Effect of Alloy Composition on the Transformation Kinetics of Delta Ferrite in Type 316 Stainless Steel Weld Metal

The presence of delta-ferrite (2 to 10%) is essential in austenitic stainless steel weld metals to avoid the problem of hot tearing during solidification. However, during elevated temperature service or post-weld heat treatment, the delta-ferrite transforms to various secondary phases. The kinetics of transformation of delta-ferrite depends mainly on the growth kinetics of secondary phases, which in turn is dictated by the alloy composition. In this paper the results of investigations for the transformation kinetics of delta-ferrite conducted on several type 316 stainless steel weld metals of different compositions are given. Based on these data, it is found that the ageing time required to transform a given fraction of ferrite varies directly with "normalized equivalent chromium content" (NECC) of the alloy and that high values of NECC lead to a rapid decay of delta-ferrite.