QuestionWhat can you tell us about nickel-based alloys?
AnswerTernary Nickel Alloys
Two primary wrought and cast compositions of ternary alloys have as their general composition Ni-Cr-Fe and Ni-Cr-Mo. These can be summarized as follows:
- Haynes Alloy 214 (Ni-16Cr-2.5Fe-4.5Al-Y) has excellent resistance to oxidation to 1205°C (2200°F) and resists carburizing and chlorine-contaminated atmospheres. Haynes Alloy 556 (Fe-20Ni-22Cr-18Co) combines effective resistance to sulfidizing, carburizing and chlorine-bearing environments with good oxidation resistance, fabricability and high-temperature strength.
- Inconel alloy 600 (Ni-15.5Cr-8Fe) has good resistance to oxidizing and reducing environments. It is intended for severely corrosive conditions at elevated temperatures.
- Incoloy 800 (Ni-46Fe-21Cr) has good resistance to oxidation and carburization at elevated temperatures. It also resists sulfur attack, internal oxidation, scaling and corrosion in many atmospheres.
Significant additions of molybdenum make Ni-Cr-Mo alloys highly resistant to pitting. They retain high strength and oxidation resistance at elevated temperatures. Many are used in the chemical industry due to their resistance to a wide variety of aqueous corrosives. In many applications, these alloys are considered the only materials capable of withstanding the severe corrosion conditions encountered. In this group, the primary commercial materials are:
- Hastalloy® alloy C-22 (Ni-22Cr-13Mo-3W-3Fe) has better overall corrosion resistance and versatility than any other Ni-Cr-Mo alloy.
- Alloy C-276 (57Ni-15.5Cr-16Mo) has excellent resistance to strong oxidizing and reducing corrosives, acids and chlorine-contaminated hydrocarbons. Alloy C-276 is also one of the few materials that withstand the corrosive effects of wet chlorine gas, hypochlorite and chlorine dioxide.
- Hastalloy® alloy C-22 has outstanding resistance to pitting, crevice corrosion and stress-corrosion cracking.
Two grades of cast Ni-Cr-Mo alloys, ACI CW-12M-1 and CW-12M-2, are also used in severe corrosion service often involving combinations of acids at elevated temperatures. The two versions of CW-12M are also produced as Hastalloy® C and Chlorimet®.
Ni-Cr-Fe-Mo-Cu is an example of a complex alloy. These alloys offer good resistance to pitting, intergranular corrosion, chloride-ion stress-corrosion cracking and general corrosion over a wide range of oxidizing and reducing environments. These alloys are frequently used in applications involving sulfuric and phosphoric acids. Important commercial grades include Hastalloy® alloys G-30 and H; Haynes® alloy No. 230; Inconel® alloys 617, 625 and 718; and Incoloy® alloy 825.
The most significant metallurgical considerations when selecting a nickel alloy are:
- Homogenous fine-grained microstructures (preferred)
- Steel cleanliness, especially non-metallic inclusions (which reduce toughness and hydrogen embrittlement resistance)
- No surface and internal defects
Nickel-based alloys used in components for gas transmission pipeline systems contain at least 50% nickel. Nickel contents up to 99+% have been used. Examples include: Nickel 200, Monel 400 and Monel 500, Inconel 600 and X-750, and Hastalloy C-276 and Inconel 625). For hydrogen gas transmission applications, where there is the potential for hydrogen embrittlement (affected by metal strength, stress state and hydrogen partial pressure), these alloys should be considered.