Inconel

Inconel is Special Metals Corporation's trade name for a family of nickel-chromium-based superalloys. When engineers reach for Inconel, they are working at the edge of what aluminum and stainless steel can survive: sustained elevated temperatures above 1200F, severely corrosive chemical environments, or applications that require both at the same time. The two most common plate and bar alloys are 625 and 718, which are related in chemistry but fundamentally different in heat treatment and mechanical behavior.

Inconel 625 is a solid-solution-strengthened alloy. It derives its properties from the nickel-chromium-molybdenum-niobium matrix without precipitation hardening. This means it does not require a specific aging treatment to develop its baseline properties, and it retains excellent ductility and toughness down to cryogenic temperatures and up to approximately 1800F for oxidation resistance. Its corrosion resistance in seawater, hydrochloric acid, sulfuric acid, and other aggressive media is outstanding. It is the standard for marine exhaust systems, chemical processing equipment, and subsea components that must survive for decades without replacement.

Inconel 718 is a precipitation-hardened alloy, strengthened by the controlled precipitation of gamma-prime and gamma-double-prime phases during aging. After solution annealing and double aging per AMS 5662 or 5663, tensile strength reaches 180,000 psi minimum with reasonable ductility. It is usable at temperatures up to 1300F under stress, which makes it the dominant material in gas turbine hot sections, rocket motor cases, and other components where aluminum is nowhere near sufficient. Machining is challenging: it work-hardens quickly, generates high cutting temperatures, and demands proper tooling selection and coolant strategy to avoid premature tool failure.