All defects that can be seen by the naked eye or low-power magnifying glass and are located on the weld surface, such as undercut (undercut), overlap, crater, surface porosity, slag inclusion, surface crack, and unreasonable weld position, are called external defects;
However, the internal pores, slag inclusions, internal cracks, lack of penetration, lack of fusion, etc. that can only be found by destructive tests or special nondestructive testing methods are called internal defects.
However, it is common that welding slag, spatter and welding scar are not cleaned after welding.
1. The weld size does not meet the specification requirements
The weld height is too large or too small during the inspection;
Or the width of the weld is too wide or too narrow, the transition part between the weld and the base metal is not smooth, the surface is rough, the longitudinal and transverse weld is not neat, and the weld concavity at the fillet weld is too large.
It is caused by poor flatness of weld groove processing, improper angle of groove or unequal size of assembly gap.
Excessive current during welding makes the welding rod melt too fast, and it is difficult to control the weld formation.
Too low current will cause the welding rod to produce “adhesion phenomenon” during arc striking, resulting in incomplete welding or weld beading.
The welder’s operation proficiency is not enough, the electrode handling method is improper, such as too fast or too slow, and the electrode angle is incorrect.
In the process of submerged arc automatic welding, the welding process parameters are improperly selected.
The weld groove shall be processed according to the design requirements and welding specifications.
Mechanical processing shall be selected as far as possible to make the groove angle, straightness of the groove edge and straightness of groove edge meet the requirements.
Manual gas cutting and manual shoveling shall be avoided.
During assembly, the weld gap shall be uniform to lay a foundation for ensuring the welding quality.
Select appropriate welding process parameters through welding process qualification.
Welders shall work with certificates, and the trained welders shall have certain theoretical basis and operating skills.
Multi layer weld In the last layer of weld on the welding surface, under the condition of ensuring the fusion with the bottom layer, the welding current shall be smaller than that between layers, and small diameter shall be used（ φ 2.0mm~3.0mm).
The strip shall be moved longitudinally and rhythmically at a uniform speed, with a certain width of lateral swing to make the weld surface neat and beautiful.
When welding, the arc will melt the concave or groove at the weld edge without being supplemented by molten metal, leaving a gap.
Excessive undercut will weaken the strength of the welded joint, cause local stress concentration, and crack will occur at the undercut after bearing.
Undercut is mainly caused by excessive welding current, too long arc, improper electrode angle control, improper electrode handling speed and too short reserved length of electrode at the end of welding.
Generally, it is a common defect in vertical welding, horizontal welding and overhead welding.
During welding, the current should not be too large, and the arc should not be drawn too long or too short. Short arc welding should be used as far as possible.
Grasp the appropriate electrode angle and skilled electrode handling method.
The electrode shall swing to the edge slowly, so that the molten electrode metal can fill the edge, and in the middle, it shall be faster.
The depth of weld undercut shall be less than 0.5mm, the length shall be less than 10% of the total weld length, and the continuous length shall be less than 10mm.
Once the depth or length exceeds the above tolerance, the defect shall be cleaned up, and the welding rod with smaller diameter and the same brand shall be used.
The welding current is slightly larger than the normal one, and the repair welding shall be carried out.
In the welding process or after welding, metal cracking occurs in the welding area, which occurs inside or outside the weld, and may also occur in the heat affected zone.
It can be divided into longitudinal cracks, transverse cracks, crater cracks, root cracks, etc;
According to the conditions of crack formation, it can be divided into hot crack, cold crack and reheat crack.
The heat affected zone of the weld seam shrinks to produce large stress.
The base metal contains many quenched structures, which are easy to crack after cooling.
There is a fairly high hydrogen concentration in the weld.
And other harmful elements and impurities, which are easy to produce cold and hot cracks.
It is mainly solved by stress relief, correct use of welding materials and perfect operation process.
Pay attention to the groove form of welding joint, and eliminate cracks caused by uneven heating and cooling of the weld due to thermal stress.
If steel plates of different thicknesses are butt welded, the thick steel plates shall be thinned.
The selected materials must meet the requirements of the design drawings, strictly control the source of hydrogen, dry the welding rods before use, and carefully remove the oil, water and other impurities from the groove.
During welding, reasonable welding parameters shall be selected to control the input heat at a cooling temperature of 800~3000 ℃, so as to improve the microstructure of the weld and heat affected zone.
