How to weld the CAST IRON?

Cast iron can often be repaired by welding when there are some casting defects.

There are many kinds of welding methods, which are mainly based on the material, size, thickness, complexity, type and size of defects, and the requirements of cutting and technical requirements.

What's the alloy?

Cast irons are a family of iron-carbon alloys. Their high carbon content (usually 2–4%) gives cast iron its characteristic hardness. However, that hardness comes at the expense of ductility. It is less malleable in comparison to steel or wrought iron. The heating and cooling cycles during welding cause expansion and contraction in the metal, inducing tensile stress. Cast irons do not stretch or deform when heated or stressed—instead, they crack—making them extremely difficult to weld. This characteristic can be improved by adding different alloys.

To facilitate a better understanding of these materials, they can be divided into five groups, based on composition and metallurgical structure: white cast iron, malleable cast iron, grey cast iron, ductile cast iron and alloy cast iron.

WELDING OF CAST IRON 

Clean the casting

Regardless of the alloy, all castings must be properly prepared prior to welding. While preparing the casting for welding, it is crucial to remove all surface materials to completely clean the casting in the area of the weld. Remove paint, grease, oil, and other foreign material from the weld zone. It is best to apply heat carefully and slowly to the weld area for a short time to remove entrapped gas from the weld zone of the base metal.

A simple technique for testing the readiness of the cast iron surface is to deposit a weld pass on the metal—it will be porous if any impurities are present. This pass can be grinded off, and the process repeated a few times until the porosity disappears.

Pre-Heating and Welding

If the welding is performed with electrode or oxyacetylene stick having the same chemical composition the whole workpiece should be homogenously pre-heated up to app. 600℃.

The welded piece should be cooled down slowly in furnace, hot sand or ash.

When performing "cold weld" on cast iron, the welding with short passes (20-30 mm) should be preferred and the welds should be immediately hammered. Overheating the workpiece during welding should be avoided.

It is recommended that the workpieces with complex geometry should be pre-heated to 300 - 350℃ before welding even if nickel content welding electrodes are used.

The cracks that are not outside the workpiece should be welded outside-in.

Welding technique

Welding techniques should be chosen based on their suitability to the cast iron alloy being welded. The most common welding processes are stick, oxy acetylene, and braze welding.

Stick welding, also known as shielded metal arc welding or MMA, makes use of a consumable electrode covered with a flux. Different types of electrode can be used depending on the application, the color match required, and the amount of machining to be done after welding.

Oxy acetylene welding also makes use of electrodes, but instead of an arc generated by current, an oxy acetylene torch provides the energy for welding. Cast iron electrodes, and copper zinc electrodes, are both suitable for oxy acetylene welding of cast iron.

Care must be taken not to oxidise the cast iron during acetylene welding, as this causes silicon loss and the formation of white iron in the weld. The welding rod should be melted in the molten weld pool, rather than directly by the flame, to minimize temperature gradients.

Braze welding is a common method for joining cast iron parts due to the minimal impact on the base metal itself. A welding rod provides the filler that adheres to the cast iron surface. Because of the lower melting point of the filler compared to the cast iron, the filler does not dilute with the cast iron but adheres to the surface.