How to Weld Cast Iron: Easy to Follow Steps & Tips

Cast iron can be challenging to weld because of the materials’ properties. Unfortunately, it is often necessary due to cracking related to metal elongation. Do not mistake cast iron for steel as the techniques for one type of iron do not always apply to cast iron. Fortunately, welding cast iron can be done successfully with some practice and experience.

Welding Cast Iron Requires Lower Heat and Small Beads (1″) At A Time to Avoid Cracking

The three important considerations when welding are:

1. Preparation – beveling or drilling holds before and after cracks, cleaning the surface
2. Amount of heat – always low and slow
3. Selecting the right rod – more nickel the better

Cast Iron Alloy Basics

Cast iron cast belongs to a family of carbon alloys with around 2.4% to 4.5% of carbon content. It is an alloy of silicon, carbon, and Ferrum (iron). The hardness of carbon makes it one of the most used materials in the construction business. The percentage of carbon mixed with the iron is what makes cast iron a desirable alloy in that it can be machined, damps vibrations, low shrinkage, and high fluidity.

Carbon in cast iron also makes the metal brittle and subject to cracking when put under a strain. For example, steel has a 20% elongation before cracking vs cast iron which has 1% elongation.

What makes cast iron one of the most challenging materials to weld is that it has fairly unusual behavior. When welding, cast irons do not stretch or deform when heated. Rather the metal can crack when exposed to heat and then cooling, which makes the entire welding process a challenge.

Cast Iron Will Crack If Stressed or Elongated 1% to 2%

Cast Iron Melting Point

Cast iron has a melting point of 1,100° to 1,300° Celcius (2,112° – 2,372° Fahrenheit) due to the high carbon content. Steel has a higher melting point at 1,450 C (2,642F). Since cast iron has a lower melting point, it is easier to use when working with molds, pipe fittings, manifolds, valve housings, pumps, transmissions, water jackets, liners, heads, and engine blocks.

Types of Cast Iron

Below are some of the most common cast iron alloy types. Each one of these alloys has special properties that may require different welding tools and techniques for the best results.

1. High alloy (made by adding alloys to ductile, white or gray cast iron)
2. Ductile
3. Malleable
4. White
5. Gray (most common and used about 90% of the time. Made with coke, limestone, steel scrap, cast iron scrap, and pig iron). The name is from the grey color of any cracks or fractures.

The Gray Iron Name Comes From the Color of Fractures or Cracks In The Iron

General composition of Grey Cast Iron:

• Silicon 2% – 2.4%
• Carbon 3%- 3.25%
• Manganese .6% – .7%
• Phosphorous .2% maximum
• Sulphur .2% maximum
• Ferrum Remainder

Cast Iron cannot be bent or forged into a specific shape, or the metal will crack. It can be machined and is wear-resistant.

Cast Iron Identification

Cast iron can be identified by the following properites:

• No visible welding joints
• Visible Casting line from two mold halves
• Chip at metal using a hammer and chisel. If the metal chips in small fragments it is cast iron. If you see one continuous chip, then the metal is steel.
• Spark testing will produce weak red sparks, followed by stars which are yellow. Steel produces small sparks at the end.

Cast Iron Welding Methods: Arc, TIG or MIG

While all three methods can be used, we recommend Arc for home welding.

MIG: MIG requires an Argon gas mixture (80% argon, 20% carbon dioxide) and a nickel wire.

TIG: Like MIG, use a nickel wire. See this video for detailed directions.

ARC: ARC is simple to do and good for basic repairs. Use high quality Nickel Rods, with a 99% composition. 55% nickel composition can be used which is cheaper. Go with the 99% if you can.

