403Cb is a niobium-modified ("columbium", the older US term for niobium) version of standard Type 403 martensitic stainless, built specifically for gas and steam turbine blading and other components that spend their working life at elevated temperature under sustained mechanical load. The base alloy is a low-nickel, roughly 12% chromium martensitic steel, similar in essence to 410 or 403 — but the small niobium addition (carbide-forming, on the order of 0.15-0.45%) ties up carbon into stable niobium carbides rather than leaving it free to form the coarser chromium carbides that plain 403 relies on for hardness.

That difference matters a lot in service: the niobium carbides raise the alloy's resistance to creep at elevated temperature and improve toughness and tensile strength compared with standard 403, without giving up much of the corrosion resistance that makes the 400-series martensitic grades useful in a turbine environment in the first place. The tradeoff is that 403Cb is a more specialized, aerospace/turbine-grade material than a general-purpose stainless — it shows up almost exclusively in gas turbine blades and high-stress steam turbine components rather than everyday stock.

403Cb machines similarly to other low-nickel martensitic stainless grades in the annealed or normalized-and-tempered condition, with the niobium carbides adding a modest abrasive component that plain 403 doesn't have. Because the alloy is almost always supplied for high-temperature, high-stress turbine service, it's common to encounter it at a higher hardness than a general-purpose 12% Cr stainless, so confirming actual hardness before committing to speeds and feeds is worthwhile.

Thermal conductivity is low, as with any chromium stainless, so heat concentrates at the cutting edge rather than dissipating into the chip. Sharp, positive-rake geometry and steady feed rates that keep the edge cutting rather than rubbing help control both work hardening and built-up edge. A tougher, wear-resistant carbide grade generally outperforms a fragile, ultra-sharp edge given the niobium carbide content and the elevated-strength condition this material typically arrives in.