We investigated the effects of Mn and S on the mechanical properties of flake graphite cast iron in detail, specifically, hardness, tensile strength, elongation, impact value, deflection, and transverse load. We also investigated the cause. With increasing Mn amount for a constant S content, a transition point appears for each mechanical property. That is, hardness, tensile strength and transverse load become the smallest, and elongation, impact value and deflection become the largest. The mechanical properties of flake graphite are determined by the intervention of four actions; MnS nucleation action, interfacial energy action of soluble S, action of soluble S and soluble Mn to eutectic solidification temperature and pearlitization action of soluble S and soluble Mn. These four actions are thought to be negated by the formation MnS, resulting in the presence of the transition points. In addition, Mn and S amount showing the transition point demonstrate the best type A graphite shape.
With increasing Mn amount for various S content, many transition points appear. When these points are connected, a new transition point appears near S0.03% and Mn0.32%. Excellent mechanical properties are seen at this new transition point. That is, hardness and tensile strength become the highest, and elongation and impact value which conflict with these properties also become the highest. At this new transition point (0.03%S, 0.32%Mn), MnS just starts to form ([S] × [Mn] = 0.01), and soluble S is the highest. Here, [S] means soluble S and [Mn] soluble Mn.
Consequently, in flake graphite cast iron, the best excellent properties are obtained at S0.03% and Mn0.32%, which is just before MnS formation.