Cast iron is a common and versatile material that is used for various applications, such as engine blocks, manifolds, pipes, fittings, and cookware. However, cast iron is also one of the most difficult materials to weld, as it is prone to cracking due to its high carbon content, low ductility, and thermal expansion and contraction. Welding cast iron requires proper preparation, technique, and post-weld treatment to prevent or minimize cracking and ensure a strong and durable weld. In this article, we will explain how to weld cast iron and avoid weld cracking.
I. What are the types of cast iron and how to identify them?
Cast iron is a general term that refers to a family of iron-carbon alloys that contain more than 2% of carbon. Depending on the amount and form of carbon, as well as the presence of other alloying elements, cast iron can be classified into different types, such as:
Gray cast iron: This is the most common and widely used type of cast iron, which has a gray fracture surface due to the presence of graphite flakes. Gray cast iron is relatively easy to weld, as it has low shrinkage and good machinability. It is often used for engine blocks, manifolds, pipes, fittings, and cookware.
White cast iron: This is the hardest and most brittle type of cast iron, which has a white fracture surface due to the absence of graphite. White cast iron is almost impossible to weld, as it has high shrinkage and poor machinability. It is rarely used for engineering purposes, but it can be found in some wear-resistant applications, such as grinding balls and mill liners.
Ductile cast iron: This is a type of cast iron that has high ductility and toughness due to the presence of graphite nodules. Ductile cast iron is moderately difficult to weld, as it has moderate shrinkage and good machinability. It is often used for automotive parts, hydraulic components, and gears.
Malleable cast iron: This is a type of cast iron that has high ductility and strength due to the presence of graphite rosettes. Malleable cast iron is moderately difficult to weld, as it has moderate shrinkage and good machinability. It is often used for pipe fittings, brackets, and agricultural implements.
To identify the type of cast iron, you can use the following methods:
Visual inspection: You can examine the fracture surface of the cast iron and look for the color and shape of the graphite. Gray cast iron has a gray color and flake-shaped graphite, white cast iron has a white color and no graphite, ductile cast iron has a gray color and spherical-shaped graphite, and malleable cast iron has a gray color and rosette-shaped graphite.
Spark test: You can use a grinder to produce sparks from the cast iron and observe the color and shape of the sparks. Gray cast iron produces red sparks with few branches, white cast iron produces white sparks with many branches, ductile cast iron produces red sparks with few branches and bright bursts, and malleable cast iron produces red sparks with few branches and dull bursts.
Hardness test: You can use a file or a hardness tester to measure the hardness of the cast iron. Gray cast iron has a low to medium hardness, white cast iron has a very high hardness, ductile cast iron has a medium to high hardness, and malleable cast iron has a medium hardness.
II. What are the challenges of welding cast iron and why does it crack?
Welding cast iron is challenging because of the following reasons:
High carbon content: Cast iron has a high carbon content, which makes it hard and brittle. When cast iron is heated, the carbon dissolves in the iron and forms iron carbides, which are even harder and more brittle. When cast iron is cooled, the carbon precipitates out of the iron and forms graphite, which occupies more volume and causes shrinkage. These phenomena create internal stresses in the cast iron, which can lead to cracking.
Low ductility: Cast iron has low ductility, which means it cannot deform or stretch without breaking. When cast iron is welded, it is subjected to thermal expansion and contraction, which create tensile and compressive stresses in the weld and the base metal. Cast iron cannot accommodate these stresses, which can lead to cracking.
Thermal conductivity: Cast iron has a low thermal conductivity, which means it does not transfer heat well. When cast iron is welded, it creates a large temperature gradient between the weld and the base metal, which causes uneven expansion and contraction, which can lead to cracking.
To prevent or minimize cracking, you need to control the heat input, the cooling rate, and the residual stresses in the cast iron during and after welding.
III. How to prepare cast iron for welding?
Before welding cast iron, you need to prepare the cast iron by following these steps:
Clean the surface: You need to remove any dirt, grease, oil, paint, rust, or other contaminants from the surface of the cast iron, as they can interfere with the welding process and cause porosity, slag inclusion, or cracking. You can use a wire brush, a grinder, a sandblaster, or a chemical cleaner to clean the surface.
