Solidification Mechanism of Austenitic Stainless Steel Weld Metals with Primary Ferrite Solidification

Solidification microstructure and crystallography of austenitic stainless steel weld metals that solidify with primary ferrite are investigated. At the fusion boundaries, austenite grows first with plane-front morphology from the base metal austenite in an epitaxial manner. Then, ferrite forms on the growing austenite by keeping the Kurdjumov-Sachs orientation relationship with the austenite. The ferrite then grows, as the primary phase, more rapidly with dendritic morphology than the planar austenite. Though the phase diagram indicates that the formation of the austenite results from the peritectic/eutectic reaction in the primary ferrite solidification mode, no unique orientation relationship was confirmed by crystallographic studies between the primary ferrite and the interdendritic austenite. It is suggested that only at the nucleation stage of new ferrite on austenite, the specific orientation relationship is established between the ferrite and the austenite. But the austenite in the dendrite boundaries is found to grow independently of the primary ferrite, growing along <100> direction, even when the primary ferrite changes its growth direction. Thus, it is concluded that the austenite in the interdendritic regions is not crystallographically restricted by the preceding primary ferrite during the growth. In fact, the <100> directions of ferrite and austenite are found almost parallel. Final ferrite morphology at room temperature is determined both by the crystallographic orientation relationship between ferrite and austenite established at the stage of ferrite nucleation and by the relationship between the welding heat source direction and the preferential growth directions of ferrite and austenite.