In the welding process of stainless steel pipes, some defects may cause stress concentration, bearing capacity, shortened service life, and may even cause brittle fracture.
Usually in the welding technical regulations, it is absolutely impossible for cracks, incomplete penetration, unfused, and surface slag inclusions; defects such as undercuts, internal slag inclusions, and pores must not be larger than the allowable value; And welding.
Here are the reasons for the formation of several common stainless steel welding defects, hazards and solutions.
Weld size not up to standard
This kind of defect mainly refers to the weld residual height and residual height difference, the weld width and width difference, the amount of misalignment, the amount of deformation after welding does not meet the standard size, the weld is uneven, uneven width, deformation Great. Inconsistent weld widths, in addition to the unsightly appearance of welds, can also affect the bond strength between the weld and the base metal. Excessive weld seam height is too high to cause stress concentration. The weld seam is lower than the base metal and the joint strength will be Insufficient; wrong side and deformation is too large, will make the transmission force distortion and stress concentration, resulting in reduced strength.
There are three reasons for the formation. One is the improper welding angle of the stainless steel pipe or the unevenness of the blunt edge and the assembly gap. The second is the unreasonable choice of the welding process parameters. The third is the low level of the welder’s operation skills.
The solution is to choose the proper groove angle and assembly clearance; to enhance the assembly quality; to select suitable welding process parameters and to improve the operating technology level.
Because the welding process parameters are not selected correctly or the operation process is not correct, the grooves or depressions that are easily formed by melting along the base material of the weld toe are called undercuts. Undercutting not only weakens the strength of welded joints in stainless steel pipes, but also causes cracks due to stress concentration.
The reasons for the formation of the main current is too large, the arc is too long, the electrode angle is not correct, improper method of transport and so on.
The solution is to choose the right welding current and welding speed during the arc welding of the electrode. The arc cannot be pulled too long. The angle of the welding rod must be correct and the method of transport should be accurate.
Incomplete penetration refers to the situation where the root of the welded joint does not completely penetrate the stainless steel pipe during welding. No penetration will lead to stress concentration and easily lead to cracks. Important weld joints may not have incomplete penetration defects.
The reason for this is that the groove angle or gap is too small, the blunt edge is too large, the assembly is poor, the welding process parameters are not properly selected, the welding current is too small, the welding speed is too fast, and the welder has poor operation techniques.
The solution is to correctly select and process the groove size, reasonably assemble, ensure clearance, select the appropriate welding current and welding speed, and improve the welder’s operating technology level.
Non-fused refers to the portion that is not completely melt-bonded between the weld bead and the base metal or between the weld bead and the weld bead during welding. The lack of fusion directly reduces the mechanical properties of the welded joints of the stainless steel pipe. Severe non-fusing causes the welded structure to be completely unsupported.
The reason is that the speed of welding stainless steel pipe is fast and the welding current is too low, the welding heat input is too low; the electrode is eccentric, the angle between the welding rod and the weldment is improper, and the arc points to the deflection; the rust and dirt on the side wall of the groove are formed; Slag is not completely clear.
The solution is to correctly select the welding process parameters, perform the operation carefully, and clean up the layers.
A weld bead refers to a metal tumor that melts on the unmelted parent metal outside the weld during the welding process. The weldment not only affects the formation of the welds in the stainless steel tubes. In the area of the welds, there are usually slag inclusions and incomplete penetrations.
The reason for the formation is that the blunt edge is too small and the root gap is too large; the welding current is large and the welding speed is fast; the welder has a low level of operational skills.
The solution is to select appropriate welding process parameters according to different welding positions and strictly control the size of the fusion hole.
The crater refers to the depressed portion of the stainless steel tube formed at the end of the weld. The craters not only severely weaken the welds at the site, but also cause crater cracks due to the concentration of impurities.
The main reasons for the formation are the short arc extinguishing residence time and the excessive current during the thin plate welding.
The solution is that when the arc welding of the electrode arcs, the electrode must stay at the pool for a short time or make a circular transport bar. After the molten metal is filled, the arc extinguishes to one side. When the tungsten electrode is welded, there must be enough. The residence time, after filling the weld attenuation arc extinguishing.
When the stainless steel pipe is welded, voids generated due to the gas in the molten pool not being able to escape during solidification are called pores. The pore is a kind of common welding defect, which is divided into two kinds, the inner pore and the outer pore of the weld. The pores are mainly round, oval, insect-shaped, needle-shaped, and dense-shaped. The existence of pores not only affects the tightness of the weld, but also reduces the effective area of the weld and weakens the relevant mechanical properties of the weld.
The main causes are the presence of oil, rust, moisture, and other contaminants on the surface and groove of the stainless steel pipe; when the electrode is arc welded, the coating of the electrode is damp, and there is no drying before use; the arc is too long or partial blow; the effect of the molten pool protection Not good, the air invades the molten pool; the welding current is too large, the welding rod becomes red, the coating peels off prematurely, and the protection function is lost; the operation method is improper, such as the arc closing movement is too fast, the shrinkage hole is easy to occur, and the arc welding action of the joint is incorrect. It is easy to form dense pores.
The solution is to remove the dirt in the 20-30mm range on both sides of the clean groove before welding; strictly according to the temperature and time specified in the electrode specification; select the correct welding process parameters and operate correctly; use short-arc welding as much as possible, There must be windproof facilities in the field construction; failure to use the welding rod.
Inclusion and slag inclusion
Inclusion refers to non-metallic inclusions and oxides formed by metallurgical reactions that remain in the weld metal. Slag inclusions are slag that remains in the weld. Stainless steel welded pipe slag is mainly divided into slag inclusions and strip inclusions. Slag inclusion weakens the effective section of the weld to reduce the mechanical properties of the weld. Slag inclusions also tend to cause stress concentration, which can easily damage the welded structure during loading.
The reason for the formation is that the inter-layer slag is not clean during the welding process, the welding current is too low, the welding speed is too fast, and the welding operation is not performed properly; the chemical composition of the welding material and the parent metal are not properly matched; the groove design and processing are not suitable, etc. .
The solution is to select the electrode with better slag removal performance; carefully remove the inter-layer slag; reasonably select the welding process parameters; adjust the angle of the electrode and the method of transport.
The burn-through defect refers to the defect that the stainless steel pipe is perforated when the stainless steel pipe is welded and the molten metal flows out from the back of the groove. This is a relatively common arc welding defect.
The reason for this is that the current during welding is too large and the welding speed is too slow, causing the stainless steel tube to be overheated; the groove gap is large and the blunt edge is too thin.
The solution is to select suitable welding process parameters and suitable groove sizes, and in addition, the welding operation level should be as high as possible.
Source: China Stainless Steel Welded Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)