Preheat the cast iron: You need to preheat the cast iron to a temperature range that depends on the type and thickness of the cast iron, the welding process, and the filler metal. Preheating reduces the temperature gradient, the cooling rate, and the residual stresses in the cast iron, which can prevent or minimize cracking. You can use a torch, an oven, an induction heater, or a furnace to preheat the cast iron. The following table shows the recommended preheating temperatures for different types of cast iron.
| Type of cast iron | Preheating temperature (°F) |
|---|
| Gray cast iron | 400-1200 |
| White cast iron | Not weldable |
| Ductile cast iron | 500-1200 |
| Malleable cast iron | 600-1200 |
Bevel the joint: You need to bevel the joint to create a groove that can accommodate the filler metal and the weld metal. Beveling also removes any cracks or defects from the edge of the cast iron, which can propagate during welding. You can use a grinder, a saw, or a chisel to bevel the joint. The angle and depth of the bevel depend on the type and thickness of the cast iron, the welding process, and the filler metal. The following table shows the recommended bevel angles and depths for different types of cast iron.
| Type of cast iron | Bevel angle (°) | Bevel depth (in) |
|---|
| Gray cast iron | 30-45 | 0.25-0.5 |
| White cast iron | Not weldable | Not weldable |
| Ductile cast iron | 30-45 | 0.25-0.5 |
| Malleable cast iron | 30-45 | 0.25-0.5 |
Drill holes: You need to drill holes at the ends of the joint and at regular intervals along the joint to relieve the stresses and prevent the cracks from spreading. You can use a drill or a punch to drill the holes. The diameter and spacing of the holes depend on the type and thickness of the cast iron, the welding process, and the filler metal. The following table shows the recommended hole diameters and spacings for different types of cast iron.
| Type of cast iron | Hole diameter (in) | Hole spacing (in) |
|---|
| Gray cast iron | 0.125-0.25 | 1-2 |
| White cast iron | Not weldable | Not weldable |
| Ductile cast iron | 0.125-0.25 | 1-2 |
| Malleable cast iron | 0.125-0.25 | 1-2 |
IV. How to choose the right welding process and filler metal for cast iron?
After preparing the cast iron, you need to choose the right welding process and filler metal for the cast iron by following these guidelines:
Welding process: You can use different welding processes to weld cast iron, such as shielded metal arc welding (SMAW), gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), flux-cored arc welding (FCAW), or oxyacetylene welding (OAW). The choice of the welding process depends on the type and thickness of the cast iron, the joint design, the welding position, the availability of equipment, and the cost and quality requirements. The following table shows the advantages and disadvantages of different welding processes for cast iron.
| Welding process | Advantages | Disadvantages |
|---|
| SMAW | Simple, versatile, portable, low cost, suitable for all positions and types of cast iron | High heat input, high spatter, high slag, requires preheating and post-weld treatment, requires skill and experience |
| GMAW | Fast, clean, low spatter, low slag, suitable for thin and ductile cast iron | High heat input, requires shielding gas, preheating, and post-weld treatment, not suitable for out-of-position and thick cast iron |
| GTAW | Precise, clean, low heat input, low spatter, low slag, suitable for thin and ductile cast iron | Slow, requires shielding gas, requires skill and experience, not suitable for out-of-position and thick cast iron |
| FCAW | Fast, versatile, portable, low cost, suitable for all positions and types of cast iron | High heat input, high spatter, high slag, requires shielding gas or flux, requires preheating and post-weld treatment |
| OAW | Simple, versatile, portable, low cost, low heat input, suitable for thin and gray cast iron | Slow, requires fuel gas and oxygen, requires filler rod, requires skill and experience, not suitable for ductile and malleable cast iron |
Filler metal: You can use different filler metals to weld cast iron, such as cast iron, steel, nickel, or copper alloys. The choice of the filler metal depends on the type and composition of the cast iron, the welding process, the service condition, and the cost and quality requirements. The following table shows the advantages and disadvantages of different filler metals for cast iron.
