The tolerance is very thight, but usally, for liquid nitriding with the proper cooling system, it is possible to not have any movement.
Only the parts with big diameter and very thin wall could move from 0 to 150 µm.
As the other recommended, the best way is to do test with the nitriding...
And other process superior to QPQ is the Arcor process. Both are salt bath process but the Arcor achieve a thicker white layer with less porosity. You should achieve 15-18µ of layer.
