Sigma Nucleation Times in Stainless Steels

Abstract

The times at which the first detectable amount of o- phase forms at temperatures between 900° and 1800°F were determined. Both X-ray diffraction and metallography were used to detect σ in highly strained filings; metallography alone to detect it in annealed bulk samples of six different types of stainless steels. The σ phase was found to form in accordance with the C-type reaction curve in both austenitic and ferritic alloys. In filings of types 304, 347, and 446, σ formed in a few hours, and in types 316, 309, and 310, it formed in a matter of minutes at temperatures corresponding to the knee of the C-curve. Only in the bulk type 309 steel, after 500 hr, and in the bulk type 310 steel, after as short a time as 25 hr at temperature, was σ detected; in both instances at approximately the same optimum temperature as in the filings. The annealing temperature affected σ phase nucleation only for the filings of type 304 and 347 steels, and for the bulk samples of type 309 and 310 steels.