The diffusivity of nitrogen in γN was determined from the stepwise denitriding kinetics of initially fully nitrided homogenous (stress-free) expanded austenite coupons. From the average diffusion coefficient for a series of narrow composition intervals, the composition-dependent diffusivity of nitrogen was reconstructed. The graph shows the diffusion coefficient of nitrogen in expanded austenite as a function of the nitrogen occupancy of the interstitial sites of the face-centered cubic lattice. Models based on the growth kinetics of the expanded austenite zone should take into consideration that the diffusivity of interstitials in expanded austenite is (strongly) concentration dependent and driven not only by a composition gradient but also by the presence of a (huge) compressive residual stress gradient over the thickness of the zone.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Low-Temperature Surface Hardening of Stainless Steel ⇒ Physical Metallurgy of Expanded Austenite
