Overview

Alloy 800HT is an austenitic, solid-solution iron–nickel–chromium alloy — the highest creep-rupture-strength variant of the 800 series. This datasheet presents the material within the American (ASTM / ASME / SAE-AMS / UNS) standard system.

It has the same base composition as alloys 800 and 800H, but with the most tightly controlled chemistry of the three: carbon restricted to 0.06–0.10% and the combined aluminium-plus-titanium content held to 0.85–1.20% (versus 0.30–1.20% for 800H). Combined with a high-temperature solution anneal at 1149 °C (2100 °F) minimum, this controlled chemistry produces a coarse grain structure and significantly higher creep and stress-rupture strength than both alloy 800 and alloy 800H. The chromium provides oxidation and corrosion resistance; iron resists internal oxidation; and the high nickel content maintains a ductile, stable austenitic structure with strong resistance to chloride-ion stress-corrosion cracking.

It is selected for the most demanding long-term, high-temperature structural service: petrochemical furnace cracker tubes, pigtails, headers and reformer tubing; ethylene and acetic-anhydride process equipment; power-plant superheater and reheater tubing; gas-cooled-reactor heat exchangers; and industrial-furnace radiant tubes, muffles, retorts and heat-treating fixtures. It can be dual-certified to UNS N08810 (alloy 800H) where required.

1. Physical Properties

Values per Special Metals official datasheet, solution-annealed (coarse-grain) condition.

2. Chemical Composition (Limiting, wt %)

Limiting composition per ASTM B408 (UNS N08811).

Special requirement: high-temperature solution anneal (≥1149 °C) producing coarse grain, required for creep-rupture performance.

3. Mechanical Properties

Typical room-temperature properties, solution-annealed (coarse-grain) condition.

The restricted Al+Ti chemistry and coarse grain give the highest creep and stress-rupture strength of the 800 series, especially above 700 °C. Room-temperature strength is lower than fine-grain alloy 800. The alloy is solid-solution and not age-hardenable. Values are typical; confirm against the mill test certificate for each delivery.

4. Corrosion and High-Temperature Resistance

5. Heat Treatment

An austenitic solid-solution alloy; not age-hardenable. The defining treatment is a high-temperature solution anneal to develop coarse grain.

Solution Anneal (defining treatment) Temperature: ≥1149 °C (2100 °F) Result: coarse grain structure required for optimum creep and stress-rupture strength.

This minimum anneal temperature, together with the restricted carbon (0.06–0.10%) and Al+Ti (0.85–1.20%) chemistry, is what gives 800HT its maximum-strength characteristics. Cool rapidly through 540–760 °C where aqueous corrosion resistance is required, to limit sensitisation.

6. Weldability and Joining

Good weldability by all common processes, similar to alloy 800. Fillers should be matched to high-temperature service.

For maximum strength, alloy 617 (ERNiCrCoMo-1) wire or 117 (ENiCrCoMo-1) electrodes are suggested. To avoid stress-relaxation grain-boundary cracking above ~540 °C, post-weld heat at ~900 °C for 1 h/inch then air cool.

7. Machinability and Fabrication

Machining Guidelines

Machining behaviour is the same as alloy 800; the alloy work-hardens during machining.

Forming Processes

8. Applications

9. Available Product Forms and Standards (ASTM / AMS System)

ASME Boiler and Pressure Vessel Code Case 1987 (higher allowable stresses than N08810). Dual-certifiable to UNS N08810 (alloy 800H) when additional requirements are met.

10. Comparison with Related Alloys (Alloy Designation System)