Monel metal is used as a filler rod when welding cast iron.

Explanation:
When it comes to welding, the choice of filler material is crucial for achieving strong and durable welds. Different metals and alloys have varying properties, and selecting the appropriate filler material depends on the base metal being welded.

1. Cast Iron:
Cast iron is a strong and brittle material that is prone to cracking during the welding process. It contains high amounts of carbon and often includes other elements such as silicon, manganese, sulfur, and phosphorus. To successfully weld cast iron, a filler material with similar properties and composition is required.

2. Monel Metal:
Monel metal is a nickel-copper alloy that is known for its excellent corrosion resistance, high strength, and good weldability. It is often used in applications where resistance to corrosive environments is essential. Monel metal contains approximately 65-70% nickel, 20-29% copper, and small amounts of iron, manganese, carbon, and silicon.

3. Welding Cast Iron:
Welding cast iron can be challenging due to its high carbon content and brittleness. The welding process must be carefully controlled to prevent cracking and distortion. Preheating the cast iron and using the appropriate filler material are crucial steps in achieving successful welds.

Monel metal, with its similar composition and properties to cast iron, is an ideal choice as a filler rod when welding cast iron. The use of Monel metal as a filler material helps to minimize cracking and distortion during the welding process. It also ensures a strong and reliable weld joint.

In summary, Monel metal is used as a filler rod when welding cast iron due to its similar composition, good weldability, and ability to provide strong and corrosion-resistant welds.