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MATERIALS SELECTION & DESIGN FIGURE 2 (Figures 1[c] and [f]). Furthermore, both the in-plane cracks and linking cracks were transgranular cracks, which means that the stress concentration can trap more hydrogen atoms (Figures 1[d] and [e]). The Morphology of Type II SOHIC Figure 2 shows the SOHIC cracks without obvious HIC cracks, which was defined as Type II SOHIC. Type II SOHIC has an obvious feature that both continuous and discontinuous linking cracks were much stronger than in-plane cracks (Figure 2). Type II SOHIC pre- sented only one main crack perpendicular to the applied stress with and without dif- ferent cracks in the thickness direction, as shown in Figures 2(a) and (b), respectively. Internal hydrogen pressure plays a different role in the formation of Types I and II SOHIC. Much more hydrogen can be gathered in the plasticity zone ahead of the crack tip, which is the latent initia- tion site proposed by Amano, et al.13 Due to the effect of stress, the direction of the internal hydrogen pressure is along the applied stress rather than vertical to it. Owing to the synergistic effect of the ap- plied stress and internal hydrogen pres- []ZM \PM TQVSQVO KZIKS[ NWZU ÅZ[\ ZI\PMZ than in-plane cracks. Moreover, the propagation rate of linking cracks could be too fast to form the in-plane cracks. The Morphology of the Fracture Surface The fracture surface of Type II SOHIC exhibited a brittle fracture with- out any dimple characteristics (Figure 3). 5IVOIVM[M []TÅLM 5V; QVKT][QWV[ _MZM found by EDS in the pit of the surface, which may be the position of initial cracks (Figure 3[a]). In addition, some tiny in- plane cracks were observed on the frac- ture surface of Type II SOHIC cracks (Figure 3[b]). It is suggested that hydro- gen can also induce the in-plane cracks during the formation of Type II SOHIC. The in-plane cracks were not so obvious NACE International, Vol. 51, No. 11 Different morphologies of Type II SOHIC. (a) SOHIC with discontinuous linking cracks; (b) SOHIC with only one main crack; (c) SOHIC with long discontinuous cracks. FIGURE 3 The SEM morphologies of the fracture surface of SOHIC cracks. (a) MnS inclusions on the fracture surface; (b) tiny in-plane cracks observed on the fracture surface. FIGURE 4 The morphologies of SOHIC cracks in inhomogeneous regions. (a) SOHIC crack deviated from the stress concentration for the banded pearlite; (b) the morphology of SOHIC cracks formed in a welding zone of L360QS. November 2012 MATERIALS PERFORMANCE 73