303 Free-Machining Stainless Steel (UNS S30300)
304 with sulfur added for fast, clean machining — the grade for complex turned fasteners and fittings where corrosion resistance can give a little.
What 303 Stainless is.
Type 303 is 18-8 stainless with a minimum of 0.15 percent sulfur. The sulfur forms manganese sulfide stringers that break chips and let the alloy be turned, drilled and tapped at production rates well above 304. The same inclusions reduce pitting resistance and make the alloy unsuitable for welding, so 303 is specified for machined parts in mild environments rather than pressure bolting.
We run 303 on the Swiss and CNC lathes for fittings, plugs, pins, set screws and instrument fasteners, from certified A582 bar. Under ASTM F593 it sits in Alloy Group 1 with 304, and under A320 it is Grade B8F.
Chemical composition.
| Element | Symbol | Min % | Max % |
|---|---|---|---|
| Carbon | C | — | 0.15 |
| Manganese | Mn | — | 2.00 |
| Phosphorus | P | — | 0.20 |
| Sulfur | S | 0.15 | — |
| Silicon | Si | — | 1.00 |
| Chromium | Cr | 17.0 | 19.0 |
| Nickel | Ni | 8.0 | 10.0 |
Values per ASTM A582/A582M-22 Tables 1 and 3 (verified against the specification); free-machining grades are covered by A582, not A276. Confirm the edition called out on your purchase order with our estimating team.
Mechanical properties.
| Condition and nominal diameter (in.) | Tensile strength, min (ksi) | Yield strength 0.2% offset, min (ksi) | Elongation, min (%) | Reduction of area, min (%) | Hardness |
|---|---|---|---|---|---|
| Condition A (annealed) — all | — | — | — | — | 262 HBW max |
Values per ASTM A582/A582M-22 Tables 1 and 3 (verified against the specification); free-machining grades are covered by A582, not A276. Confirm the edition called out on your purchase order with our estimating team. ASTM A582 controls free-machining bar by hardness only; the F593 table below gives the fastener strengths.
| F593 condition · marking | Nominal diameter (in.) | Tensile strength (ksi) | Yield strength, min (ksi) | Rockwell hardness |
|---|---|---|---|---|
| AF (annealed after forming) · F593A | ¼ to 1½ | 65 to 85 | 20 | B85 max |
| A (annealed) · F593B | ¼ to 1½ | 75 to 100 | 30 | B65 to B95 |
| CW1 (cold worked) · F593C | ¼ to ⅝ | 100 to 150 | 65 | B95 to C32 |
| CW2 (cold worked) · F593D | ¾ to 1½ | 85 to 140 | 45 | B80 to C32 |
| SH1 (strain hardened) · F593A | ¼ to ⅝ | 120 to 160 | 95 | C24 to C36 |
| SH2 (strain hardened) · F593B | ¾ to 1 | 110 to 150 | 75 | C20 to C32 |
| SH3 (strain hardened) · F593C | 1⅛ to 1¼ | 100 to 140 | 60 | B95 to C30 |
| SH4 (strain hardened) · F593D | 1⅜ to 1½ | 95 to 130 | 45 | B90 to C28 |
Values per ASTM F593-24 Table 2, full-size tests (verified against the specification). Yield is 0.2 % offset; hardness is Rockwell. Machined-specimen limits differ slightly.
Specifications that apply.
How 303 Stainless ships from Houston.
- Hex and heavy hex bolts and cap screws
- Full-thread studs and threaded rod
- Socket head cap and set screws
- Hex, jam and lock nuts
- Washers and spacers
- Machined components from certified bar
303 Stainless fasteners we make.
Every item is manufactured to your drawing or standard in 303 Stainless, imperial or metric, with an EN 10204 3.1 certificate.
What is 303 stainless steel?
303 is the free-machining member of the 18-8 family. Sulfur, held to a minimum of 0.15 percent, forms soft manganese sulfide particles through the steel that act as chip breakers and internal lubricant, so tool life and surface finish improve and cycle times fall. The sulfides also interrupt the passive film, so 303 pits more readily than 304 in chlorides, and they cause hot cracking in welds. The selenium version, 303Se, gives a better surface finish and is used for cold-headed parts.
Where 303 belongs, and where it does not
- Yes: complex turned parts, fittings, valve trim, instrument hardware and anything made in quantity on a Swiss lathe.
- Yes: indoor, fresh-water and mild-chemical service.
- No: welded assemblies, seawater and brine, and pressure-boundary bolting, where 304 or 316 should be used.
- ASTM A582 controls 303 bar by hardness; the fastener strength comes from the F593 condition ordered.
Testing, certification and marking
- Tensile and hardness tested per lot on the finished product
- Grade or marking symbol F593A to F593D and manufacturer's identification on every piece
- EN 10204 3.1 material test report with heat number and chemistry; 3.2 witness available
- PMI verification and intergranular corrosion testing (ASTM A262) arranged on request
304 Stainless Steel (UNS S30400)
The general-purpose 18-8 stainless: 75 ksi annealed bar, cold-worked bolts to 160 ksi, non-magnetic, and resistant to water, food and most mild chemicals.
ASTM A320 / ASME SA-320ASTM A320 Grade B8F (303 Free-Machining Stainless)
303 free-machining stainless bolting certified to A320 — sulfur or selenium for fast, clean machining of complex cold-service fasteners, impact exempt above −325 °F.
ASTM A582 bar · ASTM F593 bolts416 Free-Machining Stainless Steel (UNS S41600)
410 with sulfur added — the most machinable of all stainless steels, heat treatable to 190 ksi bolts, for turned pins, shafts, fittings and set screws.