Stainless Steel 303 Grade
Stainless Steel 303 Grade is an austenitic chromium-nickel stainless steel with added sulphur (0.15% min) to make it the most free-machining grade in the 300 series. The sulphur forms manganese sulphide inclusions that act as chip-breakers, giving vastly superior machinability compared with Grade 304, at the cost of somewhat reduced corrosion resistance, weldability, and cold formability. It is the preferred choice for high-volume precision-machined and screw-machine components across fittings, fasteners, and general engineering industries.
International Equivalent Designations – Stainless Steel 303 Grade
Stainless Steel 303 Grade is known by several designations depending on the country and standard. The table below lists all major international equivalents:
| Standard / Country | Designation | Description |
|---|---|---|
| AISI / ASTM (USA) | AISI 303 / SS 303 | American Iron and Steel Institute designation |
| UNS (USA) | UNS S30300 | Unified Numbering System |
| EN / DIN (Europe) | 1.4305 | European material number (Werkstoff-Nr.) |
| EN Short Name (Europe) | X8CrNiS18-9 | European chemical composition designation |
| JIS (Japan) | SUS 303 | Japanese Industrial Standard |
| BS (United Kingdom) | 303 S21 | British Standard designation |
| GB (China) | Y1Cr18Ni9 | Chinese national standard |
| ISO | X8CrNiS18-9 | International Organization for Standardization |
| AFNOR (France) | Z8CNF18-09 | French standard designation |
| SS (Sweden) | 2346 | Swedish standard designation |
Chemical Composition (Weight %)
| Element | Min % | Max % |
|---|---|---|
| Carbon (C) | – | 0.15 |
| Manganese (Mn) | – | 2.00 |
| Silicon (Si) | – | 1.00 |
| Phosphorus (P) | – | 0.20 |
| Sulphur (S) | 0.15 | 0.30 |
| Chromium (Cr) | 17.00 | 19.00 |
| Nickel (Ni) | 8.00 | 10.00 |
| Molybdenum (Mo) | – | 0.60 (optional) |
| Iron (Fe) | Balance | |
Mechanical Properties (Annealed Condition – Room Temperature)
| Property | Value | Unit |
|---|---|---|
| Tensile Strength (UTS) | 515 min | MPa (75,000 psi) |
| Yield Strength (0.2% Proof) | 205 min | MPa (30,000 psi) |
| Elongation (in 50 mm) | 50 min | % |
| Hardness – Brinell | 160 max | HB |
| Hardness – Rockwell B | 85 max | HRB |
| Hardness – Vickers | 170 max | HV |
| Elastic Modulus | 193 | GPa |
| Poisson’s Ratio | 0.27 – 0.30 | – |
Physical Properties
| Property | Value | Unit |
|---|---|---|
| Density | 8.0 | g/cm³ |
| Melting Range | 1400 – 1420 | °C (2550 – 2590 °F) |
| Specific Heat Capacity (0–100°C) | 500 | J/kg·K |
| Thermal Conductivity (at 100°C) | 16.2 | W/m·K |
| Thermal Expansion Coefficient (0–100°C) | 17.2 × 10⁻⁶ | /°C |
| Electrical Resistivity (at 20°C) | 72 | µΩ·cm |
| Magnetic Permeability | 1.008 max | Non-magnetic (annealed) |
Machinability Comparison
The defining characteristic of Grade 303 is its exceptional machinability, achieved through controlled sulphide inclusions. The table below compares relative machinability against other common austenitic grades:
| Grade | Machinability Rating (% of AISI B1112) | Notes |
|---|---|---|
| 303 | ~78% | Benchmark free-machining stainless grade |
| 303 Se | ~85% | Selenium-bearing, best surface finish, ideal for polished parts |
| 304 | ~45% | Standard austenitic, roughly half the machinability of 303 |
| 316 | ~36% | Molybdenum bearing, lower machinability, higher corrosion resistance |
Corrosion Resistance
Stainless Steel 303 Grade offers moderate corrosion resistance, lower than Grade 304 or 316, due to the manganese sulphide inclusions that act as initiation sites for pitting and crevice corrosion:
- General Corrosion: Adequate resistance in mild atmospheric, indoor, and many chemical environments, but inferior to non-free-machining grades.
- Pitting and Crevice Corrosion: Sulphide stringers reduce resistance significantly; not recommended for chloride-rich or marine environments.
