Stainless Steel Welding Defects And Countermeasures

By Author: Sinosteel Pipe
Total Articles: 6
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Stainless steel welding defects and countermeasures (I)
In the production process of stainless steel welded pipes, the emergence of welding defects will bring many serious problems. These defects will not only cause stress concentration, so that when the welded pipe is subjected to load, the stress at the defect is much higher than the average stress, greatly reducing its load-bearing capacity, and significantly shortening its service life. In severe cases, it may even cause brittle breakage and cause safety accidents. Based on this, general technical regulations have strict regulations on welding defects, and it is clear that defects such as cracks, unwelded, unfusion and surface slag inclusion are absolutely not allowed to exist; for defects such as undercuts, internal slag inclusions and pores, a certain allowable value range is also set. Once the standard exceeds the standard, it must be thoroughly removed and welded. The following will give a detailed brief description of the causes, hazards and prevention measures of common welding defects in stainless steel welded pipes.​
1. The weld size does not meet the ...
... requirements​
The weld size does not meet the requirements, which is mainly reflected in the deviation of the weld residual height and residual height difference, the weld width and width difference, the amount of edge error, and the amount of deformation after welding, which is manifested as uneven height, uneven width and large deformation of the weld. Inconsistent weld widths not only affect the appearance, but also weakens the bonding strength between the weld and the base material; too large the weld residual height can easily cause stress concentration, while lower than the base material, it cannot guarantee sufficient joint strength; too much distortion and excessive deformation will cause force transmission to cause stress concentration, resulting in a decrease in strength.
The main reasons for such problems are the improper angle of the gravure or uneven edges of the stainless steel welded pipes, which leads to abnormal metal filling and fusion during welding; the selection of welding process parameters is unreasonable, such as inappropriate current, voltage, welding speed, etc., which affects the formation of welds; the welder's operating skills are low and it is impossible to accurately control the welding process.
Preventive measures include selecting appropriate bevel angles and assembly gaps to lay the foundation for good weld molding; improving assembly quality and ensuring consistency of welding conditions; selecting appropriate welding process parameters and conducting precise debugging based on the material and thickness of the pipe; improving the welder's operating technical level, and improving his operating proficiency and accuracy through training and practice.​
2. Bite the undercut
The undercut is a groove or depression formed by burning the base material along the welding toe due to incorrect selection of welding process parameters or incorrect operation process. The presence of undercuts greatly weakens the strength of the welded joints of the welded pipe, and due to the concentration of stress, it is very easy to cause cracks, seriously threatening the safe use of the welded pipe.

The main reason for the undercut is that the current is too large, which causes the base material to melt too quickly; the arc is too long, resulting in poor protection effect and heat dispersion; the welding rod angle is incorrect, so that the arc heat cannot be uniformly applied to the base material and weld; the strip transportation method is improper, and the uniform filling and fusion of metal cannot be guaranteed.
Preventive measures are: When welding rod arc welding, appropriate welding current and welding speed should be selected according to the characteristics of the pipe and welding rod to ensure proper heat input; the arc should not be pulled too long to ensure good protection effect and concentrated heat; the angle of the welding rod should be appropriate and adjusted according to the welding position and weld shape; the strip transportation method should be correct to maintain a uniform and stable strip transportation rhythm.​
3. Unwelded
Unwelded refers to the phenomenon that the roots of the welded joints are not completely melted when welding stainless steel welded pipes. The unwelded part will become a stress concentration point. When bearing loads, the stress increases sharply and easily causes cracks, which seriously affects the quality of the welded joints. For important welded joints, unwelded defects are never allowed.

The main reasons for this are the bevel angle or gap that are too small, which makes the welding rod unable to penetrate deep into the roots and fully melt the metal; the blunt edge is too large, which hinders the fusion of the weld roots; poor assembly, which causes the roots to not be well butt during welding; improper selection of welding process parameters, the welding current is too small, unable to provide enough heat to melt the base material, and the welding speed is too fast, which makes the metal not able to fully melt; the welder's poor operation technology is unable to effectively control the welding process to ensure the roots are melted.
Preventive measures include correctly selecting and processing bevel sizes, determining appropriate angles and gaps according to pipe and welding requirements; rational assembly to ensure accurate butt and sufficient gaps of the roots of the joints; selecting appropriate welding current and welding speed to ensure that heat input and welding rhythm meet the permeability needs; improving the welder's operating technical level so that it can accurately control the welding process and ensure that the roots are completely permeable.​
4. Unfused
Unfusion refers to the part that is not completely melted and combined between the weld bead and the base material or between the weld bead and the weld bead during fusion welding. Unfused directly reduces the mechanical properties of the joints. Severe unfused will make the welded structure unable to bear at all, greatly affecting the performance and safety of the welded pipes.

