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 from different materials (usually carbon, iron and steel) in different proportions, making it difficult to ascertain how strong the metal you are working with is and how much heat it can withstand before cracking. The different grades of cast iron include malleable and ductile iron, but the most commonly used is grey cast iron. 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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Cast iron, like steel, is an alloy of iron and carbon. However, where cast iron has a carbon content of 2-4%, steel requires a carbon content of less than 2%. It is this carbon content that gives cast iron a rougher feel as well as making it more brittle than steel. The carbon also makes cast iron a poor conductor, meaning that it takes longer to heat up or cool down.
While stainless steel contains 1.2% carbon (as well as chromium to prevent corrosion), carbon steel contains 2%, placing it on the cusp of steel and cast iron. Carbon steel has a matte finish and is much harder than stainless steel, but it can corrode. It is also more expensive than cast iron.
As mentioned above, cast iron can be difficult to weld, 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 metal to cool slowly.
Most cast iron has a critical temperature of 1450°F at which point cracking can occur. Although the material can be heated above this point, it is important not to maintain this heat for too long as the carbon will cause the metal to shrink and crack. To avoid this, it is important to either let the material cool slowly after welding or it should be kept cool enough that the rate of cooling is not important.
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 irons crack when heated or stressed rather than stretching or deforming. This can be improved by adding different alloys. This means it is useful to understand which type of alloy you are working with:
Grey Cast Iron
This is the most common type of cast iron and is more ductile and weldable than white cast iron (see below). The carbon precipitates into graphite flakes during manufacture into either a ferrite or pearlite crystalline structure, giving this alloy its name. However, these graphite flakes can enter the weld pool and cause embrittlement.
White Cast Iron
White cast iron doesn’t precipitate the carbon out as graphite, but instead retains it as iron carbide. The cementine crystalline microstructure of this form or cast iron is very hard and brittle. White cast iron is generally considered to be unweldable.
Ductile, Nodular, and Malleable Iron
These irons have spheroidal carbon microstructures as a result of their unique manufacturing processes, making them less brittle and easier to weld.
The simplest way to determine which type of iron you are working with is to check the original specification. However, spectrochemical analysis can tell you which specification you are working with and an experienced metallurgist can use spark testing to determine which alloy you have.
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 alloy before welding, removing all surface materials, such as paint, grease and oil, paying particular attention to the area of the weld. Carefully and slowly heating the weld area for a short time will also remove any gas trapped in the weld zone of the base material.
A weld pass on the surface of the material will be porous and show any impurities that are still present. You can grind off and repeat this pass to keep checking until the porosity disappears.
The most important factor in avoiding stress cracking in cast iron is heat control. This is due to thermal expansion as, when metal warms, it expands. If the entire object is warmed and expands at the same time no stress is caused, however stress can build if the heat is localised in a small heat affected zone (HAZ).
Localised heating results in restricted expansion as the HAZ is contained by the surrounding cooler metal. The thermal difference between these heats will determine the resulting stress. Ductile metals like steel is able to relieve the stress by stretching, but because cast irons have low ductility they are liable to crack instead. Pre-heating reduces the thermal gradient between the HAZ and the surrounding casting body, minimising the tensile stress caused by welding. Higher temperature welding procedures require higher pre-heat temperatures.
Pre-heating cast iron before welding slows the cooling rate of the weld and the surrounding area. Where possible, heat the entire casting. Pre-heat temperatures typically range from 500-1200°F, but it is important not to heat over 1400°F since that is the critical temperature range. Any pre-heating should be done slowly and uniformly.
If hot enough pre-heating is not possible, the best alternative is to use a lower temperature welding process along with low melting point welding rods or wire.
Cast iron can be welded with or without pre-heating, but this decision will determine the best welding technique. As mentioned above, it is generally preferable to use a pre-heat to weld cast iron. Where this is not possible, it is important to keep the metal cool, but once you have decided whether to pre-heat or not, you should stay with this method.
When using a pre-heat, you should use a technique with a low current to minimise residual stress and admixture. It may also be necessary to only weld small areas of around 1 inch in length at a time so as to prevent stress build up. Peening of weld beads, such as with a ball peen hammer, can also be helpful in reducing stresses.
Once the welding is complete, allow the part to cool down slowly. You can assist with this by wrapping the casting in an insulating blanket or burying it in dry sand, slowing cooling rates and reducing the likelihood of cracking.
