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Welding defects-Blow hole

By Thomas Huang April 3rd, 2024 79 views

I. Definition

Blow hole refers to the cavity caused by the gas that is failed to secape from the molten pool before the metal solidification after welding.
The gas may be absorbed by the molten pool from the outside, or it may be generated by the reaction in the welding metallurgical process. 

photo of blow hole

II. Type

2.1 As per hole's shape

Spherical hole and worm-like hole.

2.2 As per hole's number

Single hole and group hole.  
The group hole type can be divided into uniform displayed hole, dense holes, and chain holes. 

2.3 As per gas type

Hydrogen pores, nitrogen pores, carbon dioxide pores, carbon monoxide pores, oxygen pores, etc.  
The welding pores are mostly hydrogen pores and carbon monoxide pores. 

III. Hazard

Although blow hole has less stress concentration tendency than other defects, it destroys the compactness of weld metal and reduces the effective cross-sectional area of weld metal, thus resulting in lower weld strength. 

IV. Causes and preventive measures

welding mode  causes Preventive measures
Shielded metal arc welding ( SMAW) (1)Bad or wet electrode. 
(2)There is moisture, grease or rust on the weldment. 
(3)The welding speed is too fast. 
(4)The current is too strong. 
(5)Arc length is not proper.
(6)The weldment thickness is too large and the metal cools too quick. 
(1)Use suitable and dry electrode. 
(2)Clean welded parts before welding. 
(3)Reduce welding speed, make sure the internal gas to escape easily. 
(4)Use proper current recommended by electrode manufacturer. 
(5)Adjust the proper arc length. 
(6)Perform proper preheating. 
CO2 gas shielded welding (GMAW) (1)Base metal is not clean. 
(2)Welding wire is rust or flux is wet. 
(3)Poor spot welding, improper selection of welding wire. 
(4)Dry elongation is too long, CO2 gas protection is not thorough. 
(5)Wind speed is high, no windshield. 
(6)Welding speed too fast, cooling fast. 
(7)Sparks stick to the nozzle and cause turbulence. 
(8)Gas purity is poor, containing impurities (especially moisture). 
(1)Clean the welded parts before welding. 
(2)Select proper welding wire and keep it dry. 
(3)Spot weld bead shall be free from defects. It shall be clean and use appropriate size of the welding wire. 
(4)Reduce dry elongation and adjust proper gas flow. 
(5)Install windshields. 
(6)Reduce velocity to allow internal gases to escape.  
(7)Pay attention to remove welding slag from nozzle and apply splash adhesion inhibitor to prolong nozzle life. 
(8) CO2 purity is above 99.98%, water content is below 0.005%.
Submerged arc welding (SAW) (1)Weld rust, oxide film, grease and other organic impurities. 
(2)Flux is wet. 
(3)The flux is contaminated. 
(4)The welding speed is too fast. 
(5)Insufficient flux height.
(6)Flux height is too high to allow gas to escape easily (especially in fine flux). 
(7)Welding wire rust or stained with oil.  
(8)Inappropriate polarity (especially when contaminated by docking). 
(1) weld grinding or flame burning, and then wire brush removal. 
(2)Dry at about 300℃ 
(3) Pay attention to the storage of flux and the cleaning of the area near the welding site to avoid impurities. 
(4)Reduce welding speed. 
(5)Flux outlet rubber nozzle should be adjusted higher. 
(6)Flux outlet rubber tube should be adjusted lower, in the case of automatic welding appropriate height 30- 40 mm. 
(7)Change to clean welding wire. 
(8)Change DCEN to DCEP.
Equipment problems (1) Pressure gauge cooling, gas can not flow out. 
(2)The nozzle is clogged with spark spatter. 
(3)The welding wire has oil and rust. 
(1)If gas regulator has no electric heater, install the electric heater, and checking the flow meter. 
(2)Clean nozzle spatter frequently.  And coated with splash adhesion inhibitor. 
(3)Do not touch oil when storing or installing welding wire. 
Self-shielded flux-cored wire (1) Voltage is too high. 
(2)The protruding length of welding wire is too short. 
(3)Steel plate surface has corrosion, paint, or moisture. 
(4)The torch drag angle is tilted too much. 
(5)Moving speed too fast, especially horizontal welding. 
(1)Reduce voltage. 
(2)Use according to various welding wire instructions. 
(3)Clean before welding. 
(4)Reduce the drag angle to about 0-20°. 
(5)Adjust appropriately. 
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