Delayed cracking in plasma nitriding of AISI 420 stainless steel

The phenomenon of delayed cracking in nitrided layers after DC-plasma nitriding of AISI 420 steel has been observed by optical microscopy. Prior to the plasma treatment, the samples were austenitized at 1303 K for 30 min and then oil quenched. Two tempering conditions were assessed: one group was tempered at 673 K, while another group was tempered at 943 K.All samples were subjected to sputtering, in the plasma chamber, to remove the passive oxide layer, under a 1:1 Ar/H2 atmosphere. Finally, specimens were plasma nitrided at 673 K for 20 h, with a 1/3: N2/H2 relation, at a pressure of 6.5 hPa and 700 V bias in the nitriding chamber.The nitrided layers were analyzed initially by X-ray diffraction (XRD). Detailed observations were conducted at frequent and regular intervals under optical microscopy (OM) and scanning electron microscopy (SEM) with secondary and back-scattered electrons detectors. The results revealed that after an incubation time, even without any external disturbance, cracks are formed and propagate in the nitrided layers. Both groups of samples were equally affected. The presence of precipitated particles and local residual stresses are possible causes of such a phenomenon.