Materials #10: SUS303 vs. SUS304 – Which Stainless Should You Choose for a Machine Part?

SUS303 and SUS304 are both common austenitic stainless steels. On a drawing, replacing SUS304 with SUS303 is sometimes just changing one material line; but at the workshop, that difference decides the cutting speed, tool life, weldability, the risk of cracking when bending, and even the stability of the part after machining.

A quick rule is: parts that need a lot of machining lean toward SUS303; parts that need welding, bending or general versatility lean toward SUS304. However, to choose correctly you have to look at the whole chain of operations, not just the CNC machining step.

Where do SUS303 and SUS304 differ?

Both belong to the 18Cr-8Ni austenitic stainless group. SUS304 is the general-purpose stainless grade for many applications; SUS303 is a free-machining stainless steel.

SUS303 has a higher sulfur content added to improve chip formation and reduce tool build-up when turning, milling, drilling or tapping. In return, the sulfur makes SUS303 less favorable for welding, forming and corrosion resistance in harsh environments.

Item	SUS303	SUS304
Main role	Free-machining stainless	General-purpose stainless
Machining	Better; chips easier to control	Harder to machine; prone to work-hardening and tool build-up if the cutting conditions are wrong
Welding	Not the preferred choice	More suitable for welded structures
Bending, stamping, forming	Not the preferred choice	Better for bent parts, sheet and forming
Corrosion resistance	Lower than SUS304 in many conditions	Better than SUS303; still not the default for heavy chloride environments
Common material forms	Round, hexagonal, bar for automatic turning	Sheet, tube, bar, structural and many commercial forms
Suitable applications	Spacers, small shafts, fittings, screws, connectors, turned parts	Covers, welded tanks/frames, brackets, bent sheet parts, tubing and controlled-outdoor parts

Chemical composition, mechanical properties and tolerances must be checked against the applicable standard, the material form, the delivery condition and the supplier's certificate. Do not use this comparison table in place of a material certificate.

Why is SUS303 easier to machine?

When cutting SUS304, the material tends to work-harden at the surface during machining itself. If the tool rubs, the cutting speed is too low, or the feed is too small, the next surface can be harder and wear the tool faster. Ductile, long chips also tend to wrap around the tool or the part.

SUS303 is designed to improve this characteristic. In mass production of small turned parts, threads, deep holes or tapped holes, choosing SUS303 usually makes the process more stable, reduces the risk of chip jamming, and cuts machining time.

But "easier to machine" does not mean "any tool will do." SUS303 still needs:

sharp tools with a cutting geometry suited to stainless;
enough coolant to control heat and clear chips;
avoiding tool rubbing on the surface;
cutting conditions per the tool and machine maker's recommendations;
burr control, especially at threads, intersecting holes and part edges.

When should you choose SUS303?

SUS303 is the preferred choice if the part is mainly produced by machining and has no important welding/bending step:

turned parts from round or hexagonal bar;
bushings, spacers, connectors, fittings, nuts, screws and small shafts;
parts with many threads, tapped holes or deep drilled holes;
high-volume production where cycle time and tool life clearly affect cost;
parts that need ordinary corrosion resistance but do not work in salt, strong chemicals, or under strict hygiene requirements.

A typical example is a small connector with many internal/external threads. With SUS304, the workshop can still machine it, but the time cost, chips and the risk of tap breakage can rise. For a part that is not welded and not bent, SUS303 is usually the more reasonable option.

When is SUS304 the right choice?

Choose SUS304 when the part's function needs formability, welding or corrosion resistance to be prioritized over the machining benefit:

press-bent, stamped or formed sheet parts;
covers, chutes, machine housings, tanks, frames and welded structures;
piping, brackets or assemblies with welds;
parts that need a finish such as 2B, HL, No.4 or #400;
parts used in humid, food or ordinary industrial environments that need a versatile stainless;
parts that need easier material sourcing in sheet, tube or various sizes.

