The intermediate-temperature embrittlement of a hot-rolled ferritic spheroidal graphite cast iron was studied with the consideration that triaxial stress is induced by the ellipsoidal graphite particles of three unequal axial radii. The graphite shape was changed by various rolling reductions, and the tensile tests were performed at 673 K. The results show that the elongation and flow stress are independent of rolling reduction, and intergranular fracture occurs in all specimens. In the plasticity analysis, the triaxiality ratio (σ m σeq) at a point in the ferrite matrix center can be expressed in terms of graphite shape ratio (b d ) and graphite interparticle spacing (2 a d ) as σ m /σeq = 1/3 +a d /(2 b d ) where σ m is the hydrostatic tensile stress and σeq is the equivalent stress. Accordingly, the average triaxiality ratio in the matrix center region is independent of rolling reduction and greater than one, a result that is consistent with the fact that the elongation is about constant, and all specimens undergo intergranular fracture. Moreover, the rolling reduction independent flow-stress behavior can be rationalized by the analytical result that the average σ m /σeq is unchanged with rolling reduction, where σ z is the internal stress along the tensile direction.
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