Pick 304H when the design metal temperature sits above ~525°C (977°F) and the section is governed by creep or rupture; the 0.04-0.10% carbon range gives 515 MPa minimum tensile and 205 MPa 0.2% offset yield, versus 485 MPa / 170 MPa for 304L [S2]. For welded pressure equipment exposed to corrosive condensate below that threshold, 304L (UNS S30403) is the safer call, since its ≤0.03% carbon suppresses chromium-carbide precipitation at grain boundaries [S2].
Both grades share 18-20% Cr and 8-10.5% Ni, both are austenitic and non-magnetic in the annealed condition, and both ship to ASME SA-240 / ASTM A240 plate and sheet forms [S2]. The only real compositional lever is carbon, so the spec decision collapses to a temperature-versus-welded-corrosion trade, not a metallurgy redesign.
Carbon Content Drives the Whole Decision
304L holds carbon at 0.00-0.03% by ASTM A240 chemistry limits, and 304H is held at 0.04-0.10% [S2][S3]. That single delta changes three things at once: allowable stress at temperature, sensitization kinetics in the heat-affected zone, and ASME code assignment. Chromium (18-20%), nickel (8-10.5%), manganese (≤2.00%), silicon (≤0.75%), phosphorus (≤0.045%), and sulfur (≤0.030%) are identical between the two grades [S2]. Nitrogen is capped at 0.10% in both. With iron as the balance, the corrosion envelope in clean, oxidizing service is effectively the same; the difference shows up only where carbide chemistry matters.
304H's higher carbon is not a defect: it is the deliberate addition that pins dislocations and gives the alloy its elevated-temperature strength. The trade is real, because above ~425°C plain 304 begins to precipitate chromium carbides at grain boundaries, and 304H inherits that same susceptibility but with more carbon available to do it [S3][S4]. Where welds sit in a wet or acidic environment, that sensitization becomes intergranular corrosion. The spec writer has to pick which failure mode they can live with.
Allowable Stress and Creep: Where 304H Earns Its Keep
At 1000°F (538°C) the ASME-style allowable stress for 304H sits around 25.5 ksi versus roughly 14.9 ksi for 304L, a 70% step that lets the designer drop a wall thickness or raise the design pressure for the same shell [S2]. Below ~425°C, 304 and 304H are roughly equivalent; above 525°C, 304H's creep-rupture advantage is the reason it exists at all [S3]. Masteel's technical note is more conservative, listing 304 as a grade that resists oxidation up to 925°C in continuous service and 870°C intermittent, but warning that 304 is not really a high-temperature grade above 425°C once carbide precipitation is in play [S5].
For pressure-vessel and boiler work, the practical bands used by process engineers line up as: 304L for vessels and piping designed to ASME B31.1/B31.3 where the metal temperature stays below ~425°C and welds are in mildly corrosive service, and 304H for boiler tubes, superheater sections, and reactor internals designed to ASME Section I or Section VIII Division 1 where the design metal temperature is 525-800°C [S2][S5]. Gnee's January 2026 datasheet repeats the 0.10% upper-bound carbon cap on 304H and ties it to high-temperature pressure-bearing service [S6].
Corrosion and Weldability: Where 304L Wins
304L's value is straightforward: low carbon means less chromium carbide in the HAZ after welding, so welded assemblies keep their intergranular corrosion resistance without a post-weld solution anneal [S2][S4]. The Zhongzheng 2025-12 technical brief is explicit that 304L has the strongest post-weld intergranular corrosion resistance of the three 304-family grades, while 304H has the weakest and is "not suitable for welding environments prone to corrosion" [S3]. Royal Steel's October 2026 post echoes the same boundary, noting 304H's sensitization risk in welded joints even when the parent metal is fine [S4].
For a fabricator, that translates into two operational rules. First, if the section is heavily welded and sees condensate, wet sour service, or a chloride-bearing environment during downtime, 304L is the default; 321 (Ti-stabilized) or 347 (Nb-stabilized) is the upgrade when both heat and welding are unavoidable. Second, if 304H must be welded, the procedure qualification has to limit the interpass temperature and the heat input, and a section may need a stabilization heat treatment. The May 2021 Masteel article goes further and points at flue-gas chimneys as a case where 304H is used specifically because condensed acidic liquids form, yet the metal temperature in the gas path still demands high-temperature strength, an explicit case where engineers accept the sensitization risk against the operating envelope [S5].
