Installation of stabilized austenitic stainless steel piping (Grades 321,321H, 347 and 347H) at service temperatures above 1000°F (538°C) requires an enhanced piping design that will accept the reduction in weld metal ductility associated with the formation of sigma phase. The transformation of weld metal delta ferrite to a brittle sigma phase is rapid and occurs in exposure time under 2500 hours. The reduced ductility of the weld metal at temperatures as high as 1050°F (566°C) may result in stress relaxation cracking of welds where the piping displacement stress range ~ approaches the material yield strength at temperature. The use of heat treatments to both relieve residual stress and to lower the ferrite content in weld metal should be considered in the fabrication specifications. Lowering the weld metal ferrite by solution annealing will result in a lower potential to form sigma phase at service temperatures above 1000°F (538°C).
Key Words: Stainless steel, sigma phase, stress relaxation cracking