How to weld Cast Iron


How to weld Cast Iron, Welding Rod for Cast Iron

Although cast irons are considered weldable, they are much harder to weld than mild steel/ carbon steel. The degree of brittleness and likelihood of cracking of the welded material will depend on the type of casting, the heat treatment, and the welding procedure that is followed. As an example, an SG cast iron is more likely to absorb welding stresses than grey cast iron. In general, most cast irons can be welded using all standard welding processes such as SMAW, FCAW, GMAW and Braze Welding.

How to weld Cast Iron

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Different cast iron types and their weldability

In general, most cast irons can be welded using all standard welding processes such as SMAW, FCAW, GMAW, Braze Welding. Below is a chart of the cast irons that can be welded using various consumables.

In general weldability of different cast iron is classified below:

Gray cast iron – Weldable with difficulty.

White cast iron – Unweldable, almost.

Malleable cast iron – Welding changes the properties making it useless.

Ductile cast iron – Rarely weldable, slow, and extensive process.

Microstructures of Cast Iron (Courtesy Mr. Vikram S.)

Welding electrode for Cast Iron Welding

-Fusion welding of cast irons are normally done with SMAW electrodes such as the Nickel 99 (AWS A5.15 ENi-Cl), and Nickel 55 (AWS A5.15 ENiFe-Cl) or their bare wire equivalents.

-Braze welding of cast irons is normally done with brazing rods such as the bare of flux coated Low Fuming Bronze (RBCuZn-C) or the Nickel Silver (RBCuZn-D). Additional brazing flux during welding aids in the flow characteristics of the molten puddle.

Below table summarize the various Cast Iron Welding consumables for different cast iron types with different welding processes.

How to weld Cast Iron

General Cast Iron Welding Procedure – New and Used Castings

The most important aspect in the welding of all cast irons, is to have the surface clean and free of defects prior to welding.

On new castings;

– All sand, slag and scale must be removed from the area of the casting to be welded by mechanical means such as grinding, machining, chipping or rotary burrs.

– Physical defects such as blowholes, sand inclusions, soft spots, shrinkage cracks and pin holes need to be removed. Quite often a pinhole will open up to expose a large cavity hidden underneath.

– Cracks should be ground out to their full length and depth, with no part of the crack remaining. If not done correctly, the crack could propagate after welding causing failure in the casting. 

On used castings;

– All contaminates that have impregnated the casting while in service must be removed to prevent porosity during the welding process. This can again be done by mechanical means such as grinding, machining, chipping or rotary burrs.

– Cracks should be ground out to their full length and depth, with no part of the crack remaining. If not done correctly, the crack could propagate after welding causing failure in the casting.

Cleaning before & during Cast Iron Welding or Repair

During cleaning, grinding wheels can become coated with carbon or other contaminates which can then be smeared on the finished surface, re-contaminating the part to be repaired. For this reason, the final 1/16” should be cleaned using a new grinding wheel or by chipping, rotary burrs or a coarse file. Also, an oxidizing flame from an oxy-acetylene torch can be used to burn off any surface graphite or other oily contaminates. Do so by press the cutting trigger on the torch and run the torch up and down the area to be welded, making sure that you do not overheat the casting. The added pressure caused by pressing the cutting trigger will help to clean out any contaminates that are trapped in the porous surface of the casting. This also provides light preheat which is advantageous.

When a casting has become so impregnated with contaminates (e.g. gear boxes), that standard practices do not clean off the casting adequately enough to make a sound weld, the elimination of the residual can be achieved by heating the casting to 400-600°F for 2-3 hours followed by wire brushing and/or polish grinding. This will help overcome porosity and poor welds. Cast irons that are “out-gassing” during welding should be heated to a dull red for a short period of time before welding. This will help “degasify” the cast iron prior to welding. Wire brush and/or a polish grind the part before welding to remove remaining surface contaminates. On heavier or large castings where heating is unattainable, weld a “butter pass” or a thin weld bead on the surface area of the casting to be welded. A butter pass separates the contaminates that are in the casting from the area to be welded. If porosity, comes through the butter pass, grind the butter pass down by 50% of its thickness and do another butter pass. Repeat until a clean surface is obtained, and then complete the weld that is required using proper cast iron welding techniques. Arc-air gouging is not usually recommended. However, it can be used to remove the bulk of metal providing the last 1-2mm is removed by grinding.

Welding Repair for cracks in Cast Iron

How to weld Cast Iron

To repair cracked castings, drill a hole at each end of the crack to prevent it spreading further and grind out to the bottom. Begin welding at the drilled end of the crack, where restraint is greatest and move towards the less restrained are Welding Factors to Consider;

1. Cast irons have low ductility and are in danger of cracking due to stresses setup by welding. (This is not so important when welding SC iron due to its good ductility)

2. Welding can cause formation of a hard-brittle zone in the weld area. This is caused by rapid cooling of molten metal to form a white cast iron structure in the weld area and makes the weld unsuitable for service where fairly high stresses are required.

