Glow-discharge nitriding of sintered stainless steels

The glow-discharge nitriding process is particularly suitable to harden the surface of sintered stainless steels components, owing to the high porosit y levels of these materials. On wrought austenitic stainless steels this tr eatment produces a metastable phase, known as supersaturated austenite or S phase, which has shown high hardness values and good corrosion resistance. In the present paper the influence of glow-discharge nitriding process on the microstructural and mechanical properties of AISI 316L, austenitic sint ered stainless steel has been evaluated and it is compared with the results obtained with ion-nitrided martensitic (AISI 410) and ferritic (AISI 430L) sintered stainless steels. The ion-nitriding treatment, performed at 773 K for 4 and 8 h, produces modified surface layers. The microhardness profile s show high hardness values in the modified layers and a steep decrease to matrix values, thinner hardened layers and lower hardness values are observ ed on AISI 316L, samples, in comparison with AISI 410 and AISI 430L. sample s. The S phase is detected on the modified layers of the ion-nitrided AISI 316L samples. The crystallographic characterisation has shown that a face c entred tetragonal lattice as base for this phase fits well the diffraction spectra, in respect of the 'traditional' face centred cubic lattice usually adopted, since the lattice shows a strong distortion, in spite of this, th e d-spacing values calculated with a f.c.t. base show a good agreement with literature data, when the used f.c.c. indexing is modified for the f.c.t. lattice. (C) 2001 Elsevier Science B.V. All rights reserved.