The low carbon (mild) steels include those with a carbon content of up to 0.30 percent (fig. 7-7). In most low carbon steels, carbon ranges from 0.10 to 0.25 percent, manganese from 0.25 to 0.50 percent, phosphorous 0.40 percent maximum, and sulfur 0.50 percent maximum. Steels in this range are most widely used for industrial fabrication and construction. These low carbon steels do not harden appreciably when welded, and therefore do not require preheating or post-heating except in special cases, such as when heavy sections are to be welded. In general, no difficulties are encountered when welding low carbon steels. Properly made low carbon steel welds will equal or exceed the base metal in strength. Low carbon steels are soft, ductile, can be rolled, punched, sheared, and worked when either hot or cold. They can be machined and are readily welded. Cast steel has a rough, dark gray surface except where machined. Rolled steel has fine surface lines running in one direction. Forged steel is usually recognizable by its shape, hammer marks, or fins. The fracture color is bright crystalline gray, and the spark test yields sparks with long, yellow-orange streaks that have a tendency to burst into white, forked sparklers. Steel gives off sparks when melted and solidifies almost instantly. Low carbon steels can be easily welded with any of the arc, gas, and resistance welding processes.

How steel qualities change as carbon is added.

Copper coated low carbon rods should be used for welding low carbon steel. The rod sizes for various plate thicknesses are as follows:

Plate thickness 1/16 to 1/8 in. (1.6 to 3.2 mm) 1/8 to 3/8 in. (3.2 to 9.5 mm) 3/8 to 1/2 in. (9.5 to 12.7 mm) 1/2 in. (12.7mm) and heavier Rod diameter 1/16 in. (1.6 mm) 1/8 in. (3.2 mm) 3/16 in. (4.8 mm) 1/4 in. (6.4 mm)

NOTE

Rods from 5/16 to 3/8 in. (7.9 to 9.5 mm) are available for heavy welding. However, heavy welds can be made with the 3/16 or 1/4 in. (4.8 or 6.4 mm) rods by properly controlling the puddle and melting rate of the rod.

The joints may be prepared by flame cutting or machining. The type of preparation (fig. 7-8) is determined by the plate thickness and the welding position.

Low Carbon Steels weld preparation.

The flame should be adjusted to neutral. Either the forehand or backhand welding method may be used, depending on the thickness of the plates being welded.

The molten metal should not be overheated, because this will cause the metal to boil and spark excessively. The resultant grain structure of the weld metal will be large, the strength lowered, and the weld badly scarred.

The low carbon steels do not harden in the fusion zone as a result of welding.

Metal-Arc Welding.

When metal-arc welding low carbon steels, the bare, thin coated or heavy coated shielded arc types of electrodes may be used. These electrodes are of low carbon type (0.10 to 0.14 percent).

Low carbon sheet or plate materials that have been exposed to low temperatures should be preheated slightly to room temperature before welding.

In welding sheet metal up to 1/8 in. (3.2 mm) in thickness, the plain square butt joint type of edge preparation may be used. When long seams are to be welded in these materials, the edges should be spaced to allow for shrinkage, because the deposited metal tends to pull the plates together. This shrinkage is less severe in arc welding than in gas welding, and spacing of approximately 1/8 in. (3.2 mm) will be sufficient.

The backstep, or skip, welding technique should be used for short seams that are fixed in place. This will prevent warpage or distortion, and will minimize residual stresses.

Heavy plates should be beveled to provide an included angle of up to 60 degrees, depending on the thickness. The parts should be tack welded in place at short intervals along the seam. The first, or root, bead should be made with an electrode small enough in diameter to obtain good penetration and fusion at the base of the joint. A 1/8 or 5/32 in. (3.2 or 4.0 mm) electrode is suitable for this purpose. The first bead should be thoroughly cleaned by chipping and wire brushing before additional layers of weld metal are deposited. Additional passes of the filler metal should be made with a 5/32 or 3/16 in. (4.0 or 4.8 mm) electrode. The passes should be made with a weaving motion for flat, horizontal, or vertical positions. When overhead welding, the best results are obtained by using string beads throughout the weld.

When welding heavy sections that have been beveled from both sides, the weave beads should be deposited alternately on one side and then the other. This will reduce the amount of distortion in the welded structure. Each bead should be cleaned thoroughly to remove all scale, oxides, and slag before additional metal is deposited. The motion of the electrode should be controlled so as to make the bead uniform in thickness and to prevent undercutting and overlap at the edges of the weld. All slag and oxides must be removed from the surface of the completed weld to prevent rusting.

Carbon-Arc Welding. Low carbon sheet and plate up to 3/4 in. (19.0 mm) in thickness can be welded using the carbon-arc welding process. The arc is struck against the plate edges, which are prepared in a manner similar to that required for metal-arc welding. A flux should be used on the joint and filler metal should be added as in oxyacetylene welding. A gaseous shield should be provided around the molten base. Filler metal, by means of a flux coated welding rod, should also be provided. Welding must be done without overheating the molten metal. Failure to observe these precautions can cause the weld metal to absorb an excessive amount of carbon from the electrode and oxygen and nitrogen from the air, and cause brittleness in the welded joint.