Carbon Supersaturation Research Articles

Metal Dusting Corrosion of Ni-Based High-Cr Alloys in Colossal Carbon Supersaturation

Metal dusting is a high-temperature carbon-induced corrosion phenomenon that metals and alloys undergo at temperatures in the range of 450~850°C in gaseous environments that are supersaturated with respect to carbon. Since most currently available high-temperature alloys are prone to metal dusting, understanding and thereby controlling metal dusting corrosion of structural materials is critical to many refining and petrochemical process operations and technologies, such as the production of syngas and the conversion of light hydrocarbons to high value products. The corrosion process involves the breakup of the bulk metallic structure into powder or “dust” consisting of metal particles, oxides, and carbides. Metal dusting is always accompanied by coking whereas coking can occur in process streams without metal dusting.A great deal of effort has been expended by major alloy manufacturers to develop new structural alloys that are resistant to metal dusting in the syngas processing environments. These efforts have been focused primarily on Ni-based alloys having a significant amount of Cr (~30 wt.%) and additionally small amounts of Al, Si, Cu, Mo, etc. The composition of such commercial alloys is given in Table I. All these alloys rely on the formation of Cr2O3 film on the surface to provide metal dusting resistance. Table 1. Commercially available metal dusting resistant alloys Manufacturer Name Nominal composition (wt.%) Special Metals Inconel 693 Balanced Ni:4.5Fe:29.0Cr:3.2Al:1.5Nb, others (Si, Mn, Ti, Cu) NSSMC Alloy 696 Balanced Ni:3.0Fe:30.0Cr:2.0Cu:2.0Mo:1.5Si, others (Mn, Ti) Haynes Int’l HR235 Balanced Ni:1.0Fe:31.0Cr:5.6Mo:3.8Cu, others (Si, Mn, Al) VDM Metals Alloy 699XA Balanced Ni:1.0Fe:28.0Cr:2.5Al, others (Si, Mn, Ti, Nb, Cu) The metal dusting environment is characterized by carbon activities higher than unity (ac>1) and very low oxygen partial pressures. Generally, the carbon activity decreases with increasing temperature while the oxygen partial pressure increases with temperature. There are differences in syngas mixtures used by different investigators. However, the reaction that transfers carbon to the metal surface to trigger metal dusting can be written asCO + H2 = H2O + CBy considering the pseudo-equilibrium of this reaction, where carbon is not allowed to precipitate out, one can calculate the activity of carbon from the equilibrium constant, K1 of this reaction assuming the formation of about 1 vol.% H2O. Thus, ac = PCO ×PH2 /(K1 ×PH2O )Since syngas process streams often contain significant amounts of oxygen-containing molecules such as H2O and CO2, the oxygen partial pressure of the environment is in favor of forming a protective Cr2O3 film.In contrast some petrochemical processes that deal with light hydrocarbons, often being considered as those with 1 to 6 carbon atoms, are nearly free of any oxygen-containing molecules. Under these process conditions, the activity of carbon can be calculated in a following manner.CxHy = xC + (y/2)H2 ac = [(K3 ×PCxHy )/PH2 (y/2)](1/x) The calculated carbon activities in the light hydrocarbon environments are in the order of thousands, which is about two orders of magnitude higher than those in the syngas environments.Therefore, it becomes important to understand how commercially available metal dusting resistant alloys developed mainly to control corrosion in the syngas environments behave in the light hydrocarbon environments. To address this question cyclic metal dusting tests were performed.A 49CO:49H2:2H2O gas mixture was used at 650°C and ambient pressure. Equilibrium carbon activity of the gas mixture calculated by the above equation is 39, and oxygen partial pressure is 2.2x10-26. Alloy samples were suspended within the reactor and the reaction gas mixture was flowed through the reactor. An electrically heated furnace was then driven upwards until the alloys were in its hot zone at 650°C. After 45 minutes of exposure, the furnace was driven downwards, the gas flow maintained, and the samples cooled to less than 100°C. The furnace remained in this position for 15 minutes. The entire cycle (45 min at 650°C and 15 min cooling) was repeated for 1,440 times. Similarly, a gas mixture of 20nC5H12:20H2:60CH4 was used to understand metal dusting of the same alloys at 600°C and ambient pressure. Equilibrium carbon activity of the gas mixture calculated by the above equation is 2,525, and oxygen partial pressure is 1.1x10-38 assuming presence of 100 ppm steam. In the present paper, based on the microstructure of metal-dusted Ni-based high-Cr alloy samples in both syngas and light hydrocarbon environments, a mechanistic picture of alloy corrosion is presented which also attempts to explain the role of the alloying elements.

