If all else fails, switching to a CBN insert will happily turn the hardened scale and chilled spots all day long, even taking light passes that don't cut through and under the scale in one go.
You'd ideally need to run at somewhere around 1200 sfm (so in the 600-900rpm range for a 6" workpiece), with CBN, which does raise a consideration of work-holding and machine rigidity…
So in practice it would be "as fast as you think is still safe", and don't worry about sparking or glowing chips, that's very much par for the course with CBN.
CCMW inserts suitable for small hobby lathes are available in CBN, but they're about 5 times the price of a carbide insert so it might not be the first port of call if you're cost-conscious…
But I wouldn't go back to not having a couple of them stashed in the workshop now I've used them, the flexiblity to machine really hard materials without just mullering inserts is quite a useful one (especially in a workshop with no surface or cylindrical grinding facilities).