SUS 303 vs. SUS 304: What Is the Difference?

What is SUS303 stainless steel?

SUS303 (also known as AISI 303 or UNS S30300) is a free-machining austenitic stainless steel. Derived from the widely used SUS304 grade, this steel achieves improved machinability through the addition of sulfur, although its corrosion resistance and toughness are slightly reduced, making its corrosion resistance lower than that of SUS304. In the annealed condition, the material is completely non-magnetic; however, after cold working, it may exhibit slight magnetism. Commonly referred to as "automatic lathe grade stainless steel," it is suitable for nuts, bolts, and screws; shafts and gears; bushings and fittings; and electrical switchgear components.

 

What is SUS304 stainless steel?

SUS304 is the most widely used austenitic stainless steel globally. Commonly referred to as "18/8" stainless steel, it is completely equivalent to AISI 304 or SS304 (the standard grades in the United States). Characterized by a high nickel content, it exhibits a single-phase austenitic microstructure at room temperature, possesses excellent corrosion resistance, and is highly resistant to rusting. In its annealed state, it maintains an austenitic structure and is typically non-magnetic; only after cold working may it develop slight magnetism. It can be fabricated into a wide variety of practical product forms, including sheets, coils, tubes, bars, wires, and more.

 

What is the most difference between SUS303 and SUS304?

The core difference lies in their sulfur content. SUS303 contains a higher level of sulfur (≥ 0.15%) and is known as "free-machining stainless steel." SUS304 has an extremely low sulfur content (≤ 0.030%) and serves as the standard, general-purpose stainless steel.

 

What is the equivalent materials for SUS 303 stainless steel?

JIS (Japan)	AISI/ASTM (USA)	UNS (Global)	EN/DIN (Europe/Germany)	EN Name (Chemical)	GB (China)	ISO (International)	BS (UK)
SUS 303	303	S30300	1.4305	X8CrNiS18-9	06Cr18Ni9 (Y1Cr18Ni9)	303-00-I	303S31
SUS 304	304	S30400	1.4301	X5CrNi18-10	06Cr19Ni10 (0Cr18Ni9)	304-00-I	304S31

 

SUS 303 vs. SUS 304: Chemical Composition  (wt%)

Element	SUS 303 (Free-Machining)	SUS 304 (Standard)
Carbon (C)	≤ 0.15	≤ 0.08
Manganese (Mn)	≤ 2.00	≤ 2.00
Phosphorus (P)	≤ 0.20	≤ 0.045
Sulfur (S)	≥ 0.15	≤ 0.030
Silicon (Si)	≤ 1.00	≤ 1.00
Chromium (Cr)	17.0 – 19.0	18.0 – 20.0
Nickel (Ni)	8.0 – 10.0	8.0 – 10.5
Molybdenum (Mo)	(Optional) ≤ 0.60	Not Specified

 

SUS 303 vs. SUS 304:  Mechanical Properties (Annealed Condition)

Property	SUS 303	SUS 304
Tensile Strength (min)	520 MPa	520 MPa
Yield Strength (0.2% Offset)	≥ 205 MPa	≥ 205 MPa
Elongation (in 50mm, min)	≥ 40%	≥ 40%
Hardness (Brinell - HB)	≤ 187	≤ 187
Machinability Rating	75 - 80% (Excellent)	45% (Fair)

 

SUS 303 vs. SUS 304: Machinability

Due to the addition of sulfur (typically not less than 0.15%), SUS303 offers significantly better machinability than SUS304. Using B1112 free-cutting steel as the benchmark (rated at 100%), SUS303 generally achieves a machinability rating of approximately 78%, while SUS304 is typically rated between 45% and 55%. For this reason, SUS303 is widely used in CNC machining, automatic lathe operations, and the production of precision fasteners and fittings.

 

SUS 303 vs. SUS 304: Corrosion Resistance

SUS304 provides superior corrosion resistance in humid, chloride-containing, or mildly acidic environments, making it highly suitable for food processing equipment, medical instruments, and outdoor architectural structures. In contrast, the sulfur inclusions present in SUS303 can become initiation points for pitting corrosion, especially in marine environments or applications involving exposure to chemical media. As a result, the overall corrosion resistance of SUS303 is lower than that of SUS304.

SUS 303 vs. SUS 304: Weldability

SUS304 exhibits excellent weldability and can be welded using various processes such as TIG (Tungsten Inert Gas), MIG (Metal Inert Gas), spot welding, and arc welding, with a very low risk of hot cracking. However, SUS303 has relatively poor weldability because the sulfur added to improve machinability increases the likelihood of weld cracking and reduces weld ductility.

 

SUS 303 vs. SUS 304: Cold Formability

SUS304 demonstrates outstanding cold formability and is highly suitable for deep drawing, bending, stamping, and other severe forming operations. In comparison, SUS303 has relatively poor cold forming performance. Therefore, SUS303 is mainly used for components manufactured through machining rather than heavy forming processes.

 

SUS 303 vs. SUS 304:  Application

Grade	Best Used	Typical Products
SUS 303	Applications requiring extensive machining or complex parts where corrosion risk is moderate.	Nuts, bolts, bushings, shafts, aircraft fittings, gears, and electrical switchgear components.
SUS 304	Applications requiring welding, high corrosion resistance, or food-grade hygiene.	Food processing equipment, kitchen appliances, chemical containers, architectural panels, and medical tubing.

 

 

If you have project-specific requirements for SUS304 (1.4301) stainless steel, we welcome your orders. GNEE maintains a substantial inventory of best-selling products for you to choose from. This material can be processed into a wide variety of practical product forms-including plates, coils, tubes, bars, wires, and more. For detailed chemical composition specifications or to request free samples, please contact our factory immediately. We offer competitive pricing and exceptional service.

Packaging and Shipping
Packaging: Each plate or coil is wrapped in moisture-proof paper, lined with plastic film, and secured with steel strapping. The outer layer is protected by wooden pallets or wooden crates to ensure safety during international transit.
Labeling: Each package includes information regarding the heat number, grade, dimensions, and applicable standards.
Delivery Time: Typically within 7–15 business days after receipt of the advance payment.
Shipping Methods: Sea freight (FCL/LCL), air freight, or express delivery, depending on customer requirements.