304 vs 316 Stainless Steel, and 17-4 PH: Choosing a Grade for Machined Parts

A housing that worked fine in a laboratory started showing rust streaks after six months in a food plant washdown area. The drawing said 304, the material certificate said 304, and nobody had done anything wrong — except specify an alloy for a chloride environment it was never meant to handle. The fix was 316L, and the cost of learning it that way was a recall, not a quote.

Grade selection for stainless machined parts is usually decided by the fluid the part touches, not by the strength column. Chloride level, temperature, concentration and cleaning frequency narrow the choice quickly.

Here is how the three common grades actually differ, what each costs you on the machine, and where the line sits between them.

Start with the environment

304 (UNS S30400) is the default austenitic grade: roughly 18 percent chromium, 8 percent nickel, and no molybdenum. It handles fresh water, most food products, mild acids and atmospheric exposure well, and it is the cheapest stainless you can reasonably specify for a machined part.

316L (S31603) adds 2 to 3 percent molybdenum and drops carbon to 0.03 percent maximum. The molybdenum is what resists pitting and crevice attack in chlorides; the low carbon is what prevents chromium carbide precipitation during welding. If the part is welded, use 316L; machined only, the two behave the same.

17-4 PH (S17400, also called 630) is a martensitic precipitation-hardening grade: about 16 percent chromium, 4 percent nickel, 4 percent copper and a niobium addition. It is the grade you choose when you need 40 HRC and still want better corrosion resistance than 410 or 420 can offer.

What molybdenum buys you

The standard way to compare chloride resistance is the pitting resistance equivalent number: chromium plus 3.3 times molybdenum plus 16 times nitrogen. It is a ranking tool, not a design allowable.

Read the 17-4 PH line carefully. Its PREN is no better than 304 because it contains no molybdenum. What it offers is strength, not chloride immunity: a 17-4 shaft in 2000 ppm brine pits like a 304 shaft, and faster if the surface is not passivated.

For seawater, brine or hypochlorite dosing, none of these three is right. Move to a duplex such as 2205 or a super-austenitic such as 254 SMO (S31254, PREN above 40).

Machinability differences that show up in the quote

Austenitic stainless work hardens, and that single property drives most of the cost difference between these grades.

304 and 316 have a high strain-hardening rate. Cut with a light feed and the surface under the tool hardens immediately, so the next pass cuts hardened metal instead of annealed metal. The result is tool wear, poor finish and depth-of-cut notching. Keep the chip load above roughly 0.1 mm per tooth, take a depth of cut deeper than the work-hardened layer, use sharp positive-rake carbide, and never let the tool dwell in the cut.

316 is roughly 20 percent slower to machine than 304, because the molybdenum raises hot strength and makes the chip tougher. On a part with a lot of pocketing, that alone can be 10 to 15 percent of the cycle time.

Both grades need flood coolant; recutting chips destroys an austenitic part faster than anything else.

17-4 PH: strength with a predictable shrink

17-4 is supplied in the solution-treated Condition A, around 30 HRC, which machines more easily than 316 despite being harder. You cut it in that state, then age it to the final property.

The useful conditions are H900 (1 hour at 480 °C, air cool) and H1150 (4 hours at 620 °C, air cool). H900 gives about 1310 MPa tensile and 1170 MPa yield at 40 to 44 HRC. H1150 gives about 930 MPa and 725 MPa at 30 to 34 HRC, with much better toughness and stress corrosion resistance.

Aging shrinks the part by roughly 0.0004 to 0.0006 mm per mm, which is 40 to 60 µm on a 100 mm dimension. Put that allowance in the program instead of discovering it after heat treatment: rough machine, age, then finish the fits.

Free-machining grades and their hidden cost

303 (S30300) adds sulfur, which forms manganese sulfide inclusions that break chips and let you run roughly twice as fast as 304. It is the grade to use for high-volume connectors, shafts and spacers that will live indoors.

The sulfur is also a corrosion liability. Those inclusions are where pitting starts, and passivation cannot remove them because they are part of the metallurgy. 303 is not suitable for food contact, medical, marine or outdoor exposure, and it cannot be welded.

416 (S41600) does the same trick in a martensitic grade: 12 to 13 percent chromium, sulfur added, hardenable to about 40 HRC, magnetic. It machines beautifully and corrodes readily, so keep it to oiled internal parts.

Passivation: what it fixes and what it cannot

Passivation per ASTM A967 removes free iron and surface contamination and rebuilds the chromium oxide film. It does not remove heat tint, weld scale or embedded iron from a carbon steel brush, and it does not make 303 corrosion resistant.

For 304 and 316, a citric bath at 4 to 10 percent acid, 20 to 50 °C, for 20 to 30 minutes is usually sufficient and avoids the fume handling of nitric acid. Nitric remains common where a customer specification calls it out by name. Whichever route you use, the part must be clean and free of oil first.

Passivation changes dimensions by less than a micrometer, so it will not rescue an undersized bore. Electropolishing, which removes 5 to 25 µm, is a different process with a different purpose.

Choosing the grade

If the environment on your drawing is ambiguous, note the fluid, concentration and cleaning regime, and we will tell you whether 304 is enough before you request a quote. We machine all three grades routinely and will flag the ones that need post-process passivation.

Frequently asked questions

Is 316 always better than 304?

No. It is better in chlorides and marginally better in most acids, but it costs 20 to 40 percent more and machines more slowly. In fresh water, food oils and dry indoor service, 304 performs identically for less money.

Can 17-4 PH be machined after hardening?

Only with difficulty. At 40 to 44 HRC it can be milled with carbide at reduced speeds, but tool life drops sharply and fine features suffer. Rough machine in Condition A, age, then grind or lightly mill the critical surfaces.

Does passivation improve salt spray results?

It removes contamination that would otherwise initiate corrosion, but it does not change the alloy's inherent resistance. A 304 part will still pit in a chloride environment whether or not it is passivated.

Why do 304 parts rust in storage?

Usually contamination, not the alloy: iron particles from a carbon steel rack, a grinding wheel used on both materials, or chlorinated cleaning cloths. Passivation after the last operation and clean, dedicated handling solve most of these cases.