Inert Gas Shrouding Research Articles

Continuous casting tundish quality study by mathematical & physical simulations, economics with plant result justifications

A combination of physical modeling, computational fluid dynamics modeling, and economics with plant trial studies was performed for quality improvement of Special Bar Quality (SBQ) and Oil Country Tubular Goods (OCTG) grade tundish steels. The present study consists of operating parameters like inert gas shrouding, non-isothermal conditions, and flow control devices (FCD) used on the billet product and slab quality. This work uses mathematical modeling using the fluid volume and discrete phase method (DPM) and the standard k-ε turbulence model validated with one-third scale physical water model experiments. A strong correlation between the physical model and computational simulation was found with rejection ratio and inclusion counts. Data about customer demands correlated with operating parameters for proper plant insights with an economic study to predict the cost-related issue. With the incorporation of FCD, the weight of the tundish skull was reduced by 6–10 M USD/year with a simulation studies expenditure of around 200 K. FCD also reduced the customer complaint index (CCI).

Performance Evaluation of Underwater (UW) Welding System with Friction Stir Welding (FSW) Method on 316L Stainless Steel Exposed to Sea Water

Friction Stir Welding (FSW) is a solid-state welding technique that can produce high-quality joints without defects that commonly occur in liquid welding. However, FSW has limitations in underwater applications due to the friction between the tool and the material that causes a decrease in heat efficiency and shear force. To overcome this problem, this study proposes an underwater welding (UW) system with the FSW method on 316L stainless steel exposed to seawater. This UW-FSW system uses a high-speed rotating tool surrounded by an inert gas shroud to prevent direct contact between the tool and seawater. The aim of this study is to evaluate the performance of this UW-FSW system in terms of mechanical properties, microstructure, and corrosion resistance of 316L stainless steel joints. The results of this study show that the UW-FSW system can produce joints with better tensile strength, hardness, and corrosion resistance than conventional FSW. This is due to the rapid cooling, reduction of oxidation, and formation of a thicker passive layer on the joint surface. This study provides a contribution to the development of environmentally friendly and efficient underwater welding technology.

SiC/YAG composite coatings by a novel liquid fuelled high velocity oxy-fuel suspension thermal spray

Despite recent advances in suspension-based thermal spray techniques, there is still a need to widen the capability of available thermal spray setups to handle suspension to explore new compositions and improve the properties and performance of existing ones. In this work, a novel setup for injecting liquid-based feedstock in a liquid-fuelled high velocity oxy-fuel (HVOLF) thermal spray torch is proposed, involving a new hardware (“Hybrid Nozzle”) capable of injecting suspension radially and protecting the thermal spray flame from the environmental oxygen by inert gas shrouding. The new setup was used to deposit SiC/YAG composite coatings with 85 wt% SiC content, and their wear performance was evaluated. An oxygen deficient environment was provided to the HVOLF flame, ensuring that no or minimal (<2 wt%) SiO2 was formed in the coating during the spray, as confirmed by XRD. A good wear performance was observed at low load below 20 N, with a specific wear rate < 5 × 10−5 mm3/Nm, with the onset of wear mechanisms characteristic of ceramic wear in the brittle regime between 20 and 30 N. The future applicability of this novel setup to a range of oxidation- and heat-sensitive materials and composites offers the opportunity for new coating materials to be explored with suspension and solution precursor-based HVOLF thermal spray.

Design of a Helium Vapor Shroud for Liquid Hydrogen Fueling of an Unmanned Aerial Vehicle (UAV)

Filling a vehicular liquid hydrogen fuel tank presents the potential for flammable mixtures due to oxygen concentration from liquid air condensation. Current liquid hydrogen tank designs utilize insulating paradigms such as aerogel/fiberglass materials, vacuum jackets, or inert gas purge systems to keep the outer surface from reaching the condensation temperature of air. This work examines the heat transfer at the refuelling connection of the tank to identify potential areas of condensation, as well as the surface temperature gradient. A shrouded inert gas purge was designed to minimize vehicle weight and refuelling time. The design of a shrouded inert gas purge system is presented to displace air preventing air condensation. The design investigates 3D printed materials for an inert gas shroud, as well as low-temperature sealing designs. Shroud designs and temperature profiles were measured and tested by running liquid nitrogen through the filling manifold. Materials for the inert gas shroud are discussed and experimental results are compared to analytical model predictions. Suggestions for future design improvements are made.

