Welding cast iron is not that easy due to the high carbon content. The welding process will make the carbon move to the weld metal or the zone affected by heat. This will lead to an increase in hardness and brittleness, which will, in most cases, lead to post welding cracking.
Cast iron comprises different types of materials such as iron, silicon, and carbon in varying proportions. This will make it difficult to ascertain the strength of the metal you are using. It will also make it difficult to determine the amount of heat it can withstand before it starts cracking.
Ductile iron and malleable are the different cast iron grades. However, grey cast iron is the most popular one. It can be difficult to tell the difference between the different cast-iron types.
In this article, we will be taking you through a guide on how to weld cast iron.
Is it Difficult to Weld Cast Iron?
How do you weld cast iron? This is one question most welders research. For sure, cast iron can be difficult to weld, although you can avoid weld cracks when you use the right welding techniques. The right technique will involve careful heating and cooling while including preheat, welding rods’ right choice, and allowing the metal to cool down slowly.
Most of the cast iron has a critical high temperature of about 1450°F, where cracking can occur. Since you can heat the material past this point, it is not important to maintain the heat for long since the carbon will always cause the metal to crack and shrink.
To avoid this, you can either allow the material to cool down slowly after welding or keep it cool.
Cast Iron Pre Welding Steps
There are steps you need to follow to ensure you effectively weld the cast iron. The steps include:
- Identification of the alloy
- Cleaning the cast
- Choosing the right preheat temperature
- Selection of the correct welding technique
Identification of the Alloy
The cast irons will always crack when put under lots of stress or heat instead of deforming or stretching. You can always improve this by adding different types of alloys.
It will mean it will be of great importance when you understand the alloy type you are working with.
Grey Cast Iron
It is the most popular cast iron type, and it is more weldable and ductile than white cast iron. The carbon will always precipitate into graphite flakes during the manufacturing process into pearlite or ferrite crystalline structure.
There are times when the graphite flakes can get into the weld pool, causing embrittlement.
White Cast Iron
The white cast iron will never precipitate the carbon from the graphite. It will instead retain it as iron carbide. The clementine crystalline microstructure of the cast iron is quite hard and brittle. The white cast iron is, in most cases, seen to be unweldable.
Nodular, Ductile, and Malleable Iron
They have spheroidal carbon microstructures due to their unique manufacturing process, making them easier to weld and less brittle. One simplest method you can use in determining the type of iron you are using is to the original specification.
The spectrochemical analysis will reveal the specification you are using working with and a knowledgeable metallurgist to determine the alloy you have.
Cleaning the Cast
It would help if you cleaned the alloy before you start welding. Use the following procedure to help clean the cast:
- Remove all the surface materials like oil
- Grease and paint
- Ensure you pay attention to the weld area.
- Slowly and carefully heat the weld area for a short time to remove any gas in the base material’s weld zone.
Weld pass on the material surface will be porous and show any impurities available. You can always grind off and repeat this pass to continue checking until such a time when porosity will go away.
Choosing the Right Preheat Temperature
One of the most important factors for avoiding stress cracking in cast iron is controlling the heat. It is due to thermal expansion, as when the metal warms, it expands. In case the entire objects warm and expands at the same time, then there will be no stress. Stress usually builds when the heat is localized in the small heat-affected zone.
Localized heating will result in small expansion since the surrounding cooler metal will contain the HAZ.
The thermal difference between the heat will determine the stress that results. Ductile metals such as steel are capable of relieving stress when they stretch. However, since the cast irons have low ductility, they will be liable to crack.
Preheating will reduce the thermal gradient between the surrounding casting body and the HAZ. With this, it will minimize the tensile stress. The higher temperature welding procedures will require higher preheat temperatures.
Preheating the cast iron before you weld will slow the weld’s cooling rate together with the surrounding area.
If possible, try heating the entire casting. Preheat temperatures will range from 500-1200°F. However, it is important not to heat past 1400°F as that is the critical temperature range. Any preheating needs to be uniformly and slowly.
When hot enough, then preheating will not be possible. You can consider using a low-temperature welding process together with a low melting point welding wire or rods.
Selecting the Correct Welding Technique
You can weld the cast iron with or without preheating. However, this decision will help in determining the best technique for welding. It is preferable to use preheat to weld cast iron.
When this is not possible, it is important to keep the metal cool, but after you have decided on whether to preheat or not, you should stay with this method.
When preheating, ensure you use a low current technique to help reduce residual administer and stress. It can also be necessary to only weld small areas around 1-inch in length at any particular time to help in preventing the buildup of stress.
Weld beads peening such as with the ball-peen hammer can be of great importance to reducing stresses.
