How to Weld Cast Iron

With alternatives such as titanium, ceramic, and stainless-steel coated skillet available today, it may seem a bit outdated to use a cast-iron skillet. While the popularity of cast iron has certainly gone down in the past few years, it hasn’t done as bad as some people say.  The cast-iron cookware market is expected to reach US$3,303 million from US$2,537 million in 2017.

In addition to the above, Transparency Market Research expects the global Iron and Steel Casting market to exceed US$202 billion by the year 2026. The point of all these statistics is to prove that cast iron is still being weld across the globe for different uses, like cookware. Regardless of your reason for welding cast iron, you must learn the right way of welding the material.

While there are many challenges to overcome, welding cast iron parts in the right way will help you save both time and money. If cast iron is not weld properly, then this can cause a crack or damage to the material being weld.

Often, assistance from a professional and experienced welding technician is required to weld cast iron appropriately; this is especially true when critical parts are involved in the welding process. However, you can effectively weld cast iron on your own by following a few key steps. We will be discussing these steps here to help you learn how to weld cast iron effectively.

The Applications of Cast Iron and Its Benefits as a Welding Metal

There are several applications of cast iron. Some of them include:

  • Water preps
  • Agricultural equipment
  • Automobile engine heads, blocks, and manifolds
  • On-machine tools such as covers, brackets, and bases
  • Pipe fittings
  • Cast iron pipe

In addition to the above, there are many other applications of cast iron metal. For example, it is widely used in construction machinery and other applications where weight is a key requirement. These are some of the applications of cast iron but what about the benefits? Following are some key benefits of cast iron as a weld metal:

  • Many desirable properties including robustness, rigidity, thermal conductivity, and the capacity to humidify
  • Ability to shape with sand casting
  • Low-cost material
  • A melting point lower than steel
  • Greater fluid than steel

These are some of the main reasons why cast iron makes a good metal for welding with many applications.

Why Welding Cast Iron Can be Problematic

You need to understand the difficulty of performing the job before you find out how to weld cast iron. This is because the success ratio of properly welding cast iron is just 50%; there is a high chance that that cast-iron piece you weld will have cracks or damage once you’re done welding it.

Why does this problem arise? Because several different materials of varying proportions make up cast iron. As a result, it can be difficult to evaluate the exact strength of the metal you’re working with. It can also be difficult to determine how much heat the cast iron piece can withstand before it starts to crack. The high carbon content of the cast iron is basically what causes all these problems.

While you weld, this carbon might transfer to the metal being weld or the area next to the weld metal that is heated. This can cause increased brittleness or hardness which can lead to post-weld cracks.

The Different Techniques/Methods for Welding Cast Iron

There are several methods for welding cast iron. However, we’re only going to discuss the most common techniques employed for welding cast iron. These methods include gas welding, arc/stick welding, MIG welding, and TIG welding. Following is a brief explanation of each.

1. Gas Welding

Compared to the Arc-based processes for welding cast iron, the heating process is much slower with gas welding. Moreover, a lower temperature flame than an arc is used in this technique. The good thing about this is that carbon migration is generally not an issue when cast iron is welded using the gas welding technique. However, it is important to employ a proprietary gas welding rod in the process. The only problem with this technique is that it requires a considerable amount of heat when large components are involved in the weld. Also, gas welding is slower than other techniques for welding cast iron.

2. Arc/Stick Welding

Preferred by most DIY welders, arc/stick welding can be the most effective way to weld cast iron if the right welding rods are used. The process is extremely simple and there are tons of electrodes available for the different jobs.

A special Graphite rich flux is present in cast iron welding rods. The carbon in the cast iron is chemically tied up by this special graphite. This limits migration into the heat affect zone and the weld metal. Cast iron welding rods are available in two main types: Pure Nickel and Ferro-Nickel.

Typically, Ferro-Nickel rods are made up of 47% nickel and 53% steel. Much cheaper than pure Nickel, Ferro-Nickel rods are perfect for welding cast iron to steel. On the other hand, if you want a weld deposit that is softer and more malleable, then go for Pure Nickel. However, unless the job specifically requires the use of Pure Nickel, we would recommend that you use Ferro-Nickel when welding cast iron with the arc/stick welding technique.

3. MIG welding

Although it is used often for this purpose, MIG welding is not a great choice for welding cast iron. The only real reason that you’d want to use this technique for is when you have repeating application for which you can create a procedure. In case you choose to weld cast iron this way, you should keep the following tips in mind to weld effectively:

  • Specialty nickel wire is the best choice when using the MIG welding technique. However, this option can be a bit expensive so you may want to consider stainless steel as an alternative.
  • In most situations, the best choice is an 80/20. What does this mean? It refers to 80% argon and 20% carbon dioxide. Although cast iron is naturally prone to rust, you should avoid this method if you’re worried about rust eventually developing on the metal.
  • You can bond the cast with the help of a brazing wire. However, this is likely to result in a weak weld which isn’t great for metals that are likely to experience loading or impact.
  • You can ensure the weld’s mechanical strength by utilizing studding. However, this will depend on the job being performed

4. TIG Welding

Like MIG welding, TIG welding is not preferred by many people for welding cast iron. Why is this? Because TIG is an open-arc process which means that there is no real opportunity to minimize carbon migration with this technique.

In addition to the above, the only real option available with this technique when welding cast iron is a nickel wire. Another issue with the process is that it can be expensive. On the positive side of things, the TIG welding method provides a clean and strong weld on the most cast-iron jobs. Additionally you can use the TIG welding method to cold weld provided you use the appropriate settings wire and gas.

These are the four most common methods used to weld cast iron. Now that you have basic information on each method, you can pick a technique/method that is most suited to your needs and the job being performed. However, regardless of the method you chose to weld cast iron, you need to follow some basic steps to produce an effective weld. These steps are discussed next.

