Fracture Toughness Evaluation of JCOE Pipe Under High Pressure H2 Gaseous

Abstract Hydrogen embrittlement is one of the major phenomena that deteriorates fracture toughness of pipeline steels. The stress intensity factor which is used for expressing resistance to fracture was investigated under high pressure hydrogen gaseous 21MPa according to ASTM 1820-21, using longitudinal submerged arc welded JCOE pipe of X65 grade with outer diameter (OD) 24″ and wall thickness (WT) 23.8mm rolled from a TMCP plate with Nb micro-alloying, which can create fine microstructure of base metal and HAZ. X65 UOE pipe with fine microstructure by using Nb microalloyed-TMCP plate but coarser grain size than the JCOE pipe was also tested for comparison. In this test, the loading rate was set low enough to represent high effectiveness in hydrogen embrittlement. The elastic-plastic fracture toughness JQ was derived from J-Δa curve. The stress intensity factor values converted from JQ for base metal, weld metal and HAZ of X65 JCOE pipe were all greater than 100 (MPa·√m), which met the criteria of ASME B31.12. The fracture initiation toughness of the X65 JCOE base metal in hydrogen was higher than that of the UOE pipe with coarser grain size. Weld metal of the JCOE pipe exhibited very fine acicular ferrite microstructure and showed highest toughness. While the limited experimental data was presented in this paper and further investigation should be necessary, it can be stated that finer microstructure consisting of acicular ferrite and polygonal ferrite exhibits higher fracture initiation toughness in high pressure hydrogen gaseous.