Ask a fabricator to name a stainless grade and the answer will almost always be 304 or 316. Both are austenitic, both are non-magnetic, and both behave much the same in the workshop. The grades that do the harder mechanical work sit in a different family, dismissed in the trade with two words: magnetic stainless. That label hides useful difference. Within the ferritic and martensitic groups there are grades that resist exhaust condensate for a decade, grades that hold a cutting edge at 58 HRC, and grades machined at 85 percent of the speed of free-cutting carbon steel. Choosing between them comes down to whether the part must be hardenable, how much chromium the budget can carry, and what the service environment is.

What separates ferritic from martensitic

Both families are chromium steels with little or no nickel, and both are magnetic in every condition. The difference is carbon. 430 stainless steel sits at the ferritic end with carbon capped at 0.12 percent. Its structure stays ferritic from mill to finished part, it cannot be hardened by heat treatment, and it behaves predictably through drawing and roll forming. The martensitic grades are the other way round: enough carbon is present to transform to martensite on quenching, which is what lets 420 stainless steel reach hardness levels ferritic grades never approach. That hardening capacity is bought with weldability. Ferritic grades tolerate welding within limits; martensitic grades need preheat, controlled interpass temperature and a post-weld temper, or the joint turns brittle.

The ferritic tier: 409 and 430

409 stainless steel is the grade most car owners have never heard of and most exhaust systems are made from. It carries 10.5 to 11.75 percent chromium with titanium stabilisation, 450 MPa minimum tensile and 240 MPa yield, and suits service that is hot and oxidising rather than wet. The stabilised variants S40910, S40920 and S40930 use niobium, titanium or both to lock up carbon so the weld zone does not sensitise, and they are the versions to specify when a joint sees sustained thermal cycling. 409 will not survive a salt spray cabinet the way 304 does, so it does not belong on exposed marine trim.

Where service is indoor and dry, 430 is the economical choice, at 483 MPa tensile, 310 MPa yield, 22 percent elongation and hardness under 183 HB, with enough nitric acid resistance for chemical handling equipment. Appliance fronts, dishwasher interiors, HVAC panels, sink bowls and automotive trim all run on 430. Two variants extend the family. 430 stainless steel coil and strip is the form appliance and HVAC manufacturers buy. 430F takes a different route: a sulphur addition of 0.15 percent minimum makes it free machining, supplied as bar for automatic screw machines. The sulphur improves chip breaking and cuts corrosion resistance as reliably, so 430F belongs on internal turned parts, not on components that face weather.

The martensitic tier: 410, 410S and 416HT

410 stainless steel sheet is the general purpose entry point to the hardenable family. A minimum of 11.5 percent chromium is just enough to build a passive film that holds up in mild atmospheres, steam and many mild chemical environments, and the steel is normally supplied hardened but still machinable. Maximum corrosion resistance is reached when the part is hardened, tempered and polished. Sheet is stocked in the standard 4 by 8 foot format of 1219 by 2438 mm, and in 4 by 10 foot at 1219 by 3048 mm.

410S stainless steel is the same 12 percent chromium base with carbon reduced to 0.08 percent maximum and a small alloy addition that suppresses austenite formation at temperature. The result stays soft and ductile even when rapidly cooled from above the critical temperature, so it can be welded and heated without the cracking risk that makes plain 410 awkward. In the annealed condition 410S is fully ferritic, with corrosion resistance close to 410 and better oxidation resistance. One note applies to both: the surface must be free of heat tint and oxide left by annealing or hot working, and a short dip in 10 to 20 percent nitric acid followed by a water rinse restores the passive layer.

416HT stainless steel is the grade to reach for when the part is mostly a machining job. Its machinability is the highest of any stainless grade at roughly 85 percent of that of free-machining carbon steel, it is always magnetic, and its low frictional characteristics reduce galling on threaded components. Annealed, it runs 517 MPa tensile, 276 MPa yield and 30 percent elongation at 262 HB maximum. As a hardened and tempered Condition T, tensile rises to 758 MPa and yield to 586 MPa with elongation down to 18 percent, and the curve reaches 1405 MPa tensile with 401 HB at a 427 degree Celsius temper. One warning belongs in the specification: do not temper this steel in the 400 to 600 degree band, where impact resistance collapses.

When hardness is the whole point: 420

420 stainless steel is a 410 derivative with carbon raised to 0.15 percent maximum and chromium held at 12 to 14 percent. That extra carbon turns a structural material into a cutting material. Annealed, it runs 655 MPa tensile, 345 MPa yield, 25 percent elongation and 241 HB maximum, and it responds to hardening and tempering across a wide range, which is why it is the standard for cutlery, knife blades, scissors, surgical instruments and shear blades. British standard grades 420S29, 420S37 and 420S45 spread across the hardness range, so a blade maker can order to a hardness target rather than a grade name. Do not specify it above 427 degrees Celsius, because it hardens quickly and loses corrosion resistance at temperature.

Because these grades carry less chromium than 304 or 316, the passive film is thinner and surface condition matters more: a polished or bright annealed surface stands up far better than a rough one, and weld heat tint must always be removed and the zone passivated. On selection, start with the service environment and the hardness requirement, in that order. Dry indoor service points to 430, or 430F if the part is turned on a screw machine. Hot oxidising service with no wet exposure points to 409, and to the stabilised versions where welding is involved. Where the part must be hardened, 410 is the general purpose answer and 416HT the answer when machining hours dominate the cost. Where the part must cut, 420 is the grade, ordered to the hardness band the edge needs. Where strength has to be held at temperature, the martensitic route continues into 422 stainless steel sheet, which holds useful strength to 1200 degrees Fahrenheit for aircraft and power generation hardware. In every case, specify the standard, the condition, the hardness and the finish on the order rather than the grade name alone, because these grades carry a wider spread of properties under one designation than the austenitics do. The full range of sheet, plate, coil, strip and bar sits in the stainless steel catalogue.