Is Inconel 625 a superalloy?
Inconel 625 (Alloy 625) is a wrought nickel-based superalloy strengthened primarily by the addition of carbon, chromium, molybdenum and niobium. Developed specifically for use at temperatures below 973 K, the alloy combines the high strength of age-hardenable nickel-based alloys with excellent manufacturing properties. In addition to its widespread use in the aviation, aerospace, shipbuilding, chemical and petrochemical industries, it is used in reactor core and control rod components in pressurized water reactors and in heat exchanger tubes in ammonia cracking units for heavy water production.
Excellent corrosion behavior in cracked ammonia environments and excellent creep resistance are the main features that make this alloy play an important role in heavy water plants. Although the alloy was originally designed as a solution-hardenable alloy, it was observed that precipitation of intermetallic phases and carbides occurred when the alloy was aged in the 823-1023 K range. The precipitation hardening of this alloy at high temperatures (823–923 K) mainly originates from the metastable phase γ" [Ni3(Nb,Al,Ti)] with an ordered body-centered tetragonal DO22 structure. After long-term aging, the metastable phase The γ′′ phase transforms into the orthorhombic δ phase [Ni3(Nb,Mo)]. It has been reported that upon aging above 1023 K, the δ phase forms directly from supersaturated solid solution. M23C6, M6C and MC carbides will Precipitates in the range of 1033–1253 K. It has been reported that primary MC carbides existing in an undissolved state during solution annealing decompose into M23C6 and M6C upon prolonged exposure to high temperatures.
The impact of the above transformations on mechanical properties has not been studied in detail, although there have been few previous attempts to characterize the microstructure that evolves upon long-term exposure. Recent studies have shown the precipitation of the Ni2(Cr,Mo) phase in alloy 625, which has a Pt2 Mo-type structure by long aging at temperatures below 873 K. The presence of this orthorhombic phase is believed to contribute to the low tensile and creep ductility and toughness of superalloys. In this article we describe the microstructural characteristics observed for Alloy 625 after different heat treatments and the room temperature tensile properties determined after these treatments. The heat treatment conditions examined included: (i) operating at 873 K for approximately 60 000 h, (ii) aging of the alloy for different durations at temperatures above the operating temperature, (iii) resolution annealing and aging at different temperatures.