Preparing to Weld Cast Iron

If Joining Two Pieces, Create a 45° Bevel. Leave 1/3 of the Material Depth Thickness at the Bottom

1. Clean the metal surface using either chemicals or a brush. Thoroughly remove any paint, rust, grease, and oil.
2. Removing any casting skin on both sides of the weld (approximate 25/32″ or 20 mm).
3. Remove any imperfections such as porosities, fatigued metal, cracks, and blowholes.
4. Drill 1/8″ holes approximately 1/8″ inch from each end of the crack to be repaired. The holes will keep the crack from expanding beyond the holds during the repair process.
5. “U” or “V” out the crack using a gouging electrode or grinding machine. The groove should be 90°. If welding together pieces, the fracture should be at a 45° bevel. Leave 1/3″ of thickness below the “V.” Electrode gouging is the most efficient method.
6. Edges or sharp corners should be rounded off, particularly if you are going to machine the surface or fill it after the weld. The goal is to prevent excessive base material melting during the welding process.
7. If grinding the weld area, remove any carbon (graphite) the is on the surface so that it doesn’t become part of the welding pool.

Before Welding Drill a Hole at Each End of the Crack to Avoid Additional Damage to the Base Metal

Welding Methods

There are three methods for welding cast iron. Which one you choose depends on the task.

1. “Cold” electric arc welding: used on larger parts where the item being welded cannot be completely pre-heated, making this the most common approach.
2. Hot welding (electric arc, oxy/acetylene welding, brazing) – used on parts that can be preheated with the welding torch such as smaller parts. After welding parts need a slow cool-down to room temperature. Cooling can be helping if parts are burying after welding in cinders, sand, or kieselguhr.
3. Polymer repair: used when the metal is oxidized making “hot” welding impossible. In this case, a product like plastic steel is used.

Oxy-Acetylene Welding (Hot Welding)

Oxy-acetylene welding is a type of welding used for smaller parts that can be heated wiht the torch ( referred to as gas welding).

This type of welding is performed using the following steps:

• First, you have to adjust the oxy-acetylene flame per the surface requirements. Too little flame can be lead to a fragile weld. On the other hand, a strong flame may cause the cast iron to crack. You need to stay in between these two spectra.
• Holding the torch in your right hand and the filler rod in the left hand, use a constant motion to start welding from the right-hand side end of the plate to the left-hand side.
• For perfect results, the flame has move side-to-side or in a circular motion so that the blend mixes well. The filler rod should be moved backward and forward along the plate.
• Continue doing this until you see an enclosure and no cracks or leaks!

MIG welding is another useful method, however, it can be costly because of the use of the specialty iron-nickel wire. However, it comes with a benefit, a clean weld without any residue or post welding mess. 

An alternative option is to use a stainless steel MIG wire, although there could be some irregularities in the final results.

As mentioned, this method is used when parts cannot be pre-heated, or parts are challenging to dismantle.

Cast Iron Cold Welding Steps

The “cold” in cast iron cold welding refers to the low heat needed to the base metal.

Recommended Amperage

A low amperage is needed to avoid deep fusion between the base material and filler. The reason why deep fusion is avoided is to keep graphite and the resulting iron carbide away from the weld pool. Suggested Amperages are:

Cold Welding Amperage Suggestions

These suggestions are for down hand or flat welding. If using overhead welding, reduce the numbers below by 5% to 10%. If vertical welding reduce by 5%.

Electrode Size

2,5 (3/32″)

3,2 (1/8″)

4,0 (5/32″)

5,0 (13/64″)

Approximate Amperage

50- 80

70- 110

100- 140

130- 170

Settings are not exact and will vary based on the machine, line load and job size. Try a test weld to determine if the amperage is too high, resulting in a red hot electrode. The goal is to use as low an amperage as possible.

Cast Iron Electrodes

Use the largest electrode that works with the groove size, but don’t use an electrode that is so large that you can’t get into the groove. A larger electrode will reduce heat input relative to the amount of filler metal being used.

Nickel Electrodes (AWS A5.15 E Ni-Cl): Use this type of electrode if the cast iron is old, thin, or oil impregnated. Only use this to “butter” the sides of the cast iron to create a surface seal. Then finish with a Nickel Iron Electrode.

