Stelloy HS27 is a heat-resisting nickel-cobalt-chromium-molybdenum alloy identified by the DIN designation NiCo32Cr26Mo. As the designation indicates, the alloy is built on a nickel matrix with a very large cobalt addition (nominally around 32%) and a heavy chromium addition (nominally around 26%), with molybdenum present as a solid-solution strengthener. That combination places Stelloy HS27 among the more heavily alloyed nickel-cobalt heat-resisting grades - closer in overall alloying philosophy to the high-cobalt, high-chromium superalloys used for extreme oxidation and hot-corrosion resistance than to the aluminum/titanium precipitation-hardened Udimet or Inconel grades elsewhere on this chart.

Alloys of this NiCo-Cr-Mo type are generally specified where a combination of high-temperature strength and strong resistance to oxidizing and corrosive atmospheres is required simultaneously - the large chromium fraction provides oxidation and hot-corrosion resistance, while the cobalt and molybdenum contribute solid-solution strength that is retained well into elevated-temperature service. This is an internationally-designated, less commonly documented trade name; specific mill certificates should always be consulted to confirm exact composition and heat-treatment condition for a given lot.

As a nickel-cobalt-chromium heat-resisting alloy, Stelloy HS27 shares the core machining challenges common to this material family. Thermal conductivity is low relative to steels, so heat generated at the cutting edge concentrates at the tool-chip interface rather than dissipating into the chip, which accelerates crater wear on grades not suited to superalloy cutting. The high combined cobalt and chromium content promotes rapid work hardening under cutting forces and mechanical/thermal stress, so a consistent, adequate chip load must be maintained throughout the cut - any hesitation or rubbing produces a hardened surface skin that is markedly harder to remove on the next pass.

Like other high-cobalt, high-chromium nickel alloys, Stelloy HS27 retains substantial strength and hardness at elevated temperature, so cutting edges remain under heavy mechanical load even as heat builds at the tool tip, and the alloy's chemical affinity for common tool materials makes it prone to galling and built-up edge. Rigid setups that minimize deflection, sharp positive-rake carbide grades engineered for heat-resistant superalloys, moderate-to-low cutting speeds, and generous, consistent coolant delivery are the standard countermeasures for this alloy family.