When the welding ambient temperature is low and the material is thin, in addition to increasing the operating ambient temperature, preheating shall also be carried out before welding.
Heat preservation and slow cooling and post weld heat treatment shall be carried out after welding to eliminate the delayed cracks caused by the residual stress of the weld during cooling.
It is the sliding phenomenon at the end of the weld, which not only weakens the strength of the weld, but also produces cracks during the cooling process.
The main arc extinguishing dwell time is too short, or the current used when welding thin plates is too large.
When the welding seam is closed, the welding rod shall be kept for a short time or the electrode shall be moved in a circle for several times.
The arc shall not be stopped suddenly so that there is enough metal to fill the molten pool.
Proper current shall be ensured during welding, and the arc crater can be led out of the weldment by adding an arc starting plate to the main components.
5. Slag inclusion
Non metallic inclusions such as oxides, nitrides, sulfides, phosphides, etc. are found in the welds through non-destructive testing, forming a variety of irregular shapes, and the common slag inclusions are conical, needle shaped, etc.
Slag inclusion in metal weld will reduce the plasticity and toughness of metal structure, and increase the stress, which will lead to cracks in cold and hot brittleness and damage the components.
The base metal of the weld is not cleaned, and the welding current is too small, so that the molten metal solidifies too fast, and the slag cannot float out in time.
The chemical composition of the welding base metal and electrode is impure.
If there are oxygen, nitrogen, sulfur, phosphorus, silicon and other components in the molten pool during welding, non-metallic slag inclusions are easy to form.
The welder is not skilled in operation, and the method of electrode moving is improper, which makes the slag and molten iron mixed together and cannot be separated, preventing the slag from floating.
The welding groove angle is small, and the electrode coating falls off in pieces and is not melted by the arc;
During multi-layer welding, the slag is not cleaned, and the slag is not removed in time during operation.
Welding rods with good welding process performance shall be used, and the steel to be welded must meet the requirements of design documents.
Reasonable welding process parameters shall be selected through welding process qualification. Pay attention to the cleaning of welding groove and edge range, and the welding rod groove should not be too small;
For multi-layer welds, the welding slag of each layer of welds shall be carefully removed.
When acid electrode is used, the slag must be behind the molten pool;
When using alkaline electrodes to weld vertical corner joints, short arc welding is also required in addition to correct selection of welding current.
At the same time, the electrode should be moved correctly to make the electrode swing properly to make the slag float out of the surface.
Preheating before welding, heating during welding, and heat preservation after welding to slow cooling to reduce slag inclusion.
6. Air hole
The gas absorbed in the weld metal melted during welding cannot be discharged from the molten pool before cooling, but remains in the weld to form cavities.
Internal and external pores can be divided according to the position where pores are generated;
According to the partial situation and shape of the porosity defect, the existence of porosity in the weld will reduce the weld strength, but also produce stress concentration, increasing low-temperature brittleness, hot cracking tendency, etc.
The welding rod itself is inferior, and the welding rod is not dried as required due to moisture; Deterioration or peeling of electrode coating;
Corrosion of welding core, etc.
Residual gas exists in base metal smelting;
Welding rods and weldments are stained with rust, oil and other impurities, and gas is generated due to high-temperature gasification during welding.
The welder is not skilled in operation or has poor vision to distinguish molten iron and coating, so that the gas in the coating is mixed with the metal solution.
Excessive welding current makes the electrode red and reduces the protection effect;
The arc length is too long;
The power supply voltage fluctuates too much, resulting in unstable arc combustion, etc.
Qualified welding rods shall be selected. Welding rods with cracked coating, peeled off, deteriorated, eccentric or severely rusted core shall not be used. Oil stains and rust spots near the weld crater and on the surface of the welding rod shall be cleaned.
If the current is appropriate, control the welding speed.
The workpiece shall be preheated before welding, and the arc shall be evacuated slowly at the end of welding or when it stops midway, which is conducive to slowing down the cooling rate of the molten pool and the discharge of gas in the molten pool, so as to avoid gas hole defects.
Reduce the humidity of the welding operation site and improve the temperature of the operating environment.
When welding outdoors, such as wind speed up to 8 m/s, rain, dew, snow, etc., effective measures such as wind shield and canopy shall be taken before welding.
