A soft section of material typically 2-5 mm in width between adjacent hardened zones or between the start and finish of an overlapping progressive hardening run. This may result in a flat spot during use.
Progressively hardened wheel treads, circular cams and rollers always have a “slip” which in most cases has no effect on the functionality of the component. As the diameter decreases the impact of a flat spot increases.
If this is undesirable the component should be designed for spin or progressive circular (sometimes called combined or ring) flame hardening.