| The NG-GTAW process,combines the characteristics of narrow-gap welding and gas tungsten arc welding,has a series of advantages like high welding efficiency and stable welding process,and has important application prospects in petrochemical,nuclear power and other fields.However,the regular“Bell-shape" heat source distribution of GTAW makes the sidewalls poorly fused during the narrow gap welding process.On the basis of previous research,a new type of non-axisymmetric rotating-arc NG-GTAW equipment was designed.The motor is placed inside the welding torch,and which turns to drive the tungsten to rotate.The tungsten electrode is ground with its tip deviated from the axis and located on the column wall.Once the arc is ignited during the welding process,according to the principle of minimum voltage,the arc always selects the shortest path from the tungsten tip to the base metal to burn,and in this way,the best path is established for current conduction,the arc burning is controlled and maintained periodically from the left sidewall,bottom of the weld pool,and the right sidewall.The rotating arc uniformly heats the narrow gap sidewalls,the key problem of incomplete fusion on sidewalls in narrow-gap welding is solved by this technology.The new welding equipment mainly integrates PLC electric control system,motion execution system,water circuit and electric circuit and gas circuit system,automatic welding tracking and control system,and non-axisymmetric welding torch,each component completes the welding action under the instruction of the electric control system.Welding equipment adopts dual gas protection mode.The main shaft shielding gas is used to exhaust the air in the welding area and protect the high-temperature molten pool.The rear support cover shielding gas is used to protect the formed high-temperature welds from oxidation.The left and right adjustment of the welding adopts the line structure light,and tungsten electrode height adjustment adopts arc voltage tracking technology for real-time control.After debugging,the motion control accuracy of this equipment can fully meet the needs of narrow gap welding.Visual inspection of the physical process of rotating arc NG-GTAW was carried out,and the dynamic behavior of droplets under different arc rotation frequencies was studied.When the arc rotation frequency was lower,the droplets transferred to the molten pool in a contact transition mode.As the arc rotation frequency increased,the contact transition gradually transforms into a free transition,and the droplets transition frequency was basically the same as the tungsten rotation frequency.The temperature field and residual stress field distribution of the rotating arc NG-GTAW process were studied,and it was found that during the filling welding process,the highest temperature near the weld can reached 600℃,at this time,the austenitic stainless steel is in the sensitization temperature range,staying at this temperature for a long time may cause the corrosion resistance of stainless steel to decrease.In the direction perpendicular to the weld,the area near the weld appeared tensile stress,and the stress value was slightly higher than the yield strength of the material.As the distance from the weld increased,the stress value gradually decreased.At a certain location,the tensile stress transformed into a compressive stress.In the direction parallel to the weld,the stress value was higher in the middle area of the plate,and which was lower at both ends.Narrow gap welding experiments were carried out on 321 stainless steel with this equipment.According to the actual welding requirements of 321 stainless steel,the heat input was strictly controlled below 10 kJ/cm,and welded joint with good sidewalls fusion and no obvious defects was obtained.Under the strong stirring action of the rotating arc,the weld microstructure was uniform and fine equiaxed crystals,and a small amount of columnar crystals were distributed in the base material and the heat-affected zone.XRD and EBSD tests were carried out on the weld zone,the base material zone and the heat-affected zone,it was found that the phase distribution and phase content of the three regions have a certain change,but the phase composition was all austenite and ferrite.The mechanical properties of the obtained joint were tested,and the joint showed good tensile properties and low-temperature toughness,which far meet the requirements of industrial production.For the above three areas,the polarization method was used to test the pitting corrosion resistance,and the DL-EPR method was used to test the intergranular corrosion resistance.The results showed that the corrosion resistance of the weld zone was better than of the base material and heat-affected zone,and the corrosion resistance of the three areas is not much different,which shown that the process and welding system can be applied to the welding technology needs of nuclear power,pressure vessel. |
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