Influence of steel composition and sintering parameters on glow-discharge nitriding of sintered steels

The present study evaluates the influence of steel composition, compacting and sintering parameters on the nitridability of sintered steel samples using factorial experimental design. Fe-C-Cr-Mo (Ct: 3 wt. %; Mo: 0.5 wt.%) steel powders with different carbon content (0.2, 0.35, 0.5 wt. %) were compacted at 700 MPa at 298 K (cold) or 423 K (warm) temperature; the samples were sintered at low (1393 K; t = 30 min) or high (1523 K; t = 60 min) temperature. Glow discharge nitriding treatment has been performed on sintered steel samples at 773 K for 8 and 24 hours using a 80 vol. % N2 and 20 vol. % H2 treatment atmosphere. The nitriding treatment produces a hardened surface layer, consisting in an outer compound layer, in which iron and chromium nitrides are presents and an inner diffusion layer, characterised by the presence of nitride precipitates dispersed in a nitrogen rich metal matrix. The hardened layer thickness increases as nitriding treatment time increases and ranges from ~ 200 (t = 8 h) to ~ 300 (t = 24 h) μm. The factorial experimental analysis bas shown that the case depth of the nitrided layer is significantly influenced by the sintering parameters: thicker hardened layers are obtained sintering the samples at 1393 K for 30 min.