7. Splash and welding slag are not cleaned after welding
This is the most common problem, which is not beautiful, but also very harmful. Fusion spatter will increase the hardened structure on the surface of the material, which is easy to produce such defects as hardening and local corrosion.
The welding material is damp and deteriorated after being stored, or the selected electrode does not match the base metal.
The selection of welding equipment does not meet the requirements, the AC and DC welding equipment and welding materials do not meet the requirements, the polarity connection method of the welding secondary line is incorrect, the welding current is large, there are sundries and oil stains at the edge of the weld groove, and the welding environment does not meet the welding requirements.
The operator is unskilled and fails to operate and protect according to the regulations.
Select appropriate welding equipment according to the welding base metal.
Welding rods shall be equipped with drying and thermostatic equipment.
Dehumidifier, air conditioner shall be set in the drying room, and the distance from the ground and wall shall not be less than 300 mm. Welding rods shall be distributed, used and kept (especially for pressure vessels).
The edge of the welded junction shall be cleaned to remove water, oil and sundries. In winter and rainy seasons, the protective shed shall be overlapped to ensure the welding environment.
Before welding nonferrous metals and stainless steels, protective coatings can be applied to the base metal of wires on both sides of the weld for protection.
Welding rod, thin coating welding rod and argon protection can also be selected to eliminate splashes and reduce slag.
Welders are required to clean welding slag and protect in time.
8. Arc scar
The electrode or welding handle contacts the weldment due to careless operation, or the ground wire and the workpiece are poorly contacted, causing arc for a short time, leaving arc scars on the workpiece surface.
The electric welding operator was careless and did not take protective measures and maintain tools.
Welders shall frequently check the insulation of welding handle wires and grounding wires used, and wrap up in time in case of damage.
The grounding wire shall be firmly and reliably installed.
Do not strike an arc outside the weld bead during welding.
The welding tongs shall be placed separately from the base metal or hung properly.
Cut off the power supply in time when not welding.
If electric arc abrasion is found, electric grinding wheel must be used for timely grinding.
Because on stainless steel and other workpieces with corrosion resistance requirements, arc scar will become the starting point of corrosion, reducing the performance of materials.
9. Welding scar
After welding, the welding scar is not cleaned, which affects the macro quality of the equipment, and improper treatment also causes surface cracks.
During the manufacture and installation of non-standard equipment, the fixture of positioning electric welder is removed after completion.
The slings and fixtures used in the assembly process shall be polished with a grinding wheel to be flush with the base metal after removal.
The sledge hammer shall not be used to forcibly knock off the fixtures to avoid damage to the base metal.
The ultra deep arc pits and scratches during electric welding shall be repaired and polished with a grinding wheel to be flush with the base metal.
This defect can be eliminated as long as attention is paid during operation.
10. Incomplete penetration
During welding, the root of the weld is not fully fused with the base metal or the local part between the base metal and the base metal is not penetrated.
This defect is also called incomplete penetration or incomplete fusion.
It reduces the mechanical properties of the joint, at the same time, it will produce stress concentration and cracks in this part.
No incomplete penetration is allowed in any weld during welding.
The groove is not processed as required, the thickness of the blunt edge is too large, and the angle of the groove or the gap of the pair is too small.
During double-sided welding, the back gouging is not complete or both sides of the groove and the interlayer welding are not cleaned up, so that oxides, slag, etc. hinder the full fusion between metals.
The welder is not skilled in operation. If the welding current is too high, the base metal has not melted and the electrode has melted, so that the base metal and the deposited metal of the electrode are not fused together;
When too low current is selected; If the speed of electrode moving is too fast, the base metal and the deposited metal of the electrode will not fuse well;
During operation, if the angle of the electrode is incorrect, the melting is biased to one side or the blowing is biased during welding, incomplete penetration will be formed at the place where the arc cannot act.
The clearance shall be processed and assembled according to the groove size specified in the design drawings or specifications and standards.
Before welding, carefully clean the rust and oil stain on the groove accessories, especially the root of the weld groove. For multi-layer welds, use angle grinding wheel to polish and clean the oxide between the welding layers.
The current and welding speed shall be reasonably selected, and the correct angle of the electrode shall be paid attention to at any time.
For weldments with thick thickness, high thermal conductivity and fast heat dissipation, the welding parts can be preheated before welding or heated during welding to make the base metal and electrode metal fuse.