Acciai inossidabili ferritici per elettrovalvole: effetto del trattamento termico e delle lavorazioni a freddo sulle caratteristiche magnetiche

Il settore applicativo delle elettrovalvole impone limiti molto stringenti sulle proprietà magnetiche (campocoercitivo, curva di isteresi e permeabilità magnetica) dei materiali utilizzati per la realizzazionedei componenti: tali requisiti non vengono soddisfatti dagli acciai inossidabili ferritici ricotti incondizioni “standard”. Questo lavoro descrive il complesso processo di messa a punto, su un acciaionon appartenente alla categoria appositamente creata per questa applicazione, di un trattamentotermico dedicato e di un ciclo di trasformazione delle barre finalizzato alla produzione di proprietà ottimali.In particolare vengono correlati alle caratteristiche magnetiche i diversi parametri del ciclo produttivo,le caratteristiche microstrutturali e le misure di tensioni residue ottenute mediante diffrattometria raggi X

Microstructure and magnetic properties of pure iron for cyclotron electromagnets

The microstructural and magnetic properties of pure iron ingots used as cores of cyclotron electromagnets have been investigated upon annealing sequence from 650 degrees C to 820 degrees C. Optical Microscopy, Scanning Electron Microscopy, and X-ray Diffraction were employed in the structural analysis, while magnetization curve and hysteresis loops were obtained in rod and ring samples, machined out of the large (around 4 m diameter) cast yoke, by combination of point-by-point and continuous (frequency f = 0.03 Hz) hysteresisgraph methods. Quite inhomogeneous grain structure was observed, with large and irregularly shaped grains (size of several mm), posing special constraints to the conventional magnetic testing approach (e.g., the IEC 60404-4 standard). Reproducible DC magnetic measurements could be performed using large cross-sectional area samples and suitable spatial averaging of the measured effective magnetic field, the eddy current effects being minimized at the same time. Full magnetic softening was obtained upon the annealing sequence in the absence of grain growth and change of distribution and morphology of the inclusions. The measured increase of permeability and decrease of coercivity are therefore entirely ascribed to the relief of the residual stresses, affecting the as-prepared and preliminarily annealed massive (similar to 80 x 10(3) kg) yoke. By recognizing in this way the role of stresses, we are eventually able to estimate the additional contributions to the material coercivity provided by the grain boundaries and the precipitated impurities, the latter playing a major role

Magnetic properties of silicon-iron laminations Si-enriched by a SiH4 based CVD process

Grain-oriented Fe-Si laminations have been Si enriched up to 6.5 wt% by means of Chemical Vapour Deposition processes, using either SiCl4 or SiH4 as reacting gaseous phases. The chloride based process has been applied to conventional 0.33 mm thick laminations, while the silane method has been investigated using thinned sheets (0.10 mm). In the latter case, the decomposition of SiH4 produces Si coating over a strip shaped sample. Joule heated at a temperature of 1000 °C- 1100°C. Coating is followed by Si diffusion in the bulk, up to the optimal concentration of 6.5 wt%. An important reproducible improvement of the d.c. and a.c. magnetic properties is observed after the SiCl4 treatment, which is not, however, comparably matched by the properties of the SiH4 treated samples, likely due to structural inhomogeneities residual to the deposition process. A rationale to the loss behavior in the investigated materials, based on the analysis of the loss components, is provided in this paper