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WELDING OF DUCTILE IRON

by
Hans Roedter, Sorelmetal Technical Services

Ductile Iron is the easiest of all the cast irons to weld. The these temperatures can have significant effects on the
feritic grades (lower strength) are more weldable than the final microstructure. There are a number of different
high strength grades. To accomplish proper welding the methods to do this using special types of electrodes
following information and suggestions should be carefully including wire and even shielding gases to minimize
complied with. oxidation.

Welding Metallurgy Equipment and Filler Materials

Since most welding operations are where one or more The variety of equipment and materials for welding is so
materials are repaired or joined together with a filler numerous that they cannot adequately be covered in this
material at a high temperature, the metallurgy of all these sheet. Please consult one of the references at the end for
materials and the temperature changes that they undergo more specific information.
are very important. We can create areas in the parent
material(s), called the heat affected zone (HAZ) and the Joint Design and Preparation
fusion zone (FZ) due to melting and then refreezing at rapid The preparation of the joint is dependent upon the design,
cooling rates. The amount of carbon in the matrix is then type of material, thickness, welding process and service
very important as to whether carbides, pearlite, and/or requirements. The decisions about these factors must be
martensite can be formed in these zones. Higher matrix made before the welding process begins. Figure 1 shows
carbon contents will usually give more problems. Careful some possible ways to prepare joints for the welding of two
control of the welding process will produce the best distinct components. All casting joint surfaces should be
possible final microstructure. freshly ground or machined so that they do not have rust,
dirt or oils on them.
We usually divide welding into two categories - repair and
fabrication. However, for both of these processes the new Thermal Cycling
area/material in the welded zone should have at least the The thermal cycle of the welding process can have an
same mechanical properties as the Ductile Iron parent effect on the structure produced and the stresses (possible
material so that the toughness and/or fatigue properties, as cracking of the weld) that are generated. All materials to be
well as the machinability of the part/assembly are welded that contain a high carbon content will also give
maintained. some problems. A range of matrix structures can be
produced depending upon the solidification rate, cooling
Welding Processes rate and chemistry of the parent and filler metals during
There are two basic types of welding processes and a welding. However, cracking of a weld is a function more of
number of specialized types under these categories. composition and temperature rather than of microstructure.
Figure 2 shows the relationship between carbon equivalent
1) Oxyfuel Welding (OFW) can be done with oxygen & (CE) and preheating temperatures to avoid cracks in the
acetylene gas fired torch, a Ductile Iron filler rod and welded zone.
some flux to reduce the oxidation/slag products formed
during welding. The process involves melting the area Preheating and slow cooling of the parts can make the
on the casting and the filler material with the torch, welding operation easier. In addition to reducing thermal
forming a molten pool and then allowing the mass to cracking, the hardness and residual stresses can also be
solidify and slowly cool. reduced giving very similar mechanical properties in the
HAZ and FZ. Care must be taken to avoid rapid and non-
2) Arc Welding consists of melting the parent material and uniform heating. Slow cooling may be obtained by using
an electrode with the heat generated by an electrical thermal blankets to retain heat.
arc produced between them. Very high temperatures
o o
(up to 9000 F (5000 C)) produce rapid melting, but
Weld Deposit Microstructure For more detailed information on specific welding
The weld deposit may be graphite free or contain some processes and techniques as well as examples of
nodules or flake graphite. Spheroidal (nodular) graphite fabrication and repair, the reader is referred to the following
structures can be obtained with most methods, but this will publications:
be determined by the filler material and the welding
Guide for Welding Iron Castings, American Welding
technique. If there is any unwanted cementite or
Society, 1988.
martensite in the weld, a heat treatment may have to be
done to change the microstructure. Preheating of the parts Unraveling the Mysteries of Welding Cast Iron, S.D. Kiser
can help to prevent them from forming. and R. Irving, Welding Journal, August 1993.
Volume 1, Ductile Iron Production, QIT - Fer et Titane,
After welding the welded area should be free of defects 1992.
including gas holes and slag inclusions.
Note: QIT Technical Service Representatives are there
Today you can find many examples where a Ductile Iron to answer questions about welding as well as any
casting has been welded to form a fabrication with another others about Ductile Iron.
Ductile Iron part or with a piece of steel. This is especially
true in the car manufacturing industry. Using welding
processes in this way can expand the market for Ductile
Iron castings into areas previously reserved only for steel.

Figure 1. Joint Design Improvements. Figure 2. Relationship between Carbon Equivalent and No-Crack
Temperature for Cast Irons.

REV – March 2006