking and casting processes. For ladle operations, FeO and MnO in the slag, ladle treatments, and inclusion modification are discussed. For tundish operations, tundish depth and capacity, casting transitions, refractory lining, tundish flux; gas stirring, and flow controls are discussed. Important transfer operations from ladle to tundish and from tundish to mold, such as argon protection, sealing issues, and SEN clogging are summarized. Caster operations reviewed include the effect of casting speed, fluid flow pattern control, surface level control, and caster curvature.

Modelling steel flow in a three‐strand billet tundish using a turbulence inhibitor

A three‐strand tundish belonging to a billet caster was water modelled and plant trials were performed to compare the performance of a pouring box and a turbulence inhibitor in terms of melt flow parameters and steel cleanliness.A tailor made turbulence inhibitor for this tundish is useful to accomplish with flow control of fluid turbulence and even melt redirection to all strands. The turbulence inhibitor helps to decrease nitrogen pickup during ladle changes and to float out inclusions towards the covering slag. As a consequence, rod operations to take of alumina deposits from nozzle walls are considerably decreased using a turbulence inhibitor.

Clogging in submerged entry nozzles

Casting high quality steel grades requires the use of a submerged entry nozzle (SEN) to prevent oxygen and nitrogen pick-up and to achieve favourable steel flow conditions in the mould. Clogging of the SEN leads to instability of the casting operation as well as to a decrease in slab quality. To understand the interactions between liquid steel and the SEN, 156 nozzles from the Sidmar steel plant were investigated with respect to the amount of clogging. A linear relation between the clogging volume and the clogging weight was observed. This indicates that the clogging material's density remains constant during the clogging process for the different steel grades. A distinct relationship was also found between the amount of clogging and the cast steel weight. No relation between the amount of clogging and the superheating of the steel nor the steel flow rate was noticed, however. It was also concluded that for Al killed steels the high Mn (> 3000 ppm) and high C (> 500 ppm) grades cause less clogging than Al killed steel grades with low Mn content (< 3000 ppm) and that Al/Si killed steel grades cause less clogging when compared to Al killed steel grades. When comparing these different steel grades with respect to their total oxygen concentration in the ladle, it appears that steel grades, which have a low total oxygen concentration, cause less clogging in the SEN. Based on these observations three combinations SEN/steel grade were selected for further analysis with electron microscopy.

Solid State Alloying by Plasma Nitriding and Diffusion Annealing Treatment for Austenitic Stainless Steel

Nitrogen has been added to stainless steels to improve mechanical strength and corrosion resistance, high Nitrogen steel production is limited by high gas pressure requirements and low nitrogen solubility in the melt. One way to overcome this limitation is the addition of Nitrogen in solid state because of its higher solubility in austenite. However, gas and salt bath nitriding have been done at temperatures around 550°C, where nitrogen solubility in the steel is still very low. High temperature nitriding has been, thus proposed to increase nitrogen contents in the steel but the presence of oxide layers on top of the steel is a barrier to nitrogen intake. In this paper a modified plasma nitriding process is proposed. The first step of this process is a hydrogen plasma sputtering for oxide removal, exposing active steel surface improving nitrogen pickup. This is followed by a nitriding step where high nitrogen contents are introduced in the outermost layer of the steel. Diffusion annealing is then performed in order to allow nitrogen diffusion into the core. AISI 316 austenitic stainless steel was plasma nitrided and diffusion annealed at 1423K, for 6 hours, with 0.2 MPa nitrogen pressure. The nitrided steel presented ∼ 60 μm outermost compact layer of (Fe,Cr) 3 N and (Fe,Cr) 4 N with 11 wt. % N measured by surface depth profiling chemical analysis - GDS system. During the annealing treatment the nitride layer was dissolved and nitrogen diffused to the core of the sample leaving more even nitrogen distribution into the steel. Using this technique one-millimetre thick sample were obtained having high nitrogen content and uniform distribution through the thickness.

High nitrogen steel powder for near net shape products

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Nitrogen in Spray Formed Superalloys

AbstractSpray forming is of considerable interest for potential commercial production of superalloy forging and rolling preforms. The low degree of macro segregation and the relatively small grain size (25–40 μm) make this process look attractive for compositionally complex modern day nickel based superalloys. Both argon and nitrogen have been used successfully as the atomising gas. Argon atomisation has been used when little or no oxygen or nitrogen pickup is desired and when a small amount of argon filled porosity (∼1 vol.-%) can be tolerated. Nitrogen atomization has been used when a decrease in porosity is desired and when a small amount of nitrogen pickup can be tolerated. In this case, rather than staying in the pores, the nitrogen gas dissolves into the alloy and precipitates as submicrometre titanium and niobium carbonitrides. These precipitates have been observed to have little effect on key mechanical properties (tensile, creep, and fatigue). PM/0649

