Nitrogen Pick-up Research Articles

Effect of Al-Ti complex deoxidation mode on steel cleanliness during RH refining of IF steels

The deoxidation in Ti containing Al killed IF steels is a vital parameter that directly influences the steel cleanliness. In this study, the evolution of steel compositions and inclusion characteristics during RH refining of IF steels were investigated by industrial trials involving three Al-Ti deoxidation modes, which were adding Ti-Fe 2 min after Al deoxidation in Mode I, adding Al and Ti-Fe simultaneously in Mode II, and adding Ti alloys with two equal batches at 3.2 min and 5 min after Al deoxidation in Mode III. The results showed that nitrogen pickup of several ppm was observed with Ti-Fe alloy addition. Therefore, nitrogen content in alloys should be strictly controlled. The main inclusions were single alumina particles, alumina clusters, alumina cluster with Al-Ti oxides, and Al-Ti complex oxides. Mode III had less amount of Al-Ti oxides compared with Mode I, and Mode II had a much higher proportion of Ti bearing inclusions than Mode I and Mode III at RH end. For Mode I & Mode III, the inclusion removal rate was 0.33–0.35 min−1 before adding Ti, and slowed down to 0.14 min−1 for Mode I and 0.22 min−1 for Mode III after Ti-Fe addition. While for Mode II, the rate was only 0.18 min−1. Mode III was the recommended mode for its excellent steel cleanliness and high Ti yield with slight elongation of RH treating time.

On the transformation temperatures of Ti-6Al-4V: Effect of oxygen pick-up during Laser Powder Bed Fusion

Ti-6Al-4V (Ti64) parts manufactured by Laser Powder Bed Fusion (L-PBF) exhibit a fine martensitic microstructure and significant residual stresses that are detrimental for their mechanical performance. Consequently, post-processing heat treatments are necessary to relieve these internal stresses and achieve a microstructure adequate for the mechanical requirements. Given the variations in powder compositions and processing parameters, there is a large scattering of physical properties of both as-built and heat-treated parts. In this context, this study quantifies the link between interstitial contents (specifically oxygen and nitrogen) and the transition temperatures of as-built parts, using as-atomized pre-alloyed powders for reference. The martensite decomposition temperature and beta transus temperature are assessed using high-temperature X-Ray diffraction and differential scanning calorimetry for both the powder and as-built parts. It was found that the martensite that composes the as-built microstructure can be decomposed into a stable α+β structure at temperatures exceeding 490 °C. Oxygen and nitrogen pick-up occurs during the L-PBF process, leading to an elevated beta transus temperature in as-built parts relative to the original powder. This insight provides a guide for optimizing post-processing heat treatments for Ti64 as-built components.

Failure Analysis of Boron Steel Components for Automotive Applications

The automotive industry is continuously looking for an innovative mix of new steels and manufacturing techniques in order to improve process chain efficiency and cost reduction. To this aim, boron steels are becoming increasingly popular thanks to their high hardenability and machinability. Due to their reduced finishing steps, boron steels are commonly processed using fine blanking technologies. The success of fine blanking on boron steel components is due to heat treatments which must be carefully designed to avoid precipitation of boron-rich compounds that would lower steel hardenability. At high temperature, boron is very reactive with oxygen and nitrogen. The main focus of this paper is to show some drawbacks that can occur during heat treatments of automotive components. An experimental campaign was performed on two different boron steels, namely EN 34MnB5 and EN 22MnB5. The steel samples were previously spheroidized annealed in a neutral environment (hydrogen/nitrogen atmosphere), and then fine blanked to obtain specific automotive components which were subsequently quenched and tempered. Experimental tests revealed precipitation of nanometric compounds, causing strong grain refinement and localized decrease of steel hardenability. Hardenability problems were brought back to nitrogen pick-up during initial spheroidize annealing treatments.

Complex geometry and integrated macro-porosity: Clinical applications of electron beam melting to fabricate bespoke bone-anchored implants.

