Structural applications that require specific corrosion resistance or elevated temperature strength receive the necessary properties from nickel and its alloys. Some nickel alloys are among the toughest structural materials known. When compared to steel, other nickel alloys have ultrahigh strength, high proportional limits, and high moduli of elasticity. Commercially pure nickel has good electrical, magnetic, and magnetostrictive properties.

Common nickel alloy families include: commercially pure nickel; binary systems, such as Ni-Cu, Ni-Si, and Ni-Mo; ternary systems, such as Ni-Cr-Fe and Ni-Cr-Mo; more complex systems, such as Ni-Cr-Fe-Mo-Cu (with other possible additions); and superalloys. Nickel content throughout the alloy families ranges from 32.5 to 99.5%.

At cryogenic temperatures, nickel alloys are strong and ductile. Several nickel-base superalloys are specified for high-strength applications at temperatures to 2,000°F. High-carbon nickel-base casting alloys are commonly used at moderate stresses above 2,200°F.

Alloy characteristics: Commercial nickel and nickel alloys are available in a wide range of wrought and cast grades; however, considerably fewer casting grades are available. Wrought alloys tend to be better known by tradenames such as Monel, Hastelloy, Inconel, Incoloy, etc. Casting alloys are identified by Alloy Casting Institute and ASTM designations. Wrought and cast nickel alloys are often used together in systems built up from wrought and cast components. The casting alloys contain additional elements, such as silicon and manganese, to improve castability and pressure tightness.

Commercially pure nickels and extrahigh nickel alloys: Primary wrought materials in this group are Nickel 200 and 201, both of which contain 99.5% Ni. The cast grade, designated CZ-100, is recommended for use at temperatures above 600°F because its lower carbon content prevents graphitization and attendant ductility loss. Both wrought grades are particularly resistant to caustics, high-temperature halogens and hydrogen halides, and salts other than oxidizing halides. These alloys are particularly well suited for food-contact applications.

Duranickel 301, a precipitation-hardened, 94% nickel alloy, has excellent spring properties to 600°F. During thermal treatment, Ni3AlTi particles precipitate throughout the matrix. This action enhances alloy strength. Corrosion resistance is similar to that of commercially pure wrought nickel.

Binary nickel alloys: The primary wrought alloys in this category are the Ni-Cu grades known as Monel alloy 400 (Ni-31.SCu) and K-500 (Ni-29.SCu), which also contain small amounts of Al, Fe, and Ti. The Ni-Cu alloys differ from Nickel 200 and 201 because their strength and hardness can be increased by age hardening. Although the Ni-Cu alloys share many of the corrosion characteristics of commercially pure nickel, their resistance to sulfuric and hydrofluoric acids and brine is better. Handling of waters, including seawater and brackish water, is a major application. Monel alloys 400 and K-500 are immune to chloride-ion stress-corrosion cracking, which is often considered in their selection.

Other commercially important binary nickel compositions are Ni-Mo and Ni-Si. One binary type, Hastelloy alloy B-2 (Ni-28Mo), offers superior resistance to hydrochloric acid, aluminum-chloride catalysts, and other strongly reducing chemicals. It also has excellent high-temperature strength in inert atmospheres and vacuum.

Cast nickel-copper alloys comprise a low and high silicon grade. M-35-1 and QQ-N-288, Grades A and E (1.5% Si), are commonly used in conjunction with wrought nickel-copper in pumps, valves, and fittings. A higher silicon grade, QQ-N-288, Grade B (3.5% Si), is used for rotating parts and wear rings because it combines corrosion resistance with high strength and wear resistance. Grade D (4.0% Si) offers exceptional galling resistance.

Two other binary cast alloys are ACI N-12M-1 and N-12M-2. These Ni-Mo alloys are commonly used for handling hydrochloric acid in all concentrations at temperatures up to the boiling point. These alloys are produced commercially under the tradenames Hastelloy alloy B and Chlorimet 2.

