How to Weld Cast Iron vs. Cast Steel

If you are an experienced welder, you probably already know that welding cast iron or cast steel can be challenging. Cast iron has a high carbon content of around 2 to 4 percent, approximately ten times more than most steel. It might also contain higher levels of sulfur and phosphorus, making it harder to weld without cracking.

On the other hand, the chemistry of cast steel is often similar to low-carbon cold rolled steel, so in many cases, it will be easier to weld than cast iron. Still, higher amounts of silicon and manganese in cast steel can make welding it much more difficult.

The good news is that welding experts have come up with methods that make any issues with welding either of these cast metals surmountable. Here is their advice:

How to weld cast iron

If you’re going to tackle welding cast iron, take the following four steps before welding:

1. Identify the alloy

The three primary grades of cast iron include gray iron, ductile iron, and malleable iron. Note that there is also white cast iron, but most agree it’s impossible to weld it successfully.

Gray iron is the most common form of cast iron, presenting a challenge for welders when the graphite flakes enter the weld pool and cause weld metal embrittlement.

Ductile and malleable iron are less brittle because of their microstructural differences. Both have spheroidal carbon microstructures from their manufacturing processes.

2. Clean the casting thoroughly

Castings need to be properly prepared before welding. Remove any paint, grease, oil, and other foreign materials from the weld area. Then apply heat carefully and slowly to the weld zone for a short time to remove trapped gas in the base metal.
An oft-used technique to test the cleanliness of the surface is to deposit a weld pass on it. If impurities remain on the iron, the weld will be porous. You can repeat the process until porosity is no longer present.

3. Preheat the casting

The main reason for controlling the heat is thermal expansion. When metal is heated, it expands. If the entire object warms and expands at the same rate, there is little stress. However, it creates stress and cracking when the heat is localized in a relatively small heat-affected zone (HZ). Preheating minimizes the thermal gradient between the casting body and the HZ, lowering the tensile stress caused by welding.

4. Select a suitable welding technique

Choose the welding technique that is most suitable for the iron alloy. The three most common processes are stick, oxy-acetylene, and braze welding, with stick being the most popular.

With stick welding, three primary filler electrodes work for cast iron: copper alloy, cast iron covered, and nickel alloy. The nickel alloy electrodes are quite popular for cast iron because the nickel-iron weld is stronger and has a lower coefficient of thermal expansion, reducing stresses and increasing resistance to cracking.

Direct the electric arc at the weld pool instead of at the base metal. Doing this minimizes dilution. Use the lowest current setting approved by the manufacturer to help reduce heat stress. Preheat the pieces to at least 250°F before welding with copper or cast iron or electrodes. No preheating is needed with nickel electrodes.

How to weld cast steel

Learning how to weld cast steel is less of a challenge compared to cast iron. That’s because cast steel is carbon or alloy steel that has been melted and molded into a specific shape. However, even though cast steel has similar properties to rolled steel, some differences make it harder to weld.
Although you can weld cast steel using the MIG or TIG processes, many welding experts recommend stick welding with E7018 rods for low-carbon alloys and stainless-steel rods for hard-to-weld castings.
One of the issues with welding cast steel is avoiding distortion. Here are some suggestions to help with that:

• Preheat the casting: Castings can have irregular shapes, with thinner areas heating more quickly. Concentrate the heat on thicker regions.
• Tack weld frequently.
• Set to the lowest amperage that will produce a strong enough weld.
• Use the fastest traveling speed possible.
• Use fewer passes.
• Slow down the cooldown by wrapping it in welding blankets or burying it in sand.
• A few more tips on how to weld cast steel successfully:
• Clean the edges to be welded down to the bare metal.
• Preheat cast steel if it has more than 0.40% carbon or it is low-alloy steel. Preheating to 250°F prevents hydrogen in the weld.
• If you weld in a cold environment, preheat low-carbon castings to 75°F.
• If you are repairing a crack, drill both ends (1/8″) and remove the crack by grinding or gouging.

How to weld thick cast iron

Thick castings often become hard after welding, and cold cracking may occur long after completion. Or, any added impurities can concentrate in the center of the weld, causing a crack before the weld even cools.

Here are a few hints on how to weld thick cast iron:

• Keep the welding rod vertical whenever you weld cast iron.
• Take it slow and steady, limiting the amount you use each time.
• When you repair a crack, run a small amount of beading across each end to stop it from spreading.
• Maintain a steady hand and constant speed.

Remember, the longer a weld takes to cool, the stronger the joint will probably be. Iron is brittle, and heat causes stress on the material and weakens structural integrity.