If a SUS304 part has only a few drilled holes or a few milled faces, do not rush to switch to SUS303. In that case, optimizing the tool and cutting conditions is usually more sensible than changing the material, especially if the part still has to be welded or bent.

Three cases where you should not switch SUS304 to SUS303 just for faster machining

1. Welded parts

SUS303 should not be treated as the default material for welded structures. Sulfur improves machining but reduces weld friendliness. If the assembly has functional welds or requires a stable weld/surface, use SUS304 or a grade approved by the welding engineer.

2. Bent or stamped parts

For sheet parts with press-bent corners, small bend radii or deep forming, SUS304 is more suitable. SUS303 is not a substitute just because the end of the part has a few threaded holes or some faces that must be milled.

3. Difficult corrosion environments

SUS304 is not "absolutely rustproof," and SUS303 is even less a material to use in place of SUS304 in every environment. If there are chlorides, seawater, salted detergents, water-retaining gaps, high temperature or chemicals, you need to evaluate the actual environment. You may have to move to SUS316/316L or a specialized grade, instead of just debating between SUS303 and SUS304.

Watch for distortion when machining SUS304 bar

A real-world problem is a SUS304 bar or flat bar milled on one face, opened with a long slot or deeply pocketed, and then bowing. This problem is not exclusive to carbon steel: the cold-worked state of the stock, residual stress and asymmetric material removal can all contribute.

Shapes to evaluate beforehand include:

milling almost the entire length on only one face;
milling a deep slot in the middle of the bar;
pocketing from the edge over a long length;
thinning one side of a long part;
precision machining after welding or after strong clamping.

If straightness or flatness is a functional requirement, do not wait until the workshop reports "the part is bowed." Note the geometric tolerance on the drawing and agree the machining sequence in advance: rough cut, stabilize/straighten if needed, then finish-machine the datum face.

How to note the drawing so the workshop does not misread it

When choosing SUS303 or SUS304, the drawing should show enough information that purchasing and the workshop do not substitute out of habit.

Note the correct material grade and the standard/type if the project requires it.
For precision bar, add the delivery condition or the required tolerance, e.g. cold-drawn/ground bar to an agreed tolerance class.
Specify the datum face that must be machined; do not take fit dimensions from a stock surface if it is not yet controlled.
If the part is welded, note the material per the welding requirement and do not switch to SUS303 just to make a few faces easier to machine.
If the part works in a corrosive environment, clearly note the environment, temperature, detergents or hygiene requirements when needed.
For heavily machined parts, discuss stock sourcing with the workshop early. SUS303 is not always available in flat bar or sheet the way SUS304 is.

Checklist for choosing SUS303 or SUS304

Is the part welded or bent? Yes: prefer SUS304.
Is the part mainly turning, tapping, drilling and CNC milling? Yes: consider SUS303.
Does the environment have chlorides, strong chemicals or standing water? Yes: re-evaluate the whole material choice; you may need SUS316/316L.
Is the desired stock form available? Check round, hexagonal, flat bar, sheet or tube before finalizing the drawing.
Are there special surface, tolerance or straightness requirements? Clearly note the stock condition and machined faces.
Is the machining strongly asymmetric? Yes: evaluate the distortion risk and machining sequence.
Is a material certificate needed? For important parts, require the appropriate certificate for the correct lot.

Conclusion

SUS303 is not "a better SUS304," and SUS304 is not the right choice for every stainless part either. SUS303 buys machining efficiency by accepting limits on welding, bending and the service environment. SUS304 delivers versatility, formability and better welding, but needs suitable machining technique.

Choose the material from the hardest operation of the part: if the hardest is welding/bending, start from SUS304; if the hardest is high-volume turning/tapping and the service conditions allow, start from SUS303.

References

JIS G 4303: Stainless steel bars.
JIS G 4305: Cold-rolled stainless steel plate, sheet and strip.
JIS G 4318: Cold-finished stainless steel bars.
Material certificates and machining guidance from the supplier/tool maker for each specific size.

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