AISI 304H

What is AISI 304H?

AISI 304H is an austenitic stainless steel intended for elevated-temperature service. It contains 18.0 to 20.0 percent chromium, 8.0 to 10.5 percent nickel and 0.04 to 0.10 percent carbon. The required carbon content distinguishes it from 304L. Carbon in solution and in finely distributed carbides limits grain-boundary sliding and improves creep resistance above roughly 500 degrees Celsius.

AISI 304H must have a grain size of ASTM No 7 or coarser. Coarse grains reduce the grain-boundary area per unit volume and improve creep life at elevated temperatures. Material with the specified carbon content but a finer grain size does not meet the grade requirements.

AISI 304H has the same specified room-temperature strength as standard 304. Its principal distinction is its permitted service range. Pressure codes specify it for structural and pressure-containing service between about 500 and 800 degrees Celsius, where standard 304 and 304L are not specified.

Laxcon Steels lists AISI 304H in its grades reference as an austenitic grade. The same steel is written 304H, 1.4948 and S30409.

What is the chemical composition of AISI 304H?

Composition limits in weight percent are carbon 0.04 to 0.1, manganese 2.0 maximum, sulphur 0.03 maximum, phosphorus 0.04 maximum, silicon 0.75 maximum, chromium 18.0 to 20.0 and nickel 8.0 to 10.5.

What is AISI 304H equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

What is the PREN of AISI 304H?

Computed by Laxcon Steels from the composition above as PREN = Cr + 3.3Mo + 16N, the relationship published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014). The first figure uses the minimum specified chromium, molybdenum and nitrogen, so it is the floor for a conforming heat rather than a typical value; the second uses the maxima, so the two together are the width of the band one compliant grade allows. PREN ranks resistance to chloride pitting and nothing else: it does not predict crevice corrosion, stress corrosion cracking or service life, and it is not a substitute for a corrosion test. The pitting resistance reference carries the full table and the caveats.

Mechanical properties of AISI 304H

ASTM A240 specifies annealed AISI 304H with a minimum tensile strength of 515 MPa, a minimum 0.2 percent proof stress of 205 MPa and a minimum elongation of 40 percent in 50 mm. Maximum hardness is 92 HRB or 201 HB. These values are identical to those of standard 304. The grades differ in allowable stress at 550, 650 and 750 degrees Celsius. The carbon content and coarse grain structure allow AISI 304H to carry loads for tens of thousands of hours after low-carbon material has begun to creep.

The physical properties correspond to those of the 304 family. Density is about 7,900 kg per cubic metre, and the elastic modulus is 193 GPa. Thermal conductivity is around 16.3 W per metre kelvin at 100 degrees Celsius and about 21.5 at 500 degrees Celsius. The mean coefficient of thermal expansion is near 17.2 micrometres per metre per degree Celsius over 0 to 100 degrees Celsius. This value is roughly half again that of ferritic steel. Hot austenitic pipework therefore uses expansion loops, guides and bellows designed for this expansion.

Elevated-temperature behaviour of AISI 304H

AISI 304H has good oxidation resistance in intermittent service to about 870 degrees Celsius and in continuous service to about 925 degrees Celsius. These temperatures describe scaling limits rather than strength limits. Structural and pressure applications normally use the grade from about 500 to about 800 degrees Celsius. Stabilised grades and high-chromium heat-resisting grades are used above this range.

The raised carbon content causes sensitisation between 425 and 860 degrees Celsius. This does not impair the high-temperature application itself because intergranular corrosion requires an electrolyte, which is absent at 700 degrees Celsius. Aqueous corrosion resistance is reduced after the component cools. A header that remains hot therefore behaves differently from one exposed to chloride-bearing wash water during a shutdown.

Welding and heat treatment of AISI 304H

Solution treatment consists of heating to 1,010 to 1,120 degrees Celsius followed by rapid cooling. Heat treatment cannot harden the grade. Welding uses the same processes as standard 304 and uses 308H class consumables. These consumables maintain deposited-metal carbon within the same range as the parent material. A 308L consumable would produce a low-carbon, creep-weak zone within a high-temperature joint.

Repeated cycling through 425 to 860 degrees Celsius can produce thermal fatigue and progressive carbide precipitation. Long exposure can also form sigma phase. Both effects slightly increase strength and substantially reduce toughness. A full solution anneal restores ductility, but it is rarely practical for installed pipework. Design calculations must therefore account for aged properties rather than as-supplied properties.

Applications of AISI 304H

AISI 304H is used in process-heat equipment. Applications include superheater and reheater tubing, steam headers, refinery and petrochemical piping operating above 500 degrees Celsius, furnace and boiler internals, catalytic reformer and cracking-unit components, and structural supports within hot equipment. The grade is also used as seamless mother hollow and as heavy bar for forged high-temperature fittings. These forms support pressure-containing applications.

Designations for AISI 304H

The UNS designation is S30409. The nearest European counterpart is EN material number 1.4948, with the name X6CrNi18-11. American specifications include ASTM A240 for plate, ASTM A479 for bar and ASTM A312 for pipe. Purchaser designations include 304H, SS 304H, SUS 304H, SUS304H and SS304H. The ASTM and EN composition windows align less closely than those for 304. The standard stated in the order therefore governs the material requirements.

In which product forms does Laxcon list AISI 304H?

Taken from the published specification on each product page. A form that is not listed here is not a form this reference can confirm the grade in; the enquiry route settles what can actually be rolled to an order.

Where AISI 304H is not the right choice

AISI 304H is unsuitable for welded components placed in wet service without solution annealing. Its specified carbon content sensitises the heat-affected zone and can cause intergranular attack. AISI 304L avoids this failure mode and is suitable for ambient-temperature corrosion resistance in welded fabrications.

AISI 304H is also unsuitable above roughly 800 degrees Celsius. Its elevated-temperature strength decreases, and long exposures can cause sigma phase embrittlement. AISI 321 has the same 18/8 base composition with titanium stabilisation. It is effective to about 900 degrees Celsius and retains aqueous corrosion resistance after exposure. AISI 310S contains 24.0 to 26.0 percent chromium and 19.0 to 22.0 percent nickel and is used for higher-temperature furnace service. AISI 304H provides no greater chloride resistance than 304. Its pitting resistance equivalent number is 18.0 to 20.0, and its composition contains no molybdenum. AISI 316 or a duplex grade is used where chloride pitting rather than heat governs material selection.

Other austenitic grades