Carbon Pickup Research Articles

Inductive hot-pressing of titanium and titanium alloy powders

Inductive hot-pressing is a field-assisted sintering process (FAST) in which an electrical current is used to enhance the densification of the powder. Inductive hot-pressing could be employed to enable titanium powder to reach a higher density in less time than the pressing and sintering process. In this study, titanium and titanium alloy powders with different features were processed by means of inductive hot-pressing. The influence of processing temperature on density, microstructure, quantity of interstitial elements and hardness was investigated. Generally, practically fully dense materials were obtained without any carbon pick-up, even if the materials were in contact with the graphite matrix during processing. Nevertheless, there was an increment of the nitrogen content and some oxygen pick-up, especially for the powders with smaller particle size. Hardness is not significantly affected by the pressing temperature, but it strongly depends on the amount of interstitials.

Hydrocarbon–metal reactions during metal arc welding under oil (MAW-UO)

Metal arc welding under oil is a new, revolutionary process for the repairs of pipelines which should minimise the need of hot tapping repairs and the construction of bypass lines. The process uses an automated metal arc welding set-up with a continuous wire feed. This process was developed for the in situ internal repairs of in service cross-country pipelines, tanks and vessels by using a smart pig with a modified mechanical design that allows for joint preparation and welding. The unique characteristics found in the under oil arc are discussed in detail. The large range of temperatures that exists between the arc at the order of 10 000°C (18 000°F) and the surrounding oil (ambient) allows for complex reactions between the oil hydrocarbon chains and the steel weld metal. These reactions may considerably impact the weld microstructure. Details of the dissociation of hydrocarbon chains and ionisation of the resulting species must be known to better control the process. In addition, vaporisation of the oil will form oil vapour bubble to surround the welding arc. The oil vapour bubble will also be high in decomposed products that can significantly affect the weld metal composition. Gas chromatography–mass spectrometry was used to investigate the behaviour of a welding arc under oil and the carbon pick-up. Analyses were conducted on oil samples with different welding exposure times to validate the arc behaviour mechanistic model and the gas chromatography–mass spectrometry analysis results.

Chemical Interactions of Alumina–Carbon Refractories with Molten Steel at 1823 K (1550 °C): Implications for Refractory Degradation and Steel Quality

A sessile-drop study was carried out on Al2O3-10 pct C refractory substrates in contact with molten iron to investigate possible chemical reactions in the system and to determine the influence of carbon and the role, if any, played by the presence of molten iron that can act both as a reducing agent and as a metallic solvent. These investigations were carried out at 1823 K (1550 °C) in argon atmosphere for times ranging between 15 minutes and 3 hours. We report the occurrence of chemical reactions in the Al2O3-10 pct C/Fe system, associated generation of CO gas, and carbon pickup by molten iron. Video images of the iron droplet started to show minor deviations after 30 minutes of contact followed by intense activity in the form of fine aluminum oxide whiskers emanating from the droplet and on the refractory substrate. The interfacial region also changed significantly over time, and the formation of small quantities of iron aluminide intermetallics was recorded after 30 minutes as a reaction product in the interfacial region. These chemical reactions also caused extensive penetration of molten iron into the refractory substrate. This study has shown that alumina cannot be treated as chemically inert at steelmaking temperatures when both carbon and molten iron are present simultaneously. These findings point to an additional reaction pathway during steelmaking that could have significant implications for refractory degradation and contamination of steel with reaction products.

High-temperature Interactions of Alumina–Carbon Refractories with Molten Iron

The interfacial behaviour of alumina/carbon refractories with liquid iron was investigated at 1550°C, with emphasis on the chemical interactions occurring both in the interfacial region as well as in the bulk of the refractory. The sessile drop technique was used to determine the interfacial wetting behaviour and the phase transformations during the chemical reactions were determined using SEM and EDS. Alumina–carbon refractories were prepared using two types of carbonaceous materials–synthetic graphite and natural graphite. From the experimental results, it was clearly observed that molten iron had penetrated to varying extents into all refractory substrates. The highest penetration was observed for alumina–carbon refractory substrates containing 10% synthetic graphite, while all alumina–natural graphite substrates showed much lower levels of metal penetration. Contact angles for synthetic graphite and natural graphite containing refractories showed an initial drop in the first 5 min of the experiment before increasing and stabilising at different values depending on the composition of the refractories. The initial drop in the contact angles was in direct correspondence with increasing carbon pickup values for these samples. Natural graphite samples generally showed lower carbon pickup as compared to synthetic graphite samples of similar carbon content. The differences in metal penetration and interfacial wetting behaviour were found to depend on the carbon pickup and the ash content of the refractory substrates, with the ash helping in the formation of an interfacial layer which limited reactions with the metal, and in filling the pores within the refractory.

