Chemical composition

Alloy 800HT also requires Al + Ti 0.85–1.20%. Alloys 800H and 800HT have grain size and heat-treatment requirements for elevated-temperature service, as given in the standards.

The three alloys share the same base composition. 800H requires carbon of 0.05–0.10%; 800HT narrows carbon to 0.06–0.10% and requires combined aluminium and titanium of 0.85–1.20%. These controls, with a high-temperature solution anneal, give the coarse grain needed for creep strength.

Mechanical properties

Values common to ASTM B407, B409 and B408 (alloy 800 family), B423, B424 and B425 (alloy 825) and B564 forgings; check the standard for the product form and size.

Room-temperature strength of 800H and 800HT is lower than that of alloy 800 because of the coarse grain, but at temperatures above about 600 °C their creep and rupture strength is much higher, which is what governs design in that range.

Equivalent designations

Material is often dual certified 800H/800HT. See the grade equivalent chart.

Incoloy 800H products we supply

• Incoloy pipes and tubes – ASTM B407 seamless, B514 / B515 welded
• Incoloy sheets and plates – ASTM B409
• Incoloy round bars – ASTM B408
• Incoloy flanges – ASTM B564
• Incoloy pipe fittings – ASTM B366 WPNIC10 / WPNIC11
• Incoloy fasteners – from B408 bar

All Incoloy alloys are on the Incoloy page.

Where Incoloy 800H is used

Incoloy 800H and 800HT are used for steam methane reformer outlet pigtails, manifolds and transfer lines in hydrogen and ammonia plants; ethylene cracking furnace components and transfer-line exchangers; heat-treatment furnace muffles, retorts, baskets and radiant tubes; superheater and reheater tubes and headers in power boilers; and nuclear and process heat exchangers. Incoloy 800 is used for heat exchangers, sheathing of electric heating elements and process piping below about 600 °C.

Incoloy 800H compared with other alloys

Compared with SS 310S, 800H has much better creep strength and is less prone to sigma-phase embrittlement in long service. Compared with Incoloy 825, which is designed for aqueous corrosion, 800H is designed for high temperature. For the highest temperatures and most aggressive atmospheres, Inconel 600 and 601 or cast HP alloys are used.

Fabrication

Incoloy 800H forms well hot and cold and is welded with ERNiCr-3 or matching 800H-type fillers selected for creep strength. After significant cold forming, a re-solution anneal may be required to restore the coarse-grained structure for creep service, as the design code sets out. Hot working is done in a sulphur-free atmosphere.

Sizes and supply

Incoloy 800, 800H and 800HT is supplied in the product forms listed above, in metric and imperial sizes. Because nickel alloys are produced by a limited number of mills and in smaller quantities than stainless steel, availability of each size, the delivery of special sizes and the minimum mill quantities are confirmed with each quotation. Cut lengths, cut plates and machined blanks reduce waste of an expensive alloy and can be supplied on request. Please send the full requirement – grade, specification, product, size, quantity and testing – so that the complete package can be quoted together.

Service limits

Incoloy 800H and 800HT are used up to about 1000 °C in oxidising atmospheres, with allowable stresses taken from the design code for the actual temperature. In strongly reducing sulphur-bearing atmospheres, sulphidation can limit life, and in some reformer service metal dusting must be considered. Long service in the 650–800 °C range can reduce room-temperature toughness, which matters for equipment that is shut down and restarted often. Design and material selection should take these factors into account.

Documents and inspection

Material is supplied with mill test certificates (MTC) showing heat number, chemical composition, mechanical properties and heat treatment. Third-party inspection is available on request, and positive material identification can be carried out before dispatch. Because nickel alloys look alike, every piece is marked with grade and heat number for traceability through fabrication.

For 800H and 800HT, the mill test certificate also reports the grain size, which must meet the specification minimum.