ARC MAGAZINE

Overcoming The Challenges Of Welding Cast Iron

Gray cast iron is a family of iron-based materials produced by pouring molten, high-carbon iron, alloyed with silicon, into a mold and allowing it to slowly cool and solidify into a finished product. Gray cast iron is the most common among the many varieties of cast iron materials.

Although often machined after casting, gray cast iron was never intended to be joined by welding. The metal has limited ductility and may crack if the proper welding procedures are not followed.

Typically, welding on gray cast iron is limited to repairing foundry defects, cracks developed in service, and the build up of worn parts.

While gray cast iron’s high content of carbon, silicon, sulfur and phosphorus can be beneficial to the casting process, these elements generally cause problems with weldability. The carbon content of gray cast iron is usually between 2 and 4%. By comparison, common structural steels that are usually specified for welded fabrication have a carbon content of less than 0.30%. There are also limits placed on the content of silicon, sulfur and phosphorus for these steels, further improving weldability.

Shielded metal arc welding (SMAW) and oxy-acetylene braze welding are used extensively for gray cast iron repair work, although most of the major welding processes, including TIG (GTAW) and MIG (GMAW), can be utilized as well.

Filler metals for repairs are usually designed to give a soft, ductile deposit. This is often achieved by using copper- or nickel-based filler metals, which do not harden by picking up carbon from the base metal. The idea is to minimize shrinkage tensile forces on the base metal.

Economical repairs can be made with special iron-based electrodes designed for cast iron repairs. However, iron-based electrodes produce deposits that are hard and not machinable, whereas nickel- and copper-based filler metals can be machined.

Copper-based filler metal should not be used when braze welding on gray cast iron parts that will be subjected to service temperatures above 500 degrees Fahrenheit.

Preheating is preferred when welding gray cast iron to slow the cooling rate of the weld and area surrounding the weld. This will reduce cracking potential and improve the weld area’s machinability. Preheat the entire workpiece slowly, uniformly and thoroughly to a minimum of 500 degrees Fahrenheit. The preheat should not just concentrate on the weld area. Covering the casting in an insulating material immediately after welding will further control the cooling rate and reduce the risk of cracking.

Sometimes, preheating the entire part is not feasible, such as when welding on a massive machine base or an assembled engine block. Welding repairs can still be accomplished in this case by keeping the part cool through a sequence of short passes and allowing the weld to cool between each pass. Also, lightly peen each weld as it cools to help relieve shrinkage stress.

Gray cast iron is tricky to weld, because it is not meant to be welded. But that doesn’t mean it can’t be welded. Patience by home hobbyists and repair welders will help produce quality welds to restore the workpiece to usable condition.