Pre-existing Carbides Research Articles

Influence of carbon content on static recrystallization behavior of 50% cold-rolled Fe-18Mn-9Cr-2Al-xC steels

Recrystallization behavior during annealing at various temperatures was investigated for 50% cold-rolled Fe-18Mn-9Cr-2Al-xC steels with C contents varying between 0 and 0.5 wt%. Nucleation of recrystallization occurred near randomly in the carbon-free steel, whereas increasing the carbon contents promoted preferential nucleation at carbides formed along the grain boundaries through the particle stimulated nucleation mechanism. New grains in the carbon-free steel grow freely at the expense of deformed grains until the end of recrystallization during which migration of recrystallization front was blocked by the elongated δ-ferrites. For the carbon-added steels, on the other hand, (Cr,Mn)23C6 carbides and FeAl intermetallics formed during annealing and pre-existing carbides at grain boundaries pinned the recrystallization front, and thereby suppressing the growth of new grains and developing a distinct necklace structure. Recrystallization of the carbon-added steels completed only after carbides were dissolved at higher temperatures around 900 °C. It is noteworthy that it is not the carbon atom itself in solid solution but carbides that promotes nucleation of recrystallization and subsequently suppressed growth of new grains. It was also found that tensile properties of the cold-rolled steels restored nearly to those prior to cold rolling, i.e., in as-annealed state. It is concluded that recrystallization kinetics, special distribution of new grains, and texture of the cold-rolled Fe-18Mn-9Cr-2Al-xC steels are significantly affected by the carbon contents. In addition, restoration of tensile properties of cold-rolled steels by recrystallization suggests that many repetitive thermomechanical processing are applicable to the steels for forming into a desire shape.

Investigating Cr dealloying and Li ingress in Ni-Mo-Cr alloys with different Mo/Cr ratio exposed to FLiNaK salt

Ni-Mo-Cr alloys (Hastelloy N and Hastelloy X) having different Mo/Cr ratio were exposed to FLiNaK salt at 973 K for 72 h in an in-house developed static corrosion test facility. Though both the alloys showed non-uniform attack and selective Cr dealloying, Hastelloy X showed enhanced localized corrosion owing to lower Mo/Cr ratio and pre-existing carbides. The carbides act as cathode in the alloy/precipitate micro-galvanic couple thereby accelerating the corrosion in the adjacent alloy surface. Furthermore, Li ingress into these alloys during corrosion was observed for the first time and in case of Hastelloy X, Li was found to form Li2CrO4.

Effect of pre-existing carbides prepared by different heat treatments on the nitriding behaviour during a carburizing and nitriding duplex treatment of an M50NiL steel

This article reports the effect of pre-existing carbides prepared by different heat treatments on the nitriding behaviour during a carburizing and nitriding duplex treatment of an M50NiL steel. The goal was to achieve an improved understanding of the relationship between the pre-existing carbides and nitriding behaviour. The M50NiL steel was carburized at 950 °C, heat treated at different quenching temperatures (ranging from 1000 to 1150 °C) and tempering temperatures (ranging from 500 to 580 °C), and finally nitrided at 500 °C. The surface layers were characterized by means of X-ray diffraction (XRD), laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM), Vickers microhardness measurement and transmission electron microscopy (TEM). The results show that the formation of intergranular precipitates was greatly related to the presence of pre-existing carbides, which led to a change in the carbon content in the matrix and showed different dissolving capacities. The compound layers consisted of γ′ (Fe4N) and ε (Fe2-3N) phases as well as a minor amount of Fe3O4. The pre-existing carbides had no effect on the composition of the compound layer. For the diffusion of nitrogen, a density of pre-existing carbides that was too high or too low was unfavorable, as it greatly decreased the thickness of the nitrided layer. At the quenching temperature of 1100 °C and the tempering temperature of 540 °C, the thickest nitrided layer herein and a good hardness distribution were obtained, which was attributed to the optimum density of pre-existing carbides. This study provides information regarding how to obtain a thick nitrided layer by controlling pre-existing carbides during the duplex treatment of M50NiL steels.

