Weld Metal Volume, Single-V Groove

The cross-section of a single-V groove is a rectangle and two triangles. The rectangle is the root gap running the full thickness of the plate. The triangles are what the bevel opens up on each side, and together they come to the thickness squared times the tangent of half the included angle. Multiply the area by the length of joint and you have the volume of weld metal the joint will swallow — the starting point for every consumable estimate and every argument about bevel angle.

Notice that the thickness is squared in the bevel term. Doubling the plate thickness roughly quadruples the weld metal, and everything expensive about heavy-plate welding follows from that one exponent: the arc time, the consumable bill, the number of passes, the total heat into the joint, the distortion, the inspection. It is also why double-V, U and J preparations exist. A double-V welded from both sides halves the depth each bevel has to reach, and because of the square that is a large saving; a U or J preparation replaces the straight bevel with a narrow curved groove that keeps the volume nearly constant with depth instead of growing with it. Both cost more to prepare and both pay for themselves somewhere above a thickness the fabricator learns by experience.

The included angle is the term people are most tempted to trim, and the arithmetic rewards them for it: on 12 mm plate, going from 60° to 50° saves about a seventh of the weld metal. But the angle is not free to choose. It is set by the access the process needs to fuse the root and both sidewalls. Too narrow and you get lack of sidewall fusion — the most dangerous defect on the list, because it is planar, it lies along the direction of stress, and a straightforward radiograph taken perpendicular to the plate can look right through it and see nothing. Narrow-groove welding is a real and highly economical technique, but it is a qualified process with its own consumables, its own torch geometry and its own procedure, not simply a smaller number written on the bevel machine.

The root gap contributes a plain rectangle, so its effect is linear rather than squared — but on thick plate that rectangle is still a lot of metal, and a gap that opens during tacking or from thermal movement goes straight onto the bill. It is also the dimension with the tightest tolerance in a qualified procedure: too tight and the root will not fuse, too open and it burns through or needs backing, and either way you are outside the range the procedure was qualified over.

What the formula does not count. There is no root face here, and a real preparation usually leaves a land, which subtracts a small rectangle. There is no cap reinforcement, which adds a crown above the plate surface and commonly runs 10 to 20 % of the total on a multi-pass weld. There is nothing for a back gouge, whose metal is deposited and then ground out and deposited again. And it assumes straight bevel faces. Between the reinforcement and the gouging, an estimate from this equation alone is usually low. Estimate generously, and weigh one real joint before trusting the figure for a hundred of them.

The last step is the useful one: convert the volume to a mass of deposit using the filler's density, divide by the deposition efficiency to get the mass of consumable to buy, and divide the deposit mass by a deposition rate to get arc time. Then divide the arc time by an operating factor to get shop hours. Each of those divisions makes the number worse, and every one of them is real.