PART 2 : CAST IRON WELDABILITY

Surface and joint preparation

It is important to clean the surface to be welded properly to remove any trace of oil, grease, rust, dirt, liquid-penetrant inspection solution, or paint using the appropriate solvents. Because of its porous nature, cast iron tends to become impregnated with such contaminants and should therefore be heated to between 700 and 1000°F (370 and 540°C) for about one hour or until volatizing no longer occurs. Be careful not to cause localized overheating, or to heat or cool the piece too quickly, to prevent cracking (see preheating section).

All defects or cracks must be removed. To prevent cracks from propagating during preparation or welding, it is best to block them by drilling holes ahead of their line of propagation about 3/4 inch (10 mm) from the apparent end of the cracks. The diameter of these holes should be 1/4 inch (6 mm) for lesser thicknesses and larger when the piece’s cross-section is greater. If available equipment does not allow drilling, a bead 1 to 11/2 inches (20 to 40 mm) long can be welded across the crack at each end. If a bead cracksndue to welding stresses, the end of the crack must be located again and the operation must be repeated.

If the part has already been welded, we must check for the presence of a hard layer near the welded bead. A quick and efficient method to detect a hardened zone is to use a drill to check whether the bit can penetrate the piece. All hardened zones must be removed before welding.

Since cast irons are brittle, it is better to make grooves in the shape of a ”U” and to round off their edges. Also, when nickel-based filler products are used, groove angles should be about 30% more open than for steel,  and the root face of the joint should be thinner to ensure thorough penetration. For thicknesses greater than 1/2 inch (13 mm), using ”U” or ”J” grooves on both sides will minimize the amount of filler metal and shrinkage stresses.

For grey cast irons with a rupture load less than 40 000 psi (275 MPa), you can groove only two-thirds of the thickness for pieces over 1/2 inch (13 mm) when ferronickel type welding electrodes (Sodel Ni60, Sodel 35, Sodel CU89) are used.

There are several available methods for removing defects, cracks, mill scale, or oxide films, and for chamfering. The edges to be welded can be machined, ground, chiseled or arc gouged using a Sodel 512Plus electrode. Arc gouging is the fastest method but can result in the formation of a thin hardened film. When such a hardened film forms, it must be removed by mechanical means prior to welding. When grinding is used as the preparation method, grinding traces or residue should be removed with a file or cold chisel.

If the piece shows extensive cracking, a part of it can be removed and replaced with a mild steel plate. To minimize the risks of cracking, it is important to round off the corners of the plate well. Given the high tensile strength of mild steel, the thickness of the plate can be up to 33% less than that of the piece for grey cast iron with a tensile strength less than or equal to 40 000 psi (275 MPa).

For thick pieces, brittleness in the heat-affected zone can be minimized by gouging grooves in the joint face (see cast iron diagram below). These grooves are filled with the selected filler metal, then the faces are buttered and welded. The welding shall be done as described on the next article : Cast iron welding method. This method prevents the formation of continuous weakness planes in the heat-affected zone, thereby minimizing the propagation of cracks that may occur in service, or during shrinkage of the deposited metal during colldown.

Another method of reinforcing the joint is to insert studs into the chamfer surface (see cast iron diagram below). When the studs are made of steel, there should be enough of them so that their surface area equals about 30% of the piece’s cross section in the area of the break.

The studs are screwed or pressed into pre-drilled holes to a depth equal to the diameter of the stud or more.

Furthermore, the studs should not all be inserted to the same depth and they should not be aligned across from one another. The studs should protrude from 1/8 to 3/16 inch (3 to 5 mm) above the chamfer face so that they will bond well with the deposited metal. The studs are then welded in place and the surfaces of the chamfer are buttered; the surfaces are then welded together. This technique bonds the deposited metal to the part of the piece that has not been thermally affected.

Preheating

When grey, malleable, ductile or compacted graphite cast iron is welded, part of the heat-affected zone tends to change into white cast iron or a brittle structure. This transformation occurs during cooldown and is promoted by their high carbon content, their somewhat greater alloy element content, and the speed with which heat dissipates through the piece’s cross section.

To prevent the heat from dissipating into the piece too quickly, it can be preheated to between 600 and 1200°F (315 and 650°C). It must be protected from any drafts, however, to prevent cracking problems. In addition, it is not always possible to preheat the piece because of its size or the equipment involved.

For all these reasons, it is usually better to use the cold weld method. With this method, the piece does not need to be preheated provided it is at room temperature, 68°F (20°C). Also, the temperature differential between the weld and base metal should be limited to around 90°F (50°C) so that the welder can keep his bare hand in prolonged contact with the area around the weld. In other words, for a piece at 68°F (20°C), the temperature in the vicinity of the weld should not exceed 158°F (70°C).

Even when the cold weld the cold weld method is used, it may be desirable to preheat certain parts of the piece. Doing so will cause the joint to open when these parts are heated, thereby putting the joint under compression during cooldown, thus reducing the amount of shrinkage stress. The following cast iron diagram illustrates this effect :

Next part : Cast iron welding method (3)