Overview

1.4959 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 European (DIN / EN / Werkstoff-Nummer / VdTÜV) 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 1.4958 grade. 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 1.4958 grade (W.Nr. 1.4958) where required.

1. Physical Properties

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

2. Chemical Composition (Limiting, wt %)

Composition per EN 10095 (W.Nr. 1.4959, X5NiCrAlTi31-20).

Nominal: Fe-32Ni-21Cr with Al+Ti ≥0.85%. The higher combined Al+Ti (0.85-1.20%) gives 800HT its superior creep and high-temperature strength over 800/800H.

3. Mechanical Properties

Solution-annealed (+AT) condition, per EN 10095 for W.Nr. 1.4959.

Values per EN; confirm against the inspection certificate (EN 10204).

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.

Use fillers matched to high-temperature service. For service above ~540 °C a post-weld stress-relief (~900 °C) may be used to avoid grain-boundary cracking.

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 (EN / DIN System)

DIN EN 10302 (creep-resisting steels and nickel alloys); VdTÜV-Werkstoffblatt 412 covers all product forms.

10. Comparison with Related Alloys (Werkstoff-Nummer System)