Nitriding behavior and mechanical properties of carburizing and nitriding duplex treated M50NiL steel

In this study, carburizing and nitriding duplex treatments were carried out for 10 h at various nitriding temperatures in the range of 460–540 °C on M50NiL steel. The results show that an increase in nitriding temperature greatly increases the thickness of the nitrided layer, but a further increase in the nitriding temperature to 540 °C does not obtain a deeper nitrided layer. Moreover, an increase in nitriding temperature promotes the dissolution of grainy carbides and leads to a precipitation of intergranular precipitates. The pre-existing carbides are easier to be transformed into M(N,C) at a low nitriding temperature. The compound layer is a mixed phase of γ′ (Fe4N) and ε (Fe23N). With increasing the nitriding temperature, the magnitude of the residual compressive stresses decreases, the surface roughness increases, and the proportion of γ′ phase increases. The results of the wear tests carried out at different loads demonstrate that the wear mechanism controlling the wear rate transfers from oxidative wear to abrasive wear when the load increases. The sample nitrided at 500 °C shows the best wear resistance to abrasive wear under a high load due to its thicker compound layer, high ε proportion and high surface residual compressive stress.

Decomposition modes of austenite in 9Cr–W–V–Ta reduced activation ferritic–martensitic steels

The present paper presents the results of an extensive electron microscopy investigation on the decomposition modes of high temperature austenite in 9Cr–W–V–Ta reduced activation ferritic–martensitic steels. Although the displacive martensitic transformation is predominant on austenitisation, low volume fraction of Fe rich M3C or M23C6 precipitates formed, when the tungsten content exceeded 1 wt-%. The compositional inhomogeneity introduced in the austenite by the nature, chemistry and kinetics of dissolution of the pre-existing carbides is dependent on the steel composition and austenitisation conditions. The extent of repartitioning of tungsten between M23C6 and ferrite largely influences the kinetics of austenite and martensite transformation, for the same austenitisation conditions. Supporting evidence from calorimetry analysis is also presented.

Effect of the pre-existing carbides on the grain boundary network during grain boundary engineering in a nickel based alloy

Grain boundary engineering was carried out on an aging-treated nickel based Alloy 690, which has precipitated carbides at grain boundaries. Electron backscatter diffraction technique was used to investigate the grain boundary networks. Results show that, compared with the solution-annealed samples, the aging-treated samples with pre-existing carbides at grain boundaries need longer duration or higher temperature during annealing after low-strain tensile deformation for forming high proportion of low-Σ coincidence site lattice grain boundaries (more than 75%). The reason is that the primary recrystallization is inhibited or retarded owing to that the pre-existing carbides are barriers to grain boundaries migration.

Effect of Solution Annealing on Susceptibility to Intercrystalline Corrosion of Stainless Steel with 20% Cr and 8% Ni

In general, as-received (AR) austenitic stainless steels (ASSs) contain complex carbide precipitates due to manufacturing operations, subsequent annealing treatment, or due to the fabrication processes such as welding. The presence of pre-existing carbides leads to cumulative sensitization and make the steel susceptible to intercrystalline corrosion (ICC)/intergranular corrosion (IGC) which causes premature failure during service. Solution annealing (SA) is one of the ways to deal with such situations. In this present investigation, the AR (hot rolled and mill annealed) chromium-nickel (Cr-Ni) ASS is compared with SA Cr-Ni ASS. The extent of ICC/IGC was evaluated qualitatively and quantitatively by various electrochemical tests including ASTM standard A-262 Practice A and Practice E, double loop electrochemical potentiokinetic reactivation and electrochemical impedance spectroscopy. The degree of sensitization for hot rolled mill annealed AR condition is found to be substantially higher (51.55%) than that of SA condition (26.9%) for thermally aged samples (at 700 °C). The chemical composition across the grain boundary was measured using electron probe micro-analyzer for both (AR and SA) conditions and confirms that the pre-sensitization effect was completely removed after SA treatment.