Galling-Free Dry Near-Net Forging of Titanium Using Massively Carbon-Supersaturated Tool Steel Dies.

Massively carbon-supersaturated (MCSed) tool steel dies were developed to make galling-free forging products from titanium bar feedstocks in dry conditions without lubricating oils. Two types of tool steel dies were used, SKD11 and ACD56, following the Japanese Industrial Standard (JIS). The plasma-immersion carburizing process was employed to induce massive carbon supersaturation in two kinds of tool steel dies at 673 K for 14.4 ks. A pure titanium bar was upset in a single stroke up to the reduction of thickness of 70% using the MCSed SKD11 die. Very few bulging displacements of the upset bar proved that μ = 0.05 on the contact surface of the MCSed SKD11 die to pure titanium work. Two continuous forging experiments were performed to demonstrate that an in situ lubrication mechanism played a role to prevent the contact surface from galling to titanium works in both laboratory- and industry-scaled forging processes. After precise microstructure analyses of the contact surface, the free-carbon film formed in situ acted as a lubricating tribofilm to reduce friction and adhesive wear in continuous forging processes. The MCSed ACD56 dies were also used to describe the galling-free forging behavior of manufacturing eyeglass frames and to evaluate the surface quality of the finished temples. The applied load was reduced by 30% when using the MCSed ACD56 dies. The average surface roughness of the forged product was also greatly reduced, from 4.12 μm to 0.99 μm, together with a reduction in roughness deviations. High qualification of forged products was preserved together with die life prolongation even in dry manufacturing conditions of the titanium and titanium alloys.

Role of cementite on tensile properties in auto-tempered 0.15C–5Mn martensitic steel

The effects of austenitizing temperature and cooling rate on microstructure and subsequent tensile properties were investigated in 0.15C–5Mn martensitic steels. Tensile strength and elongation increased with decreasing austenitizing temperature (1000 °C to 800 °C) in both air-cooled and water-quenched steels. Improvement of tensile properties originated from the promoted transformation-induced plasticity (TRIP) effect, as lower austenitizing temperature limited Mn diffusion to make cementite preferred site for Mn partitioning, thereby increasing the fraction of retained austenite after heat treatment. Cooling rate also affected the mechanical properties, as air-cooled samples showed lower strength and higher elongation compared to water-quenched samples. During the air-cooling, supersaturated carbon in martensite is redistributed to form additional carbides by auto-tempering effect. The formation of carbides softened the martensite to decrease strength while increased tensile elongation. Fraction of retained austenite influenced by the austenitizing temperature and formation of carbide affected by the cooling rate acted as competing mechanisms affecting the tensile properties of the martensitic steels, indicating that heat treatment should be controlled carefully to obtain the desirable mechanical properties.

Effects of coil frequency on the carbon transport in the top-seeded solution growth of SiC single crystal

The flow pattern and carbon transport in the solution for the solution growth of silicon carbide (SiC) in an induction-heating system can be effectively controlled by adjusting the coil frequency to obtain a relatively uniform distribution of growth rate. In this paper, a global heat and mass transfer model was first verified by the SiC crystal growth experiment results at the coil frequency of 8 kHz. Then, the influence of coil frequency on the fluid flow, carbon concentration, and growth rate for the growth of SiC crystals was numerically investigated. The results indicated that electromagnetic convection (at low frequencies) greatly enhances the upward convection beneath the crystal and weakens the adverse effects of Marangoni convection, resulting in a uniform distribution of radial temperature along the growth interface. Consequently, the carbon transport from the crucible wall to the crystal center is strengthened, leading to a uniform region of high carbon supersaturation with a lower coil frequency. The optimal radial uniformity of the growth rate along the growth interface is achieved at the coil frequency of 3 kHz.

Growth of diamond in liquid metal at 1 atm pressure.

