After tilting the C-grain to some other SAD patterns, we identified that the grain boundary precipitate is M3C carbide, the cementite. The zone-axis of the SAD pattern in Fig. 3(c) is along the [1View the MathML source1] direction of the cementite. We also investigated the Widmanstätten plate in Fig. 3(b), and confirmed that the Widmanstätten plate is also M3C carbide. Because of the large lattice parameters of the M3C carbide compared with the lattice parameter of austenite, some reflections of the cementite in the SAD patterns are too close and are not easy to identify. In addition, some SAD patterns from the M3C carbide are similar and easily confused. To identify the SAD patterns of the M3C carbide is arduous work, and special efforts must be put into the SAD patterns with zone-axis directions in [010] and [111]