Analysis on the Quality of Steel Bar Welds
Xin Qiaojuan
The problem of weld quality in steel drums is a problem that has long plagued steel drum companies. With the increasingly fierce market competition, quality has become the focus of competition. At present, users are increasingly demanding the quality of steel drums, and some small weld marks on the welds are not acceptable, forcing steel mills to reduce their prices, and the economic losses are considerable. So, why is the quality of steel barrel welds not solved completely? In addition to the reasons for the equipment, there are also many problems in the technology.
Steel barrel weld quality, generally have appearance inspection, tear test, strength test and airtight test. Although there are differences in inspection items at home and abroad, these items are the most basic. Some developed countries also need to carry out macroscopic metallographic examination, microscopic metallographic examination, hardness inspection, X-ray inspection and impact test. Through the above various tests, it can be found that the main quality defects of the steel barrel weld are: unfused and incomplete fusion, crack, pore, shrinkage, burn, burn through, edge burst, excessive deep indentation and the like. Whether the quality defect is allowed or not, whether the repair is allowed is determined by the product standard and the quality requirements of the user.
Now we have carried out some basic analysis on the quality problems of steel barrel welds that are common in China's steel drum enterprises, in order to obtain the consensus of the peers.
First, the weld is not fused and not fully fused
Unfused and incompletely fused are one of the more serious defects. It directly affects the weld strength, causing open welding in the barrel bulging process or local open welding in the drop test.
Unfused defects are manifested on macroscopic metallographic specimens that do not have a nugget or weld but are plastically bonded. The feature of the incompletely fused defect is that the solder joint is too small or the nugget is eccentric, forming a partial fusion in the weld and a portion not being fused. It is obvious that the joint having such a defect is low in strength.
There are two main reasons for the unfused and incomplete fusion:
1. The seam or electrode surface before welding is not cleaned. The edge of the barrel and the surface of the electrode have a high resistivity oxide or dirt layer, which greatly impedes the current. Excessive oxide and dirt layers can even make the current unconductive. Due to poor current conduction, the junction surface does not produce sufficient resistance to heat the contact surface, resulting in unfused defects.
2. The upper and lower welding wheels of the seam welder are relatively offset. Sometimes, due to the offset of the upper and lower welding wheels, the nugget may be deflected to one side, or even the other side may not be penetrated. In order to prevent the occurrence of the welding wheel offset, the width of the lower welding wheel can be designed to be larger than the upper welding.
The unfused and incompletely fused barrels in seam welding can be re-welded by increasing the welding current or by soldering points beside the weld. Gas welding repair can be used for welds that have entered the next process.
Second, weld crack
Cracks are one of the most dangerous defects, causing the barrel to produce large cracks in the bulging and flanging processes. Especially when the steel drum is subjected to a drop test, it is more dangerous if there is an external crack in the weld.
Cracks have external cracks and internal cracks. The methods for inspecting cracks are commonly used in macroscopic and microscopic metallographic methods. Some external cracks are obvious and can be found by visual inspection.
The main cause of cracking is that the steel sheet for steel drums has a high carbon content, low plasticity and high hardness. The high content of carbon in the material leads to an increase in the melting point of the material, which generally results in quenched structure or inclusions, a narrow crystallization temperature range, low high temperature strength, and small thermal expansion coefficient, so that the cracking tendency is large. The welding performance of such steel plates is not good, and the adjustment range of the process parameters such as welding current, electrode pressure and energization time is small.
For welded steel barrel welds, if found, gas welding repair can be used.
Third, the weld indentation is too deep
In general, the depth of the weld indentation should generally be less than 15% of the thickness of the sheet, and the maximum is not more than 20-30%. If it exceeds this depth, it is an excessively deep indentation. Excessive indentation has a certain influence on the strength of the weld. Sometimes, too deep indentations form obvious grooves in the weld, causing a crack in the flange of the steel drum to be squeezed, resulting in leakage.
The reason for the excessive indentation is mainly due to excessive welding current. When the welding current exceeds a certain value, continuing to increase the current can only increase the penetration rate and the overlap of the nugget without increasing the joint strength, which is uneconomical. If the current is too large, there will be defects such as excessive indentation or even penetration of the weld. Excessive indentation can also occur when the electrode pressure is too high.
