Through the comparative process test and evaluation of CO2 gas shielded welding, argon-rich gas shielded welding and electrode arc welding, it is decided to use CO2 gas shielded welding except for the argon-rich gas shielded welding of individual welds with appearance requirements. The production practice proves that this not only ensures the welding quality, but also improves the labor productivity, reduces the cost, and achieves better economic benefits.
1 Welding joint condition and weld technical requirements
1) Welded joints are available in butt joints, angle joints, T-joints and lap joints, most of which are T-joints.
2) There are butt welds and fillet welds in the form of welds, most of which are fillet welds. Due to different plate thickness, the weld feet are 6mm, 8mm, 10mm, 12mm and 15mm respectively.
3) The base metal is mainly carbon structural steel plate Q 2352A, and the specifications are 6 mm, 8 mm, 10 mm, 12 mm, 20 mm, 25 mm and so on.
4) The appearance of the weld requires that the surface of the weld and the heat affected zone shall be free from defects such as cracks, pores, slag inclusions, and fusion. The weld shape and size meet the requirements of the drawing, and the weld and the base material are smoothly transitioned. Part of the weld requires ultrasonic flaw detection.
2 Welding test
Refer to JB4708-2000 "Pressure Vessel Welding Process Evaluation" for CO2 gas shielded welding, argon-rich gas shielded welding, electrode rod arc welding butt joint mechanical properties test, T-joint fillet weld test and CO2 gas shielded weld, argon-rich gas shield Welding spatter forming process performance, comparative analysis.
2.1 Mechanical properties test of butt joints
1) Test material Q235-A, 300 mm × 125 mm × 10 mm, 2 pieces; welding rod arc welding 60V groove, CO2 gas shielded welding, argon-rich gas shielded welding 45V groove, single-sided welding double-sided forming .
2) Welding method and welding material electrode arc welding, E4303, φ3.2 mm, φ4 mm; CO2 gas shielded welding, argon-rich gas shielded welding wire ER50-6, 5 φL2; argon-rich gas: 80%A r+ 20%CO2 .
3) Visual inspection of the inspection contents, RT test, mechanical properties test (tension test, bending test).
2.2 T-joint fillet weld test
1) Material Q235-A, 300 mm × 125 mm × 10 mm, 2 pieces, no beveling, single pass welding.
2) Welding method and welding material electrode arc welding, welding rod E4303, φ3.2 mm; CO2 gas shielded welding, argon-rich gas shielded welding, welding wire ER 50-6, φL2 mm; argon-rich gas: 80% Ar + 20% CO2.
3) Visual inspection of the inspection contents, and cut 5 sections for metallographic macro inspection. It is required that the section has no cracks, no incomplete penetration, and no unfused defects.
2. 3 T-joint fillet weld forming, splash test test conditions are the same as 2. 2, through the comparative test for CO2 gas shielded welding, argon-rich gas shielded welding appearance appearance and splash size evaluation.
3 Analysis of welding test results
1) From the test of the mechanical properties of butt joint welds, the visual inspection of the welded joints of the three welding methods meets the requirements, the RT test is higher than the Ê grade, the tensile strength of the welded joint is the highest with argon-rich gas, and the CO2 gas shield The welding is the second, the electrode arc welding is the lowest, because the argon-rich gas shielded welding is less oxidizing and the alloying elements are less burnt, but they are higher than the minimum specified by the parent metal. According to the specified bending angle, each test piece has 2 bends and back bends, and the bending test is qualified. This shows that the three joint welding methods and the welding process of the welded joints have passed the mechanical test. However, the argon-rich gas shielded welding and CO2 gas shielded welding have smaller angles, larger blunt edges, higher productivity than electrode arc welding, material saving, low cost and less welding distortion. This is because the gas shielded welding wire is thin, the current density is large, the penetration depth is large, the arc penetration force is strong, and the welding is easy.
2) From the T-joint fillet weld test, the penetration depths of the three welding methods are as follows: argon-rich gas shielded weld depth is slightly larger than CO2 gas shielded weld, larger than electrode arc welding, 5 sections of each test piece No cracks, unfused, or insufficient penetration defects occurred in the roots, and the macroscopic metallographic examination passed.
3) From the spline weld of the T-joint, the forming test shows that the argon-rich gas shielded welding has less splash, and the maximum splash particle diameter is φ1. 5 mm～φ2 mm, CO2 gas shielded welding is slightly larger, the largest splash particle The diameter is from φ3 mm to φ4 mm; the surface of the argon-rich gas-welded weld is finer than that of CO2, and the shape is beautiful.
In summary: The three welding methods and welding processes can meet the mechanical performance requirements and macroscopic metallographic requirements. However, CO2 gas shielded welding, argon-rich gas shielded welding, fine wire, high current density, concentrated heat, strong arc penetration, large penetration depth, can reduce the groove angle, increase the thickness of the blunt edge, save materials, improve labor Rate, reduce welding stress and deformation. The argon-rich gas shielded welding is more beautiful than the CO2 gas shielded welding, and the splash is small, but the cost is high. Therefore, in addition to argon-rich gas-shielded welding for welds with high requirements on the appearance of extremely small numbers, the rest are CO2 gas shielded.
4 welding process
4. 1 Preparation before welding
1) Remove the dirt, rust and other dirt in the area to be welded and on the sides of 10 to 20 mm, and apply a layer of splash anti-blocking agent on the surface of the weldment, and apply a nozzle anti-blocking agent on the nozzle.
