Grain and grain boundary characters in surface layer of untreated and plasma nitrocarburized 18Ni maraging steel with nanocrystalline structure

The nanocrystallized 18Ni maraging steel was plasma nitrocarburized at 460 °C for 4 h in a mixture gas of N ₂ , H ₂ and C ₂ H ₅ OH. The surface phase compositions of the specimens were analyzed using X-ray diffractometer. The grain shape and size, and grain boundaries in the subsurface layers of the samples were characterized by electron backscattering diffraction and transmission electron microscopy. The results show that the nitrocarburized layers are composed of α-Fe, γ′-Fe ₄ N and FeN _0.049 phases. Most α-Fe and γ′-Fe ₄ N grains show in columnar shape. The major and minor axes of some α-Fe grains are elongated and shortened after nitrocarburizing, respectively. In the subsurface layers of the untreated and nitrocarburized specimens, the average areas of γ′-Fe ₄ N and α-Fe grains both with a dimension of nanometer are 0.395 μm ² and 0.397 μm ² , respectively. The γ′- Fe ₄ N grain boundaries are mainly high angle boundaries with a very small fraction of low angle boundaries. Coincidence site lattice boundaries in the subsurface layer of the untreated specimen are composed mainly of Σ3, Σ11 and Σ13b, and their fraction decreases after nitrocarburizing.