Corrosion Issues of High Temperature Reactor Structural Metallic Materials

Cooling helium of high temperature reactors (HTRs) is expected to contain a low level of impurities: oxidizing gases and carbon-bearing species. Reference structural materials for pipes and heat exchangers are chromia former nickel base alloys, typically alloys 617 and 230. And as is generally the case in any high temperature process, their long term corrosion resistance relies on the growth of a surface chromium oxide that can act as a barrier against corrosive species. This implies that the HTR environment must allow for oxidation of these alloys to occur, while it remains not too oxidizing against in-core graphite. First, studies on the surface reactivity under various impure helium containing low partial pressures of H2, H2O, CO, and CH4 show that alloys 617 and 230 oxidize in many atmosphere at intermediate temperatures (up to 890–970°C, depending on the exact gas composition). However when heated above a critical temperature, the surface oxide becomes unstable. It was demonstrated that at the scale/alloy interface, the surface oxide interacts with the carbon from the material. These investigations have established an environmental area that promotes oxidation. When exposed in oxidizing HTR helium, alloys 617 and 230 actually develop a sustainable surface scale over thousands of hours. On the other hand, if the scale is destabilized by reaction with the carbon, the oxide is not protective anymore, and the alloy surface interacts with gaseous impurities. In the case of CH4-containg atmospheres, this causes rapid carburization in the form of precipitation of coarse carbides on the surface and in the bulk. Carburization was shown to induce an extensive embrittlement of the alloys. In CH4-free helium mixtures, alloys decarburize with a global loss of carbon and dissolution of the pre-existing carbides. As carbides take part in the alloy strengthening at high temperature, it is expected that decarburization impacts the creep properties. Carburization and decarburization degrade rapidly the alloy properties, and thus result in an unacceptably high risk on the material integrity at high temperature. Therefore, the purification system shall control the gas composition in order to make this unique helium atmosphere compatible with the in-core graphite, as well as with structural materials. This paper reviews the data on the corrosion behavior of structural materials in HTRs and draws some conclusions on the appropriate helium chemistry regarding the material compatibility at high temperature.

Study of Carbide Transformations during High-Temperature Oxidation of Nickel-Base Superalloys

Microstructure phenomena resulting from high-temperature oxidation of three nickel-base superalloys were studied by microstructure examinations. Disappearance, nature modification, volume fraction evolution or precipitation of carbides were observed in the alloys near the external surface, depending on the temperature and the chemical composition of the alloys. Thermodynamic calculations allowed to better know what happened to carbon and to quantify its new distribution. The alloys studied lost a more or less great part of their sub-surface carbon content at 1200°C while carbon seemingly diffused deeper in the alloy at 1100°C and 1000°C. The latter part of carbon promoted the coarsening of the pre-existing carbides, some modifications of their natures or the precipitation of new carbides in the matrix, then the occurrence of a new-carbides zone.

PHASE TRANSFORMATION IN Fe-Mo-C AND Fe-W-C STEELS—I. THE STRUCTURAL EVOLUTION DURING TEMPERING AT 700°C

Mechanism and kinetics of carbide transformation during tempering at 700°C have been studied in Fe-Mo-C and Fe-W-C steels (with up to 2.5% W or Mo) by transmission electron microscopy (TEM) and X-ray diffraction. The sequence of carbide formation is Fe3C→Mo2C→(Fe2MoC, M23C6) in molybdenum steels and Fe3C→M6C→M23C6 in tungsten steels. Increasing the alloying element level increases the rate of carbide replacement reaction. In Fe-Mo-C steels the Fe2MoC carbides nucleate preferentially at the Fe3C-α interface and grow into cementite, whereas in Fe-W-C steels the M6C carbides usually precipitate inside cementite giving rise to the in situ transformation Fe3C→M6C. The software DICTRA and THERMO-CALC were used to simulate cementite growth and to show the possibility of the in situ transformation. The M23C6 carbide is first confined to prior austenite grain boundaries and penetrates into the grains with increasing tempering time. During growth the M23C6 carbide absorbs surrounding pre-existing carbides. As a result, after tempering for 500 h, patches of two-phase areas with (M23C6 + α) or (M6C + α) are observed in tungsten steels, and patches of (M23C6 + α) or (Fe2MoC + α) in molybdenum steels. The alloying element partitioning between α and precipitated carbides was determined using TEM-EDS. It was established that the M23C6 carbide is stable at 700°C in both investigated steels. The Fe2MoC carbide is stable in the Fe-Mo-C system at this temperature. The MC carbide was not observed even after tempering for 3000 h. © 1997 Acta Metallurgica Inc.

Characterization of medium- and low-temperature carbides in a low-carbon steel by internal friction

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Mechanical properties of vanadium-carbon alloys

The effects of carbon on the mechanical properties of 99.95% vanadium were determined. A solution heat-treatment at 1200°C followed by quenching and aging at 350°C increased the ultimate tensile strength of a vanadium-0.18 at.% carbon alloy from 30 to 75 × 10 3 p.s.i. Vanadium containing higher carbon levels could not be appreciably strengthened due to the presence of a pre-existing carbide phase which effectively precipitated the highly mobile carbon atoms from solid solution during quenching.