The last decade has witnessed rapid advancements in manufacturing technologies for biomedical implants. Additive manufacturing (or 3D printing) has broken down major barriers in the way of producing complex 3D geometries. Electron beam melting (EBM) is one such 3D printing process applicable to metals and alloys. EBM offers build rates up to two orders of magnitude greater than comparable laser-based technologies and a high vacuum environment to prevent accumulation of trace elements. These features make EBM particularly advantageous for materials susceptible to spontaneous oxidation and nitrogen pick-up when exposed to air (e.g., titanium and titanium-based alloys). For skeletal reconstruction(s), anatomical mimickry and integrated macro-porous architecture to facilitate bone ingrowth are undoubtedly the key features of EBM manufactured implants. Using finite element modelling of physiological loading conditions, the design of a prosthesis may be further personalised. This review looks at the many unique clinical applications of EBM in skeletal repair and the ground-breaking innovations in prosthetic rehabilitation. From a simple acetabular cup to the fifth toe, from the hand-wrist complex to the shoulder, and from vertebral replacement to cranio-maxillofacial reconstruction, EBM has experienced it all. While sternocostal reconstructions might be rare, the repair of long bones using EBM manufactured implants is becoming exceedingly frequent. Despite the various merits, several challenges remain yet untackled. Nevertheless, with the capability to produce osseointegrating implants of any conceivable shape/size, and permissive of bone ingrowth and functional loading, EBM can pave the way for numerous fascinating and novel applications in skeletal repair, regeneration, and rehabilitation. STATEMENT OF SIGNIFICANCE: Electron beam melting (EBM) offers unparalleled possibilities in producing contaminant-free, complex and intricate geometries from alloys of biomedical interest, including Ti6Al4V and CoCr. We review the diverse range of clinical applications of EBM in skeletal repair, both as mass produced off-the-shelf implants and personalised, patient-specific prostheses. From replacing large volumes of disease-affected bone to complex, multi-material reconstructions, almost every part of the human skeleton has been replaced with an EBM manufactured analog to achieve macroscopic anatomical-mimickry. However, various questions regarding long-term performance of patient-specific implants remain unaddressed. Directions for further development include designing personalised implants and prostheses based on simulated loading conditions and accounting for trabecular bone microstructure with respect to physiological factors such as patient's age and disease status.

Laser powder bed fusion produced Ti-6Al-4V: Influence of high-energy process parameters on in-situ martensite decomposition and prior beta grain texture

Since Ti-6Al-4V components fabricated by laser powder bed fusion achieve an undesired martensitic microstructure (α'), high residual stresses and columnar prior-β grains, post-process heat treating has become an essential operation to improve these properties. However, post-process heat treating could be considered an undesirable additional step. Thus, this study investigated the viability of in-situ heating by experimentally establishing the effect of in-situ heating on part properties. This was accomplished by incrementally increasing the laser-induced energy density (Ev) above the machine-default Ev while maintaining high part density> 99%. Microstructural changes were monitored using optical and scanning electron microscopy and quantified using X-ray and electron-backscatter diffraction analysis. The study found a decrease in α grain size with an increase in Ev, a full α' decomposition at an Ev of 167 J/mm3 and indications of prior-β grain columnar-to-equiaxed transition at higher Ev. Graded properties along the sample height, and an increase in oxygen and nitrogen pick-up was measured at high Ev, causing a decrease in material ductility. A discussion of the thermodynamics of the process parameters demonstrated the mechanisms by which a reduced laser hatch distance controls powder bed fusion texture. Process variables of interaction time and exposure rate are proposed to better quantify the interactions between process parameters and thermodynamics.

