Study On Low-Temperature Brittle Fracture Control Index Of Oil Pipes In Alpine Region

In this paper, Sharpy V-Notch Impact properties, Drop Weight Tear Test properties and Fracture Toughness properties of three pipeline materials were investigated combining fracture theory with pipeline project example. The results were used to analyze influencing factors of low-temperature brittle fracture on oil pipeline, providing some reference for making control index of low-temperature brittle fracture. By means of serial temperature CVN tests and DWTT tests of X80 tee joint material,X80 TMCP and X70 TMCP pipeline materials, Energy characteristics and fracture characteristics were used to assess the low-temperature toughness of three materials, revealing the influence of wall thickness on size effect of low-temperature toughness. Finally, The low-temperature fracture toughness tests of X80 tee joint material were carried out,which found the estimating formula of fracture toughness for given wall thickness.The sharpy V-notch impact energy is above 150 J with the shear area above 50% at -60? for X80 tee joint materials; the sharpy V-notch impact energy is above 200 J with the shear area above 70% at -80? for X80 TMCP pipeline materials; the sharpy V-notch impact energy is above 350 J with the shear area 100% at -60? for X80 TMCP pipeline materials which showed great impact properties. The results of sharpy V-notch impact tests on FL position for weld joint of X70 TMCP fluctuated tremendously. FL position is unapplicable to be chosen as assessing position for HAZ.For all the X80 tee joint materials,X80 TMCP and X70 TMCP pipeline materials,50%FATT increased with wall thicker. When wall thickness reached to some extent, shear area of the fracture were below 50%;50%FATT got thinner with temperature lower, When temperature dropped to -40?,shear area of the fracture were below 50%, tendency of brittle fracture were much serious. The ductile-brittle transition temperature range of X80 tee joint materials was wide with high onset transition temperature; The ductile-brittle transition temperature range of X80 TMCP pipeline materials was narrow with low onset transition temperature; The ductile-brittle transition temperature range of X70 TMCP pipeline materials was wide with low onset transition temperature. The size effect of X80 tee joint materials at low-temperature was most obvious.During the fracture toughness tests of X80 tee joint materials, The number of Pop-in points in the P-V curves increased with the temperature dropping down and the Pop-in points became more with specimen thickness getting thicker, which resulted in low validity of specimens. In proper specimen thickness range, the thinner specimen had better fracture toughness. The fracture toughness of this material decreased gradually with specimen thickness getting thicker and finally approached a low constant critic. Calculated by valid points fitting method, K_C=147.0x^1/2e^-0.2x+307.5(1-e^-0.2x) was the estimating formula or fracture toughness for tee joint material, which reached a conclusion that the KIC of this material was approximately 307.5Mpa·m^1/2.