Nickel Superalloys

Nickel superalloys are the metals of the gas turbine’s hot end, and they are made to do what no other common metal will: hold their strength at the temperatures where titanium alloys soften and steel gives up entirely. The family’s best-known names — Inconel 718, Inconel 625 and Hastelloy — serve the turbine blades, discs and casings of jet engines, the hot sections of power plant and the harsh process environments of oil, gas and chemicals, keeping useful strength at 650 degrees and above while resisting oxidation and corrosion. That very property is the source of their reputation in the shop, for nickel superalloys are among the most difficult of common engineering metals to machine: they work-harden at the tool point, keep their strength where the cut is hot and gall and weld to the edge, so the shop meets them with slow speeds, a rigid setup and a discipline that allows no complacency. This entry sets out the family and how it is machined.

The nickel superalloy family

The nickel superalloys are a large family of alloys built on a nickel base — commonly over half the metal — and hardened by chromium, molybdenum, niobium, titanium, aluminium and cobalt, each contributing strength, corrosion resistance or the fine precipitates that give the alloys their high-temperature strength. The most machined is Inconel 718, a precipitation-hardened alloy of nickel, chromium, iron and niobium whose strength at temperature and relative ease of fabrication have made it the turbine and aerospace workhorse; Inconel 625 is its solid-solution neighbour, strengthened without heat treatment and chosen for its corrosion resistance in chemical and marine service. Around them sit the specialised names — Hastelloy for the most corrosive chemical duties, Waspaloy, Nimonic and Rene for the hottest turbine parts — each a variation on the same theme: a metal that keeps its strength when it is hot. In machining terms the family is one problem in many grades, from the merely difficult 718 to the truly stubborn high-temperature blade alloys.

Why they are the hardest metals to machine

Nickel superalloys resist the cutter for reasons that compound, and the first is that they do not soften where they are cut. Where most metals yield to the heat of cutting, the superalloy keeps its strength at the tool point, so the cutter must shear metal that is still hard while the heat of doing so stays localised — the alloys conduct heat as poorly as titanium, and the edge runs hot. The second is severe work-hardening: the surface of a cut or a previous pass hardens under any rubbing or deformation, so a tool that dwells or takes too light a cut finds itself cutting a skin harder than the metal beneath. The third is galling and welding: the hot, strong metal welds to the tool edge, building an edge that tears and flakes, and the tough, continuous chip is cut under high force that flexes the tool and the work. Where the stainless steels and titanium share some of these traits, the superalloys combine them at higher strength, and every one of the reasons a metal is hard to cut is present at once.

Machining nickel superalloys

The practice that machines nickel is the titanium practice taken further, and its first rule is the slowest of speeds. Nickel superalloys are cut at speeds lower than steel and lower still than titanium — commonly well under half the titanium speed with the same carbide — because the edge cannot survive the heat of cutting faster. The second rule is always be cutting beneath the skin: the depth of cut is kept deep enough to pass below the work-hardened layer left by the previous pass, the feed is kept up so the tool shears rather than burnishes, and climb milling is used so the cutter takes a full chip and never rubs. The third rule is rigidity: the setup, the workholding and the tool overhang are made as stiff as possible, for a nickel cut flexes and chatters and a chattering edge is a broken one. And the fourth is coolant under pressure: a high-pressure flood aimed at the cutting zone carries the heat away and clears the strong, stringy chip, without which the cut overheats and the edge fails within minutes. The metal is machined in short, firm, well-cooled cuts — never forced, never dawdled — and its work-hardened skin is respected as the enemy it is.

Tools and their wear

Nickel superalloys are machined with sharp, positive-rake carbide, and the wear it suffers is the signature of the metal. Because the work-hardened surface of a previous pass meets the tool at the depth of cut, the tool wears a notch at the depth-of-cut line — the classic failure of nickel machining — and the edge also wears on its flank and craters where the hot, galled chip passes over it. The shop counters the notch by keeping successive cuts at different depths where it can, by chamfering the part’s edge where the tool enters and leaves, and by changing inserts at the first sign of the notch, for a worn edge in nickel fails suddenly and takes the surface with it. The feeds-and-speeds are set low, from the carbide practice of this wiki, and the tool is kept cutting in fresh metal below the hardened skin, never allowed to rub long enough to make its own new skin. Where the shape cannot be cut, nickel parts are finished by grinding or cut by the spark erosion of EDM, which cares nothing for the metal’s strength; the machine shop’s battle is fought at the cutting edge, and it is won by the fresh, sharp, well-cooled tool.

Where the alloys earn their keep

Nickel superalloys are bought for one property — strength where it is hot — and the difficulty of machining them is the price of that property, paid once at the cutter and recovered through the life of the part. The turbine disc that must spin at temperature for tens of thousands of hours, the aerospace fastener that must hold when the structure is hot, the chemical valve that must resist corrosion that would eat steel: these are not parts that could be made in a softer metal, and the shop that can machine them well holds work that others cannot. The discipline is the same discipline that this wiki has met across the stainless, titanium and work-hardening metals — sharp tools, low speeds, rigid setups, coolant, and no rubbing — taken to its highest level. The nickel superalloy is not machined by force but by respect: the shop that gives it slow, firm, cool and continuous cutting is repaid with parts that no other metal could make, and the metal that is hardest to machine becomes, in a patient shop, steady and reliable work.