Non Pre-Heated Welds
Should you be unable to pre-heat the cast iron, you should keep the part cool (not cold) during welding. Raise the casting temperature to 100°F, but never raise to a heat where you are unable to place your hand upon it, as this would be too hot.
Keep your welds short (around 1 inch in length) and use peening after welding. Allow the cast to cool naturally rather than applying compressed air or water, and fill any craters. It is also advisable to try and deposit the beads in the same direction and make sure the ends of parallel beads do not line up.
There are several types of welding process that can be used for cast iron, including arc or stick welding, MIG welding, TIG welds, oxy acetylene welding and braze welding. Each has their own advantages and drawbacks when welding cast iron.
1. Arc / Stick Welding
This type of welding, also known as shielded metal arc welding (SMAW), manual metal arc welding (MMA or MMAW), and flux shielded arc welding 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 three main filler types for cast iron stick welds are nickel alloy electrodes, copper alloy electrodes and cast iron covered electrodes. Nickel alloy electrodes are the most commonly used, offering a stronger weld with a lower coefficient of thermal expansion, thereby reducing welding stress and improving cracking resistance. Nickel electrodes can also be used without pre-heating. When using copper or cast iron electrodes, the workpiece should be pre-heated to 250°F.
In all cases, the electric arc should be directed at the weld pool rather than the base metal. This will minimise dilution as the metals melt and fuse. It is also recommended to use the lowest possible approved current setting to minimise heat stress and to weld with the stick in an upright position.
This method can also be used for welding cast iron to steel, since the steel will accept anything that works for the cast iron.
Find out more about arc welding
2. MIG Welding
Although arc or stick welding is preferable to MIG welding cast iron, it is still possible to use MIG.
Ideally, MIG welding should be performed with a nickel wire, although less expensive steel wire can also be used. An 80% argon to 20% carbon dioxide gas mix will work for most applications, although this will cause the weld to rust eventually.
Finally, while brazing wire can be used, it is not recommended for metals that will experience loading or impact since it creates a weak weld.
Find out more about MIG welding
3. TIG Welding
TIG welding can provide a strong and clean weld on cast iron, although nickel wire is the only genuinely viable option for this method, making it an expensive procedure. With the correct gas, wire and settings it is possible to skip the pre and post-heating stages and cold weld. 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 for the welding energy. When welding cast iron with this method you can use copper zinc or iron electrodes.
It is important not to allow the cast iron to oxidise during welding, since this will lead to silicon loss and the creation of white iron in the weld. As with arc welding, the rod should be melted directly into the weld pool in order to minimise temperature gradients.
Because gas welding is slower and has a lower flame temperature than with arc welding, this process typically avoids the problem of carbon migration. However, this lower heat can make welding large components more difficult and it is also a slow process.
Find out more about oxy acetylene welding
5. Braze Welding
Braze welding can be used for welding cast iron part 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, by 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.
Nickel rods include a specialised high-graphite flux to minimise carbon migration into the weld metal and heat affected zone. The two types of nickel rod are:
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 simple repairs and filling. Steel electrodes produce hard welds, which require extra grinding to finish and are not machineable. However, despite these drawbacks, steel rods can tolerate castings that are not completely clean and are more user-friendly for arc processes.
Cast iron welds can be finished by applying compressive stresses (peening) and by post-weld heating.
As a weld cools and contracts, it causes tensile stress to build, leading to weld cracks if allowed to reach a critical point. The chances of cracking can be reduced through the application of compressive stress, which opposes the tensile stress associated with cooling. 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.
Allowing cast iron to cool down too rapidly can lead it to experience stress and crack. 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.
There are some additional factors to bear in mind when welding cast iron:
Tiny cracks tend to appear alongside even when good procedure is followed while welding cast iron. If you require a water-tight cast, you can eliminate leaking by using a sealing compound that will prevent rusting in service.
Repair Using The Studding Method
Repairing major breaks in large castings can be achieved by drilling and tapping holes over surfaces that have been bevelled in order to receive the repairing weld metal. Screw steel studs into the threaded holes, making sure 3/16” (5 mm) to ¼” (6 mm) of the stud is above the surface. Slowly and cautiously weld the studs into place and cover the entire surface of the break with a good weld deposit. This deposit will allow the two sides of the crack to be welded back together. This technique can also offer increased strength in your cast iron welds.
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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