SUS304H High-Carbon Stainless Steel

What Is SUS304H?

SUS304H is a high-carbon variant of SUS304 defined in the JIS system, with carbon specified at 0.04-0.10% instead of the 0.08% maximum of SUS304. The controlled carbon content raises high-temperature strength and creep resistance, while the general corrosion resistance of the 18-8 family is retained. The grade corresponds to ASTM 304H, UNS S30409, and is used for load-bearing components in power generation and petrochemical plants that run continuously at elevated temperature.

Chemical Composition per JIS G4303 / G4305

Element (wt%)	SUS304H
C	0.04-0.10
Si	1.00 max
Mn	2.00 max
P	0.045 max
S	0.030 max
Cr	18.0-20.0
Ni	8.0-10.5

Composition limits follow JIS G4303 (bar) and G4305 (cold-rolled sheet and strip) practice and match the ASTM A240 S30409 limits. The chromium and nickel ranges are identical to SUS304; only the carbon specification differs.

Mechanical Properties

In the annealed condition the grade shows minimum tensile strength 515 MPa, minimum yield strength 205 MPa, elongation 40% minimum and typical hardness up to about 201 HB, consistent with both JIS G4305 data and ASTM A240 S30409 values. The property that matters most in service, however, is not the room-temperature tensile strength but the creep and rupture strength at 800-900 C, where the carbide-pinned microstructure clearly outperforms SUS304.

High-Temperature Performance

At temperatures above roughly 600 C, the fine chromium carbides formed by the elevated carbon act as pinning points that restrict grain-boundary movement and slow creep deformation. This is what allows SUS304H to serve in continuous 800-900 C duty in boiler superheaters and steam turbine components, where SUS304 would deform and fail far sooner. The grade also shows good thermal stability under prolonged exposure, which is why it is specified for long-service components in power generation.

Heat Treatment and Welding Considerations

The recommended heat treatment is solution annealing at 1010-1120 C followed by rapid water quenching, which dissolves carbides into the austenitic matrix and restores a uniform microstructure. Welding is the weak point of the grade: the heat-affected zone passes through the 425-815 C sensitization range, chromium carbides precipitate at grain boundaries, and intergranular corrosion resistance is lost. For welded components, post-weld solution annealing is required but is often impractical for large boiler assemblies. For welded high-temperature service, stabilized grades such as SUS321H or SUS347H are therefore the preferred choice.

Applications and Grade Equivalents

Typical applications are boiler superheater tubes, steam turbine components, industrial furnace parts and petrochemical reactor tubes. The correct equivalents are ASTM 304H (UNS S30409); in the EN system the closest heat-resisting analogue is X6CrNi18-11 (EN 1.4948). Note that EN 1.4307 (X2CrNi18-9) is a low-carbon 304L-type grade and must not be used as an equivalent of SUS304H.

FAQ

How does the high carbon content of SUS304H improve high-temperature performance?

Above about 600 C the elevated carbon forms fine chromium carbides that pin grain boundaries, restricting creep deformation under sustained stress. SUS304, with less carbon, forms fewer carbides and its grain boundaries slide more easily, so it creeps faster at 800-900 C.

What is creep resistance and why does it matter here?

Creep resistance is the ability to resist slow permanent deformation under constant stress at high temperature. For boiler superheater tubes and turbine parts it determines service life: good creep resistance keeps the component dimensionally stable for tens of thousands of hours instead of thousands.

Is SUS304H suitable for welded applications?

Not ideally. The high carbon promotes chromium carbide precipitation in the heat-affected zone during welding, causing sensitization and intergranular corrosion risk. Post-weld solution annealing at 1010-1120 C with water quenching restores resistance but is often impractical on large assemblies, so stabilized grades like SUS321H or SUS347H are preferred for welded high-temperature service.

How does SUS304H compare with SUS321H at high temperature?

SUS321H is titanium-stabilized, so it avoids sensitization entirely and needs no post-weld heat treatment, with good service up to about 900 C. SUS304H relies on carbon for strength and is the lower-cost option for non-welded components where welding constraints and corrosion requirements are not severe.

What heat treatment is recommended for SUS304H?

Solution annealing at 1010-1120 C followed by rapid water quenching. This dissolves existing carbides and prevents re-precipitation during cooling, maximizing high-temperature strength and creep resistance. Low-temperature tempering must be avoided because it promotes grain-boundary carbide precipitation.

What is the correct EN equivalent of SUS304H?

The ASTM equivalent is 304H (UNS S30409). In the EN system the closest heat-resisting analogue is X6CrNi18-11 (EN 1.4948); EN 1.4307 is a 304L-type low-carbon grade and is not an equivalent.