In general, too deep indentation has little effect on the strength of the steel drum, but for the indentation groove of the flanged part, it is better to use a sander to level it to avoid the triangle quality problem.
Fourth, weld splatter and flying thorn
Splash is a common flaw. Slight splashing of steel barrel welds is permitted and is not treated as a defect. However, large splashes are very harmful because splashing destroys the plastic zone around the solder joints, reducing joint strength and plasticity; splashing is accompanied by shrinkage cavities and cracks, affecting the processability and drop strength of the weld; Destroyed the surface quality of the weldment. For example, a full-opening bucket containing tomato sauce is filled into a plastic bag, and the burrs generated by the splash often pierce the plastic bag, causing leakage and deterioration of the contents.
The main cause of splashing is that the welding speed is too fast. The joining speed is related to the material of the barrel, the thickness of the sheet, and the requirements for the strength and quality of the weld. In general, when welding the barrel, in order to avoid splashing and obtain a dense weld, it is necessary to use a higher welding speed.
When the burr formed by the splash has a great influence on the surface, it must be trimmed with 锉)9. If a leak is caused, it must be repaired with gas welding.
Fifth, the weld is burnt through
Weld penetration directly affects the sealing performance of the steel drum, which is the most serious defect, and should not be underestimated.
There are many reasons for the occurrence of weld burn through, and there are roughly the following:
1. The welding current is too large. As with the reason that the indentation is too deep, if the current is too large, a weld burn-through defect can be generated.
2. The electrode pressure is too small. When the electrode pressure is too small, not only is the shrinkage hole likely to be generated, but also the contact resistance is too large, so that the weld contact is overheated and completely melted and burned through. Sometimes it is easy to burn the welding wheel and shorten its service life.
3. The surface of the weld bead before welding is not clean. Oxides, dirt, oil and other impurities on the surface of the workpiece increase the contact resistance. Local conduction, due to excessive current density, surface burnout. The inhomogeneity of the oxide layer also affects the inconsistency in the heating of the individual solder joints, causing fluctuations in the quality of the solder.
The defects of the weld penetration are mostly trapped by gas welding.
Sixth, the weld surface is warped
After the steel barrel body is welded, it is often found that the weld seam is undulating, which not only affects the appearance of the surface, but also affects the normal operation of the subsequent process. For example, in the process of squeezing the burring, the weld seam at both ends of the barrel is often concave, so that the barrel is not round and waste is generated.
The reason for the warpage of the weld surface is caused by the difference in the diameter of the upper and lower welding wheels. The diameter of the welding wheel affects the contact area between the welding wheel and the barrel, thereby affecting the distribution and heat dissipation of the current field and causing the position of the nugget to shift. When the diameter of the welding wheel is different, the nugget will be biased to the side of the small diameter welding wheel.
For the warped surface of the barrel, only the bulging method can be used for slight correction, and the burring process is preferably rolled and folded.
Seven, other quality defects
In addition to the above main quality defects, steel barrel welds have some minor defects that have little effect on the quality of steel drums, but sometimes minor defects can become a serious problem when not paying attention.
1. Stomata and shrinkage. Porosity and shrinkage are common defects. This defect, if it is accompanied by no cracks, has no significant effect on weld strength and air tightness. The causes of the pores and shrinkage cavities are generally such that the electrode pressure is too small and the barrel weld is not dry.
2. Welded surface of the weld. Surface burns generally do not affect the weld strength, but they affect the surface quality and corrosion resistance of the weld. When the surface of the steel drum is painted, the surface may be damaged by corrosion. Most of the causes of surface burns are caused by excessive welding current, and sometimes the pressure may be too small.
The above has a shallow analysis of the general quality problems of steel barrel welds, only the causes of the hazards of each quality defect, as well as the repair methods. The further theoretical analysis has not been carried out. The purpose is to strive for shallow fresh Easy to understand, making it more practical and operational. If there are incomplete and incorrect places, I hope that the peer experts will give criticism and correction.
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