Modificações estruturais e mecânicas de superfície pela inserção de nitrogênio e carbono em um aço inoxidável superaustenítico

Abstract

The surface nitrogen or carbon insertion into Fe-Cr-Ni alloys produce an expanded and metastable phase supersaturated by nitrogen (SN) or carbon (SC), which exist in a very large compositional range. Although their technological advantages are well known, the same is not true regarding the basic physical properties. In this scenario, the superaustenitic stainless steel (SASS) was surface-modified for investigating two different but complementary aspects. Firstly, a method for the SN-phase control through nitrogen plasma immersion ion implantation (PI3) was evaluated; secondly, the analysis focused in the nitrogen and carbon interaction in the material´s structure to produce both SN e SC phases. (i) The parameter Epulse/A (surface density of implantation energy) was evaluated for controlling the SN phase formation, in an experimental set-up where the implantation conditions varied whereas temperature was kept at 320 ºC. The layers´ thicknesses increased from 1.1 to 1.9 m with the applied voltages (6.2-10.4 kV) and Epulse/A values; however, the nitrogen saturation followed the opposite trend, which also ruled the surfaces strength against plastic deformation. Hardness was 32% higher for the 6.2 kV treatment as compared with the 10.4 kV one. Under indentation, the surfaces were brittle in the former and ductile in the latter condition, in a clear correlation with the SN phase and -´ nitrides ratio in the modified layers. To summarize, the Epulse/A parameter correlated with ion fluencies could provide structural control in SASS modified surfaces. (ii) SN and SC phases were studied in sequential nitriding (N) and carburizing (CEM) treatments, each of them carried out at 320 ºC for 3 h, whose substrates were subjected to the same annealing time. The N step was attained through PI3, whereas CEM (a unique batch for all samples) were carried out by glow discharge. In both N+C and C+N, a nitrogen rich case laid over a carbon-containing region. It was conspicuous that Epulse/A (from nitriding) directly influenced the total thicknesses of the layers (5 m), independently of the processing sequence. In the N+C, the nitrogen-rich surface partially hindered the carbon diffusion; nevertheless, it promoted a rearrangement of the top nitrided region into a thinner and more nitrogensaturated layer. Part of the SN phase decayed thermally into  and ´ precipitates. Carbon atoms in the C+N treatment displaced inward the substrate, being replaced at the surface by the nitrided layer. Such deeper carbon-rich regions provided a loadbearing effect in indentation tests, resulting in the highest hardness profiles observed here, with 10.9 GPa at near surface. This sequential C+N provided the highest nitrogen and carbon retention among all the studied treatments, including here the nitrocarburizing glow discharge.