Natural diamonds were (and are) formed (thousands of million years ago) in the upper mantle of Earth in metallic melts at temperatures of 900-1,400 °C and at pressures of 5-6 GPa (refs. 1,2). Diamond is thermodynamically stable under high-pressure and high-temperature conditions as per the phase diagram of carbon3. Scientists at General Electric invented and used a high-pressure and high-temperature apparatus in 1955 to synthesize diamonds by using molten iron sulfide at about 7 GPa and 1,600 °C (refs. 4-6). There is an existing model that diamond can be grown using liquid metals only at both high pressure and high temperature7. Here we describe the growth of diamond crystals and polycrystalline diamond films with no seed particles using liquid metal but at 1 atm pressure and at 1,025 °C, breaking this pattern. Diamond grew in the subsurface of liquid metal composed of gallium, iron, nickel and silicon, by catalytic activation of methane and diffusion of carbon atoms into and within the subsurface regions. We found that the supersaturation of carbon in the liquid metal subsurface leads to the nucleation and growth of diamonds, with Si playing an important part in stabilizing tetravalently bonded carbon clusters that play a part in nucleation. Growth of (metastable) diamond in liquid metal at moderate temperature and 1 atm pressure opens many possibilities for further basic science studies and for the scaling of this type of growth.

Carbon solute drag effect on the growth of carbon supersaturated bainitic ferrite: Modeling and experimental validations

The carbon partitioning and lengthening rate of bainitic ferrite (αb) are excellent experimental parameters to estimate our level of understanding of the mechanism of bainitic transformation from a continuum perspective and our ability to capture it in analytical expressions. For Fe-C alloys and relatively simple steels the classical Zener-Hillert theory captures the bainitic transformation rather well but mispredicts the level of carbon in solution in the bainite and overestimates the lengthening rates for transformations at lower temperatures. To address this issue, this paper presents a new thermo-kinetic model based on the Zener-Hillert theory and the Gibbs energy balance concept to simulate the lengthening behavior of αb in the Fe-C and low alloyed steels. The model incorporates the effect of the temperature dependent carbon diffusion within the migrating interface via a temperature dependent ferrite/austenite interfacial energy and a temperature dependent diffusion coefficient but does not impose local equilibrium across the interface. The good agreement between the model predictions and nine sets of published experiments indicates that both the carbon supersaturation in αb and the slower lengthening rate are caused by carbon diffusion within the migrating interface. It is found that the degree of carbon supersaturation in αb increases significantly with decreasing temperature. Consequently, the enhanced carbon solute drag effect, resulting from carbon diffusion within the interface, strongly retards the lengthening rates of αb at lower temperatures. Transformation strain is shown to have a modest effect on the lengthening rates but to lower the degree of carbon supersaturation.

In Situ Lubrication in Forging of Pure Titanium Using Carbon Supersaturated Die Materials.

A new solid lubrication method was proposed for dry forging of pure titanium with high reduction in thickness. A free-carbon tribofilm was formed in situ at the hot spots on the contact interface to protect the die surfaces from severe adhesion of work materials. This film consisted of the free carbon, which isolated from the carbon supersaturated die substrate materials, diffused to the contact interface and agglomerated to a thin film. Two different routes of carbon supersaturation process were developed to prepare carbon supersaturated ceramic and metal dies for the dry forging of pure titanium wires. A pure titanium bar was utilized as an easy-to-adherent work material for upsetting in dry and cold. The round bar was upset up to 70% in reduction in thickness with a low friction coefficient from 0.05 to 0.1 in a single stroke. Work hardening was suppressed by this low friction. SEM-EDX, EBSD and Raman spectroscopy were utilized to analyze the contact interface and to understand the role of in situ formed free-carbon films on the low friction and low work hardening during forging. Precise nanostructure analyses were utilized to describe low friction forging behavior commonly observed in these two processes. The in situ solid lubrication mechanism is discussed based on the equivalence between the nitrogen and carbon supersaturation processes.

