Abstract. Metallographic analysis is applied to the study of the chunky graphite morphology in heavy-section castings of spheroidal graphite cast irons. Three castings with different Si and Ni contents were prepared. Three positions in casting from the edge to the centre, with different cooling rates, were chosen for microstructure observation. The effect of the Si and Ni content on the graphite morphology and mechanical properties of heavy-section spheroidal graphite cast iron parts was investigated. Cerium containing commercial inoculant was used for in-stream inoculation. Chunky graphite area was estimated in micro-and macrostructure. Mechanical properties were determined on tensile test bars taken from the centre of the casting. Macro-and microstructure examination showed that the castings with high Si-content and Ni addition had chunky graphite present, while the castings produced by use of low Si and Ni containing charge had no chunky graphite. High Si-content is strong chunky graphite promoter, especially in castings with slow cooling rate. Ni addition also promotes chunky graphite formation, but only in thermal centre of the casting (where the cooling rate is the lowest). The elongation is severely lowered when chunky graphite appears in the microstructure.

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Chunky graphite is one of the most deleterious graphite degenerations in thick-walled ductile-iron castings. This defect normally appears in the thermal center of a casting and dramatically decreases the mechanical properties. Chunky-graphite formation is primarily caused by a reduction in the cooling rate and melt composition. An addition of subversive elements (Bi, Sb, Pb, Sn,...) associated with a small amount of Ce is known to be beneficial to avoid chunky graphite. In this study, the effects of Bi and Ce additions at different cooling rates on the graphite morphology and mechanical properties of ductile-cast iron EN-GJS-400-18-LT were investigated. Y-blocks with wall thicknesses of 25 mm and 75 mm and cylindrical blocks with a diameter of 200 mm and a height of 300 mm were cast. The presence of chunky graphite in the thermal center of the cylindrical blocks and a decrease in the mechanical properties were revealed, regardless of the presence of Bi. Keywords: chunky graphite, cooling rate, graphite morphology, thick-walled ductile-iron castings Grudast grafit je ena od najbolj {kodljivih degeneracij grafita v debelostenskih ulitkih iz gnetljivega`eleza. Ta napaka se obi~ajno pojavi v toplotnem sredi{~u ulitka in mo~no zmanj{a mehanske lastnosti. Nastanek grudastega grafita je posledica zmanj{anja hitrosti ohlajanja in sestave taline. Dodatek {kodljivih elementov v sledovih (Bi, Sb, Pb, Sn,...) v povezavi z majhnim dodatkom Ce je poznan kot ugoden za prepre~itev nastanka grudastega grafita. V tej {tudiji je bil prou~evan vpliv dodatka Bi in Ce pri razli~nih hitrostih ohlajanja na morfologijo grafita in mehanske lastnosti gnetljivega litega`eleza EN-GJS-400-18-LT. Uliti so bili Y-kosi z debelino stene 25 mm in 75 mm ter cilindri~ni kosi s premerom 200 mm in visoki 300 mm. Odkriti sta bili prisotnost grudastega grafita v toplotnem sredi{~u cilindri~nih kosov ter zmanj{anje mehanskih lastnosti, neodvisno od prisotnosti Bi.

Shielding gases, i.e., argon or an argon-nitrogen mixture, are normally used during the laser-beam welding of duplex stainless steel. Helium is also often added to the argon. The effect of the type of shielding gasargon, nitrogen, helium and their mixtureson the geometrical characteristics of laser-welded joints of the duplex steel W.Nr. 1.4462 was studied. The welding was carried out according to the experimental model with mixtures. The effect of the welding speed and the gas flow rate were additionally explored using a factorial experiment. Obtained data were statistically processed and mathematical modeling, applying the method of response surfaces, was carried out. The analysis revealed that the impact of the shielding gas mixtures on the geometrical characteristics of the joint is significant, while it is not so significant for the microstructure. The effect of the gas flow rate on the geometrical characteristics was registered, while the effect of the heat input is the most significant. Special cubic, quadratic and reduced quadratic models, depicting effects of the shielding gas on geometrical characteristics and microstructure of welded joint, were obtained.