Effect of cooling rate on properties of plasma nitrided AISI 1010 steel

摘要

In this work, AISI 1010 steel samples were plasma nitrided into 20% N2100 Pa and 400 Pa for N2and H2, respectively), temperatures of 500 and 580 °C, during 2 h. Three different procedures for cooling wereaccomplished after nitriding. In the first procedure the cooling occurred naturally, that is, the sample was kept on substrate holder. In the secondone the sample was pulled off and cooling in a cold surface. Finally, in the third cooling process the sample was pulled off the substrate holderdown into special reservoir filled with oil held at ambient temperature. The properties of the AISI 1010 steel samples were characterized by opticaland electron microscopy, X-ray diffraction, Mössbauer spectroscopy and microhardness tests. Thermal gradient inside the sample kept on substrateholder during cooling process was measured by three inserted thermocouples at different depths. When samples were cooled rapidly thetransformation of ϵ-Fe2 − 3N to γ′-Fe4N was inhibited. Such effect is indicated by the high concentration of ϵ-Fecompound zone. To get solid state solution of nitrogen in the diffusion zone, instead of precipitates of nitride phases, the cooling rate should behigher than a critical value of about 0.95 °C/s. When this value is reached at any depth of the diffusion zone, two distinct diffusion zoneswill appear. Temperature gradients were measured inside the samples as a consequence of the plasma treatment. It's suggested the need forstandardization of the term “treatment temperature” for plasma treatment because different nitrided layer properties could be reported for the same“treatment temperature”.