WELDING, BRAZING, DIFFUSION BONDING, ADHESIVE BONDING
Although the complex shapes produced by the casting process have enabled castings to
replace many fabricated components, there are many applications in which, for economic or
engineering reasons, castings themselves become part of a fabrication and are joined to
other castings or other materials. Although often more cost-effective than steel castings
and forgings, Ductile Irons have not been used in some applications requiring joining by
welding because they have been considered difficult to weld. This poor weldability of
Ductile Iron is partly fact but primarily misconception. When Ductile Iron castings are
repaired or joined by fusion welding their high carbon content can cause the formation of
carbides in the fusion zone (FZ) and martensite in both the FZ and heat affected zone
(HAZ) adjacent to the FZ. The formation of hard brittle phases in the FZ and HAZ can cause
a significant deterioration in both machinability and mechanical properties.
Following an investigation into the weldability of various types of
cast irons, the American Welding Society Committee on Welding Cast Irons has developed
both a weldability test and a set of recommended practices for welding cast irons. The
weldability test consists of the production of carefully controlled autogenous welds (an
autogenous weld is one made without filler metal) on test castings preheated to various
temperatures and the determination of a minimum temperature, called the "no-crack
temperature" above which there is no cracking in the test weld. The committee found
no correlation between the no-crack temperature and the carbon equivalent (CE) formula
used to determine the weldability of steels and the following formula for CE was
CECI = %C + 0.31 (%Si) + 0.33 (%P) + 0.45 (%S) +
0.028 (%Mn + %Mo + %Cr) - 0.02 (%Ni) - 0.01 (%Cu)
Figure 8.1 shows that there is a good
correlation between CEC, and the no-crack temperature for Gray, Ductile and Malleable
irons. The autogenous welding method used to obtain this correlation was chosen to
simplify and standardize test procedures and is not considered good welding practice for
cast irons. For this reason CECI should be used only to rank weldability rather than
determine either absolute weldability or specific preheating conditions. Through the use
of welding practices and consumables described in the Guide for Welding Castings
and other references used in this Section, Ductile Iron castings have been joined
successfully to other Ductile Iron castings and to steel in the fabrication of automotive
and other engineering components. In addition, nonfusion joining processes such as
brazing, diffusion bonding and adhesive bonding can be used to produce high quality joints
between Ductile Iron and a wide variety of other materials.
Welding involves the
fusion of both a filler metal (welding consumable) and the base metal
adjacent to the weld zone. The high carbon content of Ductile Iron can
lead to the formation of carbides in the fusion zone (FZ) and martensite
in both the FZ and heat affected zone (HAZ) adjacent to the FZ unless
correct procedures are followed. However, with the use of appropriate
materials and procedures, Ductile Iron castings can be successfully
joined to other Ductile Iron castings and to steel by fusion welding.
Several methods have been
employed successfully to arc-weld Ductile Iron to itself and other
materials with acceptable properties in both the weld and base metal.
The properties of shielded metal arc welded Ductile Irons were greatly
improved by the introduction over 30 years ago of the high-Ni and Ni-Fe
electrodes (AWS Ni-CI and ENi-Fe-CI). These electrodes produce
high-nickel fusion zones that are relatively soft and machinable but
have adequate tensile strength, ductility and fatigue strength. The
short arc, or dip transfer MIG welding process, by virtue of its
controlled, low heat input, reduced harmful structural changes in the
base metal HAZ. Combining the benefits of Ni-base filler wire with the
short-arc MIG process has resulted in welds with tensile properties that
are equivalent to the base Ductile Iron (Table 8. 1)
and fatigue strengths that are 65% and 75% respectively of the fatigue
limits of unwelded pearlitic and ferritic Ductile Irons (Figures
8.2 and 8.3). Although suffering
from the disadvantages of high consumable costs, low deposit rate
(1.8-3.2 kg/h (4-7 lb/h)) and a tendency toward lack-of-fusion defects,
short-arc MIG welding has been used successfully for the joining of
Ductile Iron castings for commercial applications. Recent work at BCIRA
has shown that short-arc MIG welds made with high Ni filler wire have
Charpy fracture energies that are superior to those of MIG-welded joints
made with Ni-Fe and Ni-Fe-Mn wires and flux-core arc welded joints
produced with Ni-Fe wire.
