Ideally weak nitriding kinetics during gaseous nitriding of Fe-2at.% Si alloy

Fe-2.03at.% Si alloy was internally nitrided, employing NH3/H-2 gas mixtures at 580 degrees C using a nitriding potential (rN) of 0.104 atm(-1/2). The nitrided specimens were primarily characterized by light microscopy, scanning electron microscopy and hardness measurements. Surprisingly, and for the first time, an ideally weak nitriding interaction, which is usually observed, for alloying element nitrides which are difficult to precipitate, upon nitriding only thin (about 0.1-0.3 mm thick) Fe-Me alloy specimens, was observed here upon nitriding thick (1 mm) recrystallised Fe-2.03at.% Si alloy specimens. This phenomenon can be attributed to a very large incubation time and very slow kinetics for the precipitation of (amorphous) silicon nitride. Further, an unusual hardness change with increasing nitriding time and a nitrogen-uptake rate increasing with nitriding time were observed. The hardness of the nitrided specimen first increased with increasing nitriding time up to 30 h and then decreased until 165 h and thereafter increased again. The occurrence of the first peak in the hardness versus nitriding time profile (at about 30 h) was ascribed to the development of a metastable, precursory form of silicon nitride.