Q: - How should one weld Cast Iron to Mild Steel?

A: - Cast Iron is an alloy of iron, carbon and silicon. Other elements may be added for special purposes.

• Gray Cast Iron, is probably the most common type. With slow cooling,its excess carbon solidifies as flakes of graphite. Its chief advantages are easy machinability, good damping capacity (to absorb vibrations) and relatively low cost. It is divided in further classes according to typical mechanical properties. Some types highly alloyed and with improved mechanical properties can be considered unweldable.
• Ductile Iron, due to special additions to its chemical composition, presents graphite in spheroidal form and has the highest strength and ductility of unalloyed cast irons.
• Malleable Iron is obtained with a long and specific annealing treatment that transforms iron carbides from white cast iron into irregularly shaped graphite nodules.
• White iron, that is produced by rapid cooling from the solidification temperature, contains most of its iron carbides untransformed, is very hard and brittle and practically unweldable.

The mild steels to consider should have limited content of Sulfur and Phosphorus, which are known to contribute to hot shortness or the formation of cracks at the time of solidification.

Cleanliness and weld preparation is always most important.It is known that welding might produce brittle structures and in general reduce the mechanical properties of cast iron. However successful welds can be performed for useful purposes if one acknowledges the limitation introduced by the processes.

A successful welding process should not cause the formation of cracks during or after welding and should not introduce harmful or excessive residual stresses.

There is not a single welding process capable of welding successfully any conceivable combination of iron castings and steel. Furthermore one cannot point to a single filler metal rod or electrode to cover all possible cases. Therefore the problem is not simple.

Furthermore small iron castings behave differently than large and massive cast pieces. The mass has a great influence on the self quenching capacity of the parts and on the cooling rate after welding, directly affecting the obtained structures.

Any welding process produces two zones that undergo important structural transformations:

• The weld metal is that portion of base and filler material that were melted by the welding heat, were thoroughly mixed and then solidified quite rapidly.The resulting structure is mainly a function of composition. As the dilution of cast iron into the melt contributes a large proportion of carbon that is responsible for the hard and brittle phases resulting during solidification, due attention should always be employed to melt the minimum amount of cast iron.
• The heat affected zone although not melted, was heated to high temperature by the nearby weld heat. Most of its carbon, that was in form of graphite, went into solution in the phase called austenite. Upon rapid cooling this carbon enriched austenite transforms to the hard phase martensite, which is brittle and susceptible to cracking.

To control the properties of the heat affected zone, to reduce hardness and shrinkage stresses, one has to reduce the cooling rate. This is usually achieved by preheating the iron casting, either locally with a flame, if it is very large, or preferably in a furnace. And then, after welding, by letting it cool slowly, having wrapped or buried it into insulating material, or again in a furnace.

Preheat will also control the structure of the weld metal itself to the point that the formation of martensite is minimized or avoided at all.

However, small castings can be welded sometimes without preheat if the results are acceptable. Alternative techniques consist in welding thin and short beads. Subsequent beads temper the hard structures generated by previous ones.

To reduce and redistribute residual stresses, peening of the still hot bead with a rounded ballpeen hammer should be performed. Further weld passes contribute to interpass heating that helps in preventing too rapid cooling.

In general one should try to heat to the least possible peak temperature, to introduce the minimum heat, using small electrodes and low currents, to apply suitable preheat, to control interpass temperature and to study the performance of different types of filler material until the satisfactory selection is found.

The consumables available for welding cast iron are quite varied.Filler Metals of different types can be used for welding cast iron to mild steel. The selection should be based on ease of performance and on the acceptable results achieved. Once the main factors are understood and taken care of, practice and trials can tell which is the most economic and best solution for any particular case.

When using SMAW, the steel electrode (ESt) will give a very hard weld,non machinable, useful only for very small repairs.

Standard low hydrogen electrodes like E7018 have been used successfully, provided they were dried thoroughly to minimize moisture content.Even iron powder containing electrodes like E7024 were employed with good results.

Cast Iron electrodes (E-CI) with about 2.0%C, provide a structure similar tothat of gray cast iron: the weld metal is likely to harden unless proper provisions are put in place.

For difficult cases conducive to cracks, using Ni-Fe electrodes (ENiFe-CI or ENiFe-CIA) with about 50 %Ni- 50% Fe is probably the best selection for the dissimilar welding of cast iron to mild steel, although not the most economic.

If a more ductile or machinable weld must be obtained, high nickel electrodes (more expensive) can be tried, like ENi-CI or ENi-CIA, that will result in a soft, ductile and machinable deposit.

If a more strong weld is needed, for example for nodular irons of elevated mechanical properties, ENiFeMn-CI can be used, where the addition of manganese improves strength, ductility and resistance to cracks.

The electrodes included in the AWS Specification A5.15 are not the only onesavailable. Proprietary electrodes, not classified with AWS, are available with improved properties for special applications. In difficult cases it may be worth to seek advice from electrode manufacturers, to experiment and to check results.

For certain production lines, higher deposition rates than those available by SMAW (with covered electrodes) may be preferable. In these cases GMAW (Mig) has been applied successfully, especially for ductile or malleable iron.

The wire composition is similar to that used with covered electrodes. Steel wires of types ER70S-3 and ER70S-6 have been used and also ERNiFe-CI.Also nickel containing wires (high nickel, nickel-iron, nickel-iron-manganese) are being used. All other precautions should be in place as necessary.

This presentation would not be complete without including also the solutions that employ the oxyacetylene flame. The slower heating rate of this process causes a larger heat affected zone to form but effectively avoids the development of brittle martensitic structure. Consumables are cast rods with higher levels of carbon and silicon than the castings.

RCI, RCI-A and RCI-B are used respectively for gray cast iron, higher strength alloyed iron, and for malleable and ductile iron. Suitable fluxes must be used to protect the molten metal from oxidation. Preheating must be provided. Slow cooling must be ensured.

Besides welding, in certain cases it would be useful to consider also braze-welding as a possible solution. See Braze Welding.
Here the filler metal is copper base and the cast iron is not melted. Less heat, less distortion, less cracking, machinable filler metal and generally adequate strength is provided. The most serious difficulty, that may sometime prevent its adoption, is the color mismatch of the braze-welded joint.