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.

Investigation of Eccentric Open Eye Formation in a Slab Caster Tundish

Inert gas shrouding in tundish can result in the formation of a tundish open eye (TOE) due to the presence of reversed flows on the upper surface of the tundish. Prolonged presence of open eyes promotes reoxidation of liquid steel and creates harmful inclusions, which can ultimately result in clogging of SENs. In spite of its importance, not much work has been performed on TOEs. In our series of recent works (Chattopadhyay in Modelling of Transport Phenomena for Improved Steel Quality in a Delta-Shaped Four Strand Tundish, 2011; Chattopadhyay et al. in ISIJ Int 51: 573–580, 2011; Chatterjee and Chattopadhyay in ISIJ Int 55: 1416–1424, 2015; Metall Mater Trans B 41: 508–521, 2016; Metall Mater Trans B 47: 3099–3114, 2016; Chatterjee et al. in ISIJ Int 56: 1889–1892, 2016), although substantial efforts have been made to understand the basics of TOE formation process, a lot still remains to be deciphered. The current work deals with investigating a strange phenomenon observed during tundish operations: eccentric open eye formation. It is essential to gain proper insights of this matter in order to improve the operations, enhance liquid metal cleanliness, and generate more revenues. A mathematical model was developed using ANSYS-FLUENT 16.2. The standard k-e turbulence model and the discrete phase method, coupled with the discrete random walk model was employed. Three different causes for eccentric open eye formation viz., unbalanced throughput, biased argon injection, and misalignment of ladle shroud have been analyzed. The predicted results correspond to both water model experiments and real plant observations.

Effect of laser melting on plasma sprayed WC-12 wt.%Co coatings

Tungsten carbide powder with 12% Co was deposited on AISI 321 stainless steel substrate by air plasma spraying. The coating was produced at 130mm standoff distance. The coated samples were melted using a CO2 laser with an inert gas shroud. Four different laser speeds from 100mm/min to 250mm/min were used to melt the coatings. After laser melting the treated surfaces were characterized and compared with as sprayed surfaces. It was observed that the laser melting process produced defect free surfaces. The results showed that the thicknesses of the melted surfaces and wear resistance varied as a function of laser speed.

The Role of Spraying Parameters and Inert Gas Shrouding in Hybrid Water-Argon Plasma Spraying of Tungsten and Copper for Nuclear Fusion Applications

Tungsten-based coatings have potential application in the plasma-facing components in future nuclear fusion reactors. By the combination of refractory tungsten with highly thermal conducting copper, or steel as a construction material, functionally graded coatings can be easily obtained by plasma spraying, and may result in the development of a material with favorable properties. During plasma spraying of these materials in the open atmosphere, oxidation is an important issue, which could have adverse effects on their properties. Among the means to control it is the application of inert gas shrouding, which forms the subject of this study and represents a lower-cost alternative to vacuum or low-pressure plasma spraying, potentially applicable also for spraying of large surfaces or spacious components. It is a continuation of recent studies focused on the effects of various parameters of the hybrid water-argon torch on the in-flight behavior of copper and tungsten powders and the resultant coatings. In the current study, argon shrouding with various configurations of the shroud was applied. The effects of torch parameters, such as power and argon flow rate, and powder morphology were also investigated. Their influence on the particle in-flight behavior as well as the structure, composition and properties of the coatings were quantified. With the help of auxiliary calculations, the mass changes of the powder particles, associated with oxidation and evaporation, were assessed.

Spraying of Metallic Powders by Hybrid Gas/Water Torch and the Effects of Inert Gas Shrouding

A hybrid DC arc plasma torch, combining water and gas stabilization, offers a high flexibility in plasma characteristics. These can be controlled in a wide range by the torch operational parameters, such as arc current and secondary gas flow rate. In this study, their influence on plasma spraying of tungsten and copper was investigated. To suppress the in-flight oxidation of the metals, inert gas shrouding was applied. In-flight particle diagnostics and analysis of free-flight particles and coatings was performed for spraying experiments in the open atmosphere and with argon shrouding. Both in-flight particle behavior and coating properties were found to be sensitive to the torch parameters. The application of shrouding was found to affect particle in-flight parameters, reduce the oxide content in the coatings and generally improve their properties, such as thermal conductivity. However, a different degree of these effects was observed for copper and tungsten.