The main reason is that the speed of welding stainless steel welding pipes is fast and the welding current is small, which leads to too low welding heat input, which makes the base material and the welding material fully melt and fuse; the welding rod is eccentric, which makes the arc heat distribution unevenly; the angle between the welding rod and the welded parts is improper, the arc pointing is deflected, and the base material and welding material cannot be effectively heated; there is rust and dirt on the side wall of the bevel, and the slag cleaning between layers is not thorough, hindering the fusion of metals.
Preventive measures To correctly select welding process parameters, match appropriate current, voltage and welding speed according to the requirements of pipes and welding; operate carefully and pay attention to the welding process at all times to ensure the correct angle and arc direction of the welding rod; strengthen interlayer cleaning, thoroughly remove impurities such as rust, dirt, and dirt from the bevel and layer; improve the operational technical level of the welder so that it can timely discover and solve the problem of non-fusion.
Stainless steel welding defects and countermeasures (II)
5. Welding tumor
Welding tumor refers to metal tumors formed by molten metal flowing onto unmelted base material outside the weld during welding. Welding not only affects the aesthetic shape of the welds of stainless steel welded pipes, but also often suffers from defects such as slag inclusion and infiltration in the welding tubes, further reducing the quality of the welds.

The main reasons for the occurrence of weld tumors are that the blunt edges are too small and the root gap is too large, which makes the metal easily overflow during welding; the welding current is large and the welding speed is fast, resulting in too much molten metal and not having time to distribute evenly; the welder's operating skills are low and he cannot effectively control the shape of the molten pool and the flow of metal.
Preventive measures are to select appropriate welding process parameters according to different welding positions, such as adjusting the current, voltage and welding speed for different locations such as flat welding, vertical welding, and horizontal welding; strictly control the size of the melt holes to avoid excessive metal flow; improve the welder's operating technical level, and better control the welding process through practice and experience accumulation.​
6. Arc pit
The sinking part generated at the end of the weld of sanitary grade stainless steel welded pipe is called an arc pit. The arc pit not only seriously weakens the strength of the weld at this point, but also because the impurities are concentrated in the arc pit, it is very easy to produce arc pit cracks, affecting the overall quality and service life of the weld.

The main reason for this is that the arc extinguishing residence time is too short, which causes the molten pool metal to not fully fill the arc pit; the current is too large when welding the thin plate, which causes the metal at the end to melt excessively.
The prevention measure is that when the welding rod arc welding is closed, the welding rod should stay at the molten pool or be annular strip. After the molten pool metal is filled, the arc is then directed to one side to ensure that the arc pit is fully filled; when the tungsten pole argon arc welding, there must be sufficient residence time, and the current attenuation function should be used to slowly fill the weld before extinguishing the arc.​
7. Stomatal pores
When welding sanitary grade stainless steel welded pipes, the holes formed by the gas in the molten pool failing to escape when solidified and remain are called air holes. Stomatal pores are a common welding defect, divided into internal pores and external pores of welds, and their shapes are round, oval, insect-shaped, needle-shaped and dense. The existence of air pores will not only affect the density of the weld, causing the medium to leak through the air pores, but will also reduce the effective area of ​​the weld, reduce the mechanical properties of the weld, and reduce the load-bearing capacity of the welded pipe.

The causes include oil, rust, moisture and other dirt on the surface and grooves of the sanitary grade stainless steel welded pipes, which decompose and produce gas at high temperatures of welding; the welding rod pill skin is damp during arc welding, and it is not dried before use, and the moisture in the pill skin is decomposed into hydrogen during welding, etc.; the arc is too long or blown, and the molten pool protection effect is not good, and the air invades the molten pool; the welding current is too large, the welding rod is red, the pill skin falls off early, and loses the protection effect; improper operation method, such as the arc-collapse action is too fast, which is easy to produce shrinkage holes, the joint arc-induced action is incorrect, and dense air holes are easily generated.
Preventive measures are to remove the oil, rust and moisture on both sides of the bevel before welding to reduce the source of gas; strictly bake the electrodes according to the temperature and time specified in the welding rod instructions to ensure that the electrodes are dry; correctly select welding process parameters and use appropriate current, voltage and welding speed; operate correctly and maintain a stable welding posture and movement; try to use short arc welding to enhance the protection effect, and windproof facilities should be provided in outdoor construction to avoid arc blowing; failure welding rods are not allowed, such as rust of the welding core, cracking of the powder skin, peeling, excessive eccentricity, etc.​
8. Inclusions and slag inclusions
Slag inclusions are non-metallic inclusions and oxides that remain in the weld metal due to metallurgical reactions, as well as slag remaining in the weld. Slag inclusions in stainless steel welded pipes can be divided into two types: dot-shaped slag inclusions and strip-shaped slag inclusions. Slag inclusion weakens the effective section of the weld, reduces the mechanical properties of the weld, and causes stress concentration, which easily causes damage to the welded structure during load bearing.

The main reasons are that the interlayer cleaning slag during welding process are unclear, so that the slag of the previous layer remains in the weld; the welding current is too small to fully melt and float the slag; the welding speed is too fast, so that the slag cannot float out; improper operation during welding process, such as the angle of the welding rod is inappropriate, the slag cannot be effectively eliminated; the improper matching of the welding material with the chemical composition of the base material affects the properties and discharge of the slag; the inappropriate design and processing of the bevel is not conducive to the discharge of the slag.
The prevention measures are to select welding rods with good slag removal performance to facilitate the discharge of slag; carefully remove slag between layers to ensure the cleanliness of each layer of weld; reasonably select welding process parameters, adjust current, voltage and welding speed to ensure that slag can fully float; adjust the welding rod angle and strip transportation method, and use the blowing force of the arc to blow the slag away from the weld.