Directed-energy deposition of Ti-6Al-4V alloy using fresh and recycled feedstock powders under reactive atmosphere

This research studies the directed-energy deposition of Ti-6Al-4V (wt%) in an argon atmosphere containing 9–9500 ppm oxygen (from air), using both fresh and recycled feedstock powders. For fresh powder builds, when exposed to a 3500 ppm oxygen-containing atmosphere, a yield strength (YS) of 1061 ± 0.6 MPa and an elongation-to-fracture (ETF) of 10.5 ± 1.6 % are obtained. The oxygen pickup in the build deposit tends to plateau at 1000 ppm beyond 3500 ppm oxygen exposure. Conversely, the nitrogen pickup shows an increasing trend, reaching 1000 ppm for the air exposure range investigated. Recycled powder builds are stronger but less ductile than equivalent fresh powder builds made under comparable atmospheres. A higher degree of interstitial element pickup with no apparent saturation is identified for builds made from recycled powders, due to their smaller average particle size and a correspondingly larger specific surface area. The resulting microstructure for all samples made with air exposure comprises full lamellar α + β, formed through the in-situ decomposition of martensite at a temperature close to 600 °C. This conclusion is supported by the β volume fraction at 600°C as predicted by Thermo-Calc agreeing with image analysis data. The α lath thickness is found to increase with air exposure level. The increased β-transus temperature and martensite start temperature, and the accelerated diffusion-driven phase transformation due to the presence of elevated interstitial element contents accounts for the α lath thickening behaviour. YS and ultimate tensile strength (UTS) are found to increase linearly with oxygen equivalent. Solid solution strengthening from nitrogen and oxygen pickup is responsible for this increase. YS and UTS models based on oxygen equivalent are proposed. Nitrogen and oxygen in the builds dictate the ETF, with nitrogen content being the more potent factor.

Potential of nitrogen atomized alloy 625 in the powder bed fusion laser beam process

Powder based metal additive manufacturing processes like Powder Bed Fusion – Laser Beam utilize gas atomized metal powders as feedstock material. Typically, for nickel-based alloys such as Alloy 625, argon gas is used during the atomization process. Considering the larger environmental impact of argon gas compared to nitrogen gas, and the increasing use of gas atomized metal powders, the environmental impact of powder based additive manufacturing techniques could be mitigated if gas atomization of alloys such as Alloy 625 using nitrogen was possible. This work investigates the feasibility of tailoring an alloy to allow atomization using nitrogen gas while remaining within the Alloy 625 specification. This is achieved by limiting the nitrogen pick-up during the atomization process, primarily by reducing the titanium content. The metallurgical implications of this tailored alloy and the subsequent atomization using nitrogen, as well as the attained microstructure from the Powder Bed Fusion – Laser Beam process is investigated and compared to a more common 625 alloy composition which was atomized using argon. Furthermore, the microstructural development of the alloys after heat treatments are evaluated. Lastly corrosive properties, as well as tensile and impact properties are evaluated both in the as-built and hot isostatic pressed condition.

A New Inert Gas Delivery Design for Improved Shielding of Ladle Shroud–Collector Nozzle (LS–CN) Assembly: Modeling, Design, and Industrial‐Scale Validations

Argon injection in ladle shrouds is customarily practiced during the teeming of liquid steel from ladle to tundish to minimize air ingression and thereby restrict atmospheric oxidation of steel melt. Despite such practices, nitrogen pickup results in the tundish, implying inadequate shielding of ladle shroud–collector nozzle (LS–CN) joint. Accordingly, the efficacy of different argon injection designs in shrouds is evaluated in the present study, and thereby, a novel argon injection arrangement, embodying a twin, tangential gas discharge system, developed as a curative step to ensure maximum inertization of the LS–CN assembly. Numerical prediction of argon flow and distribution around the LS–CN assembly is accompanied by a homogeneous, non‐isothermal, two‐component turbulent flow model embodied in ANSYS Fluent. These, supported adequately by water modeling results, indicated that inertization of LS–CN assembly is considerably improved with the proposed design at argon flowrates typically practiced in industries (10 Nlit min−1). To substantiate laboratory findings, a plant‐scale investigation is carried out, wherein shrouds with a new gas delivery design are fabricated and deployed in continuous bloom casting operation in a special steel plant. Industrial trials confirm the superiority of the new design over existing ones and demonstrate a 50% reduction in nitrogen pickup, during teeming, under identical operating conditions.

