303 vs 304 Stainless
303 is 304 with about 0.15% sulfur added. The sulfide inclusions break chips and lubricate the cut, so 303 machines roughly 50% faster with better finish and tool life. The same inclusions are where corrosion starts, so 303 pits sooner, and they cause hot cracking in welds, so 303 is rarely welded.
Use 303 for screw-machine and CNC parts that are not welded and live indoors or in mild service: fittings, shafts, bushings, fasteners. Use 304 for anything welded, formed, exposed to weather or food, or where corrosion is the reason for buying stainless in the first place.
| Property | 303 | 304 |
|---|---|---|
| Type | Austenitic, free-machining | Austenitic |
| Sulfur | 0.15% min | 0.03% max |
| Chromium / Nickel | 17-19% / 8-10% | 18-20% / 8-10.5% |
| Yield (annealed bar) | ~240 MPa (35 ksi) | ~240 MPa (35 ksi) |
| Tensile (annealed bar) | ~620 MPa (90 ksi) | ~585 MPa (85 ksi) |
| Density | ~8.0 g/cm³ | ~8.0 g/cm³ |
| Machinability | ~75% (of 1212) | ~45% |
| Weldability | Poor; hot cracking | Excellent |
| Corrosion resistance | Fair; below 304 | Good |
| Product forms | Bar, mainly round | Sheet, plate, bar, tube, pipe |
| Typical uses | Fittings, shafts, bushings, screw-machine parts | Sheet metal, tanks, weldments, food equipment |
Typical values for the stated conditions. Confirm against the mill certificate and your spec.
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Attach a drawing (PDF or STEP) or describe the part; a U.S. shop that fits quotes it.
Decision matrix
A check marks the side with the edge on that dimension.
| Dimension | 303 | 304 |
|---|---|---|
| Machining | ✓ Excellent for a stainless; short chips, good finish | Fair; stringy chips, work-hardening |
| Welding | Poor; sulfur causes hot cracking, avoid | ✓ Excellent; 308L filler |
| Forming | Poor; sulfides tear on bends and heading | ✓ Excellent |
| Strength | Same as 304 annealed | Same as 303 annealed |
| Corrosion resistance | Fair; pits at sulfide sites, rusts outdoors | ✓ Good in general atmospheres and foods |
| Finishing | Passivates with care; electropolishing can reveal sulfides | ✓ Passivates, electropolishes cleanly |
| Material cost | ✓ Similar per pound; cheaper per machined part | Baseline |
| Availability | Round bar in every size; little sheet | ✓ Every form |
Choose 303 when
- Screw-machine and CNC turned parts in volume
- Fittings, shafts, spacers and fasteners that are not welded
- Indoor or mild-service parts where 304 corrosion resistance is overkill
- Cycle time and tool life are the cost drivers
Choose 304 when
- Anything welded, brazed or formed
- Outdoor, marine-adjacent, food or medical service
- Sheet metal, tube and plate work
- Parts that will be electropolished or must stay bright
Frequently asked questions
- What is the difference between 303 and 304 stainless?
- Sulfur. 303 has a minimum of 0.15% sulfur added to make it free machining; 304 has a maximum of 0.03%. Strength and composition are otherwise nearly the same. The sulfur improves machining and hurts corrosion resistance, weldability and formability.
- Can 303 stainless be welded?
- It is strongly discouraged. The sulfur causes hot cracking in the weld and heat-affected zone. If a part must be welded, use 304 or 304L. Some shops weld 303 with 312 filler for non-critical joints, but it is a compromise.
- Is 303 as corrosion resistant as 304?
- No. Sulfide inclusions act as pit initiation sites, so 303 rusts and pits sooner, especially outdoors or in chlorides. It is fine for indoor, dry or oil-wetted service. Passivate it after machining.
- Is 303 magnetic?
- Slightly, after cold work, like 304. Both are austenitic and non-magnetic when annealed; cold-drawn bar of either grade can attract a magnet weakly.
