Cast iron is difficult, but not impossible, to weld. In most cases, welding on cast iron involves the repair of castings, not joining casting to other members. Improvements can be made at the foundry where the castings are produced, or can be made to correct errors discovered after the casting of this machine. Mis-cast iron parts may require welding machine improvements, such as when a hole is drilled in the wrong location. Often times, the cast iron damage repaired by welding cast iron parts Fractures are not uncommon, given the fragile nature of most cast iron.
Although there are various types of cast iron, the most common is gray cast iron, and guidelines are addressed to the type of material.
A few facts about cast iron help in understanding the welding challenges. Cast iron typically has a carbon content of 2% - 4%, roughly 10 times the steel. High carbon content causes the carbon to form graphite flakes. This graphite gives gray cast iron characteristic appearance when fractured.
When the castings are made, molten iron is poured into molds and allowed to cool slowly. When this high carbon material was allowed to cool slowly, crack free castings can be made. Considering this is helpful when welding cast iron: during and after welding, casting either be allowed to cool slowly, or should be kept cool enough that the cooling rate is not important.
Critical temperature in most cast iron is around 1450 degrees F. When the temperature conditions, which can cause cracks occur. While casting the arc will heat to temperatures above this level, it is important that the casting is held at this temperature for long periods of time.
Electrode Selection
If the part is to be machined after welding, a nickel-type electrode will be required. Use Lincoln Softweld 99Ni stick electrode to pass, high dilution welds. Softweld 55 Ni is preferred for multiple pass welds. Sometimes, through the roots put in by Softweld 99 Ni, followed by passing the contents of the Softweld 55 Ni. For welds where the machine is not required, and where welding is expected to rust like cast iron, Lincoln Ferroweld ® stick electrode can be used.
For the Heat, or Heat
In general, it is preferred to weld cast iron with preheat - and lots of it. However, another way to successfully weld cast iron is to keep it cool - not cold, but cold. Below, both methods will be described. However, once you choose a method, stick with it. Keep it hot, or cool, but do not change horses in midstream.
Welding Techniques with Preheat
Preheating the cast iron part before welding will slow the rate of cooling of the weld, and the area around the weld. It is always preferable to heat the entire casting, if possible. Typical heating temperature of 500-1200 degrees F. Do not heat over 1400 degrees F since that will put the material into a critical temperature range. Heat the part slowly and evenly.
Weld using a low current, to minimize the mixture, and residual stress. In some cases, it may be necessary to restrict the welds to small segments about 1-inch length to prevent the build up of residual stresses which can cause cracks. Weld bead peening can help in this regard as well.
After welding, allow the part to cool slowly. Wrapping the casting in an insulating blanket, or buried in dry sand, will help slow the cooling rate, and reduce the tendency to crack.
Welding Techniques without Preheat
The size of the casting, or other circumstances, may require that repairs be done without preheating. When this happens, the need to stay cool, but not cold.
Increase the casting temperature to 100 degrees F is helpful. If the part is on the machine, it is possible to run it for a few minutes to get this temperature. Do not heat the casting so hot that you can not put his bare hands on it.
Make it short, about 1 long welds. Peening after welding is important with this technique. Let the cold welding and casting. Do not accelerate the rate of cooling with water or compressed air. It is possible to weld in other areas of the casting while the previous weld cools. All craters shall be filled. Whenever possible, the beads should be stored in the same direction, and preferably the end of the bead is lined up parallel to each other.
Sealing Cracks
Due to the nature of cast iron, tiny cracks tend to appear next to the weld even when good procedures are followed. If the casting must be watertight, it can be a problem. However, leaks can usually be removed with some type of sealing compound or they may be rusted shut down shortly after returning to service.
Studding method
One method used to repair major breaks in large castings is to drill and tap holes on the surface has been tilted to receive the repair weld metal. Screw steel studs into the threaded hole, leaving 3/16 "(5 mm) to ¼" (6 mm) above the surface of the stud. Using the methods discussed above, weld studs in place and cover the entire surface of the break with weld deposit. After a good weld deposit is made, both sides of the crack can be welded together.
