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

In this work, AISI 1010 steel samples were plasma nitrided into 20% N-2 plus 80% H-2 atmosphere at a pressure of 500 Pa (partial pressure of 100 Pa and 400 Pa for N-2 and H-2, respectively), temperatures of 500 and 580 degrees C, during 2 h. Three different procedures for cooling were accomplished after nitriding. In the first procedure the cooling occurred naturally, that is, the sample was kept on substrate holder, In the second one the sample was pulled off and cooling in a cold surface. Finally, in the third cooling process the sample was pulled off the substrate holder down into special reservoir filled with oil held at ambient temperature. The properties of the AISI 1010 steel samples were characterized by optical and electron microscopy, X-ray diffraction, Mossbauer spectroscopy and microhardness tests. Thermal gradient inside the sample kept on substrate holder during cooling process was measured by three inserted thermocouples at different depths. When samples were cooled rapidly the transformation of epsilon-Fe2-3N to gamma'-Fe4N was inhibited. Such effect is indicated by the high concentration of epsilon-Fe2-3N phase observed in the compound zone. To get solid state solution of nitrogen in the diffusion zone, instead of precipitates of nitride phases, the cooling rate should be higher than a critical value of about 0.95 degrees C/s. When this value is reached at any depth of the diffusion zone, two distinct diffusion zones will appear. Temperature gradients were measured inside the samples as a consequence of the plasma treatment. It's suggested the need for standardization of the term "treatment temperature" for plasma treatment because different nitrided layer properties could be reported for the same "treatment temperature". (C) 2007 Elsevier B.V. All rights reserved.