Carburization/Oxidation Behavior of Alloy Haynes-214 in Methane–Hydrogen Gas Mixtures

The excellent resistance of Alloy Haynes-214 to carburization at elevated temperatures is attributed to the formation of a protective surface layer of Al2O3, resulting from the reaction of trace oxygen impurity in the gas phase with the Al-enriched surface. Lower carburization resistance below 1000 °C is manifested by increased carbon pick-up and degradation of mechanical properties. Exposures in CH4/H2 gas mixtures containing trace levels of oxygen below 1000 °C reduce the supply of outwardly diffusing aluminum from the bulk of the alloy to the metal surface, thus enhancing inward-oxygen diffusion and the subsequent formation of fine lamellar dispersion of internal Al2O3 within the sub-surface zone. In the meantime, inward-carbon diffusion forms internal carbides, thus leading to increased weight gain, relative loss of protection, and possible degradation of mechanical properties. Effective mitigation of carburization is limited only to exposure at extreme temperatures (1100 °C) which enhances the outward diffusion of aluminum towards the gas/metal interface as well as the nucleation of Al2O3 at the surface, thus causing an early transition from internal to external oxidation, which promotes the formation of an external protective layer of Al2O3 which effectively blocks carbon ingress in the alloy.

Carbon Dissolution Occurring during Graphite–Ferrosilicon Interactions at 1550°C

The carburisation reaction is a key reaction for the cupola process, since the metallic liquid droplets interact with coke at high temperatures. The kinetic mechanism of carbon dissolution in liquid iron had been extensively investigated. However, there is little knowledge about the kinetics of carbon dissolution when the silicon contents are over 10%, since the silicon content during the iron and steel making processes are usually well below this limit.Carbon dissolution phenomenon and associated mechanisms are established for ferrosilicon alloys and silicon at 1550°C in this study. The overall-rate constants at 1550°C for Si 98.5%, FeSi 74% and FeSi 24.7% were 3.8, 3 and 3.9×10−3 (s−1) respectively. The appearance of SiC as an interfacial product was found during the metal–graphite interactions and its role as a retarding agent during the carbon pickup was established.The kinetics of carbon dissolution from graphite is controlled by a mixed-control mechanism and this includes the diffusion of carbon and carbon transfer from the SiC interfacial layer.

Cracking of 2.25Cr–1.0Mo steel tube/stationary tube-sheet weldment of a heat-exchanger

The premature failure of a horizontal heat-exchanger, which occurred after service exposure at 580 °C for 50,000 h, revealed the occurrence of extensive through-thickness cracking in approximately 40% of the tube/stationary tube-sheet welds. Additionally, the internal surface of the welded joint featured intensive secondary intergranular cracking (up to 250 μm deep), preferential formation of a 150 μm thick layer of (Fe,Cr) 3O 4 and internal intergranular oxidation (40 μm deep). The welded region also showed intense carbon pick-up and, as consequence, severe precipitation of intergranular M 7C 3 and M 23C 6 carbides. The fracture surface was composed of two distinct regions: a “planar” region of 250 μm, formed due to the stable crack growth along by the intergranular oxidation; and a slant region with radial marks, formed by the fast crack growth along the network of intergranular carbides. The association of intergranular oxidation pre-cracks with microstructural embrittlement promoted the premature failure, which took place by an overload mechanism, probably due to the jamming of the floating tube-sheet during the maintenance halt (cooling operation).