Nickel Iron Electrodes (AWS A5.15 E NiFe-Cl-A): Used to join steel and cast iron, stainless steel, and copper alloys. Used on cast iron that is subject to sudden loads, vibrations, and strain.

Gouging Electrodes

Electrode Size Tip:

If you see porosity in the first bead, switch to a smaller diameter electrode, lower amperage setting, and higher welding speed. The goal is to reduce heat input into the base material.

Polarity

Different polarities (Straight, Reverse, AC) will have different effects on the melting and heat input of the base material.

DC Straight Polarity: This type of polarity connects to the minus pole and results in high heat input to the base. In turn, it causes excessive digging and melting. Excessive melting brings impurities into the weld, lowering weld quality.

Reverse Polarity: This type of electrode results in a shallow and wide weld zone with minimal gasses, sulfur, phosphorous, and graphite. It also reduces the formation of iron carbides.

If using direct current, a reverse polarity electrode is the best choice. However, if the iron is contaminated, you can try higher heat with straight polarity.

Using DC Reverse Polarity (electrode to the plus pole) Will Enable You to Get a Shallow Eld with Minimal Gasses, Sulfur, Phosphorous, and Graphite. Only Use Straight Polarity (Electrode to the minus pole) Should Be Reserved For Heavily Contaminated Cast Iron

Arc Length

The shortest arc possible should be used to reduce voltage and minimize heat to the base material. It could be helpful to use a long arc during the first pass, and then reduce arc size.

Welding Cast Iron

As mentioned, heat input needs to be minimized in order to reduce the chance of cracking. The metal is extremely brittle with cracking at 1% to 2% elongation.

DO NOT use long continuous beads. with a maximum 1″ length (25 mm). Do not weave and only use to “wash out” deposits.

DO NOT wave more than 1/2 electrode diameter to each side of the direction of the weld. After a bead has been placed, fill the crater and withdraw the electrode with a backward motion.

Use a round-nosed peen hammer to peen the bead while it is hot. This will stretch the beed and provide some relief from stress. Preen starting from the back of the crater to the place that you started. Use moderate strikes, so that there is only a slight indentation. Heavy strikes can result in cracks. You can use a small pneumatic hammer.

After the first beed, the next beed should not be placed until the first beed is able to be touched with a bare hand. If you feel a burn , it is too hot. DO NOT RUSH.

You can place beeds that are spaced to avoid any heat build-up. This is called skip-welding and is good for larger jobs/longer cracks.

DO NOT build up beads on top of each other. Instead, use a stepwise approach with each bead moving 1″ at a time.

A StepWise Approach is Recommended With No Bead Being Longer Than 1″

Take your time and use the “cold” method if you can. Use a high quality electrode that is large enough to work the groove.

FAQ

What kind of welding rod do you use on cast iron?

Cast iron is most commonly welded using a pure nickel or ferro nickel rod. Ferro nickel is 47% nickel and 53% steel and is cheaper.

Can you use a MIG welder to weld cast iron?

The answer is yes, but not recommended. You can use Arc, TIG or MIG. MIG nickel wire is expensive but can be used with 80% Argon, 20% Carbon Dioxide gas. TIG nickel wire is the only option for this type of welding. The preferred method is ARC welding using 99% nickel rods. You can use 55% nickel rods, but they have a lower co-efficient expansion.

What is the best way to weld cast iron?

While you can use TIG, MIG, or ARC, we recommend ARC. Preheat the cast iron before welding and go low on the heat and slow on the weld.

Can cast iron be welded or brazed?

Yes, cast iron can be welded or brazed. Brazing is often used to rebuild and machine parts. Consider using the Lincoln Electric 1/8″ flux-coated brazing rods since they fuse well with cast iron.

Reference eBook

How to Weld Cast Iron

TE Andersen Consulting

Welding Handbook