After the welding is complete, allow it to cool down. You can help it cool faster by wrapping the casing in an insulating blanket or burying it in dry sand.
In case you are unable to preheat cast iron, ensure that you keep the part cool during the welding process. Increase the casting temperature to 100°F but never rise to a heat where you are not in a position of placing your hand upon it since that would be too hot.
Ensure that you always keep your welds shot and only use peening after welding. Let the cast cool down naturally instead of applying air or water and fill any craters.
If possible, try and deposit the beads in the same direction and ensure the beads’ ends never line up.
What Welding Types to Use in Welding Cast Iron
There are different welding process types that you can always use on cast iron. Some of the common types include:
- Arc / Stick Welding
- MIG welding
- TIG welding
- Braze Welding
- Oxy-acetylene welding
Arc / Stick Welding
You can also refer to it as manual metal arc welding (MMA or MMAW) is one of the best overall processes for cast iron welding. Any top welder needs to know how to arc weld cast iron. The electrode choice will always depend on the application, color match, and the amount of post-weld machining.
Copper alloy electrodes, nickel alloy electrodes, and cast iron electrodes are the three main filter cast-iron types. Nickel alloy electrodes are the most popular as they offer a strong weld and have a low coefficient, thermal expansion.
This reduces the welding stress and improves cracking resistance. The nickel electrodes can be used without preheating. When you use copper or cast iron electrodes, the workpiece needs to be preheated to 250°F.
In all the scenarios, the electric arc needs to be directed to the weld pool instead of the base metal. With this, it will reduce dilution since the metals will melt and fuse.
It is also advisable to use the lowest possible current setting approved to help reduce heat stress and weld with the stick in an upright position.
It is also important for a welder to know how to weld cast iron with MIG welding. Despite stick or arc welding being preferable to MIG welding cast iron, it is impossible to use MIG.
You need to perform MIG welding using nickel wire, although you can always use cheaper steel wire. 80% argon to 20% carbon dioxide gas mix will work well for most of the applications. However, this can use the weld to eventually rust.
Just like in MIG welding, it is also advisable for a welder to know how to weld cast iron with TIG welding. TIG welding is capable of offering a clean and strong weld on the cast iron. However, the nickel wire is the only viable option for this method, making it a costly procedure.
With the right wire, gas, and settings, it is possible to always skip the pre and post-heating stages together with the cold weld. The welder’s skill determines the quality of the complete weld.
Oxyacetylene uses an electrode instead of the arc that is generated by the electric current. The process uses an oxy-acetylene torch for welding energy. You can use iron, zinc, or copper electrodes when welding the cast iron.
It is important never to allow the cast iron to oxidize during welding since this can easily lead to the loss of silicon and the creation of white iron in the weld. As the case with arc welding, the rod needs to be melted directly to the weld pool to reduce temperature gradients.
You can use braze welding for welding cast iron parts since it has a small impact on the metal base itself. A filler rod can be used for this process, although it only adheres to the iron surface instead of diluting it into a weld pool due to the filler’s lower melting point.
Cleaning the surface is very important when it comes to brazing welding. You can use flux to prevent oxides formation by promoting wetting and cleaning the surface, allowing the filler to flow over the metal base.
What Type of Welding Rods can you Use?
The choice of welding rod is important when it comes to welding the cast iron. However, most experts recommend using nickel rods.
The nickel rods include a special high-graphite flux that minimizes carbon migration into the weld metal and the heat zone. The two nickel rod types include:
99% Nickel Rods
They are the most expensive electrodes but offer the best results. They produce welds that can match and work best on castings with low or medium phosphorous content. The pure nickel rods produce malleable and soft weld deposits.
55% Nickel Rods
It is less expensive than the 99% nickel rods. The 55% nickel rods are machineable and are perfect for use on thick section repairs. Lower co-efficient expansion will mean that they will produce lesser fusion line cracks than the 99% rod. The ferronickel rods are great for welding cast iron to steel. You can also learn how to weld cast iron to steel.
The steel rods offer the cheapest option is best for simple repairs and filling. The steel electrodes offer hard welds that need extra grinding to complete and are not machineable.
You can finish the cast iron welds by applying comprehensive stresses and by post-weld heating.
Post Weld Heating
Letting the cast iron cool down rapidly can easily make it experience crack and stress. You can cool down the cooling process by using insulating materials or periodic application of heat.
Some of the methods include a workpiece in an insulating blanket, putting it into dry sand, putting it over the wood fire oven, and allowing the metal to cool since the fire dies down.
We hope you now know how to weld cast iron. However, it is important you know that welding cast iron is possible but needs to be done using the right techniques and care to prevent cracking. The majority of the welding methods require the material surface to be cleaned—the cast iron benefits from the pre and post-weld heating and careful cooling.