Study On Girth Welded Cracking Mechanism Of Bi-metal Composite Pipe

Bi-metal composite pipe is an important transmission pipe,which has been widely used to transport gas and oil under abominable conditions.Its cracking or breakage leads to severe economic losses.The goal of this article is to find out the reason and the cracking mechanism in the girth joint of L415QB+316L bi-metal composite pipe,which contains the root welding zone(R316)?sealing welding zone(R309L)?transition layer(R309LMo)?packing layer and covering layer(E5015).X-ray was used to determine the location of the crack and found that the cracks was in the seal welding zone.OM was used to observe the feature of fracture morphology,found that the seal welding zone and the transition layer were brittle rupture;the packed layer and cover layer were ductile fracture.SEM and EDS were applied to analysis the microstructure and the chemical composition of the girth joint,figured out that the seal welding zone contains higher content of ferrite.Microhardness tester was used to tset the hardness on the sample,the result shows that the seal welding zone and the transition layer were harder than other zone,wich cause stress concentration;the brittle microstructure of seal welding zone and the transition layer and uneven hardness distribution was an important cause of cracking.Used the finite element analysis software ABAQUS to figure out the stress distribution of the girth joint under kinds of load.The result showed: when the bending load was 15 mm,the stress of seal welding zone reaches 520MPa;under operation pressure,the stress of girth joint was low,and the operation pressure was not the main reason of craking;the stress of seal welding zone reaches 520 MPa when the tension load was 85 mm and the stess reaches tensile strenght;the stress of seal welding zone reaches 520 MPa when the load contains 13 mm bending load and 10%(the whole length of the pipe)tension load.