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 porosity levels of these materials. On wrought austenitic stainless steels this treatment 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 sintered 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 profiles show high hardness values in the modified layers and a steep decrease to matrix values, thinner hardened layers and lower hardness values are observed on AISI 316L, samples, in comparison with AISI 410 and AISI 430L. samples. The S phase is detected on the modified layers of the ion-nitrided AISI 316L samples. The crystallographic characterisation has shown that a face centred 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, the 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.