SUS303 vs SUS304 vs SUS316: Machinability and Cost Compared

Cost / 12 Aug 2026

Three grades cover most of what arrives on a JIS drawing: SUS303, SUS304 and SUS316. They look almost identical on the shelf and behave very differently under a tool. Picking between them is the single biggest cost decision on most stainless parts, and it is usually made by whoever drew the part rather than whoever pays for it.

The short version: SUS303 for turned parts that are never welded and never see chlorides. SUS304 as the default. SUS316 or SUS316L when chlorides are in the picture. Notice that the cost order under the tool is the exact reverse of the corrosion resistance order.

What is actually different in the chemistry

All three are austenitic, which means face centred cubic, non magnetic in the annealed state, not hardenable by heat treatment, and prone to work hardening as they are cut.

SUS304 is the reference point: roughly 18 % chromium, 8 % nickel, carbon at or below 0.08 %. It is AISI 304, EN 1.4301, UNS S30400.

SUS303 is SUS304 with sulphur added, around 0.15 % minimum. That sulphur forms manganese sulphide inclusions through the material. Those inclusions act as internal chip breakers and as a solid lubricant at the cutting edge. It is AISI 303, EN 1.4305.

SUS316 substitutes 2 to 3 % molybdenum and lifts the nickel to roughly 10 to 14 %. Molybdenum is what resists chloride pitting. It is AISI 316, EN 1.4401.

One number summarises the corrosion side. The pitting resistance equivalent, PREN, is chromium plus 3.3 times molybdenum plus 16 times nitrogen. SUS304 lands around 19. SUS316 lands around 25. SUS303 lands below SUS304, because the sulphide inclusions are themselves initiation sites for pitting, which is why the grade is not specified anywhere wet.

Machinability, and what the numbers mean

The standard handbook index compares each grade to AISI B1112 free machining steel at 100, for annealed bar:

Grade	Index	Relative to SUS304
SUS303	78	Roughly 1.7 times faster
SUS304	45	Reference
SUS316	36	Roughly 20 % slower

Treat those as ratios, not as cycle times. What they translate into on the shop floor:

SUS303 breaks chips on its own. A turned part runs at higher surface speed, the swarf clears instead of nesting around the tool, tool life is longer, and the surface comes off cleaner without a separate finishing strategy. On a sliding head lathe running unattended overnight, that difference is not 1.7 times, it is the difference between the job running and the job stopping.

SUS304 work hardens. Austenite transforms to martensite under strain, so a light pass or a rubbing tool leaves a hardened skin behind that the next pass has to cut through. It also conducts heat at about 16 W per metre kelvin, roughly a third of plain carbon steel, so the heat stays in the cutting edge. And it is gummy, which means built up edge on the rake face.

SUS316 does all of that, more so. The molybdenum raises hot strength, which is exactly what you do not want at the cutting edge. Insert wear is faster and the sensible surface speed is lower.

Where the money actually goes

This is the part that gets missed. On a machined part, the bar is rarely the biggest line.

Take a turned fitting, 25 mm diameter, 60 mm long, a couple of threads, a bore, no exotic tolerances, 200 pieces. Rough proportions for a job like that:

Material: somewhere around 15 to 25 % of the piece price
Machine time: 45 to 60 %
Setup, tooling, inspection, handling: the rest

Now change the grade. The bar price moves by a modest percentage between SUS304 and SUS316. The cycle time moves by 20 to 30 %, and the tooling consumption moves further. Because machine time is the bigger slice, the finished piece price gap between SUS304 and SUS316 ends up wider than the bar price gap.

Go the other way, from SUS304 to SUS303, and the effect is larger still. The bar costs roughly the same. The cycle time can nearly halve on a bar fed turned part. That is the cheapest specification change available on most stainless jobs, and it costs nothing to make at the drawing stage.

The catch is that it is only available if the part is genuinely dry, genuinely unwelded, and nobody will ever put it near salt.

Where each one fails

SUS303 fails when it is welded, when it is used outdoors or near the coast, in any chloride bearing fluid, in food or pharma wetted service, and anywhere a polished cosmetic finish matters, because the sulphide inclusions show as pinholes after polishing.

SUS304 fails in chlorides. Salt water, pool chemistry, de-icing salt, many process fluids. It pits, and pitting is a local failure that does not announce itself until something leaks. It also sensitises if it is welded and then held in the 450 to 850 degC range without a solution anneal, which is what SUS304L exists to avoid.

SUS316 fails on budget, and occasionally on delivery, because the stock range in any given size is narrower than SUS304. It is also not a cure for every corrosion problem. Strong reducing acids and high chloride at temperature need duplex or a nickel alloy, not 316.

A decision table

Service condition	Grade	Why
Turned part, volume, indoor, no welding	SUS303	Half the cycle time, same bar price
General machined part, mild environment	SUS304	The sensible default
Welded assembly, corrosive service	SUS304L	Low carbon, no sensitisation at the weld
Marine, chloride, pool, coastal	SUS316	Molybdenum, PREN around 25
Pharma, semiconductor, vacuum wetted	SUS316L	Low carbon plus a finish and passivation spec
Indoor, non corrosive, price sensitive	SUS430	Ferritic, cheaper, machines closer to carbon steel

What to put on the drawing

Whichever you choose, three notes decide whether the quote you get back is the part you wanted:

The grade and the standard the certificate must name. SUS304 to JIS G 4303 and 304 to ASTM A276 are the same material with different paperwork. Only one will pass an inspector who was told to expect the other.
Whether a substitution is acceptable. If you would take EN 1.4301 bar, say so. If you would not, say that instead. Silence gets interpreted differently by every shop.
Whether the part is welded. It changes the answer from SUS303 to SUS304, and possibly to SUS304L.

For the full JIS to AISI and EN mapping, see the stainless equivalents reference. For what happens to SUS304 under a tool and how to write the rest of the drawing, see the SUS304 machining guide.

When the grade is settled and the drawing is ready, get a quote. An engineer reads it and prices it within 24 business hours, and if you order we place it with a shop that holds what the drawing asks for, inspect it and ship it.

Questions

Which of the three is cheapest for a machined part?

SUS303, in almost every case where the part is turned and the service allows it. The bar costs about the same as SUS304 but it cuts roughly twice as fast, and on a machined part the cutting time is the larger number.

Can I weld SUS303?

No, not for anything structural or corrosion critical. The sulphur that makes it machine well also makes the weld prone to hot cracking and creates corrosion sites along the fusion line. If the part is welded, use SUS304 or SUS304L.

Is SUS316 always better than SUS304?

Better at resisting chlorides, yes. Better overall, no. It costs more to buy, more to cut, and in a dry indoor application it brings nothing SUS304 does not already give you.

How much more does SUS316 cost than SUS304 as a finished part?

Expect the bar to be meaningfully more expensive and the cycle time to be roughly 20 to 30 per cent longer on a comparable part. The finished piece price gap is usually wider than the raw material gap suggests.

Have a drawing for a stainless part?

Send it and an engineer reads the title block, quotes it within 24 business hours, and if you order we place, manage, inspect and ship the parts on one PO and one invoice.

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