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Nickel Alloy Properties

Many nickel and nickel alloys meet the compositional standards of the Unified Numbering System (UNS), a specification established by the American Society for Testing and Materials (ASTM), the Society of Automotive Engineers (SAE), and metal trade associations such as the American Iron and Steel Institute (AISI). The UNS assigns metals and alloys a lettered prefix and a five-digit number. Specialty nickel and nickel alloys belong to the UNS N category and have designations such as UNS N02200. Other standards for nickel and nickel alloys include casting grades, European Norm (EN), American Society of Mechanical Engineers (ASME) standards, and U.S. military specifications (MIL-SPEC). QQ and QQS prefixes are used to designate specific MIL-SPEC metals.

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 super-alloys. 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 super-alloys are specified for high-strength, high temperature applications at temperatures to 2,000°F. High-carbon nickel-base casting alloys are commonly used at moderate stresses above 2,200°F.

Nickel 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 trade names 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 nickel and extra high 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 trade-names Hastelloy Alloy B and Chlorimet 2.

Complex alloys

Ni-Cr-Fe-Mo-Cu is the basic composition in this category of nickel alloys. They offer good resistance to pitting, inter-granular 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. 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 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.

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