Ternary nickel alloys: Two primary wrought and cast compositions are Ni-Cr-Fe and Ni-Cr-Mo. Ni-Cr-Fe is known commercially as Haynes alloys 214 and 556, Inconel alloy 600, and Incoloy alloy 800. Haynes new alloy No. 214 (Ni-16Cr-2.5Fe-4.5Al-Y) has excellent resistance to oxidation to 2,200°F, and resists carburizing and chlorine-contaminated atmospheres. Haynes patented alloy No. 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. Intended for severely corrosive conditions at elevated temperatures, Incoloy 800 (Ni-46Fe-21Cr) has good resistance to oxidation and carburization at elevated temperatures, and it resists sulfur attack, internal oxidation, scaling, and corrosion in many atmospheres.

A cast Ni-Cr-Fe alloy CY-40, known as Inconel, has higher carbon, Mn, and Si contents than the corresponding wrought grade. In the as-cast condition, the alloy is insensitive to the type of intergranular attack encountered in as-cast or sensitized stainless steels.

Significant additions of molybdenum make Ni-Cr-Mo alloys highly resistant to pitting. They retain high strength and oxidation resistance at elevated temperatures, but they are used in the chemical industry primarily for 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 C-276, Hastelloy alloy C-22, and Inconel alloy 625. Hastelloy 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 withstands the corrosive effects of wet chlorine gas, hypochlorite, and chlorine dioxide. Hastelloy alloy C-22, the newest alloy in this group, has outstanding resistance to pitting, crevice corrosion, and stress-corrosion cracking. Present applications include the pulp and paper industry, various pickling acid processes, and production of pesticides and various agrichemicals.

Two grades of cast Ni-Cr-Mo alloys, ACI CW-12M-1 and CW-12M-2, are used in severe corrosion service, often involving combinations of acids at elevated temperatures. The two versions of CW-12M are also produced as Hastelloy C and Chlorimet.

Complex alloys: Ni-Cr-Fe-Mo-Cu is the basic composition in this category of nickel alloys. They offer good resistance to pitting, intergranular corrosion, chloride-ion stress-corrosion cracking, and general corrosion in a wide range of oxidizing and reducing environments. These alloys are frequently used in applications involving sulfuric and phosphoric acids.

Important commercial grades include Hastelloy alloys G-30 and H, Haynes alloy No. 230, Inconel alloys 617, 625, and 718, and Incoloy alloy 825.

Haynes alloy No. 230 (Ni-22Cr-14W-2Mo) has excellent high-temperature strength, oxidation resistance, and thermal stability, making it suitable for various applications in the aerospace, airframe, nuclear, and chemical-process industries.

Hastelloy alloy G-30 (Ni-30Cr-6Mo-2.5W-15Fe) has many advantages over other metallic and nonmetallic materials in handling phosphoric acid, sulfuric acid, and oxidizing acid mixtures. Hastelloy alloy H (Ni-22Cr-9Mo-2W-18Fe) is a patented alloy with localized corrosion resistance equivalent or better to alloy 625. Alloy H also has good resistance to hot acids and excellent resistance to stress-corrosion cracking. It is often used in flue gas desulfurization equipment.

Inconel alloy 617 (Ni-22Cr-12.5Co-9Mo-1.5Fe-1.2Al) resists cyclic oxidation at 2,000°F, and has good stress-rupture properties above 1,800°F.

Inconel alloy 625 (Ni-21.5Cr-2.5Fe-9Mo-3.6Nb+Ta) has high strength and toughness from cryogenic temperatures to 1,800°F, good oxidation resistance, exceptional fatigue strength, and good resistance to many corrosives. Furnace mufflers, electronic parts, chemical and food-processing equipment, and heat-treating equipment are among a few of the many applications for alloy 615.

Inconel alloy 718 (Ni-18.5Fe-19Cr-3Mo-5Nb+Ta) has excellent strength from -423 to 1,300°F. The alloy is age hardenable, can be welded in the fully aged condition, and has excellent oxidation resistance up to 1,800°F.

