Composition Comparison
The question of which alloy performs better above 900 °C is answered by two elements before any test data is quoted. Grade UNS N06617 contains 10.0 to 15.0 percent cobalt and an aluminium addition of 0.8 to 1.5 percent, which allows it to form a dense, slow-growing aluminium oxide layer at high temperature. Grade UNS N06625 contains molybdenum and niobium but no deliberate cobalt and no aluminium addition of that order, so its oxidation protection depends on chromium oxide and its strength depends on solid-solution hardening.
| Element (wt %) | UNS N06617 (ASTM B166) | UNS N06625 (ASTM B446 / B444) |
|---|---|---|
| Nickel | 44.5 min | 58.0 min |
| Chromium | 20.0-24.0 | 20.0-23.0 |
| Cobalt | 10.0-15.0 | Not specified |
| Molybdenum | 8.0-10.0 | 8.0-10.0 |
| Aluminium | 0.8-1.5 | 0.40 max |
| Carbon | 0.05-0.15 | 0.10 max |
| Iron | 3.0 max | 5.0 max |
| Niobium plus tantalum | Not specified | 3.15-4.15 |
Note the direction of the trade-off. N06617 is designed for a high-temperature load path; N06625 is designed for a corrosion load path with useful strength to about 815 °C.
Creep and Stress Rupture Above 900 °C
Creep rupture life is controlled by the rate at which dislocations can climb past obstacles and by the stability of the strengthening phases. The cobalt and molybdenum in N06617 raise the stacking fault energy and slow diffusion, and its higher chromium and aluminium content maintains a coherent oxide that does not spall under thermal cycling. In practice this means N06617 retains useful stress-rupture strength in the 900 to 1100 °C band and is a standard choice for gas turbine combustion components and industrial furnace muffles operating continuously in that range.
N06625 loses most of its engineering strength advantage above about 900 °C. Its niobium-rich carbides and its solid-solution hardening still provide good creep resistance in the 600 to 815 °C window, which makes it the preferred grade for superheater tube, expansion bellows and exhaust systems that see thermal cycling with a corrosion duty rather than sustained high-temperature load.
Oxidation, Carburisation and Corrosion
Oxide scale adherence decides service life in a furnace atmosphere. N06617 forms a chromium-aluminium oxide that remains protective through repeated heating and cooling, and it also resists carburisation better than a chromium-oxide-only alloy because the alumina layer blocks carbon diffusion. In sulphur-bearing combustion gases at high temperature the advantage narrows, because nickel-rich alloys in general form low-melting nickel sulphides; in that environment the maximum metal temperature must be limited and the atmosphere chemistry reviewed.
Wet corrosion reverses the ranking. In chloride-bearing water and in acidic halide streams N06625 resists pitting and crevice corrosion far better, and its higher molybdenum content raises the critical pitting temperature. For a heat exchanger that runs hot but also sees chloride contamination, the choice is usually resolved by deciding which of the two degradation modes is faster, and by setting an operating limit on the other.
Fabrication and Product Forms
Both alloys are forged under ASTM B564 and supplied as bar under ASTM B166, as plate under ASTM B168 or B443, and as seamless tube under ASTM B167 for N06600-class grades and ASTM B444 for N06625. N06617 is commonly supplied as bar, plate, sheet and seamless tube and is welded by gas tungsten arc with matching filler metal. Because the alloy is sensitive to heat input, weld procedures should limit interpass temperature and avoid excessive dwell in the 540 to 760 °C range, where grain-boundary precipitation can reduce ductility. Machining follows the same rules as other high-nickel alloys: low surface speed, heavy feed, rigid tooling, and no dwelling that work hardens the surface.
Selection Matrix
| Service condition | Preferred grade | Reason |
|---|---|---|
| Continuous service at 900 to 1100 °C with mechanical load | UNS N06617 | Alumina-forming oxide plus cobalt-assisted creep resistance |
| Thermal cycling furnace internals and muffles | UNS N06617 | Scale adherence and carburisation resistance |
| Seawater or chloride-bearing exchanger duty to 815 °C | UNS N06625 | Molybdenum and niobium for localised corrosion resistance |
| Sour gas, flue gas desulphurisation ducting | UNS N06625 | Pitting and crevice corrosion resistance |
| Structural component at moderate temperature with halide exposure | UNS N06625 | Higher room-temperature yield strength than N06617 |
Frequently Asked Questions
Q: Which alloy is stronger at 1000 °C?
A: UNS N06617. Its cobalt content and alumina-forming capability give it better stress-rupture life in that band, and it was developed specifically for high-temperature combustion and furnace service.
Q: Does UNS N06625 have any advantage above 900 °C?
A: Only where corrosion resistance governs and mechanical load is low. Its strength falls away in that range, so it should not be selected for a sustained-load component at 1000 °C.
Q: Can the two alloys be joined to each other?
A: Yes, with a filler metal chosen for the more highly alloyed side and a procedure qualified to ASME BPVC Section IX. Service temperature and thermal expansion mismatch should be considered in the joint design.
Q: Which grade resists seawater better?
A: UNS N06625, because of its molybdenum and niobium content. UNS N06617 is an oxidation-resistant alloy and is not the right choice for chloride-bearing wet service.
Q: What specification covers the bar form of each grade?
A: ASTM B166 covers both alloy bar stocks among other nickel-chromium-iron grades, while ASTM B446 is used for the N06625 bar form; forged product follows ASTM B564 in both cases.
Q: Is post-weld heat treatment required?
A: Usually not, but long exposure in the 540 to 760 °C range should be avoided in both grades because grain-boundary precipitation can reduce ductility and corrosion resistance.
