Bavel angle narrow gap

[lasweld]

What I know, narrow gap welding refers to the use of groove/included angle in the range between 2 to 20 degree.
Can somebody explain what should we consider to select this included angle to weld pipe (wall thickness 22 mm) in 5G position using GMAW ?
What will be the benefit and disadvantage of using included angle of 2 degree compared to 10 degree?

Thanks,

Tom

 

[metengr]

Tom;

What will be the benefit and disadvantage of using included angle of 2 degree compared to 10 degree?

In general, for narrow groove welding using a 2 degree versus 10 degree included angle;

Advantage - less filler metal and distortion

Disadvantage - potential for side wall lack of fusion defects.

 

[weldtek]

lasweld,
Two degrees included angle is going to provide you with an opening of less than 1 mm plus any gap. Using the GMAW process, you're asking for trouble.

 

[Mr168]

Narrow groove end preps are typically much tighter than the sizes you've given, and are intended for the GTAW process using specialized equipment.

However, if you're referring to going to something like a 20/10 or 37/10 bevel as opposed to a straight 37 1/2, then that makes a bit more sense in the context that you're referring to.


As Metengr mentioned, lack of sidewall fusion is the biggest concern. However, there are others that are often overlooked. On heavier wall thicknesses, too tight of an included angle can actually cause the bevel to invert before it's fully welded out due to weld shrinkage. While specialty cups are available, nozzle size can be a major concern - we strayed away from the standard ANSI preps for that purpose when machine welding. Visibility is reduced for the welding operator. Repairs are more difficult, as is interpass cleaning on heavier wall thicknesses. If a J-prep is incorporated, it can increase the difficulty level of root pass welding, though this is minimized in a shop environment.


Despite the many disadvantages, it's still worth the time savings, particularly on heavy wall, large diameter welds. In the situation presented above at less than 1" wall thickness, the differences will likely be marginal, and I wouldn't even attempt to use anything less than a 20° included angle.