Stellite 31 is the same alloy commonly known as X-40 and HS-31 (published elsewhere in this library), a well-documented cast cobalt-base superalloy standardized as AMS 5382 (investment castings), UNS R30031, and cross-referenced to DIN designation CoCr25NiW. X-40 is one of the most widely used cast cobalt superalloys in gas turbine hot sections, applied to nozzle vanes, buckets/blades, and other components exposed to combustion-gas erosion at high temperature. Kennametal's current Stellite 31 datasheet confirms the same chemistry (Cr 26%, Ni 10.5%, W 7.5%, C 0.5%, Co balance) and highlights "high tensile and creep properties" with resistance to oxidizing and reducing atmospheres up to 1150degC (2100degF).

X-40 is prized for strong stress-rupture properties at 900-980degC, good resistance to thermal shock, and good galling resistance - properties that make it a workhorse cast alloy in aerospace and industrial gas turbines, superchargers, and high-temperature tooling. For consistency, the detailed composition, machinability discussion, and cutting-speed table below match the verified data already published on this site's HS 31 page, since Stellite 31, X-40, and HS-31 all refer to the same UNS R30031 alloy.

As a cast alloy, Stellite 31/X-40 relies on a chromium- and tungsten-carbide network dispersed through the cobalt matrix for its high-temperature strength and erosion resistance, which is the dominant factor in its machining behavior rather than its moderate bulk carbon content.

Stellite 31/X-40 is one of the more demanding cast cobalt superalloys to machine, combining high hot-hardness retention with a carbide-dense cast microstructure. Chromium and tungsten carbides distributed through the cobalt matrix give the alloy its erosion and wear resistance in service, but the same hard-phase network is highly abrasive to cutting tools, driving accelerated flank wear well beyond what similar-hardness wrought alloys would produce.

Like other cobalt-base grades, X-40 retains strength and hardness at elevated temperature better than most nickel superalloys, so the cutting edge stays under continuous thermal and mechanical load rather than benefiting from thermal softening at higher speeds. It also work-hardens rapidly under cutting forces, so maintaining a steady, adequate chip load - never allowing the edge to rub or dwell - is critical to avoiding a hardened skin on the next pass.

Recommended practice is conservative cutting speeds, sharp positive-rake ground carbide inserts to reduce cutting forces and work hardening, rigid, low-deflection setups to control vibration around the hard carbide phases, and close monitoring of flank wear rather than running to a fixed tool-life estimate. As-cast surface skin (often harder and more abrasive than the bulk material) should be taken into account on the first pass.