PART 3 : CAST IRON WELDING METHOD

To minimize heating of the piece, the smallest possible diameter electrode should be used for the first passes that are in contact with the cast iron; 3/32 inch (2.5 mm) electrodes are best for this purpose. Similarly, you should select the lowest current setting possible that will provide good wetting and perfect adhesion. Adhesion is often enhanced by using alternating current. Avoid weaving the electrode when welding; direct the arc into the weld metal to minimize dilution; and make beads a minimum of 3/4 to 2 inches (20 to 50 mm) in length. The beads should be peened with a round-heated tool while they are still hot enough : over 1000°F (540°C).

The purpose of peening is to deform the bead through compression forces to reduce the effect of the shrinkage stresses created during cooldown. Be sure not to use a pointed tool such as a slag-removing hammer, because any holes it may leave behind could become sources of cracking.

If the deposited metal contains porosities, this means that the base metal is contaminated or impregnated with sand. You must then remove the bead with a cold chisel then seal this part of the piece with a steel-core electrode designed for cast iron (Sodel 352) before continuing welding.

The arc should be struck within the groove to prevent creation of a brittle spot on the piece. To minimize the effect of local overheating, successive beads should be spaced out over the entire area of the groove, taking care always to lay down the beads in the same direction, ensuring that the beads are not aligned so that they do not create a rupture plane, and making sure to fill in each crater.

It is sometimes preferable, especially with thick pieces, to butter the faces of the groove with a nickel type electrode (Sodel NI99, Sodel 355) before finishing the fill with a ferronickel type electrode (Sodel NI60, Sodel 35, Sodel CU89). This way, the metal deposited in the buttering will absorb part of the stresses by deforming readily, thus reducing the risks of cracking.

The use of nickel type electrodes (Sodel NI99, Sodel 355) is not recommended when more than three layers are required because deposits with very high nickel content are subject to hot-cracking. It is better to complete the fill using a ferronickel type electrode (Sodel NI60, Sodel 35, Sodel CU89) or to alternate nickel and ferronickel type electrodes.

Welding must always be done beginning in the most restrained areas and working toward those which are less restrained. The following diagram, below, illustrates this principle.

When replacing part of a piece with a steel plate, the best technique is to make a hole in the center of the plate, cut it into four segments, then join the four parts tothe piece before welding them together, as per the sequence shown in the diagram. It is also important to minimize heating using the method described at the beginning of this section. The holes prevent crack propagation; they are filled in when the rest of the weld complete.

When the thickness of the piece requires filling in several overlapping passes, this must be done from the outset by progessing over the full thickness of the groove to allow the opening as much play as possible during welding (see diagram below).

If the hot welding method is used, bead length can vary from 3 to 5 inches (75 to 125 mm). This welding method is otherwise the same, except that cooldown should be as slow as possible and never greater than 30-55°F (15-30°C) per hour. To achieve this cooling rate, you can use insulating blankets.

With hot welding, an oxyacetylene rod that deposits grey cast iron (Sodel 65FC) can be used. This way, the deposited metal will have a metallurgical structure and color similar to that of the piece.

You must, however, select high preheat temperatures between 1000 and 1200°F (540 and 650°C), use wider groove angles (up to 120°), and round off the edges and base of the joint well.

The interpass temperature should not fall below 600°F (315°C) and must be careful not to overheat the piece beyond 1250°F (675°C).

The flame used must be neutral or slightly reducing.

First, form a 1 inch (25 mm) weld metal pool at the root of the groove, keeping the inner flame of the torch 1/8 – 1/4 inch (3-6 mm) from the surface. Then gradually move the flame from one face to the other until they melt into the weld metal pool. Then direct the flame toward the rod to add filler metal to the weld pool. Each pass should not exceed 3/8 inch (10 mm) in thickness.

Slag cleaning

Slag on the deposit can be removed with hand tools and a stainless steel brush. When making multipass welds, all traces of slag must be removed from the weld before going over the bead.

Postheating

Post-weld annealing can be done to :

• improve the ductility of the heat-affected zone ;
• enhance the machinability of the deposit and the heat-affected zone ;
• relieve residual stresses

The temperature, time at temperature, heating and cooling rates vary according to the type of cast iron and its alloy content. The manufacturer is the best person to contract for the choice of heat cycle. Generally, for unalloyed grey cast iron, annealing at 900°F (500°C) followed by cooling in free air reduces residual stresses by 30%, while annealing at 1100°F (600°C) reduces them by 50%. To eliminate them almost completely, you must :

• raise the temperature to 1650°F (900°C) ;
• hold the piece at that temperature for 60 minutes per inch (25 mm) thickness;
• then air cool

For ductile cast iron, the cycle that gives the piece maximum ductility consists in :

• heating to between 1650 – 1750°F (900-950°C) for one hour plus one hour for each inch (25 mm) thickness ;
• oven-cooling down to 1275°F (890°C)
• holding at this temperature for five hours plus one hour for each inch (25 mm) thickness ;
• oven-cooling down to 650°F (345°C) at the rate of 100°F (55°C) per hour ;
• allowing it to air cool

PRACTICAL TIPS FOR WELDING CAST IRON

1- To clean grease or other contaminants from cast iron, heat the pieces uniformely to between 700 – 1000°F (370 – 540°C) until all volatizing stops (about one hour). Please refer to Surface and joint preparation.

2- If there are cracks on the piece before welding, stop them from propagating by making holes in their line of extension at about 3/8 inch from their apparent ends.

3- To minimize cracking risks during welding, use ”U” shaped preparations and round off the edges.

4- To detect the presence of a hardened zone, use a drill to check whether the bit can penetrate the piece.

5- When a hard layer (hardened zone) exists in the joint, remove it before welding

6- Remove any apparent traces of grinding using a chisel or file before welding.

7- To improve the mechanical strength of the joint, insert studs into the groove surface.

8- Cast iron can be welded without preheating – see the Preheating section

9- Using alternating current for welding cast iron produces very good adhesion.

10- To limit heat input, make beads about 1 inch (25 mm) long and lay them down one after another in a random and discontinuous process.

11- Buttering the pieces using a nickel electrode before welding helps lower the risks of cracking by reducing the stresses within the cast iron.