261 12.14 Gray iron is weak and brittle in tension because the tips of the graphite flakes act as points of stress concentration. 12.15 This question asks us to compare various aspects of gray and malleable cast irons. (a) With respect to composition and heat treatment: Gray iron--2.5 to 4.0 wt% C and 1.0 to 3.0 wt% Si. For most gray irons there is no heat treatment after solidification. Malleable iron--2.5 to 4.0 wt% C and less than 1.0 wt% Si. White iron is heated in a nonoxidizing atmosphere and at a temperature between 800 and 900 ° C for an extended time period. (b) With respect to microstructure: Gray iron--Graphite flakes are embedded in a ferrite or pearlite matrix. Malleable iron--Graphite clusters are embedded in a ferrite or pearlite matrix. (c) With respect to mechanical characteristics: Gray iron--Relatively weak and brittle in tension; good capacity for damping vibrations. Malleable iron
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