X 40 is the trade name most commonly used in the aerospace and gas turbine industry for the cast cobalt-base superalloy also sold as Stellite 31 and HS-31 - all three names refer to the same UNS R30031 chemistry, standardized as AMS 5382 for investment castings and cross-referenced internationally to the DIN designation CoCr25NiW and ASTM A567, a general specification covering iron, cobalt, and nickel-base castings for high-temperature service. Developed in the 1950s alongside other early cast cobalt superalloys, X 40 became one of the most widely specified materials for conventionally cast turbine vanes and nozzle segments in both aircraft and industrial gas turbines.

The alloy's value comes from a chromium- and tungsten-carbide network dispersed through a cobalt-rich matrix, which gives it strong stress-rupture properties in the 900°-980°C range, good resistance to thermal shock and thermal fatigue, and solid resistance to hot corrosion in both oxidizing and reducing combustion atmospheres. Because X 40 is essentially identical in chemistry to Stellite 31/HS-31, the composition, machinability, and cutting data below are cross-checked against this site's verified Stellite 31 (X-40) reference page for consistency.

X 40 is one of the more demanding cast cobalt superalloys to machine. Chromium- and tungsten-carbide phases distributed through the cobalt matrix are what give the alloy its erosion and wear resistance in service, but that same hard-phase network is abrasive to cutting tools and drives faster flank wear than a similarly hard wrought alloy would produce.

Like other cobalt-base grades, X 40 holds onto its strength and hardness at elevated temperature rather than softening the way steel does at higher cutting speeds, so the edge stays under continuous thermal and mechanical load throughout the cut. It also work-hardens quickly, so a steady, adequate chip load that avoids rubbing or dwelling at the cutting edge is essential to prevent a hardened skin from forming ahead of the next pass.

Conservative cutting speeds, sharp positive-rake ground carbide inserts to keep cutting forces and work hardening down, 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 are the standard countermeasures. The as-cast surface skin, which is often harder and more abrasive than the bulk material, should be accounted for on the first pass.