Choosing Between 303, 304 and 316L for a Machined Part

September 21, 2026

These three grades cover most machined stainless parts. They look the same on a drawing and behave very differently in a machine and in service. This is the comparison in the order it usually matters.

The two numbers that decide it

	303	304 / 304L	316 / 316L
Machinability (B1112 = 100)	78	45	36
PREN	18	19	24
Molybdenum	none	none	2 to 3 percent
Weldable	no	yes	yes
Typical tolerance	±0.01 mm routine	±0.025 mm	±0.025 mm

Machinability is a published typical rating against free-machining carbon steel B1112 at 100. PREN is chromium plus 3.3 times molybdenum plus 16 times nitrogen, calculated from nominal composition.

Read across the first row and the case for 303 is obvious. Read across the second and third and the case against it appears.

Why 303 cuts so well

303 is 304 with roughly 0.15 percent sulphur added. The sulphur forms manganese sulphide stringers through the material. Those stringers break the chip, which is the problem austenitic stainless otherwise has. A broken chip means it clears the cut instead of wrapping around the tool, heat leaves with it and the tool stays sharp for longer.

The practical effect on the floor is large. Threads come out clean, cross holes do not tear and cycle times drop by more than half against 316L on the same geometry. On a part with many small features, 303 can be the difference between a job that pays and one that does not.

Why 303 fails in service

The same sulphide inclusions are discontinuities in the passive chromium oxide film. Corrosion starts at a defect, and each stringer that meets the surface is a defect. So 303 pits in conditions where 304 with almost the same chromium content does not.

This is why the PREN comparison above is misleading on its own. On paper 303 sits at 18 against 304 at 19, close enough to look interchangeable. In a salt spray test or a wash down bay they are not close at all.

Two further limits follow from the sulphur:

• Welding. The sulphur causes hot cracking. 303 is not welded in normal practice.
• Polishing. The inclusions show up as pits and lines when a surface is polished, so 303 does not take a cosmetic or a hygienic finish well.

That combination is why 303 is normally kept out of food contact, medical, pharmaceutical and marine parts even when the environment looks mild.

When 304 is the right answer

304 is the default for a reason. It welds, it forms, it polishes, it passivates and it costs less than 316L. For anything dry and indoors it is usually enough.

Where it stops being enough is chloride. Salt water, swimming pools, de-icing salt, many cleaning chemicals and body fluid all attack 304 by pitting. Once chloride is in the picture, molybdenum earns its price.

304L is the low carbon version. Specify it when the part is welded, so chromium carbide does not form at grain boundaries and leave the heat affected zone able to corrode.

When 316L earns its cost

316L adds two to three percent molybdenum, which lifts PREN from around 19 to around 24. That is the jump that matters for:

• Food and dairy equipment that is cleaned with caustic or chlorinated products
• Surgical and dental instrument parts, and anything autoclaved
• Pharmaceutical and biotech parts, usually electropolished as well
• Marine hardware, pumps and valves
• Chemical process equipment

You pay for it in the machine. At a machinability rating near 36, expect longer cycles, more tool changes and more attention to finish. A breakdown of where that money goes is worth a read before you compare quotes.

For sea water at temperature, or where pitting has already been a problem in 316L, the next step is 2205 duplex at a PREN near 35. It is slower again and roughly twice as strong.

A short decision path

1. Does the part get wet with chloride, cleaned with chemicals or touch food or body fluid? If yes, 316L. Do not put 303 on that print.
2. Is the part welded? If yes, 304L or 316L. Not 303.
3. Does it need polishing or a hygienic finish? If yes, 304 or 316L. Not 303.
4. Is it dry, indoors, unwelded, with threads and small features? 303 is the cheapest correct answer, and the saving is real.
5. Do you need hardness above 30 HRC as well as corrosion resistance? That is 17-4 PH, not any of these three.

Write the environment, not just the grade

The most useful thing you can add to a stainless drawing is a sentence about where the part lives. Washed down daily with caustic. Mounted inside a sealed dry enclosure. Immersed in sea water. Autoclaved between uses.

With that sentence a shop can tell you that 304 would do, or that 303 will fail, before the first part is cut. Without it, the shop quotes what you wrote. The grade table on the home page covers the rest of the common alloys, and the buyer guide covers what else belongs on the drawing.

Frequently asked questions

Figures in this guide are published specification limits or published typical values for the alloy, not measurements of any part. Machinability is rated against free-machining carbon steel B1112 at 100 and PREN is chromium plus 3.3 times molybdenum plus 16 times nitrogen. The conventions are set out in thereference section and the editorial standards.

Keep reading

• When to Machine 17-4 PH and When to Age It
• Duplex 2205 and the Parts That Actually Need It