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MATERIALS SELECTION & DESIGN Stress Oriented Hydrogen-Induced Cracking of L360QS Steel FIGURE 1 by an Olympus† metallographic micro- scope, and the fracture surface was ana- lyzed by scanning electron microscopy (SEM) and energy dispersive spectros- copy (EDS). Results and Discussion The Morphology of Type I SOHIC Figure 1 shows the typical morpholo- gies of Type I SOHIC. It can be seen that the in-plane cracks initially formed at the position ahead of the notch tip at a certain distance where the stress concentration was the maximum, which can be ex- plained by the latent initiation site model proposed by Amano, et al.13 HIC cracks Different morphologies of Type I SOHIC. (a) SOHIC with discontinuous HIC cracks and linking cracks; (b) SOHIC with HIC cracks linked by continuous cracks; (c) SOHIC with only HIC cracks; (d) the small crack A at the border of a HIC crack; (e) the morphology of linking crack B; and (f) the small crack C at the border of a HIC crack. Cracks A, B, and C appear in Figure 1(b). Several authors16-18 have investigated the effect of inclusion and microstructure on HIC, and showed that inclusions and pearlite colonies play the part of initiation sites for HIC. Meanwhile, the effect of microstructure on SOHIC was also in- vestigated. Studies by Amano, et al.13 show that SOHIC resistance is improved by dispersing pearlite colonies through thermo-mechanical controlled process- ing. Studies by Coudreuse, et al.19 dem- onstrate that a quench and temper heat treatment of A516 Grade 60 (UNS K02100) normalized steel can signifi- cantly reduce or eliminate cracking. The effect of inclusions and microstructural inhomogeneity on SOHIC has not yet JMMV QV^M[\QOI\ML []NÅKQMV\Ta PW_M^MZ Starting with the insights noted previ- ously, an experimental study was launched with the objective of developing further understanding of the mechanism and influence factors of SOHIC on L360QS steel. 72 MATERIALS PERFORMANCE November 2012 Experimental Procedure The samples used for this study were cut from crude oil pipelines meeting the API X52 standard for L360 steel used in [W]Z WQT ÅMTL[ _Q\P \PM KPMUQKIT KWUXW- sition (wt%) C 0.13, Si 0.4, Mn 1.5, P 0.02, S 0.003, Cr 0.3, Mo 0.1, Ni 0.3, Ti 0.04, and Fe balance. Both welding and matrix samples came from rolled seamless steel tubes. The experiments were per- formed following NACE TM0103- 2003.20 notch in the tension surface were used and soaked in NACE A solution with saturated H2 S gas at 25 °C for 168 h. The stress at the notch tip was formulated at 80% yield stress for each sample. After the test, the central notch zone of each sample was cut into different small samples to investigate the crack morphologies. An ultrasonic technique was used to clean the cracks after polish- QVO \PM [XMKQÅK XTIVM WN \PM [UITT [IUXTM[ The crack morphologies were observed can be initiated within the plastic zone associated with the notch through hydro- gen transport along an activated slip plane. The distance between the notch tip IVL \PM ÅZ[\ 01+ KZIKS _I[ LM\MZUQVML by the stress level. The higher the applied stress, the greater the extent of the plastic zone in the thickness direction. In addi- tion, the in-plane cracks aligned parallel to the applied stress, while the linking cracks were diverse. Both discontinuous and continuous linking cracks can be observed in Figures 1(a) through (c). It is important to emphasize that the linking cracks were not always strictly perpen- LQK]TIZ \W \PM IXXTQML [\ZM[[ J]\ LMÆM`ML at some angle toward the thickness direc- tion, which was consistent with the theory proposed by Iino, et al.14 The shear stress Double beam samples with a imposed by an externally applied in-plane stress actually opposes those generated by pressure within the crack, thereby leading to stepwise linking. As the applied stress increases, the location of maximum shear stress moves from near the crack tip, to a little way from the crack in the thickness direction. As a result, the initial linking cracks between two in-plane cracks are not located at the mid of the in-plane cracks but the edge of the in-plane cracks †Trade name. NACE International, Vol. 51, No. 11