Effect of Slag Compositions on Change Behavior of Nitrogen in Molten Steel

The problem of nitrogen pickup in the smelting process of the electric arc furnace (EAF) has not been solved well. Using seven slag–steel equilibrium experiments and theoretical analysis, the relation of the foaming index and optical basicity with the nitrogen capacity of slag was clarified. Meanwhile, the effect of slag composition on the equilibrium distribution ratio of nitrogen and the mass transfer coefficient of nitrogen pickup was also studied. The results show that, with the increase in slag basicity, the nitrogen pickup amount, nitrogen pickup rate, and nitrogen equilibrium distribution ratio LN increase. Increasing the foaming index of slag and reducing its optical basicity will increase the nitrogen capacity of slag, which is conducive to hindering the nitrogen pickup of molten steel. The relationship between slag optical basicity and nitrogen capacity can be expressed as lgCN = −5.59lgΛ − 12.41. With the increase in the Al2O3 content of slag, the nitrogen pickup amount of molten steel decreases and the nitrogen pickup rate decreases. The test with MgO = 7.5% showed the highest nitrogen pickup rate and the highest nitrogen pickup mass transfer coefficient, which were 0.21 × 10−4%/min and 1.97 × 10−4 cm/s, respectively. The test with Al2O3 = 7.5% in slag showed the lowest nitrogen pickup rate and the lowest nitrogen pickup mass transfer coefficient, which were 0.08 × 10−4%/min and 1.35 × 10−4 cm/s, respectively.

Effect of the Process Atmosphere Composition on Alloy 718 Produced by Laser Powder Bed Fusion

The detrimental effect of nitrogen and oxygen when it comes to the precipitation of the strengthening γ’’ and γ’ phases in Alloy 718 is well-known from traditional manufacturing. Hence, the influence of the two processing atmospheres, namely argon and nitrogen, during the laser powder bed fusion (L-PBF) of Alloy 718 parts was studied. Regardless of the gas type, considerable losses of both oxygen of about 150 ppm O2 (≈30%) and nitrogen on the level of around 400 ppm N2 (≈25%) were measured in comparison to the feedstock powder. The utilization of nitrogen as processing atmosphere led to a slightly higher nitrogen content in the as-built material—about 50 ppm—compared to the argon atmosphere. The presence of the stable nitrides and Al-rich oxides observed in the as-built material was related to the transfer of these inclusions from the nitrogen atomized powder feedstock to the components. This was confirmed by dedicated analysis of the powder feedstock and supported by thermodynamic and kinetic calculations. Rapid cooling rates were held responsible for the limited nitrogen pick-up. Oxide dissociation during laser–powder interaction, metal vaporization followed by oxidation and spatter generation, and their removal by processing atmosphere are the factors describing an important oxygen loss during L-PBF. In addition, the reduction of the oxygen level in the process atmosphere from 500 to 50 ppm resulted in the reduction in the oxygen level in as-built component by about 5%.

Control of residual oxygen of the process atmosphere during laser-powder bed fusion processing of Ti-6Al-4V

The effect of the residual oxygen concentration in the process atmosphere during laser-powder bed fusion (L-PBF) of Ti-6Al-4V was investigated, using an external oxygen monitoring system equipped with two types of oxygen sensors typically used in L-PBF hardware: a lambda probe and an electrochemical oxygen sensor. The recordings of the oxygen variations during L-PBF highlighted that the electrochemical sensor is more reliable than the lambda probe, whose signal showed a maximum deviation of about 700 ppm O2 after 7 h, attributed to its sensitivity to hydrogen present in the system. The study revealed that proper monitoring of the oxygen in the laboratory scale L-PBF system used is necessary to limit oxygen and nitrogen pick-ups by the built material. Concentrations as high as 2200 ppm O2 and 500 ppm N2 in the Ti-6Al-4V part built under standard conditions were measured, compared to maximum levels of 1800 ppm O2 and 250 ppm N2 with the external oxygen control. In addition, the findings underline the critical effect of the component design, such as the high aspect ratio columns or the lattice structures, on the heat accumulation in case of Ti-6Al-4V, leading to enhanced oxygen and nitrogen pick-up, as high as 600 ppm O2 and 150 ppm N2 difference between the bottom and top of the cylindrical samples of 70 mm height used in this study. The determination of tensile properties of samples built at different heights put in evidence the detrimental effect of the oxygen increase with build height on the ductility, which decreased from 12% to below 6% between the bottom and top positions. This work highlights that the possible presence of impurities in the L-PBF atmosphere can have harmful impact on the properties of Ti-6Al-4V components, which can be mitigated adjusting the oxygen control system.

