Alloy structural steels for nitriding are used for surface hardening by the saturation with nitrogen the surface of the 
finished product. To the process of nitriding are used steels whose main ingredients are aluminum and chromium as well 
as molybdenum, titanium and vanadium, which together with nitrogen create very hard nitrides AlN, CrN, Mon, TiN, VN. 
Nitriding is used in case of midcarbon steels with a carbon content of 0.25 - 0.45%. 
Apart from the steels intended for nitriding, it is also possible to use nitrogen to harden the surface in case of some 
steels to hardening and tempering , corrosion resistant and hight-speed steels.

Symbol of steel grade according to the PN marking the digit at the beginning refers to the average carbon 
content in hundredths %, followed by letters denoting
H - Chrome
M - Molybdenum
F - Vanadium
J - Aluminium

then if there are numbers after the letters they refer to the average content of a certain component in %

38HMJ - chrome-molybdenum-aluminium
25H3M - chrome-molybdenum
33H3MF - chrome-molybdenum-vanadium