Oxidation behavior of a V–4Cr–4Ti alloy during the commercial processing of thin-wall tubing

Because of the high solubility and mobility of oxygen in vanadium, composition control during the fabrication of thin (0.25 mm) wall tubing from vanadium alloys by cold drawing and annealing, presents a technological challenge. During intermediate annealing at 1000 °C in the 10 −4 Torr vacuum regime, oxygen penetration into the tube wall is controlled by the development of a semi-protective surface oxide (linear-parabolic oxidation conditions); oxygen-hardened surface layers lead to a high incidence of surface cracking during the final stages of cold drawing. In the 10 −5 Torr regime, under linear kinetic oxidation conditions, rapid oxygen penetration results in unacceptably high levels of oxygen pick-up (∼1500 wppm). In the 10 −7 Torr vacuum regime, molecular impingement rates are reduced to the point where overall oxygen pick-up is reduced to <100 wppm. Improved cleaning/gettering procedures also restrict carbon and nitrogen pick-up to very low levels.

Role of Ash Impurities in the Depletion of Carbon from Alumina-Graphite Mixtures in to Liquid Iron

Due to their excellent thermal shock and slag resistance at high temperatures, alumina–graphite refractories are used extensively in the steel industry. The degradation of carbon based refractories through carbon depletion is an important issue and a fundamental understanding of refractory behaviour at high temperatures is crucially important. Natural flake graphite, with ash impurity levels ranging from 1 to 10%, is used extensively in the commercial preparation of alumina–carbon refractories. This study investigates the role played by ash impurities in the depletion of carbon from the refractory composite. Two natural graphites, respectively containing 2.1% and 5.26% ash, were used in this study. Substrates were prepared from mixtures of alumina and carbon over a wide concentration-range. Using a sessile drop arrangement, carbon pick-up by liquid iron from alumina–natural graphite mixtures was measured at 1 550°C and was compared with the carbon pick-up from alumina–synthetic graphite mixtures. These studies were supplemented with microscopic investigations on the interfacial region. Very high and similar levels of carbon dissolution were however observed from both alumina–natural graphite mixtures, with carbon pickup by liquid iron from mixtures with up to 30 wt% alumina reaching saturation. A sharp reduction to near zero levels was observed in the 30 to 40 wt% alumina range. Along with implications for commercial refractory applications, these results are discussed in terms of poor wettability between alumina and liquid iron, interactions between ash impurities and alumina, and formation of complexes in the interfacial region.

Dissolution of carbon from alumina-carbon mixtures into liquid iron: Influence of carbonaceous materials

Due to their excellent thermal shock and wear resistance at high temperatures, alumina-carbon based refractories are used extensively in the steel industry. A clear understanding of factors affecting the dissolution of carbon from refractories is of crucial importance, as carbon depletion from the refractory can significantly deteriorate refractory performance and metal quality. Atomistic simulations on the alumina-graphite/liquid iron system have shown that nonwetting between alumina and liquid iron is an important factor inhibiting the penetration of liquid metal in the refractory matrix and limiting carbon dissolution. This study investigates the role played by the carbonaceous material in the dissolution of carbon from the refractory composite. Two carbonaceous materials, namely, petroleum coke and natural graphite, respectively, containing 0.35 and 5.26 pct ash, were used in this study. Substrates were prepared from mixtures of alumina and carbon over a wide concentration range. Using a sessile drop arrangement, carbon pickup by liquid iron from alumina-carbon mixtures was measured at 1550 °C and was compared with the carbon pickup from alumina-synthetic graphite mixtures. These studies were supplemented with wettability measurements and microscopic investigations on the interfacial region. For high alumina concentrations (>40 wt pct), carbon dissolution from refractory mixtures was found to be negligible for all carbonaceous materials under investigation. Significant differences however were observed at lower alumina concentrations. Carbon dissolution from alumina-petroleum coke mixtures was much lower than the corresponding dissolution from alumina synthetic graphite-mixtures and was attributed to poor wettability of petroleum coke with liquid iron, its structural disorder, and the presence of sulfur. Very high levels of carbon dissolution, however, were observed from alumina-natural graphite mixtures, with carbon pickup by liquid iron from mixtures with up to 30 wt pct alumina reaching saturation. A sharp reduction to near zero levels was observed in the 30 to 40 wt pct alumina range. Along with implications for commercial refractory applications, these results are discussed in terms of material characteristics, interactions between ash impurities and alumina, and formation of complexes in the interfacial region.