The role of Directed Energy Deposition atmosphere mode on the microstructure and mechanical properties of 316L samples

Laser-Directed Energy Deposition was used to produce AISI 316L stainless steel samples. The effect of the protective atmosphere on the microstructure and mechanical performance of AISI 316L deposited parts was investigated by building samples using a simple nitrogen shielding gas or using a nitrogen-filled build chamber. The effect of the different processing conditions on the microstructure was evaluated by X-ray analysis, optical and scanning electron microscopy. Only slight differences in the cellular dendrites morphology of samples built under different protective atmosphere conditions were observed. However, the presence of oxides was monitored too: the oxides composition and area fraction were analysed and compared by image analyses, and it was demonstrated that the protective atmosphere mainly affects the oxides dimensions. The effect of the oxides and nitrogen pick-up on the mechanical performance of the samples was evaluated by tensile tests. The results revealed that the nitrogen-filled build chamber allowed the achievement of slightly higher tensile strength and elongation with respect to the other processing conditions as a consequence of the reduced size of the oxide inclusions.

Numerical Simulation and Experimental Investigation of Nitrogen Transfer Mechanism from Gas to Liquid Steel During Pressurized Electroslag Remelting Process

Since the gas–slag–metal three-phase transfer mechanism of nitrogen is not suitable to account for the nitrogen pickup during pressurized electroslag remelting process, the laboratory experiments have been carried out to study the transfer mechanism of nitrogen using different nitrogen partial pressures and electrode immersion depths. Then, two possible transfer mechanisms of nitrogen have been proposed, and a 2D transient numerical model has been developed to investigate the behavior of nitrogen based on the new proposed mechanisms. The simulated results are compared with the experiment to validate the feasibility of new proposed mechanism. The results show that the nitrogen content in ingot increases with the increasing nitrogen partial pressure and decreasing electrode immersion depth. At the current experimental condition, the electrode immersion depth seems to play a more important role in the nitrogen pickup than the nitrogen partial pressure. Due to the limits of thermodynamic and kinetic conditions, the nitrogen pickup is negligible via the transfer from gas to liquid metal through molten slag whether nitrogen chemically or physically dissolves into slag. The mechanism that nitrogen directly reacts with the partially exposed liquid metal film under the electrode tip due to the fluctuation of gas/slag interface could reasonably account for the experimental results. Furthermore, based on the mechanism, the predicted variation tendency of nitrogen content in ingot is in agreement with the experiment.

Effect of argon and nitrogen atmospheres on the properties of stainless steel 316 L parts produced by laser-powder bed fusion

The role of the inert gas during laser powder bed fusion (L-PBF) is to remove the process by-products and the air that is initially present in the process chamber. On this purpose, different gas supply options are available. The effect of the process gas and its purity, using argon and nitrogen, on the properties of the 316 L stainless steel produced by L-PBF was studied. The results obtained showed that utilization of argon and nitrogen result in residual oxygen levels that vary over the course of the process sequence in the process chamber. It can be concluded that 316 L stainless steel is a robust alloy to process by L-PBF. A limited effect of the residual oxygen or the gas type (argon or nitrogen) on the tensile properties of the 316 L stainless steel parts was registered. The oxygen and nitrogen pick-up within the produced parts are limited. However, when processing 316 L stainless steel with lower purity gas supply such as a nitrogen generator, risks related to powder degradation arise. Out of the available gas options, the findings highlighted that processing with high purity argon ensures limited powder degradation and high toughness of the produced parts.