N-containing dissolved organic matter promotes dissolved inorganic carbon supersaturation in the Yangtze River, China

Dissolved inorganic carbon (DIC) represents a major global carbon pool and the flux from rivers to oceans has been observed to be increasing. The effect of weathering with respect to increasing DIC has been widely studied in recent decades; however, the influence of dissolved organic matter (DOM) on increasing DIC in large rivers remains unclear. This study employed stable carbon isotopes and Fourier transform ion cyclotron mass spectrometry (FT-ICR MS) to investigate the effect of the molecular composition of DOM on the DIC in the Yangtze River. The results showed that organic matter is an important source of DIC in the Yangtze River, accounting for 40.0 ± 12.1 % and 32.0 ± 7.2 % of DIC in wet and dry seasons, respectively, and increased along the river by approximately three times. Nitrogen (N)-containing DOM, an important composition in DOM with a percentage of ∼40 %, showed superior oxidation state than non N-containing DOM, suggesting that the presence of N could improve the degradable potential of DOM. Positive relationship between organic sourced DIC (DICOC) and N-containing DOM formulae indicated that N-containing DOM is crucial to facilitate the mineralization of DOM to DICOC. N-containg molecular formular with low H/C and O/C ratio were positively correlated with DICOC further verified these energy-rich and biolabile compounds are preferentially decomposed by bacteria to produce DIC. N-containing components significantly accelerated the degradation of DOM to DICOC, which is important for understanding the CO2 emission and carbon cycling in large rivers.

THE SIGNIFICANCE OF RETAINED AUSTENITE IN THE HIGH STRENGTH AND PLASTICITY OF MEDIUM CARBON Q&P STEEL

The Fe–0.44%C–1.8%Si–1.3%Mn–0.82%Cr–0.28%Mo steel treated by the quenching-partitioning process showed a product of strength and elongation of 30 GPa·% with yield stress of 1350 MPa. Such combination of high ultimate tensile strength and good ductility is attributed to a high portion of retained austenite (≥20%) transforming to martensite under tension. The high yield stress is provided by carbon supersaturation of austenite and a high dislocation density in this phase.

Wide tuning the carbon supersaturation within Fe-C lath martensite via high pressure martensitic transformation of Fe-0.45C alloy

The carbon supersaturation within lath martensitic lattice was tuned via thermal treatment of a Fe-0.45C alloy under hydrostatic pressure of 1–6 GPa. The content of supersaturated carbon increases linearly with high pressure till 84.4% of the nominal carbon content under 6 GPa. The strengthening of carbon supersaturation is remained proportional to the square root of its content but the contribution was underestimated.

Deposition of an adherent diamond film on stainless steel using Cr/Cr[sbnd]Al[sbnd]N as an interlayer

Cr/CrAlN films were chosen as an interlayer to deposit an adherent diamond film on stainless steels by using hot filament chemical vapor deposition (HFCVD). The results show that diamond film with good adhesion is successfully deposited on 3Cr13 stainless steel when the interlayer with Al content from 0 to 18 at.%, which is ascribed to high chemical adhesion by a certain amount of chromium carbide. When Al content in the interlayer further increases, an excessive Al2O3 is formed on the surface of the interlayer, carbon supersaturation is easily arrived, so that the amount of the carbide at the diamond/interlayer interface decreases evidently. This decreases the strength of the covalent bonding from the carbide, producing poor adhesion. Moreover, with the raise of Al content in the interlayer, the hardness of the multilayer coated stainless steel increases due to the increase of the interlayer's hardness and the slight increase of the diamond content in the film.

Enhanced carbon solubility in solvent for SiC rapid solution growth: Thermodynamic evaluation of Cr–Ce–Si–C system

The carbon dissolution in solvent plays a key role in the process of solution growth route for SiC single crystal, which could determine the growth rate and quality of the products. However, the carbon dissolving ability of binary alloy solvent still needs to be improved. Here, we demonstrate the improved carbon dissolution and enlarged carbon supersaturation in Cr–Ce–Si ternary solvent, showing great potential for SiC solution growth. The phase relations of Cr–Ce–Si–C system were determined by using CALPHAD method based on thermodynamic parameters of CeCr2Si2C. It is indicated that the Cr–Ce–Si ternary solvent shows much larger carbon solubility in temperature range from 1700 to 2000 °C compared to Cr–Si binary one. Furthermore, the carbon supersaturation in solvent is also significantly increased in low temperature range after the addition of Ce, leading to a rapid growth rate. Our work not only demonstrates the feasibility of adding Ce in the alloy solvent for rapid growth of SiC crystal, but also provides an example for investigating the C solubility in ternary solvent.

