SUS304 vs SS304 Stainless Steel: Composition, Properties and Interchangeability

SUS304 and SS304: one alloy, two standard systems

SUS304 is the Japanese Industrial Standard designation, defined for bar in JIS G4303, for plate and sheet in JIS G4304 and G4305, and for pipe in JIS G3459. SS304 as used in trade means ASTM Type 304, registered as UNS S30400 and standardised in Europe as EN 1.4301 / X5CrNi18-10. Both describe the same 18/8 austenitic family: nominally 18% chromium and 8% nickel in a face-centred cubic structure that cannot be hardened by heat treatment.

That is why SUS304 is often called the parent grade of the 300 series. Once you add molybdenum you get 316, once you add titanium or niobium you get a stabilised grade, and once you cut the carbon you get the L versions. The base chemistry and the welding, forming and corrosion behaviour stay recognisable in all of them.

Chemical composition compared

Element	JIS SUS304	ASTM A240 Type 304
Carbon (C)	≤ 0.08%	≤ 0.07%
Silicon (Si)	≤ 1.00%	≤ 0.75%
Manganese (Mn)	≤ 2.00%	≤ 2.00%
Phosphorus (P)	≤ 0.045%	≤ 0.045%
Sulfur (S)	≤ 0.030%	≤ 0.030%
Chromium (Cr)	18.00–20.00%	17.50–19.50%
Nickel (Ni)	8.00–10.50%	8.00–10.50%

The composition windows are almost identical, but the ranges are not interchangeable on paper. JIS allows a slightly wider chromium band and more silicon, while ASTM tightens carbon to 0.07% and silicon to 0.75%. A heat that sits at 20.00% Cr or 1.00% Si is a valid SUS304 but falls outside the ASTM Type 304 window, so it cannot be dual-certified without a chemistry adjustment. Buyers who need both certificates should specify the overlapping band: Cr 18.00–19.50%, C ≤ 0.07%, Si ≤ 0.75%.

Physical and mechanical properties

Property	Typical value for 304
Density at 20 °C	7.93 g/cm³
Melting range	1,398–1,454 °C
Elastic modulus	193 GPa
Specific heat at 20 °C	500 J/(kg·K)
Thermal conductivity at 100 °C	16.2 W/(m·K)
Electrical resistivity at 20 °C	0.72 µΩ·m
Mean thermal expansion, 0–100 °C	17.3 × 10⁻⁶ /K
Relative magnetic permeability, annealed	About 1.02 (effectively non-magnetic)

Mechanical minimums – ASTM A312 pipe requires 515 MPa tensile and 205 MPa yield; JIS G3459 SUS304 pipe requires 520 MPa tensile and 205 MPa yield.

Hardening – 304 hardens only by cold work; heat treatment is limited to solution annealing at 1,010–1,120 °C followed by rapid cooling.

Low temperature – the austenitic structure keeps useful toughness well below zero; 304 is widely used for cryogenic service to −196 °C.

High temperature – oxidation resistance is good to about 870 °C in intermittent service, but continuous service in the 425–815 °C band should use 304H or a stabilised grade.

Corrosion behaviour

304 performs well in rural, urban and most industrial atmospheres and in many organic and oxidising media. In warm chloride environments above roughly 60 °C it becomes vulnerable to pitting, crevice corrosion and stress corrosion cracking, and stagnant chloride pockets are the usual failure point. In potable water, 304 is commonly accepted up to about 200 mg/L chloride at ambient temperature. Its pitting resistance equivalent number is about 18 (Cr + 3.3 Mo + 16 N), which is the reason 316L, with roughly 2–3% molybdenum, is specified wherever chlorides are present.

Grade equivalence and interchangeability

Grade	UNS	EN / Werkstoff	JIS	Old BS	Product form
304	S30400	1.4301 / X5CrNi18-10	SUS304	304S31	General purpose
304L	S30403	1.4306 / X2CrNi19-11	SUS304L	304S11	Low carbon, heavy-section welds
304H	S30409	1.4948 / X6CrNi18-11	SUS304H	304S51	High carbon, elevated temperature

The older tables that list 1.4301 against a Swedish SS number and a separate "304S31" column refer to withdrawn national systems. For new projects, quote UNS plus EN, and add JIS only when the end user is in Japan or specifies SUS marking.

Interchangeability in service versus in purchasing

In service, almost always yes: corrosion resistance, weldability, formability and strength are equivalent within the composition windows. In purchasing, no. The dimensional standards differ, so a SUS304 pipe to JIS G3459 and an SS304 pipe to ASTM A312 will not necessarily share the same wall tolerance, outside-diameter tolerance, length tolerance or marking requirements. Pressure equipment also ties the design allowable stress and the material test certificate to a named specification. The practical rule is to buy the standard that the design code references and to accept the other grade only through a documented substitution approved by the engineer.

For a dual-certified order, state the tighter of the two chemistry windows, name both product standards, and require the mill certificate to show heat number traceability, mechanical test results and, for pressure service, the applicable impact or flattening test.

FAQ

Q: Is SUS304 the same as 304 stainless steel?
Chemically they are the same family and in most applications they perform identically. SUS304 is the JIS designation and 304 is the ASTM and EN designation, so the difference sits in the specification and the certificate rather than in the metal.

Q: Which is better, SUS304 or SS304?
Neither is stronger or more corrosion resistant in general. Choose SUS304 when the drawing or the customer references JIS, and SS304 when the project follows ASTM or EN. Only the specification system decides which one belongs on the purchase order.

Q: Can 304 be used in seawater?
Not for continuous immersion. Chloride levels in seawater cause pitting and crevice attack above about 60 °C, and even at ambient temperature stagnant seawater attacks 304. Use 316L, a duplex grade or a higher-molybdenum alloy for marine service.

Q: Why does 304 sometimes show slight magnetism?
Fully annealed 304 has a relative permeability close to 1.02, so it is effectively non-magnetic. Cold working, welding or bending forms a little strain-induced martensite and the affected area can respond to a magnet without any change in grade.

Q: Do 304, 304L and 304H differ in corrosion resistance?
At room temperature the three are close. The difference appears after welding or in high-temperature service: 304L avoids chromium carbide precipitation at the grain boundary, and 304H keeps higher creep strength for continuous service above about 500 °C.

Q: What heat treatment is used for 304?
Solution annealing at 1,010–1,120 °C followed by rapid cooling to keep carbon in solution. Stress relief by slow cooling in the sensitisation range of 425–815 °C must be avoided, because it precipitates chromium carbides and reduces corrosion resistance.