Allan, every ferritic steel can be nitrided, but generally speaking, the higher alloyed the steel the higher the final hardness value. The best results are achieved with the high-alloy nitiriding steels that contain the correct alloying elements to create enough hard nitrides.
The common case-hardening or QT steels which would be used for cranks, cams and followers definitely can be nitrided.
Here's a picture of mine, having covered 2000 miles in twisty mountain roads (and several ton-up runs
).
Sorry for the bad quality, hard to get a good photo if you can't turn the barrels over without the followers disappearing in their guides, so had to use the flashlight, which didn't work too well.
However, you can see where the lobes have run, but there's no visible or sensible ridge whatsoever.
Cheers