Weldability of materials
Cast irons
Cast irons are iron based alloys containing more than 2% carbon, 1 to
3% silicon and up to 1% manganese. As cast irons are relatively inexpensive,
very easily cast into complex shapes and readily machined, they are an important
engineering and structural group of materials. Unfortunately not all grades are
weldable and special precautions are normally required even with the so-called
weldable grades.
Material types
Cast irons can be conveniently grouped according to their structure which
influences their mechanical properties and weldability; the main groups of
general engineering cast irons are shown in the first figure.
Grey cast irons
Grey cast irons contain 2.0 - 4.5% carbon and 1 - 3% silicon. Their structure
consists of branched and interconnected graphite flakes in a matrix which is
pearlite, ferrite or a mixture of the two. The graphite flakes form planes of
weakness and so strength and toughness are inferior to those of structural
steels.
Nodular cast irons
The mechanical properties of grey irons can be greatly improved if the
graphite shape is modified to eliminate planes of weakness. Such modification is
possible if molten iron, having a composition in the range 3.2 - 4.5% carbon and
1.8 - 2.8% silicon, is treated with magnesium or cerium additions before
casting. This produces castings with graphite in spheroidal form instead of
flakes, known as nodular, spheroidal graphite (SG) or ductile irons. Nodular
irons are available with pearlite, ferrite or pearlite-ferrite matrices which
offer a combination of greater ductility and higher tensile strength than grey
cast irons.
White cast irons
By reducing the carbon and silicon content and cooling rapidly, much of the
carbon is retained in the form of iron carbide without graphite flakes. However,
iron carbide, or cementite, is extremely hard and brittle and these castings are
used where high hardness and wear resistance is needed.
Malleable irons
These are produced by heat treatment of closely controlled compositions of
white irons which are decomposed to give carbon aggregates dispersed in a
ferrite or pearlitic matrix. As the compact shape of the carbon does not reduce
the matrix ductility to the same extent as graphite flakes, a useful level of
ductility is obtained. Malleable iron may be divided into classes. Whiteheart,
Blackheart and Pearlitic irons.
Whiteheart malleable irons
Whiteheart malleable castings are produced from high carbon white cast irons
annealed in a decarburising medium. Carbon is removed at the casting surface,
the loss being only compensated by the diffusion of carbon from the interior.
Whiteheart castings are inhomogenous with a decarburised surface skin and a
higher carbon core.
Blackheart malleable irons
Blackheart malleable irons are produced by annealing low carbon (2.2 - 2.9%)
white iron castings without decarburisation. The resulting structure, of carbon
in a ferrite matrix, is homogenous with better mechanical properties than those
of whiteheart irons.
Pearlitic malleable irons
These have a pearlite rather than ferritic matrix which gives them higher
strength but lower ductility than ferritic, blackheart irons.
Weldability
This depends on microstructure and mechanical properties. For example, grey
cast iron is inherently brittle and often cannot withstand stresses set up by a
cooling weld. As the lack of ductility is caused by the coarse graphite flakes,
the graphite clusters in malleable irons, and the nodular graphite in SG irons,
give significantly higher ductility which improves the weldability.
The weldability may be lessened by the formation of hard and brittle
microstructures in the heat affected zone (HAZ), consisting of iron carbides and
martensite. As nodular and malleable irons are less likely to form martensite,
they are more readily weldable, particularly if the ferrite content is high.
White cast iron which is very hard and contains iron carbides, is normally
considered to be unweldable.
Welding process
Bronze welding is frequently employed to avoid cracking. As oxides and other
impurities are not removed by melting, and mechanical cleaning will tend to
smear the graphite across the surface, surfaces must be thoroughly cleaned, for
example, by means of a salt bath.
In fusion welding, the oxy-acetylene, MMA, MIG/FCA welding processes can all
be used. In general, low heat inputs conditions, extensive preheating and slow
cooling are normally a pre-requisite to avoid HAZ cracking.
Oxy-acetylene because of the relatively low temperature heat
source, oxy-acetylene welding will require a higher preheat than MMA.
Penetration and dilution is low but the wide HAZ and slow cooling will produce a
soft microstructure. Powder welding in which filler powder is fed from a small
hopper mounted on the oxy-acetylene torch, is a very low heat input process and
often used for buttering the surfaces before welding.
