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Welding defects-undercut

By Thomas Huang March 29th, 2024 108 views
Welding is a very important and widely used joining process for metal products in the manufacturing process of reactors. 
However, in the actual operation process, welding parts may have undercut problems. 

Weld undercut refers to a situation where the weld edge is too high or too low.  The welding undercut will lead to the quality degradation of the weld, and even affect the strength and sealing of the welded structure. 

I. What is welding undercut

Welding undercut refers to the phenomenon that the filler metal fails to completely fill the toe of the base metal and the edge of the weld during welding. 

photo of undercut

II. Causes of welding undercut and preventive measures 

Welding mode Causes  Precautions
Shielded Metal Arc Welding (SMAW) (1)The current is too strong. 

(2)The welding rod is not suitable. 

(3)The arc is too long. 

(4)Improper operation method. 

(5)The base metal is unclean. 

(6)Base metal overheating.
(1) Use a lower current. 

(2)Select the right type and size of electrode. 

(3)Maintain proper arc length. 

(4)Weld with the right angle, slower speed, shorter arc, and narrower route. 

(5)Remove grease or rust from base metal. 

(6)Apply electrodes with smaller diameter. 
CO2 gas shielded welding causes  (GMAW) (1) Arc is too long, welding speed is too fast. 

(2)The electrode alignment is incorrect in fillet welding. 

(3)Vertical welding swing or poor operation, resulting in undercut of weld bead. 
(1) Reduce arc length and speed. 

(2)For horizontal fillet welding, the welding wire should be 1- 2 mm away from the intersection. 

(3)Correct the operation. 

III. Mend for undercut

3.1 Grinding 

It is to use grinding wheel or sandpaper to polish the edge of the weld, and smooth the too high or too low part to be basically flush with the weld surface. 
During grinding, it should maintain the appropriate angle and strength to avoid additional damage to the weld.  
Evaluate the result with the naked eye or with Detection Tools after grinding.
The 2nd grinding could be done if necessary. 

3.2 Change welding parameters 

Usually, the undercut is often caused by wrong welding parameters. 
Adjusting of welding parameters can prevent the arising of undercut  to some extent.  
The parameters include welding current, voltage and welding speed.  
It is necessary to conduct repeated trials and adjustments according to specific conditions to find the best combination of welding parameters.

3.3 Re-weld

This is a thorough method to repair the undercut. 
It is to to break or cutt the original weld, then make new weld.  
We should use suitable welding method and welding material, to ensure the welding quality.  
It is suitable for serious undercut or high repair requirement. But, we need to take care to avoid the introduction of new defects caued by reweld.

IV. Prevention from the perspective of process management 

In order to solve the undercut, the following measures can be taken: 

4.1 Choose appropriate welding process: 

Select the process according to the welding material, weldment thickness and structural characteristics.

4.1.1 Adjust welding parameters: 

Generally, reducing welding current, voltage and speed, increasing gas flow, can make the welding process more stable, reduce the occurrence of undercut.

4.1.2 Replacement of welding equipment: 

Choosing high quality welding equipment can improve the stability and controllability of the welding process and reduce the occurrence of undercut.  

4.2 Improve welder skills: 

Welder's skill level directly affects welding quality. 
By strengthening the welder training, improve their operating skills, can reduce the occurrence of undercut.  
In the welding operation, mastering the correct welding posture and current voltage setting can effectively avoid undercut.

4.3 Strengthen quality control 

Strengthen the weld quality monitoring, timely detection and correction of undercut phenomenon, can ensure the welding quality. 
Regular inspection of the welding process can effectively avoid the problem occurrence. 

4.4 Regular maintenance and inspection of equipment: 

Ensure the welding equipment is running properly, check and maintain the equipment regularly. 
For some seriously worn equipment parts to be replaced in time to ensure the stability and reliability of the welding process, is also one of the important measures to avoid the phenomenon of undercut.
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