304H vs 304L Stainless Steel: Composition, Properties and Application Differences

What Are 304H and 304L Stainless Steel?

Both 304H (UNS S30409) and 304L (UNS S30403) are austenitic stainless steels derived from the classic 18-8 grade 304 (UNS S30400). They share the same chromium-nickel base, with 18.0-20.0% chromium and 8.0-10.5% nickel, but they differ in one decisive parameter: carbon control. Under ASTM A240 the two grades are specified as plate, sheet and strip; in pipe and tube form they appear as TP304H and TP304L under ASTM A312, and as boiler and heat-exchanger tubes under ASTM A213.

304H is the carbon-controlled high-temperature version of 304. Instead of a simple maximum, carbon is specified within a range of 0.04% to 0.10%, which guarantees the minimum carbon level needed to develop creep-rupture strength above about 500 °C. 304L, by contrast, caps carbon at 0.030% so that chromium carbides do not precipitate at grain boundaries during welding, protecting the heat-affected zone from intergranular corrosion. In short, 304H is selected for hot strength; 304L is selected for weldability and corrosion resistance.

Chemical Composition Comparison

The table below lists the composition limits of both grades as specified in ASTM A240 (values in weight percent).

Element	304H (UNS S30409)	304L (UNS S30403)
Carbon (C)	0.04 - 0.10	≤ 0.030
Manganese (Mn)	≤ 2.00	≤ 2.00
Phosphorus (P)	≤ 0.045	≤ 0.045
Sulfur (S)	≤ 0.030	≤ 0.030
Silicon (Si)	≤ 0.75	≤ 0.75
Chromium (Cr)	18.0 - 20.0	18.0 - 20.0
Nickel (Ni)	8.0 - 10.5	8.0 - 12.0
Nitrogen (N)	≤ 0.10	≤ 0.10

Mechanical Properties in the Annealed Condition

At room temperature the strength difference between the two grades is modest, as shown by the minimum values for plate in ASTM A240.

Property	304H (UNS S30409)	304L (UNS S30403)
Tensile strength, min (MPa)	515	485
Yield strength 0.2%, min (MPa)	205	170
Elongation in 50 mm, min (%)	40	40

The real difference appears above approximately 500 °C. Pressure-equipment design at elevated temperature relies on time-dependent properties such as creep and rupture strength, and these require a minimum carbon content. That is why 304H is specified for headers, superheater and reheater tubes, hot piping and furnace components, while 304L is normally not used for load-bearing hot service beyond roughly 425 °C.

Weldability and Corrosion Resistance

304L is the preferred grade when thick sections are welded without post-weld heat treatment. Its 0.030% carbon maximum keeps the heat-affected zone practically free of chromium carbide precipitation, so the corrosion resistance of the joint remains close to that of the parent metal. 304H welds acceptably, but when the welded component must also resist corrosive media, a post-weld solution anneal at 1010-1120 °C followed by rapid cooling is normally required, or a low-carbon filler is used to compensate for carbon pickup in the weld zone.

In aqueous service both grades behave like standard 304: they resist atmospheric corrosion, fresh water and many organic and mildly oxidizing media, and both can pit in chloride-rich environments. Where chlorides are significant, molybdenum-bearing grades such as 316L (UNS S31603) are the normal choice, not 304H or 304L.

How to Select: 304H or 304L?

Choose 304H when the component is designed for continuous elevated-temperature service - superheater and reheater tubing, boiler headers, furnace parts - and the design is based on time-dependent strength at temperature. Choose 304L when welding dominates fabrication, when the part will meet corrosive media after welding without a full solution anneal, or when the maximum service temperature stays below roughly 425 °C. In practice the two grades are ordered as distinct products: TP304H or TP304L pipe per ASTM A312, 304H or 304L tubes per ASTM A213, and UNS S30409 or UNS S30403 plate per ASTM A240.

FAQ

Is 304H the same as 304?

No. 304H is a carbon-controlled version of 304 with carbon specified in the range 0.04-0.10% so that elevated-temperature strength is guaranteed. Standard 304 has a carbon maximum of 0.08% and no minimum, so its creep-rupture properties are not assured.

Can 304L be used at high temperature?

For load-bearing service, 304L is normally limited to about 425 °C because its low carbon content reduces creep strength. It is selected for welding and corrosion reasons, not for hot strength.

Which grade resists intergranular corrosion better after welding?

304L. The 0.030% carbon maximum minimizes carbide precipitation in the heat-affected zone, so it is the standard choice when post-weld annealing cannot be performed.

What is the solution annealing temperature for these grades?

Both are normally solution annealed at 1010-1120 °C followed by rapid cooling, which keeps carbides in solution and restores corrosion resistance.

Are 304H and 304L magnetic?

Both are austenitic and essentially non-magnetic in the annealed condition. Slight magnetism can appear after cold working because of strain-induced martensite, which is normal for the 18-8 family.

Which grade should be used for a welded food-processing tank?

For welded food-contact equipment where post-weld annealing is impractical, 304L is the usual choice because of its superior weld-zone corrosion resistance. For food-contact applications, compliance with GB 4806.9-2016 should also be confirmed.