This work aims to characterize the phases, thickness, hardness and hardness profiles of the nitride layers formed on the CA-6NM martensitic stainless steel which was plasma nitrided in gas mixtures containing different nitrogen amounts. Nitriding was performed at 500 ºC temperature, and 532 Pa (4 Torr) pressure, for gas mixtures of 5% N2 + 95% H2, 10% N2 + 90% H2, and 20% N2 + 80% H2, and 2 hours nitriding time. A 6 hours nitriding time condition for gas mixture of 5% N2 + 95% H2 was also studied. Nitrided samples results were compared with non-nitrided condition. Thickness and microstructure of the nitrided layers were characterized by optical microscopy (OM), using Villela and Nital etchants, and the phases were identified by X-ray diffraction. Hardness profiles and hardness measured on surface steel were determined using Vickers hardness and nanoindentation tester, respectively. It was verified that nitrided layer produced in CA-6NM martensitc stainless steel is constituted of compound layer, being that formation of the diffusion zone was not observed for the studied conditions. The higher the nitrogen amounts in gas mixture the higher is the thickness of the nitrided layer and the probability to form different nitride phases, in the case γ'-Fe4N, ε-Fe2-3N and CrN phases. Intrinsic hardness of the nitrided layers produced in the CA-6NM stainless steel is about 12-14 GPa (~1200-1400 HV)

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Resumo (0,03%C; 0,35% N;13,2%Cr;11,8% Mn;2,8%Si; bal (0,03%C; 0,35% N;13,2%Cr;11,8% Mn;2,8%Si; bal %Fe) showed a cavitation erosion resistance quite near the most resistant commercial alloys.

Brazing has been increasingly used to join metals to advanced ceramics. Brazing covalent materials requires either the use of active filler alloys or the previous metallization of the surface. To that end, a new and simple mechanical technique has been applied to metallize advanced ceramics, thus avoiding the use of costly Ti-based active filler alloys. The mechanical metallization of Si 3 N 4 with Ti was employed as an alternative route to deposit active metallic films prior to brazing with stainless steel using 72% Ag-28% Cu or 82% Au-18% Ni eutectic alloys. The brazing temperatures were set to 40 • C or 75 • C above the eutectic temperature of each filler alloy. Ti-films of average thickness 4 µm produced adequate spreading of both filler alloys onto Si 3 N 4 substrates, which were subsequently brazed to stainless steel. The interface of Si 3 N 4 /310 stainless steel basically consisted of a reaction layer, a precipitation zone and an eutectic microconstituent. Mechanically sound and vacuum-tight joints were obtained, especially upon brazing at relatively lower temperatures. Increasing the brazing temperature resulted in thermal cracking of the Si 3 N 4 , possibly due to increased thermal stress.

Este é um artigo publicado em acesso aberto (Open Access) sob a licença Creative Commons Attribution Non-Commercial, que permite uso, distribuição e reprodução em qualquer meio, sem restrições desde que sem fins comerciais e que o trabalho original seja corretamente citado. Diffusion Welding of Stainless Steels for Fabrication of Compact Heat ExchangersAbstract: The joining of dissimilar materials or components with particular geometry often requires special union procedures, for instance a high density energy source as laser or electron beam for fusion welding or even necessarily a solid state welding process. In particular, this work describes the recent experience on applying the solid-state diffusion bonding (SSDB) to different types of stainless steels: austenitic AISI 316L, duplex UNS 31803 and superduplex UNS 327250. The SSDB was realized under high vacuum at 1050 °C for 60 min, employing uniaxial hydrostatic pressure of 18-35 MPa. The quality of the joints was evaluated by optical microscopy and mechanical tests. The preliminary results indicate the necessity of optimization of the process parameters for the duplex and superduplex steels. The positive results of the present study allowed the fabrication of compact heat exchanger prototype in stainless steel, employing the new manufacturing method, developed by LABTUCAL/UFSC, whereby a water jet is applied to precisely cut the channels of the heat exchanger core.