1, the appearance of defects:
Appearance of defects (surface defects) is not by means of instruments, can be found from the surface defects. Common appearance of defects undercut, weld, dents and welding deformation, and sometimes the surface of pores and surface cracks. Single-sided welding of the root incomplete penetration and so on.
A, is undercut along the weld toe in the base metal part of the formation of depressions or grooves, it is due to the edge of the weld arc to melt the base metal has not been deposited after the full complement of metal left by the gap. Have undercut the main reason is the arc heat is too high, ie a current is too large, too small transport of velocity caused. The angle between the electrode and the workpiece is incorrect, irrational swing arc is too long, the order is unreasonable, and this will result in welding undercut. DC arc welding produces magnetic blow also undercut one of the reasons. Some of the welding position (Li, Wang, Yang) will increase undercut.
Undercut reduces the effective cross-sectional area of ??the base material, reducing the carrying capacity of the structure, while also resulting in stress concentration, the development of the crack source.
Correct operating position, the choice of appropriate standards, using good methods of operation will help to eliminate undercut. Fillet weld, welding instead of AC DC welding also can effectively prevent the biting edge.
B, weld seam in the heated liquid metal to flow less than the base metal is not melted or overflow from the weld root, not formed after cooling of the metal and base metal fusion welding tumor is the tumor. Welding specification is too strong, electrode melting too fast, poor quality of welding (such as partial core), welding power source characteristics and operating posture instability and so easy to bring weld. In the horizontal, vertical, overhead welding position to form more tumors. Often accompanied by lack of fusion weld, slag defects, easily lead to cracks. Meanwhile, the weld seam to change the actual size, will bring stress concentration. Welded inside the tube reduces its diameter tumor, animals may cause flow blockage.
Measures to prevent the weld: the weld in the flat welding position, the correct selection of standardized, unbiased selection of the core rod, a reasonable operation.
C, pits
Pits refers to the weld surface or the back of the lower part of the base metal part.
More than is due to close pits arc welding rod (wire) were not caused by a short stay (known as pit crater at this time), Yang Li, horizontal welding, often resulting in the weld root of the back concave.
Pit reduces the effective cross-sectional area of ??the weld, often with crater crater crater cracks and shrinkage.
Measures to prevent the pits: use of a welding current decay system, try to use the flat welding position, choose the appropriate welding standard, close to arc welding in the pool or stay inside the ring swinging a short time, fill the crater.
D, at least not welding
Man is not welded on the surface of the weld groove continuous or intermittent. Filler metal is to produce not less than the root cause of solder filled. Specification is too weak, too small rod, improper transport of lead solder is not full. Man is not the same weakened welded seam, prone to stress concentration at the same time, the standard is too weak to make the cooling rate increases, likely to result in pores, cracks and so on. Not welded to prevent full measures: to increase the welding current, welding increases weld surface. E, burn
Burn is the welding process, the penetration depth exceeds the thickness of the workpiece, the molten metal from the weld back out to form a perforated missing. Welding current is too large, too slow, the arc in the weld stays too long, will have a burn defects. Workpiece gap is too large, too blunt edge also prone to burn-through phenomenon. Burn is not allowed on the boiler and pressure vessel defects in the product, it completely destroyed the weld joint to lose its connection to flying and carrying capacity. Use a smaller current and welding speed with the right, reducing the assembly space, additional pads in the back of the weld pad or drugs, the use of pulse welding, can effectively prevent the burn.
F, other surface defects:
(1) forming the appearance of bad geometry refers to the weld does not meet the requirements. There are Weld high, the surface is not smooth, and the weld is too great, weld to base metal transition is not smooth and so on.
(2) refers to the wrong side of the workpiece in the thickness direction of two staggered a certain position, both regarded as weld surface defects, and may be regarded as forming assembly defects.
(3) collapse of the single-sided welding, because excessive heat input, leaving too much molten metal liquid metal to weld on the back of collapse, after forming the weld on the back of protruding front collapsed.
(4) holes and crater surface shrinkage.
(5), such as angular distortion of welding distortion, distortion, wave soldering defects belong to the deformation angle deformation O forming part of the assembly defect.
2, porosity, and slag
A, stomatal
Holes is welded, the pool is not in the gas escape before the metal solidified, remaining in the weld into the hole formed. The gas may be absorbed from the outside pool, welding metallurgy may be generated during the reaction.
(1) the classification of stomatal pores on the points from their shape, have spherical pores, a worm-like pores; from the pore volume can be divided into individual and group-like pores. Group-like pores have uniform porosity, density distribution of pore-like holes and chain of points. Classified according to the gas composition within the pores, holes with hydrogen, nitrogen holes, pores of carbon dioxide, carbon monoxide, porosity, oxygen holes and so on. Welding holes are mostly hydrogen and carbon monoxide vent hole. (2) the formation mechanism of pores in gas-solid metal at room temperature solubility of only high-temperature gas solubility in liquid metal a few to a few one-tenth of one percent in the bath of metal solidification process, there are a lot of gas out from the metal in the Plaza . When the solidification rate is greater than the rate of gas escape when the formation of pores.
(3) have pores filled with the main base material or metal surface rust, oil, etc., electrode and flux will increase the pore volume is not drying as rust, oil and drug skin electrodes, flux in the water decompose at high temperatures for the gas increase the gas content in hot metal. Welding heat input is too small, bath cooling rate is not conducive to gas escape. Weld metal oxygen deficiencies will increase the oxygen holes.
