Cast Iron is considerably less weldable than low-carbon
steel. Cast iron contains much more carbon and silicon than
steel, with the result that cast iron is less ductile, and
is more metallurgically deformed when welded. However, there have been
many successful cast iron repair welds performed in maintenance
and casting reclamation applications. The degree of brittleness
and likelihood of cracking of the welded material will depend
on the type of casting the heat treatment and the welding
The most important aspect of welding cast iron is to have
the surface clean and free of defects prior to welding,
since castings that have been in service are likely to be
impregnated with oil or grease. All surface contaminations
should be removed with solvents, commercial cleaners, or
paint removers. Casting skin should be removed from surfaces
to be welded. Blind cracks and pits must be completely dressed
out to sound metal by mechanical means such as grinding,
chipping, rotary filling or shot blasting. Cracks should
be excavated to their full length and depth. Excavate spongy
areas and pinholes.
Impregnated oil or other volatile matter can
be eliminated by using an oxidizing oxy-acetylene flame
to heat the casting or weld groove to approximately 900
F for about 15 minutes and then wire brushing, grinding
or rotary filling to remove the residue. This method has
the advantage of de-gassing the casting and removing some
of the surface graphite as well.
New castings present less of a cleaning problem
than castings that have been in service. However, casting
skin, sand, and other foreign materials must be removed
from the joint to be welded and the adjacent surfaces of
To repair cracked castings, drill a hole at
each end of the crack to prevent it spreading further and
grind out to the bottom. Begin welding at the drilled end
of the crack, where restraint is greatest and move towards
the free end.
Casting which have to transmit fairly heavy
working loads often have the weld joint assisted by mechanical
means, such as bolt straps, or hoops which are shrunk on.
Broken teeth of large cast iron gears are sometimes repaired
by studding. Holes are dilled and tapped in the face of
the fracture and mild steel studs screwed in. These are
then covered with weld metal and build up to the required
dimensions. They are machined afterwards or ground to shape.
Precautions when welding cast irons
Factors to consider are the same whatever the type of cast
1. Low ductility with a danger of cracking due to stresses
set up by welding. (This is not so important when welding
SG iron due to its good ductility)
2. Formation of a hard brittle zone in the weld area. This
is caused by rapid cooling of molten metal to form a white
cast iron structure in the weld area and makes the weld
unsuitable for service where fairly high stresses are met.
3. Formation of a hard, brittle weld bead due to pick-up
of carbon from the base metal. This does not occur with
weld metals which do not form hard carbides such as Monel
and high nickel alloys. These are used where machinable
welds are desired.
Although much can be done without preheating,
to avoid cracking (due to lack of ductility of castings, especially complicated
may be minimized by suitable preheating.
In general all cast irons need to be
when oxyacetylene welding. This pre-heating reduces the
welding heat-input requirements.
High pre-heat is needed when using a cast iron filler
because the weld metal has low ductility near room temperature.
To avoid such pre-heating requirements, you may use Aufhauser NickelRod #99,
with the base metal
at or slightly above room temperature. The weld readily
yields during cooling and relieves welding stresses that might
otherwise cause cracking in the weld.
1. Local preheating occurs where parts not
held in restraint may be preheated to about 500°C in
the area of the weld, with slow cooling after welding is
completed. Cracking from unequal expansion can take place
during the preheating of complex castings or when the preheating
is confined to a small area of a large casting> This is
why local preheating should always be gradual.
2. Indirect preheating involves preheat of
200°C for other critical parts of the job in addition
to local preheating. This is done so that they will contract
with the weld and minimize contraction stresses. Such a
technique is suitable for open frames, spokes etc.
3. Complete preheating is used for intricate
casings, especially those varying in section thicknesses
such as cylinder blocks. It involves complete preheating
to 500°C followed by slow cooling after welding. The
preheating temperature should be maintained during welding.
A simple preheating furnace may be made of bricks into which
gas jets project. Another may be filled with charcoal which
burns slowly and preheats the job evenly.
Post weld Heating:
Post weld heat treatment may consist of either full annealing
or stress relieving: when heat treatment is not applied,
the welded casting is usually cooled slowly from the welding
temperature to room temperature by covering it with insulating
material such as lime, ground asbestos, or vermiculite.
Stress relieving at 1150°F and then furnace cooling
to at least 700°F is recommended whenever feasible.
Full annealing at 1650°F is sometimes employed to produce
greatest softening of the weld zone or a more complete stress
relied. However, annealing lowers the as-cast tensile strength
of all but the softest irons.
In critical applications that require radiographic or ultrasonic
inspection after heat treatment, castings often are inspected
before treatment also, to save unnecessary costs if an internal
defect should be present.
Satisfactory welds may be made on cast iron without preheating
by using electrodes depositing soft metals and peening the
weld with a blunt tool (such as a ball hammer) immediately
after welding. This spreads the weld metal and counteracts
the effects of contraction. Good practice is to deposit
short weld runs (50 mm at a time) and then peen before too
much cooling takes place. (Aufhauser NickelRod #99 is soft
and allows peening).
