Phosphorous eutectic

Structural components in gray cast iron alloys, also referred to as steatite.

Due to the higher phosphor content in flake graphite cast iron, ternary phosphorous eutectic occurs here much more often than in nodular graphite cast iron. This is only then regarded as a defect if steatite does not occur as connected net as intended (e.g. in wear-resistant cast iron as illustrated in Fig. 2) but individually or accumulated in gussets (Fig. 1).

Phosphorous eutectic can only be visualized in the metallographic specimen after etching. It is then present as bright phase, in ferritic matrixes often enclosed by pearlite residues. The nodular formation of nodular graphite cast iron is impaired in the range of the steatite.

After solidification, ternary phosphorous eutectic consists of three phases: Iron phosphide (Fe3P), cementite (Fe3C) and austenite (γ MC), i.e. consisting of 41 % Fe3P+ 30 % Fe3C + 29 % γ MC, the eutectic point lies at 90.7 % Fe + 6.9 % P + 2.4 % C. The two hard components Fe3P and Fe3C (Figures 3 and 4) may improve the wear resistance of cast iron. However, it is necessary that the phosphorous eutectic has a suitable arrangement in the structure. A uniformly connected network is regarded the most favorable form. Accumulated gussets are unfavorable

The high segregation of phosphor (Fig. 5) is the cause of formation of this ternary phosphorous eutectic that solidifies at 952 °C. Despite a solubility of 1.1 %, phosphor is already formed in the austenite if the alloy contains as little as 0.1 % phosphor. The critical concentration depends on the cooling conditions during solidification.

Phosphorous eutectic forms hard inclusions in its structure. The residual melt that has solidified last has to adapt to the contours of the previously solidified structural components (dendrites, eutectic grain) and takes up the shape of a network in the structure. The phosphorous eutectic that is diffused in the specimen structure as bright phase is often only the phosphorous phase of the eutectic. The austenite proportion of the phosphorous eutectic can hardly be measured, as it accumulates at the already present austenite.

At a high degree of saturation and slow cooling, the phosphorous eutectic can also solidify after the stable system. In this case, graphite is formed instead of iron carbide. It accumulates at the already existing flake graphite. In this case, the phosphorous eutectic only consists of iron phosphide and disintegrated austenite and is therefore referred to as pseudo-binary phosphorous eutectic.

Explanation (fig. 5):
SE = Secondary electron image (SEM image). Bright ranges indicate accumulation of the respective element.

Examples (fig. 5):
1st line of figures, 2nd figure from the left; the bright range indicates carbon, i.e. graphite in this case; or 1st line of figures, right image; strong silicon accumulation around the nodular graphite; or 2nd line of figures, left figure; strong phosphor accumulation, i.e. phosphorous eutectic in this case. In the range of the phosphorous eutectic, accumulation of the elements titanium, vanadium and manganese are also clearly visible.

  • Fig. 1: Common formation of the phosphorous eutectic at slow solidification. Here, in a pearlite matrix, magnification 500:1, etched with HNO3.
  • Fig. 2: Phosphide network at wear-resistant flake graphite cast iron, P content = 0.5 %, magnification 100:1, etched with HNO3.
  • Fig. 3: SEM image from ranges of the phosphide network from Fig. 2 with marked ranges for EDX surface analyses
  • Fig. 4: EDX surface analysis from range 1 in Fig. 3.
  • Fig. 5: Element distribution between two spherolites in nodular graphite cast iron, ranges with phosphorous eutectic are clearly visible.  (Source: FT&E)
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