(4) vent holes to reduce the dangers of the effective cross-sectional area of ??the weld, the weld loose, thereby reducing the joint strength, lower plasticity, can also cause leaks. Pores also cause stress concentration factors. Holes may also contribute to hydrogen cold cracking.
(5) measures to prevent the holes A, clear wire, working near the surface of the groove and the oil, rust, moisture and debris. B, the use of basic welding rod, flux, and thoroughly dried. C, DC reverse and use a short arc welding. D, preheating before welding, slow cooling rate. E, with the strong side of the welding specifications. B, slag is a slag after welding slag remaining in the weld of the phenomenon.
(1), the classification of slag A, metal slag: refers to tungsten, copper and other metal particles being left in the weld, used as clip on tungsten, copper clip. B, non-metallic slag: that drugs are not skin or melt the welding rod flux, sulfide, oxide, nitride residue on the weld being. Metallurgical reaction is not complete, good slag removal.
(2) the distribution and shape of the slag with a single point-like slag, slag strip, chain and dense slag slag
(3) the causes of slag A, groove dimensions unreasonable; B, groove with dirt; C, multi-layer welding, the slag layer is not complete; D, welding heat input small; E, weld heat too fast, too fast solidification of liquid metal; F, drug skin electrodes, flux unreasonable chemical composition, melting point is too high; G, tungsten inert gas welding, improper polarity power supply, electricity, current density, tungsten melting off in the pool in. H, manual welding, the welding rod swinging bad, is not conducive to floating slag. According to the above reasons were taken corresponding measures to prevent the production of slag.
(4) the dangers of slag slag spots and holes similar to the hazard, with sharp corners will produce slag tip stress concentration, but also the development of the crack tip source, great harm.
3 crack
The destruction of the weld atoms to form a new interface known as the crack arising from the gap.
A, the classification of crack
According to the crack size, divided into three categories:
(1) macro-crack: visible cracks.
(2) micro-cracks: can be found under the microscope.
(3) ultra-micro crack: The high-expansion can be found under the microscope, generally refers to intergranular cracks and intragranular cracks.
From the resulting temperature point of view, the crack is divided into two categories:
(1) Thermal cracks: AC3 line produced in the vicinity of the crack. Welding Electrode is completed usually appears, also known as crystal crack. This two cracks occurred mainly in the grain boundary, the crack surface has oxidized color, loss of metallic luster.
(2) Cold crack: refers to the completion of the cold welding to below the martensitic transformation temperature M3 produced cracks in the weld is generally a period of time (hours, days or even longer) to appear, it is also known as delayed cracking.
Divided by the causes of cracks, the crack can be divided into:
(1) reheat crack: joint cooling after heating to 500 ~ 700 ℃ generated when the crack. Reheat cracks in precipitation hardened materials (such as those containing CR, MO, V, TI, NB metal) welding heat-affected zone of coarse-grained area, generally from the fusion line to the heat-affected zone of coarse-grained area development, was intergranular cracking characteristics.
 (2) mainly due to lamellar tearing in steel rolling process, the sulfide (MNS), silicate and other impurities in the folder in which the formation of anisotropy. Or outside the welding stress the use of restraint stress, the metal debris along the rolling direction of the crack.
(3) stress corrosion cracking: the role of stress and corrosive media co-produced under the crack. In addition to residual stress or restraint stress factors, stress corrosion cracking and weld metal composition and the main form of the.
 B, the dangers of crack cracks, especially in cold crack, the harm is catastrophic. The pressure vessel accidents in the world except in very limited due to poorly designed, inappropriate selection of other causes, the vast majority is due to cracks caused by brittle failure.
C, thermal cracking (crack crystal)
(1) crystalline crack formation mechanism of hot cracking occurred at the end of the weld metal solidification, the temperature sensitive area roughly in the vicinity of solidus temperature zone, the most common is the crystallization of the thermal crack crack, because the generation process in the weld metal solidification , the segregation of the impurities generated crystalline low melting point eutectic enrichment at grain boundaries, forming so-called "liquid film", in particular the temperature-sensitive area (also known as brittle temperature zone), the strength is extremely small, because the weld solidification contraction by the tensile stress, eventually cracking the formation of cracks. The most common crystal crack is cracking along the weld center length for vertical cracks, and sometimes also occurred within the two columnar grains in the weld between the transverse cracks. Crater cracks are another form of common thermal cracking. Thermal cracks are cracks along the grain boundaries, usually in the impurities of carbon steel, low alloy steel, austenitic stainless steel welds in air and other materials
(2) the factors that affect crystal crack
A, effects of alloying elements and impurities carbon and sulfur, phosphorus and other impurities increases, the temperature sensitive area will be expanded so that more opportunities crystalline cracks.
B, the cooling rate affect the cooling rate increases, the first crystals have increased segregation, and second, to increase the crystallization temperature range, both of which increase the opportunity for the emergence of crack crystal;
C, crystal stress and restraint stress in the brittle temperature region, the intensity of very low metal, welding stress and to make this fly part of the metal in tension, when the tensile stress reaches a certain level, there will be crystal cracks.
(3) measures to prevent crystal cracking
A, reduced sulfur, phosphorus and other harmful elements in the content, materials with low carbon welding. B, adding some alloying elements, reducing the columnar crystals, and segregation. Such as aluminum, sharp, iron, mirror, etc. can be refined grains. C, using shallow weld penetration and improve thermal conditions in which low melting point material floating on the surface of the weld does not exist in the weld. D, a reasonable selection of Nickel Welding Electrodes, and the use of preheat and post heat, reducing the cooling rate. E, using reasonable assembly sequence, reducing the welding stress.