NHML Resources - Welding Cast Iron

Richard Avery

Proper in-house repair of cast iron parts can save you money and time. Unlike welding low carbon steel, the welding of cast iron requires special training. We have highlighted some tips on welding cast. Invest time in evaluating your material and planning your work. This investment will result in a reliable part repair.

Determining the Metal

The type of material determines how we approach the repair. A chemical analysis and metallographic check of the structure is the only positive identification but this may delay your schedule.

Check your engineering or purchasing records. They may specify the metal. If not, is it magnetic? If so, and it's a structural casting, then it is likely to be grey cast iron, ductile case iron, low carbon steel or low alloy casting. The application may offer additional insight.

Grey cast iron is particularly good for compressive loads such as machine bases. Where higher tensile loads prevail, ductile cast iron or steel castings are usually used. While ductile cast iron can be welded very successfully, steel castings are more widely used for welded applications. Therefore, if the casting has been installed by welding (e.g. valve in a pipe line), the odds are high that it is a steel casting.

Grey cast iron gets its name from the characteristic grey color of a fractured surface. A freshly fractured surface of ductile iron or cast steel has a bright, lighter color.

Grey cast iron has unique damping properties, a feature that is readily apparent when the metal is struck. Grey iron gives a dull, thud-like sound versus a clear ring like most steels. The last check is to compare the unknown cast steel part with known grey iron, ductile iron and cast steel parts. Use these check points, plus other common techniques, like the grinding wheel spark test, to compare the materials and identify the materials.

White cast iron, Ni-Hard and other extremely hard castings used for wear resistant applications are usually considered un-weldable, although some repairs are possible with special techniques. These alloys can be identified by metallographic examination or by a hardness test. The special welding procedures required for these alloys are not covered in this discussion.

Preparing the Part

Assuming the metal is grey cast iron, the casting must be prepared. A crack ending in solid metal must be prepared to prevent further propagation. Two techniques are commonly used: drilling the end of the crack or running a weld bead at the end and at right angles to the crack.

Joint preparation may include chipping, grinding, air carbon arc gouging or similar techniques. If arc gouging is used, the arc surface should be removed by grinding or chipping prior to welding. Where possible, it is preferable to weld both sides of the joint rather than a single side weld.

The sides of the weld joint should be more open than for carbon steel. A 40° to 45° bevel is normal. "U" grooves may be used for thick joint repairs to save welding time.

Grey iron castings often have a "casting skin" containing foreign matter that can cause weld defects. The skin should be removed by grinding or chipping when the joint is being prepared.

Low carbon steel is ready to weld after the joint has been prepared and the weld area has been wire brushed. Grey iron castings, particularly those that have been in service, may require special treatment, as outlined below.

It is common for oil and grease to seep into the grey iron casting. If they are not removed, gross weld porosity and other defects will result. One approach to eliminating this problem is to burn out the oil or grease by slowly heating the casting to 700-800 °F for 30 minutes or 1000 °F for a shorter time.

An alternative to heating is the "weld and remove" technique. A weld bead is deposited and, if there are weld defects, the bead is removed by grinding and chipping. Two or three tries may be required until the weld bead is free of defects. This indicates that the oil or grease has been removed and the repair process can proceed.

Heat Treatments

Often stresses are introduced to the metal through welding or cold work. Try to eliminate activities which cause tensile stress like bending, crimping or shrink fitting. A thermal treatment may help to reduce these residual stresses.

It is impractical to provide rigid rules for when to use pre and post weld heat treatments. Countless cast iron welds are made with E Ni-Cl or E NiFe-Cl electrodes without any weld heat treatment. On the other hand, some highly stressed, complex welds are successfully made only with the best of techniques including carefully controlled pre and post weld heat treatments.

With welds of moderate to high restraint, use 500 to 600 °F pre weld heating. This heating reduces the hardened heat affected zone which is more prone to cracking. Conditions often dictate preheating by local heat sources, such as with a gas torch. Slow and uniform heating and cooling rates should be used, particularly with complex castings to avoid unequal expansion and the risk of cracking.

Preheating the wrong areas of a casting can introduce unwanted stresses. Yet with proper planning, preheat placement can become a useful tool to reduce weld stresses. The need for post weld heat treatment also varies. For most field weld repairs, slow cooling after welding is sufficient. Slow cooling may be accomplished by periodic torch heating to retard the rate of cooling. In larger castings with a large heat sink, try covering the casting with insulating material after welding. If dimensional stability is essential, in a machined part for example, a full stress relief of the entire casting at 1100 to 1150 °F and a slow cool to 700 °F may be required.

Weld Filler Metal

Cast iron can be joined by processes such as oxyacetylene, shielded metal arc, gas metal arc welding, brazing, etc. However, field repair welding with nickel cored (AWS E Ni-Cl) and nickel-iron cored (AWS E NiFe-Cl) electrodes is the most common process. For most applications, the E Ni-Fe-Cl electrode is preferable over the E Ni-Cl electrode. The nickel-iron weld is stronger and has a lower coefficient of thermal expansion which reduces welding stresses and improves resistance to cracking. The E Ni-Cl electrode, with its nickel core results in a softer, more machinable weld deposit in thin section welds.

Many cast iron electrodes are sold by companies specializing in maintenance welding products. Often they do not use the nickel cored, nickel-iron cored or AWS designations. The nickel cored electrodes are often identified as "maximum machinable welds" while the nickel-iron cored electrodes are described as "machinable, high strength welds." If maximum machinability is not a concern, and it seldom is, it is best to use a nickel-iron type electrode.

Follow the welding currents recommended by the electrode manufacturer. Welders will find the weld metal to be somewhat more sluggish than low carbon steel weld metal. Some weave or oscillation is preferred. Wide weaving, over 3 times the electrode diameter, is excessive and should be avoided. Electrode diameters are dictated by the welding conditions. Slow heat input is desired, 1/8" diameter electrodes are most commonly used.

Welding Sequence&Peening

The final planning step for a successful weld is the weld sequence or weld bead placement and whether or not to use peening. The optimum sequence is one that will reduce distortion and weld stresses. Often a backstep weld technique where the weld bead increments are deposited in the direction opposite to the progress of welding the joint, will minimize distortion and weld stresses. Experience is valuable in selecting the optimum technique but a little preplanning in advance of striking an arc is next best.

Peening of the weld (while the metal is still hot) can be a powerful tool to minimize distortion and insure compressive stress inside the weld. Peening should be done with a rounded tool with moderately intense blows.

See our Industry Definitions for further insight.

 

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