Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.
Issue link: http://mp.epubxp.com/i/89686
MATERIALS SELECTION & DESIGN Stress Oriented Hydrogen-Induced Cracking of L360QS Steel
as those in Type I SOHIC due to the fast propagation of linking cracks. The crack propagation rate associated with applied stress and stress gradient in the thickness direction plays a primary role in deter- mining the type of SOHIC. In the pres- ence of high stresses, the linking crack could propagate too fast to leave enough time to form in-plane cracks, leading to Type II SOHIC. Otherwise there is enough time to form the in-plane cracks and linking cracks at a low crack propaga- tion rate, leading to Type I SOHIC.
Effects of Microstructural Inhomogeneity
Figure 4(a) shows the effect of pearlite
colonies on SOHIC. The in-plane crack was located in the zone of pearlite colo- nies. The pearlite colonies could provide crack initiation sites for the in-plane cracks only in a case where the pearlite colony conforms to the latent initiation site model. When the linking cracks propagate to the HIC crack site, the stress at the crack tip will be relaxed. As a result, the new crack could initiate at a weak area, such as pearlite colonies. From the aforementioned results, it can be recog- nized that the morphology of SOHIC cracks was determined by the synergistic effect of applied stress and microstruc- tural inhomogeneity. Furthermore, the effect of columnar
crystals in a welding zone on the crack propagation was also remarkable (Figure 4[b]). It can be seen that the crack in the columnar zone was disrupted to a differ- ent shape with the orientation of the co- lumnar crystals. This can be attributed to the fact that the inhomogeneity of the columnar crystals can trap more hydro- gen atoms, therefore creating hydrogen embrittlement.
Conclusions z ;701+ Q[ KTI[[QÅML QV\W \_W \aXM[
according to the shape of the cracks: Types I and II. Type I SOHIC is a traditional SOHIC crack with in-
74 MATERIALS PERFORMANCE November 2012
plane cracks and linking cracks while Type II SOHIC has no obvi- ous in-plane cracks.
z The crack propagation rate associ- ated with applied stress and stress gradient in the thickness direction may play an important role in the formation of Types I and II SOHIC. If the in-plane crack initi- I\M[ ÅZ[\ I TW_ KZIKS XZWXIOI\QWV rate of a linking crack could lead to the formation of Type I SOHIC. Otherwise it leads to the formation of Type II SOHIC.
z The effect of microstructural inho- mogeneity bands on the morphol- ogy of SOHIC is remarkable.
Acknowledgments