2) Invert the CO2 cylinder for 1 to 2 hours to allow the water to sink. Discharge the water once every 0.5 hours and put it 2 to 3 times.
3) Prepare the welding process according to the welding process test. Welding wire ER 5026, φ1. 0 mm, φL 2 mm, welder KRII350.
4) Use the left welding method.
4. 2 welding operation process
4. 2. 1 butt weld operation process
1) Due to the large penetration depth of CO2 gas shielded welding, double-sided single-pass welding can be used for the profile groove (without bevel) when the plate thickness is less than 12mm. For the butt joint of the groove opening, if the groove is narrow, multiple layers of single pass welding can be used; if the groove is wider, multi-layer multi-pass welding can be used.
2) During the welding process, when the welding gun is oscillated laterally, it is necessary to ensure a certain penetration of the bevel on both sides, so that the bead is flat and has a certain concave, avoiding the middle protrusion, which will cause the weld to be between the sides and the groove surface. The angle is formed, and defects such as incomplete penetration and slag inclusion are generated.
3) To control the thickness of each layer of weld bead, make the weld bead of the front bead of the cover bead 1.5 to 2.5 mm lower than the base metal, and must not melt the edges on both sides of the groove, so that the groove can be seen when covering the surface , create good conditions for the cover.
4) When welding the cover, the surface of the previous layer should be flattened before welding. The amplitude of the welding gun is larger than that of the filling layer. The amplitude should be the same when swinging, and the speed should be uniform. Pay special attention to the groove. In the case of side melting, ensure that the edge of the molten pool exceeds the edges on both sides of the groove and is not more than 2 mm to avoid undercut.
5) If multiple welds are used for each layer, the wire shall point to the angle bisector of the weld bead and the groove, weld bead and weld bead, and the weld bead overlaps each other not less than 1 to 3 of the width of the weld bead.
4. 2. 2 fillet weld operation process
1) When the fillet weld is welded, it is prone to defects such as undercut, incomplete penetration, and sagging of the weld, so the angle of the wire should be controlled. When the thick plate is welded, the angle between the welding wire and the horizontal plate is 40° to 50°. When the slab is not equal, the inclination of the wire should be such that the arc is biased toward the thick plate. The thicker the plate, the larger the angle between the wire and the wire.
2) For fillet welds with a solder fillet of 6 to 8 mm, use a single layer of single pass welding with the torch pointing (wire) 1 to 2 mm from the root. For welds with a weld foot of 6 mm, use a linear movement method. For welds with a weld foot of 8 mm, the torch should be oscillated laterally and welded by a slanted circle.
3) For fillet welds with a solder fillet of 10 to 12 mm, due to the large solder fillet, multi-layer soldering and soldering are required. When welding, the first layer operation is the same as that of single layer welding. The angle between the welding gun and the vertical plate is reduced and pointed at 2 to 3 mm from the root. At this time, the current is slightly larger than usual, in order to obtain the bead of the unequal soldering foot. When soldering the second layer, the current is slightly less than that of the first layer, and the torch should point to the recess of the first pass of the bead until the desired solder fillet is reached.
4) For fillet welds with a fillet of 15 mm, multi-layer multi-pass welding shall be used, ie welding 3 layers. It should be noted that: When operating, the size of the solder fillet should be controlled to about 6 ~ 7 mm. Otherwise, the solder fillet is too large, so that the deposited metal will sag, and the solder joint will be generated on the horizontal board. Undercut. The angle and orientation of the torch should ensure that the equal foot and smooth and even weld are obtained.
5 Problems to be aware of in the welding process
In production, we found that many people, not only welders, inspectors, and even welding technicians, confuse the relationship between the solder fillet and the size of the solder fillet and the thickness of the weld. The welder mistakes the weld foot for the size of the weld foot. The inspector measures the weld thickness as the weld foot, so that the actual weld foot exceeds the size required by the design, and records it as the weld foot size in the quality record. Other technicians require the size of the solder fillet in the welding process file, etc. These are all wrong.
In fact, the solder fillet refers to the minimum distance from the weld toe on one right angle surface to the other right angle surface in the cross section of the fillet weld. The fillet size is the maximum isosceles drawn in the cross section of the fillet weld. The length of the right-angled side of a right-angled triangle, and the thickness of the weld is the distance from the front of the weld to the back of the weld in the weld cross-section. Therefore, the required fillet weld dimensions on the process documentation and weld symbol are the weld feet, not the fillet dimensions, and not the weld thickness.
6 Conclusion
1) After testing and production practice, the mechanical properties and macroscopic metallographic examination of CO2 gas shielded welded joints meet the requirements, and the CO2 gas shielded welding has smaller angles than the electrode arc welding groove, and the blunt edge is larger, the welding heat influence The narrower area saves materials and energy, improves labor productivity, and improves welding quality. It should be promoted and used.
2) Compared with CO2 gas shielded welding, the argon-rich gas mixture welding is finer, the weld bead is smooth, the appearance is beautiful, the splash is small, the penetration depth is large, but the cost is relatively high, so it is suitable for welding with high weld appearance requirements. Seam. The argon-rich gas shielded welding operation process is similar to the CO2 gas shielded welding operation process.
3) Distinguish the relationship between the solder fillet, the size of the solder fillet and the thickness of the weld, and pay attention to the solder fillet, not the fillet size and weld thickness, as required on the process documentation and in the weld symbol.
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