Microstructure And Mechanical Properties Of Friction Taper-plug Welds Lap Welding For X65 Pipeline Steel

Friction hydro-pillar processing(FHPP)process developed by The Welding Institute(TWI)is a solid-state joining technology.This advanced technique is mainly employed to repair the defects of the structures and components.FHPP have a lot of advantages such as low cost,no defects relative to melting,and high efficiency.However,FHPP is only used to repair the defects which depth is less than the half thickness of the repaired structures due to the high welding force.In order to repair the through cracks using this solid-state technique,the friction hydro-pillar processing overlap welding(FHPPOW)has been proposed in this paper.X65 pipeline steel was selected as the base and lap plate material.The orthogonal test parameters and different experiment environments were considered.During the welding,the processing parameters were acquired by a data collection system.After welding,the macro morphology,metallographic structure,and mechanical properties have been investigated.The main findings are as follows:In accordance with the applied downward force variation during the welding process,the whole process can be divided into four phases: preparation stage,friction stage,filling stage,and forge stage.FHPPOW performed in the air has a large process window.The welding joints have none defect with 6000 and 7000 rpm rotating speed and 20~40 kN axial forces respectively.Under water,the FHPPOW joints with 6000 rpm rotational speed,20~30 kN downward force and 6500 rpm revolving speed,20 kN axial force have defects on the edge of the hole bottom.The plasticity material deforms difficultly under the water due to the high heat transfer coefficient.Hence,the FHPPOW performs under the water is easier to produce the welding defects than that in the air.The macro morphology of the FHPPOW welding joint is consisted of welding zone,heat affected zone(HAZ),inner flash,outer flash and base material.The microstructure of the base material is fine pearlite and ferrite.The welding zone and HAZ is mainly consisted of upper bainite texture.The hardness of the welding zone is the highest in the whole joint and the value of the hardness in the base metal is lowest.The hardness of the HAZ is between with the welding zone and base material.Compared with the welding joint performed in the air,the hardness value of the joint conducted under the water is higher due to the hardenability.In the pull-out test,the welding joints performed whether in the air or under thewater are fractured in the stud.But in the crossed uniaxial tensile test,the welding joints conducted under water with the defects in macrograph are failure in the fusion line and the joints performed both in the air and under water with defect-free are failure in the welding zone located in the interface of the lap plate and substrate.The tensile test in the different depth of the weld indicated that the welding quality in the hole bottom of the FHPPOW joint performed in the air is good than that of joint conducted under the water.The shear test shows that all the FHPPOW welding joints are failure in the interface between the lap plate and the substrate.Charpy impact absorbed energy of the FHPPOW weld zone at 0°C is lower than that of the base material.The fracture mechanism is mainly cleavage fracture.The fracture surface have a little dimples in the welding joint with the higher Charpy impact absorbed energy.