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Generation and control of porosity and slag inclusion defects in LASW straight seam welded pipe
Jan 30, 2019

1, noun explanation: porosity -- the bubble in the molten pool failed to overflow during solidification, and the residual hole formed in the weld metal.

2. Harmfulness: the porosity of LASW submerged arc welding joints is a voluminous defect, which can reduce the effective cross section of welding seams, reduce the density of joints, and reduce the bearing capacity and tensile strength of joints.Is a serious quality defect.

3. Stomatal sources :(1) rust produces CO stomata and hydrogen stomata.(2) water and air produce (H2O) hydrogen and nitrogen pores.

4. Causes and preventive measures of the porosity of LASW welded pipe:

(1) welding groove not clean, rust, scale, and impurity water, etc.For example, the steel plates before welding of LASW and after milling edge of steel plates are stored for several days, they are affected by damp, the edges of the plates are polluted, and they are not well kept (the steel plates are often trampled).

(2) wet flux and insufficient drying.It is stipulated that the drying temperature of the special flux for submerged arc welding shall be above 300 c and the drying time shall be more than 2 hours. However, in actual production, it is only allowed to dry at 30 c, which is a serious violation of the process regulations.

(3) flux composition and particle size, entered without inspection and testing.Complete screening and magnetic separation, sundries, too many times of flux recovery and repeated use, adding new flux quantity is out of proportion.(site and flux tank are not clean)

(4) height of flux accumulation during submerged arc welding.General in 25-45mm this, the accumulation of thickness is too thin, the emergence of arc.The principle is :(1) the bulk thickness is 45mm when the density is large, and 25mm when the density is small.(2) with high current and low welding speed, the stacking thickness is taken as the maximum value and vice versa.(3) dry in summer or rainy season before use.

(5) the flux viscosity is too large, the particle size is too small, the pool gas is not easy to overflow.Commonly used flux code SJ101, ST301.

(6) welding wire surface cleaning is not enough, the chemical composition is not qualified, or do not match the raw materials.There is no inspection and test when welding materials enter the factory.Dongguang and cangzhou wire quality difference is very big, cangzhou wire flexibility, discount bending, swing too big, pool protection poor.

(7) unstable network voltage and excessive arc voltage.Insufficient dissolved transformer, unstable voltage need to be replaced.

(8) excessive welding speed and low welding current.The rate of welding should be reduced and the current increased to delay the rate of crystallization of the metal in the weld pool so that the gas can escape quickly.

(9) reduce the influence of magnetic blow.The connection position of the welding cable on the workpiece should be as far away from the welding terminal as possible, so as to avoid the magnetic field generated by the welding cable again when working.

(10) pre-welding post.First hand welding the pipe end, then gas shielded welding, can not guarantee the quality, gas protection is not good, gas flow should be increased.

(11) arc.The welding process of the clipper plate is reversed.The welding quality is very poor, the length and width are not standard enough, and the blow-out and blow-out holes are produced.The pool is too hot and absorbs a lot of air.

(12) the staff quality is too low.Excessive extension of welding wire, process to be adjusted at any time.

(13) weld morphology.The molding coefficient of the welding seam is too small, the shape of the welding seam is narrow and deep, the gas and slag inclusion are not easy to surface, easy to form pores and slag inclusion, the general molding coefficient is controlled in 1.3mm to 1.5mm thick wall pipe take the large value, the thin wall pipe take the small value.

5. LASW welding slag inclusion

(1) primer welding root is not complete, failed to make the main slag completely overflow.

(2) improper method of root cleaning in the bottom of the weld leads to a large and deep groove on both sides of the root forming a W shape. In the case of rapid high-temperature welding, slag in the deep groove is not easy to overflow and form slag inclusion.

(3) in 6 kinds of position arc too fast, also easy to produce slag inclusion.

6. Preventive measures:

(1) grinding wheel shall be used for slag removal for pre-welding and backing welding. Roots shall be thoroughly removed and each joint point shall be leveled to form a u-shaped groove to avoid a w-shaped opening.

(2) when the arc extinguishing at 6 o 'clock, fill the welding pool and extinguish the arc on the welding seam.

7. LASW weld quality review

(1) the concept of improving quality first.Only under the premise of ensuring quality can benefit.

2) welding pores and slag inclusion are the problems existing in lambing.The welding parts of the shenzhou vii spacecraft also showed blowhole.The main thing is that we should strictly control the quality and let the welding defects to the lowest position.Stomata, slag inclusion is a comprehensive problem, to comprehensive treatment.

(3) the probability of porosity in straight seam welded pipe of pipeline steel is 0.4-14%. The probability of porosity in Q235B straight seam welded pipe is 12-35%.