SUS304 vs SUS304L: Carbon Content, Strength and Weldability

SUS304 and SUS304L in the JIS System

SUS is the material prefix used by the Japanese Industrial Standards for stainless steel. SUS304 is the JIS designation for the standard 18-8 austenitic grade that the market calls 304, and SUS304L is the low-carbon variant. They correspond to ASTM and AISI Type 304 and Type 304L, and to the European grades 1.4301 and 1.4307 respectively.

Depending on the product form, SUS304 and SUS304L are ordered to JIS G4303 for bars, JIS G4304 for hot rolled plate, JIS G4305 for cold rolled sheet and coil, or JIS G3459 for pipes; the equivalent American order would quote ASTM A276, A240 or A312. SUS304 is one of the most widely traded stainless grades in the world, and it appears in everyday consumer goods as well as in demanding industrial plants because of its combination of corrosion resistance, heat resistance and strength.

The One Intentional Difference: Carbon Content

The two grades share the same chromium and nickel basis; chromium is 18.0 to 20.0% in both, with the nickel range set by the applicable product standard, which is slightly wider for the low-carbon grade. The deliberate difference is carbon. SUS304 is limited to 0.08% maximum carbon, while SUS304L is limited to 0.030% maximum, and that is what the letter L stands for: low carbon.

Item	SUS304	SUS304L
Carbon content	0.08% max	0.030% max
Chromium	18.0 - 20.0%	18.0 - 20.0%
Nickel	Per product standard	Per product standard, slightly wider range
Intergranular corrosion after welding	Higher risk in heavy sections	Strongly reduced risk
Minimum yield strength	205 MPa	175 MPa
Minimum tensile strength	520 MPa	480 MPa

The low carbon level is also reported to improve oxidation resistance at elevated temperature, because less carbon is available to form carbides that degrade the protective oxide layer.

Mechanical Properties Compared

The table below compares typical annealed mechanical property requirements for the two grades.

Grade	Yield strength (MPa, N/mm2)	Tensile strength (MPa, N/mm2)	HBW	HRB	HV
SUS304	205	520	187 or less	90 or less	200 or less
SUS304L	175	480	187 or less	90 or less	200 or less

Because carbon is a strengthening element in austenitic stainless steel, removing it costs strength. SUS304L is therefore the lower-strength grade, with a minimum yield strength of 175 MPa against 205 MPa for SUS304 and a minimum tensile strength of 480 MPa against 520 MPa. For components carrying high pressure, heavy impact or high mechanical load, SUS304 is the better choice, while SUS304L is preferred wherever corrosion resistance after welding dominates the design.

Sensitization, Welding and Intergranular Corrosion

Sensitization is the reason the low-carbon grade exists. When austenitic stainless steel is held in the carbide precipitation range, commonly quoted as roughly 450 °C to 850 °C, carbon combines with chromium at the grain boundaries. Chromium is drawn out of the adjacent metal, chromium-depleted zones form, and in an aggressive medium those zones corrode preferentially, producing intergranular corrosion.

SUS304L suppresses this mechanism simply by keeping carbon at 0.030% maximum, so that far less chromium carbide can form. This is why SUS304L is the preferred grade for welded vessels, tanks and pipework, particularly in heavy sections where the weld thermal cycle holds a wide band of material in the sensitizing range and post-weld solution annealing is impractical.

Welding consumables: 308L filler metal is used for both grades, matching the low-carbon deposit.

Stabilized alternatives: where service temperature is high, 321 with titanium or 347 with niobium resists sensitization by tying up carbon as stable carbides.

Post-weld treatment: solution annealing at approximately 1040 °C to 1120 °C followed by rapid cooling restores corrosion resistance, but is limited by component size and distortion risk.

Applications and Selection Guidance

SUS304 is chosen for its all-round balance of properties and its wide availability: kitchen equipment and sinks, domestic appliances, architectural trim, food and dairy processing equipment, and general fabrication. SUS304L is chosen for chemical process vessels, storage tanks, welded pipework, pulp and paper plant, and food processing lines where welds are present and corrosion resistance must be maintained in the heat-affected zone.

In practice the two grades are often interchangeable. If a component is welded and cannot be annealed afterwards, or if the wall is thick, or if the service medium is aggressive, SUS304L is the safer specification. If the component is not welded, or if strength and cost are the dominant factors, SUS304 is sufficient. Because the low-carbon grade is produced in smaller quantities, availability and price should also be checked before it is written into a specification.

Frequently Asked Questions

Q: What does the L in SUS304L mean?
The L stands for low carbon. SUS304L is the low-carbon version of SUS304, with a maximum carbon content of 0.030% instead of 0.08%.

Q: What is the carbon content difference between SUS304L and SUS304?
SUS304L is limited to 0.030% maximum carbon, while SUS304 is limited to 0.08% maximum. The lower carbon level is the only intentional difference between the two grades.

Q: Is SUS304L weaker than SUS304?
Yes, slightly. SUS304L has a minimum yield strength of 175 MPa and a minimum tensile strength of 480 MPa, against 205 MPa and 520 MPa for SUS304, while the hardness limits are the same.

Q: Why is SUS304L better for welding?
Its very low carbon content prevents the formation of chromium carbides during the weld thermal cycle, so the heat-affected zone avoids chromium depletion and resists intergranular corrosion far better than SUS304.

Q: What is sensitization in stainless steel?
Sensitization is the precipitation of chromium carbides at grain boundaries when the steel is held at roughly 450 °C to 850 °C, which depletes chromium locally and reduces corrosion resistance.

Q: Should SUS304L always be selected instead of SUS304?
No. SUS304L is unnecessary when parts are not welded or when post-weld annealing is carried out, and its lower strength and more limited availability should be weighed against its welding benefits.