Nitrided Steels

The carbon content of nitrided steels lies between 0.2 and 0.45 %. They are heat — treatable and are alloyed with Cr and Mo (for improved hardenability) as well as with aluminium or vanadium (nitride formers). Typical nitrided steels and their re­spective compositions are shown in table 4-4. Nitriding is carried out at tempera­tures between 500 and 600 °C, i. e. below the a-y-conversion temperature of steel [VDI80].

As opposed to case-hardened steel, for which high levels of hardness are reached by means of a y-a-phase conversion and the production of martensite, ni­trided steel has a very hard surface thanks to the brittle metal nitrides. The nitro­gen diffusing into the surface layer during the nitriding process forms with the al­loying elements Cr, Mo and Al special nitrides, which mostly precipitate in submicroscopic form and cause high latticework tension, i. e. high surface hard­ness. Fig. 4-8 shows a photo of the structure of nitrided steel 31CrMo12.

However, chip removal of this material takes place before nitriding, usually in a heat-treated state with a structure consisting of tempered martensite and fine, evenly distributed carbides. Nitrided steels are used in a similar range of applica­tions as case-hardened steels (gears, guide strips).