Control of the Mg-Treated Iron Casting Skin Formation by S-Diffusion Blocking at the Metal–Mould Interface

Abstract

Having established that sulphur presence in the mould materials appears to have an important contribution in graphite degeneration at least in the casting surface layer, a research program is undertaken to explore the possible beneficial effect of sulphur diffusion blocking at the metal–mould interface. Test samples, with and without a thin steel sheet (up to 3 mm thickness) application on the inner surface of the mould cavity, before iron melt pouring, are considered for structure analysis. A higher nodulizing potential (0.048% Mgres, 0.015% Ceres, and 0.006% Lares) decreases the occurrence of surface graphite degeneration in castings obtained in rigid chemically bonded resin sand moulds, using P-toluol sulfonic acid (PTSA) hardener (S-including), but it is not enough to avoid this phenomenon (200–400 μm skin in present experimental conditions). The casting skin appears to have different values, depending on the evaluation technique (un- and Nital-etching direct measurement, or graphite parameters variation on the casting section). In the presence of a thin steel sheet at the metal–mould interface, the casting skin thickness decreases or is just excluded. It is supposed that it acts as a barrier, blocking S-diffusion from the mould media into the iron melt. Without this S-diffusion, the graphite degeneration in the casting surface layer could be avoided, or at least diminished. For industrial application, the increasing of residual content of nodulizing elements is a limited solution, and it is recommended to use barriers to block S transfer on the mould/metal surface, such as dense coatings or coatings with desulphurization capacity.

Experimental Study Regarding the Possibility of Blocking the Diffusion of Sulfur at Casting-Mold Interface in Ductile Iron Castings

“…The thermal regime of iron melt processing was as follows: superheating temperature: Ts = 1550 • C; Mg-treatment temperature: Tm = 1530 • C; pouring temperature: Tp = 1350 • C. The inoculated Mg-treated iron was poured in a pyramid trunk with square section samples and with steel sheets (0.5, 1, and 3 mm thickness) inserted at the casting-mold interface (Figure 1). For comparison, a sample without a steel sheet was cast (for more details, see paper [10]). The structure evaluation in the surface layer and the casting body was made according to Figure 2.…”

“…The structure evaluation in the surface layer and the casting body was made according to Figure 2. Graphite morphology analysis [10] was conducted by using a professional automat image analyzer (OMNIMET ENTERPRISE and analySIS ® FIVE Digital Imaging Solutions software version 5.0, Waukegan, IL, USA), using both the standard cast iron modulus and particle analysis software.…”

“…The sulfur concentration in the steel sheet after the diffusion of sulfur from the mold, C S = 0.0564 wt.% (calculated); • Sulfur concentration in modified cast iron, C 0 = 0.015 wt.%; • Diffusion layer thickness is considered equal to the demodified layer average thickness (for the present example, on the 0.5 mm steel sheet, a demodified layer determined by means of the shape factors, δ str = 0.13 mm, resulted [10]);…”

“…Recently, it was found that in the presence of a thin steel sheet at the metal-mold interface, the thickness of the surface layer decreases or is just avoided. It was suggested that it acts as a barrier, blocking sulfur diffusion into the iron melt [10]. The present paper aims to elucidate some aspects of the demodifiyng process related to the mechanism of the transition of sulfur from the mold into liquid cast iron through the prism of the diffusion blocking effect at the interface.…”

“…Previous works [1,[6][7][8][9][10] have pointed out that decreasing the residual magnesium content aggravates the surface graphite degeneration five times more in mold including sulfur, compared with no-sulfur mold media. If the sulfur contribution of the mold is diminished, such as by blocking its transfer to the iron melt, the graphite degeneration in the casting surface layer can be avoided or, at least, diminished.…”