Metallurgical changes and mechanical behaviour during high temperature aging of welds between Alloy 800 and 316LN austenitic stainless steel

Abstract

In the use of ferritic to austenitic stainless steel transition joints for power plant applications, the difference in coefficients of thermal expansion constitutes a serious problem. One way to mitigate this is to use a trimetallic configuration by interposing a material with a coefficient of thermal expansion intermediate between the ferritic and austenitic steels. Modified 9Cr - 1Mo steel has been joined to 316LN austenitic stainless steel using Alloy 800 as an intermediate piece. In the work herein reported, welds between Alloy 800 and 316LN have been produced using Inconel 182 filler material. These have been subjected to high temperature exposure for up to 5000 h at 625 ° C. Results have shown that up to 500 h of aging the structure and mechanical properties remain unaffected. On treatment for 2000 and 5000 h, however, there is a noticeable increase in hardness and reduction in toughness. These have been found to be caused by precipitation of Ni₃Ti and carbide phases including NbC and M₂₃C₆.