Study On Formation Mechanism And Microstructure Performance Of Solid-Phase Welded Joints For Fine-Grained Pipes Used In Oil And Gas Engineering

With the rapid development of the world economy,the increasing demand for energy,oil and gas engineering has developed rapidly,and at the same time,steel for oil and gas engineering has entered a period of rapid development worldwide.Oil and gas engineering steels have higher requirements for the service performance of steels due to the particularity of the environment in which they are used.Fine-grained steel has been well used for its high strength and toughness.However,fine-grained steel has very small grains which tend to grow during the thermal cycle of welding.Therefore,traditional welding processes such as fusion welding are more likely to cause coarsening of grains in the heat affected zone and reduce joint quality.As a result,a solid-phase welding process is proposed.Friction welding,as a typical solid-phase welding,has significant advantages such as narrow welding heat-affected zone,good welding quality,and high welding efficiency.It has been widely used in aerospace,automotive,and petroleum drill rod welding.The thesis first uses different fine-grained tubing for oil and gas engineering,namely X100 and a 90-grade oil well tubing,to conduct a friction welding suitability study.It was found that the fine-grained tubing used in the oil and gas engineering in the experiment had good applicability to the solid-phase friction welding process.The performance of the formed welded joint can be matched with the main pipe,which to meet the requirements.Secondly,the formation mechanism of friction welding was analyzed.It was found that there was dynamic recrystallization in the friction welding process.Under the effects of thermo-mechanical coupling,dislocation slip occurred,resulting in plastic flow of the crystal and diffusion of metal atoms in the thermoplastic zone,thereby achieving the combination of the friction welding physical interface.Then,X100 was taken as the main research object to optimize the friction welding process parameters.Taking the maximum strength value as the test index,the optimal process parameters were obtained: the speed was 1000 r / min,the friction pressure was 170 MPa,the upset pressure was 255 MPa,and the shortening was 7mm.Finally,in order to conduct a more in-depth study on the microstructure and properties of friction welding joints of fine-grained pipes for oil and gas engineering,X100 was taken as the main research object,and the post-weld heat treatment process was studied.The results showed that after heat treatment of X100 friction welded joints,the structure recrystallized,the grains were refined,the streamline characteristics of the heat affected zone basically disappeared,and the structure performance was more uniform.As the tempering temperature increased,the hardness of the welded joint continues to decreased.When the tempering temperature was 600 ?,the hardness of the welded joint changes the smallest range,about233HV-243 HV,the hardness of each area of the joint was almost equal to the hardness of the base material,the performance was the most uniform,and the matching with the parent pipe was more higher.