A Comprehensive Guide: Mastering the Art of Welding Cast Iron

Welding cast iron has a unique set of challenges for even the most seasoned welders. However, understanding the aspects of this process can be immensely rewarding. This is because it allows for cost-effective repairs of expensive cast iron components commonly found in engine parts. In this guide, we will delve into the methods of welding cast iron, with a focus on grey cast iron. 

Types of Cast Iron and Weldability:

Grey Casting Iron

While all cast iron types are weldable except for white cast iron, grey cast iron is the most prevalent in various applications. Luckily, grey cast iron handles welding the best, but that doesn’t mean welding it is easy. Gray cast iron, with its grey fracture surface, is the least challenging to weld. But it does come with its own set of considerations, such as its tendency to absorb oils due to its porosive structure.

Ductile And Malleable Cast Iron

Ductile and malleable cast irons are something “in-between” the grey and white cast irons. They are weldable, but both have specific heat requirements before, during, and after welding. For example, if the malleable cast iron is heated above its critical temperature of about 925°C, the carbon will recombine with the iron and transform into white cast iron. 

White Cast Iron

White cast iron is so hard and brittle that it’s considered unweldable. This cast iron type simply doesn’t have the necessary ductility to accommodate the welding stresses. This would always result in the weld cracks. The carbon atoms are combined with iron atoms into iron carbides, which produces its characteristic white fracture colour, making it a bit easier to identify visually.  

We will now focus on the grey cast iron for the rest of this guide since this is the cast iron type you are most likely to see in your welding career. While challenging, it’s not impossible to master welding grey cast iron. 

Challenges of Welding Cast Iron:

The inherent hardness and brittleness of cast iron make it susceptible to cracking during the welding process. Unlike ductile materials like mild steel, cast iron lacks the flexibility to accommodate welding-induced stresses. This leads to cracks in the weld and base metal, especially when cooled rapidly. DO NOT quench (cool with water) the welded cast iron parts; they will immediately crack. If you hear tinkling noises (like breaking thin icicles) coming from the welded cast iron element as it cools down, the weld and the heat affected zone (HAZ) is likely cracking internally, and the part needs to be re-welded by removing the compromised metal and welding it again.

How to Weld Cast Iron Like a Pro:

 1. Welding Process Selection

  – We strongly recommend using the stick welding process to repair or join cast iron parts. While you can MIG and TIG weld cast iron, stick welding is the industry’s go-to choice for a good reason. Stick welding is recommended for repairing or joining cast iron parts. This is due to its high-temperature arc, fast travel speeds, and lower preheating requirements. It’s crucial to weld cast iron quickly, which is possible with the stick welding process. In addition, you can switch polarity to DCEN and reduce the HAZ and lessen the HAZ brittleness as a consequence of welding. Cast iron is sensitive to input temperature fluctuations. So make sure that your stick welding machine has a reliable and consistent arc output.

2. Preparing the Casting and Joint

-The first thing you need to do is remove the casting skin over the entire welding area. Next, remove dye penetrants if they were used to test for weld cracks. They are also used to clean the surface from all oils, grease, paints, markings, etc.

We mentioned earlier that grey cast iron can soak in oil. So, it’s essential to bake out the embedded oil before welding engine cast iron parts. Otherwise, the trapped oil and other compounds will burn during the welding process and emit gases which will cause porosity. To prevent porosity, preheating the casting to temperatures above 260°C is necessary. But we’ll discuss preheating in far more detail soon.

If you are repairing a crack on the cast iron part like an engine block, drill two holes at both ends of the crack to prevent the crack from spreading any further as you weld. Welding temperature will cause local expansion and contraction and the crack will propagate unless you drill holes at both ends of the crack.

You should also consider using carbide tools to gouge out the crack to make a V or U shape before welding. But the need to do this will vary from case to case depending on the cast iron type, crack depth, and the service life conditions of the part you are repairing. To use common sense, consider if the part can serve its purpose if the weld is mostly a surface weld. If not, gouge out the crack to provide a deeper weld penetration. 

If joining two cast iron parts into a butt joint configuration, it’s best to make a U joint on both ends when possible.

3. Preheating-A Crucial Step:

-Preheating the entire part before welding reduces the thermal gradient between the weld zone and the surrounding metal, minimising cracking risks. This is because preheating allows for better realignment of the material during cooling. Which in turn helps prevent cracks caused by constrained heat-affected zones (HAZ).

But, there is a catch. You should preheat the casting evenly. If you heat one part of the casting at a higher temperature than the rest of the casting, that part will expand more and push into the surrounding material that has not expanded at the same rate. So now, colder parts and hotter parts will push and pull on each other, causing the brittle cast iron to crack. 