The Key Steps in Welding Cast Iron

When welding cast iron on your own, you need to know the critical steps involved in effectively welding cast iron. There are four steps involved in DIY cast iron welding. By performing these steps in the following order, you will be able to produce an extremely durable cast iron weld.

1. Identify the Alloy

Cast irons are part of the iron-carbon alloy family and they have a high carbon content; this is where they get their hardness characteristic from. While the cast iron metal is hard, this quality comes at the expense of ductility. Compared to wrought iron or steel, cast iron is far less malleable.

As a result of the above, the metal expands and contracts when it is heated and cooled during the welding process. This creates tensile stress. It can be extremely difficult to weld cast iron because when they are stressed or heated, they crack instead of deforming or stretching. However, you can improve this situation by adding different alloys. Following are the two options available to you.

Grey Cast Iron

The most common cast iron alloy, grey cast iron is more weldable and ductile than white cast iron. During manufacturing, carbon on grey cast iron advances into graphite flakes and changes into either a ferrite or pearlite crystalline microstructure. The only problem with this cast iron ally is that the graphite flakes inside it can enter the weld pool; this causes embrittlement in the weld metal which can pose a challenge to welders.

White Cast Iron

The other cast iron alloy option available to you is white cast iron. Unlike grey cast iron, this form of cast iron can keep hold of the carbon as iron carbide without expediting it out as graphite. The result is a brittle and hard cementite crystalline microstructure. However, a major cause of concern with white cast iron is that it is mostly considered unweldable.

2. Clean the Casting

You must prepare all castings properly before you start to weld; this is regardless of which cast alloy you’ve chosen for welding. Another important thing to do here is removing all surface materials to prepare the casting for welding. This will ensure that the casting in the weld area is completely clean.

Once this is done, the next step would be eliminating oil, grease, paint, and other obstructive material from the area of the weld. One of the most important things to ensure here is that heat is applied slowly and carefully to the welding area for only a short period. This will help eliminate any gas trapped in the base metal’s weld area. A technique that you can use to test the preparedness of the cast-iron surface is resting a weld pass on the metal. If impurities are present on the metal, then it will be porous. To ensure the porosity has been eliminated, you can repeat the process several times while grinding off the pass.

3. Pre-Heat

When put under stress, all cast irons can become vulnerable to tracking. Therefore, the single most important thing to avoid cracks in cast iron is controlling the heat during welding. There are three steps involved in welding cast iron. The three steps are:

  • Pre-heating
  • Low-heat input
  • Slow cooling

Thermal expansion is the main reason why you’d want to control heat during welding. Metal is likely to expand when it is warmed. However, warming and expanding an entire cast iron piece at the same rate will help to avoid stress in the metal. On the other hand, localizing heat in the heat-affected zone can cause stress to build in the metal.

The problem with localized heating is that it can lead to restricted expansion. Here, the cooler metal around the heat-affected zone controls the HZ. Additionally, the thermal gradient between the casting body and the heat-affected zone determines the degree of resulting stress.

Stretching relieves stress caused by restricted expansion and contraction in steel and other ductile metals. However, this can lead to cracks in the metal during contraction; this is because the ductility of cast irons is rather poor.

With pre-hearing, you can reduce the thermal gradient between the heat-affected zone and the casting body. This, in turn, will reduce welding-induced tensile stress. Generally, a higher temperature pre-heat is needed by higher temperature welding methods. Therefore, the best strategy to reduce heat input in the absence of adequate pre-heating is choosing a low-temperature welding process. Additionally, you should go for wires or welding rods with a low melting point.

Another factor impacting the stresses induced at the weld is the cooling rate. Contraction can result from rapid cooling, leading to fragile and easily cracked welds. On the other hand, low cooling can minimize the stress caused by contraction and hardening.

To conclude this step, we would that, while all cast irons can develop cracks when they are put under stress, you can lower the chances of this with pre-heating. This indicates the importance of this step in the welding of cast iron.

4. Choose a Welding Technique

We’ve already gone through the four main techniques/method used to weld cast iron. Based on the information provided above, you need to pick a welding technique that is best suited to welding job at hand. In other words, you need to choose a welding technique based on the size of the metal and alloy welded.

5. Finishing

Once you’ve picked and used the most appropriate welding technique for the cast iron you need to weld, the next and final step would be finishing the weld. As the weld cools and contracts, tensile stress can be built into the cast iron metal being welded.

Therefore, the thermal contraction phase is when the iron cast metal is most likely to develop cracks. It is almost certain that the weld will begin cracking once the stress reaches a critical point. The good news is that there is a way to reduce the chances of the weld developing a crack; this involves the application of compressive stress during cooling to oppose tensile stress.

While the weld is still soft, a technique called peening is employed by welders to a weld bead that can be deformed. The peening technique involves the use of a ball-peen hammer to perform moderate strikes.

Why is this technique useful? Because it can help lower the chances of a crack developing in the heat-affected zone and the weld. However, an important thing to ensure is that this method should be used only when the metal being welded is relatively ductile. The final step that you need to perform while welding cast iron control is cooling control.

What does this process entail? It is a process that looks to slow cooling as much as possible by using insulating materials. Another way used to slow down the natural cooling process is by applying heat to weld periodically. All of these things ensure a good finish and an effective cast iron weld. You now know how to weld cast iron.

About Pierre Young

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Hey, I'm Pierre Young a qualified AWS Certified Welder. I got into welding in 2009 as a side hustle. Ever since then, I've been doing all kinds of welds - both for business and pleasure. While immersing myself in this wonderful hobby, I've learned from hands-on experience what welding gear works and what doesn't. Welding Headquarters is the site where I share everything I've learned.

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