The Comprehensive Guide to Brazing Cast Iron

Picture of patterned cast iron. Photo courtesy of Rupert Kittinger Sereinig from Pixabay.

Cast iron is characterized by its low ductility, which makes it uniquely challenging to weld. Welding may result in embrittlement in the heat-affected zone, which may lead to failure in the completed weld. Brazing is often the preferred method to repair cast iron parts.

Types of Cast Iron for Brazing

There are several types of cast iron, each of which has unique characteristics that make them well-suited to different applications. The five main types are:

Gray Cast Iron

This is the most common type. It has a graphite microstructure, which gives it its characteristic gray color. Gray cast iron is popular for its good machinability and thermal conductivity.

White Cast Iron

White cast iron is hard and brittle, which makes it difficult to machine. It is used in applications where hardness and wear resistance are necessary.

Ductile Cast Iron

Ductile cast iron has higher strength, toughness, and ductility than gray cast iron. It is often used in applications where greater ductility is required.

Malleable Cast Iron

Malleable cast iron is produced by heat-treating white cast iron or adding trace amounts of elements to temper the carbon. This gives it good ductility and impact resistance, making it ideal for applications that offer higher levels of ductility and the capability to deform without fracturing.

Compacted Graphite Iron

Often called CGI, compacted graphite iron has a microstructure that’s somewhere between gray cast iron and ductile cast iron. It is known for its high strength, high thermal conductivity, and wear resistance, which makes it well-suited for use in engine components.

Types of Filler Materials

As with any other type of welding, brazing, or joining process, the choice of filler metal will affect the quality of the completed joint.

Brazing rods come flux-coated or bare, with either a flux added in the form of a paste or a granular form. Cast iron will almost always require flux in one form or another to keep the brazed joint free from impurities and prevent the detrimental effects impurities may have on the completed joint.

Choosing the Right Filler Materials

Copper-alloy filler metals are common for brazing cast iron parts. They are chosen for their capability to produce durable joints. Copper alloy can also join dissimilar metals together.
Silver-based brazing rods are also chosen for their capability to form strong bonds at relatively lower temperatures. Silver-based alloys are a good choice when joining materials such as carbide to a cast iron piece.
A silicon-bronze wire is generally used as a TIG welding rod but can braze cast iron with good results. Note that flux may be necessary with a silicon bronze rod.

Safety Precautions

Safety is as important in brazing as it is in welding processes. You should consider the following items carefully.

Personal protective equipment

Brazing generates heat, which may cause severe burns. For this reason, it is important to wear welding gloves, a long-sleeved welding shirt or jacket, sturdy work pants, and safety-toe work boots. Safety glasses, face shields, or welding goggles are a must to protect the eyes from the intensity of the flame. Be sure the shade is rated for the process — generally, a shade 5 lens is suitable.

The photo below shows a welder using a shade 5 face shield to perform brazing operations on a metal sculpture.

Ventilation

Since brazing cast iron may give off fumes, working in a well-ventilated area is critical. Consult the filler metal manufacturer’s recommendations for respiratory protection.

Step-by-Step Guide

Choosing the Right Brazing Technique

There are four main ways to perform brazing:

Torch or manual brazing

This method is the most common for its low cost and simplicity. Torch brazing only requires a torch (typically oxy-acetylene fueled) and the filler metal.

Induction brazing

Induction brazing uses a high-frequency current to produce localized heat sufficient to melt the brazing alloy into the joint.

Resistance brazing

Small parts are often joined by resistance brazing, which is performed when an electric current passes through two parts with a brazing alloy between them. The heat generated by the electric current melts the brazing alloy, which flows through the joint forming the desired bond.

Vacuum brazing

This process uses heat in an enclosed chamber where air is evacuated and the part is heated up to melt the brazing filler metal to achieve the desired bond. It is then brought down to ambient temperature in a slow, controlled manner. The absence of air prevents the formation of oxides and porosity-type defects.

Preparing the Cast Iron Surface

There are many different methods to clean a cast iron part prior to brazing. Many people like the clean surface provided by sandblasting, which may be advantageous for large parts of odd shapes and dimensions. When sandblasting is not an option, a simple wire brush may be sufficient for smaller parts. Cast iron parts that are dirty or have paint, grease, or oils on the surface may require some preheating to remove the contaminants.

Heating the Cast Iron

Perhaps the most important step in the brazing process is the heating of the cast iron part to be brazed. It’s important to heat the cast iron throughout to avoid stress cracking that may result due to cast iron’s lack of ductility. In the immediate area of the joint, it’s necessary to heat the iron to the point where it is nearly red-hot. Only then should you start to add the filler metal.