8.1 Average transverse tensile properties of short-arc mig-welds
between 25mm (1 inch) thick plates.
in 50 mm (2 in.)
|1 - 3
Cored Arc Welding
Flux cored arc welding (FCAW),
utilizing a flux cored wire developed specially for the welding of cast
irons, has improved upon the metallurgical advantages provided by the
Ni-rich consumables and offers the additional advantage of much higher
metal deposit rates (6-9 kg/h (13-20 lb/h)). The key to the success of
the FCAW process is the consumable, marketed under the trade name
"Ni-Rod FC55", which consists of a nickel-iron tubular wire
filled with carbon, slagging ingredients, and deoxidizers. In addition
to the advantages offered by the high nickel content, Ni-Rod FC55
provides the additional benefits of a high carbon content, which produce
graphite precipitates during the solidification of the weld metal. It
has been claimed that the expansion resulting from the formation of
graphite counteracts weld-metal shrinkage, reducing stress-induced
cracking of the weld. The high productivity of the FCAW method, and the
good mechanical properties of welded joints (Table
8.2) have resulted in its use in the production of critical, high
volume automotive components such as drive shafts,
"half-shafts" and wheel spindles on off-road vehicles. This
ability to economically produce high quality welds has given foundries
the added freedom to employ cast-weld techniques for the production of
nickel-iron-manganese alloy, "Ni-Rod 44", with a nominal
composition of 44 % Fe, 44 % Ni and 11 % Mn was developed to further
reduce the risk of cracking in the HAZ. Available as both filler wire
and manual electrodes, Ni-Rod 44 has been evaluated using various
welding procedures on both ferritic and pearlitic Ductile Irons. Table
8.3 shows that Ni-Rod 44 welded joints have good strengths but lower
ductility, compared to MIG-welded joints produced with a high-nickel
8.2 Mechanical properties of joints welded with
flux-cored wire Ni-Rod FC55: base material ASTM grade 60/45/10
of area %
Properties of welds made between 19mm (0.7 in.) thick Ductile iron
a 44% FE-44% Ni-11% Mn filler wire.
in area %
||Argon 2% O2
||75% Ar - 25% CO2
*SAW: submerged arc welding
tTrademark of Linde (Union Carbide).
MMA: manual metal arc
Gas welding can be used to join Ductile
Iron components by the creation of either fusion or diffusion bonds. Gas
fusion welding is a well established welding method for joining Ductile
Iron. The process simply involves fusion of the base metal and filler
rod by heat generated from an oxyacetylene flame. The weld pool is
constantly being fluxed. When Ductile Iron filler rods are used and
fluxes of suitable composition (usually incorporating cerium and/or
other rare earth elements) are used the weld deposit solidifies like a
Ductile Iron, with the formation of graphite spheroids. Successful gas
fusion welding depends upon controlled preheating of the workpiece,
maintenance of a controlled and well fluxed weld pool and the use of
suitable consumables. The major disadvantages of this process are low
productivity, dependence on operator skill and the distortion of complex
castings by excessive heat input. However, when correct procedures and
materials are employed, gas fusion welding can produce joints with
strength and ductility properties comparable to the base metal.
Powder welding is a non-fusion
form of gas welding in which a modified oxyacetylene torch serves as
both a powder supply and heat source. The melting point of the deposited
powder is below that of the base iron and when the base iron surface
reaches a certain temperature, the deposited powder coating melts and
"wets" the casting surface. Subsequently the weld is built up
as the preheated powdered alloy continuously melts as it impinges on the
wetted surface. Powder welding does not fuse the base iron and the
success of the weld is determined by the development of a diffusion
bond. Powder welding has several limitations. It is slow, expensive and
is restricted to horizontal welding. Although the casting is not heated
to its melting point, sufficient heat may be applied to cause distortion
in complex castings. Powder welding is used for defect repair, cladding
and joining high alloy irons. Work at BCIRA has shown some promise for
the joining of ferritic and pearlitic Ductile Irons.
Selection of the correct welding
procedure and consumable is a necessary but not sufficient condition for
the production of high quality welds in Ductile Iron. Other critical
- type and composition of the base
- design and preparation of the welded
- control of the thermal history of
the component before, during and after welding.
Type and Composition
Although the cast iron
weldability test indicates that the "no-crack temperature" is
related to composition but not microstructure (Figure
8. 1), ferritic Ductile Irons are generally considered to have the
highest weldability of all grades of Ductile Iron. Composition
influences weldability primarily through CECI - the higher CECI is, the
more susceptible the casting is to cracking. Composition also affects
weldability through its influence on the hardenability of the HAZ.
Manganese and chromium strongly increase hardenability, which reduces
weldability through the increased tendency to form martensite in the HAZ.
Although silicon increases hardenability slightly, this effect on
weldability is offset by the strong graphitizing effect of silicon,
which improves weldability by reducing carbide formation.
Design and Preparation
The design of a welded joint is
dependent upon factors such as metal thickness, casting geometry,
welding process and service requirements. Whenever possible, the design
should ensure that the components being joined, rather than the weld,
carry most of the load. With a welded assembly the designer can often
position the weld in an area of low stress. Figure
8.4 provides examples of joint designs which have been improved to
reduce joint stress and increase weld penetration, while Figure
8.5 illustrates recommended joint designs for both welding and
brazing. To ensure sound, gas-free welds, the casting skin adjacent to
the joint should be removed and the joint surfaces should be freshly
ground or machined and any scale, rust, dirt, grease and oil removed.
