After shot peening some leaf springs we've noticed that there appear to be some spots. Like the color of the peening is different (darker or lighter). Anyway, we were wondering what was the cause of these spots and we found that this spots ressemble those of the scale on the leaf before peening. As a matter of fact, when the peening intensity (or perhaps the hardness of the shot is not that high) is lower these spots seem to have a certain height. I wopuld imagine that it's because the sclae is still attached to the leaf spring and therefore I would assume that the spring itself has not been peened in those areas or if the scale has been removed the peening intensity and/or coverage would be lower than those places where there was no scale before peening.

Wouldn't it be best to remove the scale from the surface of the leaf BEFORE peening?

We've had a lot of variability in some endurance tests with shot peened leaf springs and we were wondering if perhaps this scale has something to do with it.

There is no discussion that peening is actually a very good process to improve fatigue behaviour of metallic materials.
Nevertheless, peening brings better improvements when :
- Material is showing high mechanical properties,
- When shot is harder than the target material,
- Material have been elaborated in order to avoid any defect or heterogeneity (ex. cracks, cavities, foreign embedments...
That is a good reason why scale oxide has to be removed before peening.
The easiest and most effective way to simultaneously descale and peen is to use ceramic beads!
This works properly on connecting rods, hot formed springs, transmission gears...

Hope this helps,
F-Xavier