Effect of welding conditions and a long term heat-treatment at 650°C on metallurgical changes in austenitic stainless steels (SUS 304 and SUS 316) weld metals have been investigated. Results were as follows; 1) The solidifying structure at center parts of weld metals tend to grow parallel to the direction of plate thickness of the 1 pass 1 layer welded joint. 2) δ-ferrite forms in the order to the solidifying directions. 3) δ-ferrite changes those morphologies from granular and rod-like shape to net-like shape with increase of the amount of δ-ferrite. In the case of large amount δ-ferrite, this net-like δ-ferrite takes the form into densely three-dimentional network in the inner part of each pass. 4) There is no δ-ferrite network in the boundary zone between pass and the following pass, because δ-ferrite in these boundary zone are mixed granular and rod-like shape. 5) Networks of δ-ferrite in high heat input weld metals are larger than that in low heat input weld metals. 6) Carbides and δ phase precipitate in δ-ferrite subjected to heating for a long term at 650°C. 7) Carbides and δ phase take the form of network with a long term heating at 650°C in the weld metal containing a large volume of δ-ferrite.
From above results and the correlated research results of creep test and notch toughness of austenitic stainless steel weld metals subjected to a long term heat-treatment, it is considered that the following processes are advisable for the prevention measure of material worse at high temperature; 1) Selecting of the weld metals in which have the amount of δ-ferrite under about 5%, for the reason of dispertion of δ-ferrite and for destruction of δ-ferrite networks. 2) Making use of 2 pass 1 layer welding sequence to disperse the solidifying direction and applying the low heat input welding process to destroy δ-ferrite networks.
