Welding cast iron has the reputation of being a difficult task to perform. Although it is possible, how do you weld cast iron. It is a problematic material to weld due to the high carbon content that makes it brittle and highly prone to cracking.
It is doable and there are ways to make it work. Let’s look into the details, including the best welding process and the best way to weld cast iron.
Is it Hard to Weld Cast Iron?
In short, this is not a straight ahead job for anyone really, but even more difficult for the beginner welders. Cast iron is an alloy of iron, 2-4% carbon, and varying amounts of other metals such as silicon, manganese, and traces of sulfur and phosphorus.
The metal is generally brittle, but the type of cast iron will determine whether it’s weldable or not. After reading this article you will be at least better informed of how to go about this tricky, but not impossible, process.
Types of Cast Iron
Below is a brief description of the most common forms of cast iron, including their properties that determine the welding process.
This is the most common form of cast iron used throughout the world. It is called “Gray Iron” because of it’s graphite flaky structure. While it is more ductile than White Cast Iron it is still not easy to weld and requires some special techniques to be successful. The major challenge for welders is the possibility of causing a weld metal embrittlement (process of making it brittle) when the graphite flakes enter the weld pool during the welding process.
White Cast Iron
White cast iron is almost unweldable thanks to carbon making up most of its form. It retains the cementite crystalline structure making white cast iron super hard and brittle. It has very high resistance to abrasion and very good compressive strength but this also makes it difficult to machine and cut.
Malleable Cast Iron
Malleable Cast Iron is a product of white iron after some serious heat treatment and left to cool slowly thus changing the composition of the metal. It is similar to ductile cast iron in that it is more pliable and user friendly when it comes to welding.
Ductile Cast Iron
Ductile cast iron, also known as ductile iron, was created in 1943. The definition of “Ductile” is pliable and not brittle. In short ductile cast iron is comprised of a higher percentage of carbon making it more flexible than the standard cast iron or gray iron, which consequently makes ductile cast iron more machinable and weldable.
The Best Ways to Identify Different Cast Irons
- Consulting the manufacturer, not always convenient to do this method.
- Checking the color on the crack. Grey iron will show grey while white iron is brighter along the fracture.
- Spark test, each type will give off different shapes and colors of sparks when grinding. This is the most popular method of identifying what type of cast iron you are dealing with.
- Use a metal file to make a deep mark in the metal, may need some experience here.
Steps to Weld Cast Iron
Once you have identified the particular type of cast iron you are working with, there are some simple general pre-welding steps to follow.
- Cleaning the cast iron.
- Choosing the correct pre-heat temperature.
- Selecting the right welding technique.
Step 1. Cleaning the Welding Area
As with most metals you always will want to start with a clean surface and cast iron is no different. This means things like rust and other impurities on the iron.
You need to be careful that the target area is perfectly dry of all types of foreign materials including water, oils or grease.
You will also need to go in with mineral spirits to remove any surface graphite before welding. Pre-heating the welding area will ensure any entrapped gases are released before the welding process begins.
Step 2. Choosing the correct pre-heat temperature
Learning proper heating and cooling control is a crucial skill to have under your belt before attempting to weld cast iron. It’s an important step to prevent stress cracking and minimize any residual stress build-up that may lead to cracking.
When a metal is heated, it expands. If the metal is uniformly heated, no stress would be caused, but localizing the heat in a small region causes stress build-up, leading to cracking.
Ductile metals can handle it by stretching from the stress, but cast iron has low ductility meaning it will crack. Pre-heating reduces the thermal gradient between the cooler surrounding area and the target area. If possible, heat up the whole casting but if not, heat up as large an area as possible.
Preheat the iron to 500-1200 degrees Fahrenheit to minimize internal stresses in the material. You can use an infrared thermometer or observe when the metal starts glowing dull red, which happens at around 900 degrees Fahrenheit. Perform the welding in a dark room so you can notice when the target area starts glowing. Allow it to just glow so then you know that you have achieved the right temperature for welding.