Incoloy 825 (42Ni-30Fe-21.5Cr-3Mo-2.25Cu) offers excellent resistance to a wide variety of corrosives. It resists pitting and intergranular corrosion, reducing acids, and oxidizing chemicals. Applications include pickling-tank heaters and hooks, spent nuclear-fuel-element recovery, chemical-tank trailers, evaporators, food-processing equipment, sour-well tubing, hydrofluoric-acid production, pollution-control equipment, and radioactive-waste systems.

Superalloys: One class of Ni-based superalloys is strengthened by intermetallic compound precipitation in a face-centered cubic matrix. The strengthening precipitate is gamma prime, typified by Waspaloy (Ni-19.5Cr-13.5Co-4.3Mo-3.0Ti-1.4Al-2.0Fe), Udimet 700 (Ni-15Cr-18.5Co-5Mo-3.4Ti-4.3Al-<1Fe), and the modern but complex Rene 95 (Ni-14Cr-8Co-3.5Mo-3.5W-3.5Nb-2.5Ti-3.5Al).

Another type of Ni-based superalloy is represented by Hastelloy alloy X (Ni-22Fe-9Mo-22Cr-1.5Co). This alloy is essentially solid-solution strengthened, but probably also derives some strengthening from carbide precipitation through a working-plus-aging schedule.

A third class includes oxide-dispersion-strengthened (ODS) alloys such as IN MA-754

(Ni-20Cr-0.6yttria) and IN MA-6000 (Ni-15Cr-2Mo-4W-2.5Ti-4.5Al), which are strengthened by dispersions such as yttria coupled (in some cases) with gamma prime precipitation (MA-6000).

Nickel-based superalloys are used in cast and wrought forms, although special processing (powder metallurgy/isothermal forging) often is used to produce wrought versions of the more highly alloyed compositions (U-700, Astroloy, IN-100).

An additional dimension of Ni-based superalloys has been the introduction of grain-aspect ratio and orientation as a means of controlling properties. In some instances, grain boundaries have been removed. Wrought powder-metallurgy alloys of the ODS class and cast alloys such as MAR M-247 have demonstrated property improvements due to grain morphology control by directional crystallization or solidification. Virtually all uses of the cast and wrought nickel-base superalloys are for gas-turbine components.

Fabrication: Most wrought-nickel alloys can be hot and cold worked, machined, and welded successfully. The casting alloys can be machined or ground, and many can be welded and brazed.

Nearly any shape that can be forged in steel can also be forged in nickel and nickel alloys. However, because nickel work hardens easily, severe cold-forming operations require frequent intermediate annealing to restore soft temper. Annealed cold-rolled sheet, not stretcher leveled, is best for spinning and other manual work. In general, cold-drawn rods machine much more cleanly and readily than hot-rolled or annealed material.

Nickel alloys can be joined by shielded metal-arc, gas tungsten-arc, gas metal-arc, plasma-arc, electron-beam, oxyacetylene, and resistance welding; silver and bronze brazing; and soft soldering. Resistance welding methods include spot, seam, projection, and flash welding.

Special nickel alloys, including superalloys, are best worked at about 1,800 to 2,200°F. In the annealed condition, these alloys can be cold worked by all standard methods. Required forces and rate of work hardening are intermediate between those of mild steel and Type 304 stainless steel. These alloys work harden to a greater extent than the austenitic stainless steels, so they require more intermediate annealing steps.

Both cold-worked and hot-worked Ni-Cu require thermal treatment to develop optimum ductility and to minimize distortion during subsequent machining. Stress relieving before machining is recommended to minimize distortion after metal removal. Stress equalizing of cold-worked Cu-Ni increases yield strength without marked effects on other properties.

Many Hastelloy alloys can be upset forged if the length of the piece is no greater than twice its diameter. However, upsetting should never be attempted on a cast ingot. Cast ingots must be reduced at least 75% before hot upsetting.

Most wrought nickel-based alloys can be formed from sheet into complex shapes involving considerable plastic flow. These alloys are processed in the annealed condition.