Nitride precipitation in salt-bath nitrided interstitial-free steel

Nitride precipitation and its effect on microstrain in salt-bath nitrided interstitial-free steel were investigated using transmission electron microscopy and neutron diffraction. As the cooling rate after nitriding decreased, two nitrides, γ′-Fe4N and α"-Fe16N2, were identified in diffusion zone. Combined analyses using Rietveld whole-profile fitting and size–strain analysis revealed that the microstrain in the nitrided specimen increased due to nitrogen supersaturation and then decreased after nitride precipitation, whereas the effective particle size continuously decreased. It was found that microstrain is the dominant factor in peak broadening of the nitrided specimen.