Abstract
The objective of this study is to investigate the effects of Maganese (Mn) on continuous cooling transformation behavior and heat treatment characteristics of low alloyed shperoidal graphite cast iron containing 0.5 to 3mass%Mn. Austenitizing was performed at 1223K and 900s, and the cooling time from Ac3 to 773K was varied about 3 to 4000s. All of the ferritic, pearlitic, bainitic and martensitic transformations were observed for small (0.5%) Mn content. On the other hand, ferritic transformation did not occur when Mn was more than 3mass% and the bainitic one never when more than 1.5%. This was due to the fact that manganese inhibited the precipitation of ferrite for bainite nucleus. Manganese lowers A3 temperature and increases carbon content in the matrix, thereby lowering Ms temperature. As the C content in the matrix was consumed for graphite or cementite growth at a cooling time of more than 100s, the Ms temperature tended to elevate. Gross residual austenitic structures appeared in quenched microstructures of 1.5 and 3%Mn cast irons. As higher Mn content and shorter cooling time produces larger austenites, the formation of these gross resudual austenite structures is considered to be induced by the drop in Ms temperature. The amount of residual austenites decreased when the cooling time was more than 100s and the amount of martensites decreased when 500s in 1.5 and 3%Mn cast iron. The hardening curve was therefore convex at cooling time ranges of 40 to 70s or 100 to 500s. The hardenability of the cast irons improved by increasing the Mn content.