Low cycle fatigue of four stainless steels in 20% CO–80% H 2

Four stainless steels, 253 MA, 304L and Kanthal APM and APMT have been subjected to low cycle fatigue (LCF) at 750 °C. The testing was carried out in a newly developed test rig that allows for testing in a controlled environment from room temperature up to 850 °C. The atmospheres used in the testing were laboratory air, a carburising atmosphere (20% CO–80% H 2) and an inert gas (Ar). The results from the testing showed that the fatigue life for 253 MA decreased in the carburising atmosphere and in argon compared to normal air. This feature was insignificant at low strains but increased with higher strains. For APM, APMT and 304L the carburising atmosphere did not significantly alter the fatigue life. 304L showed a significant pick-up of carbon at the surface of the specimen but this effect did not influence the material properties. No significant pick-up of carbon could be detected in APM or APMT. It is demonstrated that the crack size increases exponentially with increasing number of cycles and the rate at which this takes place increases exponentially with the plastic strain range. The strain enhancement factor for the crack propagation is about 50% higher in 20% CO + 80% H 2 than in air.

Vacuum Induction Melting of Ternary NiTiX (X=Cu, Fe, Hf, Zr) Shape Memory Alloys Using Graphite Crucibles

In the present study we investigate vacuum induction melting (VIM) of prominent ternary NiTiX (X ¼ Cu, Fe, Hf, Zr) shape memory alloys using graphite crucibles. We apply a melting procedure which was recently developed for binary NiTi alloys and which keeps the carbon pick-up during melting at a minimum. We investigate the microstructures of the as-cast and homogenized alloys using scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis (EDX). Differential scanning calorimetry (DSC) was performed to study the phase transformation temperatures of the as-cast and homogenized materials. The results show that VIM processing in graphite crucibles provides ternary NiTiX shape memory alloys with good chemical homogeneity and acceptable impurity contents.

Dissolution of carbon from coal-chars into liquid iron at 1550 °C

Carbon-dissolution studies were carried out on four coal-chars (ash content ranging from 9.04 to 12.61 wt pct), using the carburizer-cover method, and the rates of carbon transfer into liquid iron at 1550 °C were determined. A theoretical model was developed for estimating the interfacial area of contact between the chars and the liquid iron. Using a force-balance approach, the partial penetration of the particles was calculated numerically and the total solid/liquid contact area was evaluated for a range of system parameters. The wettability was found to have a very significant effect on the area of contact. An improvement in wetting reduced the upward force due to surface tension, thereby increasing the downward penetration of particles in the liquid and the contact area. While two chars showed a monotonic increase in carbon pickup by the liquid iron, a two-stage behavior was observed for the remaining two chars. Stage I, which corresponds to short times of contact, showed a much higher rate of carbon dissolution, as compared to stage II during later times. The slow rate of carbon dissolution in stage II was attributed to high levels of interfacial blockage by reaction products, which resulted in many fewer areas of contact between the carbonaceous material and the liquid iron. First-order dissolution rate constants (×103 ms−1) were computed for stage I in all chars, and the observed trend was as follows: 0.01795 (Char 1)>0.00954 (Char 4)>0.0061 (Char 3)>0.00274 (Char 2). These results compare well with the dissolution rate constants quoted in the literature. Char 1, which had the highest rate constant, also had the lowest concentration of reducible oxides (e.g., silica) among all chars. The consumption of solute carbon through silica reduction could affect the carbon levels in the liquid iron. Due to reduction reactions, the experimentally measured rates of carbon dissolution are expected to be slower than the inherent rates of carbon dissolution into the liquid metal. This study shows strong evidence that chemical reactions at the interface play an important role in determining the rate of char dissolution into liquid metal.

Interfacial Phenomena Occurring during Iron/Char Interactions in a Blast Furnace

Using the sessile drop approach, interfacial reactions taking place in the iron/carbon interfacial region were investigated at 1550°C in a horizontal tube resistance furnace with an argon atmosphere. Two coalchars, labelled as 1 and 2 with respective ash concentrations of 10.88 wt% and 9.04 wt%, and electrolytically pure iron were used in this study. Liquid iron droplets were exposed to chars at high temperatures for times ranging between 1 to 180 min and the assembly was then withdrawn into the colder section quenching the droplet. To examine the time dependant growth of new phases formed in the interfacial region, FESEM and EDS investigations were carried out on the underside of the droplet, which effectively represents the iron/char interface. The transfer of carbon and sulphur into the iron droplet was also determined using a LECO Analyser. Interfacial regions for both chars showed a high occurrence of ash deposits, which were found to increase with time. Al, Ca, S, O, Fe were also detected in EDS analysis of the interface. However very low levels of Si were found in the interfacial region despite high concentrations of silica in the chars initially suggesting chemical reactions involving silica. After three hours of contact, carbon pick-up by liquid iron reached only 0.12 wt% and 0.28 wt% for Char 1 and Char 2 respectively, both of which were much below the saturation level of 5.6 wt%. These results are discussed in terms of the formation of interfacial products, the consumption of solute carbon by reducible oxides and low intrinsic rates of carbon dissolution from non-graphitic Chars.

