304H Stainless Steel: High-Temperature Properties and Code Applications

Written by: Emma, Technical Sales Engineer  |  Reviewed by: Ethan, Materials Engineer  |  Updated: August 2026

304H is the higher-carbon member of the 304 family, and its “H” points to a specific job: elevated-temperature service where strength and creep behavior matter more than weldability or low-temperature corrosion resistance. This article explains what 304H is, how its carbon content relates to high-temperature performance, and what ASTM material compliance does — and does not — mean for ASME code applications.

1. What Is 304H Stainless Steel?

304H (UNS S30409) is a chromium-nickel austenitic stainless steel derived from 304, with one decisive difference: its carbon content is specified as a controlled range rather than a simple maximum. Where 304 allows carbon up to roughly 0.07–0.08% and 304L caps it at 0.030%, 304H specifies carbon in a band — typically about 0.04% to 0.10% — giving it both a minimum and a maximum.

That minimum carbon is the point. Carbon contributes to the strength that 304H needs at elevated temperature, and a minimum limit ensures the steel retains enough carbon to deliver it. The “H” designation therefore signals an intended use — higher-temperature service — rather than a claim of generally superior corrosion resistance over 304 or 304L.

2. 304H vs 304 and 304L

The three grades share the same austenitic chromium-nickel base, and their practical separation is driven by carbon. The table below summarizes how each grade is typically positioned.

The relationship is a matter of emphasis, not a simple ranking. 304L is chosen where welding and post-weld corrosion resistance are the priority, because its low carbon reduces sensitization risk. 304H is chosen where the component will carry load at elevated temperature for long periods, because its higher carbon supports strength and creep-related performance. Neither grade is “better” in the abstract; each fits a different service condition.

3. Why Carbon Matters at High Temperature

At elevated temperature, carbon strengthens the austenitic matrix through solid-solution hardening and through the formation of fine carbides, both of which help the material resist deformation over time. This is why 304H carries a minimum carbon limit that 304 and 304L do not: the carbon is there deliberately, to support load-bearing performance when the metal is hot.

The trade-off is sensitization. Higher carbon makes the material more prone to chromium carbide precipitation when it is held or cooled through the sensitizing temperature range, which is why 304H is generally not the first choice for welded structures that must resist intergranular corrosion in service. High-temperature strength and low-temperature weld corrosion resistance pull in opposite directions, and the carbon content is the lever between them.

In plain terms, the same carbon that helps 304H hold its strength when hot is the carbon that makes it less forgiving when welded and then exposed to a corrosive environment. The grade is therefore selected for what it will be asked to do in service, not simply because it is “the high-temperature one.”

4. High-Temperature Strength and Creep Considerations

Room-temperature mechanical properties are not a basis for high-temperature design. As temperature rises, short-term tensile and yield values fall, and time-dependent mechanisms — creep and stress rupture — become the controlling factors. Creep is slow, permanent deformation under sustained stress; stress rupture is eventual fracture under the same conditions. For a component that will operate hot for years, long-term creep data and the code allowable stress matter far more than an ambient tensile number.

Creep and stress rupture are closely related but measured differently. Creep is the gradual accumulation of strain under a sustained load, while stress rupture is the time to failure at a given stress and temperature. Both are time-dependent, which is why a good short-term tensile value, on its own, says little about long-term hot service.

The table below lists the properties that matter most when 304H is evaluated for elevated-temperature service.

There is no single “maximum service temperature” that applies to 304H across the board. The usable limit depends on the applied stress, the required service life, the atmosphere, and the governing code — and design values such as allowable stress must be taken from the applicable code edition and material tables, not from a general-purpose figure.

5. Oxidation and Corrosion at Elevated Temperature

304H’s carbon content is about strength, not corrosion resistance. In fact, the higher carbon makes 304H more susceptible to sensitization than 304L, so it should not be assumed to offer better corrosion performance. Oxidation resistance at elevated temperature is governed primarily by the chromium content, which is broadly similar across the 304 family, and by the atmosphere — air, combustion gases, and other environments behave differently.

The practical point is that atmosphere, load, time, and corrosive species all influence material selection. 304H is not automatically suitable for “any high-temperature environment”; it is a candidate for elevated-temperature strength, and its corrosion and oxidation limits must be assessed for the specific service.