Minimizing the heat during the actual welding process is the next best strategy when pre-heating is not possible. This may require you to stop and start a few times to prevent over heating the welding area. Welding cast iron at high temperatures is a recipe for disaster.
In the past I have used different ways of pre heating cast iron before welding. You could simply use an electric heat gun or if it is a hot day just leave the job in the sun for awhile. Anything to heat the metal is better than leaving it cold.
Step 3. Selecting the right Welding Technique
Choosing the right welding technique comes down to preference, provided it’s suitable for the alloy being welded. You also need to choose the best filler. The most common welding techniques are stick, TIG, MIG, and oxy-acetylene welding.
What is the Best way to Weld Cast Steel?
Once you have cleaned the metal, the crack will be visible as a dark line. Drill a hole on either end of the crack to prevent the crack from extending. Failure to drill on both ends of the crack creates the possibility of cracking as you weld, or the part will break over time. I also recommend to create a small channel in the metal to hold the weld by using an angle grinder.
Caution: do not cool the finished product by immersing into water! This will instantly create a major crack.
How to Weld Cast Iron Using Different Types of Welders
The kind of welder to use will be determined by the surface you are welding, and the end result expected. For instance, if you are welding a machined surface, you would use the TIG weld as a spatter from the stick, and MIG welding would cause unnecessary damage to the surface.
1/ Stick Welding
Also known as metal arc welding (MMA), this method uses an electrode covered with flux, and it’s the best welding process for cast iron. The electrode required will depend on its application, color match required, and the machining to be done post welding.
The main types of electrodes used are iron, nickel, or copper-based. Nickel-based electrodes are the most popular for cast iron welding as they are stronger, therefore reducing the resistance to cracking.
The process involves melting an electric arc between the electrode and the welding area. Ensure the arc is directed towards the weld pool to minimize dilution that would occur when directed toward the base metal. You should also use the lowest setting to reduce heat stress and avoid damaging the metal.
2/ Oxy-acetylene Welding
This process uses an electrode, but instead of an electric current, the method utilizes oxy-acetylene to generate heat. The slow heating and low amount of heat used in the process results in a large HAZ (heat affected zone), so care must be taken to avoid oxidizing the cast iron. The rod should be welded in the molten pool rather than directly by the flame.
3/ Braze Welding
Brazing is commonly used for welding cast iron parts as it causes minimal impact on the base metal. The process involves using a filler with a lower melting point than cast iron. The filler, therefore, adheres to the surface of the cast iron rather than diluting into the weld pool.
For this technique to work seamlessly, you need to start off with a clean surface. You can also use flux to prevent oxidizing and promote the wetting of the surface.
4/ TIG Welding
TIG welding is generally not recommended due to the highly localized heating characteristics. The skills of the welder also determine the overall outcome of this welding technique.
Frequently Asked Questions about How to Weld Cast Iron
How can I weld cast iron exhaust manifold?
The key to welding a cast iron exhaust manifold is to go low and slow to avoid creating stress that will lead to cracking.
What kind of welding rod do you use on cast iron?
Nickel alloy rods are the most popular for welding cast iron. They are stronger with a low thermal coefficient which reduces welding stress.
Can cast iron be welded to steel?
It’s possible but not recommended due to the differences in these metals’ chemical and mechanical properties. For instance, steel has higher tensile strength than cast iron, meaning it can withstand pressure before cracking, but cast-iron cracks under stress.
How do you weld cast iron without cracking it?
It’s recommended to preheat the surface you want to work on or use minimal heat where preheating is not possible.
Conclusion–How to Weld Cast Iron
While welding cast iron is challenging, it’s possible as long you understand the properties of cast iron. The welder also needs to undertake the process with utmost care to avoid cracking. Pre and post-weld heating is a must, and high heat is a no-no during the welding process. Finally, always start with a clean surface.