For example, if the thickness of the plates or pipe sections is more than a quarter inch, one or both sides may have to be beveled before the weld. (Beveling means cutting pieces diagonally.) This procedure is similar to mitering and provides more surface area for fitting the sides together. Sections may also be reshaped to provide a secure pocket for depositing weld metal, so it won't spill out.
In structural welding, a backing plate may be attached to the back side of joint to stop the through-flow of molten metal. Conversely, in pipe welding, an opening between the two sides is usually left that way, allowing the welder to weld through to the back and leave a weld bead on the outside.
With these different objectives in mind, there are six basic types of welds performed in the shop and field that every student should learn.
A fillet is a closed weld, which means the base metal is not cut through to accommodate weld metal. In most cases, no beveling or orther extra steps are needed before welding, just good fit-up and clean surfaces for welding. Fillets are commonly performed on two plates where one is perpendicular to the other, as shown in the drawing above.
Here the two sides to be welded require full penetration in order to achieve a strong connection. The gray area in the photo shows the place where the weld is performed. (Although it's not shown, you may recall from the anatomy section that both sides of the base metal must be fused together, creating a fusion zone, with reinforcement at the top and bottom.) The drawing does show that both sides are beveled to widen the gap, and that an opening is left at the bottom of the plates to be joined. This makes for a nice pocket, or groove, to hold the molten metal as it gets deposited.
Tack welds are standard practice in all welding. Since metal expands or distorts as it gets heated, it's essential to anchor a joint with quick spot welds. This is known as tacking. When the permanent weld is performed, it's also important to remelt those tacks as you lay down your first bead. Otherwise cracks or holes may end up in the weld.
This is a standard practice when welding a beveled groove and other large joint. Because one bead of weld can't do the job, the welder lays down a series of passes, using the order indicated in the drawing. Notice how the circles overlap. To prevent any missed gaps in the weld, the beads must be "wedded" together. In addition, a multipass weld has a tempering affect on the heat-affected zone. When you study metallurgy you'll learn that bringing metal to high heat in this manner causes grain refinement in the steel microstructure.
There are a few terms associated with multipass welding you should remember. In a groove weld, for example, the first bead deposited is called the "root pass". The second bead (in pipe welding) is known as a "hot pass", which means it should be performed within five minutes of the root pass. Subsequent beads are known as "fill" passes. The last pass or passes (at the top) are known as "cap" or "cover" beads.
While not an everday task in most shops, welders are sometimes called upon to join metal plates together with plug welds. This involves drilling or gouging one or more holes on the interior of the work plates, then depositing weld metal into the openings.
Similar to a plug weld, the slot weld involves a longer, narrower opening but achieves the same goal of fastening two pieces of metal together on the inside, rather than on the edges.
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