Backside oxidation during welding of stainless steel has been a difficult problem to solve in the welding process for many years. Under normal circumstances, the back side is filled with argon for protection, but when the container is large, the pipeline is long, or there is no gas storage space on the back side, a lot of argon gas will be wasted, and the protection effect is not good, which directly affects the welding quality. In order to avoid oxidation on the back of the weld bead and ensure the welding quality, a new process of self-protected stainless steel flux-cored welding wire on the back is adopted. During welding, the protective coating will penetrate into the back of the molten pool to form a dense protective layer. This paper takes the paraformaldehyde project as an example to illustrate the characteristics of flux-cored welding wire.
In the chemical industry, a large number of stainless steel pipes are welded every year. According to the requirements of "Steel Structure Manufacturing Specification", the process of argon arc welding single-sided welding and double-sided forming is adopted. At present, the two most commonly used processes in the construction process are:
(1) TIG welding with solid wire;
(2) TIG welding with flux-cored wire.
The first method is most used in the construction process, while the second method is the most advanced welding method at present, but it has not been popularized in the construction process. A comparison of the two methods can be found in Table 1.
|
Welding method Project |
Back welding quality | Difficulty of construction | Technical requirements for welders | Comprehensive construction cost | Construction efficiency | Inspection pass rate | Welding manufacturability | Weld Mechanical Properties | Suitable weld |
| Solid wire | Good | Difficult | Higher | High | Lower | Easy | Good | Good | Rotating port |
| Flux cored Welding wire | Good | Easy | High | Higher | High | Easier | Good | Good | Rotating fixing port |
Table 1 Comparison of backing welding process of solid wire and flux-cored wire
It can be seen from Table 1 that flux-cored wire welding has the characteristics of high efficiency and low cost, and is suitable for popularization and use.
Characteristics of stainless steel flux-cored welding wire
Stainless steel flux-cored wire highlights the beneficial properties of many welding methods, such as the role of the flux part and the coating of the electrode to improve the chemical composition and mechanical properties of the filler metal. In terms of production efficiency, it has the characteristics of gas shielded metal arc welding and submerged arc welding.
Compared with solid wire, it has the following advantages:
(1) It has good welding process performance, and the welding seam is beautifully formed. The gas-slag joint protection is used to obtain good forming. Add arc stabilizer to stabilize the arc and make the droplet transfer evenly;
(2) It has good chemical metallurgical properties, high mechanical properties of deposited metal, and strong anti-porosity and crack resistance;
(3) All-position welding can be performed with larger welding current;
(4) High production efficiency and easy operation;
(5) Wide range of uses, not only for connection, but also for surface protection, as well as for filling materials for various metal rapid prototyping.
Technological properties of stainless steel flux-cored welding wire
Stainless steel flux cored wire is a special coating wire. During welding, the protective coating will penetrate into the back of the molten pool to form a dense protective layer, so that the back of the weld bead will not be oxidized. . The use of this wire is basically the same as that of ordinary argon arc welding solid wire, the coating will not affect the shape of the front arc and molten pool, and the weld metal can meet the requirements in terms of performance.
The use of flux-cored welding wire is not limited by factors such as pipe specifications, welding positions, etc., the operation is flexible, and the preparation work for argon filling is reduced. However, due to the thin coating on the surface of the welding wire, there are some incompatibility in the welding operation, and defects such as concave often appear, so the operation level of the welder is relatively high. The self-shielded welding wire is suitable for the bottoming of the weld, and should not be used for the weld bead above the second layer, otherwise it is easy to cause slag inclusion, and the weld formation is not beautiful. When using full argon arc multi-layer multi-pass welding, it should be used in conjunction with solid wire.
Precautions for using flux cored wire
1. The DC power supply with flat characteristics is used, and it is directly reversed (DC+).
2. The protective gas flow rate is generally 10~11L/Min.
3. The dry elongation of the welding wire should not be too short or too long, otherwise defects such as pits, air grooves, and arc instability will easily occur.
4. There must be practical and effective wind blocking measures. When the wind speed exceeds 2m/s, the protective effect of the gas will be destroyed, resulting in the increase of nitrogen in the weld metal, which will lead to the generation of pores and hot cracks.
5. The groove and its surrounding water, oil, rust and other impurities must be cleaned up.
Defects, causes and preventive measures of flux-cored welding wire
2.1, stomata
1) Causes: The voltage is too high; the protruding length of the welding wire is too short; the surface of the pipe has rust, paint, moisture, etc.; the migration speed is too fast.