Carbon pickup of interstitial free steel from Al2O3–C refractories

Samples of interstitial free (IF) steel were heated in alumina–graphite crucibles with flux cover at 1600°C for 120 min in a medium frequency induction furnace. The carbon contents of IF steel samples taken from the molten steel at times 0, 5, 10, 20, 30, 45, 60, 90 and 120 min were examined and the refractories after heating were investigated by electron probe microanalysis (EPMA) with energy dispersive spectroscopy (EDS). The steel carbon content increased rapidly in the first 10 min, and then decreased because of the formation of a decarburising layer on the refractory. This layer separated the molten steel from the bulk refractory and stopped pickup of carbon. At the same time, decarburisation via oxygen through the flux and liquid layer started to reduce the carbon content in the molten steel.

Numerical Analysis of Reactions in a Cupola Melting Furnace

Composition variations of metal and slag in a cupola melting process were investigated by a reaction model for gas/liquid iron/coke and slag/liquid iron systems. A significant amount of alloy elements is lost due to the gas oxidation in furnace shaft. Carbon in metals is determined by gas oxidation and carbon pickup from coke in the shaft. Manganese and silicon are mainly lost by gas oxidation in shaft. Sulfur increases in shaft by pickup from coke but decreases in siphon by slag desulphurisation. There is no significant phosphorus change in shaft, but phosphorus increases in siphon. Oxides formed in shaft contribute 20% of total tapped slag, in which SiO2 contribute 40% of total SiO2 in the tapped slag.

Carbon Pickup in Continuous Casting Processes

The quality of the as‐cast strand is influenced by a recarburization reaction since mould powder contains carbon particles necessary for a controlled melting behaviour. Intensive testing reveals a carbon pickup in the surface of the as‐cast strand depending on content and type of the carbon in the mould powder. Using the combination of laboratory trials and numerical simulations the effect of carbon pickup from mould powder to the strand shell was investigated. Laboratory experiments show that the recarburization of the liquid steel through the mould powder can be explained by Marangoni convection. This can happen in the slag rim when the carbon is entrapped by the solidifying steel and starts to diffuse inwards the strand.

Carbon Pickup Phenomenon of Fe-C System Alloys in Evaporative Pattern Casting Process

Carbon Pickup Phenomenon of Fe-C System Alloys in Evaporative Pattern Casting Process

A plasma tundish heater

The world's first industrially employed plasma system with graphite electrodes for the continuous heating of a tundish was commissioned at the end of March 2001. The system is equipped with 2 graphite electrodes so that a floor electrode is no longer needed. The process data recorded during commissioning give cause for optimism for the future applications of this technology. With an efficiency of 67 %, the system is better than the normal 55 % achieved by the metal torches. In addition, the results show that no carbon pickup takes place and the nitrogen absorption of the melt is far less than 10 ppm and thus well inside the tolerance range of the plant operator.

Chemical interaction processes at the interface between mild steel and liquid magnesium of technical grade

Magnesium alloys are commonly melted and held in steel or cast iron crucibles and small but important amounts of Fe and C can dissolve into Mg-rich melts. Here, carbide formation is studied during interface reactions between solid Fe-xC alloys (x = 0–3.6 wt%) and liquid Mg-9Al-0.7Zn-0.2Mn (wt.%, AZ91) at temperatures from 700 to 800 °C. Two ternary carbides, AlFe3C and Al2MgC2, formed in the reaction layers between the Fe-C and AZ91, and T2-Al2MgC2 additionally formed within the AZ91 alloy due to carbon pickup. T2-Al2MgC2 grew in liquid AZ91 as hexagonal plates that were commonly twinned. A reproducible orientation relationship was measured between T2-Al2MgC2 and α-Mg, and the grain refinement of magnesium by heterogeneous nucleation on T2-Al2MgC2 is explored.