304H vs 304L Stainless Steel: High-Temperature Strength vs Weldability

Standards and Designations

304H (UNS S30409) and 304L (UNS S30403) are carbon-controlled variants of Type 304 (UNS S30400). Both are covered for plate, sheet and strip by ASTM A240, for seamless pipe by ASTM A312, for boiler and superheater tubes by ASTM A213, and in the EN 10088 series; the JIS designations are SUS304H and SUS304L. The two grades are identical in chromium and nickel content but differ in carbon, and that single difference drives entirely different service envelopes: 304H is a high-temperature grade, while 304L is a welding-first grade.

Chemical Composition

The composition limits below follow ASTM A240.

Element (wt %)	304H (S30409)	304L (S30403)
Carbon	0.04-0.10	0.03, max
Manganese, max	2.0	2.0
Silicon, max	0.75	0.75
Phosphorus, max	0.045	0.045
Sulfur, max	0.030	0.030
Chromium	18.0-20.0	18.0-20.0
Nickel	8.0-10.5	8.0-12.0
Nitrogen, max	0.10	0.10

304H requires a minimum carbon of 0.04%, which is the opposite of the low-carbon design of 304L. This minimum carbon is deliberate: it guarantees the precipitation of carbides that pin grain boundaries and support creep strength at elevated temperature.

Mechanical Properties

Minimum room-temperature values for annealed plate per ASTM A240 are listed below.

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

At room temperature the difference is modest, but it grows with temperature. Because of the guaranteed carbon content, the ASME design allowable stress for 304H remains useful up to about 815°C in pressure piping, while 304L is limited to about 425°C in the same applications; above that temperature the low-carbon grade softens and creeps rapidly.

High-Temperature Performance and Creep Strength

Creep is time-dependent plastic deformation under sustained load at elevated temperature. The chromium carbides formed by the 0.04-0.10% carbon of 304H precipitate both at grain boundaries and within grains, resisting grain-boundary sliding and giving the grade markedly better creep rupture strength than 304L above about 500°C. Components operating continuously between about 500°C and 815°C benefit from 304H: industrial furnace trays and fixtures, steam boiler superheater and reheater tubes, gas turbine and engine exhaust components, thermal processing equipment, and high-temperature piping. In cyclic heating and cooling duty, the stable structure of 304H also reduces thermal fatigue damage compared with the softer low-carbon grade.

Weldability, Fabrication and Selection

304L exists for welding. The 0.03% carbon limit keeps chromium carbides from precipitating in the heat-affected zone, so large welded structures such as chemical storage tanks, food processing vessels and pharmaceutical equipment can be used in the as-welded condition without post-weld heat treatment. 304H, by contrast, is at higher risk of sensitization in the weld heat-affected zone and is generally not selected for welded corrosive assemblies; when 304H is welded, solution annealing may be needed to restore full ductility and corrosion resistance. 304H typically costs more than 304L because of the tighter carbon control. Selection rule: choose 304H for load-bearing high-temperature components above about 500°C, and choose 304L for welded vessels, tanks and piping in corrosive service below about 425°C.

Frequently Asked Questions

What does the H in 304H mean? H indicates a controlled carbon range of 0.04-0.10% specified for high-temperature strength, particularly creep resistance.

Why is 304L called low carbon? Its carbon is limited to 0.03% max, about one-third of standard 304, to prevent sensitization during welding.

Can 304L be used at high temperature? Not for load-bearing service; 304L is limited to about 425°C in pressure applications because its creep strength is low.

Is 304H suitable for welded chemical tanks? Generally no; the higher carbon raises sensitization risk in the heat-affected zone, so 304L is preferred for welded corrosive service.

Are 304H and 304L magnetic? Both are essentially non-magnetic in the annealed condition; cold working can induce a slight magnetic response.

Which standard covers 304H boiler tubes? ASTM A213 TP304H for seamless boiler, superheater and heat exchanger tubes, and ASTM A240 for plate and sheet.

Frequently Asked Questions

Q1. What are the carbon limits of 304H and 304L?
304H (UNS S30409) requires a controlled carbon range of 0.04-0.10 percent, while 304L (UNS S30403) limits carbon to 0.03 percent maximum; chromium 18.0-20.0 and nickel 8.0-10.5 are common to both, with 304L allowed up to 12.0 percent nickel.

Q2. How do their room-temperature mechanical properties compare?
Per ASTM A240, 304H shows a minimum tensile strength of 515 MPa and 0.2 percent yield strength of 205 MPa, while 304L shows 485 MPa and 170 MPa respectively; both have 40 percent minimum elongation in 50 mm.

Q3. What temperature limits apply in pressure design?
Because of its guaranteed carbon content, the ASME design allowable stress for 304H remains useful up to about 815 C in pressure piping, while 304L is limited to about 425 C in the same applications; above that temperature the low-carbon grade softens and creeps rapidly.

Q4. Why does carbon give 304H its creep strength?
The chromium carbides formed by the 0.04-0.10 percent carbon precipitate both at grain boundaries and within grains, resisting grain-boundary sliding and giving markedly better creep rupture strength than 304L above about 500 C.

Q5. When should each grade be selected?
Choose 304H for load-bearing high-temperature components above about 500 C, such as furnace trays, boiler superheater and reheater tubes covered by ASTM A213 TP304H, and exhaust components; choose 304L for welded vessels, tanks and piping in corrosive service below about 425 C, where it can be used as-welded without post-weld heat treatment.