Valenite has a new grade of carbide insert for milling high-temp alloys. I've had good luck with their PDMX 0905ZEER51 grade VP2003 inserts on their V556 high-feed mill. It's not depth of cut that keeps the material from work-hardening, it's staying in the feed. NO DWELLS!
I'm running 200 sfm in Inco 718 @ 42Rc. Though it's a shallow depth of cut (.060), it's feeding at .024 inches per tooth. Gets the job done quick! There are ceramic grades that work well too, so I hear, but I'm afraid to apply the speeds they specify for fear of changing the temper of the workpiece. I can't imagine what a shower of sparks would do to the workpiece (or the cutter for that matter.)