AISI 304H / 1.4948 (X6CrNi18-10) Heat Resistant Steel

What Is AISI 304H Heat-Resistant Steel?

AISI 304H is the high-temperature version of 304 stainless steel, with carbon deliberately controlled to the upper part of the specification range, 0.04-0.10%. The higher carbon content increases creep strength and rupture resistance above 500°C, which makes the grade suitable for boilers, superheaters, heat exchangers and power generation equipment. Its European designation is 1.4948 (X6CrNi18-10), the American designation is UNS S30409, and the corresponding Chinese grade is 06Cr19Ni10H.

Designation system	Grade
AISI	304H
EN	1.4948, X6CrNi18-10
UNS	S30409
JIS	SUS304H
GB	06Cr19Ni10H

Chemical Composition

The composition below follows ASTM A240 (and ASME SA-240 for pressure equipment) and EN 10028-7 for pressure purposes.

Element	ASTM A240 / SA-240	EN 10028-7
C	0.04-0.10%	0.04-0.08%
Mn	≤2.00%	≤2.00%
Cr	18.0-20.0%	17.00-19.00%
Ni	8.0-10.5%	8.00-11.00%
Mo	-	≤0.10%
N	≤0.10%	-

Mechanical and Physical Properties

Minimum mechanical values for plate and sheet are given below together with typical physical constants.

Property	ASTM A240	EN 10028-7
Rp0.2	≥205 MPa	≥230 MPa
Rm	≥515 MPa	530-740 MPa
Elongation	≥40%	≥45%
Hardness	≤92 HRB / ≤201 HBW	-

Physical properties: density ≈7.9 g/cm³, elastic modulus ≈200 GPa at 20°C, thermal expansion ≈16.3 μm/(m·K) at 100°C, thermal conductivity ≈17 W/(m·°C). The density of 1.4948 is approximately 7.9 g/cm³, the same family value as other 18/8 austenitic grades.

High-Temperature Performance and Creep Resistance

The defining property of 304H is creep strength. Because carbon strengthens the austenitic matrix at elevated temperature, 304H maintains load-carrying capacity where 304 and especially 304L soften. In continuous oxidizing service, the practical upper limit is about 815°C; the exact allowable stress must be taken from the governing design code, for example ASME BPVC Section II. Above this temperature, or where scaling resistance is critical, higher-alloy grades such as 310S (1.4845) should be considered.

304H Compared with Related Grades

Grade	Carbon	Key difference
304H, 1.4948	0.04-0.10%	Enhanced creep strength for high-temperature pressure service
304, 1.4301	≤0.07%	Standard general-purpose grade, lower creep strength
304L, 1.4307	≤0.030%	Best weld corrosion resistance, lowest high-temperature strength
316H, 1.4919	0.04-0.10%	Adds molybdenum for chloride and pitting resistance at temperature
310S, 1.4845	≤0.08%	Much higher Cr and Ni, for service above 815°C

Applications

304H is specified for boilers and steam generators, superheater and reheater tubes, heat exchangers, refinery furnace tubes, pressure vessels in high-temperature service and power plant components. It is normally supplied in the solution-annealed condition in plate, sheet, coil, strip, tube and bar forms.

Frequently Asked Questions

What is 1.4948 equivalent to?

1.4948 (X6CrNi18-10) corresponds to ASTM AISI 304H, UNS S30409, JIS SUS304H and Chinese 06Cr19Ni10H.

What is the density of 1.4948?

Approximately 7.9 g/cm³, the same as other 18/8 austenitic stainless steels.

What is the difference between 304H and 304?

304H specifies carbon in the range 0.04-0.10%, which improves creep strength at elevated temperature; standard 304 has a carbon maximum of 0.07% and is not optimized for high-temperature creep design.

What temperature can 304H withstand?

In continuous oxidizing service about 815°C is the practical limit, but allowable stresses must be verified against the governing pressure design code such as ASME BPVC Section II.

What is the difference between 1.4948 and 1.4301?

Both are 18/8 austenitic grades; 1.4948 carries slightly higher carbon and is optimized for elevated-temperature strength, while 1.4301 is the general-purpose 304 grade.

What is the difference between 304H and 316H, 1.4919?

316H adds 2.0-3.0% molybdenum, which improves resistance to chlorides, pitting and crevice corrosion in elevated-temperature chemical and marine environments.