materials Article Bifilm Inclusions in High Alloyed Cast Iron Marcin Stawarz * and Malwina Dojka Department of Foundry Engineering, Silesian University of Technology, 7 Towarowa Street, 44-100 Gliwice, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-32-338-5532 Abstract: Continuous improvement in the quality of castings is especially important since a cast without defects is a more competitive product due to its longer lifecycle and cheaper operation. Producing quality castings requires comprehensive knowledge of their production, crystallization process, and chemical composition. The crystallization of alloyed ductile iron (without the addition of magnesium) with oxide bifilm inclusions is discussed. These inclusions reduce the quality of the castings, but they are a catalyst for the growth of spheroidal graphite that crystallizes in their vicinity. The research was carried out for cast iron with a highly hyper-eutectic composition. Scanning electron microscopy and EDS analysis were used in the research. A detailed analysis of the chemical composition was also carried out based on the spectrometric method, weight method, etc. Based on the obtained results, a model of spheroidal graphite crystallization near bifilm inclusions was proposed. The surface of the analyzed graphite particles was smooth, which suggests a primary crystallization process. The phenomenon of simple graphite and bifilm segregation towards the heat center of the castings was also documented. Keywords: bifilms; spheroidal graphite; alloyed cast iron; crystallization Citation: Stawarz, M.; Dojka, M. Bifilm Inclusions in High Alloyed Cast Iron. Materials 2021, 14, 3067. 1. Introduction https://doi.org/10.3390/ Foundry engineering processes are prone to many issues during casting manufactur- ma14113067 ing that may influence the final casting quality. Many are often ignored by manufacturers or even unknown. It is extremely important to realize the cause of poor casting quality Academic Editor: before buying expensive additives for metal improvement and waste money on defective Francesco Iacoviello castings. In many works, researchers explain that the design of a proper gating system, the way that liquid metal is poured into a mold, and melt treatments are important factors that Received: 24 May 2021 affect a casting’s quality [1–6]. Most of this research is based on Professor John Campbell’s Accepted: 2 June 2021 bifilm theory [6–8]. In short, as one of the researchers explains [5], according to this theory, Published: 4 June 2021 turbulence during the pouring generates defects inside the alloy as air bubbles and oxide films form on the surface of the liquid metal. The oxide film on the surface may fold over on Publisher’s Note: MDPI stays neutral itself. The double oxide ‘bifilm’ acts as a crack in the liquid metal, leading to the initiation with regard to jurisdictional claims in of additional cracks and hot tears in the casting. Figure1 presents a schematic illustration published maps and institutional affil- iations. of surface turbulence causing the entrainment of bifilms and associated bubbles. Dispinar D. et al. [1] and G. Gyarmati et al. [2] explained that the bifilm content in the melt is much more important than the dissolved gas content such as hydrogen, which has always been considered to be the major source of defects. They proved that with proper degassing, bifilm inclusions can be eliminated from the melt, and regardless of the Copyright: © 2021 by the authors. hydrogen content, aluminium castings without defects can be manufactured. Similarly, Licensee MDPI, Basel, Switzerland. Tiryakio˘gluM. [4] concluded that if air entrainment defects are eliminated, there is no This article is an open access article reason to measure or control the hydrogen levels in the melt. He clarified that due to the distributed under the terms and conditions of the Creative Commons presence of bifilms with two unbonded interfaces, nucleation is bypassed during pore Attribution (CC BY) license (https:// formation. Hydrogen diffuses to oxide bifilms and inflates them, therefore, hydrogen creativecommons.org/licenses/by/ serves as an agent to make entrainment defects visible. What is even more interesting, 4.0/). Uluda˘gM. et al. [3] studied Al-7%Si-Mg alloy and discovered that the best melt quality Materials 2021, 14, 3067. https://doi.org/10.3390/ma14113067 https://www.mdpi.com/journal/materials Materials 2021, 14, 3067 2 of 11 was obtained when the melt was degassed without additives. Moreover, their research showed that all examined melt additions decreased the melt quality. They explained that Ti-containing intermetallic compounds were nucleated on bifilms. On the other hand, the addition of Sr to A356 increased the number of bifilms in the melt and