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 N2, H2 and C2H5OH. 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′-Fe4N and FeN0.049 phases. Most α-Fe and γ′-Fe4N 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′-Fe4N and α-Fe grains both with a dimension of nanometer are 0.395 μm2 and 0.397 μm2, respectively. The γ′-Fe4N 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.