One Minute Mentor: Nitrogen-Expanded Austenite

July 07, 2017
Source: ASM International

The graph shows hardness-depth profiles of nitrided AISI 316. A profile for carburizing is shown for comparison.

Lines obtained by fitting a sigmoidal function are given to guide the eye. The hardness values measured directly at the surface (with the same load) were 1585 HV for KN = 2.49;  and 1417 HV for KN= 0.293.

The effect of the nitriding potential is also reflected in the hardness-depth profiles. A smooth decrease near the surface is  followed by a sharp drop at a position close to the transition from γN to γ. Nitrogen-expanded austenite is very hard, and the transition from the hardened case to the (soft) core occurs within a few microns.

Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Low-Temperature Surface Hardening of Stainless Steel ⇒ Gaseous Low-Temperature Surface Hardening of Austenitic Stainless Steel 

Subject Classifications

Materials Processing and Treatment | Heat Treating

Metals and Alloys | Carbon and Alloy Steels

Metals and Alloys | Stainless Steels