How to Braze Cast Iron
Cast iron (gray,white and malleable) is a man made alloy of iron, carbon, and silicon. A
portion of the carbon exists as free carbon or graphite. Total carbon
content is between 1.7 and 4.5 percent.
Cast iron is used for water pipes, machine tool castings,
transmission housing, engine blocks, pistons, stove castings, etc.
The metal can be brazed or bronze welded, gas and arc welded, hardened, or machined. In terms of limitations, the cast iron must be preheated prior to welding. It cannot be worked cold.
Often brazing is the best approach for repairing cast iron, particularly when the parts need to be rebuilt and then machined to the right tolerance.
If you want to braze cast iron, melting the base metal is not necessary since the rod has a built-in flux.
Clearance should be around .003 for maximum tensile and shear strength.
The rod takes extreme heat, above 1400 degrees to penetrate the pores of the metal.
The rod required to braze cast iron welds all cast iron, brass,
nickel, copper, and more than 50 different kinds of steel, including
Open the pours around the area to be repaired, drill, and grind out a U shape in the metal, so it is no longer a hairline crack.
Practice with cast iron by grinding a slot 1/16th to 1/8th of an inch
into the metal. Use a brazing or cutting tip to bring the metal to 1400
The metal will start to turn red at 600 degrees, but continue heating.
Test the rod on the metal 1/4 inch outside the bulk of the flame.
The rod has a flux core that will melt much lower than the rod, so if the rod itself will not melt, have patience.
When you are close to 1400 degrees, it will look like you're going to burn a hole in the metal but do not worry.
Once you see the rod itself is melting, scratch the rod all around the area to be repaired.
The rod will melt and, with the help of the flux core, will fuse down into the metal.
When you pull the torch, it will be cherry red for about 20 seconds.
If you have followed the instructions explicitly and scratched
the rod all around the area to be repaired thoroughly, you will have the
strongest weld possible on that particular metal.
You may then grind, shape, polish, and paint as desired.
For More information see this video of how to braze cast iron and the rod you need.
Mig Cast Iron Weld
Photo Credit: Docs Machine
Cast Iron Properties
Cast iron has a Brinell hardness number of 150 to 220
(no alloys) and 300 to 600 (alloyed); tensile strength of 25,000 to
50,000 psi (172,375 to 344,750 kPa) (no alloys) and 50,000 to 100,000
psi (344,750 to 689,500 kPa) (alloyed); specific gravity of 7.6; high
compressive strength that is four times its tensile strength; high
rigidity; good wear resistance; and fair corrosion resistance.
Agriculture combine part being rebuilt using cast iron brazing process with machining to the right specification.
Photo Credit:Paul Abshire Welding Works
Gray Cast Iron
If the molten pig iron is permitted to cool
slowly, the chemical compound of iron and carbon breaks up to a certain
extent. Much of the carbon separates as tiny flakes of graphite
scattered throughout the metal. This graphite-like carbon, as
distinguished from combined carbon, causes the gray appearance of the
fracture, which characterizes ordinary gray cast iron.
Since graphite is
an excellent lubricant, and the metal is shot throughout with tiny,
flaky cleavages, gray cast iron is easy to machine but cannot withstand a
heavy shock. Gray cast iron consists of 90 to 94 percent metallic iron
with a mixture of carbon, manganese, phosphorus, sulfur, and silicon.
Special high-strength grades of this metal also contain 0.75 to 1.50
percent nickel and 0.25 to 0.50 percent chromium or 0.25 to 1.25 percent
Commercial gray iron has 2.50 to 4.50 percent carbon. About
1 percent of the carbon is combined with the iron, while about 2.75
percent remains in the free or graphitic state. In making gray cast
iron, the silicon content is usually increased, since this allows the
formation of graphitic carbon. The combined carbon (iron carbide), which
is a small percentage of the total carbon present in cast iron, is
known as cementite.
In general, the more free carbon (graphitic carbon)
present in cast iron, the lower the combined carbon content and the
softer the iron.
Tests for Gray Cast Iron
The unmachined surface of gray cast iron
castings is a very dull gray in color and may be somewhat roughened by
the sand mold used in casting the part. Cast iron castings are rarely
machined all over. Unmachined castings may be ground in places to remove
Nick a corner all around with a chisel or
hacksaw and strike the corner with a sharp blow of the hammer. The dark
gray color of the broken surface is caused by fine black specks of
carbon present in the form of graphite. Cast iron breaks short when
fractured. Small, brittle chips made with a chisel break off as soon as
they are formed.
A small volume of dull-red sparks that follow a
straight line close to the wheel are given off when this metal is spark
tested. These break up into many fine, repeated spurts that change to a
The torch test results in a puddle of molten metal
that is quiet and has a jelly like consistency. When the torch flame is
raised, the depression in the surface of the molts-puddle disappears
instantly. A heavy, tough film forms on the surface as it melts. The
molten puddle takes time to harden and gives off no sparks.
Brazing Cast Iron Manifold
White Cast Iron
When gray cast iron is heated to the molten
state, the carbon completely dissolves in the iron, probably combining
chemically with it. If this molten metal is cooled quickly, the two
elements remain in the combined state, and white cast iron is formed.
The carbon in this type of iron measures above 2.5 to 4.5 percent by
weight, and is referred to as combined carbon. White cast iron is very
hard and brittle, often impossible to machine, and has a silvery white
Malleable Cast Iron
Malleable cast iron is made by heating
white cast iron from 1400 to 1700°F (760 and 927°C) for abut 150 hours
in boxes containing hematite ore or iron scale. This heating causes a
part of the combined carbon to change into the free or uncombined state.
This free carbon separates in a different way from carbon in gray cast
iron and is called temper carbon. It exists in the form of small,
rounded particles of carbon which give malleable iron castings the
ability to bend before breaking and to withstand shock better than gray
cast iron. The castings have properties more like those of pure iron:
high strength, ductility, toughness, and ability to resist shock.
Malleable cast iron can be welded and brazed. Any welded part should be
annealed after welding.
The surface of malleable cast iron is very
much like gray cast iron, but is generally free from sand. It is dull
gray and somewhat lighter in color than gray cast iron.
When malleable cast iron is fractured, the
central portion of the broken surface is dark gray with a bright,
steel-like band at the edges. The appearance of the fracture may best be
described as a picture frame. When of good quality, malleable cast iron
is much tougher than other cast iron and does not break short when
When malleable cast iron is ground, the outer,
bright layer gives off bright sparks like steel. As the interior is
reached, the sparks quickly change to a dull-red color near the wheel.
These sparks from the interior section are very much like those of cast
iron; however, they are somewhat longer and are present in large volume.
Molten malleable cast iron boils under the torch
flame. After the flame has been withdrawn, the surface will be full of
blowholes. When fractured, the melted parts are very hard and brittle,
having the appearance of white cast iron (they have been changed to
white or chilled iron by melting and fairly rapid cooling). The outside,
bright, steel-like band gives off sparks, but the center does not.
Video of How To Braze a Cast Iron Exhaust Manifold
References for Braze Cast Iron
Author: Jeff Grill