Effects of δ Ferrite Morphology on Low Temperature Fracture Toughness of SUS304L Steel Weld Metal

The effect of δ ferrite morphology and grain boundary property on the low temperature toughness of austenitic stainless steel SUS304L weld metal has been studied. The types of δ ferrite were classified into three categories : vermicular, lacy, and globular. When Charpy tests were performed at low temperatures, brittle fracture easily occurred in a vermicular type of δ ferrite because the longitudinal direction of this δ corresponds to [100] δ, that is a cleavage plane. On the other hand, brittle fracture scarcely occurred in both a globular type and a lacy type δ ferrite, because the stress concentration must be low at globular type δ and the Kurdjumov-Sachs (K-S) relationship holds between lacy type δ and γ phase. Grain boundaries holding the K-S relationship are coherent to grains and hardly cause the decohesion. The present data indicate that changing the δ ferrite morphology from a vermicular type to a lacy type prevents the cleavage fracture and increases the fracture toughness at low temperatures.