Stainless Steel Parts

One name, several grades

Stainless steel is a family, not a single metal. What unites the grades is chromium, which forms a thin, self-healing oxide layer that keeps the iron underneath from rusting. What separates them is everything else: how easily they cut, whether they weld, how they handle salt, and how strong they get. Picking the right grade matters more than any other choice on a stainless part, and the wrong one is how a part either costs too much or corrodes too soon.

Most work falls into two groups. The austenitic grades 303, 304, and 316 are the corrosion-resistance workhorses. The precipitation-hardening grade, 17-4 PH, is for parts that need strength as well.

The grades you’ll usually pick from

303 is the free-machining grade: a little added sulfur makes it cut fast and finish clean, which lowers the price on turned fittings, shafts, and standoffs. The same sulfur is why it welds poorly and is not for food contact. 304 is the general-purpose default: weldable, food-safe, and a touch more corrosion-resistant, used for brackets, covers, enclosures, and fluid parts.

316 is 304 with molybdenum added, which buys real resistance to chlorides. Salt water, road salt, coastal air, and aggressive washdown all eventually pit 304; 316 holds up where they would. It costs more and cuts slower, so it is specced when the environment demands it rather than by default.

When stainless also has to be strong

The austenitic grades resist rust well but are not especially strong, and they cannot be hardened by heat treatment. When a part has to hold up under load as well as resist corrosion, the answer is 17-4 PH, a heat-treatable stainless that ages to roughly 190 ksi tensile while keeping corrosion resistance close to 304. It is the grade of pump shafts, valve internals, actuator parts, and tooling that has to stay accurate under stress.

Choosing a grade, plainly

A short decision tree covers most parts. Turned and machining-heavy, not welded, no food contact: 303. Welded, food-contact, or general service: 304. Salt, marine, or chloride washdown: 316. Strength plus corrosion resistance: 17-4 PH. And if the part lives somewhere dry and budget is the binding constraint, carbon or alloy steel with a finish is often the cheaper answer than any stainless. If you are not sure, describe the environment in the quote notes and the grade gets flagged for you.

Finishes and the magnet question

Machining can leave traces of free iron on a stainless surface, and that iron is what rust-stains first. Passivation, a citric- or nitric-acid bath commonly specced to ASTM A967, removes it so the chromium layer can do its job; it is standard practice for food, fluid, and outdoor parts. Beyond that, bead blast gives a uniform matte, and brushed or polished finishes are there where looks matter.

One common worry: a stainless part that tugs a magnet is not fake. Cold work makes 303 and 304 mildly magnetic, and 17-4 is fully magnetic by nature. A magnet tells you roughly which stainless family a part belongs to, not whether it is genuine. If non-magnetic behavior is a hard requirement, call it out on the print.