Cast iron has a very high carbon content (usually 2-4%), which easily produces cracks and pores during welding, so nickel-based electrodes are commonly used for manual arc welding (MMA) or TIG welding. As for MIG welding, it can only be used for non-structural repairs or fillings. MIG welding can also be used to weld cast iron.
Why cast iron is not suitable for MIG welding
1. Characteristics of cast iron: high carbon content easily forms hard and brittle white structure after welding, which is very easy to crack; because the extremely low plasticity of cast iron will produce a large contraction force after MIG welding, further causing cracks.
2. High heat input and fast cooling of MIG: The process characteristics of MIG welding are high-efficiency continuous wire feeding, high heat input, deep melting, large molten pool, and fast cooling after welding, which will aggravate the embrittlement of the structure and shrinkage stress. At the same time, MIG welding is difficult to perform short-section welding and timely hammering like manual welding. Temperature control and stress release during welding are limited, and the risk of cracks is higher.
3. Ordinary carbon steel welding wire used for MIG is not compatible: The chemical composition of the commonly used MIG carbon steel welding wire (such as ER70S-6) is very different from that of the cast iron base material, and it is impossible to form a transition layer with both strength and toughness. The joint area between the weld and the base material is very easy to be brittle. Therefore, expensive nickel-based welding wire or stainless steel welding wire must be used for welding cast iron, and MIG equipment is less adaptable to such welding wire.
4. Cast iron welding process requires flexible control: Cast iron welding requires strict process measures, including sufficient preheating at 200-400℃, short weld segmented welding, weld hammer stress release, and slow cooling after welding, etc. However, the MIG welding process is continuous and highly automated, and it is not easy to flexibly meet these requirements. The process control is not flexible enough and the adaptability is poor.
5. Mismatched application scenarios: MIG welding is more suitable for mass welding of materials such as low-carbon steel, stainless steel, and aluminum alloys, and pursues high efficiency and automation. However, cast iron welding is mostly repair and maintenance work, which requires small-batch, flexible, and highly controlled welding processes. Therefore, it does not match cast iron welding itself in terms of application scenarios.
How to use mig welding for cast iron
- Selection of welding wire and shielding gas: Ordinary carbon steel welding wire should not be used when welding cast iron, but nickel-based welding wire (such as ERNi99) should be preferred, which can significantly improve the crack resistance and toughness of the weld. If conditions are limited, stainless steel welding wire (such as ER309) can also be used, but the effect is still inferior to nickel-based welding wire. For shielding gas, pure argon or a mixed gas of 75% argon + 25% carbon dioxide can be used to obtain better gas protection and arc stability.
- Preheating and cooling control: The high carbon characteristics of cast iron make it very easy to crack during welding, so preheating is a key step. It is usually necessary to preheat the welding area to 200-400°C to reduce thermal stress and cooling rate. Immediately after welding, the weld should be covered with insulation wool or asbestos cloth to allow it to cool slowly to avoid shrinkage stress and white embrittlement caused by rapid cooling.
- Welding process operation: When using MIG to weld cast iron, the weld must be welded in sections as short as possible, and the length of each weld should be kept at 3-5 cm. After welding a section, the welding slag should be cleaned and checked for cracks. The current should be controlled in a low range to avoid excessive heat input and prevent the molten pool from being too large, causing embrittlement of the base material and cracking of the weld. Tap each weld with a hammer while it is hot to help release shrinkage stress.
For cast iron welding, MMA welding uses cast iron containing iron, which is the most suitable. MIG welding can only be an alternative choice. In addition to welding techniques, you must also choose a suitable welding machine.