The Role of Selenium on the Formation of Spheroidal Graphite in Cast Iron

Abstract

Sulfur, an element that belongs to group 16 (chalcogens) of the periodic table, is an excellent promoter of nucleation substrates for graphite in cast iron. In ductile iron, sulfur favors a higher nodule count, which inhibits the risk of carbides and of microporosity. It is reasonable to expect that other elements from group 16, such as selenium or tellurium, play similar roles in the nucleation of graphite. The objective of this paper was to investigate the effect of selenium on the process of graphite formation. Thermal analysis cups were poured to evaluate the nodule count and size distribution. Some of the cups were not inoculated, while others were inoculated with a Ce-bearing inoculant, or with the Ce inoculant and additions of Se. Cross-shaped castings were also poured to quantify the microporosity regions by tomography. It appears that selenium additions modify the number and size of graphite particles, as well as the volume of microshrinkage. Direct correlations between these three parameters were found. Advanced Extensive Field Emission Gun Scanning Electron Microscope (FEG-SEM) techniques were used to identify the nature of the main nucleation compounds. Selenides, combined with Mg and rare earths, were observed to serve as nuclei for graphite. Their presence was justified by thermodynamics calculations.

Metastable versus stable solidification of silicon cast irons, effects of magnesium and tellurium

“…In this regard, bismuth has been shown to increase the number of nodules in spheroidal graphite cast irons when added as pure metal, oxide or sulfide or in an inoculant, and Bi-rich particles have indeed been observed in the center of graphite spheroids [43][44][45][46]. Similarly, Alonso et al [47] have studied the effect of addition of Se to a spheroidized melt and observed that some of the graphite nuclei did contain Se and rare earth rich phases. Nodule count was also found higher on the samples cast with addition of Se as compared with the same sample without addition and the authors concluded that addition of Se triggers the formation of graphite nuclei.…”

Effect of the porosity on the fatigue strength of metals

“…The most common solidification defects contained in heavy section DCI components are represented by nonnodular graphite elements (such as exploded, spiky, and chunky graphite), non-metallic inclusions, slag inclusions, macro-and micro-shrinkage porosity, and micro-pores related to the presence of gas. 22 Such defects, which have a detrimental effect on the mechanical properties of DCIs (both static and cyclic), can be partially avoided by means of some optimization strategies, 23,24 although their presence is unavoidable. Alonso et al 24 have recently investigated the effect of selenium additions on the formation process of graphite, proving the substantial reduction of the solidification defects.…”

“…22 Such defects, which have a detrimental effect on the mechanical properties of DCIs (both static and cyclic), can be partially avoided by means of some optimization strategies, 23,24 although their presence is unavoidable. Alonso et al 24 have recently investigated the effect of selenium additions on the formation process of graphite, proving the substantial reduction of the solidification defects. Special nano-carbon additives have been used in the experimental research work by Ahmed et al 25 in order to promote the tendency of spheroidal graphite formation.…”

“…12-18), and biaxial loading (test No. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. Note that, for tests performed at the same stress level (for instance, test No.…”

“…12-18), and biaxial loading (test No. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. Moreover, also the experimental values of fracture plane orientation, θ exp , 9,10 (see Section 4) are reported in Table 5.…”