Method for hardening metal and specifically steel surfaces. The component to be treated acts as a cathode and is exposed to a nitrogen/hydrogen plasma at elevated temperatures. This first removes surface oxide and other contaminations; next, the nitrogen ions are incorporated in the metal structure close to the surface, resulting in an increased surface hardness Furthermore, slip properties and corrosion resistance are improved in many cases. Plasma nitriding can be used not only on steel but also on aluminium and titanium. Also referred to as glow nitriding.