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

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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.

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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

It is possible to weld cast iron, although it can be problematic due to the high carbon content. This carbon content is often around 2&#;4%, which is about ten times that of most steels. The welding process causes this carbon to migrate into the weld metal and/or the heat affected zone, leading to elevated brittleness/hardness. This, in turn, can lead to post weld cracking.

Cast iron is made up of iron and carbon in different proportions, with additional elements such as manganese, silicon, chromium, nickel, copper, molybdenum, etc. to enhance specific properties. In addition, it may contain significantly higher levels of sulphur and phosphorus as impurities making it difficult to weld without cracking. The different grades of cast iron include grey iron, white iron, ductile (nodular) iron, and malleable iron with widely varying weldability. All categories of cast iron except white iron are considered as weldable, although the welding can be significantly more difficult compared with carbon steel welding. However, it can be difficult to tell the difference between these different types of cast iron without detailed metallurgical analysis. Despite this, cast iron is a durable, wear resistant metal that has been used for centuries.

Contents

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As mentioned above, cast iron can be difficult to weld because of its specific composition, but it is not impossible if you use the correct welding technique to avoid weld cracks. This involves careful heating and cooling, often including pre-heat, the correct choice of welding rods, and allowing the part to cool slowly.

There are a number of key steps that can be taken to help ensure effective welding of cast iron. These include:

Cast iron has poor ductility hence it could crack due to thermal stresses when rapidly heated or cooled. The susceptibility to cracking depends on the cast iron type/category. This means it is required to understand which type of alloy you are working with:

Grey Cast Iron

This is the most common type of cast iron. It is basically an iron-carbon-manganese-silicon alloy with 2.5-4% carbon. The carbon precipitates into graphite flakes during manufacture into either a ferrite or pearlite crystalline structure. However, these graphite flakes can dissolve during welding and precipitate as high carbon martensite, embrittling the heat affected zone and the weld metal.

White Cast Iron

White cast iron is free of graphite and contains carbon in combined form as metal carbides making the microstructure brittle.  White iron is generally considered as unweldable.

Ductile (Nodular) Iron

Ductile iron is similar in composition as grey iron but the impurity levels are low compared with grey iron. Unlike the grey iron, which contains carbon in the form of graphite flakes, the ductile iron contains the graphite as spheroids in its matrix. The rest of the matrix is mostly pearlite with a ferrite region surrounding the graphite spheroids.

Malleable Iron

Malleable iron is heat treated white iron with substantially lower carbon content compared with the white iron. Depending on the heat treatment employed, it generally has as mixture of ferrite or pearlite structure with nodules of graphite and hence has more ductility compared with the standard white iron.

The simplest way to determine which type of iron you are working with is to check the original specification. Chemical and metallographic analysis can also help in identifying the category of the cast iron that you are working with.There are some other ways to tell the difference between alloys; grey iron will show grey along a fracture point, while white iron will show a whiter colour along a fracture due to the cementite it contains. However, ductile iron, for example, will also show a whiter fracture, yet is much more weldable.

It is important to clean the cast iron before welding, removing all surface materials, such as paint, grease and oil, paying particular attention to the area of the weld. The casting skin may be removed through grinding. It is essential that the cleaned surface is wiped with mineral spirits to remove the residual surface graphite prior to the welding.  Slowly preheating the weld area for a short time will help remove any moisture trapped in the weld zone of the base material.

The most important factor in avoiding stress cracking in cast iron is heating/cooling control. This is to minimise the residual stresses build-up during the heating and cooling process.

Localised heating, such as the one encountered during welding, results in restricted expansion as the HAZ is contained by the surrounding cooler metal. The thermal gradient will determine the resulting stress. Ductile metals like steel is able to relieve the stress by stretching, but because cast irons have poor ductility they are liable to crack instead. Pre-heating reduces the thermal gradient between the HAZ and the surrounding casting body, minimising the residual stresses caused by welding. Pre-heating cast iron before welding slows the cooling rate of the weld and the surrounding area. Where possible, heat the entire casting. Typical minimum pre-heat temperatures are from 100-400°C, depending on the type of the cast iron and the allowable HAZ hardness . Any pre-heating should be done slowly and uniformly.

Theoretically, any of the common arc welding processes such as manual metal arc welding, flux cored arc welding, metal active gas welding, submerged arc welding, tungsten arc welding, etc. can be used, a process which facilitates slow heating and cooling is generally preferred.