Applying the Brazing Filler

Once the cast iron is heated sufficiently, add the appropriate filler metal. It is important to keep in mind that the cast iron part itself should melt the filler metal when brought into close contact. Do not use the torch to melt the filler metal. If the filler metal does not melt or “draw” into the joint, you may need more heat.

Cooling Process

It is important to bring a cast iron part down to ambient temperature slowly to eliminate the risk of cracking. There are a variety of ways to do this.

If an industrial oven or furnace is available, simply place the finished part in the oven and bring the temperature down in a slow, controlled manner. This may take several hours, depending on the size and thickness of the part. It may also be possible to use a forge or even a gas grill to assist in the cooling process.

If an oven or similar equipment is unavailable, an alternative method involves heating the part using a torch. In this case, the flame size and distance to the workpiece should be adjusted, ensuring the entire part reaches a uniform temperature to avoid cracking.

A common and cost-effective method for controlling the cooling rate of a completed cast iron brazed joint is to fill a barrel or large bucket with vermiculite and completely submerge the completed cast iron part. You can purchase vermiculite in many hardware stores. It undergoes expansion when heated, giving excellent insulating properties that maintain heat in a cast iron part, which enables it to cool slowly.

Cleaning and Finishing

Depending on the application of the end user, it may be necessary to clean the brazed part. You can do this in several ways and to varying degrees.

Cleaning the Brazed Cast Iron

Oftentimes, a simple wire brush may be all you need to clean the surface of the brazed joint. You can brush off any residue or debris baked out of the finished part. Inspect the brazed connection, looking for any voids, cracks, and other defects that may prove detrimental to the strength of the piece.

The photo below shows a completed and cleaned brazed connection. Note the shiny surface — free of impurities.

Finishing the Brazed Cast Iron

If you need a more aesthetic or uniform surface finish, it may be necessary to grind the connection down flush to the base metal. You can accomplish this with a simple angle grinder with a sanding pad or Tiger disc attachment.

Common Problems and Solutions

There are some inherent challenges to joining cast iron with the brazing method.

Porosity and Cracking

Porosity is the entrapment of gas pockets in a welded or brazed connection. Porosity is usually the inevitable result of contaminants in or on the material’s surface or sometimes in the filler metal itself. It is important to clean the joint and surrounding area thoroughly to minimize the chances of porosity defects in the completed part. You should always store filler metals in a warm, dry place, free from moisture that they and flux could absorb.

Cracks are another common problem. They are typically caused by a lack of uniformity in heating, which results in stress in the microstructure of the metal. Keeping the cast iron part you will be brazing uniformly hot often minimizes the chances of crack formation.

Incomplete Wetting and Flow

Incomplete wetting and flow in the brazed joint is usually due to insufficient heating of the base metal or a lack of uniformity across the entirety of the brazed joint. If the filler metal is not drawn into the entire joint (resulting in voids or the filler metal falling out), stop adding filler metal and apply more heat to the problematic area.

Routine Maintenance

Routine maintenance is essential for achieving reliable, consistently high-quality bonds when brazing cast iron. You can achieve this by:

Inspecting and cleaning equipment like torches, induction coils, furnaces, and heating elements to ensure they are free from dirt and debris that may adversely affect the brazing process.
Check temperature control devices or torch hoses and regulators to ensure accurate and consistent heating.
Inspect fixtures, clamps, and fit-up tools to ensure the parts to be joined will align and fit up properly.

Expert Tips

One of the most common errors in brazing is overheating the part. The cast iron material must be hot enough to absorb the brazing filler metal but not so hot that the iron reaches a molten state. This error occurs mostly with experienced welders who are accustomed to molten iron.

FAQ

Which is stronger: brazing or welding cast iron?

The answer depends on the application. In many cases, it is possible to join materials that would become embrittled, due to the higher temperatures involved. A brazed joint is often advantageous due to the lack of a heat-affected zone, which results from most common welding processes.

What metals can brazing join?

In addition to cast iron, brazing is suitable for many other materials, including brass, copper, stainless steel, carbon steel, gold, nickel, tungsten, and carbides.

Are there metals it is not possible to braze?

Some alloys are not well suited to brazing, including titanium and metals with oxide coatings. This is due to the fact it is difficult to heat an entire joint with the uniformity required to form a strong bond by brazing.

Featured photo courtesy of Michael Kauer // Pixabay