Extraction technique and evolution of carbides in serviced T91 steel
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Carbide particles in T91 steel after service for different time were obtained by electrolytic extraction method and their phase,amount,morphology and composition were analyzed by SEM,XRD and EDS.The results show that the carbides in T91 steel exist mainly in two forms: M23C6,which is the majority,and MX.With increasing service time,the M23C6 carbides coarsen gradually along grain-boundary,while the MX carbides are relatively stable.No significant increase of size and amount of MC carbides are observed in the T91 steel even after service for 105296 hours.Morphologically,the initial rod-like carbides are spheroidized gradually.Mo,Cr,V in solid solution diffuse into M23C6,which leads to the increase of Mo,Cr,V and the decrease of Fe in the carbides as service time increases.Keywords:
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Using optical microscope, SEM and image analyzing software, the microstructures and precipitation carbides morphology of GCr15 steel under different heat treatment conditions were studied. The results show that GCr15 steel consists of pearlite and carbides, and the lamellar spacing of pearlite is around 0.143 μm without net and bands shaped carbides precipitated out. After annealing with process a, the carbides in the steel distribute uniformly; with process b, the numbers of precipitating carbides in the steel reduce with the poorest roundness but biggest size; with process c, a large amount of carbides precipitate out in the steel with big size and good roundness. Compared with above three annealing processes, the process a has the shortest time consuming, therefore it is the best annealing method.
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The structure and hardness of SCrWMoV steel with multiple types of ultra fine carbides are studied after annealing, quenching and tempering in this paper. The results show that multiple types of carbides M3C, M7C3, M23C6, M6C and MC were observed in the annealed steel. Nucleation and coalescence of new carbides, partial dissolution of original carbides in phase region during annealing at 800-840 °C, result in ultra-fine carbides. Average size of the carbides is 0.33~0.34(im in the steel annealed at 800-840°C. Because M3C and M23C6 dissolve easily in austenite, the high hardness HRC63-65 can be obtained by quenching at 840-860 °C. Un-dissolved carbides M6C and MC (VC) can effectively prevent the coarsening of austenitic grain, and conduce to obtain very fine martensite. The retained austenite can be easy to decompose during tempering at low and middle temperature due to the precipitation of multiple types of carbides and the good tempering-resistance of the steel is obtained. The microstructure and property of the steel after heat treatment can be accurately explained by calculating based on phase equilibrium thermodynamic.
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The carbides piled-up in M2 high-speed steel were investigated by means of SEM and the type of carbides in the pile-up was analyzed by EDS,the effects of as-cast structure,heat-compression process and different annealing processes on carbide pile-up were studied.The results show that the carbides in the pile-up are M6C,and the main causes for the piling up of carbides are the improper choices of the cooling speed in continuous annealing and the isothermal temperature and time in isothermal annealing.The carbides piled-up in the steel can be eliminated by re-annealing at lower temperature.
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Low alloy Cr–Mo–V steels are often used in steam power generation units. The evolution of carbide often leads to embrittlement during elongated service. Therefore, the determination of carbide evolution mechanism during long time service is important to understand and prevent premature failures such as temper embrittlement. In this study, low alloy Cr–Mo–V steels used as main steam pipes in a thermal power plant were studied after various service times as well as in the as fabricated condition. Electron microscopic analyses were carried out on extraction replicas to observe and analyse the morphology and composition of the carbides. Predominant plate-like vanadium rich carbides were observed in the as fabricated condition. When exposed to high temperature as a result of the use, the V rich carbides transformed to Mo rich carbides, which have a typical H type morphology. The change in morphology and composition of the carbide is mainly due to the gradual depletion of Mo from the solid solution, and a non-destructive carbide extraction method was established for the examination of the precipitates in the working turbine rotor.
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In this paper, the precipitation of carbide and wear loss of high-carbon 8 mass% Cr tool steel at two tempering conditions (i.e., 773⁻803 K and 823⁻853 K) were studied by INCA Steel, EPMA-1720H, XRD, and ML-10 tester. The results show that the particles of test steels include the carbides (Cr₇C₃ and Cr23C₆) and carbides nucleated on Al₂O₃. When carbides are of the same size, the number of carbides in test steel at a tempering temperature of 773⁻803 K is greater than that at a tempering temperature of 823⁻853 K, especially when the size of carbides is less than 5 μm. Compared with the test steel tempered at 823⁻853 K, the distance between adjacent actual particles reduced by 80.6 μm and the maximum amount of reduction was 9.4% for single wear loss at the tempering temperature of 773⁻803 K. It can be concluded from thermodynamics results that Al₂O₃ inclusions began to precipitate in liquid, and the precipitation of carbides was at the solid⁻liquid region. Al₂O₃ can be used as the nucleation interface of carbide, thus promoting the formation of carbides. During the cooling of molten steel, a lower temperature can increase the difference of actual solubility product bigger than equilibrium solubility product, thus promoting the carbide formation.
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The properties of carbides, such as morphology, size, and type, in H13 hot work die steel were studied with optical microscopy, transmission electron microscopy, electron diffraction, and energy dispersive X-ray analysis; their size distribution and quantity after tempering, at different positions within the ingot, were analyzed using Image-Pro Plus software. Thermodynamic calculations were also performed for these carbides. The microstructures near the ingot surface were homogeneous and had slender martensite laths. Two kinds of carbide precipitates have been detected in H13: (1) MC and M6C, generally smaller than 200 nm; and (2) M23C6, usually larger than 200 nm. MC and M6C play the key role in precipitation hardening. These are the most frequent carbides precipitating at the halfway point from the center of the ingot, and the least frequent at the surface. From the center of the ingot to its surface, the size and volume fraction of the carbides decrease, and the toughness improves, while the contribution of the carbides to the yield strength increases.
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