About 416|The best-machining stainless steel. This is a martensitic grade with sulfur added for free machining. Rated at 100% on the stainless machinability scale (the benchmark). Cuts like butter with sharp tools, chips break clean, and tool life is excellent. HSS works fine on it, though carbide obviously lasts longer. The tradeoff is corrosion resistance. It will rust, and 'stainless' is generous. Passivation helps but it's not a substitute for a more corrosion-resistant grade. Also cannot be welded due to the sulfur content. Typically used for motor shafts, gears, valves, fasteners, and any high-volume screw machine work where the part just needs basic corrosion resistance. Compare to
303 (better corrosion resistance, slightly harder to machine) and
304 (much better corrosion resistance but dramatically worse machinability).