2) Prevention and control measures: reduce the voltage; perform welding according to the instructions of the welding wire; clean the groove before welding; the moving speed should be appropriate.
2.2. Slag inclusion
1) Causes: The arc voltage is too low; the arc swing of the welding wire is improper; the welding wire is too deep; the current is too low, the welding speed is too slow; the first welding slag is not fully removed; the first welding is poor;
Bevel too narrow; weld sloping down.
2)Precautionary measures: the arc voltage should be adjusted properly; the welder should be skilled in operation, and the welding should be carried out according to the instructions of the welding wire; the first welding must be cleaned up; The gap between the openings must be appropriate; the pipes must be leveled when assembling, and the moving speed can be appropriately accelerated when welding the fixed openings.
2.3, undercut
1) Causes: The current is too strong; the arc is too long, the welding speed is too fast; the operation method is improper.
2) Preventive measures: use lower current; reduce arc length and speed; use correct angle, slower speed, shorter arc and narrower running method, etc.
2.4. Incomplete penetration
1) Causes: current is too low; welding speed is too slow; voltage is too high; improper arc swing; improper groove angle.
2) Preventive measures: appropriately increase the current; appropriately increase the welding speed; reduce the voltage; the welder can use the stainless steel flux-cored wire skillfully; try to make the angle larger when grinding the groove
2.5. Splash
1) Causes: The arc is too long; the current is too high or too low; the arc voltage is too high or too low; the welding wire protrudes too long; the welding torch is too inclined and the drag angle is too large;
2) Preventive measures: use a short arc; it is possible to use an appropriate current; the arc voltage should be adjusted properly; strictly follow the instructions for use of the welding wire; keep the welding torch as vertical as possible and avoid excessive inclination; pay attention to the storage conditions of the welding wire; the welding machine Pay attention to regular maintenance, and repairs must be made if there is a problem.
Welding points and technology of stainless steel flux cored wire
3.1. Operation points
The operation method of flux-cored wire welding is similar to that of solid-core welding wire, but there are some differences. Special attention should be paid to the actual operation, otherwise it will affect the welding quality and cause defects such as slag inclusion.
(1) Arc length requirements: the ideal state of arc length is that the shorter the better;
(2) Formation of dissolved holes: TIG welding with stainless steel flux-cored wire is used. In order to obtain a good welding bead on the back of the root, the molten hole must be seen during bottoming welding, which will allow sufficient slag to penetrate into the back bead. Allows complete protection of the back bead surface. If there is no molten hole formation, the molten slag cannot penetrate into the backside weld bead for protection, which will cause the backside weld bead to oxidize and not get a high-quality weld bead appearance. On the contrary, if the melting hole is too large, the back weld bead becomes too wide, the weld bead metal is too thick, and the weld temperature is too high, the weld bead is easily oxidized, which reduces the welding quality and also reduces the welding efficiency. Therefore, in order to obtain a good backside weld bead, the size of the fusion hole should be accurately controlled.
(3) Key points of wire feeding operation: The key points of stainless steel flux-cored welding wire feeding are small amount and fast-paced feeding, and the distance between the wire head movement should be short. When filling and feeding quickly, special attention should be paid to whether the stainless steel flux-cored welding wire is fully melted. Generally, it should be kept The molten pool is clear, and the molten pool is fully stirred by the swing of the tungsten electrode to make the flux completely melt and float out, and make the welding seam fused well, otherwise, slag inclusions will remain and defects will be formed. Experience has shown that there are slag inclusion defects in the weld bead, which can be judged from the shape of the inner weld bead. If it is found that the back weld bead has welding thorns, generally there are slag inclusion defects in the weld, and the defects can be removed by grinding immediately.
(4) Key points of joint operation: when starting the arc joint welding after stopping the arc, it is best to back up about 10mm to start the arc and connect by lap joint; if the temperature of the welding bead is still high, it is not necessary to knock out the welding slag of the previous welding pass. arc welding. If the temperature of the previous weld bead has cooled, the slag in the groove must be knocked off, but the slag of the back pass must not be knocked off, because the slag of the back pass can prevent the back pass from oxidizing when the arc is re-started.