Formation of lower bainite in a high carbon steel – an in-situ synchrotron XRD study

The microstructural evolution of and simultaneous dimensional changes in high-carbon SAE 52100 bearing steel were monitored continuously during austempering for 120 min at selected temperatures in the range of 210 °C-270 °C, and also during its subsequent tempering to 340 °C for an additional 120 min, via high-energy X-ray diffraction in real time and in-situ dilatometry. The austenite-to-bainitic ferrite transformation induces lattice defects and internal lattice stresses that increase with austempering time and at lower austempering temperatures. These changes are evidenced by the increase in the full-width half-maximum of the relevant reflections in X-ray diffraction. The lattice parameter of bainitic ferrite takes its highest value during the early stages of austempering, and then gradually decreases as the transformation progresses. This observation points to an initial state of carbon supersaturation in the ferritic lattice that is likely reducing due to carbon segregation close to dislocations, fine carbide precipitation within the bainitic ferrite, and carbon partitioning into the surrounding austenite. The carbon partitioning into austenite is evidenced in particular at the higher austempering temperatures of 240 °C and 270 °C, at which there is a noticeable increase in the lattice parameter of the remaining austenite at longer times. The dimensions of the bearing steel specimens are governed by the volume change due to the formation of bainitic ferrite during austempering and by the relaxation of its lattice distortion during tempering at 340 °C in the absence of further phase transformation.

Defects of diamond crystal structure as an indicator of crystallogenesis

Based on the study of a representative collections of diamonds from diamondiferous formations of the Urals and deposits of the Arkhangelsk and Yakutian diamond provinces, we established patterns of zonal and sectoral distribution of crystal structure defects in crystals of different morphological types, identified the specifics of crystals formed at different stages of crystallogenesis and performed a comprehensive analysis of constitutional and population diversity of diamonds in different formations. We identified three stages in the crystallogenesis cycle, which correspond to normal and tangential mechanisms of growth and the stage of changing crystal habit shape. At the stage of changing crystal habit shape, insufficient carbon supersaturation obstructs normal growth mechanism, and the facets develop from existing surfaces. Due to the absent stage of growth layer nucleation, formation of new {111} surfaces occurs much faster compared to tangential growth mechanism. This effect allows to explain the absence of cuboids with highly transformed nitrogen defects at the A-B1 stage: they have all been refaceted by a regenerative mechanism. Based on the revealed patterns, a model of diamond crystallogenesis was developed, which takes into account the regularities of growth evolution, thermal history and morphological diversity of the crystals. The model implies the possibility of a multiply repetitive crystallization cycle and the existence of an intermediate chamber; it allows to explain the sequence of changes in morphology and defect-impurity composition of crystals, as well as a combination of constitutional and population diversity of diamonds from different geological formations.

A novel NbTaW0.5 (Mo2C)x refractory high-entropy alloy with excellent mechanical properties

In this study, a refractory high-entropy alloy (RHEA) NbTaW0.5 was selected as the master alloy and carbide Mo2C was added to prepare the NbTaW0.5(Mo2C)x (x = 0–0.25) RHEAs. The effect of Mo2C addition on the phase composition, microstructure evolution, and mechanical properties of the as-cast and annealed RHEA was investigated. The microstructure and mechanical properties of the as-cast and isothermally annealed (1673 K/12 h) NbTaW0.5(Mo2C)x RHEAs were studied. The results showed that the as-cast RHEAs consisted of a disordered body-centered cubic solid solution phase and a second M2C carbide phase with hexagonal close-packed (HCP) crystal structure. The HCP–M2C carbides were enriched with Nb, Ta, and C elements and tended to achieve a similar eutectic morphology in the interdendritic regions. After isothermal annealing, orthorhombic carbides were formed near the primary carbides, which apparently depleted the carbon supersaturation of the matrix. The compression mechanical properties of as-cast alloys were determined at room temperature, 1473 K, and 1673 K. The yield strength of NbTaW0.5(Mo2C)0.2 RHEA exceeded 1000 MPa at 1473 K, and even reached 697 MPa at 1673 K, far exceeding some of the currently reported RHEAs. These RHEAs had good plasticity and showed excellent application prospects.

