Stress-corrosion-crack initiation and growth-rate studies on titanium grade 7 and alloy 22 in concentrated groundwater

The stress-corrosion-crack initiation and growth-rate response was evaluated on as-received, cold-worked, and aged Alloy 22 (UNS N06022) and titanium Grade 7 in 105 °C to 110 °C, aerated, concentrated, high-pH groundwater environments. Time-to-failure experiments on actively loaded tensile specimens evaluated the effects of applied stress, welding, surface finish, shot peening, cold work, crevicing, and aging treatments in Alloy 22. Titanium Grade 7 and stainless steels were also included in the matrix. Long-term crack-growth-rate data showed stable crack growth in titanium Grade 7. Alloy 22 exhibited stable growth rates under “gentle” cyclic loading, but was prone to crack arrest at fully static loading. No effect of Pb additions was observed. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees.     

Influence of halide ions and alloy microstructure on the passive and localized corrosion behavior of alloy 22

Alloy 22 (N06022) is the current candidate alloy used to fabricate the external wall of the high-level nuclear waste containers for the Yucca Mountain repository. It was of interest to study and compare the general and localized corrosion susceptibility of Alloy 22 in fluoride and chloride solutions at 90 °C. Standard electrochemical tests such as cyclic potentiodynamic polarization, amperometry, and electrochemical impedance spectroscopy were used. Studied variables included the solution pH and the alloy microstructure (thermal aging). Results show that Alloy 22 is highly resistant to general corrosion in all the solutions tested. Thermal aging is not detrimental and even seems to be slightly beneficial for general corrosion at the higher solution pHs. Pitting corrosion was never observed. Crevice corrosion was found only for high chloride-containing solutions after anodic polarization. The presence of fluoride ions together with chloride ions seems to increase the susceptibility of Alloy 22 to crevice corrosion compared to pure chloride solutions. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees.     

Grain-boundary chemistry and intergranular corrosion in alloy 825

Alloy 825, a former candidate material for radioactive high-level waste containers, was investigated to assess its thermal stability and the time-temperature conditions for sensitization. Alloy specimens with a carbon content of 0.01 wt pct in the mill-annealed (MA) and solution-annealed (SA) conditions were studied after thermal exposure to temperatures ranging from 600 °C to 800 °C for periods of up to 1000 hours. Sensitization was evaluated by using corrosion tests that were correlated to grainboundary chemistry analyses. Sensitized microstructures were found to contain M₂₃C₆-type carbides and a chromium-depleted region in the vicinity of the grain boundaries. Thermal aging at 700 °C for 100 hours resulted in the highest sensitization. While heat treatment at 640 °C showed a progressive development of sensitization with time, healing was found to occur after aging at 800 °C for 100 hours. The degree of sensitization, quantified by an equivalent chromium-depleted-zone size, correlates well with the corrosion rate in the nitric acid test. Thermodynamic models were used to calculate the interfacial chromium concentration, chromium depletion profile, and the depleted-zone width. Comparisons between experimental measurements and model calculations indicate that reliable prediction depends on the selection of key model parameters.     

The effect of annealing on the microstructure and mechanical properties of Cu-X microcomposites

The effect of annealing on the microstructure, texture, and room-temperature mechanical properties ofin situ processed copper-based microcomposites has been investigated. These copper microcomposites, containing 15 vol pct Nb, Cr, or Ta, were produced by rolling of cast material. Annealing was carried out in vacuum for 10 hours at 250 °C, 400 °C, and 650 °C. Evidence of microstructural coarsening was found even at the lowest annealing temperature. The through-thickness microstructure of the composites was examined by transmission electron microscopy both before and after the annealing treatments. Texture of the as-processed micro-composites was assessed using X-ray diffraction methods. The strength of the composites following annealing was found to scale with the melting point of the second component. This article is based on a presentation made in the symposium “High Performance Copper-Base Materials” as part of the 1991 TMS Annual Meeting, February 17–21, 1991, New Orleans, LA, under the auspices of the TMS Structural Materials Committee.     