Nitrogen pick-up during sintering of stainless steel

Nitrogen pick-up during sintering of stainless steel

Nitrogen pick-up of steels through a plasma process

Nitrogen pick-up of steels through a plasma process

Metallurgical results from a 30‐t AC plasma ladle furnace

In a 30‐t ladle furnace equipped with three A.C. transferred arc‐plasma torches for a power input of 6 MW, the influence of argon plasma heating on the chemical composition of a number of steel grades from carbon to chromium‐nickel steels was investigated. The concentrations of carbon, manganese, chromium and nickel remained virtually constant even for extra‐long heating periods of 2 hours. Silicon melting loss was appr. 0.03%/h, comparable to conventional ladle furnaces. Nitrogen pickup proved to be negligibly small, although the N2 partial pressure in the furnace was about 0.5 bar. Since the slag is well heated up by the plasma flames, a metal desulphurization is noticed, the extent of which depends on the basicity of the slag. At the slag/gas interface, equilibrium is established with the slightly oxidizing furnace atmosphere, leading to sulphur transfer from slag to gas via SO2.

Processing of Sintered Stainless Steel Parts

AbstractStainless steel components are made from fully prealloyed powders that require high pressures to obtain high densities. When parts are double pressed and sintered, nitrogen pickup from the presintering atmosphere can affect the recompactibility. During the high temperature sintering step chromium oxide should be reduced if good ductilities are to be achieved. Automotive applications will normally require a salt spray test survival time of ≥10 days. Sintered versions of standard grades fail under these conditions. Experiments were performed to determine the way in which production parameters affect salt spray test performance. PM/0568

Absorption of gaseous contaminants by welds and weld simulations in ferritic stainless steels

AbstractThe factors influencing absorption by Type 444 ferritic stainless steel of interstitial elements from gaseous environments deliberately contaminated with nitrogen and oxygen have been studied for induction melted weld simulations and actual tungsten inert gas welds. For weld simulations, nitrogen pick-up is limited by factors such as the formation of stable TiN surface films; induction melting is therefore considered unsatisfactory as a weld simulation for the purpose of studying gas absorption. It is found that titanium is consumed by both gaseous species, whereas niobium is not; dual stabilization is therefore recommended. Welds show greater nitrogen pick-up than simulations, the effect increasing with increasing heat input, while at the highest heat inputs the quantity absorbed exceeds predictions based on solubility equilibria. These effects are explained in terms of the enhanced gas transfer conditions that obtain and the dissociation of the gas that occurs in the welding arc.MST/416

Absorption of gaseous contaminants by welds and weld simulations in ferritic stainless steels

The factors influencing absorption by Type 444 ferritic stainless steel of interstitial elements from gaseous environments deliberately contaminated with nitrogen and oxygen have been studied for induction melted weld simulations and actual tungsten inert gas welds. For weld simulations, nitrogen pick-up is limited by factors such as the formation of stable TiN surface films; induction melting is therefore considered unsatisfactory as a weld simulation for the purpose of studying gas absorption. It is found that titanium is consumed by both gaseous species, whereas niobium is not; dual stabilization is therefore recommended. Welds show greater nitrogen pick-up than simulations, the effect increasing with increasing heat input, while at the highest heat inputs the quantity absorbed exceeds predictions based on solubility equilibria. These effects are explained in terms of the enhanced gas transfer conditions that obtain and the dissociation of the gas that occurs in the welding arc.MST/416

Sintering with Nitrogen Based Atmospheres

The use of nitrogen based atmosphere systems for sintering is now well established both worldwide and in the UK. On the basis of this experience, the atmosphere systems that can be used to replace conventional endothermic and cracked ammonia atmospheres are briefly described and their practical implementation reviewed in terms of advantages and limitations. In particular, the recent application in the UK of a nitrogen-methanol atmosphere system as an alternative to endothermic gas is discussed. In addition to the practical experience gained, a number of laboratory development programmes have been carried out to gain a better understanding of the effect of nitrogen based atmospheres on sintered components. The results of work on high temperature sintering, on the effect of hydrogen level on sulphur content in ferrous compacts, and on nitrogen pick-up in ferrous compacts are reviewed. PM/0345

