Cast iron is difficult, but not impossible, to weld. In most
cases, welding on cast iron involves repairs to castings, not
joining casting to other members. The repairs may be made in the
foundry where the castings are produced, or may be made to
repair casting defects that are discovered after the part is
machined. Mis-machined cast iron parts may require repair
welding, such as when holes are drilled in the wrong location.
Frequently, broken cast iron parts are repaired by welding.
Broken cast iron parts are not unusual, given the brittle nature
of most cast iron.
While there are a variety of types of cast iron, the most common
is gray cast iron, and these guidelines are directed toward this
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 as much as most steels. The high carbon content
causes the carbon to form flakes of graphite. This graphite
gives gray cast iron its characteristic appearance when
When castings are made, molten iron is poured into a mold and
allowed to slowly cool. When this high carbon material is
allowed to cool slowly, crack free castings can be made.
Remembering this is helpful when welding cast iron: during and
after welding, the casting must either be allowed to cool
slowly, or should be kept cool enough that the rate of cooling
is not important.
A critical temperature in most cast iron is about 1450 degrees
F. When at this temperature, conditions that can lead to
cracking occur. While the arc will heat the casting to
temperatures above this level, it is important that the casting
not be held at this temperature for long periods of time.
If the part is to be machined after welding, a nickel-type
electrode will be required. Use Lincoln Softweld® 99Ni stick
electrode for single pass, high dilution welds. Softweld 55 Ni
is preferred for multiple pass welds. Sometimes, root passes are
put in with Softweld 99 Ni, followed by fill passes with
Softweld 55 Ni. For welds where machining is not required, and
where the weld is expected to rust like the cast iron, Lincoln
Ferroweld® stick electrode can be used.
To Heat, or not to Heat
In general, it is preferred to weld cast iron with preheat--and
lots of it. But, another way to successfully weld cast iron is
to keep it cool--not cold, but cool. Below, both methods will be
described. However, once you select a method, stick with it.
Keep it hot, or keep it cool, but don't change horses in the
middle of the stream.
Welding Techniques with Preheat
Preheating the cast iron part before welding will slow the
cooling rate of the weld, and the region surround the weld. It
is always preferred to heat the entire casting, if possible.
Typical preheat temperatures are 500-1200 degrees F. Don’t heat
over 1400 degrees F since that will put the material into the
critical temperature range. Preheat the part slowly and
Weld using a low current, to minimize admixture, and residual
stresses. In some cases, it may be necessary to restrict the
welds to small, approximately 1-inch long segments to prevent
the build up of residual stresses that can lead to cracking.
Peening of weld beads can be helpful in this regard as well.
After welding, allow the part to slowly cool. Wrapping the
casting in an insulating blanket, or burying it in dry sand,
will help slow cooling rates, and reduce cracking tendencies.
Welding Techniques without Preheat
The size of the casting, or other circumstances, may require
that the repair be made without preheat. When this is the case,
the part needs to be kept cool, but not cold.
Raising the casting temperature to 100 degrees F is helpful. If
the part is on an engine, it may be possible to run it for a few
minutes to obtain this temperature. Never heat the casting so
hot that you cannot place your bare hand on it.
Make short, approximately 1” long welds. Peening after welding
is important with this technique. Allow the weld and the casting
to cool. Do not accelerate the rate of cooling with water or
compressed air. It may be possible to weld in another area of
the casting while the previous weld cools. All craters should be
filled. Whenever possible, the beads should be deposited in the
same direction, and it is preferred that the ends of parallel
beads not line up with each other.
Because of 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 water tight, this can be a problem. However,
leaking can usually be eliminated with some sort of sealing
compound or they may rust shut very soon after being returned to
The Studding Method
One method used to repair major breaks in large castings is to
drill and tap holes over the surfaces that have been beveled to
receive the repair weld metal. Screw steel studs into the
threaded holes, leaving 3/16” (5 mm) to ¼” (6 mm) of the stud
above the surface. Using the methods discussed above, weld the
studs in place and cover the entire surface of the break with
weld deposit. Once a good weld deposit is made, the two sides of
the crack can be welded together.
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