Comparison of deformation induced precipitation behaviour in Alloy 718 under two microstructural conditions

Alloy 718 samples under two initial microstructural conditions, viz., solution annealed to form only γ phase (ST) and aged to precipitate only δ particles (DELTA), were deformed in tension till fracture in the temperatures range from 200°C–700°C. From the comparison of the evolved microstructure of deformed and undeformed specimens that have been subjected to similar thermal history, deformation induced precipitation could be identified. Deformation in the range of 550°C to 650°C promoted the precipitation of γ′ and γ″ phases in both structures. In case of DELTA alloy, the γ′ precipitation was found to precede the γ′ phase precipitation while no such preference for precipitation could be identified in ST specimens. This difference in the precipitation behaviour and the sequence of precipitation has been explained on the basis of the relative concentration of solutes in the matrix of the starting microstructures of ST and DELTA specimens.     

General corrosion of alloy 22: Experimental determination of model parameters from electrochemical impedance spectroscopy data

Models used to predict general corrosion damage of Alloy 22 high level nuclear waste (HLNW) containers must be deterministic, relying upon the time-invariant natural laws, because of long time scales involved in the predictions compared with the time over which the corrosion of this alloy has been studied. The point defect model (PDM) is one such model and requires high-accuracy experimental data to determine model parameters that will accurately predict corrosion over the long times required for HLNW disposal. Electrochemical impedance spectroscopy (EIS) and steady-state polarization data were collected in deaerated 4 M NaCl, pH 6 solutions at 30 °C and 60 °C. This work, which was carried out under a quality assurance program, emphasized data reproducibility and adherence to the steady-state condition in order to ensure confluence between theory and experiment. Thus, fresh electrolyte was continuously flowed through test cells allowing test specimens to remain undisturbed for up to 6 weeks during data collection. Parameters for the PDM have been estimated by optimizing the PDM on the impedance data, and the ability of the parameter values to account for the steady-state passive current density and barrier oxide layer thickness has been evaluated using phase space analysis. The calculated impedance gives generally good agreement with experimental data showing that the PDM is a valid approach. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees.     

The effect of potential and aging on the Pb-assisted stress corrosion cracking susceptibility of alloy 22 gas tungsten arc-welded weldments

The susceptibility of as-received, solutionized, and short-term thermally aged mill-annealed (MA) and gas tungsten arc-welded (GTAW) alloy 22 to Pb-assisted stress corrosion cracking (PbSCC) was evaluated in supersaturated, deaerated, acidic PbCl₂ solutions at 95 °C. Anodic polarization tests in acidic PbCl₂ solutions showed that 16,000 ppm of Pb produced a strong anodic peak and an order of magnitude greater passive current density for both MA and GTAW alloy 22 as compared to pure NaCl solutions. Current spikes were also observed in the anodic polarization plots for the PbCl₂ solutions, suggesting periodic events of passivity breakdown and repassivation. Constant deformation SCC tests were conducted using double U-bend samples of as-received, solutionized, and thermally aged MA and double U-groove welded alloy 22 plates. The results indicate that as-received, solutionized, and thermally aged MA and GTAW alloy 22 were resistant to PbSCC in supersaturated PbCl₂ solutions at 95 °C, pH 0.5, and applied potentials near the anodic peak ranging from −100 to 50 mV_SCE. Enhanced dissolution of alloy 22 was also observed in the crevice region of the double U-bend samples tested in the 16,000 ppm PbCl₂ solutions. This Pb concentration is seven orders of magnitude greater than that found in the anticipated repository environments, and chemical speciation modeling showed that Pb²⁺ is strongly immobilized in J-13 Yucca Mountain waters through the precipitation of PbCO₃ solids. Therefore, although enhanced dissolution of the inner U-bend did occur in our tests, the overall results from this PbSCC investigation suggest that as-fabricated, solutionized, and aged MA and GTAW alloy 22 are resistant to SCC in extremely aggressive, acidic, and supersaturated PbCl₂ solutions at 95 °C. Provided that these high Pb concentrations are not attainable in the anticipated repository environments, alloy 22 is unlikely to be susceptible to SCC, localized corrosion, and enhanced dissolution by the presence of Pb. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees.     

