Vpliv oblike in deleža grafita na električno upornost sivih litin s kroglastim grafitom

Sevšek, Primož (author), Petrič, Mitja (mentor), Mrvar, Primož (comentor)

Strjevanje sivih litin se danes najpogosteje spremlja z enostavno termično analizo, s pomočjo katere se lahko oceni stanje mikrostrukture v smislu velikosti, oblike in razporeditve mikrostrukturnih sestavin. Poleg termične analize se za karakterizacijo zlitin lahko uporabljajo tudi meritve električne upornosti kovin. Te so pomemben del analiznih metod pri razvoju in testiranju materialov. Meritve električne upornosti se uporabljajo za spremljanje reakcij, ki potekajo v materialih pri spremembi temperature, kot so izločanje ali premene v trdnem, oziroma pri spremljanju poteka strjevanja kovin in zlitin. V okviru magistrskega dela smo z meritvami električne upornosti poizkušali ugotoviti vpliv mikrostrukture na električno upornost sivih litini. Za merjenje temperature in električne upornosti smo uporabili merilno celico izdelano po postopku Croning. Ulitih je bilo več vzorcev z različnim časom po nodulaciji, oziroma z različnim deležem magnezija in/ali ogljika. Zaradi razlik v kemijski sestavi zlitin, smo dosegli spremembe v mikrostrukturi ter v vrednostih električne upornosti med strjevanjem. Električna upornost materialov se spreminja s temperaturo in je odvisna tudi od mikrostrukture. Mikrostrukturne faze, oblika, količina in velikost faz imajo velik vpliv na električno upornost materiala. Večje kot so faze in bolj kot so lamelne, višja je električna upornost, saj je s tem prisotnih več ovir za gibanje elektronov. Če pa so faze manjše in bolj zaobljene, je električna upornost nižja. Vpliv na električno upornost pa ima tudi matrica. Upornost je višja, če je matrica perlitna, kot pa če je feritna. Ugotovljeno je bilo, da oblika grafita nima velikega vpliva na električno upornost, dokler izločeni grafit ni čisto lamelni. Sprememba upornosti med strjevanjem je pri sivi litini s kroglastim grafitom podobna kot pri sivi litini z vermikularnim grafitom. Med strjevanjem se upornost v obeh primerih zmanjša, med tem ko se pri sivi litini z lamelnim grafitom električna upornost med strjevanjem poveča. Za vzorce sive litine s kroglastim in/ali vermikularnim grafitom se specifična električna upornost med strjevanjem zmanjša za približno 0,1 µΩm. Pri strjevanju vzorcev sive litine z lamelnim grafitom pa se specifična električna upornost poveča za približno 0,3 µΩm.

• Language: Slovenian
• Keywords: strjevanje, nodulacija, specifična električna upornost, grafit, siva litina
• Work type: Master's thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2019
• PID: 20.500.12556/RUL-106888
• COBISS.SI-ID: 1802079
• Publication date in RUL: 24.03.2019

Secondary language

• Language: English
• Title: Influence of shape and amount of graphite on electrical resistivity of spheroidal cast iron

Abstract

Solidification of grey cast irons is nowadays most commonly monitored by simple thermal analysis. By thermal analysis we can get assessment of the state of microstructure in terms of size, shape and distribution of microstructural components. Beside thermal analysis, the electrical resistivity measurements of metals can also be used to characterize alloys. These are an important part of analytical methods in the development and testing of materials. By electrical resistivity measurements it is possible to follow reactions such as precipitation, transformation and solidification of metals and alloys. In the master's thesis, we tried to determine the influence of the microstructure on the electrical resistivity of cast irons by measuring electrical resistance. A measuring cell, which measured temperature and electrical resistance, was made using the Croning process. Several samples were made with different time after nodularisation, or with a different proportion of magnesium and/or carbon. Due to differences in the chemical composition of the alloys, changes in the microstructure and in the values of the electrical resistivity were achieved during solidification. The electrical resistivity of the materials varies with temperature and depends on the microstructure. Microstructural phases, shape, amount and size of phases have a major influence on the electrical resistivity of materials. Larger phases or more lamellar graphite, brings higher electrical resistivity, as there are several obstacles for the movement of electrons. However, if the phases are smaller and more rounded, the electrical resistivity is lower. Matrix also have influence on the electrical resistivity . Resistivity is higher if the matrix is perlitic. It was found that the shape of the graphite does not have a major influence on the electrical resistivity until graphite is completely lamellar. The change of electrical resistivity during the solidification of gray cast iron with a nodular graphite, is similar as of vermicular graphite. During solidification, the resistance in both cases decreases, while in the case of gray cast irons with lamellar graphite, electrical resistivity increases during solidification. For cast iron samples with spheroidal and/or vermicular graphite, the specific electrical resistivity decreases by approximately 0,1 μΩm during solidification. At the solidification of cast iron samples with lamellar graphite, the specific electrical resistivity increases by approximately 0.3 μΩm.

• Keywords: solidification, nodularisation, electrical resistivity, graphite, cast iron