Sintering Atmospheres and DC Magnetic Properties of Iron

The de magnetic properties of iron test-rings sintered under nineteen different atmospheres have been determined. High-purity iron powder rings were pressed to a density of 7·2 Mg m−3 and sintered in a laboratory tube furnace at 1120°C. The types of atmosphere studied were (i) hydrogen, nitrogen, and their mixtures, (ii) endothermic-base, (in) exo-endo, (iv) exothermic-base, and (v) nitrogen-base. Neither magnetic saturation induction B 25 nor remanence was greatly affected by the type of atmosphere chosen, but both were decreased when the atmosphere was a carburizing one or when nitrogen pick-up could occur. The carbon content after sintering had about twice as much influence as nitrogen on B 25 and remanence. Coercivity and maximum permeability were very strongly influenced by the type of atmosphere chosen. Nitrogen had the most potent influence on coercivity; carbon and nitrogen had about equal effects on maximum permeability. Compared with carbon and nitrogen, the levels of oxygen and sulphur and the grain size had, in general, secondary or insignificant influences on magnetic properties. A multiple regression analysis indicated that about 90% of the variation in magnetic properties due to atmosphere composition could be explained by the resulting chemical analysis and grain size of the sintered rings. Hydrogen and hydrogen/nitrogen atmospheres gave by far the best magnetic properties.

The interaction between silicon nitride and several iron, nickel and molybdenum-based alloys

Solid-solid reactions have been studied between silicon nitride and AlSl 316 and 20/25/Nb austenitic stainless steels, Fecralloy ferritic stainless steels (with and without yttrium), PE 16, Nimonic 75, Hastelloy X nickel-based alloys and a TZM molybdenum alloy. The reactant couples were heat-treated, in gettered inert gas, for up to 5161 h, at 800 to 1100° C. The temperature for the onset of measurable reaction with the iron and nickel-based alloys was between 825 and 900° C. Interaction was appreciable at 1000° C, being greatest with 20/25/Nb and least with the Fecralloy steel. The overall pattern of these reactions was similar, in that selected alloy constituents (chromium, together with iron and/or nickel where appropriate) reacted with the silicon nitride to form an adherent product, which was basically a silicide, although it also contained nitrogen. Some of the silicon and/or nitrogen released by subsequent decomposition of the primary reaction product was taken up by the alloys. In PE 16 and Hastelloy X alloys silicon was associated with molybdenum. There were several types of nitrogen pick-up: in the Hastelloy X alloy it followed a diffusion profile, while with other alloys it reacted with the constituents Ti, Al or Y to form nitrides. The surface layers on the austenitic stainless steel were denitrided, with nitrogen being transferred, via the gas phase, to a tantalum getter. With the TZM alloy no constituent was transferred to the silicon nitride. However, a silicon layer built up at the alloy surface and nitrogen was picked up, with its penetration following a diffusion profile.

The dissolution of nitrogen in metallurgical slags

AbstractNitrogen is soluble in CaO-SiO2-Al2O3 slags under a variety of highly reducing conditions. With graphite as reducing agent, the solubility increases with increasing CaO and decreasing SiO2 contents, dropping off, however, at very low SiO2. At 1550°C and 1 atmos. N2, it lies between 0.25 and 2.5 wt.%. It is steeply temperature dependent, and reaction is slow. Equilibrium-constant calculations for possible dissolution reactions indicate that there is no single mode of dissolution covering all compositions. However, it appears that at intermediate CaO and SiO2 contents, carbon participates directly, probably forming CN− ions, whereas in SiO2-rich melts, nitrogen enters the silicate network. In the absence of graphite, using excess Si or Al as reductants, nitrogen pick-up of the same order of magnitude is observed, but with less marked composition and temperature dependence. Reaction is rapid. Large quantities (> 3 wt.%) can be dissolved via Si3N4, and the presence of CaC2 also promotes appreciable di...

Nitrogenation and decarburization of stainless steel

The rate of nitrogenation of iron alloys by nitrogen bubbling was determined. The rate of nitrogen pickup in iron with high oxygen activities was controlled by a chemical reaction at the gas bubble-metal interface. For an 18-8 type stainless steel and for iron containing between 50 and 400 ppm oxygen, the rate is controlled by a chemical reaction and liquid-phase mass transfer in series. The rate equation for this case was developed. The rates calculated from existing rate data and the fluid dynamic properties of the system were in good agreement with the experimental results. When argon-oxygen mixtures are bubbled through shallow (7.5 cm) stainless-steel melts, the rate of oxidation of chromium is considerably faster than that of carbon. It is suggested that oxygen primarily oxidizes chromium and iron and as the oxides are carried through the bath by argon bubbles they oxidize the carbon.

Phoenix-Lance steelmaking

Here is an interesting new steelmaking technique being developed in Germany for the production of open-hearth quality steels from high-phosphorus pig irons. It utilizes the classical basic-bessemer converter with top-blown oxygen, and has the advantages of early phosphorus removal and, consequently, less nitrogen pick-up in the final steel.