A lot of technical information has been written on the root pass. But for a beginner, it can be difficult to digest. Hopefully, we can boil it down to the basics. Most welds, whether MIG, TIG or Stick are made in multiple passes, particularly when the metal is over ¼” thick. Even on 1/8” to 3/16” metal two passes may be a good idea. The first pass is called the “root” pass. It anchors and supports all the other welds. In truth, in many cases the root weld may end up only a few thousandths thick after proper cleaning and grinding takes place, but it does serve as a foundation for the other welds.
A root pass is made on beveled material, or in filet welds to achieve 100% fusion without having to weld both sides. A proper root weld will actually melt through the other side, forming a bead on the backside of the weld. It should more or less resemble the front side of the weld in appearance. Typically it will not be as wide on the backside of the weld as the front, but the width of the bead on the backside should be consistent. Usually, it is performed with an ER 6010 or an ER 6011, which offer great penetration to achieve the fusion needed. However, these welding rods are somewhat difficult to manage, and create problems with cleaning up the welds. In this case a grinder may even be used to erase some of the root weld to make sure the slag is completely gone.
Some places are now forbidding the use of 6010/6011 welding rods, and are requiring that the weld be performed with a 7018 all the way out. That is fine, as long as it is called for by the engineer. The slag releases easily but penetration is difficult to achieve. A key hole is usually made in front of the weld with the 6011 so that you can see the penetration. But the 7018 doesn’t allow for the same kind of rod manipulation which creates the key hole, so it can be tricky. A simple stringer bead will be used. A small rod welded at the upper end of the rod’s amp range, and a good root gap are required. Only a few places are requiring this type weld now, and the 6010 is still the standard. The 6010 requires a stepping or whipping motion that moves the welding rod in and out of the weld in a forward to back motion to create a keyhole and fill the hole with the weld. Whatever the type of weld, or position, root pass welding will be necessary. Careful practice of root technique is required.