High Temperature Resistant Stainless Steel Pipe: Grades, Chemistry and Service Limits

What Makes a Stainless Steel Pipe Suitable for High-Temperature Service

High temperature resistant stainless steel pipe is an austenitic chromium-nickel product chosen for two properties that a standard 304 pipe cannot deliver at the same time: a stable, adherent oxide scale that resists scaling and spalling in hot gas, and creep rupture strength that keeps the wall from thinning under sustained stress. The metallurgical basis is a face-centred cubic austenite matrix, which is why these grades keep toughness and ductility at temperatures where ferritic and martensitic steels have already lost most of their strength.

Selection is driven by three inputs: maximum continuous metal temperature, atmosphere (oxidising, reducing, carburising or sulphur-bearing) and required design life. The purchase specification matters as much as the grade name. Seamless tube for boiler, superheater and heat exchanger service is normally ordered to ASTM A213/A213M, while high-temperature and corrosive piping systems are ordered to ASTM A312/A312M, with general requirements covered by ASTM A999/A999M. Flat products such as furnace baffles and tube sheets follow ASTM A240/A240M.

Standard Grades and Specified Chemistry

The four grades below cover most high-temperature orders. Values are the specified ranges of ASTM A240/A240M and are the same base compositions used in ASTM A213/A213M and ASTM A312/A312M.

The carbon floor in 304H and 347H is deliberate: it is the carbon that provides solid-solution strengthening and raises high-temperature strength, and it is exactly the element that would cause sensitisation in a welded 304 component. Titanium in 321 and niobium plus tantalum in 347H are added to combine with carbon as stable carbides, which is why those two grades can be used in the sensitisation window between 425 °C and 815 °C without a solution anneal after welding. 310S takes a different route, using 24-26 % chromium and 19-22 % nickel to build a more protective scale rather than relying on stabilisation.

Mechanical Properties and Creep Behaviour

At room temperature the four grades share the same minimum values in ASTM A240/A240M: tensile strength 515 MPa, yield strength 0.2 % offset 205 MPa and elongation 40 %. Room-temperature values are therefore useless for differentiating them. The real difference appears in the creep range, and this is why design above roughly 425 °C must use allowable stresses from ASME BPVC Section II Part D rather than a tensile figure taken from a mill certificate. ASME B31.3 treats the creep range for austenitic stainless steels as beginning at 538 °C (1000 °F), and above that temperature the wall thickness must be justified against stress rupture data for the specific grade and heat treatment.

321 and 347H are normally selected in the 550-750 °C band, where both oxidation and creep are active. 304H is economical for the same band for less aggressive atmospheres. Where metal temperature exceeds about 900 °C, scaling resistance dominates the design and 310S is the usual answer, although its creep rupture strength is not proportionally higher, so support spacing and thermal expansion must be reviewed at the same time.

Oxidation Resistance and Practical Temperature Limits

Scale behaviour is cycle-dependent, not only temperature-dependent. A 321 tube running continuously at 800 °C in dry air may build a thin, protective scale, while the same tube cycled daily between ambient and 800 °C will spall repeatedly because the oxide and the metal have different thermal expansion coefficients. For cycling service, thicker walls, lower peak temperature or a higher-chromium grade are the practical remedies. In sulphur-bearing or reducing flue gas, chromium oxide is not protective and a higher nickel alloy is required instead of an 18-8 grade.

Verification also has a corrosion dimension. If pipe is to be welded and used in the sensitisation range, the order should state an intergranular corrosion test to ASTM A262 - Practice E (copper-copper sulfate) for unstabilised grades, or Practice A (oxalic acid etch) for screening. Certified mill test reports should list actual heat analysis, tensile properties and, where required, the results of the A262 test.

Ordering and Quality Verification

A high-temperature pipe order should state the product specification with its year of issue, the grade and UNS number, the manufacturing route (seamless or welded), outside diameter and wall thickness or schedule, length, heat treatment condition, and the supplementary requirements that apply. Hydrostatic or non-destructive examination, eddy current or ultrasonic testing, and the required marking system are the usual supplemental items. Where the pipe will be bent or formed, the forming operation will cold work the material and may require a post-forming solution anneal for grades that are not stabilised; that requirement should be written into the specification before the pipe leaves the mill, not negotiated after fabrication has started.

Frequently Asked Questions

Q: Is 321 or 347H better for a flue gas line at 700 °C?
A: Both resist sensitisation, and both sit in the creep range at 700 °C. 347H carries the higher carbon range plus niobium, so it is normally preferred where stress rupture life dominates. 321 is often adequate at moderate stress, and it is easier to source in small diameters.

Q: Can 304 pipe be substituted when 304H is specified?
A: No, not for creep service. 304 has a maximum carbon of 0.07 % with no minimum, so the actual carbon may be around 0.02 %, and the elevated-temperature strength of that heat may fall below the value assumed in the design. The 0.04-0.10 % window in 304H is the reason it exists.

Q: Why does the mill certificate show three different property sets?
A: A dual or triple certified heat is tested against each grade in the specification. Read the carbon value and the low-temperature or elevated-temperature results, which are the ones that change between 304, 304L and 304H.

Q: Does a higher chromium grade always last longer?
A: Only in oxidising atmospheres. In reducing or sulphur-bearing gas, and in molten salt or slag contact, higher chromium alone does not protect the surface. Check the actual gas chemistry and the metal temperature profile before upgrading to 310S.

Q: Does high-temperature pipe need a post-weld heat treatment?
A: Stabilised grades 321 and 347H and low-carbon grades generally do not, provided the weld and heat-affected zone are not left in the sensitisation window for long periods. Unstabilised 304H welds in the 425-815 °C range are the normal candidates for a solution anneal if the code of construction permits it.