DWTT of pipeline steel

DWTT (drop weight tear performance) is an important performance parameters of pipeline steel. Pipeline steel pipe includes seamless steel pipe and welded steel pipe (LSAW and SSAW steel pipe) and for the pipeline steel used for the production of oil pipeline, it not only requires a certain strength, but also a certain toughness, so the production pipeline are all required a DWTT test, and this parameter has a close relationship to its body ferrite.
For the impact of ferrite type on DWTT: pipeline generally form the inner polygonal ferrite or acicular ferrite.
The role of polygonal ferrite is:
1) coordination deformation. Due to the polygonal ferrite belongs to a low strength and good ductility phase, there is an appropriate amount of polygonal ferrite can improve coordination among various organizations deformation capacity, in addition, the presence of grain boundaries of pearlite also makes coordination ferrite deformation limited, the deformation resistance to increase, and expansion work successfully shaped crack increases, thereby improving toughness.
2) passivation crack tips. Polygonal ferrite belong to the toughness organization with low strength, it can make a greater plastic deformation in the crack tip, impeding crack propagation, meanwhile the crack tip stress gets relaxed and the crack tip passivated. In addition, a certain amount of polygonal ferrite grains will continue to force the crack go through the pearlite structure and polygonal ferrite, a large number of the interface suppresses the crack propagation, so that the toughness is improved.