6. ASTM and ASME Code Applications

It is important to keep two different things separate. An ASTM material specification such as A240 (for plate, sheet, and strip) or the equivalent product-form standard defines the chemistry, mechanical properties, and other acceptance criteria for the material itself. An ASME code requirement — such as the allowable stresses in Section II or the design rules in Section VIII — governs how the material may be used in a pressure-retaining component.

Material compliance with an ASTM specification does not by itself constitute ASME design approval. For pressure equipment, the material must be supplied in a form and condition recognized by the applicable code, and the design allowable stresses and temperature limits must be read from the code edition in force. A buyer should not assume that “ASTM A240 304H” automatically authorizes use at any particular temperature or stress.

The chain of responsibility runs from the melt to the finished component. The mill certifies the material to the ASTM product specification, while the designer applies the code to set allowable stress and temperature limits. A buyer asking for 304H should understand which link of that chain they are purchasing and which must still be satisfied through design and fabrication.

7. Typical 304H Applications

304H is commonly specified where elevated-temperature strength and long-term thermal exposure are expected, such as in high-temperature process equipment, heat-exchange components, pressure equipment, furnace parts, and elevated-temperature piping. The precise suitability in each case depends on the design temperature, stress, atmosphere, and applicable code, so these are typical categories rather than a guarantee of fitness.

8. Common Purchasing and Specification Mistakes

• Assuming a fixed maximum temperature — the usable limit depends on stress, time, atmosphere, and code.
• Treating 304H as a “more corrosion-resistant” 304 — its carbon improves strength, not corrosion resistance.
• Ranking 304H above 304L — they serve different conditions and are not a simple high/low pair.
• Using room-temperature properties for hot design — elevated-temperature and creep values govern.
• Equating ASTM compliance with ASME approval — material specification and design code are separate requirements.
• Applying one product standard to every form — plate, pipe, and bar fall under different ASTM specifications.

9. How to Specify 304H

A clear request states the grade, the applicable product standard, and the service conditions. A practical example might read:

For a complete purchase, the RFQ should also cover the UNS designation, product form, heat treatment and delivery condition, design or service temperature, whether service is continuous or intermittent, the applicable ASME or code requirements, and any supplementary inspection or testing. The example above is a model phrasing, not the text of any ASTM or ASME standard; the actual material standard and design requirements must be confirmed against the product form and the project code.

FAQ

Q1: What is 304H stainless steel?
It is the higher-carbon variant of 304, with carbon specified as a range (typically about 0.04–0.10%) to support elevated-temperature strength and creep-related service.

Q2: How does 304H differ from 304L?
304L has low carbon (0.030% maximum) to reduce sensitization in welded fabrication, while 304H has higher carbon to improve elevated-temperature strength. They serve different conditions.

Q3: Is 304H more corrosion resistant than 304?
No. Its higher carbon makes it more prone to sensitization, and its advantage is strength at elevated temperature, not improved corrosion resistance.

Q4: What is 304H’s maximum service temperature?
There is no single universal limit. The usable temperature depends on stress, service life, atmosphere, and the applicable code, and design values must come from the code edition in force.

Q5: Does ASTM compliance mean ASME approval?
No. ASTM defines the material requirements, while ASME codes govern design and allowable stresses. Meeting the material specification is not the same as design approval.

Q6: Where is 304H typically used?
In elevated-temperature service such as high-temperature process equipment, heat-exchange components, pressure equipment, furnace parts, and high-temperature piping, subject to design and code review.

Q7: Is 304H suitable for welded high-temperature equipment?
It can be used in suitable elevated-temperature applications, but its higher carbon content increases sensitization risk compared with 304L. Welded equipment must therefore be evaluated against the service environment, welding procedure, corrosion requirements, and applicable code.

Q8: Does 304H require a specific ASTM product standard?
Yes. The applicable ASTM specification depends on the product form, such as plate, pipe, or bar. The purchase specification should identify the 304H grade, UNS designation, applicable product standard, dimensions, delivery condition, and required documentation.

Q9: What properties should be checked when selecting 304H for high-temperature service?
The evaluation should consider elevated-temperature strength, creep and stress-rupture behavior, oxidation resistance, thermal cycling, design stress, service temperature, and the allowable stresses specified by the applicable code. Room-temperature tensile properties alone are not sufficient.

Q10: What should a 304H stainless steel MTC show?
The MTC should provide the applicable grade and specification, heat number, chemical composition, mechanical test results, and required traceability information. The reported carbon content is particularly important because 304H has a controlled carbon range rather than simply a maximum carbon limit.