Precipitation effects in austenitic stainless weld metals

Abstract

Creep‐rupture specimen of similar welded joints of X6 CrNi 18 11 (1.4948/AISI 304) and X6 CrNiMo 1713 (1.4919/AISI 316) show only low elongation after longtime testing. The reason for this loss of ductility was found by metallographic investigations.The weld metal of five joints had ferrite contents from 3 to 7.5%, due to a primary ferritic solidification. During creep testing in the temperature range from 500 to 800°C carbide precipitation takes place at the austenite‐ferrite grain boundaries, because delta ferrite is not in a state of equilibrium at these temperatures. After carbon has been used up, the remaining delta ferrite changes into sigma phase, if its alloying element content is high enough. In the upper temperature range, coagulation of sigma phase is dominating. At these large particles grain boundary migration is hindered. During the grain boundary sliding sigma phase particles break and initiate creep cracks. In fully austenitic weld metal sigma precipitation starts at austenite – austenite grain boundaries. During creep testing, void formation starts at sigma particles. Growing of voids leads to grain separations.