Physical and Mathematical Modelling of Inert Gas Shrouding in a Tundish

Inert gas shrouding practices in water model tundishes were mathematically simulated using the finite volume based program ANSYS 12. The fluid within the tundish and the ladle shroud was assumed to be Newtonian and incompressible (water), and the flow within the shroud was assumed to be predominantly bubbly i.e. discrete bubbles are formed, and these move down the shroud along with the down-flowing water. Thus, the numerical model was developed using the Discrete Phase Modeling (DPM) approach, along with the standard k-e turbulence model with two way turbulence coupling. Predicted flow fields, slag behaviour and bubble trajectories were investigated and then compared with experiments in the full scale and one third scale water model tundishes. Various experimental measurements compared well with predictions from the mathematical model, which was shown to slightly over-estimate depths of penetration of the bubble column by 5–15%.

Oxidation control in atmospheric plasma spraying coating

Thermal spray coatings are formed by successive impingements and interbonding materials among the splats, solidified individual molten particles. Depending on the processing conditions employed during the spray process, deposits are produced with an assortment of microstructures and properties. This study highlights how the coating oxidation differences are influenced by the mechanisms involved during the spray process. The commercial steel powder referenced Amdry XPT 512 is chosen for a systematic study of comparison across different spraying techniques. Steel particles were sprayed with a F4 plasma torch and with a shrouded plasma spray process used in order to protect particles against atmospheric oxidation. The plasma jet was successively shielding by an inert gas shroud and by an oxidizing gas shroud. In-flight oxidation and post impact oxidation present in coatings are discussed in detail and the effects of these mechanisms on coating properties are addressed. The comparison was made on in-flight particle characteristics and on coating properties in terms of oxide content and porosity level. Using shrouded gas, in-flight characteristics are quite similarly independent of the nature of the shrouded gas. This way, the comparison of oxide contents present in the coatings corresponds to in-flight oxidation and is completely dependent on the nature of the shrouded gas. Comparing these results to those obtained by APS, a decrease in both velocity and temperature of in-flight particles was observed leading also to a significant decrease in oxide contents and to a slight increase in porosity level compared to coatings sprayed with air shrouding.

Development of Low-Oxide MCrAlY Coatings for Gas Turbine Applications

Advanced high-energy plasma systems are being used to achieve the benefits of the high-velocity oxy-fuel (HVOF) system without losing the inherent advantages of plasma for coating of gas turbine parts. MCrAlY coatings play a very important role in the performance and reliability of gas turbine components. One of the important considerations for next generation of gas turbines, which have more demanding conditions and need to withstand ever increasing operating temperatures, is that they should possess very low oxygen content levels in the coating. Low oxygen content coatings are applied by the expensive low-pressure plasma spray (LPPS)/vacuum plasma spray (VPS) technique for critical components in aero- and land-based gas turbines. This work deals with the development of low-cost LPPS equivalent coatings (having low oxygen content) using the high-energy high-velocity plasma spray (HEHVPS) gun and inert gas shroud. A comparison has also been made with CoNiCrAlY coatings by HVOF.

Evaluation of the electrochemical behavior of HVOF-sprayed alloy coatings

High-velocity oxy-fuel (HVOF) spraying is a versatile technique that can yield high-density coatings with porosities less than 1% by optimization of the process variables. Oxidation of the coating material can be greatly reduced by inert gas shrouding during the spraying process. This work evaluates the electrochemical behavior of HVOF-sprayed coatings aimed to have better or similar corrosion behavior as stainless steel AISI 316 in acidic medium. A total of eight different chromium-containing coatings with varying proportions of alloying elements such as Ni, Mo, Si, Fe, Co, W, B and C have been studied in hydrochloric acid medium. Porosity measurements, gravimetric analysis, open-circuit potential measurements, potentiodynamic polarization and optical microscopy have been performed. The electrochemical behavior of each coating has been compared with that of bulk AISI 316. The results indicate that HVOF-sprayed AISI 316 coating offers a lower corrosion resistance compared to bulk AISI 316. High nickel- and chromium-containing coatings appear to offer a corrosion resistance comparable to bulk AISI 316. However, results indicate that other alloying elements like molybdenum might be essential for obtaining higher corrosion protection.

