X17CrNi16-02 is the DIN/EN counterpart of AISI 431, and the designation lays out the composition directly: about 0.17% carbon, roughly 16% chromium, and a small 2% nickel addition. That combination puts it in a different category from the ferritic grades in this family — with enough carbon and chromium in the right balance, this alloy hardens by heat treatment (quench and temper) the same way a martensitic tool steel does, and the added nickel is there specifically to improve toughness and impact resistance in the hardened condition, something plain 12-13% chromium martensitic grades like 410 lack.

The higher chromium content (16% versus roughly 12-13% in 410/420) also gives this alloy meaningfully better corrosion resistance while still allowing it to reach a useful hardened strength level, typically in the 30-40 HRC range depending on tempering. That combination of strength, toughness, and improved corrosion resistance is why this grade shows up in shafts, valve components, fasteners, and aerospace and marine hardware that need more durability than a straight 410 can offer without giving up machinability entirely.

Because it's martensitic, this alloy is magnetic and its hardness — and therefore its machining behavior — depends heavily on what condition (annealed or hardened and tempered) the bar stock is supplied in.

Unlike the ferritic and austenitic grades in this family, this alloy's machining behavior is driven mainly by hardness rather than gumminess or work hardening. In the annealed condition it cuts reasonably well — comparable to a mid-alloy martensitic stainless — but once quenched and tempered to service hardness, cutting forces, heat generation, and abrasive wear all increase substantially, and insert selection needs to account for whichever condition the stock is actually in.

The nickel addition that improves toughness also makes the material somewhat tougher to shear cleanly than a plain 410 or 420 of the same hardness, so a sharp edge and adequate rigidity matter more here than on lower-alloy martensitic grades. Chip formation is generally well-behaved and segmented rather than long and stringy, which is one advantage this grade has over the austenitic members of the family.

Because hardness varies so much with heat treatment condition, coolant and speed selection should always be checked against the actual supplied hardness rather than assumed from the alloy name alone — a hardened-and-tempered bar can require dramatically lower speeds and a tougher insert grade than the same alloy in the annealed state.