304H vs 304 Stainless Steel: Heat Resistance for High-Temperature Applications

304 and 304H: Same 18-8 Base, Different Carbon Control

304 (UNS S30400) and 304H (UNS S30409) are both austenitic stainless steels of the classic 18% chromium, 8-10.5% nickel family. The only intentional difference is carbon: 304 is limited to 0.08% maximum, while 304H is specified with a controlled range of 0.04-0.10%. That small change turns a general-purpose grade into a heat-resisting grade, because carbon is what stabilizes the microstructure against creep at high temperature.

Chemical Composition per ASTM A240

Element (wt%)	304 UNS S30400	304H UNS S30409
C	0.08 max	0.04-0.10
Mn	2.00 max	2.00 max
P	0.045 max	0.045 max
S	0.030 max	0.030 max
Si	0.75 max	0.75 max
Cr	18.00-20.00	18.00-20.00
Ni	8.00-10.50	8.00-10.50
N	0.10 max	0.10 max

The H designation in ASTM standards also specifies a minimum grain size requirement that promotes creep resistance. Both grades are covered by ASTM A240 for plate, A312 for pipe and A213 for boiler and superheater tube, which is the product form where 304H is most often specified.

Why 304H Outperforms 304 at High Temperature

Above roughly 500 C, the low carbon of standard 304 allows grain boundaries to soften and slide under sustained load, and creep deformation accelerates. In 304H the higher carbon forms fine chromium carbides that pin grain boundaries, resisting deformation under prolonged thermal stress and repeated heating-cooling cycles. This same mechanism improves resistance to thermal fatigue, which matters for furnace trays, heat exchanger tubes and other components that cycle in temperature. For this reason pressure-vessel and boiler codes recognize higher allowable stresses for 304H than for 304 at elevated temperatures.

Mechanical Properties and Trade-Offs

In the annealed condition both grades show minimum tensile strength 515 MPa, yield strength 205 MPa and elongation 40% per ASTM A240. The trade-off of the higher carbon appears in service behavior: 304H has slightly lower room-temperature ductility and is less suited to severe cold forming such as deep drawing, and it is marginally more susceptible to intergranular corrosion in certain chemical environments after welding. In high-temperature service these effects are secondary, but they matter for fabrication planning.

Applications and Limitations

Specify 304H for components in continuous service around 800-1100 C: industrial furnace parts, superheater and reheater tubes, steam lines, exhaust manifolds and high-temperature conveyor components. Do not use 304H where 304 is the rational choice: low-temperature service, severe chloride or seawater environments where carbon offers no benefit and only adds cost, or complex cold-formed parts that need maximum ductility.

Welding and Fabrication Guidance

Weld 304H with low heat input to avoid excessive carbide precipitation at grain boundaries, and use 308H-type filler metal to match the high-temperature strength of the base material. 304 is more forgiving in fabrication and can be welded with standard GTAW and GMAW procedures without special post-weld treatment for most applications. Both grades machine well with carbide tooling, though 304H may need slightly reduced cutting speeds to control work hardening.

FAQ

What does the H in 304H mean?

H designates a high-temperature variant with a controlled carbon range of 0.04-0.10% and a specified minimum grain size, giving higher creep and rupture strength in continuous high-temperature service. It is recognized in codes such as ASTM A240 and A213.

When must 304H be chosen over 304?

When the component operates continuously in the roughly 800-1100 C range and creep resistance governs the design, for example boiler tubes, superheaters, furnace parts and steam components. Above about 500 C the H grade carries higher allowable design stresses.

Does the higher carbon of 304H hurt corrosion resistance?

Slightly, in certain chemical environments, because higher carbon increases sensitization risk in the heat-affected zone of welds. In high-temperature service this is not the governing factor; in chloride service the difference is irrelevant because neither grade is suited to chlorides.

Which filler metal is used for 304H?

308H-type filler is recommended so the weld metal retains high-temperature strength comparable to the base material. Keep heat input low and control interpass temperature.

Can 304 replace 304H at high temperature?

Only with a derating of allowable stress, which usually forces thicker walls. Where creep and rupture strength are design drivers, 304H is the specified grade and 304 is not an equivalent substitute.

Is 304H suitable for chloride or low-temperature service?

No. Neither 304 nor 304H resists chloride pitting, and 304H offers no advantage at low temperature, where its carbon content is irrelevant and its lower ductility is a drawback. Use 316L or a higher-alloyed grade for chlorides.