How can multi layer multi pass welding avoid defects such as porosity and slag inclusion?

1.The welding materials should be dried as required.

2.The workpiece is cleaned as required.

3.The level of the welder must be identified before the corresponding workpiece can be welded.

Porosity refers to the cavities formed by the gas in the molten pool that remains in the weld before the metal solidifies during welding. The gas may be absorbed by the molten pool from the outside or reacted during the welding metallurgical process.

1) Classification of stomata Stomata are divided into spherical stomata and striped worm-shaped stomata according to their shapes; In terms of quantity, it can be divided into individual stomata and group stomata. The group stomata are divided into evenly distributed stomata, dense stomata and chain-like distributed stomata. According to the classification of gas composition in the pores, there are hydrogen pores, nitrogen pores, carbon dioxide pores, carbon monoxide pores, oxygen pores, etc.

The pores of fusion welding are mostly hydrogen pores and carbon monoxide pores.

2) The formation mechanism of pores: The solubility of gas in solid metal at room temperature is only one-tenth to one-hundredth of the solubility of gas in high-temperature liquid metal, and a large amount of gas has to escape from the metal during the solidification process of molten pool metal. When the solidification rate is greater than the gas escape rate, a stomata is formed.

3) The main reason for the occurrence of pores is rust and oil stains on the surface of the base metal or filler metal, and the amount of pores will be increased if the electrode and flux are not dried, because the rust, oil stain and the moisture in the electrode coating and flux decompose into gas at high temperature, which increases the content of gas in the high-temperature metal. The energy of the welding line is too small, and the cooling rate of the molten pool is large, which is not conducive to gas escape. Insufficient deoxidation of the weld metal can also increase oxygen holes.

4) The hazard of porosityThe porosity reduces the effective cross-sectional area of the weld, makes the weld loose, thereby reducing the strength of the joint, reducing the plasticity, and causing leakage. Porosity is also a factor that causes stress concentrations. Hydrogen porosity can also contribute to cold cracking.

5) Measures to prevent stomata aRemove oil, rust, moisture and debris from the welding wire, working bevel and its nearby surfaces. b.

Alkaline electrodes, fluxes are used, and dried thoroughly. c.DC reverse connection is used and short arc welding is used.

d.Preheat before welding to slow down the cooling rate. e.

Weld with a strong specification. Slag inclusion refers to the phenomenon that the residue of slag in the weld after welding.

1）.Classification of slag inclusionsaMetal slag inclusions:

It refers to the residue of tungsten, copper and other metal particles in the weld, which is customarily called tungsten and copper. b.Non-metallic slag inclusions:

Refers to the unmelted electrode coating or flux, sulfide, oxide, nitride residue in the weld. The metallurgical reaction is incomplete, and the slag removal is not good.

2) The distribution and shape of slag inclusion include single point slag inclusion, strip slag inclusion, chain slag inclusion and dense slag inclusion.

3) Causes of slag inclusionThe size of the groove is unreasonable; b.There is dirt at the bevel; c.

In the case of multi-layer welding, the interlayer slag removal is not thorough; d.The energy of the welding line is small; e.The weld dissipates heat too quickly, and the liquid metal solidifies too quickly; f.

The chemical composition of the electrode coating and flux is unreasonable, and the melting point is too high; g.When tungsten electrode inert gas shielded welding, the power supply polarity is improper, the electricity and current density are large, and the tungsten electrode melts and falls off in the molten pool. h.

When manual welding, the welding rod swings poorly, which is not conducive to the floating of slag. Corresponding measures can be taken according to the above reasons to prevent the generation of slag inclusions.

4) The hazards of slag inclusion are similar to those of porosity, and the slag inclusion with sharp corners will produce tip stress concentration, and the tip will also develop into a crack source, which is more harmful.