Effects of Grain Size and Homogenization Heat Treatment on the HAZ Cracking Susceptibility of Cast Alloy 718

Abstract

The materials used were cast alloy 718 plates with three different grain size levels. The plates were heat-treated at 1368K×5.4ks or 1378K×10.8ks before welding. The results of the longitudinal Varestraint test indicated that the fine-grained and heat-treated (1378K×10.8ks) specimens were considerably less sensitive to HAZ cracking. Based on the microscopic observations of specimens heated at various temperatures, the grain boundary liquation mainly occurred in the laves cluster which had a low melting point. In addition, the degree of grain boundary liquation in the fine-grained and heat-treated (1378K×10.8ks) specimens were less than that of the coarse-grained and non heat-treated specimens. The EPMA analysis revealed that S was severely segregated at the laves cluster in the interdendritic region. The amount of S segregation decreased with decrease in grain size or homogenization heat treatment. A theoretical calculation suggested that the elevated grain boundary liquation temperature was caused by decreasing the S segregation in the laves cluster. On the basis of these results, the HAZ cracking susceptibility of cast alloy 718 could be reduced by decreasing grain size or homogenization heat treatment before welding. The decreased HAZ cracking susceptibility was considered to be attributed to the reduction in grain boundary liquation due to decrease in the amount of laves cluster and S segregation which resulted in an increased grain boundary liquation temperature.