Temperature on its own is not a specification. A carbon steel pipe that gives ten years of service at 400 degrees Celsius may fail in a single turnaround at 560 degrees, and the answer is not a thicker wall. What changes the material choice is the failure mechanism the service condition activates: creep rupture, high temperature oxidation, sulphidation or naphthenic attack from the process stream, or thermal fatigue from repeated cycling. Selecting an alloy means identifying which of those is in play, then picking the cheapest grade family that survives it. This article covers the four families that cope with most high temperature and aggressive process duty, and the checks that prevent mix-ups.

Creep is the reason temperature limits exist

Below roughly 400 degrees Celsius design is governed by yield strength. Above that, atoms diffuse fast enough for the metal to stretch slowly under steady load, so failure is governed by creep rupture strength over the design life. Creep strength falls steeply with temperature, which is why an alloy comfortable at 540 degrees may have almost no useful life at 620 degrees. Design codes publish allowable stress against temperature and life, which turns selection into one question: which family holds enough creep strength to keep the wall thickness economic.

Chromium-molybdenum steels carry most of the load

Chromium and molybdenum additions raise creep strength substantially at modest cost, and Cr-Mo steels remain the workhorses of power plant and refinery piping. ASTM A335 P22 seamless pipe at 2.25 percent chromium and 1 percent molybdenum is the classic choice for steam lines and process piping up to around 580 degrees. Where higher temperature or thinner walls are needed, ASTM A335 P91 seamless pipe at 9 percent chromium with vanadium and niobium holds far better creep strength, so wall thickness can often be cut by a third or more, which on a large bore header means a significant weight saving.

The trade-off is fabrication discipline. P91 requires controlled preheat, careful interpass temperature, post weld heat treatment to a specified window, and hardness verification, because an incorrect heat treatment produces a brittle heat affected zone. P91 also shows Type IV cracking in the fine grained region of weldments after long service, so weld detail and inspection planning belong in the specification. Filler metal must be matched to the base metal and recorded on the weld map, because a P91 joint made with P22 filler will not survive its design life and hardness testing alone will not reveal it.

Stabilised austenitics for the 550 to 850 degree band

Where temperatures fall in the range that causes chromium carbide precipitation in standard 304 or 316, stabilised austenitic grades take over. Titanium or niobium additions lock up the carbon as stable carbides, so chromium stays in solution and the material keeps its corrosion resistance and creep strength after long exposure. 321 and 321H stainless steel plate and 347H stainless steel seamless pipes are standard selections for superheater tubing, reformer outlet systems, thermal oxidisers, and refinery transfer lines where metal temperature runs into the 600 degree range. The H suffix denotes a higher carbon range that improves creep strength at elevated temperature.

Austenitics expand roughly 40 percent more than carbon and Cr-Mo steels, which matters at dissimilar joints between austenitic and ferritic sections where cycling concentrates strain. The usual answer is a transition piece with a graded or nickel-based weld and a shorter design life allowance at that joint; ignoring it produces cracking after a few dozen cycles, often where inspection is difficult.

Above the stabilised grades sits 310 and 310S stainless steel pipe, a 25 percent chromium, 20 percent nickel austenitic that resists oxidation and scaling in continuous service up to roughly 1000 degrees Celsius, less under heavy cycling. Furnace rolls, radiant tubes, recuperators and kiln internals are typical uses, with Alloy 310S plate and bar covering the fabricated parts around them. Its low carbon content limits creep strength, so it is chosen for oxidation resistance rather than load carrying.

Nickel alloys for combined high temperature and corrosion

Once the process stream is also aggressive, nickel based alloys become competitive despite their cost. Alloy 600 combines high temperature strength with strong resistance to chloride stress corrosion cracking, which is why it appears in nuclear steam generator tubing and chloride bearing process heaters. Alloy 625 adds molybdenum and niobium for considerably better creep strength along with resistance to oxidation and seawater, and is used for bellows, expansion joints and high temperature ducting. Alloy 825 is a lower cost nickel-iron-chromium grade aimed at reducing acid and chloride environments rather than the highest temperatures, while Alloy 718 is the precipitation hardening option where high strength must be retained to around 700 degrees, typically for fasteners, valve stems and turbine hardware.

For wet chloride and mixed acid service the molybdenum rich grades lead. Hastelloy C-276 handles chlorides, hypochlorite, and mixed acid streams where no stainless survives, and Hastelloy B is specified for hydrochloric acid service in the absence of oxidizing species, a limitation worth observing because the wrong choice corrodes quickly. On the iron based side, stainless steel 904L and the duplex family cover the middle ground: duplex 2205 for general chloride duty with roughly twice the yield strength of 316, super duplex 2507 for higher chloride concentration and sour service, and super duplex S32760 for seawater systems with a PREN above 40. Where only thin wall tubing is needed, 317LMN stainless steel tube offers better chloride resistance than 316L at moderate cost.

Procurement checks that prevent a substitution

High alloy material is where mix-ups happen: 304 and 321 look identical, and 316 and 904L differ in nickel and molybdenum content that no visual inspection can reveal. Three controls prevent almost all of these problems. Require material test certificates to EN 10204 3.1 with actual chemistry and mechanical results, not just grade statements. Perform positive material identification on every heat and every length going into high temperature or corrosive service, and record the results against the line number. Confirm the heat treatment condition on the certificate, because solution annealed and stress relieved material can carry the same grade designation with entirely different properties.

Fabrication then has to match the material. Pickling and passivation after welding restores the passive layer on austenitic and duplex surfaces; skipping it is the most common cause of early staining on correctly specified material. For sour service the hardness and chemistry limits of NACE MR0175 or ISO 15156 apply.

A short selection path

Start with the design metal temperature and the governing failure mechanism. Up to about 580 degrees in clean service, Cr-Mo steels from P22 upward are usually the economic choice. Between 550 and 850 degrees, move to stabilised austenitics, or to 310S where oxidation rather than load is the limit. Where the stream carries chlorides, sulphur species, or acids at temperature, go directly to the nickel alloys instead of trying to make a stainless work. Then confirm the wet corrosion case at start-up, shutdown and upset, because condensation during a shutdown often sets the specification more than normal operation does. Choosing the family on mechanism rather than temperature alone is what keeps a line in service for its full design life.