Microstructural and Chemical Changes of a Ti-Stabilized Austenitic Stainless Steel After Exposure to Liquid Sodium at Temperatures Between 500 °C and 650 °C

Ti-stabilized austenitic stainless steel was carburized in sodium containing a high carbon activity at three different temperatures, 500 °C, 600 °C, and 650 °C during 1000 hours and 5000 hours. The carbon profile, the carbide volume fraction, and the lattice parameter evolution as function of depth were determined using high-energy X-ray diffraction and electron probe microanalysis. At 650 °C and 600 °C, the carbon precipitated as M23C6 and M7C3 carbides in the sample. The volume fraction of M7C3 carbides was lower than predicted by thermodynamic equilibrium using Thermo-Calc software®. At 500 °C, carbides almost did not form in the steel. Instead, high carbon supersaturation of the austenitic matrix occurred. Both results demonstrate that the carburization profile was strongly influenced by the kinetics of carbide formation at temperatures lower than 650 °C. High-energy X-ray diffraction measurements demonstrated that the austenite and carbide lattice parameters evolved along the carbon profile. Both measured lattice parameter profiles of austenite and M23C6 carbide were compared to the ones predicted from chemical changes of austenite and carbides.

LAMMPS molecular dynamics simulation of methane decomposition on nickel thin films at high temperatures

The activation energy of methane decomposition by nickel was calculated using models of nickel thin films, a large-scale atomic/molecular massively parallel simulator (LAMMPS) and a ReaxFF potential. The rate constants of the first-order chemical reaction of methane decomposition were obtained by analyzing the number of undecomposed methane molecules in individual movie frames. The obtained activation energies of methane decomposition on the Ni (111) surfaces showed values close to those reported in the literature. The structural change of a Ni thin film with (111) surfaces was simulated to investigate how carbon and hydrogen atoms are deposited on the Ni thin film upon the decomposition of methane molecules at a high temperature. The catalytic activity of the (111) Ni thin film was found to decrease from the time point of critical supersaturation of carbon in the Ni thin film.

Free-Forging of Pure Titanium with High Reduction of Thickness by Plasma-Carburized SKD11 Dies.

A tool steel type SKD11 punch was plasma carburized at 673 K for 14.4 ks at 70 Pa to make carbon supersaturation. This carburized SKD11 punch was employed for upsetting the pure titanium wire with the diameter of 1.00 mm up to the reduction of thickness by 70% in a single shot. Its contact interface to titanium work was analyzed to describe the anti-galling behavior in this forging. Little trace of titanium proved that the galling process was suppressed by the in situ solid lubrication. The isolated free carbon agglomerates are wrought as a solid lubricant to sustain the galling-free forging process. This anti-galling upsetting reduced the residual strains in the forged wires. A long titanium wire with a length of 45 mm was incrementally upset to yield the titanium ribbon with a thickness of 0.3 mm, the width of 2.3 mm, and the length of 50 mm. The grain size of original pure titanium was much reduced to 2 μm on average. A micro-pillared microtexture was imprinted onto this forged titanium ribbon.

Chemical Condensation Wave Initiating Oxygen-Free Combustion and Detonation

It is well known that the propagation of traditional combustion and detonation waves was determined by the branched chain reactions discovered by Academician N.N. Semenov. This article discusses a new type of detonation waves initiated by chemical condensation processes. Chemical condensation waves arise as a result of the heat released during the explosive condensation of a highly supersaturated carbon vapor formed as a result of the dissociation of the initial carbon-containing molecules behind the front of the initiating shock wave. Unlike traditional combustion and detonation waves, the mechanism of chemical condensation does not include branched chain reactions; nevertheless, the laws of propagation of detonation waves of condensation are clearly described by the Zel’dovich–Neumann–Döring theory.

Detonation wave of condensation

In a number of recent studies by the authors, a new physical phenomenon has been discovered and investigated in detail — the formation of a detonation wave during the condensation of highly supersaturated carbon vapor. The essence of the phenomenon is that an initiating shock wave propagating through an exothermic carbon compound (for example, carbon suboxide C3O2 or acetylene C2H2), as a result of its rapid dissociation, yields a highly supersaturated carbon vapor; the energy released during its condensation forms and maintains a detonation wave. This review describes in detail the results of experimental studies and their analysis based on the detailed kinetics and thermodynamics of the processes occurring and the one-dimensional Zeldovich–Neumann–Döring (ZND) theory of detonation. The potential to practically use the discovered phenomenon is discussed.