3.2 Selection of base metal, welding wire and electrode
1) Base metal: The base metal is a stainless steel tube, the material is 0Cr18Ni9, the specification is φ219×6.0mm, and the implementation standard is GB/T 14976-2002. Its chemical composition is shown in Table 2 below.
Table 2. Chemical composition of 0Cr18Ni9
| C | Si | Mn | Cr | Ni | S | P |
| ≤0.07 | ≤1.0 | ≤2.0 | 17.0～19.0 | 8.0～11.0 | ≤0.03 | ≤0.035 |
2) Welding wire: The welding wire,The grade is TGF-308L, the specification is φ2.5×1000mm, and the executive standard is GB/T 17853. Its chemical composition is shown in Table 3.
Table 3. Chemical composition of stainless steel flux-cored wire TGF-308L
| C | Si | Mn | Cr | Ni | S | P | Mo |
| ≤0.08 | ≤0.65 | 1.0～2.5 | 19.5-22.0 | 9.0～11.0 | ≤0.03 | ≤0.03 | ≤0.75 |
3) Welding rod: choose the welding rod from Beijing Jinwei, the grade is A102, the specification is φ3.2mm, and its chemical composition is shown in Table 4.
Table 4. Chemical composition of stainless steel electrode A102
| C | Si | Mn | Cr | Ni | S | P | Mo | Cu |
| ≤0.08 | ≤0.90 | 0.5～2.5 | 18.0-21.0 | 9.0～11.0 | ≤0.03 | ≤0.04 | ≤0.75 | ≤0.75 |
3.3 Selection of welding parameters
| Welding layers | Power characteristics | Diameters(mm) | Current (A) | Arc voltage (V) | Argon flow (L/min) |
| 1 | Positive | 2.5 | 110-140 | 11~12 | 10~11 |
| 2 | Negetive | 3.2 | 80-90 | 22~24 |
3.4 Preparation before welding
1 Groove form and requirements
The groove type cannot be ignored, and the groove parameters of argon arc welding have strict requirements. The groove type is shown in Figure 1
2. Baking of the electrode: the baking temperature of the stainless steel electrode A102 is 150℃, the baking temperature is 30min, the heat preservation temperature is 100℃, and the heat preservation time is 60min.
3. Cleaning on both sides of the groove: Use mechanical methods to clean up the oil, embroidery, dirt and other debris that affect the welding quality within 20mm of the groove and its inner and outer walls.
4. Pipeline alignment: When piping is aligned, the bevel angle, gap, misalignment, edge angle, etc. should meet the requirements of the corresponding specifications. The inside of the pipe is clean and free of debris, the inner wall is flush, and the maximum amount of misalignment does not exceed 10% of the wall thickness, and is not more than 2mm.
5. Spot fixing: Spot fixing should be carried out symmetrically, using manual tungsten argon arc welding, with a length of 20 (mm). After spot fixing, carefully check the quality of spot fixing. If there are defects such as surface cracks, pores, and incomplete penetration, must be completely removed.
6. Environmental protection: Welding is prohibited in any of the following situations:
a. Relative humidity＞80%
b. Wind speed during manual arc welding: ≥8m/s.
c. ≥2m/s during argon arc welding.
d. Temperature: the temperature of the weldment is lower than 5℃.
e. Weather: Outdoor operations without snow and rain protection measures (when there is rain or snow). If the on-site environment cannot meet the above requirements, measures such as setting up a protective shed, using oven heating to increase the ambient temperature and reducing the relative humidity of the air should be taken. Welding can be carried out after the construction, and a temperature and humidity meter is placed in the protective shed to facilitate the observation of ambient temperature and humidity.
3.5. On-site welding
According to the selected materials and welding parameters, make full preparations before welding, and select 5 welders on site to carry out welding for one day at the same time, with a total of 30 welding joints with a welding specification of φ219×6.0mm. Randomly inspect and take pictures of each welding joint on site, some of which are shown in the following figure:
3.6. Post-weld inspection
1. Visual inspection: Defects that can be found by visual inspection are: surface pores, slag inclusions, weld penetration, undercuts, cracks and similar surface defects and the surface color of welded joints. Check the available magnifying glass within 10 times. Before the inspection, the slag should be removed and cleaned within a width of 20mm on both sides of the weld, and then the inspection should be carried out along both sides of the weld.
2. Radiographic inspection: The radiographic inspection of the weld shall comply with the current industry standard "Non-destructive Testing of Pressure Equipment Part 2 Radiographic Inspection" JB/T 4730.2.