If you are repairing equipment you cannot preheat because of size or other constraints, consider powering the equipment and running it until it reaches the maximum service temperature. Keep in mind that the closer you get to the necessary preheating temperature, the better the chances of a successful weld. If you cannot preheat the part, the welds will be much harder and more likely to re-crack. But it’s often far cheaper to repair industrial equipment and keep repairing it than to replace it, especially if the crack is not critical.

Typical preheating temperatures for stick welding cast iron are between 315°C-815°C, but try never to weld parts below (200°C). Heat the casting slowly. Keep in mind too much heat may cause it to expand too quickly on the surface but not fast enough deep in the metal and again cause conflicting forces making it to crack.

After welding, place the part in an enclosed metal box to slow down the cooling. Or cover it with sand or a welding blanket. 

4. Filler Metal Selection:

-The key to successful cast iron repair lies in using nickel-based filler metal. Nickel rods increase ductility by blending with the base metal, making the weld more flexible. Additionally, rods like ENi-CI-A and ENiFe-CI-A contain carbon levels exceeding nickel’s solubility limit, expanding weld volume and counteracting shrinkage stresses during cooling. Nickel rejects solid carbon from the base metal, reducing the volume of cast iron in the joint.

However, you must be careful when using the nickel-iron and nickel-iron-manganese electrodes because they may introduce more cast iron into the weld. This may result in transverse and centerline weld cracking in certain conditions. 

When in doubt, we recommend using nickel-base stick electrodes. Of course, if you are welding expensive industrial equipment, consult the engineer and look for suitable filler metal for that specific cast iron alloy. 

5. Laying the Welds:

-First, use the lowest amperage output on your stick welder that you can for the thickness of the welded part and the diameter of the nickel-based electrode. The lower the current you can use, the least heat you’ll introduce into the HAZ. If you don’t need maximum penetration, consider switching the polarity to DCEN (electrode negative, ground clamp positive). You can do this when making surfacing welds on worn cast iron parts that only require surface repair. Or to weld cracks where penetration depth is not a major concern.

Restrict the welds to 1-inch segments and never make welds longer than 10X the electrode’s diameter. 

If you are repairing a crack on the engine surface or something similar, start the weld on the casting surface slightly behind the drilled holes. Weld towards the centre of the crack but don’t make a weld longer than 1 inch. Repeat the process from the other side of the crack, and then weld from the centre towards both welds.

Avoid re-striking on the base metal. Instead, re-strike the arc on the previously laid weld. 

Consider grinding out areas of the casting with the carbide bits where you may get a high welding stress concentration. As well as buttering in the area with the ductile welding material, like nickel-based electrodes, before joining two cast iron parts. For example, if you were to make a T-joint where one cast iron part is ground to form a 30-degree bevel to the other part. The other part would experience a lot of stress as the weld cools and pulls it towards the beveled part. But, if you grind out both parts and fill the weld groove with nickel-based filler metal, you would have better chances to avoid cracking because both parts are filled with ductile material.

Cascade Welding

When welding thick V, U, or J, type joints, use a cascade welding sequence. First, lay the bottom first layer and then follow with cascading layers on top of it. You can see an example of this as in the image below. Continue laying welds until you fill the joint. The cascade sequence should help in preventing overheating of the HAZ.

6. Stress Relief

-The peening technique involves applying numerous blows perpendicular to the weld surface using a rounded peening, ball-point hammer. 

Hitting the weld bead as it cools will counteract the shrinkage forces and minimise the distortion and stress concentration. Quite literally, peeing the weld will push the weld metal into the surrounding metal and prevent the weld from pulling the surrounding metal. Combine peening with preheating and nickel-based electrodes, and you are golden. 

The number one mistake to avoid is hitting the weld metal with too much force. In addition, you must maintain a temperature above  538°C. So, as soon as you deposit the weld, start hammering it down while it’s still warm. Don’t wait for a second more than what it takes you to exchange the stinger for the hammer.

Finally, slowly cooling down the entire casting in the sand or an enclosed box will help relieve stress. This will help minimise the chances of weld cracking internally or externally.

Final Thoughts:

Mastering cast iron welding requires practice and attention to detail. Through careful preparation, proper preheating, and selecting the right filler metals, welders can navigate the challenges and broaden their skill set in this specialised field. Welding cast iron demands a thorough understanding of its properties and meticulous attention to each step in the welding process. By honing these skills, welders can confidently tackle cast iron repair projects, opening up new opportunities in their welding careers.

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