Cast irons are extremely difficult to weld because of their tendency to crack under stress.
Well executed in-house welding of cast iron parts can save you time and money, but it’s difficult to do right, and failure usually results in cracking or other damage. Inexperienced welders are usually better off getting a professional shop to do the work—especially for critical parts.
When sending your part off to a shop isn’t an option, it’s research time. The best way to achieve a quality cast iron weld is to ask questions first and weld second.
Before starting, make sure to identify your alloy, clean the casting thoroughly, decide on a pre-heat temperature, and select an appropriate welding technique.
Know your metals
Cast irons include a family of iron-carbon alloys. Their high carbon content (usually 2 to 3 percent) gives cast iron its characteristic hardness; however, that strength comes at the expense of ductility. Instead of stretching, cast irons relieve the stress caused by localized heat by cracking. Cast irons are extremely difficult to weld as a result, to a greater or lesser degree depending on the alloy.
There are different welding techniques depending on the specific cast iron alloy you are working with.
White cast iron is so hard and brittle that few would even attempt to weld it, however it can be heat treated to become malleable. Depending on the exact heat treatment used, malleable cast iron gains a pearlitic or ferritic microstructure. These structures result in a more ductile alloy; however, that ductility is severely reduced in the heat-affected zone (HZ) during welding.
Grey cast iron is the most common variety. It is more ductile and weldable than white cast iron, but still poses a challenge to prospective welders— the carbon within grey cast iron can enter the weld pool to cause weld metal embrittlement.
Clean the casting
Regardless of the alloy, all castings must be properly prepared prior to welding. The casting should be clean and completely free of defects. If defects are discovered during inspection, they must be removed by grinding or flame gouging.
Wetting – adhesion of liquid metal to the solid casting surface – is essential to a strong weld. Graphite residue, dirt, oil, and grease can all inhibit wetting, so the first step of any weld should be a thorough cleaning (solvents and steam cleaning are good at removing grease, while acetone is effective at removing any residual graphite). Castings that have been in extended service should also be degassed: heat the welding zone with a weld pass, then grind the area smooth.
It is a good idea to conduct a wetting test before starting a weld. Apply the intended filler to a flat surface; if it is not uniformly wetted, the casting needs further cleaning.
All cast irons are vulnerable to cracking under stress; high carbon content makes cast irons hard, but brittle. Heat control is the single most important factor in avoiding cracks. A cast iron weld should combine:
- Low heat input
- Slow cooling
The primary reason for heat control is thermal expansion. When metal warms, it expands. No stress is caused when an entire object warms and expands at the same rate, but stress does build when heat is localized in a small heat-affected zone (HZ).
Localized heating causes restricted expansion – the HZ is contained by the cooler metal around it. The degree of resulting stress depends on the thermal gradient between the HZ and the casting body.
In steel and other ductile metals, stress built by restricted expansion and contraction is relieved by stretching. Because cast irons have relatively poor ductility, they tend to relieve stress by cracking during the contraction period. Obviously, this is an undesirable outcome.
Pre-heating decreases the thermal gradient between the casting body and the HZ, thereby minimizing the tensile stress caused by welding. Pre-heat temperatures vary depending on the welding process to be used. In general, higher temperature welding methods require a higher temperature pre-heat.
When adequate preheating isn’t possible, the best strategy is to minimize heat input: select a low temperature welding process and low melting point welding rods or wires.
The second major consideration is cooling rate. Rapid cooling creates brittle, easily cracked welds, while slow cooling reduces hardening and contraction stress.
Preheating, multiple-pass welding, and casting insulation are routinely used to slow cooling and prevent the formation of brittle microstructures.
In MIG welding, an electric arc forms between a consumable wire electrode and the part being welded
Specific welding techniques should be suited to the cast iron alloy being welded. The most common welding processes are stick, MIG, flux core, and TIG.
Stick welding – also known as shielded metal arc welding or MMA – makes use of a consumable electrode covered with a flux. An electric arc between the electrode and welding area melts the metals and causes fusion.
There are three main filler types that work well for cast iron stick welding:
- Cast iron covered electrodes
- Copper alloy electrodes
- Nickel alloy electrodes
Preheat pieces to least 250 °F prior to welds with cast iron or copper electrodes. Nickel electrodes can be used without a preheat. A mild steel electrode can be used in a pinch, but it isn’t recommended due to poor ductility and machinability.
In MIG (gas metal arc welding), an electric arc forms between a consumable wire electrode and the part being welded. The arc heats the metal, which melts and joins. Nickel or copper base filler will provide a good balance of ductility and strength; mild steel can be used if color match is important.
A small diameter electrode wire should be used at low current to minimize penetration. A thorough preheat is recommended to avoid cracking.
Flux core arc welding is similar to MIG, but requires a continuously fed consumable electrode containing a flux and a constant voltage. It can be used successfully on cast irons with a nickel bass flux-cored wire. It is most successful in welding cast iron when used with a nickel base flux-cored wire and CO2 shielding gas.
TIG welding of cast iron should only be attempted after a thorough preheat.
TIG (tungsten arc welding) is usually not recommended for cast iron because it applies localized, intense heat. It should only be attempted after a thorough preheat.
Whatever the welding technique, always minimize penetration and thermal gradients. The welding current should be kept as low as possible, with short circuit dip-transfer modes. It is important to plan the weld sequence carefully to reduce distortion and weld stress.
While not as strong as a full weld, brazing can be used to join cast iron. Because of the lower temperatures involved, brazing doesn’t require pre-heating. It can also be used to join dissimilar metals.
Cracking usually occurs during the thermal contraction phase – tensile stress builds as the weld cools and contracts. If the stress reaches a critical point, the weld cracks.
The chances of cracking can be decreased by opposing tensile stress with compressive stress. Welders usually apply compressive stress by peening (moderate strikes with a ball-peen hammer) a deformable weld bead while the weld is still soft. Peening decreases the risk of cracking in the weld and HZ, but should only be attempted when working with relatively ductile weld metal.
The final step of the weld is cooling control. Use insulating materials to slow cooling as much as possible.
While contracting your cast iron welding to a professional can guarantee a quality weld, it is possible to complete a weld-repair in-house with careful prep. When preparing for a cast iron weld, always plan out your materials and techniques before getting started.