Cracked Cast Iron Crankcases
The front wall of a cast iron crankcase cracked at the transition from the comparatively minor wall thickness to the thick bosses for the drilling of the bolt holes. Metallographic examination showed the case was aggravated by the fact that the casting had a ferritic basic structure and the graphite in part showed a granular formation, so that strength of the material was low. In a second crankcase with the same crack formation the structure in the thick-wailed part was better. But it also showed granular graphite in the ferritic matrix in the thin-walled part between the dendrites of the primary solid solution precipitated in the residual melt. A third crankcase had fractures in two places, first at the frontal end wall and second at the thinnest point between two bore holes. In all three cases casting stresses caused by unfavorable construction and rapid cooling were responsible for the crack formation. A fourth crankcase had cracked in the bore-hole of the frontal face. In this case the cause of the fracture was the low strength of a region that was caused by a bad microstructure further weakened by the bore hole.