Development of dense corrosion resistant coatings by an improved HVOF spraying process

For 6 years, we have developed corrosion resistant coatings to protect steel structures in the marine environment by using a thermal spray technique. This paper summarizes the major developments and results obtained. Such a coating requires primarily impermeability and secondarily homogenous and clean microstructure. In order to make denser and highly corrosion resistant coatings, we selected spray materials and improved fabrication processes. HastelloyC was a suitable material for High Velocity Oxy-Fuel (HVOF) spraying to form corrosion resistant coatings because of its high resistance against thermal oxidation as well as seawater corrosion, especially crevice and pitting corrosion. An inert gas shroud system was attached with a commercial HVOF apparatus and this attachment increased the in-flight velocity of spray particles over 750 m s−1 and simultaneously suppressed oxidation significantly. In addition, some new methods were designed to evaluate the sprayed particle’s state and the coating properties with high accuracy and sensitivity. Thermal energy of in-flight spray particles was revealed by molten fraction of spray particles, determined by quantitative analysis of melted and unmelted particles captured in an agar gel. Through-porosity of the coatings with open porosity below 0.1% was determined by using Inductively Coupled Plasma analysis of dissolved substance from substrate through the penetrating path of the coatings. The coating of HastelloyC nickel base alloy by the HVOF spraying with the gas shroud attachment had zero through-porosity and 0.2 mass% of oxygen content. The laboratory corrosion tests showed that the on-shroud HastelloyC coating was comparable to the bulk material of HastelloyC in terms of corrosion resistance. This coating, formed on steel, demonstrated an excellent protective performance over 10 months in the marine exposure test.

Application of HVOF thermal spraying to solve corrosion problems in the petroleum industry—an industrial note

Commercially available thermal spray coatings have seen limited use in corrosion applications due to the presence of interconnecting porosity and oxide networks. Use of vacuum chambers or post-treatments can eliminate most defects, but these methods are costly and impractical on a large scale. The ability to produce such high-quality coatings by thermal spraying in atmosphere and without post-treatments would offer important advantages as a means of building and repairing process equipment. A modified HVOF process using unique inert gas shrouding has resulted in highly dense, low-oxide coatings of metallic alloys. These coatings were extensively evaluated for severe petroleum industry corrosion applications in laboratory and plant testing, with exposures as long as 5 years. Coatings of corrosion-resistant alloys, such as type 316L stainless steel and Hastelloy C-276, were shown to act as true corrosion barriers. They were protective to underlying base metals in severe environments and in most cases exhibited corrosion resistance comparable to the corresponding wrought alloy. The process was scaled up for on-site plant use and successfully applied to numerous corrosion problems in petroleum industry plant equipment. Significant technical and economic advantages can be realized by use of thermal spray coatings to solve plant equipment problems.

Graphite as a candidate material for a rotary glass fiberising apparatus

Heat resistant metallic alloys are currently the best materials to use for the rotating dishes or spinners with perforated walls which transform a stream of hot glass into thermally insulating glass wool by a centrifugal process. A material even more resistant to high temperature creep could give a longer service life. Since graphites are virtually creep proof materials and their mechanical strength actually increases with temperature, a laboratory assessment was carried out on a fine grain moulded graphite as a candidate material for these spinners. Corrosion by hot glass was found to be minimal and oxidation of parts not submerged in glass could be prevented by the use of an inert gas shroud. Strength was determined by bend tests, internal pressurisation of hollow shapes, and by spinning disc and dish shapes to destructive speeds. At room temperature the effective strength of graphite was found to decrease as the volume under stress increased. Adequate strength was retained to give a very low instantaneous probability of failure at the operational speed of rotation. Failure when it does occur is catastrophic and this may represent a limiting feature to the use of graphite for this purpose.