The effect of solution annealing on the microstructural behavior of alloy 22 welds

Multipass gas tungsten arc welds of alloy 22 were subjected to solution annealing durations of 20 minutes, 24 hours, 72 hours, and 1 week at temperatures of 1075 °C, 1121 °C, 1200 °C, and 1300 °C. The specimens were studied in cross section by secondary electron microscopy to determine the effect of solution annealing on tetrahedrally close-packed (TCP) precipitate stability. Electron backscatter diffraction mapping was also performed on all of the specimens to determine the recrystallization behavior of the welds. It was found that complete TCP precipitate dissolution occurs after solution annealing at 1075 °C and 1121 °C for 24 hours, and at 1200 °C and 1300 °C for durations of 20 minutes. Regions of most rapid recrystallization were correlated to the regions of lowest solute content, highest plastic strain, and highest residual stresses. Texture analysis indicated that while the columnar dendrites originally present in the weld grew with a 〈001〉 orientation in the transverse direction (approximately opposite the heat flow direction), the recrystallized grains adopt a 〈101〉 orientation in the transverse direction when recrystallization and TCP phase dissolution occur simultaneously.     

Effects of Ru on the high-temperature phase stability of Ni-base single-crystal superalloys

Microstructural instabilities associated with the precipitation of refractory-rich topologically-close-packed (TCP) phases within the microstructure of advanced Ni-base single-crystal superalloys were quantified in two nominally identical alloys with and without additions of Ru. Differences in the microstructural kinetics associated with the formation of TCP precipitates in these experimental single-crystal superalloys enabled the influence of Ru to be assessed. Detailed microstructural investigations were carried out on specimens subjected to prolonged isothermal exposures at elevated temperature. Even after 1000 hours at temperatures in excess of 1100 °C, the microstructure of the Ru-bearing alloy was highly resistant to the formation of TCP phases. Transmission electron micro-analysis (TEM) coupled with X-ray diffraction (XRD) was used to identify the characteristic crystal structures of the TCP precipitates in both alloys as being primarily the orthorhombic P and tetragonal σ phase. The sluggish precipitation kinetics of TCP phases in the Ru-bearing single-crystal Ni-base superalloy prevents the breakdown of the parent γ-γ′ microstructure and greatly enhances the high-temperature creep characteristics.     

Electrochemical methods to detect susceptibility of Ni-Cr-Mo-W alloy 22 to intergranular corrosion

Alloy 22 (UNS N06022), a Ni-Cr-Mo-W based alloy, is a candidate material for the outer wall of nuclear waste package (NWP) containers. Even though the alloy is highly stable at low temperatures, it could undergo microstructural changes during processing such as welding and stress relieving. Formation of topologically close-packed (TCP) phases such as μ, P, σ, etc. and Cr-rich carbides could make the material susceptible to localized corrosion. Hence, it is important to correlate the microstructural changes with the corrosion resistance of the alloy by nondestructive and rapid electrochemical tests. In this investigation, different electrochemical test solutions were used to quantify the microstructural changes associated with aging and welding of the wrought alloy 22. The results of double-loop (DL) electrochemical potentiodynamic reactivation (EPR) tests in 1 M H₂SO₄+0.5 M NaCl+0.01 M KSCN solution indicated Cr depletion during initial stages of aging of wrought alloy 22. Results of EPR tests in 2 M HCl+0.01 M KSCN solution at 60 °C correlated well with the Mo depletion that occurred near TCP phases formed during aging of both weld and wrought alloy 22 materials. The EPR test results were compared with standard chemical weight loss measurements specified by ASTM standard G-28 methods A and B.     

