329 is one of the earliest commercially significant duplex stainless steels — an austenitic-ferritic alloy that splits its microstructure roughly evenly between the two phases instead of committing fully to one, the way 300-series austenitic or 400-series ferritic/martensitic grades do. With around 25.5% chromium, 3.5% nickel, and 1% molybdenum, it carries far more chromium and far less nickel than an austenitic grade like 304 or 316, which is what stabilizes the ferrite phase alongside the austenite. That dual-phase structure is the whole appeal: duplex grades combine the corrosion resistance of austenitic stainless with meaningfully higher strength and better resistance to chloride stress-corrosion cracking, a failure mode that can catch straight austenitic grades off guard in hot chloride service.

The strength difference shows up directly in the numbers — 329 typically runs 640-900 N/mm² tensile, well above the 500-730 N/mm² range typical of standard austenitic stainless. That extra strength and the mixed two-phase microstructure are also exactly why duplex grades are treated as a distinct, more demanding machining category rather than lumped in with austenitic stainless. 329 shows up in heat exchangers, chemical process tanks, pump and valve components, and other equipment where both mechanical strength and resistance to chloride-bearing environments matter — often as the predecessor to more modern duplex grades like 2205.

Duplex stainless steels machine noticeably differently than either of the phases they're made of. The ferrite phase is stronger and less ductile than austenite, and the two phases have different hardness and thermal behavior, so the cutting edge is constantly transitioning between slightly different materials as it moves through the workpiece. That mixed structure, combined with 329's higher strength and chromium content, produces higher cutting forces and faster tool wear than a standard austenitic grade of similar hardness would suggest. Work hardening is still a factor, as it is with any stainless alloy, but on duplex material it compounds with the alloy's inherently higher base strength.

Because of this, duplex grades are generally cut significantly slower than austenitic stainless of comparable hardness — as a rule of thumb, duplex material should be run at roughly 30-40% lower cutting speed than a 316-class austenitic grade to keep tool wear and heat generation in check. Rigid setups, sharp positive-rake tooling, and generous coolant all matter even more here than on 304 or 316, since duplex's lower thermal conductivity and higher shear strength both push more heat and load into the cutting edge. Chip control tends to be reasonable since the alloy isn't especially gummy, but tool life is the main constraint operators need to plan around.