Oxide-scale stability and creep strength decide it, not melting point. Every grade here melts above 1260 °C, but the temperature a part can actually run at is far lower. Above 900 °C the shortlist is Hastelloy X, Inconel 617 and Inconel 602CA, with cast Inconel 713C for turbine hardware.
Melting point is the wrong number
A melting range tells you when the alloy turns to liquid. It tells you nothing about when it stops being usable. Between those two points sits the whole engineering problem: the metal creeps under load, the surface oxidises, and the microstructure coarsens until the part no longer carries its design stress.
For reference, 713C melts around 1260-1290 °C and is rated to roughly 982 °C in service. That gap is normal, and it is why specifying against melting point leads to parts that fail early.
What actually governs it
The oxide that forms. Chromia scales are good to roughly 1000-1100 °C; alumina formers such as 602CA go further and spall less under thermal cycling. Above the point where the scale breaks down, metal loss accelerates faster than any strength calculation predicts.
Creep rupture strength at the design temperature and the design life. A grade that is strong at 800 °C may be unusable at 1000 °C, and the data you need is the rupture curve at your temperature, not the room-temperature tensile figure on the certificate.
Thermal cycling. A part that cycles daily needs a scale that survives spalling far more than a part held at temperature continuously. This is where 602CA and 617 separate themselves from plain 600.
The shortlist, by duty
Continuous furnace furniture, retorts and muffles: 602CA and 601, chosen for alumina scale adhesion. Hastelloy X where both oxidation and moderate strength are needed.
Combustion and turbine hot sections: 617 where wrought sections are possible; 713C where the part is cast and the stress is high.
Where the environment also carries sulphur or chlorides at temperature, the answer changes again — those species attack the scale chemically, and the grade has to be picked for that rather than for temperature alone.