| Filler metal | Advantages | Disadvantages |
|---|
| Cast iron | Compatible with cast iron, low cost, suitable for repairing cracks and defects | High shrinkage, high cracking tendency, requires preheating and post-weld treatment, low ductility and strength |
| Steel | Low shrinkage, low cracking tendency, high ductility and strength, suitable for joining cast iron to steel | Not compatible with cast iron, requires preheating and post-weld treatment, may cause hardening and embrittlement |
| Nickel | Compatible with cast iron, low shrinkage, low cracking tendency, high ductility and strength, suitable for repairing and joining cast iron | High cost, may cause dilution and porosity, may cause corrosion and stress corrosion cracking |
| Copper | Compatible with cast iron, low shrinkage, low cracking tendency, high ductility and strength, suitable for repairing and joining cast iron | High cost, may cause dilution and porosity, may cause corrosion and stress corrosion cracking |
V. How to weld cast iron and avoid weld cracking?
After choosing the right welding process and filler metal, you need to weld the cast iron by following these steps:
Tack weld the joint: You need to tack weld the joint at several points to hold the cast iron pieces in place and prevent them from moving during welding. You can use the same welding process and filler metal as the final weld, or a different one that is compatible with the cast iron. You should use a low heat input and a short arc length to minimize the distortion and stress in the cast iron.
Weld the joint: You need to weld the joint using the chosen welding process and filler metal. You should use a low heat input and a short arc length to minimize the distortion and stress in the cast iron. You should also use a short weld bead and a skip welding technique, which means welding in short segments and alternating the welding direction. This reduces the heat concentration and the cooling rate in the cast iron, which can prevent or minimize cracking. You should also avoid excessive penetration and overlap, which can cause porosity and slag inclusion.
Peen the weld: You need to peen the weld while it is still hot to relieve the stresses and prevent the cracks from forming. Peening is a process of hammering the weld with a ball-peen hammer or a pneumatic hammer to create small indentations on the surface. This compresses the weld metal and the base metal, which reduces the shrinkage and the tensile stresses in the cast iron.
Post-weld heat treat the weld: You need to post-weld heat treat the weld to reduce the residual stresses and improve the mechanical properties of the weld. Post-weld heat treatment is a process of heating and cooling the weld at controlled rates and temperatures. There are different types of post-weld heat treatments, such as:
Stress relief: This is a process of heating the weld to a temperature below the lower critical temperature of the cast iron, which is about 1300°F and holding it for period of time, then cooling it slowly. This reduces the residual stresses and the cracking tendency in the cast iron.
Annealing: This is a process of heating the weld to a temperature above the upper critical temperature of the cast iron, which is about 1700°F and holding it for a period of time, then cooling it slowly. This softens the weld metal and the base metal, which improves the ductility and the toughness of the cast iron.
Normalizing: This is a process of heating the weld to a temperature above the upper critical temperature of the cast iron, which is about 1700°F and holding it for a period of time, then cooling it in air. This refines the microstructure and the grain size of the weld metal and the base metal, which improves the strength and the hardness of the cast iron.
VI.Conclusion
Welding cast iron is a challenging task that requires proper preparation, technique, and post-weld treatment to prevent or minimize cracking and ensure a strong and durable weld. Cast iron is a family of iron-carbon alloys that have different types, such as gray, white, ductile, and malleable cast iron. Cast iron is prone to cracking due to its high carbon content, low ductility, and thermal expansion and contraction. To weld cast iron, you need to clean the surface, preheat the cast iron, bevel the joint, drill holes, choose the right welding process and filler metal, tack weld the joint, weld the joint, peen the weld, and post-weld heat treat the weld. By following the guidelines and tips in this article, you can weld cast iron and avoid weld cracking.
Related articles:
1. About Tungsten in TIG Welding: Types, Selection and Use
2. Identifying and solving tungsten electrode and arc issues
3. How to Solve 10 Common TIG Welding Problems [Guide]
4. TIG Welding: A Guide to Top-Quality Connections
5. MIG/MAG, MMA, TIG Welding: Choosing the Right Technique
