chatters on the way back, fine on the way in

Atlas, carbide, facing. Cut in toward center is acceptable. When I feed back out it chatters and leaves a pattern. Same tool, same rpm. I don't change anything except direction.

You're likely climbing with the insert when you reverse. A tool with rake wants to be fed so the cut loads it into the holder, not out of it. Face from the center out, or use a tool meant for that direction. The pattern is the insert bouncing, not the spindle changing its mind.

Faced from the center out. Pattern's gone. I was feeding the convenient direction and calling it the cut. Noted, without converting to HSS. The Atlas is still an Atlas.