Cast iron welding is possible but problematic due to the high carbon content. This carbon content is often 2-4%, which is about ten times that of most steels. During the welding process, this carbon migrates into the weld metal and/or the heat-affected zone, which leads to increased brittleness/ hardness. This, in turn, can lead to cracks after welding.
Cast iron consists of iron and carbon in varying proportions, with additional elements such as manganese, silicon, chromium, nickel, copper, molybdenum, etc. to improve certain properties. In addition, it can contain significantly higher sulfur and phosphorus contents than impurities, which makes welding difficult without cracking.
The different types of cast iron include gray cast iron, white cast iron, ductile (nodular) iron, and malleable cast iron with widely varying weldability. All categories of cast iron with the exception of white iron are considered weldable, although welding can be significantly more difficult compared to welding carbon steel.
However, it can be difficult to tell the difference between these different types of cast iron without a detailed metallurgical analysis. Even so, cast iron is a long-lasting, wear-resistant metal that has been used for centuries.
Pre-Welding Steps for Cast Iron
Four key steps should be taken before getting started to ensure the effective welding of cast iron. These include:
- Determining the Type of Cast Iron
- Cleaning the Cast
- Selecting the Correct Pre-Heat Temperature
- Choosing the Right Welding Technique
1. Determining the Type of Cast Iron
The first step of this kind of welding is identifying what type of cast iron it is. There are numerous types, and you won’t want to weld most of them.
Cast iron has poor ductility hence it could crack due to thermal stresses when rapidly heated or cooled. The susceptibility to cracking depends on the cast iron type/category. This means it is required to understand which type of alloy you are working with:
- Grey cast iron: It is the most common form of cast iron. Carbon precipitates out into graphite flakes during manufacturing into either a pearlite or ferrite crystalline microstructure. It is more ductile and weldable than white cast iron. However, it still poses a challenge to prospective welders as the graphite flakes within grey cast iron can enter the weld pool to cause weld metal embrittlement.
- White cast iron: White cast iron retains the carbon as iron carbide without precipitating it out as graphite. The cementite crystalline microstructure is very hard and brittle. White cast iron is generally considered un-weldable.
- Ductile, nodular, or malleable iron: These cast irons are all less brittle due to microstructural differences due to manufacture. All three have spheroidal carbon microstructures created by their unique manufacturing processes.
The simplest way to determine which type of iron you are working with is to check the original specification. Chemical and metallographic analysis can also help in identifying the category of cast iron that you are working with.
There are some other ways to tell the difference between alloys; grey iron will show grey along with a fracture point, while white iron will show a whiter color along a fracture due to the cementite it contains. However, ductile iron, for example, will also show a whiter fracture, yet is much more weldable.
Gray cast iron is the most common type of cast iron welded and should be the only cast iron you attempt to fuse unless you’re experienced with casting or welding it. Having someone experienced to assist you is another good option.
2. Clean the casting
Regardless of the alloy, all castings must be properly prepared prior to welding. While preparing the casting for welding, it is crucial to remove all surface materials. The casting must be completely clean in the area of the weld. Remove paint, grease, oil, and other foreign material from the weld zone. It is best to apply heat carefully and slowly to the weld area for a short time to remove entrapped gas from the weld zone of the base metal.
A simple technique for testing the readiness of the cast-iron surface is to deposit a weld pass on the metal—it will be porous if any impurities are present. This pass can be grinded off, and the process is repeated a few times until the porosity disappears.
3. Pre-Heat Temperature
All cast irons are vulnerable to cracking under stress. Heat control is the single most important factor in avoiding cracks.
A cast-iron weld requires three steps:
- Pre-heating
- 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 will 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.
Unfortunately, this can cause cracking during the contraction period since cast irons have relatively poor ductility. Pre-heating decreases the thermal gradient between the casting body and the HZ, thereby minimizing the tensile stress caused by welding. In general, higher temperature welding methods require a higher temperature pre-heat.
When adequate preheating is not possible, the best strategy is to minimize heat input select a low-temperature welding process, and low melting point welding rods or wires.
The cooling rate is another factor that has a direct impact on the stresses induced at the weld. Rapid cooling causes contraction, which creates brittle, easily cracked welds. Contrastingly, low cooling reduces hardening and contraction stress.
4. Welding Technique Use To Weld Cast Iron
Theoretically, any of the common arc welding processes such as manual metal arc welding, flux-cored arc welding, metal active gas welding, submerged arc welding, tungsten arc welding, etc. can be used, a process that facilitates slow heating and cooling is generally preferred.
1. Manual Metal Arc Welding (MMA)
This type of welding, also known as shielded metal arc welding (SMAW), is generally believed to be the best overall process for cast iron welding – provided that the correct welding rods are used. The choice of the electrode will depend on the application, the required color match, and the amount of post-weld machining.
The two main electrode types for manual metal arc welding are iron-based and nickel-based. The iron-based electrode will produce weld metal with high carbon martensite, hence generally limited to minor repair of casting and when color matching is required.
Nickel alloy electrodes are the most commonly used, offering a more ductile weld metal. Nickel electrodes can also help to reduce the pre-heating and HAZ cracking by providing a lower-strength weld metal.
In all cases, care should be taken to minimize the parent metal melting. This will minimize the dilution.