The evolution of microstructure in Al-2 Pct Cu thin films: Precipitation,dissolution, and reprecipitation

The precipitation, dissolution, and reprecipitation processes of Al₂Cu (θ phase) in Al-2 wt pct Cu thin films were studied. The films were characterized in the as-deposited condition, after annealing at 425 °C for 35 minutes, and after rapid thermal annealing (RTA) at 345 °C, 405 °C, and 472 °C. In the as-deposited samples, the precipitates had a fine even distribution throughout the thin film both at aluminum grain boundaries and within the aluminum grains. Annealing below the solvus temperature caused the grain boundary precipitates to grow and precipitates within the center of aluminum grains to diminish. Annealing above 425 °C caused the θ-phase precipitates to dissolve. Upon cooldown, the θ phase nucleated at aluminum grain boundaries and triple points in the form of plates.In situ heating and cooling experiments documented this process in real time. Analytical microscopy revealed that there is a depletion of copper at the aluminum grain boundaries in regions free of precipitates. The θ-phase precipitates nucleated and grew at the grain boundariesvia a collector plate mechanism and drew copper from the areas adjacent to the aluminum grain boundaries. This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

The microstructural response of mill-annealed and solution-annealed INCONEL 600 to heat treatment

Samples of INCONEL* 600 were examined in the mill-annealed and solution-annealed states, and after isothermal annealing at 400 °C and 650 °C. The corrosion behavior of the samples was examined, analytical electron microscopy was used to determine the microstructures present and the chemistry of grain boundaries, and Auger electron spectroscopy was used to measure grain boundary segregation. Samples of different alloys in the mill-annealed state were found to have quite different microstructures, with Cr-rich M₇C₃ carbides occurring either along grain boundaries or in intragranular sheets. The corrosion behavior of the samples correlated well with the occurrence of grain boundary chromium depletion. Solution annealing at 1190 °C caused dissolution of all carbides, whereas at 1100 °C the carbides either dissolved or the grain boundaries moved away from the carbides, depending upon alloy carbon content. Low-temperature annealing at 400 °C had little effect on millannealed or fully solutionized samples, but in samples with intragranular carbides present, the grain boundaries moved until intersecting or adjacent to the carbides. Isothermal annealing at 650 °C caused carbide nucleation and growth at grain boundaries in fully solutionized samples. Chromium depletion at grain boundaries accompanied carbide precipitation, with a minimum chromium level of 6 wt pct achieved after 5 hours. Healing was found to occur after 100 hours. Solution-annealed samples with intragranular carbides present had more rapid corrosion kinetics since the grain boundaries moved back to the existing carbides. Thermodynamic analysis of the chromium-depletion process showed good agreement with experimental measurements. The Auger results found only boron present at grain boundaries in the mill-annealed state. Aged samples had boron, nitrogen, and phosphorus present, with phosphorus and nitrogen segregating to the greatest extent. The kinetics of phosphorus segregation are much slower at 400 °C compared with 650 °C.     

Influence of time and temperature dependent processes on strain controlled low cycle fatigue behavior of alloy 617

Strain controlled low cycle fatigue tests have been conducted in air to ascertain the influence of strain rate(ε = 4 × 10^-6'to 4 × 10^-3 s^-1) and temperature(T = 750/850/950 °C) on LCF behavior of Alloy 617. A strain range of 0.6 pct and a symmetrical triangular wave form were employed for all the tests. Crack initiation and propagation modes were studied. Microstructural changes that occurred during fatigue deformation were evaluated and compared with the results obtained on isothermal aging. Deformation and damage mechanisms which influence the endurance have been identified. A reduction in fatigue life was observed with decreasing ε at 850 °C and with increasing temperature at ε = 4 × 10^-5 s^-1. Cyclic stress response varied as a complex function of temperature and strain rate. Fatigue deformation was found to induce cellular precipitation of carbides at 750 and 850 ‡. Dynamic strain aging characterized by serrated flow was observed at 750 °C (ε = 4 × 10^-5 s^-1) and in the tests at higher ε at 850 °C. Strengthening of the matrix due to dynamic strain aging of matrix dislocations by precipitation of M₂₃C₆ carbides led to fracture of grain boundary carbide films formed at 750 °C, producing brittle intergranular crack propagation. At 850 °C transgranular crack propagation was observed at the higher strain rates ε≥4× 10^-4 s^-1. At 850 and 950 °C even at strain rates of 4 × 10^-5 s^-1 or lower, life was not governed by intergranular creep rupture damage mechanisms under the symmetrical, continuous cycling conditions employed. Reduction of endurance at lower strain rates is caused by increased inelastic strain and intergranular crack initiation due to oxidation of surface connected grain boundaries. formerly Guest Scientist at the De-partment for Reactor Materials of the Nuclear Research Centre, Juelich (IRW/KFA),     

