Study On Stress Corrosion Cracking Resistance Of X65 Pipeline Joints With Large Deformation Based On Thermal Simulation Technology

During the reel-lay laying and service process,under the action of the corrosive medium containing hydrogen sulfide and large deformation,the welded pipeline,especially the weakest heat-affected zone in the welded joint,will face a severe test.Therefore,the development and design of a reasonable welding process and the improvement of the overall performance of the zone with coarse grains are of great significance for improving the service life of the pipeline and reducing costs.In this paper,X65 pipeline steel with large deformation was selected,the coarse-grained zone（CGHAZ）during the primary thermal cycle and the critical coarse-grained zone（ICCGHAZ）during the secondary thermal cycle with different parameters were obtained by thermal simulation.Their microstructure,fracture toughness and sulfide stress corrosion cracking（SSCC）sensitivity before and after pre-strain were studied,and the well-designed welding process was developed.The main research results of this paper are as follows:（1）After primary thermal cycle,with the increase of t_8/5,the main microstructure of X65 pipeline steel with large deformation transforms from martensite to acicular ferrite,granular bainite and acicular ferrite,proeutectoid ferrite and granular bainite.As t_8/5 increases,the prior austenite grain size increases from 92.72μm to 133.08μm,and the hardness decreases from 307 HV₁₀ to 194 HV₁₀.（2）After 5%pre-strain,the dislocation density and hardness of CGHAZ increase slightly,and the elongation of the specimens in the air and corrosive media reduced to varying degrees,and the maximum percentage of reduction is 23%.（3）After primary thermal cycle,the specimens have excellent hydrogen embrittlement resistance,corrosion resistance and toughness when t_8/5=20,25,30 and60 s.Among them,when t_8/5=20 s,CGHAZ with dense acicular ferrite laths has ideal stress corrosion resistance and fracture toughness with pre-strain,which are 79.89%and 0.20 mm,respectively.（4）Compared with CGHAZ,ICCGHAZ has larger lath gap and there are coarse necklace-type M-A constituents at the prior austenite grain boundaries.With the increase of t_8/5,the size of M-A constituents increased from 2.69μm to 3.74μm,the dislocation density increased from 4.99×10¹⁴ m^-2 to 5.52×10¹⁴ m^-2,and the fracture toughness decreased from 0.013 mm to 0.0028 mm.When t_8/5=30 s,it is beneficial to realize the synergistic improvement of the fracture toughness and stress corrosion resistance of ICCGHAZ with pre-strain.