Strategies for Welding of Cast Iron

Cast iron is a broad family of iron–carbon alloys containing 1.7% to 4.5% carbon. In addition to carbon, it typically contains:

• Silicon: 0.5% – 3%
• Manganese: 0.2% – 1.3%
• Phosphorus: up to 0.8%
• Sulphur: up to 0.2%

The carbon content is the defining difference between cast iron and steel. It directly influences structure and properties.

• Lower carbon levels tend to promote the formation of white cast iron (hard and brittle).
• Higher carbon levels encourage the formation of grey cast iron, where carbon appears as graphite flakes.

This variation in carbon form and distribution is why cast iron behaves very differently during welding compared to mild steel.

Properties of Cast Iron

Here are the properties of cast iron:

1. Hardness

Apart from being naturally hard, the ability of cast iron to be hardened further through heat and rapid cooling helps to increase its resistance to wear, and also increases its brittleness.

2. Melting Point

The melting point of cast iron is approximately 1200°C, whereas mild steel has a melting point of approximately 1300°C and 1400°C. This lower melting point affects heat control during welding and makes overheating a real risk.

3. Castability

Because of its high percentages of carbon and silicon, molten cast iron has excellent fluidity. This allows manufacturers to cast intricate and complex shapes with relative ease. It is one reason cast iron remains widely used in:

• Engine blocks.
• Pump housings.
• Machine bases.
• Heavy equipment components.

4. Machinability

Grey cast iron produces discontinuous chips that can easily break off during machining, making them very useful when high degrees of machinability and dimensional stability are needed.

5. Porosity and Damping

Cast iron has a porous, sponge-like structure. That porosity:

• Retains oil and grease, giving partial self-lubrication.
• Provides excellent vibration damping.

That is why machine beds and bases are often made from cast iron.

Types of Cast Iron Used in Industry

Cast iron is not a single material. Its classification depends on carbon content, graphite form, and manufacturing process.

1. Grey Cast Iron

• Contains graphite in flake form.
• Good machinability.
• Excellent damping properties.
• Most commonly welded in repair work.

2. White Cast Iron

• Carbon present as cementite (iron carbide).
• Extremely hard and wear-resistant.
• Very brittle.
• Generally not weldable except in special cases.

3. Chilled Cast Iron

• Rapid cooling during casting.
• Hard outer layer with tougher inner core.

4. Nodular (Ductile) Cast Iron

• Graphite present in spherical form.
• Improved strength and ductility compared to grey iron.

5. Malleable Cast Iron

• Heat-treated white cast iron.
• Better toughness and machinability.

6. Alloyed Cast Iron

• Using alloys such as chromium, nickel, or molybdenum
• Better wear, heat, and corrosion resistance.

Problems in Welding Cast Iron

Welding cast iron presents several issues related to its structural and metallurgical characteristics.

The following issues can negatively affect the prep of welding:

• Brittleness: High carbon content makes cast iron prone to cracking.
• Porosity and contamination: Contaminants such as oil, grease, and foreign materials reduce prepability due to their porosity.
• Carbon migration: During the welding of cast iron, excessive carbon can migrate into the base metal, increasing its hardness and leading to cracking.
• Heat sensitivity: Rapid heating and cooling can produce thermal stresses in the welded joint.
• HAZ cracking: Hard and brittle microstructures can develop in the heat-affected zone through carbon pickup.

If heat input is not controlled, cracking is almost guaranteed.

Techniques to Weld Cast Iron

Because of these challenges, two primary techniques are used:

• Hot Welding Technique
• Cold Welding Technique

Each serves a different repair scenario.

Hot Welding Technique

This approach treats cast iron like a casting process.

Procedure:

• Preheat the component to 350°C–400°C.
• Maintain temperature above 350°C throughout welding.
• Perform welding at controlled heat input.
• Allow for slow cooling after completion.

Cooling guidelines:

• Reduce temperature gradually at approximately 50°C per hour per inch of thickness.
• If cooling in a furnace, turn the power off and allow for natural cooling for the most gradual temperature transition while reducing residual stresses.

This method reduces thermal gradients and minimises residual stresses. It is typically preferred for:

• Large castings.
• Structural repairs.
• Thick sections.

Cold Welding Technique

Cold welding focuses on minimising heat input.

Key practices include:

• Use low current and smaller diameter electrodes.
• Avoid weaving; use stringer beads only.
• Maintain a short arc length (preferably touch-and-weld technique).
• Deposit short bead lengths of 25–30 mm maximum.
• Allow cooling between beads.
• Perform hot peening using a ball peen hammer to relieve residual stress.

Cold welding is often chosen for:

• In-situ repairs.
• Large components that cannot be uniformly preheated.
• Maintenance welding.

Electrode Selection for Cast Iron

For weldable grades of cast iron, nickel-based electrodes are widely used. These may be:

• Pure nickel electrodes – for soft, machinable deposits.
• Ferro-nickel electrodes – for higher strength and better dilution tolerance.

Ador’s Solution for Cast Iron Welding

While repairing cast iron using welding, you need to be precise, disciplined and use the right consumables.

Ador Welding has an exclusive range of Low Hydrogen welding electrodes (LH) has been designed and developed specifically for cast iron repair applications. These electrodes are suitable for both the hot welding and cold welding techniques.

Whatever your needs may be, whether you need to repair damaged housing, or you have a casting that has broken, or you are doing maintenance on a cast iron component, the LH electrode from Ador will give you a reliable solution.

FAQs

1) Why is welding cast iron difficult?

Cast iron is brittle and high in carbon, which makes it prone to cracking during rapid heating and cooling.

2) Is preheating necessary before welding cast iron?

Yes, preheating helps reduce thermal shock and minimizes cracking by allowing more uniform heat distribution.

3) What type of filler rod is used for welding cast iron?

Nickel-based filler rods are commonly used because they reduce cracking and accommodate cast iron’s expansion and contraction.

4) What are common mistakes when welding cast iron?

Common mistakes include skipping preheating, cooling the part too quickly, using the wrong filler material, and applying excessive heat input.