Effect of minor alloying element variation on the properties of alloy 800

Application of Alloy 800 in steam generator tubing of fast reactors, where continuous service temperature of the order of 550° is experienced, has been analyzed with respect to small variations in its chemical composition. Several laboratory melts of Alloy 800 have been prepared and their microstructural and mechanical property changes during simple aging and creep tests at 500 to 600° have been studied. It has been found that in the above temperature range precipitation of M₂₃C₆on the grain boundaries is independent of the Ti : C ratio generally specified for Alloy 800. Gamma prime precipitation occurred in alloys containing as low as 0.5 pct Ti+ Al after 1000 h of aging and was accompanied with a creep ductility decline. Upon γ precipitation creep rate was retarded and its reacceleration for test times up to 8500 h at 550° was not observed. Based on the findings, increased Ti concentration at the expense of Al within the specified chemical composition range with carbon content of 0.030 to 0.050 has been suggested. A. A. Tavassoli is on sabbatical leave from the Arya-Mehr University of Technology, Tehran, Iran.     

The compctition between the alpha and omega phases in aged Ti-Nb alloys

The compctition between the stable α phase and metastable ω phase to precipitate in a metastableβ phase matrix was investigated in the present study. Four binary Ti-Ni alloys with compositions between 20 and 35 at. pct Nb were air cooled to room temperature from 1000 °C and then aged at temperatures between 300 and 500 °C. For aging temperatures of 400 °C and lower it was found that the quench before aging enabled ω phase precipitates to grow to the exclusion of α precipitates. When specimens were directly aged at 400 °C only α precipitates were observed. Precipitates which could not be identified using SAD were observed in specimens of the 30 and 35 at. pct Nb alloys. All precipitation reactions became more sluggish as the niobium content of an alloy was increased. The results reported here form the second part of a study of the stable and metastable equilibria of the Ti-Nb alloy system. Formerly a Graduate Student in the Materials Science Program at the University of Wisconsin-Madison     

The kinetics of carbide precipitation in silicon-aluminum steels

The kinetics of carbide precipitation in a fully processed 2.3 wt Pct silicon, 0.66 wt Pct aluminum electrical steel with carbon contents of 0.005 to 0.016 wt Pct were investigated over the temperature range from 150 to 760 °C and times from 30 seconds to 240 hours. The size, morphology, and distribution of the carbide phases, as functions of aging time and temperature, were determined by optical and transmission electron microscopy. The 1.5T core loss was also evaluated and correlated with the changes in precipitation. Distinct C curves were observed for the formation of grain-boundary cementite at temperatures above 350 °C and a transition carbide ({100} _α habit plane) at temperatures below 350 °C. Grain-boundary cementite had a relatively small effect on core loss. The large increases in core loss that accompanied transition carbide precipitation peaked at specific aging temperatures depending on the carbon content of the steel. Once a transition carbide dispersion was initially established at a given aging temperature, particle coarsening and core loss changes were generally insensitive to aging time. The influence of a combined addition of silicon and aluminum on the solubility of cementite and the transition carbide in iron was estimated and discussed. This paper is based on a presentation made at the symposium “Physical Metallurgy of Electrical Steels” held at the 1985 annual AIME meeting in New York on February 24–28, 1985, under the auspices of the TMS Ferrous Metallurgy Committee.     

