Lincoln Electric Cast Iron and Aluminum Arc Welding Electrodes manual Guidelines for Welding Cast

GUIDELINES FOR WELDING CAST

IRON (CONT.)

blackboard chalk. After a few minutes, even cracks not visible to the naked eye are evident because the kerosene from the cracks bleeds back through the chalk.

Preparation for Welding - For weld soundness, the cracks must be properly prepared before welding. Drill a hole at the end of each crack to prevent it from extending further. Grind, chip, machine or saw the crack to create a bevel. Gas cutting or arc gouging can be used on castings that are preheated for welding. Be sure to get to the bottom of the crack. On sections more than 3/16” (4.8mm) thick, bevel the edges so the root of the joint is 1/8” (3.2mm) to 3/16” (4.8mm) wide. If the crack extends through the section, leave about a 1/8” (3.2mm) gap and 1/16” (1.6mm) land.

1/16”

1/8”

Remove surface scale by grinding wherever welds are to be placed. Be sure the work is clean and dry. Preheating drives moisture and oil out of the casting.

Types of Electrodes to Use - For machinable welds, use Softweld 99 Ni or Softweld 55 Ni. However, the fusion line between Softweld deposits and the cast iron may be too hard

to machine unless the casting is preheated over 500°F. Ferroweld deposits are not machinable.

Welding Techniques with Preheating - Although the welding of gray cast iron has been made to sound “mysterious”, it can be readily done if a few facts about the metal and how it is made are understood.

Most of the difficulty is caused by the high carbon content in cast iron - usually between 2% and 4% carbon. This fact, plus an understanding of how cast iron is made, indicates the heating-cooling cycles needed for successful welding.

A gray iron casting is made by pouring molten iron into a mold. It is allowed to cool slowly. As it cools, the carbon in the metal becomes a flaky form of graphite. This graphite gives the iron its characteristic gray color and some of its properties.

When it is welded, part of the gray iron is melted and an area adjacent to the weld is raised above a temperature called a critical temperature - about 1450°F. The mass of the casting around the weld tends to draw the heat from the weld area rapidly. If this cooling of the heat-affected zone and the weld is more rapid than it was when the casting was originally made, a highly brittle, crack-sensitive area forms.

The best method to slow the cooling rate is to preheat the casting to prevent the iron from rapidly absorbing heat from the weld area.

This requires a preheat temperature between 500° and 1200°F. Do not preheat over 1400°. Preheat temperature can easily be determined by using special heat indicating crayons.

Preheat the entire casting slowly and uniformly. Do not attempt to preheat only the area to be welded.

Always use low currents for minimum penetration and low ad-mixture. Weld with short 3” (76.2mm) to 5” (127mm) beads. Immediately after welding, peen each bead to relieve stresses. Be sure the preheat temperature is maintained until the entire welding job is completed. After welding, allow the casting to cool slowly to room temperature overnight just as it did when it was originally made. To do this, furnace cool or cover the casting with dry sand, powdered lime, or a

fireproof blanket. This will produce a strong, ductile deposit and fusion zone which is relatively free of cracks.

Welding Techniques without Preheat - Because of the size or other problems, preheating the casting is often impractical or impossible. In such cases, the cracks can still be repaired. Prepare the crack for welding with the same methods described previously.

When heat is applied suddenly to glass, it cracks. In this way, cast iron resembles glass. Cast iron should never be welded cold.

When the high preheat temperatures cannot be applied, heat the casting to about 100°F. Do not try to weld outdoors in cold weather. If the part being welded is an engine block or head, a uniform preheat can be obtained by running the engine, if possible, for a few minutes. Never heat the casting so hot that you cannot place your bare hand on it.

Make very short beads - not over

1” (25.4mm) long. Immediately peen each bead while it is hot to relieve shrinkage stresses and retard cracking. While one bead is cooling, deposit others at scattered points throughout the joint. All weld craters must be filled. Whenever possible, this is done by ending a bead by blending its crater into the start of a previously deposited bead. All beads should be deposited in the same direction. Ends of adjacent parallel beads should not line up with each other.

4	2
6	5
3	1

Let each bead cool to the point where it can be touched with a bare hand before starting an adjacent bead.

Sealing Cracks - Because of the nature of cast iron, tiny cracks tend to appear next to the weld even when