304 Stainless Steel Pipe Welding

1, process
Construction preparation → Material input, inspection → Cutting → Drainage line, hanger manufacturing / construction → Welding → Welding polishing → Pickling → Welding inspection → Pipeline pressure test, flushing
As shown in Figure 2.
2.1 Construction preparation
(1) Develop a construction plan and construction schedule, and establish high-quality work standards.
(2) Workers are mainly pipeline engineering, argon arc welders, and other works, and argon arc welders require certificates from related departments.
(3) Construction materials are prepared according to the material plan, delivered to the site, and guaranteed to be supplied as planned.
(4) Field workers prepare technical tests, field technicians, and safety tests.

2.2 Preparation of welding material
(1) Piping and fitting selection is based on environmental factors, chemical composition and working pressure, and the appropriate product grade is selected to ensure the metal structure and mechanical properties of the weld.
(2) Argon gas should meet the requirements of the national standard “Argon” GB4842. Use argon gas with a purity of 99.96%. Too much impurities can compromise the protection of argon and directly affect weld quality.
(3) Use tantalum tungsten electrode material.
The shape and diameter of the tantalum tungsten tip have a great influence on the stability of the welding process and the formation of the weld. The conical flat end works best (see Figure 1.1). Tungsten electrodes are selected according to the thickness of the welded pipe and the welding current.
(4) The welding gun uses a PW-150 type air gun, and the nozzle opening size directly affects the protective effect, but the nozzle opening is generally Φ10 mm.
3, welding method and technical parameters

The socket type argon arc welding method integrates the pipes and is a “jointless connection” connection.
3.1 Specific construction steps
3.1.1 Once a stainless steel pipe enters the field, it must not come into direct contact with other materials such as cement slurry, cement, mortar, concrete and welded steel pipe. Cutting, cutting pipe and cross-section according to site and drawing: Make sure there is no damage or deformation before cutting the pipe. The pipe shaft is cut vertically using a pipe cutter (a special cutting device for stainless steel). If the slit is tilted, the insertion amount will be inaccurate. After removing, remove the burr end, debris, and foreign matter from the tube end.
3.1.2 Stainless steel pipe has good cut, insert pipe socket, give opinion 1.2, socket socket size is shown in Table 1.2

3.1.3 When welding, the socket and pipe should be on the same axis and should be horizontal and not angled. Brush the chalk powder with a width of 40 to 50 mm on the outside of the end of the socket, and start welding after drying. For the first spot weld, 3-5 points are suitable.
3.1.4 Welding work is centered on the waist, the upper body is powered and the upper body rotates from right to left. This weld makes the weld pool easier to observe and better protects the weld pool. After the normal combustion of the arc forms a melting pool, the arc moves horizontally forward. The socket end of the ring is used to control the size of the bath and the temperature of the bath. The length of the tungsten electrode is 4 to 8 mm, the distance between the tungsten electrode and the workpiece is 1 to 1.5 mm, and the distance from the nozzle to the seam is 5 to 10 mm. If welding is stopped or completed during welding, the arc extinguishing method is used as the arc extinguishing method. That is, when the arc is discharged, the torch gently moves to the outside of the groove, the arc disappears, the torch closes immediately to the arc stop position, and air is supplied for 3 to 5 seconds. The weld pool is cooled under delayed gas protection to prevent shrinkage and cracking. After welding, it is cooled in the natural environment.

3.1.5 When welding, observe the color of the weld. If the weld colors are gray and black, it means that the argon protection is inadequate. The argon flow rate should be adjusted until the weld color is golden yellow, blue or red.
3.1.6 Current is adjusted to the right during the welding process. The specific technical parameters are shown in Table 1.1. Not only is the welding current too high, it is easy to bake, but it is also prone to welding, tungsten and burning. If the welding current is too small, the arc will become unstable and defects such as incomplete penetration, slag and pores will be likely to occur. The welding speed is selected according to the welding current (see Table 1.1). If the speed is too high, the gas protection effect will be impaired, and the weld metal and tungsten are also susceptible to oxidation defects. If it is too late, welding will be easier.
3.1.7 During the welding process, the principle of small current and robustness should be followed. The corresponding current requirements apply to pipes of different wall thicknesses. Relying on airflow protection, rapid temperature rises and temperatures are designed to avoid post-chemical temperatures of 600 ° C to 800 ° C to avoid intergranular corrosion.
3.1.8 In the absence of short circuit, short arc welding, gas protection effect, heat concentration, arc stability, uniform penetration, deformation;
If the arc voltage is too high, the gas protection effect becomes insufficient, which may cause oxidation of the weld metal and cause problems such as incomplete penetration.
3.1.9 The wind speed during welding must not exceed 2 m / s. Beyond that, the windshield will occupy the building area.
3.1.10 When welding in a damp place, the relative humidity within 1 m of the welding arc should not exceed 90%. If it is greater than 90%, welding should be stopped.

4, welding polishing, passivation, cleaning
4.1 After welding, use a grinding machine to wear the pipe and weld the excess weld.
4.2 After soldering is complete and cooled to natural temperature, passivation paste is used for passivation at the soldering position. Stainless steel pickling passivation paste is a chemical reagent that removes yellow, blue and black solder joints and scales from soldering and stainless steel after high temperature treatment. Suitable for ferrite, austenite and other stainless steels. The stainless steel is completely pickled, passivated to improve corrosion resistance and the stainless steel surface is new and bright.
Pickling paste Paste features: 1. The reaction rate is fast, it is completely removed, the viscosity is medium, it is easy to observe, and there is no corrosion on the substrate.
2. CL, P, etc. for destroying the surface of stainless steel are not included.
(1) Use the completed stainless steel passivation paste for construction and stir the passivation paste sufficiently.
(2) Apply the passivation paste evenly to the surface of the pipe weld. The thickness of the paste layer is about 2 to 3 mm. Typical time is 5 to 15 minutes. If the scale is thick or the temperature is below 0 ° C, the reaction time should be extended.
(3) After pickling, wash with water (if the scale remains after washing with water, remove the scale with a stainless wire brush or a hard plastic nylon brush), and test with Lithomas paper. If there is no response, wipe off any water stains for passivation.

Welding inspection
1. The welder must self-diagnose the surface quality of all welds 100%.
2. No defects such as cracks, pores, unmelted or oversized undercuts are found on the welded surface.
3. The external dimensions of the weld must meet the design requirements and the edges of the weld must be rounded on the substrate.
4. The weld must not be significantly oxidized or burned (meaning that the front or back of the weld is black, slag, etc.).
5. To judge the welding effect, silver and gold are best shown in Figure 1.4, blue is good, red is good, gray is not good, black is the worst.

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