Physical and Numerical Modeling of Exposed Slag Eye in Continuous Casting Mold using Euler–Euler Approach

Gas injection through the submerged entry nozzle (SEN) into the continuous casting mold can be an effective approach for preventing SEN clogging and promoting the floatation of the non‐metallic inclusions. However, sometimes the exposed slag eyes due to gas injection appear on the top surface of the liquid slag layer, resulting in heat losses, re‐oxidation, and nitrogen pickup in the molten steel. An Eulerian multiphase‐flow model is developed to predict the argon‐steel‐slag three‐phase flow in a slab continuous casting mold. All the phases are treated based on Eulerian approach. The mathematical model is compared with the industrial observations and the water model experiments. Both of physical and numerical results reproduce the phenomenon of the high gas concentration at the SEN exit port. Most of the argon bubbles stay below the slag layer for quite long time because the slag blocks their floatation. Furthermore, the argon bubbles would gradually gather in a dense plume while escaping through the slag layer. Scattered argon exit spots are found at the top surface of slag layer. Two main locations of the exposed slag eye are found: 1) adjacent to the SEN; 2) at the mold's mid‐section at the position where a concentrated argon plume breaches through the slag layer. The near‐SEN exposed eye occurs under any of considered conditions. The one at the mid‐section is formed when the meniscus convex reaches a critical level, been dependent on the casting conditions.

Strategies for nitrogen control during the production of interstitial-free steel

ABSTRACTInterstitial-free (IF) steel is a micro-alloyed steel that contains significantly low concentrations of interstitial elements such as carbon and nitrogen. In this context, nitrogen control in liquid steel is a major issue, particularly in cases where the nitrogen content in hot metal or other charged material is high. This study describes the practice of IF steel production at No.3 steelmaking plant of WISCO using hot metal containing high nitrogen due to hot metal desulphurisation treatment with nitrogen as the carrier gas. By adapting new technology for nitrogen removal in a combined blown converter and an Ruhrstahl Heraeus vessel as well as controlling nitrogen pick-up during transfer operations, final product can be generated with an average nitrogen content of about 18 ppm and with some values as low as 10 ppm.

Tundish Open Eye Formation: A Trivial Event with Dire Consequences

Inert gas shrouding is a traditional practice in tundish metallurgy and has several benefits such as, protecting the melt stream from air aspirations, aiding inclusion flotation with argon bubbles, and also possible thermal and chemical homogenization. On the down side, it displaces the protective slag layer on the top of the melt, and exposes the steel to the ambient atmosphere. This region is often referred to as the slag eye or open eye. This exposed area leads to higher radiative heat losses, reoxidation of the liquid steel, nitrogen pickup, and subsequent inclusion formation. Although perceived as a trivial event by many, Tundish Open Eye (TOE) has dire consequences. In the present work, TOE formation and its consequences have been investigated. The mathematical modeling of this turbulent multiphase system is performed using the Volume of Fluid (VOF) method, and discrete phase method (DPM), coupled with the standard k‐ϵ turbulence model. The mathematical model is compared with the water model results and plant trials. The main objective is to ensure that the steelmaking tundish acts as a refiner and not as a contaminator.

Reaction of Unalloyed and Cr-Mo Alloyed Steels with Nitrogen from the Sintering Atmosphere

Abstract Nitrogen is usually regarded as an inert sintering atmosphere for PM steels; however, this cannot be taken for granted in particular for steels alloyed with nitride forming elements. Among those elements, chromium has become more and more important as an alloying element in sintered low alloy structural steels in the last decade due to the moderate alloying cost and the excellent mechanical properties obtainable, in particular when sinter hardening is applied. The high affinity of Cr to oxygen and the possible ways to overcome related problems have been the subject of numerous studies, while the fact that chromium is also a fairly strong nitride forming element has largely been neglected at least for low alloy steel grades, although frequently used materials like steels from Cr and Cr-Mo prealloyed powders are commonly sintered in atmospheres consisting mainly of nitrogen. In the present study, nitrogen pickup during sintering at different temperatures and for varying times has been studied for Cr-Mo prealloyed steel grades as well as for unalloyed carbon steel. Also the effect of the cooling rate and its influence on the properties, of the microstructure and the composition have been investigated. It showed that the main nitrogen uptake occurs not during isothermal sintering but rather during cooling. It could be demonstrated that a critical temperature range exists within which the investigated CrM-based steel is particularly sensitive to nitrogen pickup.

