Plasma Nitriding of ISO 5832-1 Stainless Steel with Intermittent Nitrogen Flow at 450°C

Abstract

ISO 5832-1 stainless steel specimens were plasma nitrided at different nitrogen potentials. The main goal was to obtain the S-phase with different nitrogen concentrations and free of chromium-based precipitates. The control of nitrogen potential was made by pulsing the gas at predetermined times: 10/10, 05/15, 02/18, and 01/19, where the numbers represent the time in minutes that the nitrogen flow was kept on/off, respectively. For all pulsing conditions, the nitriding was carried out at 450 °C for 2 hours. After nitriding, specimens were characterized by means of optical (OM) and electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and microhardness. Results show that the nitrided layer thickness decreases with decreasing times of nitrogen pulse, and that lower times of nitrogen flow lead to lower precipitation of chromium nitrides. It can be thus concluded that the use of intermittent nitrogen flow is an alternative to control the nitrided layer in terms of thickness, hardness, and the amount of nitrogen present in phase ɣ N (S-phase).

Plasma Nitriding of ISO 5832-1 Stainless Steel at 425 ºC with Intermittent Nitrogen Flow

“…When the nitrogen flow is turned on, some of the carbon released from the reactor walls reacts with nitrogen, forming CN, which, in its turn, is eliminated from the atmosphere by the vacuum system. This behavior is similar to those reported by Reis et al 22 and Sphair 21 . Figure 3 shows cross-sections micrographs of specimens 8C and 8P0218.…”

“…It is possible to observe that, for all conditions, a double layer is present, where the outer layer is nitrogen-rich (γ N ) and the inner one is carbon-rich (γ C ) 5,16,21 . Similar results were obtained in previous work, where the nitriding process were carried out at 450ºC 22 . Nitrogen and carbon concentration profiles measured by GDOES by Sphair et al 21 showed that there is a high concentration of nitrogen close to the material surface, and a significant decrease at distances around 7 to 10 µm from the surface.…”

“…When comparing the austenite expansion that occurred for condition 2C with the work of Reis et al 22 , in which nitriding was carried out at 450ºC, the results obtained in this work are very similar, indicating that the hypothesis that the solubility limit was reached is valid. It is also possible to see that the degree of precipitation is lower in the present work when compared to the work of Reis et al 22 .…”

“…The thicknesses of outer, inner, and total layers were measured, and the results are shown in Table 3. It can be observed that the thickness of γ N increases with increasing treatment times 8,13,24 , and decreases when intermittent nitrogen flow is used 22 . When comparing the thicknesses values with those obtained by Sphair 21 , who reported layer thicknesses of 6.4 μm for the condition 2C and 5.0 μm for the condition 2P0218, both obtained at 400ºC, it observed that the increase in temperature to 425º resulted in a significant increase of the nitrided layer thickness.…”

“…The use of gas pulses, as shown by Sphair 21 , has economic advantages when compares to the use of continuous gas flow, since it is possible to produce less brittle layers with lower probability of crack formation at the same time that it is possible to make the process at temperatures higher than 400ºC, allowing to obtain higher thickness layers with minimum nitrides percipitation 22 . Regarding gas consumption, Sphair 21 observed that the savings are not significant for a 2 hours process carried out according to the experimental conditions of the referred work.…”