Some comparative observations of carbide precipitation morphology related to coherent twin boundaries and high-energy grain boundaries in stainless steels

Abstract

Stickler and Vinckier showed more than three decades ago that there is a very consistent relationship between the boundaries upon which carbides (M23 C6) precipitate in 300 series stainless steels (having carbon contents ranging from 0.02 to 0.08 wt%). For example, carbides first appear on regular (high-energy) grain boundaries, then non-coherent boundaries and steps on annealing twins, and finally on coherent twin boundaries at a constant temperature above about 600° C, and at aging times which, correspondingly, change by orders of magnitude (1,10, 100 hrs. respectively at 675°C for 304 stainless steel). We have examined a wide range of precipitation features on these various boundaries in 316 and 304 stainless steels which have not been described previously because there have been limited observations comparing carbide morphologies in the transmission electron microscope (TEM) for high-energy grain boundaries (γgb ∼ 800 mJ/m2), non-coherent steps on twin boundaries (γTB ∼ 200 mJ/m2), and coherent twin boundaries (γtb ∼ 20 mJ/m2) for 316 and 304 stainless steels.