Hottest welding defects and preventive measures

2022-08-12
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Welding defects and preventive measures

1. Appearance defects: appearance defects (surface defects) refer to defects that can be found on the surface of the workpiece without the help of instruments. Common appearance defects include undercut, weld beading, depression, welding deformation, and sometimes surface pores and surface cracks. The root of single-sided welding is not fully penetrated, etc

a. Undercut refers to the depression or groove formed in the base metal part along the weld toe. It is the gap left by the insufficient supplement of deposited metal after the arc can change the size of the base metal melting range at the weld edge. The main reason for undercut is that the arc heat is too high, that is, the current is too large and the strip conveying speed is too small. Incorrect angle between welding rod and workpiece, unreasonable swing, too long arc, unreasonable welding sequence, etc. will cause undercut. Magnetic bias blow of arc during DC welding is also a cause of undercut. Some welding positions (vertical, horizontal and overhead) will aggravate undercut

undercut reduces the effective cross-sectional area of the base metal. 5. Effective width: 150mm, which reduces the bearing capacity of the structure. At the same time, it will also cause stress concentration and develop into a crack source

correct the operation posture, select reasonable specifications, and adopt good strip transportation methods will be conducive to the elimination of undercut. When welding fillet welds, using AC welding instead of DC welding can also effectively prevent undercut

b, the liquid metal in the weld bead flows to the base metal that is not melted due to insufficient heating or overflows from the root of the weld. The metal bead that is not fused with the base metal after cooling is called the weld bead. Too strong welding specification, too fast melting of welding rod, poor quality of welding rod (such as eccentric core), unstable characteristics of welding power supply and improper operation posture are easy to bring about weld beading. It is easier to form weld beading in horizontal, vertical and overhead positions

weld beading is often accompanied by incomplete fusion and slag inclusion defects, which is easy to cause cracks. At the same time, the weld bead changes the actual size of the weld, which will bring stress concentration. The weld beading inside the pipe reduces its inner diameter, which may cause blockage of flow animals

measures to prevent weld beading: make the weld in the flat welding position, correctly select the specification, select the non eccentric core electrode, and operate reasonably

c, pits can be used for polypropylene, polyamide and engineering thermoplastic polyurethane polymer pits, which refer to the part lower than the base metal on the surface or back of the weld

most of the pits are caused by the welding rod (welding wire) not staying for a short time when the arc is stopped (at this time, the pit is called an arc pit). When the welding is carried out vertically and horizontally, the concave is often produced at the root of the back of the weld

the crater reduces the effective cross-sectional area of the weld, and the crater often has crater cracks and crater shrinkage

measures to prevent pits: select the welding machine with current attenuation system, try to select the flat welding position, select the appropriate welding specification, and allow the welding rod to stay in the molten pool for a short time or swing in a ring during arc termination, so as to fill the arc pit

d. incomplete welding refers to continuous or intermittent grooves on the weld surface. Insufficient filler metal is the root cause of incomplete welding. The specification is too weak, the electrode is too thin, and the electrode is not transported properly, which will lead to incomplete welding

incomplete welding also weakens the weld, which is easy to produce stress concentration. At the same time, because the specification is too weak, the cooling rate increases, which is easy to bring pores, cracks, etc

measures to prevent incomplete welding: increase the welding current and weld the cover weld

e, burn through burn through refers to that during the welding process, the penetration exceeds the thickness of the workpiece, and the molten metal flows out from the back of the weld to form a perforated defect

burn through defects will occur if the welding current is too large, the speed is too slow, and the arc stays at the weld for too long. The workpiece gap is too large and the blunt edge is too small, which is also prone to burn through

burn through is an impermissible defect in boiler pressure vessel products, which completely destroys the weld and makes the joint lose its connection and bearing capacity

choose a small current and cooperate with the appropriate welding speed to reduce the assembly gap, add a backing plate or medicine pad on the back of the weld, and use pulse welding, which can effectively prevent burn through

f, other surface defects:

(1) poor formation refers to the appearance and geometric size of the weld does not meet the requirements. There are weld superelevation, unsmooth surface, too wide weld, unsmooth transition from weld to base metal, etc

(2) staggering refers to that two workpieces are staggered at a certain position in the thickness direction, which can be regarded as both weld surface defects and assembly forming defects

(3) collapse during single-sided welding, due to excessive heat input and too much molten metal, the liquid metal collapses towards the back of the weld. After forming, the back of the weld protrudes and the front collapses

(4) surface porosity and crater shrinkage

(5) various welding deformations such as angular deformation, distortion, wave deformation, etc. belong to welding defects. O angular deformation also belongs to assembly forming defects

2. Air hole and slag inclusion

a. air hole refers to the hole formed in the weld when the gas in the molten pool does not escape before the metal solidifies. The gas may be absorbed by the molten pool from the outside, or it may be generated by reaction in the process of welding metallurgy

(1) classification of stomata stomata can be divided into spherical stomata and tapeworm stomata from their shape; It can be divided into single pores and group pores in quantity. The group pores can be divided into evenly distributed pores, dense pores and chain distributed pores. According to the gas composition in the pores, there are hydrogen pores, nitrogen pores, carbon dioxide pores, carbon monoxide pores and oxygen pores. The bottom bearing plate of the module is made of ultradur B 4300 G4, etc. Fusion welding pores are mostly hydrogen pores and carbon monoxide pores

(2) formation mechanism of pores the solubility of gas in solid metal at room temperature is only one tens to one hundreds of times that in high-temperature liquid metal. During the solidification process of molten pool metal, a large amount of gas will escape from the metal. When the solidification rate is greater than the gas escape rate, pores are formed

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

(4) harm of porosity porosity porosity reduces the effective cross-sectional area of the weld and loosens the weld, thus reducing the strength of the joint, reducing plasticity, and causing leakage. Porosity is also a factor causing stress concentration. Hydrogen pores may also contribute to cold cracks

(5) measures to prevent pores A. remove oil, rust, moisture and sundries from welding wires, working grooves and their adjacent surfaces. b. Alkaline electrode and flux shall be used and thoroughly dried. c. DC reverse connection and short arc welding are adopted. d. Preheat before welding and slow down the cooling rate. e. Use stronger specifications for welding

b, slag inclusion refers to the phenomenon that slag residue exists in the weld after welding

(1). Classification of slag inclusion A. metal slag inclusion: refers to the residual metal particles such as tungsten and copper in the weld, which is customarily called tungsten and copper inclusion. b. Non metallic slag inclusion: refers to the residue of unmelted electrode coating or flux, sulfide, oxide and nitride in the weld. Metallurgical reaction is not complete and 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) the causes of slag inclusion a. the groove size is unreasonable; b. Dirt on the groove; c. During multi-layer welding, slag removal between layers is not complete; d. Low welding line energy; e. Weld heat dissipation is too fast, and liquid metal solidifies too fast; f. The chemical composition of electrode coating and flux is unreasonable, and the melting point is too high; g. During tungsten inert gas welding, the polarity of the power supply is improper, the electric and current density is high, and the tungsten electrode melts and falls off in the molten pool. h. During 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 slag inclusion

(4) the harm of slag inclusion is similar to that of pores. Slag inclusion with sharp corners will produce tip stress concentration, and the tip will also develop into a crack source, which is more harmful

3. Cracks are cracks caused by the destruction of atomic bonding in the weld and the formation of new interfaces

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