Introduction
SUS303 stainless steel is a free-machining austenitic stainless steel offering excellent corrosion resistance combined with outstanding machinability. Often described as the "machinable version of SUS304", it is the go-to material for automatic lathe and CNC turning operations where high-speed, high-precision machining is required.
SUS303 is primarily composed of chromium (Cr), nickel (Ni), with controlled additions of sulfur (S) and phosphorus (P) to enhance chip-breaking and cutting performance. It belongs to the 18-8 austenitic stainless steel family and delivers reliable performance in corrosive environments, particularly in acidic conditions.
In general usage, 303 refers to the ASTM A959 designation, while SUS303 refers to the JIS G4303 designation. Since their chemical compositions are essentially identical, the two designations are used interchangeably in most industrial and commercial contexts. This page uses JIS G4303 as the primary reference.
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SUS303 & SUS303Cu Chemical Composition
Reference standard: JIS G4303
| Grade | C (%) | Mn (%) | Si (%) | P (%) | S (%) | Ni (%) | Mo (%) | Cr (%) | Cu (%) |
|---|---|---|---|---|---|---|---|---|---|
| SUS303 | ≤ 0.15 | ≤ 2.00 | ≤ 1.00 | ≤ 0.20 | ≥ 0.15 | 8.00–10.00 | ≤ 0.60 (opt.) | 17.00–19.00 | — |
| SUS303Cu | ≤ 0.15 | ≤ 3.00 | ≤ 1.00 | ≤ 0.20 | ≥ 0.15 | 8.00–10.00 | ≤ 0.60 (opt.) | 17.00–19.00 | 1.50–3.50 |
Notes:
- Sulfur (S) content is typically controlled within 0.15–0.35% in practice.
- SUS303Cu contains 1.5–3.5% copper (Cu), making it even more suitable for high-speed automatic machining than standard SUS303.
Physical and Mechanical Properties
Mechanical Properties (JIS G4303)
| Grade | Tensile Strength σb (MPa) | Yield Strength σ0.2 (MPa) | Elongation δ5 (%) | Reduction of Area ψ (%) | Hardness | Density (g/cm³) | Melting Point (°C) |
|---|---|---|---|---|---|---|---|
| SUS303 | ≥ 520 | ≥ 205 | ≥ 40 | ≥ 50 | ≤ 187 HB / ≤ 90 HRB / ≤ 200 HV | 7.93 | 1398–1420 |
Other Physical Properties
| Property | Value |
|---|---|
| Density | 8.03 g/cm³ |
| Melting Point | 1398–1420 °C |
| Thermal Conductivity | 16.3 W/(m·K) |
| Specific Heat Capacity | 0.50 J/(g·K) |
| Electrical Resistivity | 720 nΩ·m |
| Thermal Expansion Coefficient | 17.3 µm/(m·K) |
| Elastic Modulus | 193 GPa |
| Magnetic Permeability | Non-magnetic when annealed; may become slightly magnetic after cold working |
Applications of SUS303 Stainless Steel
SUS303 is a versatile, high-performance stainless steel widely used across demanding industries:
- Precision machined parts — shafts, bushings, fittings, and connectors produced on automatic lathes
- Fasteners — precision nuts, bolts, screws, and studs requiring tight tolerances
- Automotive components — fuel system parts, sensor housings, valve bodies
- Aerospace components — connectors, brackets, and structural fittings
- Electronics & instrumentation — terminal blocks, sensor bodies, precision housings
- Chemical processing equipment — fittings and connectors in mildly corrosive environments
Browse our Stainless Steel Helical Inserts, Stainless Steel Wood Screws, and Castle Nuts (DIN 935) – Stainless Steel A2 for related stainless steel fastener products.
Machinability and Fabrication
Machinability
The primary advantage of SUS303 is its exceptional machinability. The addition of sulfur (S) and phosphorus (P) promotes chip breaking and reduces tool wear, enabling high-speed cutting operations with excellent surface finish. It is the preferred grade for automatic screw machines and CNC turning centers.
Weldability
SUS303 has limited weldability due to its sulfur content, which can cause hot cracking in the weld zone. It is not recommended for applications requiring welding. For weldable applications, SUS304 is the appropriate choice.
Formability
SUS303 can be formed using bending, stamping, roll forming, spinning, extrusion, and forging. However, its formability is noticeably inferior to SUS304, and it is not suitable for deep drawing or complex cold-forming operations.
SUS303 vs SUS304: Key Differences
Chemical Composition Comparison
| Grade | C (%) | Mn (%) | Si (%) | P (%) | S (%) | Ni (%) | Mo (%) | Cr (%) |
|---|---|---|---|---|---|---|---|---|
| SUS304 | ≤ 0.08 | ≤ 2.00 | ≤ 1.00 | ≤ 0.045 | ≤ 0.03 | 8.00–11.00 | — | 17.00–19.00 |
| SUS303 | ≤ 0.15 | ≤ 2.00 | ≤ 1.00 | ≤ 0.20 | ≥ 0.15 | 8.00–10.00 | ≤ 0.60 (opt.) | 17.00–19.00 |
Performance Comparison
- Corrosion resistance: SUS304 performs slightly better than SUS303. However, with sufficient passivation time, both grades show significantly improved salt spray test results — enough to largely close the gap between them.
- Mechanical properties: SUS303 has lower hardness and strength compared to SUS304. SUS304 is preferred for high-strength, high-corrosion-resistance engineering applications.
- Machinability: SUS303 is far superior for CNC turning, automatic lathe work, and precision cutting. SUS304, while less machinable, can achieve higher strength and hardness through cold working and heat treatment — making it the standard choice for cold-headed fasteners such as screws and stamped parts.
- Weldability: SUS304 is significantly better than SUS303 for welded assemblies.
In summary: choose SUS303 when machinability is the priority; choose SUS304 when weldability, formability, or maximum corrosion resistance is required.
For a related material guide, see our article on C3604 Free-Cutting Brass Chemical Composition and Standards — another popular material for automatic lathe machining.
Standards reference: JIS G4303 | ASTM A959 | GB/T 1220 | GB/T 20878
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