Low temperature active screen plasma nitriding of 17–4 PH stainless steel

In the present work, low temperature active screen plasma nitriding of a 17–4 PH precipitation-hardening stainless steel was investigated. The active screen technique has been used to avoid undesirable plasma concentration, edge effects and arching during D/C plasma nitriding. Solution treated (S) and Solution treated and aged (S+A) 17–4 PH stainless steel samples were Active Screen Plasma Nitrided (ASPN) for 20h at low temperature (400°C), in order to avoid undesirable precipitation of chromium nitrides. Formation of these nitrides impairs corrosion resistance of the stainless steel because they act as Cr sinks, reducing the overall amount of Cr available in the matrix. The main objective of this work was to characterize the effect of the starting conditions on the microstructure of the nitrided layers. Besides, the chemical gradients and hardness evolution during low-temperature, long term active screen plasma nitriding were also studied.A homogeneous nitrided layer was obtained along the entire surface after the nitriding process. Hardness measurements along the nitrided surface showed virtually constant hardness values, indicating that ASPN was effective to avoid edge effects. Moreover, the nitrided layer could not be etched by Villela reagent, suggesting that the corrosion resistance was not impaired.Nitrogen supersaturation after plasma nitriding was indicated for both starting conditions (S) and (S+A) by X-ray diffraction patterns, as the expanded martensite peaks were broadened and shifted to lower 2θ angles compared to the martensite peaks of the substrate. The nitrided layer of the (S+A) specimen was thicker (9.2μm) than the nitrided layer of the (S) specimen (5.7μm). Also, the maximum nitrogen content (measured by WDX) of the (S+A) specimen (3.7%) was greater than the maximum nitrogen content measured on the simply solution treated (S) specimen (1.9%). The difference in nitrogen pick-up was related to greater hardness values for the (S+A) specimens (max. Hardness 1130HV0.01) in comparison with the (S) condition (max. hardness 950HV0.01).The latter results were discussed in terms of the effect of Cu in the activity of Fe-N solid solutions. Thermo-Calc® simulations showed that when copper is in solid solution, it increases the nitrogen activity in iron-nitrogen alloys, decreasing the maximum nitrogen solubility in the steel. On the other hand, when copper is precipitated as nanoparticles in the matrix — as in the (S+A) condition — nitrogen activity decreases, implying a greater solubility of nitrogen.The substrate hardness after aging was not changed by the 400°C/20h nitriding treatment, indicating that the surface treatment can be carried out without affecting the bulk properties.

Source and Control of Nitrogen for X70 Pipeline Steel

The effects of some key factors on nitrogen absorption during the smelting process of X70 pipeline steel were studied, and the source of nitrogen pick-up was analyzed to find the bottleneck for nitrogen control. A series of measures were put forward to decrease the nitrogen pick-up. The results indicated that an exponential relationship existed between the nitrogen absorption index and the free oxygen in molten steel. Nitrogen absorption index could decrease below 0.3 when free oxygen in molten steel was above 100 × 10−6 after tapping. For low sulfur killed steel, the nitrogen absorption ratio and sulfur content satisfied a linear relationship with a slope of −0.007. Low free-oxygen and sulfur were beneficial to the deep desulfurization during vacuum treatment. The contradiction of high desulfurization ratio and low nitrogen pick-up during LF process could be resolved by skimming oxidizing slag after tapping and making new high basicity top slag. After optimization, the average content of nitrogen in final product decreased from 46 × 10−6 to 35 × 10−6.