Shield metal-arc welding of cast irons
The most suitable electrodes for Shield metal-arc welding
is Aufhauser NickelRod #99 and NickelRod #55.
Grey Cast Iron
NickelRod #99 is more suitable for single layers and for
filling small defects as the deposit remains highly machinable.
Single-layered welds of NickelRod #55 are not as machinable
as NickelRod #99, however they do have increased strength
and ductility. NickelRod #55 welds are more tolerant towards
contaminants such as sulphur and phosphorous and are superior
to NickelRod #99 electrodes when welding casting high in
Peening is a must for grey cast irons.
Joining of cast iron to steel can be performed with either
cast NickelRod #55 or NickelRod #99, but NickelRod #55 is
preferred. Ferrous based electrodes, including hydrogen
controlled types are generally not recommended fro welding
cast irons. Brackets, lungs and even wear plates can be
attached to casting using the correct parameters and NickelRod #55.
Ductile cast iron
Ductile cast iron can only be repaired using NickelRod #55
due to its higher tensile strength and better ductility.
When welding ductile cast irons, penetration should be low
and wide joints or cavities should be built up fro the sides
towards the centre. Stringer beads or narrow weaves should
be used. Deposits short beads and allow cooling to preheat
temperature. Peening is advisable but not as critical as
when welding grey cast iron.
Austenitic cast irons
These are usually welded with NickelRod #55. Although austenitic
castings can be welded with NickelRod #55 the weld may be
unsuitable for applications where corrosion/hear resistance
qualities do not match the parent metal.
Cast irons are generally considered unweldable using the
FCAW welding of cast irons
Flux cored welding of cast iron is carried out using higher
current than that for Shielded metal-arc welding. This is
offset by faster travel speeds as for normal Flux Cored
Arc Welding. Both grey, ductile and malleable cast irons
can be welded using the Flux Cored Arc Welding process.
Preparation and heat treatment are much the same as for
shield. NickelRod #55 and NickelRod #99 are most suitable
for FCAW welding of cast irons.
Oxy-acetylene welding of cast irons
For successful oxy fusion welding, it is essential that
the part be pre-heated to a dull, red heat (approximately 650°C).
A neutral or slightly reducing flame should be used with
welding tips of medium or high flame velocity. The temperatures
should be maintained during welding. As with Shielded Metal
Arc Welding preparation it is necessary to use a furnace
to ensure even heating of large castings. It is important
that the casting be protected from draught during welding
and provision should be made to ensure that the required
preheat is maintained. It is important to avoid sudden chilling
of the casting otherwise white cast iron may be produced
which is very hard and brittle. This may cause cracking
or make subsequent matching impossible.
Oxy welding is suitable for grey cast irons
with an AWS A5.15 RCI (Aufhauser
RCI), RCI-A type electrode and should used with a suitable
flux such as Aufhauser Cast Iron Flux.
Austenitic cast irons can only be oxy welded
with an AWS RCI-B type consumable.
Braze welding of cast irons
Braze welding should only be used to repair old casting
because of the poor color match achieved with newer castings.
Braze welding is suitable for grey, Austenitic and malleable
cast irons, however joint strength equivalent to fusion
welds are only possible with grey cast iron. A neutral or
slightly oxidizing flame should be used.
Technical and trade information
Braze welding has advantages over oxy welding in that the
consumable melts at a lower temperature than the cast iron.
This allows lower preheat (320-400°C). As with other
forms of welding the surface must be properly cleaned so
that carbon doesn't contaminate the weld deposit.
The application consumables to use are AWS RBCuZn-C (Aufhauser
Bronze) Types and AWS RBCuZn-D (Aufhauser 773
Nickel Silver) Types.
Brazing of cast irons
Any brazing processes suitable for steel are applicable
to cast irons. Pre- and Post- braze operations should be
similar to that of a standard brazing processes.
Consumables suitable for brazing carbon steel can be used
for cast irons.
Powder Spraying of cast irons
Powder spraying is particularly suited to edges, corners,
shallow cavities and thin sections as there are usually
no undercut marks. Porous areas must be ground out o a saucer
or cup shape with no overhanging edges. Sharp corners, edges
and protruding points must be removed or radiuses as they
may go into solution in the molten metal causing hard spots.
Spraying and fusing should be as per the normal powder spraying
Poor quality or difficult irons can be joined by coating
both parts separately with 1-2 mm of spray-fused alloy and
then joining the coating together with a suitable nickel
Shielded Metal Arc Welding electrode. Consumables are based
on a nickel-silicon-boron mixture.
Soldering of cast iron is usually limited
to the repair of small surface defects, often sealing areas
from leakage of liquid or gases. The casting must be thoroughly