2. MAG Welding
MAG welding is generally carried out with a nickel consumable. An 80% argon to 20% carbon dioxide gas mix will work for most applications. While brazing wire can be used, it is generally not recommended as braze metal will be significantly weaker than the casting.
3. TIG Welding
TIG welding can provide a clean weld on cast iron but is not generally preferred due to its highly localized heating characteristics. As with all TIG welding, the quality of the finished weld is largely determined by the skill of the welder.
4. Oxy-Acetylene Welding
Oxyacetylene welding also makes use of electrodes, but instead of an arc generated by the current, an oxy-acetylene torch provides the energy for welding. Cast iron electrodes, and copper-zinc electrodes, are both suitable for oxy-acetylene welding of cast iron.
Care must be taken not to oxidize the cast iron during acetylene welding, as this causes silicon loss and the formation of white iron in the weld. The welding rod should be melted in the molten weld pool, rather than directly by the flame, to minimize temperature gradients.
5. Braze Welding
Braze welding is a common method for joining cast iron parts due to the minimal impact on the base metal itself. A welding rod provides the filler that adheres to the cast iron surface. Because of the lower melting point of the filler compared to the cast iron, the filler does not dilute with the cast iron but adheres to the surface.
Cleanliness of the surface is critical for this welding technique since the join is dependent on the quality of the filler wetting the surface of the base metal. According to Machine Design, using flux to prevent the formation of oxides during brazing is common.
It is a liquid that promotes wetting, which lets the filler flow over the metal parts to be joined. It also cleans the parts of oxides so that the filler bonds more tightly to the metal parts. In addition, fluxes are used in welding to clean metal surfaces.
Welding Techniques without Preheat
The size of the casting, or other circumstances, may require that the repair be made without preheating. When this is the case, the part needs to be kept cool, but not cold.
Raising the casting temperature to 100 degrees F is helpful. If the part is on an engine, it may be possible to run it for a few minutes to obtain this temperature. Never heat the casting so hot that you cannot place your bare hand on it.
Make short, approximately 1” long welds. Peening after welding is important with this technique. Allow the weld and the casting to cool. Do not accelerate the rate of cooling with water or compressed air.
It may be possible to weld in another area of the casting while the previous weld cools. All craters should be filled. Whenever possible, the beads should be deposited in the same direction, and it is preferred that the ends of parallel beads not line up with each other.
Filler Wire or Electrode used for Cast welding
As mentioned above, the choice of welding rod is important for welding cast iron, although most experts would advise using nickel rods.
1. 99% Nickel Rods
These electrodes are more expensive than other options but also provide the best results. 99% nickel rods produce welds that can be machined and work best on castings with a low or medium phosphorous content. These pure nickel rods produce a soft, malleable weld deposit.
2. 55% Nickel Rods
Less expensive than 99% rods, these are also machinable and are frequently used for thick section repairs. A lower co-efficient expansion means that these produce fewer fusion line cracks than the 99% rod. These ferro-nickel rods are ideal for welding cast iron to steel.
Less expensive options are available, such as steel rods, although these are not as effective as nickel rods:
3. Steel Rods
Steel rods provide the cheapest option of the three and are best for minor repairs and filling. Steel electrodes produce hard welds, which require extra grinding to finish and are not machinable. However, despite these drawbacks, steel rods provide color matching and can better tolerate castings that are not completely clean than nickel rods.
4. Brazing bronze
You can oxy-acetylene braze or use the rods with a TIG welder. It’s a good way to provide a stronghold in a crack or between two components needing joining, while not causing cracking and a change of the cast iron properties.
Cleaning and Peening Weld Surfaces
Like most welding, the cleaner the surface is, the better your welds will be. Sometimes the material you’re welding has had a lifetime’s worth of fumes or oil throughout the casting, and this will cause challenges in welding. There are certain things to be aware of when you weld cast iron.
Using welding consumables, like the MG-289 Cast Iron Welding Alloy rods, that are designed for contaminated cast iron is the best way to ensure the weld is sufficient.
Peening is the process of tapping the soft weld as it cools with a ball-peen hammer to prevent cracking. This should be done with caution and only when the weld can deform, but it’s not always necessary, as what primarily causes cracks is the irregular or rapid heating or cooling of the component. Preheating and slowly cooling your project is what’s most important.
Preheat or cold-weld?
There are a few different opinions on whether you should weld hot or cold. Everyone agrees that preheating is a good option, but there are a few people suggesting welding with minimal heat is sufficient too.
When deciding on how to weld cast iron, taking one of these approaches is necessary, because, unlike most metals, cast iron is brittle and has very minimal ability to deform through bending or expansion and contraction. Heat always has a deforming effect on metal, and if one part of the metal heats or cools faster than another, stress and cracking occur in the cast iron or weld.
Preheating minimizes this effect by bringing the surrounding area of the weld nearer to the welding temperature, making the whole component change uniformly. Cast iron changes characteristics over 1400 degrees Fahrenheit, so it’s crucial not to put excessive heat into the component.
Welding cast iron in a cool manner without preheating and using minimal heat is thought to help this by reducing the overall temperature. This can be beneficial in not producing visible cracks, and it will be a stronger weld than a hot weld without preheating. However, internal stress still occurs, which can show later on in the component’s life, and it produces an overall weaker weld than properly preheating before welding.
Experienced cast iron welders who work on repairing or manufacturing cast iron products regularly always and only preheat. This is the best way to ensure the weld is strong internally, not just visibly crackless.