Grade 405 is a ferritic stainless steel with roughly 12–13% chromium and a small aluminum addition, typically around 0.10–0.30%. That aluminum content is what defines the grade: it stabilizes the ferrite phase across a wide temperature range and specifically prevents the material from hardening when it air-cools after welding, which is the main reason 405 exists as a distinct grade from plain 12% chromium alloys. Like other ferritic stainless steels, it carries little to no nickel, stays magnetic, and cannot be strengthened by heat treatment the way a martensitic grade can.

Because it resists the brittle, crack-prone hardening in the heat-affected zone that plain chromium martensitic steels are prone to, 405 is a practical choice for welded stainless assemblies, heat-exchanger and furnace components, and other equipment where weldability matters as much as corrosion resistance. Published machinability ratings put it around 60% relative to a free-machining reference steel, which is a modest improvement over its chromium-only ferritic relatives thanks to its relatively soft, easily formed structure.

405 is one of the easier ferritic stainless grades to machine, mainly because it's soft, relatively low in carbon, and doesn't work-harden the way an austenitic grade does. The main challenge isn't tool wear but chip control: like other low-carbon ferritic and martensitic stainless steels, 405 tends to produce long, stringy chips that resist breaking cleanly, especially at lower cutting speeds or with a dull edge.

A sharp cutting edge paired with a tight chip-breaker geometry is the most reliable way to manage this. Because 405's machining behavior in this respect is similar to austenitic grades such as 304 and 316, insert grades developed for those alloys often perform well here too, even though the metallurgy driving their machinability is different. Keeping the tool cutting cleanly rather than rubbing also helps avoid the smeared, built-up-edge finish that soft ferritic and low-carbon martensitic stainless grades are prone to.

At stable cutting conditions, a semi-hard substrate with a thin CVD coating is a good general-purpose choice for turning. Below roughly 390 SFM (120 m/min), a semi-hard substrate with a PVD coating tends to hold an edge better. For milling, a semi-hard substrate with a thin PVD coating is generally the preferred starting point.