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Can you weld cast iron?

Can you weld cast iron? Your complete guide

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Welding cast iron is quite a challenge, but if you strictly follow the recommended guidelines, you can repair cast iron parts with little or no cracking.

Find out why welding cast iron is so difficult, how to weld it, and which welding rods are best to use on cast iron.

The biggest challenge with welding cast iron

Cast irons are iron-carbon alloys with a carbon content of 2 to 4%, about 10 times the amount of carbon in most steels. There are many types of cast iron, but our guide focuses on gray cast iron because it is the most commonly used. Remember that it is impossible to weld white cast iron, while all other types are weldable.

Although cast irons are hard, strong, and machinable, they have low ductility, which makes them incredibly difficult to weld. Welding any metal causes expansion and contraction as the material heats and cools. However, as cast iron has low ductility, especially in the fusion zone, residual expansion and contraction stresses can cause cracks in the weld.

When you weld carbon steel, a ductile material, as the weld and fusion zone expand and contract, the surrounding material can “flex.” However, cast iron doesn't deform before it breaks – it just breaks. Thus, when the weld cools and begins to pull on the surrounding cast iron mass, it can easily crack.

To make matters worse, the welding process can create locally enriched carbon zones and, upon rapid cooling, the heat affected zone (HAZ) can become extremely hard and brittle. This is why cast iron is susceptible to HAZ cracking and generally considered difficult to weld.

How to weld cast iron

It is possible to weld cast iron using oxy-acetylene and electric arc welding (SMAW). You can also join cast iron parts by brazing, but the bond cannot match the strength of welding.

Welders generally use the stick welding process for cast iron. Its high temperature arc allows for fast travel speeds and you can use lower preheat temperatures. Additionally, electro-electronic welding can be used with DCEN (electrode negative) polarity, which can help reduce HAZ. Make sure you use a quality arc welder like the Kemppi Minarc Evo 140VRD, or the more powerful version, Kemppi Minarc Evo 180VRD. Your welder must have a stable and predictable arc output to avoid inconsistencies. Since cast iron is a sensitive material, you should not use poorly optimized machines.

Prepare the part for welding

Removing all oil, paint, grease and cast film from the surface is crucial. Additionally, remove residual graphite from the surface with mineral spirits. Pay special attention to the soldering area.

If the joint requires grooves, use a V-groove with an included angle of 60-90°. It is helpful to create grooves around cracks and joints to ensure maximum penetration and avoid surface-only welds.

Controlling the heat

Preheating the cast iron is the fundamental step to avoid cracks in the weld. You should preheat the entire casting slowly and evenly if possible. As the part is heated, it expands. As a result, it can crack easily if there is a thermal gradient across its volume. However, if heat is applied evenly, there will be no localized spots that resist expansion and may crack.

Preheating will also allow the casting to cool slowly, decreasing the thermal gradient between the HAZ and the rest of the part and reducing residual stresses. Typical preheat temperatures for cast iron are 260-650 degrees Celsius (500-1200°F). Never preheat above 760°C (1400°F).

Stagger welds during the welding process whenever possible. Use the lowest amount of amperage and avoid making welds larger than ten times the electrode diameter.

Allow the casting to cool slowly after welding to prevent residual stresses and cracking. You can place it on sand or cover it with specialized welding blankets to prevent rapid heat loss.

Welding Rods for Cast Iron

Nickel alloy welding rods (AWS A5.1 Eni-CL) are the most commonly used electrodes for welding cast iron, and for good reason. Nickel electrodes create a ductile weld metal that remains soft and machinable. The welds will have reduced tensions and greater resistance.

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