also formed Materials 2021, 14, x FOR PEER REVIEWlarger pores. Sr reacts with Al2O3, and the formation of complex compounds results in2 of the 11 fracturing of oxides into smaller pieces through breakaway oxidation. FigureFigure 1.1. SchemeScheme ofof bifilmsbifilms andand bubblesbubbles formationformation [[8].8]. MostDispinar research D. et in al. this [1] area and has G. beenGyarmati conducted et al. [2] on nonferrousexplained that alloys, the but bifilm bifilm content defects in arethe alsomelt a is cause much of more problems important with castthan iron.the dissolved Our previous gas content work showed such as thathydrogen, bifilm inclu-which sionshas always may cause been difficulties considered with to be the the inoculation major so ofurce high-chromium of defects. They cast proved iron [9 ].that During with inoculationproper degassing, with Ti addition,bifilm inclusions the TiC compoundscan be eliminated that serve from as the underlays melt, and for regardless M7C3 carbides of the arehydrogen transported content, on the aluminium bifilms with castings the crystallization without defects front can to thebe manu shrinkagefactured. cavities. Similarly, Thus, theyTiryakio are noğlu longer M. [4] anconcluded active nucleus that if inair this entrainm location,ent which defects is are very eliminated, unfortunate there from is anno economicreason to viewpoint.measure orAccording control the to hydrogen previous leve investigations,ls in the melt. bifilm He defectsclarified are that also due present to the inpresence the microstructure of bifilms with of greytwo castunbonded iron. Professorinterfaces, J. nucleation Campbell wroteis bypassed [10] that during graphite pore mayformation. growon Hydrogen bifilm inclusions. diffuses to He oxide explains bifilms that and the silicainflates film, them, when therefore, entrained hydrogen into the alloyserves as as a bifilm,an agent first to attractsmake entrainment the oxy-sulfide defects nuclei visible. to nucleate What is on even it. Then, more these interesting, nuclei serveUluda asğ M. the et underlay al. [3] studied for the Al-7%Si-Mg nucleation ofalloy flake and and discovered nodular graphite.that the best This melt theory quality of graphitewas obtained nucleation when is the also melt mentioned was degassed in the works without of I. additives. Riposan et Moreover, al. [11]. F. Hsutheir et research al. [12] andshowedR. Dojka that all et al.examined [13] noted melt the additions importance decreas of aed gating the melt system quality. for manufacturing They explained good- that qualityTi-containing castings intermetallic according tocompounds John Campbell’s were nucleated bifilm theory. on bifilms The conducted. On the other research hand, onthe ductileaddition irons of Sr revealed to A356 that increased turbulent the fillingnumber from of bifilms the top in results the melt in oxide and also bifilms formed entrained larger inpores. ductile Sr ironreacts castings. with Al F.2O Hsu3, and et the al. [formation12] concluded of complex that floating compounds bifilms results are the in sources the frac- of intrinsicturing of cracks oxides in into castings. smaller Ductile pieces ironthrough castings breakaway may be oxidation. weakened by bifilm inclusions, resultingMost in research their premature in this area fracture. has been These conducted results show on nonferrous that turbulent alloys, filling but bifilm significantly defects influencesare also a cause the elongation of problems of ductile with cast iron. iron The. Our authors’ previous recent work work showed presented that inbifilm the paper inclu- confirmssions may the cause results difficulties of cast iron with investigations the inoculation in theof high-chromium area of bifilm defects. cast iron In [9]. the During paper, theinoculation crystallization with Ti of addition, alloyed ductile the TiC iron compounds (without the that addition serve as of underlays magnesium) for with M7C3 oxide car- bifilmbides are inclusions transported is discussed. on the bifilms The phenomenon with the crystallization of simple graphite front to and the bifilmshrinkage segregation cavities. towardsThus, they the are heat no center longer of an the active castings nucleus was in also this documented. location, which is very unfortunate from 2.an Materials economic and viewpoint. Methods According to previous investigations, bifilm defects are also pre- sent in the microstructure of grey cast iron. Professor J. Campbell wrote [10] that graphite Tests were carried out based on a two-stage metallurgical process of a liquid metal. may grow on bifilm inclusions. He explains that the silica film, when entrained into the Experimental melting was carried out in a medium-frequency induction furnace (PI25, alloy as a bifilm, first