The Effects of .DELTA.-Ferrite Morphology on Low Temperature Fracture Toughness of Austenitic Stainless 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 test were performed at low temperature, brittle fracture easily occurred in a vermicular type δ ferrite because the longitudinal direction of this δ perallel to [100]δ, that is a cleavage plain.On the other hand, brittle fracture scarcely occurred in both a globular type and a lacy type δ ferrite, because the 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 boundary holding the K-S are coherence to grains and hardly cause the decohesion.The present data indicate that changing the δ ferrite morphology from a vermicular type to lacy type prevents the cleavage fracture and increases the fracture toughness at low temperature.