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 table includes alloys 800, 800H and 800HT for comparison; 825 is the last row. Its combination of nickel, chromium, molybdenum and copper is what sets it apart from the high-temperature alloy 800 family.

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.

Incoloy 825 is supplied in the annealed condition. It keeps good mechanical properties from cryogenic temperatures up to about 540 °C, above which it is not normally used for pressure parts.

Equivalent grades and physical properties

Equivalent designations are similar but not identical; order to the standard you need.

Typical values from recognised industry references, for guidance only; they are not specification limits.

See the full grade equivalent chart for other grades.

Corrosion resistance

Sulphuric and phosphoric acids: Incoloy 825 is one of the standard alloys for sulphuric acid service, including pickling solutions, and for phosphoric acid evaporators and wet-process phosphoric acid plants.

Chlorides: its high nickel makes it highly resistant to chloride stress corrosion cracking, a common failure mode of austenitic stainless steels; its molybdenum gives better pitting resistance than 316L, though in warm seawater, 625 or super duplex is preferred.

Sour and oil and gas service: 825 resists hydrogen sulphide and carbon dioxide environments and is used for downhole tubing, flowlines, and as the corrosion-resistant layer in clad and lined pipe. It can be supplied to NACE MR0175 / ISO 15156 on request.

Oxidising media: it also resists nitric acid and other oxidising chemicals.

Incoloy 825 products we supply

• Incoloy pipes and tubes – seamless to ASTM B423, welded to B705; heat-exchanger tubes to B163.
• Incoloy sheets and plates – ASTM B424.
• Incoloy round bars – ASTM B425.
• Incoloy flanges – from ASTM B564 forgings.
• Incoloy pipe fittings – ASTM B366 WPNICMC and forged fittings.
• Incoloy fasteners – from B425 bar.

All Incoloy alloys are on the Incoloy page.

Comparable alloys

Incoloy 800H shares the nickel-iron-chromium base but is designed for high-temperature strength rather than aqueous corrosion. SS 904L is a lower-nickel alternative for sulphuric acid at moderate conditions, and Alloy 20 is a similar nickel-iron-chromium alloy developed for sulphuric acid. Inconel 625 and Hastelloy C276 are used where 825 is not resistant enough – in hot, concentrated chlorides, reducing acids or the most severe sour service.

Typical applications of Incoloy 825

Incoloy 825 is used for sulphuric acid pickling tanks, coils and piping, phosphoric acid evaporators, chemical process equipment, pollution-control and flue-gas desulphurisation systems, seawater-cooled heat exchangers in moderate conditions, nuclear fuel reprocessing, oil and gas downhole tubing and flowlines, clad pipe and vessels for sour gas, and explosives and fertiliser production.

Fabrication

Incoloy 825 forms well cold and hot and is welded by TIG, MIG and manual arc processes. ERNiCrMo-3 filler is commonly used because it gives a weld at least as corrosion resistant as the base metal; matching 825-type fillers are used where specified. Joints must be clean and free of sulphur and lead. Titanium stabilisation means post-weld heat treatment is not needed for intergranular corrosion resistance. Machining needs rigid set-ups and sharp tools.