Case reviews on the effect of microstructure on the corrosion behavior of austenitic alloys for processing and storage of nuclear waste

This article describes the corrosion behavior of special austenitic alloys for waste management applications. The special stainless steels have controlled levels of alloying and impurity elements and inclusion levels. It is shown that “active” inclusions and segregation of chromium along flow lines accelerated IGC of nonsensitized stainless steels. Concentration of Cr⁺⁶ ions in the grooves of dissolved inclusions increased the potential to the transpassive region of the material, leading to accelerated attack. It is shown that a combination of cold working and controlled solution annealing resulted in a microstructure that resisted corrosion even after a sensitization heat treatment. This imparted extra resistance to corrosion by increasing the fraction of “random” grain boundaries above a threshold value. Randomization of grain boundaries made the stainless steels resistant to sensitization, IGC, and intergranular stress corrosion cracking (IGSCC) in even hot chloride environments. The increased corrosion resistance has been attributed to connectivity of random grain boundaries. The reaction mechanism between the molten glass and the material for process pot, alloy 690, during the vitrification process has been shown to result in depletion of chromium from the reacting surfaces. A comparison is drawn between the electrochemical behavior of alloys 33 and 22 in 1 M HCl at 65 °C. It is shown that a secondary phase formed during welding of alloy 33 impaired corrosion properties in the HCl environment. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees.     

Microstructure-mechanical property relationship to copper alloys with shape memory during thermomechanical treatments

The phase transformations during fabrication and aging after cold deformation in three polychrystalline copper alloys of the Cu-Al-Ni system with shape memory effect (SME) were characterized. Some phase transformations were identified with clear repercussion in their mechanical properties during thermomechanical treatments. Around 430 °C, mutual effects of β-phase recrystallization and precipitation of γ₂ and NiAl phases were observed. Close to 600 °C the dissolution of phase α was observed, beginning transformation into β phase process. Brittle phases such as γ₂ and NiAl began to precipitate during a short exposure time at 380 °C, 585 °C, 600 °C, and 700 °C temperatures. The phase transformations were intensified due to the plastic deformation that acted as a driving force for the diffusion processes. The introduction of chemical elements inhibited the grain growth and increased the structural disorder generating an elevation in the hardness property.     

Correlation of structure and superconducting properties in Nb(Cb)-Ti quaternary alloys

The aging behavior of a Nb-Ti alloy containing 60 wt pct Ti and small additions of oxygen and erbium or scandium was characterized and related to superconducting properties. The ternary and quaternary alloys were cold reduced and aged for various times at temperatures between 250° and 1000°C. ω and α phase transformations and oxide precipitation processes were followed by lattice parameter, diffraction intensity, resistivity, and metallographic studies, and correlated with superconducting critical magnetic field and critical current density measurements. The optimum 1 hr aging temperatures for producing ω and α phase precipitation were found to be 400° and 500°C, respectively. Aging at 1000°C produced only oxide precipitation. It was found that oxygen, erbium, and scandium stabilize the α phase but have little effect on ω precipitation. The ω phase proved the most effective fluxoid pinning precipitate. The fine scale dispersoid provided an extremely high number density of pinning sites. Formerly Research Assistant, Department of Metallurgy and Materials Science, M.I.T., Cambridge, Mass. Formerly Research Assistant, Department of Metallurgy and Materials Science, M.I.T. Formerly Research Associate, Department of Metallurgy and Materials Science, M.I.T.     

A model to predict carburization profiles in high temperature alloys

A mathematical model has been developed which, by means of finite difference computation techniques, permits the prediction of carbon concentration profiles in carburized high temperature alloys. It is assumed that a proportion of the carbon which diffuses into the alloy reacts with elements such as chromium to form carbide precipitates. The amount of carbon remaining in solution is determined from the solubility product of the carbide. Only this carbon in solution is able to diffuse through the alloy matrix, and thus the carbide precipitation process reduces the rate of carburization. Applying the model, the diffusion coefficient of carbon in Alloy 800 H at 900 °C has been determined as (3.3 ± 0.5) × 10⁻⁸ cm²/s. The model can also treat the carburization of an alloy containing two carbide-forming elements, but application to alloys containing both chromium and niobium (columbium) was successful only to a limited extent, probably as a result of the slow, complex kinetics of carbide precipitation. The model can be used to adapt carbon concentration profiles from one geometrical configuration to another. On the basis of profiles determined experimentally on small, cylindrical test specimens, carbon concenration profiles have been predicted for thick section tubes of Alloy 800 H exposed to a carburizing environment for up to 100,000 h. Formerly of the Institute of Reactor Materials, Nuclear Research Centre (KFA), Jülich