320 sits at the far end of the 316/317/318 progression, pushed beyond standard austenitic stainless territory by a substantial copper addition made specifically to fight sulfuric acid. Chromium and molybdenum alone — the workhorses behind 316 and 317's corrosion resistance — actually struggle in dilute-to-moderate sulfuric acid; copper is one of the few alloying elements that meaningfully improves resistance in that specific environment. But copper only does its job properly when the base alloy is also pushed to a much higher nickel content than standard 316-family stainless, which is why 320 carries roughly 32-38% nickel versus the 10-14% typical of 316 — a big enough shift that it's often considered a step out of standard austenitic stainless and into "super austenitic" or nickel-iron-chromium alloy territory (best known under the trade name Alloy 20 / 20Cb-3).

The payoff is real: 320 handles sulfuric acid service — pickling operations, chemical processing, and fertilizer production — that would rapidly attack 316 or even 317. Like the rest of this family it stays austenitic and non-magnetic, and niobium stabilization (similar in principle to 316Ti and 318) protects weld zones from carbide precipitation. The higher nickel content also makes 320 noticeably tougher and gummier to machine than the chromium-nickel grades earlier in this family, so tooling and parameter choices need to shift accordingly.

320 is meaningfully harder to machine than 316 or 317, and the reason traces directly back to the same nickel content that gives it its corrosion resistance. High-nickel austenitic alloys are notorious for being gummy: they work-harden aggressively, generate high cutting forces, and conduct heat away from the cutting edge even more poorly than chromium-nickel stainless. The result is a material that behaves closer to a nickel alloy than a standard stainless in terms of the punishment it puts on a cutting edge.

Every principle that applies to the rest of the 316/317 family applies here with less margin for error. Sharp, positive-rake geometry that shears the material cleanly is essential — a dull or negative-rake edge will simply plow through 320 and leave a work-hardened surface that makes the next pass progressively worse. Feed rates need to stay high enough to consistently cut beneath any hardened layer, and rigid, low-deflection setups matter more here than almost anywhere else in this family, since 320's high cutting forces will push a marginal setup into chatter or edge rubbing quickly.

Chip control is also more demanding: 320 produces long, tough, gummy chips that resist breaking even more stubbornly than standard 316, so an aggressive stainless/high-nickel-alloy chipbreaker geometry and generous coolant flow are important to keep the cut zone clear and manage the substantial heat this alloy generates.