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Microstrutture in Leghe Ni-Cu Ottenute mediante Elevato Sottoraffreddamento
No full textVengono descritte le tecniche impiegate per la realizzazione della solidificazione di leghe metalliche a temperature largamente inferiori a quelle di equilibrio: levitazione magnetica, «drop-tube» e atomizzazione. La modellizzazione e la soluzione numerica delle leggi del moto e della trasmissione del calore, unitamente alle modalità di rilascio del calore latente di solidificazione, hanno consentito di ottenere il profilo di velocità e di temperatura per le gocce atomizzate nell'ipotesi di moto rettilineo.
Tali parametri sono stati utilizzati per il progetto della camera di raffreddamento dell'atomizzatore realizzato presso il DIMEG della Università di Padova. Si sono finora prodotte polveri nel sistema Ni-C 11. Il confronto tra le strutture ottenute in tali polveri e quelle presenti in campioni massivi di cui era noto il sottoraffreddamento ha permesso di stimare il sottoraf-freddamento nel processo di atomizzazione. Si può valutare che'in taluni casi questo sia superiore a 195 K
Phase transformation in duplex stainless steels after isothermal, treatments, continuous cooling and cold working
No full textThis paper concerns the phase transformation induced by heat treatment and cold rolling in four duplex stainless steel. In 2205 and 2507 , during the isothermal heat treatments, chi-phase precipitates as small particles at the ferrite/austenite boundaries, followed by sigma precipitation. At the lowest temperature the formation kinetic of chi-phase is favoured, with the increasing of time and temperature a progressive transformation of chi to sigma occurs and the kinetic of sigma is favoured. During continuous cooling, the chi -phase appears at low cooling rates. In low Ni grades the grain boundaries precipitation of chromium nitrides were detected , but no sigma and chi. In 2101 the austenite transforms to martensite both after cold rolling and quenchin
Precipitazione di fasi secondarie in un acciaio duplex 2205 trattato isotermicamente (Precipitation of secondary phases of 2205 duplex stainless steel after isothermal
No full textA duplex stainless steel (DSS) containing nominally 22Cr, 5Ni, 3.2Mo and 1.5Mn has been investigated for secondary phases precipitation, after ageing at 780, 850 and 900°C. Selective etching with different reagents, OM and SEM-BSE have been used to quantify ferrite, austenite and secondary phases. A critical discussion of different metallographic techniques (use of OM and/or SEM-BSE) applied to measure intermetallic content is presented, including an attempt to distinguish chi and sigma. The chemical composition of the phases has been determined by means of EDS, the effects on mechanical properties have been studied with a hardness test. The hardness increases with holding time for all temperatures. The rate of secondary phases increases with the holding time. The 850°C treatment is the most effective for increasing hardness and secondary phase precipitation. The SEM-BSE method seems to be more reliable than OM for measuring the secondary phase contents and for identifying chi and sigm
Precipitation of secondary phases in super duplex stainless steel ZERON100 isothermally aged.
No full textIn the present paper the results of secondary phases determination and quantification in ZERON100 duplex steel, heat treated at 850-1000°C for 180-2400 s, are presented. During the isothermal heat treatments, at 850°C, the χ phase is the only phase to precipitate at α/γ boundaries and triple points, while at 900 and 950°C, χ phase is the first phase to precipitate after 180 s, followed by σ phase 300 s later. At all the temperatures the total amount of secondary phases is ∼3% after ,600 s aging. (17 refs.
Studio dell'effetto delle fasi secondarie della tenacita' a frattura in un 2205 DSS
No full textIt is well known that the fracture toughness of DSS is strongly reduced by the precipitation of various inter- metallic phases occurring in the temperature range 600-1000°C. A large decrease in impact fracture toughness occurs even at room temperature for volume fractions of intermetallic phases lower than 1%, when only small and rare particles are present. In the present investigation, the influence of the intermetallic phases on the impact fracture behaviour of a 2205 grade DSS has been investigated. Samples containing different amounts of the intermetallic phases have been obtained by isothermal aging treatments in the range 800-950°C. The results of the impact tests confirm that the dangerous phase content determine both the toughness and the fracture behaviour of the DSS examined. At content lower than 1%, when precipitates are rare and small, their effect is a reduction of the absorbed energy for the ductile fracture. But the 1% appears as the critical content, when some particles became large enough to operate the nucleation of the brittle fracture. Indeed, at higher content, a number of large particles are present, well sufficient to induce a general brittle fracture. The obtained results allow correlating the absorbed energy values with the intermetallic phases content and dimensions
.Microstructure of a nitrided steel previously decarburized
No full textIn this study the effects of a surface-controlled decarburization on the structure of a nitrided steel are analyzed. Samples of a quenched and tempered 42CrMo4 steel were decarburized by heating in air at different depths and submitted to gaseous nitriding. After decarburization and nitriding, the microstructure of surface layers was investigated by optical microscopy (OM) and scanning electron microscopy (SEM). The nitrogen and carbon profiles in the diffusion layers were determined by SEM equipped with a wavelength dispersive spectrometer (EPMA-WDS) and by glow discharge optical spectrometry (GDOS). The effect of nitriding was determined by microhardness measurements. Our results indicate that a previous decarburization only slightly affects the surface hardness, but reduces the conventional nitriding depth. The decarburization also favors the nitrogen take-up and produces increased nitrogen concentrations in the compound layer and in the narrow zone beneath it
Influence of small amount of secondary phases on impact toughness of UNS S32205 and Zeron® 100 Duplex Stainless Steel
No full textDuplex Stainless Steels (DSS) possess improved mechanical and corrosion properties, if compared to other stainless steels grades. Moreover, DSS retain good impact fracture toughness even at low temperatures, and, if compared with ferritic stainless steels, the ductile-to-brittle transitions is more gradual. However, DSS are sensitive to secondary phase's precipitation during isothermal heat treatments, even for very short time, mainly in the temperature range 550-950°C, or during quenching treatments at insufficient cooling rates in the same temperature range. The aim of this work is to investigate, using a regression analysis of experimental data, the effect of the precipitation of a slight amount of secondary phases (more or less 1%) on the impact toughness behaviour in the ductile-to-brittle transition region in two DSS: the UNS S32205 and the Zeron-100 grades. In aged materials, small amounts (more or less 1%) were present: mainly chi-phase in Zeron-100 and sigma in UNS S32205. Materials were tested in wrought ("as received" and "sigma-free") and aged (isothermally heat-treated) states by means of Charpy-V impact tests, in a temperature range between 20 and-196°C. The results show that in the UNS S32205 grade the small amounts of secondary phases reduce the impact toughness at room temperature to over 70%. This difference increase with lowering the temperature also because the wrought material maintain a good impact toughness response to about-40°C, with a successive decrease to 100 KJ at-90°C. Instead the Zeron-100 retain a good impact toughness even at-100°C, but a slight amount of secondary phases affects the toughness behaviour by lowering the absorbed energy by about 80 J at room temperature, and this difference in toughness can be considered almost constant for all the experimental temperature range. The lateral expansion was evaluated in order to achieve a further parameter to characterize the plastic deformation in the transition region
Analysis of phase stability in Cr-Ni and Cr-Mn DSS.
No full textAnalysis of phase stability in Cr-Ni and Cr-Mn DSS: the Cr-Ni are more prone to sigma precipitation than Cr-Mn, where nitrides were detected
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