Abstract
The precipitation behavior of the sigma phase was studied for 8 kinds of austenitic stainless steel weld metals. The obtained results indicate: (1) the sigma phase replaces delta-ferrite in the welds during aging. (2) The sigma formation rate dx⁄dt in the temperature range 723–1023 K is expressed as follows:
dx⁄dt=H(xf−x)2⁄xf=H(y−yf)2⁄(1−yf)
xf−x=xf⁄(1+Ht), y−yf=(1−yf)⁄(1+Ht), H=H0exp(−Q⁄RT)
where x and y are the ratios of the sigma-phase content and the untransformed ferrite content to the initial ferrite content at aging time t; xf and yf are the x and the y after aging for a long time such as 36 Ms; and H is the rate constant. (3) The values of activation energy Q are about 170 kJ/mol for 20Cr–10Ni steels and 220 kJ/mol for 20Cr–10Ni–2Mo or –1Nb steels and 30Cr–9Ni steel.
