Which materials can be plasma nitrided and which hardness can be achieved in the process?

Nitridable materials are steels, high-alloy steels and titanium.
You will find an overview of the typical hardness values of different material groups here:

material group	material number	material name	OH (HV)	NHD (mm)
cast iron	0.6025	GJL-250	300 – 500	0,1 – 0,2
	0.7060	GJS-600	400 – 600	0,1 – 0,3
constructional steel	1.0116	S235 J2G3	150 – 350	0,2 – 0,8
	1.0570	S355 J2G3	300 – 550	0,2 – 0,6
microalloyed steel	1.0976	S355 MC	200 – 350	0,1 – 0,6
case hardening steel	1.7131	16MnCr5	550 – 700	0,2 – 0,6
heat-treatable steel	1.0503	C45	350 – 500	0,2 – 0,8
	1.7225	42CrMo4	550 – 700	0,2 – 0,6
nitriding steel	1.8519	31CrMoV9	700 – 900	0,2 – 0,6
	1.8550	34CrAINi7	900 – 1100	0,2 – 0,6
cold work steel	1.2379	X155CrVMo12-2	900 – 1300	0,05 – 0,2
hot forming tool steel	1.2344	X40CrMoV5-1	900 – 1300	0,05 – 0,3
high-speed steel	1.3343	HS 6-5-2	1000 – 1400	0,005 – 0,1
stainless steel	1.4021	X20Cr13	900 – 1200	0,01 – 0,2
	1.4122	X35CrMo17	950 – 1300	0,01 – 0,2
	1.4301	X5CrNi18-10	900 – 1300	0,01 – 0,1
	1.4571	X6CrNiMoTi17-12-2	900 – 1300	0,01 – 0,1
valve steel	1.4718	X45CrSi9-3	900 – 1250	0,05 – 0,2
	1.4882	X50CrMnNiNbN21-9	900 – 1300	0,01 – 0,1
duplex steel	1.4462	X2CrNiMoN22-5-3	900 – 1300	0,01 – 0,1
maraging steel	1.6359	X2NiCoMo18-8-5	850 – 1150	0,05 – 0,15