Nickel alloy

What defines the nickel alloy family?

A nickel alloy is an alloy based on nickel rather than iron. Chromium, molybdenum, copper, cobalt, niobium, titanium and aluminium may be added for specific environments or temperatures. Its nickel matrix remains austenitic from cryogenic temperatures upward. It undergoes no embrittling transformation or ductile to brittle transition. It also remains stable at temperatures far above those at which steel loses useful strength.

Nickel provides two principal properties. Resistance to chloride ion stress corrosion cracking increases with nickel content. At the 63 to 75 percent nickel levels typical of this family, the tendency to crack in hot chlorides effectively disappears. Nickel also retains large amounts of chromium and molybdenum in solid solution without forming the brittle phases that limit the alloy content of stainless steel.

The Laxcon Steels grade reference lists 50 nickel alloy grades with their full composition. The table below carries every one of them that has a cross-standard equivalent on record.

Alloying elements and corrosion behaviour

Each alloying element provides a specific property. The resulting combinations distinguish the grades within the family.

Chromium provides resistance to oxidation and high-temperature corrosion. It permits service in air and oxidising acids. Inconel 600 contains 14 to 17 percent chromium. Inconel 601 contains around 23 percent chromium and includes aluminium, which produces a more adherent oxide. Inconel 690 contains 27 to 31 percent chromium and is used in caustic and high-temperature water service.

Molybdenum provides resistance to reducing acids and to localised attack in chlorides, including pitting and crevice corrosion. Inconel 625 contains around 9 percent molybdenum. Molybdenum and niobium also strengthen its matrix without a hardening treatment.

Copper provides resistance in seawater and hydrofluoric acid service. Monel 400 contains 28 to 34 percent copper. This nickel-copper alloy resists hydrofluoric acid at all concentrations up to the boiling point and also resists flowing seawater.

Aluminium, titanium and niobium can provide strength through precipitation of an ordered gamma prime or gamma double prime phase during ageing. This process hardens Inconel 718, Inconel X-750 and Monel K500. Age-hardenable alloys are suited to highly loaded components such as turbine discs. Solid solution alloys are suited to applications such as corrosion-resistant vessel walls.

Mechanical properties and processing

Nickel alloys are classified as solid solution or precipitation strengthened. Solid solution alloys include Inconel 600, Inconel 625 and Monel 400. They obtain their annealed properties without ageing and gain additional strength only through cold work. They can be welded without post-weld ageing. Inconel 625 retains its strength and toughness after welding from cryogenic temperatures to 982 degrees Celsius, without thermal treatment after joining.

Age-hardenable alloys require solution treatment and ageing. The ageing cycle must account for each welding and forming operation because welding heat overages some regions and solution treats others. These alloys attain strength levels above those of solid solution nickel alloys.

Nickel alloys work harden rapidly, conduct heat poorly and retain strength at cutting-edge temperatures. Machining therefore requires rigid setups, sharp positive-rake tools, low surface speeds, continuous flood coolant and feeds sufficient to cut below the work-hardened layer. Rubbing hardens the material beneath the cutting edge. Free-machining variants can reduce machining difficulty where corrosion resistance is not the primary requirement. Monel R405 is a nickel-copper example, but it has lower corrosion resistance and weldability.

Selection and applications

Nickel alloys are used when service conditions exceed the practical range of stainless steel. Relevant conditions include temperatures at which steel loses useful strength or scaling resistance. They also include chloride environments that cause stress corrosion cracking in austenitic stainless steel or pitting in high-molybdenum grades. Chemical conditions may include reducing acids, hydrofluoric acid or process streams that prevent a protective chromium oxide film from surviving.

Below these thresholds, duplex or super duplex stainless steel, or a six percent molybdenum austenitic grade, usually provides the same service life with lower material cost and shorter machining time. Selection therefore depends on whether a less costly material would fail in the intended environment.

Questions about nickel alloy grades

Nickel alloy equivalents across standards

Basis "verified" means every value in that row was checked against the governing standard on 2026-08-18; "inherited" means it came from the previous website and has not been checked. The complete mapping table is on the equivalents reference.

Where the nickel alloy family is not the right answer

Nickel alloys are unsuitable where stainless steel provides adequate service life. Their material cost is substantially higher. Slow machining, long lead times and difficult procurement in small quantities add further constraints.

Individual grades also have specific limitations. Inconel 600 can crack under stress in hot concentrated caustic and requires full stress relief before such service. Oxidising media such as nitric acid, wet chlorine and ammonia in the presence of oxygen attack Monel 400. It can also undergo crevice attack in stagnant seawater despite its resistance to flowing seawater. Inconel 625 hardens and loses ductility after prolonged exposure between roughly 650 and 870 degrees Celsius. These limitations are grade-specific rather than general properties of nickel alloys. Each alloy must therefore be matched individually to its service environment.

Nickel alloy grades in detail