| High-frequency welding(HFW)steel pipe is one of the methods to product oil and gas linepipe,oil casing pipe and structural pipe because of its good size accuracy,high production efficiency and low cost.However,due to the early welding quality problems of HFW steel pipe in China,the promotion and application of HFW steel pipe in the oil and gas pipeline market have been seriously affected.With the improvement of modern HFW equipment and technology,the welding quality of HFW steel pipe has been significantly improved.How to comprehensively evaluate the quality of HFW steel pipe and regain the recognition of the quality of HFW steel pipe from the oil and gas pipeline market is the key problem that the HFW steel pipe industry urgently needs to solve.In this thesis,based on the analysis of main problem in the HFW steel pipe produted in the early stage,the process specifications to improve and stabilize the quality of HFW pipe have been establish and perfect.The technical method of evaluating HFW welding quality by combining the methods of mechanicl properties statistics and fatigue tests have been discussed.Consequently,the mechanicl properties and fatigue life of HFW pipes was evaluated.The aim is provide comprehensive tested data for control the HFW steel pipe quality and application market expansion,and the_following results are obtained:High speed edge-milling equipment,electrode and its contact device were improved,meanwhile,the welding temperature monitoring,burr monitoring and welding video system were built,some key factors of forming and welding to affect the welding quality of HFW steel pipes were found.The forming technical specification,such as the frame layout,forming angle,extruding degree,sizing rate and so on,were established;and the welding specification of the matching relation of steel pipe size,welding speed and welding power was established.As a result,the stability of electrode power input were improved,and the quality of HFW steel pipe were improved.The effects of post-weld heat treatment processes on microstructure and mechanical properties of serial pipeline steels and weld seams of the HFW steel pipes were studied by means of thermal simulation test machine.For the low carbon microalloy pipeline steels,the complete austenitizing temperature is higher than 900?C.In the range of normalizing temperature from 850 to1000?C,the lower strength of the pipe body and weld seam of the HFW pipes appears as heating temperature is at 925?C and 950?C,respectively,however,the impact toughness of weld seams is improved significantly.As the normalizing temperature continuely rises,the impact toughness of the low-strength steels has a little change,while,the impact toughness of the high-strength steels shows increasing trend.The statistical analysis results of the 550 batches of HFW pipes with different steel grades show that the HFW steel pipes have good quality,and their mechanical properties are far higher than the requirements of relevant technical standards.However,with the increase of steel grade,the difference of mechanical properties between the weld seam and the pipe body increases,the weakening phenomenon appears in the weld seam zone,and the lowest hardness zone for the high strength X70 HFW steel pipe appears in the heat treatment zone of the inner weld.The fatigue test results show that,for the specimens with or without the surface finishing.the fatigue resistance of S-N curves of weld seams of HFW steel pipes are lower than that of the pipe bodies.The fatigue properties of the specimens without surface finishing are lower than that of the specimens with surface finishing.However,the fatigue resistance of S-N curves of weld seams with surface finishing are similar to that of the pipe bodies without surface finishing,the results indicate that the HFW pipes has good welding quality.The decrease of fatigue performance is related to the weld defects,the weakening of performance and the surface quality,because those defects accelerate fatigue crack initiation.The fatigue cracks of weld seams of low-strength HFW steel pipes with surface finishing are mainly generated at the welding defects,while the fatigue cracks of the high strength steel pipes are mainly generated in the softening zone of heat treatment.The fatigue cracks of the weld seams of HFW steel pipes without surface finishing initiate along the longitudinal machining marks of the inner burring,the results indicate that the defects caused by the burring of the welding have a greater effect on the fatigue performance of the HFW steel pipes.The weld fatigue resistance of the weld seem of four HFW steel pipes is about 80%of the pipe body,which can meet the service life requirements. |
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