Alloy 925 is a nickel based alloy containing Iron and Chromium as major additions, and also additions of Molybdenum, Copper, Titanium and Aluminium. A combination of high strength and corrosion resistance make it suitable in applications such as valves, hangers, packers, and tubulars, in sour gas environments. It can also be used for fasteners, and for pump shafts in marine environments.
The combination of Nickel and Chromium renders protection in both oxidizing and reducing atmospheres. The high Nickel content also gives resistance to chloride ion stress-corrosion cracking. Pitting and crevice corrosion resistance is also excellent. The alloy is produced by a vacuum remelt process, but can be supplied using an air melt single consumable electrode (VAR or ESR) practice if the material specification allows.
Metallurgy INCOLOY alloy 925 is an austenitic nickel-iron-chromium alloy made precipitation hardenable by additions of titanium and aluminum. The precipitation-hardening (agehardening) heat treatment causes precipitation of gamma prime phase, Ni3 (Al, Ti). The phase greatly increases both the hardness and strength of the alloy. Exposure to elevated temperatures also causes formation of other phases, including eta and sigma. Figure 4 is a time-temperature-transformation diagram, and Figure 5 shows effects of the phases on impact strength of the solution-annealed plus aged alloy.
Corrosion Resistance INCOLOY alloy 925 has a high level of corrosion resistance. In both reducing and oxidizing environments, the alloy resists general corrosion, pitting, crevice corrosion, intergranular corrosion and stress-corrosion cracking. Some environments in which INCOLOY alloy 925 is particularly useful are “sour” (H2S containing) crude oil and natural gas, sulfuric acid, phosphoric acid, and seawater. The performance of INCOLOY alloy 925 under conditions representing sour gas wells is indicated in Figure 6 and Tables 8, 9 and 10. Figure 6 shows resistance to stresscorrosion cracking in a sour environment at high pressure and temperature. Table 8 shows that the alloy resists sulfide stress cracking, a form of hydrogen embrittlement. The tests involve exposure of stressed C-ring specimens (made from a portion of tubing cross section) to a solution containing hydrogen sulfide, sodium chloride and acetic acid.
Condition: Solution Annealed and Age Hardened
|U.T.S||140 KSI Min||(965 N/mm2)|
|0.2% Proof Stress||110 KSI Min||(758 N/mm2)|
|Red of Area||20 %|
|CVN Impacts @ -60°C (-75°F)||47 Joules||(35 ft-lbf)|
|NACE MR0175 limits the hardness to 38 HRC max.|
Many different industries rely on Alloy 925 for a number of uses including:
- Valves, hangers, packers, and tubulars for sour gas production
- Marine fasteners and pump shafts