3. The formation of a hard, brittle weld bead due to pick-up of carbon from the base metal. This does not occur with weld electrodes or wires which do not form hard carbides such as Monel and high nickel alloys. These are also used where machinable welds are desired.

Preheat/ Interpass for Cast Iron Welding

-For Arc Welding

Although preheating is not always necessary when welding cast irons, due to the lack of ductility in cast irons, cracking can be minimized using a preheat.

-For Brazing

In general, all cast irons need to be preheated when oxy acetylene braze welding to reduce the heat input requirements.

1. Local preheating occurs where parts not held in restraint may be preheated to about 1000°F in the area of the weld, with slow cooling after welding is completed. Cracking from unequal expansion can take place during the preheating of complex castings or when the preheating is confined to a small area of a large casting which is why local preheating should always be gradual.

2. Indirect preheating involves a preheat of 400°F on other areas of the part, in addition to local preheating. This is done so that the overall part will contract with the weld and minimize contraction stresses. Such a technique is suitable for open frames, spokes etc.

3. Complete preheating is used for intricate castings, especially those varying in section thicknesses such as cylinder blocks. It involves complete preheating to 1000°C in a furnace, followed by slow cooling after welding. The preheating temperature should be maintained during welding. A simple preheating furnace may be made of bricks into which gas jets project.

Stress-relief, PWHT of Cast Iron

After welding on a cast iron part, especially with welds intended for use in severe service or subject to close machining tolerances, cool the casting as slow as possible by putting the part back into the preheating furnace or encapsulating it in an insulating blanket or an insulating powder/sand. It is sometimes the practice to post heat welded joints to relieve stresses and soften hard areas. If done, heat the casting to a temperature 1100- 1150°F. The casting should be held at this temperature for one hour per 1” of thickness. The cooling rate should not exceed 50°F per hour until the casting has cooled to about 700°F. (For maximum softening and stress relief, heat to 1650°F followed by slow cooling to 1000°F or lower.) To obtain optimum ductility, the above heat treatment should be carried out immediately following welding.

For the best results with SG cast iron, the casting should be placed in a furnace (1100-1200°F) and the temperature brought up to 1650°F. The casting should be held at temperature for 2-4 hours, then cooled to 1300°F, held there for 5 hours, cooled again to 1100°F, and finally cooled to room temperature. Malleable cast iron may be reheat-treated after welding.

Peening after welding for Cast Iron Welding

Satisfactory welds may be made on cast iron without preheating, by using proper electrodes that deposit soft metal and peening the weld with a blunt tool (such as a ball hammer) immediately after welding. This spreads the weld metal and counteracts the effect of the contraction of the cooling weld metal. Good welding practice is to deposit short weld runs (~2” at a time) and then peen before too much cooling takes place.

Grey Cast Iron Welding

Single-layered welds using the Nickel 55 are not as machinable as Nickel 99, however they do have increased strength and ductility. Nickel 55 welds are more tolerant towards contaminants such as Sulphur and phosphorous and are superior to Nickel 99 electrodes when welding castings that are high in phosphorous.

-Peening is a must for grey cast irons.

-Joining of cast iron to steel can be performed with either Nickel 55 or Nickel 99, but Nickel 55 is preferred.

-Ferrous based electrodes, including hydrogen-controlled types (E7018) are generally not recommended for welding cast iron.

-Brackets, lags and even wear plates can be attached to castings using the correct welding parameters and Nickel 55.

SG Cast Iron Welding

SG cast irons should only really be repaired using Nickel 55 due to its higher tensile strength and better ductility.

-Penetration should be low and wide joints or cavities should be built up from the sides towards the center.

-Stringer beads or narrow weaves should be used.

-Deposit short beads and allow the weld to cool back to preheat temperature.

-Peening is advisable but not as critical as when welding grey cast iron.

Austenitic Cast Iron Welding

-When attempted, Austenitic cast irons usually welded with Nickel 55.

-The weld may be unsuitable for applications where corrosion/heat resistance qualities do not match the parent metal.

Cast Iron Brazing/ Braze Welding

Cast Iron Brazing

-For successful braze welding of cast iron, it is essential that the part be preheated to a dull red heat.

-A neutral or slightly oxidizing flame should be used with a medium or heavy-duty welding nozzle.

-The temperature should be maintained during welding. It is necessary to use a furnace to ensure even heating of large castings.

-It is important that the casting be protected from drafts during welding. Sudden chills to the cast iron can cause the formation of white cast iron, which is very hard and brittle. This may cause cracking or make subsequent machining impossible.

Braze welding should only be used to repair old castings because of the poor color match. Braze welding is suitable for grey, SC and malleable cast irons, however joint strength equivalent to fusion welds are only possible with grey cast iron.

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