Frank – well yes and no, depending on how you went about nitriding.
To be precise, yes nitrogen hardens surfaces, but it can be applied in different ways and at different temperatures, so you can get an improvement in the grain or toughness out of the process – but to be absolutley fair to Nicholas Farr , it is the temperature rather than the nitrogen that has that effect. The nitrogen migrates into the surface layers and alters the crystalline structure and natrue of the alloy.
Controlability – I’m not a process engineer, so I’ll make a suggestion raher than go out on a limb. Controlling the depth of case in any process is not usualy a problem – (even dunking in Kasenit gives a moderately known quantity!) What made our people specify a particular process and us in DQA police it was other factors – vulnerability to surface cleaniness, ordimensional stability., and whether you want to treat the whole item, or just some of it.
However, once with steel, as far as I know, you start adding carbon, you are involved in controlling cooling because carbon alters the crystalline structure of a solid solution. That, generally means changes in dimension (in complex shapes and near finished items?), because some kind of “locking” of the structure is required. Depends on temperatures of course, so one might well choose the process according to the item and how it is made. Nowadays there are so many processes that one ought to be very precise about what is being done before going any further than a pretty wide generalisation, and at that point one wants to talk to specialists.
Kasenit, at our level gives a depth of about .003 per dunk, so a couple of dunks followed by a quench does the trick. it is VERY hard, but if you do need to be rid of it, you can eiher he-heat and air cool before machining, and then reheat and quench if need be, or at that sort of thickness get under it in one, given a decent tool.