What Do You Know About Stainless Steel?

• 1
  Key takeaways
• 2
  What do you know about stainless steel passivation
• 3
  How the main stainless families differ
• 4
  Machining behavior on the floor
• 5
  What to check before you release the drawing
• 6
  Stainless grade selection at a glance

Ordinary steel is iron with carbon, usually under 2.11% carbon by weight. Add chromium and the behavior changes. Once chromium passes roughly 10.5%, a chromium oxide layer forms on the surface within minutes of exposure to air. That layer is a few nanometers thick and it rebuilds itself if scratched, as long as oxygen is present.

That is the whole trick. Stainless steel is not coated and it is not rust-proof in every environment. It is passive. The oxide film is stable in neutral air and fresh water, less stable in chloride-rich conditions, and it needs free oxygen to repair. A sealed joint with no oxygen and standing salt water is where even a good grade starts to pit.

Nickel, molybdenum and nitrogen change how stable that film is. Molybdenum helps against chlorides, which is why 316 and 316L hold up better than 304 near coastal air or de-icing salt. Nitrogen raises strength in duplex and some austenitic grades. None of these elements make the part immune. They raise the threshold at which the environment wins.

One practical consequence: passivation is not a coating you buy. Citric or nitric acid passivation only removes free iron and embedded contamination left by machining, then lets the natural oxide reform. If you machine with a carbon steel brush or leave iron dust on the surface, the part will rust even though the certificate says 304.

Austenitic grades carry the most chromium and nickel: 303, 304, 316, 316L, 17-4PH is not austenitic but is often grouped with them in shop talk. They are non-magnetic in the annealed state, tough, and weld well. Their weakness is work hardening. The surface gets harder as the tool rubs it, so a dull insert or a light feed makes the next pass harder than the last.

Ferritic grades like 430 hold less nickel and cost less. They are magnetic, resist stress corrosion cracking better than 304 in some chloride service, and form less readily. Martensitic grades such as 420, 431 and 440C carry more carbon and can be hardened by heat treatment. That is why 440C appears in bearings and 420 in cutlery and shafts.

Duplex and precipitation-hardening grades sit at the high end. 17-4PH (SUS630) machines in the solution-treated condition and then ages to high strength, which lets you cut a part soft and finish it hard. Inconel and titanium are not stainless at all, but they show up in the same conversations because they solve the same problem in hotter or more aggressive service.

Pick by environment first, then by strength, then by machinability. The reverse order is how projects end up with a 316L part that will not hold a thread because the shop ran it like 303.

Austenitic stainless work hardens fast. Keep the tool engaged, take a real depth of cut, and never let the insert rub. A light finishing pass after the surface has hardened will not cut cleanly; it will polish and push. Feed per tooth matters more than spindle speed. If the chips turn blue and stringy, you are generating heat in the cut instead of removing material.

Coolant choice is not cosmetic. Flood coolant carries heat away from the edge and the part. High-pressure through-tool coolant helps in deep holes and pockets where chips pack. For 303, sulfur additions make chips break short, so you can push feed and get a good finish. For 316L and 17-4PH, expect lower cutting speeds, more passes, and a higher risk of built-up edge.

Thermal growth is real on long parts. A 500 mm shaft can move several hundredths of a millimeter as it warms during roughing. Rough, let it cool, then finish. On our 16 simultaneous 5-axis centers we can hold ±0.005 mm and Ra 0.8–1.6 μm on stainless when the setup is rigid and the tool path keeps engagement steady. Fine finishes down to Ra 0.2–0.8 μm are possible with the right inserts and a separate finishing pass.

Galling is the other trap. Stainless threads and sliding surfaces can cold-weld to mating stainless. Use a lubricant, choose dissimilar grades where the design allows, or specify a surface treatment. Tapping 316L by hand is slow; rigid tapping with the right lubricant is far more reliable.

Start with the service environment. Fresh water, coastal air, food contact, body contact, high temperature, and strong chlorides each push you toward a different grade. A part that sees only indoor air rarely needs 316L. A part that sees salt spray and cyclic load often needs more than 304. Write the environment on the drawing so the shop can question a mismatch.

Then check the features. Thin walls, deep pockets, long threads and tight true position on stainless cost more than the same features in aluminum. If a wall is under 1 mm on a 316L part, expect chatter and plan a support or a change in geometry. If a thread is smaller than M3, consider a thread mill or a forming tap rather than a cutting tap.

Finally, decide the finish and the heat treatment before quoting. A passivated surface is not the same as a polished one. Laser marking on stainless needs a minimum character height of 1.5 mm to stay legible after passivation. If the part is 17-4PH and will be aged after machining, tell the shop, because aging changes dimensions slightly and the finishing allowance has to account for it.

Documentation matters too. We keep raw material check, in-process monitoring and final inspection records, and we can supply reports on request. If your industry needs traceability, say so up front rather than after the parts ship.