It is possible to weld cast iron, although it can be problematic due to the high carbon content. This carbon content is often around 2–4%, which is about ten times that of most steels. The welding process causes this carbon to migrate into the weld metal and/or the heat affected zone, leading to elevated brittleness/hardness. This, in turn, can lead to post weld cracking.
Cast iron is made up of iron and carbon in different proportions, with additional elements such as manganese, silicon, chromium, nickel, copper, molybdenum, etc. to enhance specific properties. In addition, it may contain significantly higher levels of sulphur and phosphorus as impurities making it difficult to weld without cracking. The different grades of cast iron include grey iron, white iron, ductile (nodular) iron, and malleable iron with widely varying weldability. All categories of cast iron except white iron are considered as weldable, although the welding can be significantly more difficult compared with carbon steel welding. However, it can be difficult to tell the difference between these different types of cast iron without detailed metallurgical analysis. Despite this, cast iron is a durable, wear resistant metal that has been used for centuries.
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As mentioned above, cast iron can be difficult to weld because of its specific composition, but it is not impossible if you use the correct welding technique to avoid weld cracks. This involves careful heating and cooling, often including pre-heat, the correct choice of welding rods, and allowing the part to cool slowly.
There are a number of key steps that can be taken to help ensure effective welding of cast iron. These include:
- Identifying the Alloy
- Cleaning the Cast
- Selecting the Correct Pre-Heat Temperature
- Choosing the Right Welding Technique
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
This is the most common type of cast iron. It is basically an iron-carbon-manganese-silicon alloy with 2.5-4% carbon. The carbon precipitates into graphite flakes during manufacture into either a ferrite or pearlite crystalline structure. However, these graphite flakes can dissolve during welding and precipitate as high carbon martensite, embrittling the heat affected zone and the weld metal.
White Cast Iron
White cast iron is free of graphite and contains carbon in combined form as metal carbides making the microstructure brittle. White iron is generally considered as unweldable.
Ductile (Nodular) Iron
Ductile iron is similar in composition as grey iron but the impurity levels are low compared with grey iron. Unlike the grey iron, which contains carbon in the form of graphite flakes, the ductile iron contains the graphite as spheroids in its matrix. The rest of the matrix is mostly pearlite with a ferrite region surrounding the graphite spheroids.
Malleable iron is heat treated white iron with substantially lower carbon content compared with the white iron. Depending on the heat treatment employed, it generally has as mixture of ferrite or pearlite structure with nodules of graphite and hence has more ductility compared with the standard white iron.
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 the cast iron that you are working with.There are some other ways to tell the difference between alloys; grey iron will show grey along a fracture point, while white iron will show a whiter colour 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.
It is important to clean the cast iron before welding, removing all surface materials, such as paint, grease and oil, paying particular attention to the area of the weld. The casting skin may be removed through grinding. It is essential that the cleaned surface is wiped with mineral spirits to remove the residual surface graphite prior to the welding. Slowly preheating the weld area for a short time will help remove any moisture trapped in the weld zone of the base material.
The most important factor in avoiding stress cracking in cast iron is heating/cooling control. This is to minimise the residual stresses build-up during the heating and cooling process.
Localised heating, such as the one encountered during welding, results in restricted expansion as the HAZ is contained by the surrounding cooler metal. The thermal gradient will determine the resulting stress. Ductile metals like steel is able to relieve the stress by stretching, but because cast irons have poor ductility they are liable to crack instead. Pre-heating reduces the thermal gradient between the HAZ and the surrounding casting body, minimising the residual stresses caused by welding. Pre-heating cast iron before welding slows the cooling rate of the weld and the surrounding area. Where possible, heat the entire casting. Typical minimum pre-heat temperatures are from 100-400°C, depending on the type of the cast iron and the allowable HAZ hardness . Any pre-heating should be done slowly and uniformly.
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 which 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 electrode will depend on the application, the required colour match and the amount of post-weld machining.
The two main electrode types for manual metal arc welding are iron based and nickel based. Iron based electrode will produce weld metal with high carbon martensite, hence generally limited to minor repair of casting and when colour 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 minimise the parent metal melting. This will minimise 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 not generally preferred due to its highly localised heating characteristics.. As with all TIG welding, the quality of the finished weld is largely determined by the skill of the welder.
Find out more about TIG welding
4. Oxy Acetylene Welding
As with arc welding, oxy acetylene uses an electrode, but rather than an arc generated by electrical current, this process uses the oxy acetylene torch to generate the heat. The low heat intensity and slow heating associated with the process will result in a large HAZ, but the slow heating in beneficial in preventing the formation of high carbon martensite in the HAZ. The low heat intensity of the process will require preheating to a higher temperature, typically in the region of 600°C, to make the welding feasible. A neutral or slightly reducing flame is used for the welding.
Find out more about oxy acetylene welding
5. Braze Welding
Braze welding may be used for welding cast iron parts, since it has a minimal impact on the base metal itself. Once again, a filler rod is used for this process except it adheres to the surface of the iron rather than diluting into a weld pool due to the lower melting point of the filler.
As with other techniques, cleaning the surface is important with braze welding. A flux can be used to prevent oxides forming, promoting wetting, cleaning the surface and allowing the filler to flow over the base metal.
TIG brazing is also possible, using a lower amperage to heat the workpiece while avoiding melting the cast iron. The argon gas shroud of the torch shields the brazing zone, meaning that there is no need to use flux as with oxy-fuel.
Find out more about braze 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 machineable. However, despite these drawbacks, steel rods provide colour matching and can better tolerate castings that are not completely clean than the nickel rods.
As a weld cools and contracts, it causes residual stress to build, leading to cracking. The chances of cracking can be reduced through the application of compressive stress. Compressive stress is applied by peening (using a ball peen hammer to deliver moderate strikes), which deforms the weld bead while still soft. However, peening should only be used with relatively ductile weld metal, that is on welds produced with nickel consumables.
Allowing cast iron to cool down too rapidly can lead to cracking. The cooling process can be slowed down by using insulating materials or the periodic application of heat. Some methods include placing the workpiece in an insulating blanket, placing it into dry sand, or even putting it over a wood fire oven and allowing the metal to cool as the fire dies down.
It is possible to weld cast iron, but it needs to be done using the correct techniques and with care to avoid cracking. Most welding methods require the surface of the material to be cleaned and cast iron benefits from pre and post-weld heating as well as careful cooling.
TWI has decades of expertise in all aspects of welding and joining, including working with cast steel. Please contact us, below, if you have any questions and feel we could assist you with your project.
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