MMA widely used in the fabrication and repair of cast iron
because the intense, high temperature arc enables higher welding speeds and
lower preheat levels. The disadvantage of MMA is the greater weld pool
penetration and parent metal dilution but using electrode negative polarity will
help to reduce the HAZ.
MIG and FCA MIG (dip transfer) and especially the FCA
processes can be used to achieve high deposition rates whilst limiting the
amount of weld penetration.
Filler alloys
In oxy-acetylene welding, the consumable normally has slightly higher carbon
and silicon content to give a weld with matching mechanical properties. The most
common MMA filler rods are nickel, nickel - iron and nickel - copper alloys
which can accommodate the high carbon dilution from the parent metal and
produces a ductile machinable weld deposit.
In MIG welding, the electrode wires are usually nickel or Monel but copper
alloys may be used. Flux cored wires, nickel-iron and nickel-iron-manganese
wires, are also available for welding cast irons. Powders are based on nickel
with additions of iron, chromium and cobalt to give a range of hardnesses.
Weld imperfections
The potential problem of high carbon weld metal deposits is avoided by using
a nickel or nickel alloy consumable which produces finely divided graphite,
lower porosity and a readily machinable deposit. However, nickel deposits which
are high in sulphur and phosphorus from parent metal dilution, may result in
solidification cracking.
The formation of hard and brittle HAZ structures make cast irons particularly
prone to HAZ cracking during post-weld cooling. HAZ cracking risk is reduced by
preheating and slow post-weld cooling. As preheating will slow the cooling rate
both in weld deposit and HAZ, martensitic formation is suppressed and the HAZ
hardness is somewhat reduced. Preheating can also dissipate shrinkage stresses
and reduce distortion, lessening the likelihood of weld cracking and HAZ.
Table 1: Typical preheat levels for welding cast irons
| Cast iron type |
Preheat temperature degrees C |
|
MMA |
MIG |
Gas (fusion) |
Gas (powder) |
| Ferritic flake |
300 |
300 |
600 |
300 |
| Ferritic nodular |
RT-150 |
RT-150 |
600 |
200 |
| Ferritic whiteheart malleable |
RT* |
RT* |
600 |
200 |
| Pearlitic flake |
300-330 |
300-330 |
600 |
350 |
| Pearlitic nodular |
200-330 |
200-330 |
600 |
300 |
| Pearlitic malleable |
300-330 |
300-330 |
600 |
300 |
RT - room temperature
* 200 degrees C if high C core involved.
As cracking may also result from unequal expansion, especially likely during
preheating of complex castings or when preheating is localised on large
components, preheat should always be applied gradually. Also, the casting should
always be allowed to cool slowly to avoid thermal shock.
An alternative technique is 'quench' welding for large castings which would
be difficult to preheat. The weld is made by depositing a series of small
stringer weld beads at a low heat input to minimise the HAZ. These weld beads
are hammer peened whilst hot to relieve shrinkage stresses and the weld area is
quenched with an air blast or damp cloth to limit stress build up.
Repair of castings
Because of the possibility of casting defects and their inherent
brittle nature, repairs to cast iron components are frequently required. For
small repairs, MMA, oxy-acetylene, bronze and powder welding processes can all
be used. For larger areas, MMA or powder technique can be used for buttering the
edges of the joint followed by MMA or MIG/FCA welding to fill the groove.
- Remove defective area preferably by grinding or tungsten carbide burr. If
air arc or MMA gouging is used, the component must be preheated locally to
typically 300 degrees C.
- After gouging, the prepared area should be lightly ground to remove any
hardened material.
- Preheat the casting to the temperature given in Table 1.
- Butter the surface of the groove with MMA using a small diameter (2.4 or
3mm) electrode; use a nickel or Monel rod to produce a soft, ductile
'buttered' layer; alternatively use oxy-acetylene with a poder consumable.
- Remove slag and peen each weld bead whilst still hot.
- Fill the groove using nickel (3 or 4mm diameter) or nickel-iron electrodes
for greater strength.
Finally, to avoid cracking through residual stresses, the weld area should be
covered to ensure the casting will cool slowly to room temperature.
If you would like more information on any aspect of cast irons, contact Bill Lucas .
Copyright by TWI, 1999
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