[en] The main objective of this doctoral thesis is to improve and control the metallurgical quality to obtain cast parts of ductile iron. In order to achive this target, it is necessary to consider all the elaboration process of the metal, beginning with the melting of the metallic charge, continuing with the nodulization treatment and finishing with the inoculation. The revision of the state of the art was focused on metallurgical aspects, enumerating different parameters that influence the microstructure formation of ductile iron. Thermal analysis possibilities have been described, in order to evaluate the metallurgical quality of the melt. In the same way, the last publications about increasing the metallurgical quality using conditioners have been examinated. The experimental work was carried out in the factory Victorio Luzuriaga Usurbil S. A. (VLU), therefore in the first stage the different phases of the melt elaboration process were studied using a thermal analysis equipment. It was found that the metallurgical quality in the pouring furnaces was very deficient and that the way the Magnesium treatment was done was affecting in a negative way. Therefore and as a result of the state of the art study, it was decided to introduce a new conditioning phase before the Magnesium treatment. It was decided to use a conditioner in development from FOSECO Company, whose main agent is Barium. After several tests it was fixed a proper procedure to carry on the new phase, adapted to VLU´s facilities, showing higher temperatures of the eutectic reaction in the treatment ladle and in the pouring furnace. In addition, a new conditioner has been developed, based on Barium and Boron, for which a patent request is being transacted. The castings produced with conditioned melt show an increment of nodules numbers, better distribution of them and higher ferrit content. Moreover mechanical properties are affected, losing hardness, decresing tensile strength, and increasing elongation and impact behaviour. During the characterization work, Barium presence inside of the graphite nuclei has been reported and it was also observed that Boron forms carbides around the graphite. In this way it was deduced the manner they contribute to the observed microstructural improvements.

[eu] Doktoretza lan honen helburu nagusia grafito esferoidaleko fundiziozko piezak ekoizteko kalitate metalurgikoa hobetzea eta kontrolatzea da. Horretarako saldaren elaborazio prozesu guztia kontutan hartu behar da, zeinek karga metalikoaren galdaketa, nodulizazio tratamendua eta inokulazioa barneratzen dituen. Egindako bibliografia azterketan aspektu metalurgikoak azpimarratu dira. Alde batetik grafito esferoidaleko fundizioaren mikroestrukturaren formazioan parte hartzen duten parametroak zehaztu dira. Bestalde, analisi termikoak saldaren kalitate metalurgikoa ebaluatzeko eskaintzen dituen aukerak aztertu dira. Azkenik, kalitate metalurgikoa hobetzeko egokitzaileen erabilerari buruzko behaketa egin da. Esperimentazio atala Victorio Luzuriaga Usurbil S. A.-n (VLU) burutu da. Lehenengo fase batean analisi termikoko ekipamendu baten laguntzaz saldaren elaborazio prozesuaren fase ezberdinak aztertu ziren. Magnesio tratamendua gauzatze moduaren ondorioz kalitate metalurgikoa galdaketa labeetan oso urria zela nabarmendu zen. Bibliografia azterketaren emaitzak kontutan hartuz, Magnesio tratamenduaren aurretik egokitze etapa berri bat barneratzea adostu zen arazoari aurre egiteko asmoz. FOSECO enpresan garatzen ari ziren produktu egokitzaile bat erabiltzea erabaki zen, bere eragile nagusia Barioa delarik. Hainbat proba ondoren, VLU-ko instalazioetara moldaturiko etapa berri hau burutzeko prozedura egoki bat ezarri zen. Bestalde, Bario eta Boroan oinarritutako egokitzaile berri bat garatu da, honen inguruko patente eskaera bat tramitatzen ari delarik. Egokitutako saldarekin galdaturiko piezek nodulu kopuruaren handitzea, beraien banaketa hobea eta matrizearen ferrita portzentaiaren gehikuntza erakusten dute. Ondorioz, piezek gogortasun galera bat, trakzio-saiakuntzan erresistentzia txikiagoa eta luzapenaren handitzea eta Charpy-saiakuntzan emaitza hobeak erakusten dituzte. Burututako karakterizazio lanean Karbonoa grafito esferoidal erara hazten den germenetan Bario aurkitu da. Gainera, Boroak grafitoaren inguruan karburoak osatzen dituela ikusi da. Horrela, bi elementuek aztertutako mikroestrukturaren hobekuntzetan nola laguntzen duten argitu delarik.