ABSTRACT200 μm thick solution annealed AISI 316L stainless steel foils were implanted with Ar ions to produce a 0.25 at. % concentration-depth plateau extending from the near surface to a depth of ≈ 250 nm, and then annealed at 550°C for 2 hours to form small Ar bubbles and Ar-vacancy clusters. Distinct sets of samples (including control ones without Ar) were irradiated at the temperature of 550 °C with Au ions accelerated at 5 MeV to produce an average damage content about ≈36 dpa at the region containing the Ar plateau. These samples were investigated by transmission electron microscopy using plan-view specimens prepared by ion milling. In contrast with the control samples where the irradiation causes the formation of a high concentration of extended defects and large cavities, carbonite precipitation of 1:1 metal-carbon (MC) content with a cubic structure occurs only in the samples containing the Ar bubbles. This precipitation phenomenon is not commonly observed in the literature. The results are interpreted considering that the precipitate growth process requires the emission of vacancies which are synergistically absorbed by the growth of the Ar bubbles.
The combination of the austempered ductile iron mechanical properties strongly depend on the parameters used on the austempering cycle. On this study, the influence of austempering time and austenitizing temperature on the properties of a ductile iron were evaluated. A metallic bath of Zamak at 380°C was used as an austempering mean. A set of ductile iron blocks were austenitized at 900°C for 90 minutes and submitted to different austempering times in order to determine the best combination of microstructural and mechanical properties. After the definition of the time of austempering, the reduction of the austenitizing temperature was evaluated. The best combination of properties was obtained with austenitizing at 860°C and austempering during 60 minutes.
Resumo Este trabalho reporta a viabilidade de realizar o tratamento térmico de austêmpera, para obtenção de um Ferro Dúctil Austemperado (ADI), utilizando uma liga de Zn-Al-Mg-Cu, como meio de resfriamento e permanência em patamar, durante o processo de tratamento térmico. Por meio da obtenção da curva de resfriamento da liga Zamac 5, constatou-se a viabilidade de realização da austêmpera em temperaturas superiores a 380ºC. A partir de corpos de prova de ferro fundido nodular de matriz predominantemente perlítica, instrumentados e austenitizados a 870°C e 900°C, avaliou-se a taxa de resfriamento promovida pela liga Zamac 5, até a temperatura de 400°C. O tratamento mostrou-se eficiente, utilizando a liga como meio de austêmpera, sendo registradas taxas de resfriamento superiores a 6°C/s. A análise morfológica constatou a formação de ausferrita, sem a presença de perlita, caracterizando assim a formação do ADI. Palavras-chave: Austêmpera; Zamac; ADI; Ausferrita OBTAINMENT OF ADI BY AUSTEMPERING TREATMENT USING ZAMAK 5 BATH Abstract This work reports the feasibility of conducting austempering heat treatment to obtain an Austempered Ductile Iron (ADI), using a Zn-Al-Mg-Cu alloy, as means of cooling and sustaining the thermal arrest during the heat treatment process. By obtaining the alloy cooling curve, with a composition corresponding to Zamak 5, it was verified the feasibility of austempering at temperatures above 380ºC. From samples of nodular cast iron with predominantly perlite matrix, instrumented and austenitized at 870 °C and 900 °C, the cooling rate promoted by the Zamak 5 alloy was evaluated, until the temperature of 400 °C. The treatment was efficient, using the alloy as austempering medium, with cooling rates higher than 6 °C/s. The morphological analysis verified the formation of ausferrite, without the presence of perlite, characterizing the formation of ADI.
Precipitation of large Z-phase particles, Cr(V, Nb)N, replacing fine MX carbonitrides, Nb(C,N) or V(N,C), has recently been identified as a major cause for premature losses in long-term creep strength in high chromium martensitic steels for power plants applications. For developing a new heat-resistant steel with better creep performance for applications in modern power plants with service temperatures around 650 °C, martensitic steels with 9 wt.%Cr were studied to achieve a chemical composition without the presence of a reversion/formation of detrimental Z-phase from consumption of thermally stable carbonitrides. The alloy composition was design to control deleterious Z-phase precipitation based on extrapolation of conventional vanadium content. The modelling of the alloy content is relied on thermodynamic equilibrium calculations using MatCalc software and Schaeffler modified diagram to achieve a fully martensitic microstructure.
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