PROCEDURE FOR WELDING CAST IRON OR IRON CASTING.

Cast irons are alloys of iron, carbon, and silicon in which elements such as phosphorus, sulfur, manganese, etc. are generally present. Their carbon content is normally greater than 2%, ranging between 2.5 and 4.5%.
They are characterized by acquiring their shape directly through casting, as these alloys cannot be subjected to cold or hot plastic deformation processes.

These types of materials are more common than believed; they are found in machine bases, pump housings, transmission cases and engines, gears, cast liners, cast and malleable couplings, manifolds, mills, and monoblocks. They are classified according to their microstructure, which is formed based on the alloying elements that comprise them. Among the most widely used are:

• Gray cast irons.
• White cast irons.
• Nodular cast irons.

Gray cast iron: Gray iron is widely used in various machine components, many types of gearboxes, housings for electrical equipment, pump and turbine casings, gears, engine monoblocks, and many other automotive parts. The ASTM identification system for gray cast irons establishes that the identifying number corresponds to the minimum tensile strength in thousands of pounds per square inch, so that ASTM Gray Iron No. 20 has a minimum tensile strength of 20,000 lb/in². This type of cast iron is weldable using a special procedure; heat input must be controlled, as cracking may occur.

White cast iron: It is used in grinding components due to its high resistance to wear and impact. This type of cast iron contains a high percentage of phosphorus and sulfur, which makes its weldability very complex and nearly impossible; furthermore, its low cost makes it easily replaceable.

Nodular cast iron: It retains greater tensile strength than gray cast irons, features similar wear resistance and hardness, exhibits high thermal shock resistance, and offers greater machinability.

Some typical applications of nodular iron are: crankshafts, pistons, and cylinder heads for automobiles and diesel engines. In steel mills: work rolls, furnace doors, table rollers, and bearings. Wrenches, levers, cranks, clamping frames, and chucks. Various dies for forming steel, aluminum, bronze, brass, and titanium. These components are weldable by controlling heat input and using nickel-based coated electrodes.

Basic weldability guide for cast iron: to carry out the welding process on gray or nodular cast iron, the following steps must be taken into account:

1. Base metal identification. (Type of cast iron). The first step is to identify what type of cast iron it is, in order to properly select the filler material and preheat temperature.
2. Part dimensions. Verify what type of work is to be performed; if cracks or fissures are found, they must be completely removed. Keep in mind that cracks propagate with heat; it is recommended to drill a 3 or 4 mm hole at the crack tips to prevent further propagation.
3. Mechanical service conditions. Analyze what type of service the part to be welded will perform. Remember that this is a repair, and the resulting loads may cause damage to the part again. Sometimes it is better to drill and fasten with bolts or reinforcement plates instead of welding.
4. Failure or wear analysis. Verify what caused the failure, as in the vast majority of cases it may recur; therefore, the welded joint or repair must be done perfectly.
5. Filler material selection and welding procedure. Gray and nodular cast irons should be welded using 100% nickel-based electrodes; depending on the type of application, they may also be welded with 60% nickel electrodes. They can likewise be welded with copper or copper alloys — in this case, carefully analyze the type of loads the part will be subjected to. The welding procedure must be controlled; preheat must be applied according to material thickness and chemical composition — consult the carbon equivalent tables for this purpose. After preheating, deposit short weld beads no longer than 25 millimeters, and perform stress relief by peening. Once the welding procedure is complete, apply post-weld heat treatment according to the carbon equivalent tables, and allow slow cooling in a bed of completely dry lime or sand, or use thermal blankets, in order to avoid abrupt temperature changes.