347 is a niobium-stabilized version of the 18-8 austenitic stainless family, built on the same roughly 18% chromium / 11% nickel base as 304 but with a small, deliberate niobium (columbium) addition — typically about 10 times the carbon content, commonly landing in the 0.4-0.8% range. That niobium is the entire point of the grade: it ties up carbon as stable niobium carbides during welding and other high-temperature exposure, so chromium stays in solid solution instead of precipitating out as chromium carbide at the grain boundaries. That precipitation, called sensitization, is what leaves plain 304 vulnerable to intergranular corrosion in and around a weld's heat-affected zone; niobium stabilization is what makes 347 the grade of choice when a stainless weldment needs to hold up in service without post-weld solution annealing.

347 sees heavy use in chemical processing equipment, high-temperature exhaust systems, and boiler and superheater piping, anywhere heavy-section welded stainless has to survive the 800-1500°F (425-815°C) sensitizing range without a follow-up heat treatment. It also offers somewhat better creep resistance than plain 304 or 316 at elevated temperature, another reason it turns up in high-temperature pressure piping. Like all austenitic grades, it is not hardenable by heat treatment and gains strength only through cold work; its corrosion and creep advantages come entirely from alloy chemistry, not microstructural hardening.

347 machines much like standard 304 in terms of gumminess and work-hardening tendency — it's austenitic, non-heat-treatable, and prone to smearing and hardening under a rubbing or dull tool exactly the way 304 is. The difference is the niobium carbides distributed through the matrix: they're hard, abrasive particles that increase flank wear compared with an unstabilized 18-8 grade at the same hardness, similar in effect to the titanium carbides in 321.

Sharp, positive-rake tooling with adequate coating wear resistance handles this combination well — the sharp edge shears cleanly and limits work hardening, while the coating resists the added abrasive wear from the niobium carbides. Feed rates should stay high enough to cut under any hardened layer left by a previous pass, and because 347 is frequently used in heavier welded sections, interrupted cuts near weld reinforcement or scale are common and call for a tougher edge preparation than a light finishing pass would need.

Chip control follows the familiar austenitic pattern: expect long, stringy, continuous chips unless the insert's chipbreaker geometry is matched to stainless rather than borrowed from a general carbon-steel line. Adequate coolant helps offset the alloy's low thermal conductivity, which otherwise concentrates cutting heat right at the tool tip.