Effect of Subcritical Treatment (Tempering) on Hardness and Wear of High Chromium White Cast Iron

Abstract

High chromium white cast iron (HCWCI) is used in mining, crushing plants, as mill liners, and in the manufacture of grinding balls. The grinding ball is the essential element in fine fragmentation. It must have an excellent life to counter extreme wear and impact conditions that it undergoes. Hence it is important to improve its mechanical properties. This study aims to find the effect of tempering temperature on microstructure, hardness, and abrasive wear of High Chromium White Cast Iron used in the manufacture of grinding balls. In this study, the balls in high chromium white cast iron with 12 to 17% Cr, of 50mm and 70mm in diameter were austenitized at 950°C for 45 min and 55 min for the balls of 50 and 70mm then were quenched in forced air. The balls were tempered at 250°C, 400°C, and 600°C for 120 min and 180 min for both diameters 50 and 70mm respectively, and cooled in the furnace. The results showed that tempering practiced at 250°C and 400°C gives an excellent hardness than balls austenitized at 950°C. It is about 60 HRC and it drops to less than 50 HRC for the tempering at 600°C. This tempering causes a significant mass loss (short life) but tempering at 250°C and 400°C decreases this loss by up to 1.6%. The results of XRD showed the presence of the martensite and carbides type M7C3 after tempering at 250°C and a ferritic matrix after tempering at 600°C.