Inconel 617 is good at 700 °C, especially in oxidation resistance and creep. But it’s expensive, and you’ll want experienced welders, as it’s unforgiving. 740H is a newer alloy, tailored for these conditions. It offers great creep strength and weldability, especially with post-weld heat treatment. Sourcing can be hard, tho. Alloy 263 is a compromise, excellent for formed components and decent for creep, but not quite as tough as 617 or 740H at 700 °C. Some prefer it for ease of fabrication, but you’ll need to assess whether its strength is enough for you.

Super 304H is cheaper and easier to fabricate, but it’s the outlier in long-term creep. Fine for <650 °C, but at 700 °C, you’re at its boundary.