ASTM 536-84 lists minimum acceptable tensile strength, yield strength and percent elongation for the four grades of as-cast ductile iron. SAE J434C specifies the Brinell hardness ranges for irons expected to have the same yield strength and elongation minima as equivalent ASTM grades. Casting producers have a problem with these specifications when users request a combination of low hardness in a specified ASTM range with high tensile and yield strength in that range. The specifications fail to accept the existence of an increasing strength and decreasing elongation continuum, with increasing hardness over the range of 150-300 BHN. The continuum is verified by data from several foundry companies and their affiliated plants. Equations with statistical values that describe the continuum are provided. Image analysis reveals the gradual change in ferrite and pearlite microstructures with increasing BHN. A parameter defined as tensile strength minus yield strength provides insights to the image analysis and continuum results. All equations are based on BHN measurements made by the optical scan methods.
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