HS 31 is the same alloy commonly known as X-40 or Stellite 31, a well-documented cast cobalt-base superalloy standardized as AMS 5382 (investment castings) and identified as UNS R30031. It is cross-referenced in this project's original data 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.

Correction: the prior version of this page carried a boilerplate commented-out description calling HS 31 "nickel-based" with "57.7% nickel" - copied verbatim across several unrelated cobalt-alloy pages in the old dataset. HS 31/X-40 is unambiguously cobalt-base; nickel is a minor alloying addition (roughly 10%), not the matrix element. The composition table below has also been corrected: the old page's table omitted tungsten and carbon entirely, despite both being defining elements of X-40's real specification (the DIN name itself, CoCr25NiW, indicates tungsten content that the old table did not list), and did not sum to 100%.

X-40 is prized for resistance to oxidizing and reducing atmospheres up to roughly 1150°C (2100°F), strong stress-rupture properties at 900-980°C, and good resistance to thermal shock and galling - properties that make it a workhorse cast alloy in aerospace and industrial gas turbines, superchargers, and high-temperature tooling.

X-40/HS 31 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.