- Marine and Outdoor Exposure: Should be avoided; Grade 304 or 316 are preferred for such conditions.
- Intergranular Corrosion: Can be sensitised if heated in the 425–860°C range; avoid prolonged exposure in this band.
Heat Treatment
- Annealing: Heat to 1010–1120°C and cool rapidly by water quenching or rapid air cooling to restore maximum corrosion resistance and relieve stresses.
- Hardening by Heat Treatment: Not possible. Cold working is the only method available to increase strength and hardness.
- Stress Relieving: Not generally recommended due to sensitisation risk; if required, cool rapidly through the 425–860°C range.
Fabrication and Weldability
- Machinability: Excellent – approximately 78% of AISI B1112, the best of any austenitic stainless grade. Permits high cutting speeds, excellent surface finish, and long tool life.
- Formability: Poor to moderate – sulphide inclusions reduce ductility in severe cold-forming operations such as deep drawing; not recommended for heavy forming applications.
- Weldability: Poor – sulphur content promotes hot cracking (hot shortness). Welding is generally not recommended; mechanical fastening or brazing is preferred. If welding is unavoidable, use low heat input and low-sulphur filler metal, and expect reduced joint properties.
- Recommended Filler Metal (if welding is unavoidable): ER312 or ER308, with awareness of increased crack risk near the fusion zone.
Related Sub-Grades of Stainless Steel 303 Grade
| Grade | AISI / UNS | EN / DIN | JIS | Key Difference |
|---|---|---|---|---|
| 303 Se | UNS S30323 | 1.4305 (Se variant) | SUS303Se | Selenium-bearing – superior surface finish, better hot workability, used for cold-headed and polished parts |
| 303 Cu | – | – | – | Copper-added modification for further improved machinability in cold-heading operations |
| 304 | AISI 304 / S30400 | 1.4301 | SUS 304 | Standard austenitic grade – better corrosion resistance and weldability, lower machinability |
| 304L | AISI 304L / S30403 | 1.4307 | SUS 304L | Low carbon version of 304 – improved weldability, no free-machining additive |
| 316 | AISI 316 / S31600 | 1.4401 | SUS 316 | Molybdenum bearing – superior corrosion resistance for demanding environments |
Applicable Standards & Specifications
| Standard | Scope |
|---|---|
| ASTM A582 / A582M | Free-machining stainless steel bars, hot-rolled and cold-finished |
| ASTM A314 | Stainless steel billets and bars for forging |
| ASTM A276 / A276M | Stainless steel bars and shapes |
| AMS 5640 | Bars, wire and forgings – Condition A (303) |
| AMS 5641 | Bars, wire and forgings – annealed (303) |
| EN 10088-3 | Stainless steels – semi-finished products, bars, rods, wire (1.4305) |
| JIS G4303 | Stainless steel bars – SUS 303 |
| ISO 683-13 | Heat-treatable steels, alloy steels and free-cutting steels – stainless steels |
Typical Applications
Stainless Steel 303 Grade / 1.4305 / SUS 303 is used wherever high-volume, precision-machined components are required and mild-to-moderate corrosion resistance is acceptable:
- Fasteners: Screws, bolts, nuts, studs, and washers produced on screw machines and CNC lathes
- Precision Components: Bushings, gears, shafts, spacers, and pins requiring tight tolerances and fine surface finish
- Fittings & Valve Parts: Threaded fittings, valve stems, valve bodies, and pump components requiring extensive machining
- Electrical & Electronics: Connectors, terminals, and small precision hardware
- Aerospace (Non-Welded): Non-critical, non-welded fittings and machined hardware
- General Engineering: Textile machinery components, nozzles, and instrumentation hardware
Supply Forms – Stainless Steel Fittings India (SSF)
At Stainless Steel Fittings India (SSF), we manufacture and export Stainless Steel 303 Grade / AISI 303 / 1.4305 / SUS 303 components in the following forms:
- Precision Machined Components
- Threaded Fittings (NPT, BSPT, BSPP)
- Fasteners (Bolts, Nuts, Washers, Studs, Screws)
- Instrumentation & Valve Fittings
- CNC Turned Parts & Custom Components
- Investment Castings
For technical queries, RFQs, or custom requirements in Stainless Steel 303 Grade (AISI 303 / SUS 303 / 1.4305 / X8CrNiS18-9 / UNS S30300), contact us today. Our engineering team is ready to assist with material certifications, mill test reports, and export documentation.