Ni-Cr-Mo alloy

Ni-Cr-Mo type corrosion-resistant alloy, which is resistant to both reducing medium and oxidizing medium, is a kind of corrosion-resistant alloy system with wide applicability. Among the most representative and widely used alloys of this type are Hastelloy® c with 16 070 Mo, introduced in the 1930s, and Inconel 625 with 9% Mo, introduced in the 1960s. The latter is insensitive to intergranular corrosion because it contains a large amount of cerium. The former, due to its poor thermal stability, will exhibit severe intergranular corrosion when exposed to sensitization or welding. The test results confirm that Hastelloyc alloy has two temperature intervals sensitive to intergranular corrosion, ie, two sensitization zones. The low temperature sensitization zone is 700-800°C, the most sensitive temperature is 760°C, the high temperature sensitization zone is 850-1100°C, and the most sensitive temperature is 871°C. The two sensitized zones correspond to precipitation temperatures of the M6C-based carbides (including M2c, M23c6) and the metal phases containing the u-phase (q, p-phase), respectively. These precipitation phases are not molybdenum-rich and Tungsten is chromium-rich, and some of them are all very high in content. They precipitate along the grain boundary and cause the depletion of Cr, Mo, and W that determine the corrosion resistance of the surrounding alloy. In some corrosive environments, the depleted areas are preferentially corroded. The intergranular corrosion is generated. In addition, the selective dissolution of the Mo-rich carbide and the Mo-rich intermetallic phase in some strong oxidizing environments is also another cause of the intergranular corrosion of such alloys.
After confirming the mechanism of intergranular corrosion of high-molybdenum Ni-Cr-Mo alloys, technical measures were taken to reduce carbon and reduce silicon, and a 2nd generation containing 16% improved in resistance to intergranular corrosion of Hastelloy C alloy was born. Mo’s Ni-Cr-Mo corrosion resistant alloy, because this alloy only controls the carbon and silicon of Hastelloy-C alloy, is effective for intergranular corrosion caused by precipitation of carbides, but the crystals caused by precipitation of intermetallic phases The interstitial corrosion has not yet been solved. To this end, the Hastelloy C-4 alloy, which is a third-generation alloy that reduces carbon, silicon, iron, and tungsten in the alloy, was introduced. The problem of intergranular corrosion of the Hastelloy C alloy has been solved so far. The two-phase precipitation behavior is the relationship between temperature, time and precipitation phase. Hastelloy C-4 alloy has the best thermal stability. As the C-4 alloy removes the tungsten in the original alloy, its resistance to pitting and crevice corrosion decreases.