304H vs 304L vs 304: Comparison Matrix
The three grades are not interchangeable, and the right pick is set by temperature band and welded-versus-parent-metal exposure: [S4]
304 (UNS S30400, ≤0.08% C): the general-purpose default. 485 MPa tensile, 170 MPa yield, 40% elongation, 201 HB max, continuous service to ~925°C oxidation but only ~425°C for sustained structural use, weldable with post-weld heat treatment recommended for corrosive service [S3][S5].
304L (UNS S30403, ≤0.03% C): 485 MPa tensile, 170 MPa yield, 40% elongation, best post-weld intergranular corrosion resistance, lowest allowable stress at temperature (about 14.9 ksi at 1000°F), the right call for chemical tanks, pharmaceutical piping, and welded vessels below ~425°C [S2][S3].
304H (UNS S30409, 0.04-0.10% C): 515 MPa tensile, 205 MPa yield, 40% elongation, ~25.5 ksi allowable at 1000°F, best creep and rupture strength above 525°C, weakest in welded corrosive service, the right call for boiler tubes, superheater headers, and refinery furnace components per ASME Section I [S2][S3][S7].
The decision rule for a process engineer: if the design metal temperature is below 425°C and the section is welded into a corrosive loop, 304L; if the design metal temperature is above 525°C and the section is mostly parent-metal (boiler tube, furnace tube, header), 304H; if the temperature band crosses 425-525°C and welds cannot be avoided, step up to 321H or 347H rather than fighting 304H's sensitization. For broader stainless selection context outside the 304 family, the spec-side tradeoffs in 303 vs 304 stainless show how a similar carbon-versus-corrosion decision plays out in machining-grade alloys, while surface finish choices that change field corrosion behavior, not base alloy, are covered in the 2B vs BA vs No.4 finish comparison.
Failure Modes the Spec Has to Acknowledge
Three failure modes drive almost every field incident with these grades. First, sensitization in 304H welds: chromium combines with the extra carbon at 450-850°C and leaves the grain boundary chromium-depleted, which then corrodes in service. Second, creep rupture in 304L at high temperature: 304L simply does not have the carbon to hold dislocation structures at 600-800°C, and the wall thins over time. Third, sigma-phase embrittlement after long exposure in the 550-900°C band, which hits all three 304-family grades but is more punishing on 304H because of its higher ferrite potential at those temperatures [S3][S4].
Two test-trackable signals for incoming material: confirm the carbon product analysis on the MTC against the 0.04-0.10% range for 304H or ≤0.03% for 304L, and confirm the solution-anneal temperature (typically 1040-1100°C) so the metallurgical condition matches the ASME code case. Pressure-service temperature measurement at the design point, not at the skin, is the second signal: a temperature sensor embedded in the lagging or a temperature recorder on the skin during a startup ramp is the cheapest way to catch a section that is creeping above 525°C while the datasheet still says "304L." For a continuous readout on a superheater outlet, a temperature monitor tied to a temperature controller trip is the standard safeguard, and the calibration logic is laid out in the temperature measurement reference. None of this is a substitute for getting the grade right in the MTR, but it buys time if the spec was wrong.
Standards, Sourcing, and What to Put on the MTR
Both 304L and 304H ship to ASME SA-240 / ASTM A240 for plate and sheet, with 304L dual-certified to ASME SA-240 GR 304L and 304H to SA-240 GR 304H [S2]. Pressure-vessel and piping rules under ASME Section VIII (Division 1) and B31.1 reference these SA-240 grades directly, and the allowable-stress tables in Section II Part D are what produce the 25.5 ksi vs 14.9 ksi gap at 1000°F. For European work, EN 1.4307 (304L) and 1.4948 (304H) are the equivalents, and the same carbon split applies [S2].
The minimum doc set a QA engineer should demand on every heat: full product analysis including C, Cr, Ni, Mn, Si, P, S, N, and Fe balance; a statement of the solution-anneal condition; a grain-size reading (ASTM E112) since coarse grain hurts creep life in 304H; and a traceability link from the plate or tube to the original heat. If the operating temperature is above 525°C, the spec should also call out ASME Section I instead of Section VIII where the component is a boiler tube, and the fab should be qualified to the higher creep-aware welding procedure. For broader cost-side context on stainless heavy sections during 2026 procurement, the 304 vs 304L vs 304H detailed analysis and the 304L vs 304H buyer guide line out the per-kilo spread and lead-time differences by region.
Bottom line: 304H is the pressure-vessel and boiler grade above ~525°C, and 304L is the welded-corrosion grade below ~425°C. Between those two bands, the right call is usually a stabilized grade, not a 304-family variant.