Högskolan i Skövde

The applied austempering heat treatment to an as-cast ferritic CGI alloy resulted in a significant increase in hardness and micro-scratch resistance of the heat-treated samples compared to the as-cast sample. In the present study, the samples were extracted from as-cast pieces and heat treated by austempering under austenitizing temperature of 900 oC for 60 minutes, which followed by quenching in salt-bath at 325oc for various austempering holding time 30, 60, 90 and 120 minutes. The effect of austempering time was investigated on the final microstructure, hardness, and micro-scratch properties of the achieved samples. Light optical and scanning electron microscopic together with EBSD analysis of the samples showed the ausferritic matrix formed in all heat-treated samples without affecting the graphite forms. The Brinell hardness measurements of the ausferrite matrix showed a significant increase in hardness compared to the ferritic CGI, however, the maximum increase was observed for the shortest austempering time. In addition, nano-hardness values showed a quite significant variation in the matrix due to the presence of acicular ferrite and carbon-saturated retained austenite. Increase of austempering time led to decrease of carbon saturated retained austenite. However, for austempered samples above 60 minutes austempering time, the scratch hardness showed an increase trend with austempering time, a constant value was observed for the ferritic CGI alloy.