321 is an austenitic stainless steel built on the same 18% chromium / 9-10% nickel base as 304, with one deliberate addition: roughly 0.40% titanium. That titanium is the whole point of the grade. During welding or any prolonged exposure in the 425-870°C (800-1600°F) range, plain austenitic stainless can suffer "sensitization," where chromium migrates out of solution to form chromium carbides at the grain boundaries, leaving those boundaries chromium-depleted and vulnerable to intergranular corrosion. Titanium has a stronger affinity for carbon than chromium does, so it ties up the alloy's roughly 0.08% carbon as stable titanium carbides instead, protecting the chromium that would otherwise be stripped out. This makes 321 the natural choice for welded assemblies that will see sustained heat in service rather than a single post-weld anneal.

With a tensile strength typically in the 500-730 N/mm² range and a machinability rating around 36% relative to a free-machining reference steel, 321 sits close to 304 in raw cutting difficulty but carries an added complication: the titanium carbide particles formed during stabilization are hard and abrasive, and they show up unevenly through the matrix. Typical applications include exhaust and furnace components, expansion joints, aircraft manifolds, and pressure vessels and piping that operate continuously at elevated temperature — anywhere post-weld corrosion resistance and heat service both matter at once.

321 machines much like 304 in most respects — it work-hardens quickly if the tool rubs instead of cutting, it has relatively low thermal conductivity that concentrates heat at the edge, and it forms long, stringy chips that need a chipbreaker matched to stainless. What sets it apart is the titanium carbide network formed during stabilization. Those carbide particles are considerably harder than the surrounding austenite, so as the cutting edge passes through, it's alternately shearing soft matrix and grinding against small hard inclusions. The net effect is accelerated abrasive flank wear that doesn't always track with the alloy's bulk hardness or tensile numbers.

Because of that abrasive component, edge preparation and coating selection matter even more on 321 than on a plain 304 part. A tool that would hold up fine on 304 can wear out early on 321 if it isn't built to resist abrasion as well as adhesion. Keeping feeds high enough to cut beneath any previously work-hardened layer, using sharp positive-rake geometry to minimize rubbing, and applying coolant generously to control heat at the tool-chip interface all remain important. Chip evacuation also deserves attention, since stringy chips loaded with hard carbide fragments can mar a finished surface or damage the tool if allowed to recut.