800 Alloy

Alloy name: 800
Diagram No.: 754
Type of diagram: TTS
Chemical composition in weight %: 30.0-35.0% Ni, 19.0-23.0% Cr, min. 39.5% Fe, max. 0.10% Cr, 0.15-0.60% Al, 0.15-0.60% Ti, Al+Ti=0.30-1.20%
Alloy group: Nickel-based alloys
Note: Time-temperature sensitization diagrams for various heats of alloy 800. The relative change of the magnetic susceptibility is reported below each point. The dotted lines define the conditions for chromium depletion.
Time-temperature sensitization diagrams for two alloys that form M23C6 carbides are shown in Fig. Although sensitization diagrams indicate the kinetics of the effect of carbide precipitation on intergranular corrosion and not the kinetics of carbide precipitation (as do transformation diagrams), it can be seen that alloy 825, which contains 3% Mo, exhibits sensitization at higher temperatures than does alloy 800, which contains no molybdenum. The refractory-metal carbides take on the The refractory-metal carbides take on the form of MC, M6C, or M12C, depending on alloy composition. In the MC-type carbides, M is a mixture of niobium, titanium, and tantalum, but may also contain some molybdenum or tungsten. If nitrogen is present in the alloy, it may substitute for some of the carbon, and the resulting carbonitrides may appear yellow or orange when examined metallographically, in contrast to the grayish color of the carbides. The MC-type carbides are extremely stable and generally do not break down. The M6C-type carbides, also known as ç-carbides, are formed by molybdenum and tungsten. However, major substitutional alloying elements, such as nickel, iron, and chromium, also are present in these carbides. Some typical compositions include Mo6C, (Ni,Co)3Mo3C6, and (Mo,Ni,Cr,W)6C. Typically, these carbides dissolve at higher temperatures than the M23C6-type carbides. It must be noted that since these carbides are not usually rich in chromium, the depletion zones associated with them are molybdenum or tungsten depletion zones. “Sensitization” in these alloys means enhanced intergranular corrosion as a result of depletion of molybdenum or tungsten. Other types of refractory carbides are Mo12C and Mo2C, which may form in a nickel-molybdenum system depending on the carbon and molybdenum contents.
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