Molybdenum carbide precipitates

All of the alloys listed in Tables 1 and 2 may become sensitized that is, form various precipitates at grain boundaries when exposed to certain temperatures and thereby become subject to intergranular attack. Chromium-rich carbides are the most common precipitates in the alloys of Tables 1 and 2, except in the Ni-Cr-Mo alloys in which molybdenum carbide is formed. All of the evaluation tests discussed below detect susceptibility to intergranular attack associated with chromium and molybdenum carbide precipitates. 

Microstmcture of Rodent after casting is composed of large grains/den-drites of the austenite matrix, which is the solid solution of nickel strengthened chromium and molybdenum. The matrix is additionally strengthened by dispersion of molybdenum silicide precipitates. A small number of carbide and aluminum oxide particles were also observed. The presence of eutectics y-P rather weakens the structure of the alloy. 

Hof Hoffineister, H., Microsegregations and Eutectic Carbide Precipitations in Iron-Carbon-Molybdenum Alloys (in German), Arch. Eisenhuettenwes., 43(9), 689-692 (1972) (ExperimentaL Phase Relations, 9)... 

Sonication of a solution containing metal precursors can also generate precipitation. For instance, the sonication of a solution of molybdenum hexacarbonyl (Mo(CO)g)) leads to the precipitation of a molybdenum oxide under air (40 2) and of molybdenum carbide under argon atmosphere (41). In the presence of a silica support, sonication of Mo(CO)g solution in decalin leads to molybdenum carbide (Mo2C)-silica material under argon atmosphere and to molybdenum blue oxide (M02O5.2H2O)-silica under... 

Remove the Nitric Acid Test from ASTM A 262 and establish it as a separate ASTM Test At present, the boiling 65 % nitric acid test (Practice C) is specified for materials to be used in nitric acid service. Only this test is sensitive to sigma-phase in molybdenum-bearing austenitic stainless steels. Also, problems such as end-grain corrosion associated with hexavalent chromium derived from corrosion products are unique to this solution. While this test also detects susceptibility to intergranular attack associated with chromium carbide precipitates, there are other tests that perform this function in less time and with greater simplicity. 

One of the most important attributes of nickel with respect to the formation of corrosion-resistant alloys is its metallurgical compatibility with a number of other metals, such as copper, chromium, molybdenum, and iron. A survey of the binary phase diagrams for nickel and these other elements shows considerable solid solubility, and thus one can make alloys with a wide variety of composition. Nickel alloys are, in general, all austenitic alloys however, they can be subject to precipitation of intermetallic and carbide phases when aged. In some alloys designed for high-temperature service, intermetallic and carbide precipitation reactions are encouraged to improve properties. However, for corrosion applications, the precipitation of second phases usually promotes corrosion attack. The problem is rarely encountered because the alloys are supplied in the annealed condition and the service temperatures rarely approach the level required for sensitization. 

Although columbium (niobium) stabilized alloy G from formation of chromium-rich carbides in the heat-affected zones of the welds, secondary carbide precipitation still occurred when the primary columbium carbides dissolved at high temperatures, and the increased carbon in the matrix increases the tendency of the alloy to precipitate intermetallic phases. Alloy G-3 has lower carbon (0.015% maximum vs. 0.05% maximum for alloy G) and lower columbium (0.3% maximum vs. 2% for alloy G). The alloy also possesses slightly higher molybdenum (7% vs. 5% for alloy G). 

Nickel-based alloys can also be subjected to carbide precipitation and precipitation of intermetallic phases when exposed to temperatures lower than their annealing temperatures. As with austenitic stainless steels, low-carbon-content alloys are recommended to delay precipitation of carbides. In some alloys, such as alloy 625, niobium, tantalum, or titanium is added to stabilize the alloy against precipitation of chromium or molybdenum carbides. Those elements combine with carbon instead of the chromium or molybdenum. 

Some metastable carbides such as MeC were identified, but their occurrence was only confirmed for materials annealed for very long periods ( 60 h). The identification of hyperfine molybdenum carbide-nitrite precipitation (< 0.5 nm) was performed using the APFIM technique. 

For example, sensitization due to carbide precipitation in the HAZ is a potential problem in both classes of alloys. However, in the case of nickel-based alloys, the high content of such alloying elements as chromium, molybdenum, tungsten, and niobium can result in the precipitation of other intermetallic phases, such as //, cr, and r. ... 

Represented by HasteUoy AUoys B and B-2, nickel-molybdenum aUoys have been primarUy used for their resistance to corrosion in non-oxidizing environments such as HCl. HasteUoy AUoy B has been used since about 1929 and has suffered from one significant limitation weld decay. The welded structure has shown high susceptibility to knifeline attack adjacent to the weld metal and to HAZ attack at some distance from the weld. The former has been attributed to the precipitation of molybdenum carbide (MojC) the latter, to the formation of MgC-type carhides. This necessitated postweld annealing, a serious shortcoming when large structures are involved. Many approaches to this problem... 

Both the intermetaUic phases and the carbides are rich in molybdenum, tungsten, and chromium and therefore create adjacent areas of alloy depletion that can be selectively attacked. Carbide precipitation can be retarded considerably by lowering carbon and silicon this is the principle behind HasteUoy Alloy C-276. 

AISI 321 and 347 are stainless steels that contain titanium and niobium iu order to stabilize the carbides (qv). These metals prevent iatergranular precipitation of carbides during service above 480°C, which can otherwise render the stainless steels susceptible to iatergranular corrosion. Grades such as AISI 316 and 317 contain 2—4% of molybdenum, which iacreases their creep—mpture strength appreciably. In the AISI 200 series, chromium—manganese austenitic stainless steels the nickel content is reduced iu comparison to the AISI 300 series. 

Another type of nickel alloy with which problems of intergranular corrosion may be encountered is that based on Ni-Cr-Mo containing about 15% Cr and 15% Mo. In this type of alloy the nature of the grain boundary precipitation responsible for the phenomenon is more complex than in Ni-Cr-Fe alloys, and the precipitates that may form during unfavourable heat treatment are not confined to carbides but include at least one inter-metallic phase in addition. The phenomenon has been extensively studied in recent years . The grain boundary precipitates responsible are molybdenum-rich M C carbide and non-stoichiometric intermetallic ix... 

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