When practical, the casting
should be preheated in order to prevent thermal cracking, reduce
hardness in the HAZ and reduce residual stresses and distortion. It is
preferable that the entire casting be preheated but when casting size or
the lack of facilities makes this impractical, castings can be preheated
with burners or an oxyacetylene torch. When local preheating methods are
employed, extreme care is required to avoid rapid, non-uniform heating
to avoid cracking and distortion in complex castings. Ferritic Ductile
Irons require only a mild preheating in the range 300-400oF
(150-200oC). Pearlitic Ductile Iron requires higher
preheating temperatures, 600-650oF (315-340oC).
Low heat input welding methods such as short-arc MIG minimize the
harmful effects of the HAZ. Post-weld thermal treatments such as slow
cooling and postheating may be required to reduce residual stresses.
Depending upon service requirements, the welded assembly may be
subjected to annealing or normalizing heat treatments to dissolve
carbides and produce the desired mechanical properties.
The formation of less than
optimum microstructures in both the FZ and HAZ during the fusion welding
of Ductile irons makes non-fusion joining techniques attractive
alternatives. Brazing is "the joining metals by the fusion of
non-ferrous alloys that have melting points above 800oF (425oC)
but lower than those of the metals being joined". During the
brazing process the melted filler metal flows by capillary action into a
narrow gap between the components and solidifies to form a bond. Brazing
is related to soldering, braze-welding, and powder-welding, but is
distinguished from these processes either by the type and melting range
of the filler metal or by the design of the joint (Table
8.4 Non fusion joining processes.
(or braze welding)
melting-range of filler-
Large gap or
Large gap or
Joint Design and
Unlike welded joints, the joint-gap for
brazing (Figure 8.5) is narrow and of
controlled thickness to maximize joint strength, induce penetration of
the brazing alloy by capillary flow, and reduce the amount of brazing
alloy consumed. The joints should preferably be designed to operate in
compression or shear. Although brazed joints can have excellent
mechanical properties under pure tensile loading, any bending moment
will severely reduce the mechanical properties. Ductile Iron should be
prepared for brazing by removal of the casting skin, roughening of the
surface with an abrasive, degraphitization of the joint surfaces with an
oxidizing oxy-acetylene flame or a salt bath and degreasing and cleaning
with a suitable solvent.
The choice of a heating method
for brazing depends on the component size, joint design, brazing alloy,
and production rate. Brazing Torches can be hand operated, which is
flexible but requires considerable operator skill, or used as fixed heat
sources in a mechanized brazing line. Induction brazing is a rapid and
reproducible heating method generally used on long production runs.
Batch or continuous furnaces are frequently used when the entire
component is heated to the brazing temperature. Brazing furnaces may
have inert or reducing atmospheres or a vacuum to prevent oxide
formation on both the workpiece and brazing alloy, or an air atmosphere
may be used, in which case a brazing flux is required.
Diffusion bonding, in which both
similar and dissimilar metals can be joined by solid state diffusion
processes, can be used to overcome the microstructural problems related
to fusion welding while providing a joint that is significantly stronger
than that produced by other non-fusion processes. The use of a Ni foil
varying in thickness from 10-100 um (0.0004-0.004 in.), with bonding
temperatures and times of 820oC (1510oF) and 30
minutes has resulted in bonds between Ductile Iron and carbon steels
with exceptional mechanical properties. These bonds have impact
properties and endurance ratios equal to the base Ductile Iron (Figures
8.6 and 8.7) and a joint
efficiency (ratio of the tensile strength of the joint to that of the
base metal) which decreases from 98% to 92% as the strength of the
Ductile Iron increases from 400 MPa (58 ksi) to 700 MPa (100 ksi).
Adhesive bonding is being used
increasingly for the joining of engineering materials, especially sheet
metals. In addition to the elimination of structural changes in the base
metal, the absence of heat input in adhesive bonding also eliminates the
problem of distortion and permits the bonding of Ductile Iron to a wide
variety of metallic and non-metallic materials, regardless of their
melting points or physico-chemical properties. The most common adhesives
used in structural metal-to-metal bonds are: anaerobics, toughened
acrylics and epoxy resins. Adhesive bond strengths are significantly
lower than the strength of ferritic Ductile Iron and as a result,
careful consideration must be given to joint design in applications in
which strength is a requirement. Figure
8.8 illustrates typical examples of adhesive joints. Enhanced joint
performance can be obtained through specialized joint designs which
convert tensile and shear stresses into compressive stresses. Other
limitations to the use of adhesive joints are their limited operational
temperature range and a general lack of data on the performance of
adhesive bonding in long term applications involving different loading
and environmental conditions.
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