1. Manual Metal Arc Welding (MMA)

This type of welding, also known as shielded metal arc welding (SMAW), is generally believed to be the best overall process for cast iron welding &#; provided that the correct welding rods are used. The choice of electrode will depend on the application, the required colour match and the amount of post-weld machining.

The two main electrode types for manual metal arc welding are iron based and nickel based. Iron based electrode will produce weld metal with high carbon martensite, hence generally limited to minor repair of casting and when colour matching is required. Nickel alloy electrodes are the most commonly used, offering a more ductile weld metal. Nickel electrodes can also help to reduce the pre-heating and HAZ cracking by providing a lower strength weld metal.

In all cases, care should be taken to minimise the parent metal melting. This will minimise the dilution. 

2. MAG Welding

MAG welding is generally carried out with a nickel consumable. An 80% argon to 20% carbon dioxide gas mix will work for most applications. While brazing wire can be used, it is generally not recommended as braze metal will be significantly weaker than the casting.

3. TIG Welding

TIG welding can provide a clean weld on cast iron, but not generally preferred due to its highly localised heating characteristics.. As with all TIG welding, the quality of the finished weld is largely determined by the skill of the welder.

Find out more about TIG welding

4. Oxy Acetylene Welding

As with arc welding, oxy acetylene uses an electrode, but rather than an arc generated by electrical current, this process uses the oxy acetylene torch to generate the heat. The low heat intensity and slow heating associated with the process will result in a large HAZ, but the slow heating in beneficial in preventing the formation of high carbon martensite in the HAZ. The low heat intensity of the process will require preheating to a higher temperature, typically in the region of 600°C, to make the welding feasible. A neutral or slightly reducing flame is used for the welding.

Find out more about oxy acetylene welding

5. Braze Welding

Braze welding may be used for welding cast iron parts, since it has a minimal impact on the base metal itself. Once again, a filler rod is used for this process except it adheres to the surface of the iron rather than diluting into a weld pool due to the lower melting point of the filler.

As with other techniques, cleaning the surface is important with braze welding. A flux can be used to prevent oxides forming, promoting wetting, cleaning the surface and allowing the filler to flow over the base metal.

TIG brazing is also possible, using a lower amperage to heat the workpiece while avoiding melting the cast iron. The argon gas shroud of the torch shields the brazing zone, meaning that there is no need to use flux as with oxy-fuel.

Find out more about braze welding

As mentioned above, the choice of welding rod is important for welding cast iron, although most experts would advise using nickel rods.

1. 99% Nickel Rods

These electrodes are more expensive than other options but also provide the best results. 99% nickel rods produce welds that can be machined and work best on castings with a low or medium phosphorous content. These pure nickel rods produce a soft, malleable weld deposit.

2. 55% Nickel Rods

Less expensive than 99% rods, these are also machinable and are frequently used for thick section repairs. A lower co-efficient expansion means that these produce fewer fusion line cracks than the 99% rod. These ferro-nickel rods are ideal for welding cast iron to steel.

Less expensive options are available, such as steel rods, although these are not as effective as nickel rods:

3. Steel Rods

Steel rods provide the cheapest option of the three and are best for minor repairs and filling. Steel electrodes produce hard welds, which require extra grinding to finish and are not machineable. However, despite these drawbacks, steel rods provide colour matching and can better tolerate castings that are not completely clean than the nickel rods.

Peening

As a weld cools and contracts, it causes residual stress to build, leading to cracking. The chances of cracking can be reduced through the application of compressive stress. Compressive stress is applied by peening (using a ball peen hammer to deliver moderate strikes), which deforms the weld bead while still soft. However, peening should only be used with relatively ductile weld metal, that is on welds produced with nickel consumables.

Post-Weld Heating

Allowing cast iron to cool down too rapidly can lead to cracking. The cooling process can be slowed down by using insulating materials or the periodic application of heat. Some methods include placing the workpiece in an insulating blanket, placing it into dry sand, or even putting it over a wood fire oven and allowing the metal to cool as the fire dies down.

It is possible to weld cast iron, but it needs to be done using the correct techniques and with care to avoid cracking. Most welding methods require the surface of the material to be cleaned and cast iron benefits from pre and post-weld heating as well as careful cooling.

TWI has decades of expertise in all aspects of welding and joining, including working with cast steel. Please contact us, below, if you have any questions and feel we could assist you with your project.

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