uneven hardness

Casting or structural defects that are up to now only known for cast iron and here particularly occurs at flake graphite cast iron at parts with medium wall thickness. The defect is a structural defect that can only be identified by a hardness text and/or a metallographic specimen.

The defect occurs spontaneously although nothing has changed in the actual manufacturing process. The used composition materials do not show any peculiarities, the melting recipe is identical and no deviations can be identified regarding the magnesium treatment. In the matrix (see metal matrix of cast iron, basic structural mass) of the metallographic specimen, no hard spots, chill or carbide inclusions are found, however, a pearlite net distributed across the entire cross section.

The crystallization of hypoeutectic and often also eutectic cast iron is initiated by primary solidification at austenite dendrites. The following eutectic crystallization is then mushy and shell forming (see solidification type). In the eutectic grain of the nodular graphite cast iron, the carbon has to be transported to the spherulite by means of diffusion via the austenite. This only allows low crystallization rates. If the number of eutectic grains is low (see Eutectic grain count), e.g. in insufficiently treated iron, the residual melt is undercooled along the liquidus balance of the austenite crystallization. This leads to an increased tendency for segregation for nodular graphite cast iron with. Important accompanying elements show different behavior (figure 1). At ferritic nodular graphite cast iron , particularly micro segregation (or depletion) of silicone may have dramatic consequences with respect to pearlite stabilization.

Respectively, the cause of the defective basic matrixes discussed here originates from micro segregation or depletion of silicon at the grain boundaries. The tendency of manganese enrichment at the grain boundaries also acts similarly. Within the residual melt as the most important micro segregation zone, some elements form additional sub-segregations and may therefore additionally promote the negative effect of impoverishment in silicon.

  • Fig. 1: Concentration profile of micro segregation at an actual casting (according toS. Hasse, FT&E)