Abstract
A study was made on the relation between the microstructure and the mechanical properties of the HAZ which was produced by a thermal simulation test for a high strength steel. It was found that the HAZ toughness deterioration of the high strength steel is closely related to the amount and existing form of martensitic islands in the matrix. Blocky martensitic islands surrounded by ferrite grains did not deteriorate toughness, but lath shape martensitic islands formed in a large matrix packet of the same crystal orientation were quite detrimental to toughness depending on the amount of martensite. The latter structure was formed in the unaltered coarse-grained zone (UACGHAZ), in the intercritically reheated coarse-grained zone (ICCGHAZ), and in the sub-critically reheated coarse-grained zone (SCCGHAZ). But among these structures, the structure in the ICCGHAZ had relatively short lath martensitic islands with softer lath matrix. This structure was especially apt to create micro-cracks. The dependence of strength and toughness on the martensite morphology was considered to result from the degree of restriction against the plastic deformation of matrix due to the microscopic heterogeneity in strength.
