For a flare bevel groove T-joint with radii of 12 mm that is flush to the edge, what is the approximate effective throat?

• A. 3.6 mm
• B. 1.2 mm
• C. 6 mm
• D. 12 mm

Answer: (A)

In a flare bevel groove on a T-joint, the edge radius plays a big role in the effective throat, which is the smallest weld section that must fill to achieve proper fusion. When the edge is rounded with a radius, the weld has to reach from the root into that rounded edge, so the throat is essentially the projection of that edge into the weld axis.

A practical way to estimate this is to think of the edge radius R being projected along the weld axis. The throat length is roughly R times the sine of half the bevel angle (the angular portion the weld must fill). With a radius of 12 mm and a typical flare bevel geometry, that projection works out to about 0.31 × 12 mm, which is roughly 3.7 mm. Rounding gives about 3.6 mm, which matches the closest approximation.

So the correct approximate throat is about 3.6 mm because it reflects how the edge radius reduces the effective weld throat in a flare bevel groove flush to the edge. The other values would require significantly different bevel geometry or would imply an unrealistically large or tiny projection for the given edge radius.